Kuhi no ka lima, hele no ka maka.

Where the hands move, there let the eyes follow.

(A rule in hula.)

‘Ōlelo No‘eau, compiled by Mary Kawena Pukui, #1868 

Figure 13.1 Shave Ice: A rainbow of sensation; Shave Ice

If you are riding a wave on your favorite board or waiting for that ‘ono fish to bite the hook, be grateful to your reflexes, at least in part, for your success. Your reflexive response to the swift jolt of your surfboard, or the tug on your fishing line, happens so fast you do not even need a moment to plan your movement. A reflex is an involuntary response to a stimulus. We are born with some impulses, and others are learned. If the body is abruptly thrown off balance, reflexes cause the optimal muscle contractions to regain your footing. That is only a part of what happens because we also have the voluntary (conscious) movements that we plan and control in addition to the movements of a reflex. If there is a tug on the fishing line, the reflex may be to quickly pull up with the rod, but then there will be a planned sequence of subtle movements to lure in the fish.

Figure 13.2 Reflexes in Action: Surfing and fishing in Hawai‘i

These movements, involuntary and voluntary, rely on sensory receptors, sensory nerves, interneurons, motor nerves, and muscles. Most of the signals travel through the pathways of the spinal cord white matter and connect and become integrated into the spinal cord gray matter. Sensory information enters the spinal cord on the dorsal side, is integrated with other information in the gray matter and motor information leaves from the ventral side in a reflex arc. In this chapter, you will learn how all this information is processed to help you maintain balance in more ways than you may imagine.

13.1 Spinal Cord

Spinal Cord Anatomy

The spinal cord, along with the brain, makes up the central nervous system (CNS). They are enclosed within a protective 3-layered covering called the meninges, which will be discussed in more detail in Chapter 14, The Brain. For now, you should understand the basic structure of the meninges which includes a thin delicate inner layer called the pia mater (pia = tender), a middle spider web-like layer called the arachnoid mater (arachnoid = spider-like), and a tough protective outer layer called the dura mater (dura = hard) (Figure 13.3). In addition to protecting the spinal cord, the dura mater extends to cover and protect the pairs of spinal nerves that extend from the cord at the level of each vertebra. There is a space associated with the arachnoid mater called the subarachnoid space filled with cerebrospinal fluid which adds another level of protection to the meninges. Outside of the dura mater is a space called the epidural space filled with fat and large blood vessels. The adipose tissue contributes to the protection of the spinal cord. Large blood vessels in the epidural space provide a convenient route to the spinal cord during the administration of an epidural analgesic injection which provides pain relief during childbirth.

Human spinal cord with meninges	Cross Section of the Spinal Cord

Meningeal Layers	Arachnoid Mater and Dura Mater

Figure 13.3: Meningeal layers The pia mater, arachnoid mater and dura mater layers of the meninges surrounding the spinal cord

Figure 13.4: Spina bifida and subtypes (occulta, meningocele, myelomeningocele)] (OpenStax)

Within the meninges, the spinal cord is located in the vertebral canal. The bone of the vertebrae, along with the vertebral ligaments provides a great deal of protection and support for the spinal cord. In some children, the vertebrae fail to develop caudally (towards the tail) leaving the cord vulnerable and, in the worst cases, completely exposed. This condition is called spina bifida and is associated with a lack of folic acid intake during fetal development. There are varying levels of severity of this disorder. In the least severe form that involves a hidden defect is called occulta (occult = hidden) (Figure 13.4). The more serious conditions that involve the projection of meninges or meninges and spinal cord through the skin are called meningocele or meningomyelocele, respectively.

Although all the causes of spina bifida are not known, it is clear that the risk of having a baby with spina bifida can be significantly reduced with access to healthcare and nutrition. Adequate prenatal care can help to screen out any supplements or medications that could cause spina bifida. Also, healthcare practitioner-guided supplementation of folic acid is important. Spina bifida occurs in the first few weeks of pregnancy, so ideally a woman would talk to her healthcare practitioner before she gets pregnant to learn about folic acid supplementation ahead of time. Given that nearly half of all pregnancies are unplanned in the U.S. it makes early prenatal care even more important.

To help stabilize the cord in some regions of the canal, the pia mater extends laterally, forming the denticulate ligaments to prevent the cord from moving back and forth. The spinal cord is divided into spinal segments which give rise to the axons of the dorsal roots and ventral roots that come together to make up the paired spinal nerves that exit from each intervertebral foramen (figure 13.5). A thickening of the cord at the cervical enlargement and lumbar enlargement accounts for the extra tissue required for sensory and motor functions of the upper and lower extremities. The cord does not extend through the entire length of the vertebral canal. Rather, it ends at about L1/L2 (1st or 2nd lumbar vertebrae) in adults or L3/L4 in infants and small children. The difference between adults and infants is because the bones continue to grow long after the nervous tissue has stopped. The spinal cord ends in a point called the conus medullaris and the pia mater extends as a fiber called the filum terminale which, along with the denticulate ligaments, contributes to the stability of the cord in the vertebral canal. Along with the filum terminale, the spinal roots hang off the end of the cord on their way to the intervertebral foramen of the lower vertebrae and, because they appear similar to a horse’s tail, are called the cauda equina.

Vertebral Column	Numbering order of the vertebrae of the human spinal column

Figure 13.5: Spinal cord anatomy

The subarachnoid space that fills the vertebral canal after the cord ends, is a convenient location for medical professionals to collect samples of cerebrospinal fluid in a procedure called a lumbar puncture (LP) or spinal tap. In this procedure, patients curl into a tight ball on their sides to lengthen the intervertebral space so that the physician can insert a needle between the vertebrae and into the subarachnoid space to withdraw the fluid. It is important to do this below the level of the spinal cord (lower than L1/L2 in adults and L3/L4 in children) to avoid causing damage. This is a fairly common procedure used to withdraw fluid to look for blood, bacteria, or signs of inflammation. It can also be used to inject medications such as antibiotics or chemotherapeutic agents to an otherwise inaccessible area due to the blood-brain barrier.

Figure 13.6 Cross-section of Spinal Cord (OpenStax)

Figure 13.7 Dorsal root ganglion microscope slide (OpenStax)

The nervous tissue of the spinal cord itself is arranged into gray matter shaped like a butterfly with white matter surrounding it (figure 13.6). The white matter, composed of myelinated axons, is where we find the ascending sensory pathways delivering sensory information from receptors to the brain and the descending motor pathways carrying motor commands from the brain to the muscles. The gray matter, composed of cell bodies and unmyelinated axons, is where we find certain sensory neurons, interneurons involved in the integration of information, and the cell bodies of the lower motor neurons that give rise to the axons that control our muscles.

The wings of butterfly-shaped gray matter in the spinal cord cross-section have dorsal and ventral regions. Towards the dorsal surface are called the posterior (dorsal) horns and contain cell bodies of sensory neurons. The neurons that bring sensory information into the cord from the body enter here as dorsal roots. These sensory neurons have cell bodies in the dorsal root ganglia which are outside of the CNS (figure 13.7). The parts of the wings towards the ventral surface are called the anterior (ventral) horns and are where we find the large cell bodies of the lower motor neurons. The axons of these lower motor neurons exit each spinal segment as ventral roots. Finally, in the thoracic spinal segments, we find lateral horns composed of the cell bodies of certain autonomic neurons that are part of the fight or flight response. The autonomic nervous system will be discussed in a separate chapter.

