Kau ka lā i ka lolo, hoʻi ke aka i ke kino.

The sun stands over the brain, the shadow retreats into the body.

Said of high noon, when the sun is directly overhead and no shadows are seen — an important time for some ancient rites and ceremonies.

‘Ōlelo No‘eau, compiled by Mary Kawena Pukui, #1611

Introduction

Figure 14.1 Lateral View of the Human Brain

14.1 The Brain

Overview: Meninges, Ventricles, Cerebrospinal Fluid, Blood-Brain Barrier

Overview

The nervous system is responsible for controlling much of our bodily functions. The nervous system can be separated into two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is composed of the brain and the spinal cord, and the peripheral nervous system consists of the nerves that branch off of the brain and spinal cord. In this chapter, our focus will be on the brain. The adult brain has four main regions: the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain is protected by multiple structures, starting with the cranium (skull bones) that gives solid support, followed by the meninges, made of connective tissue, that forms a series of membranes that surround the brain. In between these layers, the cerebrospinal fluid (CSF) works as a cushioning fluid while circulating amongst specific layers of the meninges. The blood-brain barrier (BBB) is another level of protection for the  central nervous system and prevents the entrance of dangerous materials from the blood into the brain.

Figure 14.2 Meningeal Layers of Superior Sagittal Sinus The layers of the meninges in the longitudinal fissure of the superior sagittal sinus are shown, with the dura mater adjacent to the inner surface of the cranium, the pia mater adjacent to the surface of the brain, and the arachnoid and subarachnoid space between them. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage.

Cranial Meninges

From deep to superficial, the meningeal layers are pia, arachnoid, and dura. [memorization trick: From deep to superficial layers, the meninges form a P.A.D. Pia, Arachnoid, Dura]. Notice in Figure 14.2 the makeup of the three meningeal layers.

The pia mater (tender mother) is the innermost (deepest) layer adhering to the brain surface. It is made of delicate areolar connective tissue. The pia mater follows the contours of the brain and provides a scaffolding or passageway for blood vessel penetration. The arachnoid mater is localized externally (superficial) to the pia mater and is made of collagen and elastic fibers. Deep in the arachnoid mater is the subarachnoid space