Figure 13.8 Locations of ascending sensory and descending motor spinal fiber tracts (OpenStax)

Motor Pathways

As mentioned, the white matter contains the myelinated axons of the ascending sensory and descending motor axons (figure 13.8). As with the gray matter, it can be divided into different regions. Structurally it can be divided into the dorsal columns between the dorsal horns, the lateral columns between the dorsal and ventral horns, and the ventral columns between the ventral horns. Functionally we can divide the white matter into groups of axons, called tracts, carrying specific types of information. These tracts are usually named for the origin and destination of the axons and may include a modifier to explain wherein the spinal cord they are found.

Figure 13.9 Upper and lower motor neurons of the corticospinal tracts (Openstax)

The major descending tract that controls skeletal muscle movements is the corticospinal tract (figure 13.9). It is composed of two neurons, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the primary motor cortex of the frontal lobe of the brain and synapses on the lower motor neuron, which is in the ventral horn of the spinal cord and projects to the skeletal muscle in the periphery.

The lateral corticospinal tract and anterior corticospinal tract (both also called pyramidal tract) are composed of the axons of upper motor neurons. As their names imply they travel from the cortex (cortico-) to the spinal cord (spinal) and travel in either the anterior or lateral columns. These two motor pathways allow the primary motor cortex in our brains to control the lower motor neurons that control the motor units of muscles. One other pathway in the brain called the corticobulbar tract allows the brain to control muscles of the head and neck by innervating lower motor neurons in the brainstem which travel in the cranial nerves. Together, these three tracts are called the direct pathways or pyramidal pathways since they give the motor cortex direct control over the lower motor neurons. Since the left side of the brain controls (and senses) the right side of the body and vice versa, sensory and motor neurons need to cross the midline, or decussate, at some point.

When talking about pathways it is often convenient to describe parts that are traveling on the same side of the stimulus or muscle, or ipsilateral (ipsi- = same, -lateral = side), and the parts that are traveling on the opposite side, or contralateral (contra- = opposite). The lateral corticospinal tract decussates in the medulla of the brain at a point called the pyramidal decussation. The anterior corticospinal tract decussates in the spinal cord. In addition to the direct pathways, there are also several indirect pathways or extrapyramidal pathways (extra = outside of) involving indirect motor control often involving complex circuits. The tectospinal tract involves reflexive movements of the head, equilibrium, and coordination of visual reflexes. The vestibulospinal tract controls muscle tone and posture that responds to vestibular input from the inner ear allowing us to maintain balance. The rubrospinal tract controls limb movement on the basis of cerebellar processing and originates in the red nucleus of the brain. Finally, the reticulospinal tract controls body posture as a response to the reticular system of the brain which controls, among other things, our level of alertness. This is the pathway that causes your body to sag in your chair and your head to nod as your coffee wears off during fascinating lectures involving spinal pathways.

Figure 13.10 Anterior View of Ascending Sensory Pathways of the Spinal Cord The dorsal column system and spinothalamic tract are the major ascending pathways that connect the periphery with the brain. (OpenStax)

Sensory Pathways

Like the motor pathways, the sensory pathways are also divided into different pathways (figure 13.10). The spinothalamic tracts carry information about pain and temperature while the dorsal column-[pb_glossary id="4231"]medial lemniscus tract[/pb_glossary] carries general somatosensory and proprioceptive information. As with the motor pathways, the sensory pathways must decussate at some point. Unlike the motor pathway which involves just two neurons (upper and lower motor neurons) the sensory pathways involve three neurons. The first-order sensory neuron delivers the information from the receptor to the spinal cord. The cell bodies of these neurons are located in the dorsal root ganglia. The second-order sensory neurons decussate and synapse on neurons in the thalamus. These third-order sensory neurons project to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain where the stimulus is finally perceived. Although the two pathways have the same three types of neurons, they travel in different locations and decussate at different levels. The first-order sensory neurons of the dorsal column-medial lemniscus pathway travel in the spinal cord ipsilaterally to the stimulus before synapsing in the medulla. That means that some general sensory neurons travel from your big toe all the way to your brain and are the longest cells in your body. These neurons travel in two bundles in the dorsal columns called the fasciculus gracilis and the fasciculus cuneatus that carry information from your lower body and upper body, respectively. After synapsing on neurons in the nucleus gracilis and nucleus cuneatus of the medulla, the second-order sensory neurons decussate and travel to the thalamus in a tract called the medial lemniscus. In the spinothalamic tract, on the other hand, first-order sensory neurons synapse immediately on second-order sensory neurons in the dorsal horn which decussate and travel in the spinal cord contralaterally to the stimulus before synapsing in the thalamus. Third-order sensory neurons then project to the primary somatosensory cortex in the parietal lobe of the brain where the stimulus is perceived.

The Shingles virus	A child with a case of Shingles

Figure 13.11 The varicella-zoster virus] (a) The Shingles virus (b) A child with a case of Shingles 

Shingles

You may remember having chickenpox as a child. Shingles is caused by the same virus, varicella-zoster virus (VZV), that causes chickenpox (figure 13.11). It is a disease that affects your nerves. After recovering from chickenpox, the virus travels from the skin retrogradely inside of the axons of sensory neurons and stays dormant in the dorsal root ganglia. For most individuals, the virus will remain dormant and they don’t even know that it’s there. But one in three adults may experience shingles when the virus becomes active again and is transported anterogradely inside of the axon back to the skin. We do not yet understand the reasons why the virus switches from inactive to active state, but a weakened immune system or stress may trigger the switch. A patient with shingles may experience tingling and numbness of the skin, develop blisters, pain, chills, and fever. Once the blisters dry up, the patient will see the scab clearing up. Most cases last from three to five weeks. Some people choose to get vaccinated for shingles, depending on their situation.

13.2 Spinal Nerves

The spinal nerves that exit from between the vertebrae via the intervertebral foramen consist of fused dorsal (incoming sensory or afferent) and ventral (outgoing motor or efferent) roots. After they come together every subsequent division is considered mixed motor and sensory with the addition of some autonomic fibers from the lateral horns at the thoracic levels. Because the first 7 spinal nerves are named for the vertebra below it and the rest are named for the vertebra above it, there are 8 instead of 7 cervical spinal nerves. There are also 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal for a total of 31 pairs of spinal nerves (figure 13.2).

Figure 13.12 Nerve Structure The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers (tissue source: simian). LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) (Openstax)

Figure 13.13 Close-Up of Nerve Trunk Zoom in on this slide of a nerve trunk to examine the endoneurium, perineurium, and epineurium in greater detail (tissue source: simian). LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) (Openstax)

Nerve Anatomy and Organization

Because the axons within a nerve are very delicate, they require some protection (figures 13.12 and 13.13). As mentioned above, the tough dura mater extends from the meninges to wrap around each spinal nerve. This protective covering is called the epineurium. As in muscles, the axons are divided into bundles called fascicles. These are also covered with connective tissue called the perineurium. Finally, the individual axons of the peripheral nervous system are covered with an endoneurium. It is because of the endoneurium that severed peripheral axons can sprout and regrow through this “tunnel” to find their targets. The endoneurium is empty after an axon is cut because the peripheral portion degenerates in a process called Wallerian degeneration.

After spinal nerves form, they quickly form branches called rami (singular ramus). The dorsal ramus sends sensory and motor neurons to the skin and the muscles of the dorsal surface of the trunk while the ventral ramus from different spinal segments (except the thoracic segments) merge into a large branching structure called a nerve plexus. At the thoracic level, separate rami called communicating rami branch into autonomic fibers which connect to the sympathetic chain ganglia that run along either side of the spinal cord.

Figure 13.14 Nerve Plexuses of the Body There are four main nerve plexuses in the human body. The cervical plexus supplies nerves to the posterior head and neck, as well as to the diaphragm. The brachial plexus supplies nerves to the arm. The lumbar plexus supplies nerves to the anterior leg. The sacral plexus supplies nerves to the posterior leg. (Openstax)

Emerging from the plexuses that form from the ventral rami are peripheral nerves (figure 13.14). These are the named nerves that go to different regions of the body and carry sensory afferent and motor efferent fibers. There are several plexuses each with many named nerves but we will just look at some of them. First, the cervical plexus which receives input from spinal nerves C1-C5 gives rise to, among others, the phrenic nerve. This is a particularly important nerve because it controls the diaphragm. The spinal segments that contribute to this nerve are C3-C5 giving rise to the mnemonic often used by medical students C3, C4, C5 keeps the diaphragm alive. You could also say it keeps us alive because without the muscles of respiration we cannot breathe. This is why trauma to the neck region is so serious. Any damage above C5 will likely involve the patient being on mechanical ventilation for the rest of their lives. Next, the brachial plexus, which receives input from C6-T1 gives rise to the peripheral nerves of the arm. These include the axillary nerve, the radial nerve, the median nerve, and the ulnar nerve. As with other nerves, these can be damaged. Damage to the radial nerve can cause wrist drop while carpal tunnel syndrome results from compression of the median nerve.

Most people are familiar with the ulnar nerve and know it better as the “funny bone” due to the sensation resulting from impacting the nerve where it passes over the medial epicondyle of the humerus. In this case, not humorous. As with any part of the body, a plexus can be injured. A good example of this is brachial plexus avulsion. During birth, sometimes the baby’s shoulders get stuck, which is called shoulder dystocia. This can cause a tremendous amount of force to be exerted on the arm of the baby which can cause damage to the brachial plexus leading to motor and sensory impairments of the arm. Next, the Lumbar plexus (L1-L5) and the sacral plexus (S1-S5) give rise to several large nerves including the femoral nerve, the obturator nerve, and the sciatic nerve. The sciatic nerve is the source of pain for those who suffer from sciatica, a peripheral neuropathy which can cause pain, paresthesia (abnormal sensation) and/or muscle weakness along the back of the leg to the feet. This pain can be debilitating and can be caused by nerve compression from a ruptured or bulging intervertebral disc, plexus damage, or muscle compression of the nerve often involving the piriformis muscle.

13.3 Sensation

General senses

Sensation and Perception

Describing sensory function with the term sensation or perception is a deliberate distinction. The sensation is the activation of sensory receptor cells at the level of the stimulus and perception is the central processing of sensory stimuli into a meaningful pattern where we become aware of the stimulus. Receptors are the cells or structures that detect stimuli and convert the energy of the stimulus into an action potential in a process called sensory transduction. Perception is dependent on sensation, but not all sensations are perceived.

Sensory Modality: General and Special Senses

Within the realm of physiology, senses can be classified as general senses or special senses. In general, perception of different types of sensation such as light, sound, touch, temperature etc… are called sensory modalities. Special senses are associated with a specific organ structure and include taste, smell, vision, hearing and balance (vestibular). General senses are distributed throughout the body and have receptor cells within the structures of other organs that are not dedicated to the function of sensation. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. While the mechanoreceptors are dedicated to detecting a stimulus, the skin, muscles, and blood vessels are organs that serve another purpose. General senses often contribute to the sense of touch, as described above, proprioception (body position), or to a visceral sense, which is most important to autonomic functions.

Somatosensation is considered a general sense, as opposed to the special senses which are covered in another chapter. Somatosensation (somato- or soma- = body) is the group of sensory modalities that are associated with touch, proprioception, and interoception, which is the sensation of the internal state of the body. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, and proprioception. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and the walls of visceral organs.

Figure 13.15 Tactile receptors in the skin and hypodermis.

Tactile Sensations

If you drag your finger across a textured surfboard, the skin of your finger will vibrate. Such low-frequency vibrations are sensed by mechanoreceptors called tactile epithelial cells (Merkel cells; type I cutaneous mechanoreceptors) (figure 13.15). Tactile epithelial cells are located in the stratum basale of the epidermis. Deep pressure and vibration are transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the hypodermis, or subcutaneous tissue. A light touch is transduced by the encapsulated endings known as tactile (Meissner) corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair root plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles (Ruffini corpuscles; type II cutaneous mechanoreceptors).

Thermal Sensations

Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors (noci- = pain) to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from body temperature. In the human body, we have cold and warm thermoreceptors which are sensitive to just cold and others to just heat, respectively. Extremely cold or warm temperature is sensed by nociceptors which will be discussed in the next section. Hawai’i has many microclimates and believe it or not, there are places on the islands where you need jackets and fireplaces in the house. You can go from laying down at a warm sunny beach and sitting in front of a fireplace sipping a cup of hot chocolate in one day. You can feel and enjoy these differences all thanks to the thermoreceptors!

Pain Sensations

Do you remember the time when you had a cut on your hand and you felt the pain right away? Pain is a very important sensation and it helps us respond to changes in our bodies. Nociception is the sensation of potentially damaging stimuli and we have nociceptors (pain receptors) in every tissue of the body except the brain. Mechanical, chemical, thermal or even photo (bright light) stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of heat associated with spicy foods involves capsaicin, the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C (98.6°F). The dynamics of capsaicin binding with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic (an- = without; -alges = pain), like in products such as Icy Hot™.

There are two types of pain, fast and slow. Fast or acute pain occurs rapidly, within 0.1 seconds after a stimulus is applied. This information is processed and propagated by the myelinated Type A nerve fiber. Recall that myelinated neurons have the insulation layer and thus information can travel faster than the unmyelinated neurons. An example of fast pain is when you are getting a flu shot at the clinics. On the other hand, toothaches are examples of slow or chronic pain which occurs more slowly, usually a second or more after the stimulus is applied. The nerve impulse of a slow pain is processed and propagated by the unmyelinated Type C nerve fibers. Slow pain tends to increase in intensity and can last a longer time.

Figure 13.16 Dermatomes and their corresponding spinal nerves (OpenStax)

Dermatomes

A dermatome (derma- = skin; -tome = segment) is an area of the skin that provides sensory input via the spinal nerves and one cranial nerve to the central nervous system (figure 13.16). The trigeminal nerve (V) is the cranial nerve that serves most of the skin of the face and scalp. We have 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, and 5 sacral nerves. Each of these nerves (C1 being an exception with no dermatome) relays the sensory information from the specific region of the skin to the brain. For the spinal nerves, their sensory neurons arise from the dorsal root ganglion. Therefore, pain from a certain area of the skin may indicate problems with a specific spinal nerve or its dorsal root or root ganglion.

Figure 13.17 Referred pain. OpenStax)

Localization of Pain

Referred Pain

When you read about the warning signs of a heart attack, you will find that patients may have shortness of breath, chest discomfort, and pains in the arms, neck, or jaw. The latter descriptions are known as referred pain where the pain is felt in a surface area near or far from the injured organ. In figure 13.17, you will find that these areas are served by the same segment of the spinal cord. For example, the sensory nerves which serve the heart and the skin area medial of the left arm enter the T1 to T5 segments of the spinal cord. Another example of a referred pain is “ice cream freeze” or “brain freeze” where you develop a headache after eating shaved ice or a cold drink too quickly. The nerve responds by causing rapid constriction and dilation of the blood vessels.

A rainbow of sensation; Shave Ice

Phantom limb is another example of referred pain where the sensation of the missing or phantom limb is felt. This is known as the phantom limb sensation. If the severed nerve endings are activated, the cerebral cortex interprets the sensation as coming from the non-existing limb. This condition may resolve in most individuals within two to three years without treatment. However, phantom limb pain can last much longer and is notoriously difficult to treat.

Pain Relief

Aspirin is a non-steroidal anti-inflammatory drug (NSAID) (figure 13.18). It inhibits pain and inflammation by inhibiting the enzyme called cyclooxygenase (COX). Normally, the COX produces a local hormone, prostaglandins, which stimulate the nociceptors and thus lead to pain sensation and inflammation. With the intake of aspirin, there is less prostaglandin present and less inflammation and pain sensation. Since aspirin is an acid it may damage the linings of the gastrointestinal tract causing gastric ulcers and bleeding. It can also damage the lining of the stomach through several other mechanisms. The risk is particularly high with alcohol consumption. One must be cautious and consult with your doctor in determining the right dosage.

Figure 13.18 Aspirin.

Acupuncture

You might have also heard about using acupuncture as a means to relieve pain. Acupuncture is a form of alternative medicine and it is a component of traditional Chinese medicine where a metallic hair-thin needle is inserted into the skin. The traditional Chinese medicine practitioners believe that the human body has an energy, called qi (pronounced chee), flowing in the pathways or meridians. By inserting the needle in specific acupuncture points of the troubled area, the energy can be re-balanced. Western practitioners view the acupuncture points as locations to stimulate nerves, muscles, and connective tissues and sometimes apply an electrical current to the needles. Another theory suggests that acupuncture relieves pain by activating the sensory neurons and boosts your body’s natural painkillers. Not everyone is suitable for acupuncture and you should consult with your doctor before starting acupuncture.

Proprioception and Balance

Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For example, have you ever stretched your muscles before or after exercise and noticed that you can only stretch so far before your muscles spasm back to a less stretched state? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent the over-contraction of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Another type of receptor called tendon organs (Golgi tendon organs) similarly transduce the tension levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure pressure in the components of the skeletal system within the joint providing an awareness of limb and body position or proprioception. Lamellated corpuscles, also found in joints, detect the vibrations that result from movement. The detection and awareness of movement is called kinesthesia.

Sensory Receptors

Stimuli in the environment activate first order sensory neurons in the peripheral nervous system. Different types of stimuli are sensed by different types of receptors. Receptors can be classified into types based on three different criteria: structure, location, and function. Receptors can be classified structurally based on cell type and their position concerning stimuli they sense. They can also be classified functionally based on the transduction of stimuli, or how the mechanical, light, or chemical stimulus changes the cell membrane potential.

Figure 13.19 Receptor Classification by Cell Type Receptor cell types can be classified based on their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters into a bipolar cell, which then synapses with the optic nerve neurons. (Openstax)

Structural Classification

The receptor cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus (Figure 13.19). The pain and temperature receptors are examples of receptors found in free nerve endings. Lamellated and tactile corpuscles are neurons with encapsulated nerve endings located in the dermis of the skin that respond to pressure and touch. The photoreceptor cell in the retina that responds to light stimuli, such as the rod cell in the image, is an example of a specialized receptor cell.

Location Classification

Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located in the skin or near a moving part of the body, such as a muscle or joint, that interprets the positions of the tissues or their movements.

Functional Classification

The third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals bind to receptors or diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.

Sensory receptors can be further categorized based on the type of stimuli they transduce. Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor. However, some nociceptors respond to other types of stimuli such as mechanical, thermal or light stimuli. Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. As mentioned above, photoreceptors detect light. Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.

Sensory Adaptation of Sensory Receptors

When you stepped into the ocean water at Ala Moana Beach Park, did it feel so cold at first that you wanted to step out of the water? The strange thing is that the feeling would eventually go away if you stayed in the water for a little bit longer. Although the water temperature remained cold, your body had adapted. This process is called sensory adaptation or neural adaptation. Although the constant stimulus is there, our bodies adapt to the stimuli and the sensory receptors develop a gradual decrease in their nerve impulses. As a result, you will experience less sensation and less perception over time. We have fast and slow adaptations. Fast adaptation is conducted by rapid or phasic sensory receptors that respond immediately after the stimuli, usually within hundreds of milliseconds. The responses of the cell diminish very quickly and then stop, as if in phases. Examples of these phasic sensory receptors include those responsible for certain kinds of touch, temperature, vibration, and smell.

When you wake up in the morning, even before you open your eyes you can sense that your body is laying down. Even though the stimuli have been there all night, you can still detect them. This is a type of slow adaptation and it is conducted by the slowly adapting or tonic sensory receptors. In this case, the receptors adapt slowly and only partially to continue triggering nerve impulses as long as the stimulus remains. Examples of these adaptations include those associated with body position, pain, and the chemical composition of the blood. That’s why some pain doesn’t subside rapidly but may persist for a long period.

13.4 Reflexes

You finally finished your A&P homework and are enjoying a walk along the beach, then…ouch! That piece of coral was sharp.

Walking on the Beach

When this happens, sometimes your foot jerks up before you fully comprehend the pain. The signal that moved your foot needed to travel only between your foot, spine, and leg. Those quick, unplanned, automatic movements in response to stimuli are called reflexes. Reflexes that have integration of their neuronal signal in spinal cord gray matter are called spinal reflexes and differ from cranial reflexes that integrate their signals in the brainstem. Withdrawing the foot from something sharp is a spinal reflex. If something starts blows into your eye, you will involuntarily blink and that is a cranial reflex. Notice skeletal muscle is the effector in these examples. That makes these somatic reflexes. You will learn several reflexes in the autonomic nervous system chapter, and the effectors for autonomic reflexes are cardiac muscle, smooth muscle, and glands. If you are ever trying to decide if a reflex is somatic or autonomic, you need to look only at the effector. So while the eye’s involuntary blinking (skeletal muscle) is a somatic reflex, the eye’s pupillary constriction (smooth muscle) is an autonomic reflex. For this chapter, we will focus on somatic reflexes so the effector will be skeletal muscle.

Stepping on Glass	Heat Source

Figure 13.20: Reflex Arc: All 5 components sensory receptor, sensory neuron, integrating center, motor neurons (agonist and antagonist), effector

Reflex Arc

The signal of a reflex travels from the site of the stimulation, through the central nervous system, and out to the effector. The name of this pathway is the reflex arc. Here are the five components of the reflex arc (figure 13.20):

1. Sensory receptors: respond to changes in the environment by detecting a particular stimulus. If the stimulus is large enough, an action potential will be triggered in the sensory neuron.
2. Sensory neurons: carry signals from the sensory receptors to gray matter in the spinal cord or brainstem. Other neurons relay the signal here and carry it to the brain to provide stimulus awareness.
3. Integration (reflex) center: is the gray matter of the brain and spinal cord (CNS). In a monosynaptic reflex (mono- = one), there is a single synapse between the sensory neuron and the motor neuron (there are no interneurons). In monosynaptic reflexes, the integration happens at the axon hillock of the motor neuron where temporal and spatial summation occurs. In most cases, there is a polysynaptic reflex (poly- = many) that has at least one interneuron in between the sensory neuron and the motor neuron. Interneurons carry signals to other interneurons and motor neurons.
4. Motor neurons: carry signals from the CNS to the effector.
5. Effector: creates the action at the end of the reflex arc. For a somatic reflex, this is skeletal muscle and for the autonomic reflex, it is cardiac muscle, smooth muscle, or glands.

Figure 13.21: Stretch Reflex: reflex arc labeled pathway with leg, thigh, muscle spindle, patellar ligament, hammer, a section of spinal cord

Somatic Spinal Reflexes

The stretch reflex is a monosynaptic reflex and is one of the ways your body protects you from overstretching a muscle (figure 13.21). The most likely stretch reflex you may have witnessed is the patellar reflex, which is kicking of the foot in response to tapping the patellar ligament while the patient sits on a bench with legs dangling off the edge. SImilar reflexes occur in other places such as the ankle, elbow, and wrist.

When the patellar ligament is tapped, it tugs, via the patella, on the tendon that attaches the quadriceps femoris muscle. That tug on the muscle is detected as a stretch by the muscle spindle, which is the sensory receptor of our stretch reflex arc pathway. The sensory signal travels into the spinal cord, synapsing with a motor neuron that causes the stretched muscle to contract. Why does it contract? Think about how your bones are moved. When the muscle contracts, it shortens, and that moves the bones. Now go back in your mind to the beginning of this reflex arc. It was the stretching that set the whole reflex in motion. The shortening of the muscle (the reflex) is relieving that stretch. Of course, the shortening of the quadriceps femoris will also cause the foot to kick forward and that is the most observable part of this reflex. You can see that if a patient does not have the foot kick response, then there may be damage somewhere along the pathway of this reflex arc.

Another way you may have experienced the stretch reflex is when stretching your calves (soleus or gastrocnemius) and a coach or physical education teacher shouts, “Keep holding that stretch!” It is because during the initial moments of a stretch, the muscle spindle in the muscle you are trying to stretch is sending the signal for the stretch reflex, causing that muscle to shorten (tighten) and you need to hold the stretch a moment so the reflex will stop and you can then get some stretch as you intended in that muscle. It takes a moment for the reflex to stop, almost as if your body is saying, it’s okay, we’re not stretching too far, we can let it go.

There are two additional pathways activated during the stretch reflex. One of them is inhibition of the antagonistic muscle. An antagonistic muscle counteracts or works against another muscle (the agonist). For our example, the sensory neuron that carries the excitatory signal into the spinal cord and to the motor neuron to cause the contraction of the quadriceps femoris also excites an interneuron that inhibits the motor neuron that projects to the hamstring muscle. Through hamstring inhibition, the reflex of the quadriceps femoris contracting can be more effective. This type of circuit is called reciprocal innervation and is essential to muscle coordination. If the agonist and antagonist muscles contracted at the same time, it could cause the leg to be immobilized because the muscles would be trying to move the bones in opposite directions. In order for any muscle to shorten its antagonist must lengthen or there will be no movement. The other pathway in this scenario is a signal that branches off the sensory neuron to send a signal up the spinal cord and to the brain to provide awareness of the contraction and the reflex.

Figure 13.22 Golgi tendon reflex

The tendon reflex (Golgi tendon reflex)  is your body’s way of protecting you from holding on to too much weight or generating too much force and damaging your muscle. If the weight or force becomes too great, the muscle relaxes (figure 13.22). Rather than relying on muscle spindles, the tendon reflex relies on tendon organs (Golgi tendon organs) receptors that are inside the tendon near the attachment to its muscle. Let’s imagine you are sitting on the edge of a table with your leg dangling and a heavy bucket hanging from the top of your foot. If you tried to lift the bucket by straightening your knee, you would create the force for that lift by contracting (shortening) your quadriceps femoris muscle. But what if someone added some rocks to the bucket, making it too heavy? When the force is too great, creating too much tension on the tendon attached to your quadriceps femoris, the tendon organ sends a signal into the spinal cord. The sensory signal activates an interneuron that inhibits the motor neuron that projects to the muscle associated with that tendon (quadriceps femoris in our example). That is the agonist muscle. The sensory neuron also activates an interneuron that excites the antagonistic muscle, in this case, the hamstring muscle. The result is that your extended leg, holding the bucket up, would suddenly bend at the knee because your quadriceps femoris would relax (recall it was inhibited) and the hamstring would contract. As in the stretch reflex example, this is reciprocal innervation and there is also a branch of the sensory neuron that carries the signal to the brain where we perceive the sensation.

Figure 13.23: Flexor Withdrawal and Crossed Extensor Reflex: all components for both legs with a cross-section of spinal cord showing neuronal pathways

At the beginning of this reflex section, we mentioned how you quickly jerk up your foot if you step on a sharp piece of coral. But if you suddenly lift one foot, it is important that you just as swiftly place down your other foot so you don’t fall over. For this, we need a reflex pathway that crosses to the other side of the body. Notice the stretch reflex and tendon reflex pathways are contained within a single side of the body. If the sensation comes in on the left side, the motor activity goes out to the left side. That is an ipsilateral reflex. That is not enough for when you step on a sharp piece of coral. We also need a contralateral reflex. These two simultaneous reflexes are called the flexor withdrawal and crossed extensor reflexes (figure 13.23). The sensory neuron in your foot sends a pain signal to the spinal cord to activate interneurons. Some of the interneurons send signals to motor neurons that travel out through that spinal cord segment to thigh flexor muscles. Other interneurons send signals to additional spinal cord segments and synapse with more motor neurons to activate leg flexor muscles. Together, these flexor muscles work to withdraw the foot from the sharp coral. This is called the (flexor) withdrawal reflex. That takes care of only half the challenge, and all that was on the same side of the body. Now we need to get your other foot firmly placed on the ground. In response to the pain signal from that sensory neuron, interneurons in your spinal cord send signals that not only cross to the other side of the spinal cord but also travel to additional spinal cord segments to create the signals necessary to extend your other leg and place your foot on the ground. This is the crossed extensor reflex and is the part of the circuitry that makes this a contralateral reflex.

13.5 Spinal Cord Injury and Neurologic Exam

Do you remember when we talked about jumping off the side of the Waikiki Walls in Chapter 7 the axial skeleton? The vertebral column protects the spinal cord. Spinal cord injury is a common result of trauma (such as jumping and hitting the ocean floor) and can lead to many types of neurological impairments. In the U.S. nearly 20,000 people suffer a spinal cord injury each year and almost 300,000 individuals are living with spinal cord injury. Because the average age of individuals suffering from spinal cord injury is in the early forties and damage is usually permanent, this means that they will live with their injuries for many years. The most common cause of spinal cord injury is motor vehicle accidents (MVAs) followed closely by falls.

Spinal cord injury can cause many different types of neurological deficits. After the initial trauma patients may suffer from any combination of loss of sensation, paresthesia, neuropathic pain, muscle weakness, and paralysis. Forms of weakness or paralysis include paresis, which is partial paralysis or weakness of a limb, paraplegia, or paralysis of the lower extremities, quadriplegia, or paralysis of all four limbs, and hemiplegia which is paralysis of one side of the body.

One useful exercise to understand the results of spinal cord trauma is the examination of a spinal cord hemisection or Brown-Sequard hemisection. In this type of injury, usually caused by injury to a vertebra, the left side or the right side of the spinal cord is transected (severed). If you recall from the section on spinal cord pathways, the axons for different types of sensory information travel in different pathways. Pain and temperature second-order neurons are located in the dorsal horns and decussate immediately and travel contralaterally to the stimulus in the spinothalamic tract of the spinal cord. Touch and proprioception second-order neurons, on the other hand, are in the brain and therefore the first-order neurons in the dorsal column of the spinal cord travel ipsilateral to the stimulus. Descending upper motor neurons decussate in the medulla and therefore travel ipsilateral to the muscle in the corticospinal tracts of the spinal cord. This means that, following a hemisection, patients will suffer from paralysis on the same side as the injury. They will also suffer from loss of sensation and proprioception on the same side as the injury. However, they will lose pain and temperature sensation on the opposite side of the injury.

Tactile and Tonicity Exams

The neurologic exam is used to assess the function of the nervous system. In this section, we will consider only the portion that is relevant to the content of the spinal cord. To assess sensory function there are several tests. One of those tests is called two-point discrimination and involves testing a patient’s ability to distinguish between two points of contact on the skin as being separate. As the points move closer together, when the patient can no longer distinguish them as two separate sensations you have reached their limit of discrimination. When there is neurological damage, a patient’s level of discrimination may be significantly impaired. Due to our detailed understanding of dermatome structure, as illustrated in the figure (see dermatome figure 13.16), it is usually possible to isolate the location of the lesion within one or two spinal segments. Other sensory tests involve other sensory modalities such as pain and temperature and involve testing a patient’s ability to distinguish hot versus cold or to sense painful stimuli such as a needle prick.

Like sensory deficits, motor function can also be impaired. Motor function is tested by instructing the patient to push against your hand and pull using different muscles. For example, pushing down on a patient’s shoulders and telling them to shrug or push against your hand. Although detecting major impairment is easy, detecting subtle impairment takes years of practice and testing on many different types of patients. One extreme example of motor impairment is paralysis. Paralysis can be described as spastic or flaccid. Spastic paralysis is due to upper motor neuron damage whereas flaccid paralysis is due to lower motor neuron damage.

Reflex Examination

Another extremely important clinical test is testing reflexes. The most common spinal reflex tested is the stretch reflex. However, because the test involves tapping the muscle tendon with the reflex hammer, we call these the deep tendon reflexes (DTRs) clinically. As you can imagine, this name can lead to some confusion since there is another type of reflex called the tendon reflex (Golgi tendon reflex). Common deep tendon reflexes tested include the patellar reflex (i.e. the knee jerk reflex), the triceps brachii reflex, the biceps brachii reflex, the brachialis reflex, and the Achilles reflex (i.e. the ankle jerk reflex). When tapping on the tendon, the reflex that is elicited can be described as normal, hyporeflexic, or hyperreflexic. Hyporeflexia, like spastic paralysis, is often due to lower motor neuron damage whereas hyperreflexia is often due to upper motor neuron damage. Although it is tempting to think that reflex abnormalities must be due to nervous system impairment it should be noted that there are many other possible causes which include drugs, electrolyte imbalances, endocrine disorders, cancer, and many others. In addition to testing the stretch reflex, sometimes cutaneous reflexes are used for testing. Cutaneous reflexes are elicited by stimulating the skin. One example of this is the plantar reflex which is tested by dragging the sharp end of the reflex hammer across the bottom of the foot from heel to toe. Normally this causes the toes to flex. However, if it causes the toes to dorsiflex and fan-out (the Babinski sign) then this can be a sign of upper motor neuron damage. In infants, however, the Babinski sign is considered a normal reflex due to incomplete myelination in newborns.

Proprioception

Proprioception is another modality that is often tested in a neurological exam. Simple tests can involve touching the nose with a finger or touching a location on the skin after it has been touched by the clinician. Other tests involve observing the patient’s gait (the way that they walk) or walking heel to toe. Finally, the Romberg test is used as an overall measurement of balance. Coordination and balance require input to the cerebellum from proprioceptive information in the spinocerebellar tract, vestibular information from the inner ear, and visual information. The loss of one or more of these sensory inputs will cause impairments in balance and loss of proprioception or vestibular input will cause swaying when a patient stands with feet together and their eyes closed. The severity of the damage will be reflected by the impairment of the patient’s ability to maintain an upright posture.

Chapter Summary

Key Terms

afferent

moving toward (the central nervous system)

anterior corticospinal tract

division of the corticospinal pathway that travels through the ventral (anterior) column of the spinal cord and controls axial musculature through the medial motor neurons in the ventral (anterior) horn

anterior (ventral) horn

gray matter of the spinal cord containing multipolar lower motor neurons

arachnoid mater

middle layer of the meninges named for the spider-web–like trabeculae that extend between it and the pia mater

autonomic reflex

reflex carried by autonomic nerves with effectors consisting of smooth muscle, cardiac muscle or glands

axillary nerve

systemic nerve of the arm that arises from the brachial plexus

Babinski sign

abduction and dorsiflexion of the toes in response to testing of the plantar reflex 

brachial plexus

nerve plexus associated with the lower cervical spinal nerves and first thoracic spinal nerve

brachial plexus avulsion

trauma that results in partial or complete tearing of part of the brachial plexus

Brown-Sequard hemisection

injury in which one half (left or right) of the spinal cord is severed

bulbous (Ruffini) corpuscles

encapsulated nerve endings that detect skin stretch

capsaicin

molecule that activates nociceptors by interacting with a temperature-sensitive ion channel and is the basis for “hot” sensations in spicy food

cauda equina

bundle of spinal nerve roots that descend from the lower spinal cord below the first lumbar vertebra and lie within the vertebral cavity; has the appearance of a horse’s tail

central nervous system (CNS)

anatomical division of the nervous system located within the cranial and vertebral cavities, namely the brain and spinal cord

cerebrospinal fluid (CSF)

circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood

cervical enlargement

region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of and finer control of muscles of the upper limb

cervical plexus

nerve plexus associated with the upper cervical spinal nerves

chemoreceptor

sensory receptor cell that is sensitive to chemical stimuli, such as in taste, smell, or pain

communicating rami

the autonomic branches of a spinal nerve consisting of either preganglionic or postganglionic sympathetic neurons

contralateral

word meaning “on the opposite side,” as in axons that cross the midline in a fiber tract

contralateral reflex

reflex in which receptor and effector are on opposite sides of the body

conus medullaris

the end of the spinal cord that tapers into a point

corticobulbar tract

connection between the cortex and the brain stem responsible for generating movement

corticospinal tract

connection between the cortex and the spinal cord responsible for generating movement

cranial nerve

one of twelve nerves connected to the brain that are responsible for sensory or motor functions of the head and neck

cranial reflex

reflex carried by cranial nerves with an integration center within the brain

crossed extensor reflex

reflex that causes limb extension and occurs simultaneous and contralateral to the withdrawal reflex

cutaneous reflex

reflex elicited by stimulating receptors located in the skin

decussate

to cross the midline, as in fibers that project from one side of the body to the other

deep tendon reflexes

clinical procedure used to test the stretch reflex in various muscles by tapping on the muscle tendon to stretch the muscle

denticulate ligaments

lateral extensions of the pia mater that stabilize the spinal cord in the vertebral canal

direct pathways

the corticospinal and corticobulbar tracts that give conscious control over movement consisting of upper motor neuron originating in the primary motor cortex and lower motor neuron located in the ventral horn or brainstem, respectively

dorsal column

white matter tracts carrying touch and proprioception sensory information and located between the dorsal horns of the spinal cord

dorsal column-medial lemniscus tract

ascending tract of the spinal cord associated with fine touch and proprioceptive sensations

dorsal ramus

the dorsal branch of a spinal nerve consisting of both sensory and motor neurons

dorsal (posterior) root

axons entering the posterior horn of the spinal cord

dorsal (posterior) root ganglion

sensory ganglion attached to the posterior nerve root of a spinal nerve

dura mater

tough, fibrous, outer layer of the meninges that is attached to the inner surface of the cranium and vertebral column and surrounds the entire CNS

effector

the structure (skeletal muscle, cardiac muscle, smooth muscle or gland) that carries out the action at the end of a reflex arc

efferent

moving away from (the central nervous system)

encapsulated ending

configuration of a sensory receptor neuron with dendrites surrounded by specialized structures to aid in transduction of a particular type of sensation, such as the lamellated corpuscles in the deep dermis and subcutaneous tissue

endoneurium

innermost layer of connective tissue that surrounds individual axons within a nerve

epidural space

space above the dura mater of the meninges

epineurium

outermost layer of connective tissue that surrounds an entire nerve

exteroceptor

sensory receptor that is positioned to interpret stimuli from the external environment, such as photoreceptors in the eye or somatosensory receptors in the skin

fascicle

small bundles of nerve or muscle fibers enclosed by connective tissue

fasciculus cuneatus

lateral division of the dorsal column system composed of fibers from sensory neurons in the upper body

fasciculus gracilis

medial division of the dorsal column system composed of fibers from sensory neurons in the lower body

femoral nerve

systemic nerve of the anterior leg that arises from the lumbar plexus

filum terminale

thin extension of the pia mater from the caudal end of the spinal cord that helps to stabilize the spinal cord in the vertebral canal

flaccid paralysis

paralysis resulting from lower motor neuron damage in which muscles are completely relaxed

free nerve ending

configuration of a sensory receptor neuron with dendrites in the connective tissue of the organ, such as in the dermis of the skin, that are most often sensitive to chemical, thermal, and mechanical stimuli

general sense

any sensory system that is distributed throughout the body and incorporated into organs of multiple other systems, such as the walls of the digestive organs or the skin

gray matter

regions of the nervous system containing cell bodies of neurons with few or no myelinated axons; actually may be more pink or tan in color, but called gray in contrast to white matter

hair root plexus

nerve endings that are wrapped around hair follicles to detect hair movement

hemiplegia

paralysis of one side of the body

hyperreflexia

exaggeration of reflexes often due to upper motor neuron damage

hyporeflexia

decreased reflexes often due to lower motor neuron damage

indirect pathways

several complex circuits that innervate lower motor neurons and are involved in subconscious control over movement

integration (reflex) center

gray matter of the spinal cord or brain that integrates incoming sensory and other information in the control of a reflex

interoceptor

sensory receptor that is positioned to interpret stimuli from internal organs, such as stretch receptors in the wall of blood vessels

ipsilateral

word meaning on the same side, as in axons that do not cross the midline in a fiber tract

ipsilateral reflex

reflex in which receptor and effector are on the same side of the body

kinesthesia

sense of body movement based on sensations in skeletal muscles, tendons, joints, and the skin

lamellated (Pacinian) corpuscles

encapsulated nerve endings located in the deep dermis or subcutaneous layer that detect deep pressure or vibration

lateral column

white matter of the spinal cord between the posterior horn on one side and the axons from the anterior horn on the same side; composed of many different groups of axons, of both ascending and descending tracts, carrying sensory and motor commands to and from the brain

lateral corticospinal tract

division of the corticospinal pathway that travels through the lateral column of the spinal cord and controls appendicular musculature through the lateral motor neurons in the ventral (anterior) horn

lateral horn

region of the spinal cord gray matter in the thoracic and upper lumbar regions that is the central component of the sympathetic division of the autonomic nervous system

lower motor neuron

second neuron in the motor command pathway that is directly connected to the skeletal muscle

lumbar enlargement

region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of muscles of the lower limb

lumbar plexus

nerve plexus associated with the lumbar spinal nerves

lumbar puncture

procedure used to withdraw CSF from the lower lumbar region of the vertebral column that avoids the risk of damaging CNS tissue because the spinal cord ends at the upper lumbar vertebrae

mechanoreceptor

receptor cell that transduces mechanical stimuli into an electrochemical signal

medial lemniscus

fiber tract of the dorsal column system that extends from the nuclei gracilis and cuneatus to the thalamus, and decussates

median nerve

systemic nerve of the arm, located between the ulnar and radial nerves

meninges

protective outer coverings of the CNS composed of connective tissue

monosynaptic reflex

rapid reflex arc where the incoming sensory neuron synapses directly on the lower motor neuron minimizing synaptic delay

muscle spindle

specialized intrafusal muscle fiber that detects stretch of a muscle

nerve plexus

network of nerves without neuronal cell bodies included formed from the ventral rami of spinal nerves

nociception

the detection of painful stimuli

nociceptor

receptor cell that senses pain stimuli

nucleus cuneatus

medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the upper body and arms

nucleus gracilis

medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the lower body and legs

obturator nerve

peripheral nerve that carries sensory information from the medial thigh and motor innervation of most adductor muscles

osmoreceptor

receptor cell that senses differences in the concentrations of bodily fluids on the basis of osmotic pressure

paraplegia

paralysis of the lower extremities

paresis

partial paralysis or weakness of a limb

patellar reflex

stretch reflex of the quadriceps muscle elicited by tapping the patellar ligament

perception

becoming aware of a stimulus

perineurium

layer of connective tissue surrounding fascicles within a nerve

peripheral nerve

nerve in the periphery distal to a nerve plexus or spinal nerve

peripheral neuropathy

damage to a peripheral nerve, spinal nerve or the nerve roots that results in sensory and or motor impairments to the region innervated by the nerve

peripheral nervous system (PNS)

anatomical division of the nervous system that is largely outside the cranial and vertebral cavities, namely all parts except the brain and spinal cord

phantom limb pain

erroneous perception of pain as originating from an amputated limb

phantom limb sensation

erroneous perception of stimuli as originating from an amputated limb

phasic sensory receptor

sensory receptors that adapt rapidly and more completely

photoreceptor

receptor cell specialized to respond to light stimuli

phrenic nerve

systemic nerve from the cervical plexus that innervates the diaphragm

pia mater

thin, innermost membrane of the meninges that directly covers the surface of the CNS

plantar reflex

cutaneous reflex elicited by scraping the plantar surface of the foot from heel to toe

polysynaptic reflex

reflex arc in which at least one interneuron lies between the afferent sensory neuron and the efferent motor neuron

posterior (dorsal) horn

gray matter region of the spinal cord in which sensory input arrives

primary motor cortex

location of upper motor neurons in the precentral gyrus of the frontal lobe

primary somatosensory cortex

postcentral gyrus of the parietal lobe that initially receives somatosensory input from an ascending pathway from the thalamus and begins the processing that will result in conscious perception of that sensory modality

proprioception

sense of position and movement of the body

proprioceptor

receptor cell that senses changes in the position and kinesthetic aspects of the body

pyramidal decussation

location at which corticospinal tract fibers cross the midline and segregate into the anterior and lateral divisions of the pathway

pyramidal tract

the lateral and anterior corticospinal tracts

quadriplegia

paralysis of all four limbs

radial nerve

systemic nerve of the arm, the distal component of which is located near the radial bone

receptor cell

cell that transduces environmental stimuli into neural signals

reciprocal innervation

the simultaneous inhibition of all antagonist muscles during stimulation of any muscle

referred pain

when pain is perceived at a location other than the site of the painful stimulus

reflex

automatic and consistent response to a stimulus

reflex arc

reflex pathway consisting of receptor, sensory afferent neuron, integration or reflex center, motor efferent neuron and effector

reticulospinal tract

extrapyramidal connections between the brain stem and spinal cord that modulate movement, contribute to posture, and regulate muscle tone

Romberg test

clinical test of balance requiring vestibular, proprioception and visual stimuli to maintain posture

rubrospinal tract

descending motor control pathway, originating in the red nucleus, that mediates control of the limbs on the basis of cerebellar processing

sacral plexus

nerve plexus associated with the lower lumbar and sacral spinal nerves

sciatic nerve

systemic nerve from the sacral plexus that is a combination of the tibial and fibular nerves and extends across the hip joint and gluteal region into the upper posterior leg

sciatica

painful condition resulting from inflammation or compression of the sciatic nerve or any of the spinal nerves that contribute to it

sensation

nervous system function that receives information from the environment and translates it into the electrical signals of nervous tissue

sensory adaptation

the gradual decrease in responsiveness of a receptor to a constant stimulus

sensory transduction

process of changing an environmental stimulus into the electrochemical signals of the nervous system

shoulder dystocia

condition where a baby’s shoulders get stuck during a vaginal delivery

somatic reflex

reflexes with effectors consisting of skeletal muscle

somatosensation

general senses related to the body, usually thought of as the senses of touch, which would include pain, temperature, and proprioception

spastic paralysis

paralysis resulting from damage to upper motor neurons in which muscle tension  is maintained and may spasm

special sense

any sensory system associated with a specific organ structure, namely smell, taste, sight, hearing, and balance

spina bifida

caudal neural tube defect resulting in exposure of the spinal cord

spinal nerve

one of 31 nerves connected to the spinal cord

spinal reflex

reflex carried by spinal nerves with an integration center within the spinal cord

spinal segment

a region of the spinal cord that supplies dorsal and ventral roots for one spinal nerve

spinothalamic tract

ascending tract of the spinal cord associated with pain and temperature sensations

stimulus

an event in the external or internal environment that registers as activity in a sensory neuron

stretch reflex

response to activation of the muscle spindle stretch receptor that causes contraction of the muscle to maintain a constant length

subarachnoid space

space between the arachnoid mater and pia mater that contains CSF and the fibrous connections of the arachnoid trabeculae

sympathetic chain ganglia

autonomic ganglia in a chain along the anterolateral aspect of the vertebral column that are responsible for contributing to homeostatic mechanisms of the autonomic nervous system

tactile (Meissner’s) corpuscles

encapsulated nerve endings located in the dermal papilla that detect discriminative touch and vibration

tactile epithelial cells (Merkel cells)

cells located in the stratum basale of the epidermis that detect light touch

tectospinal tract

extrapyramidal connections between the superior colliculus and spinal cord

tendon organ (Golgi tendon organ)

encapsulated receptor within muscle tendons that detect tension

tendon reflex (Golgi tendon reflex)

inhibitory reflex that uses the tendon organ as a receptor to prevent damage to muscle or tendon by stimulating muscle relaxation as a response to excessive force

thermoreceptor

sensory receptor specialized for temperature stimuli

tonic sensory receptors

sensory receptors that adapt slowly and only partially

two-point discrimination

the ability to discern two separate points touching the skin as being separate

ulnar nerve

systemic nerve of the arm located close to the ulna, a bone of the forearm

upper motor neuron

first neuron in the motor command pathway with its cell body in the cerebral cortex that synapses on the lower motor neuron in the spinal cord

ventral column

white matter of the spinal cord located between the ventral horns

ventral ramus

the ventral branch of a spinal nerve consisting of both sensory and motor neurons

ventral (anterior) root

axons emerging from the anterior or lateral horns of the spinal cord

vestibulospinal tract

extrapyramidal connections between the vestibular nuclei in the brain stem and spinal cord that modulate movement and contribute to balance on the basis of the sense of equilibrium

visceral sense

sense associated with the internal organs

Wallerian degeneration

active process of retrograde degeneration of the distal end of an axon after it has been severed

white matter

regions of the nervous system containing mostly myelinated axons, making the tissue appear white because of the high lipid content of myelin

withdrawal reflex (flexor withdrawal reflex)

reflex in which a painful stimulus causes withdrawal of the injured body part

Sources

1. OpenStax A&P textbook (Ch 13 and 14): https://openstax.org/books/anatomy-and-physiology/pages/14-1-sensory-perception?query=sensation&target=%7B%22index%22%3A0%2C%22type%22%3A%22search%22%7D#fs-id2124989
2. Wikipedia:
   1. Neural adaptation: https://en.wikipedia.org/wiki/Neural_adaptation
   2. Tonic sensory receptors: https://en.wikipedia.org/wiki/Tonic_(physiology)
   3. Dermatome: https://en.wikipedia.org/wiki/Dermatome_(anatomy)
   4. Referred pain: https://en.wikipedia.org/wiki/Referred_pain
   5. Phantom limb sensation: https://en.wikipedia.org/wiki/Phantom_limb#Phantom_limb_pain

1. American Heart Association: https://www.heart.org/en/health-topics/heart-attack/warning-signs-of-a-heart-attack

1. Fundamentals of Anatomy & Physiology, 11th edition, Frederic Martini; Judi Nath; Edwin Bartholomew

1. Shingles virus: By Photo Credit:Content Providers(s): CDC/Dr. Erskine Palmer/B.G. Partin — This media comes from the Centers for Disease Control and Prevention Public Health Image Library (PHIL), with identification number #1878. https://commons.wikimedia.org/w/index.php?curid=816522

Child with shingles: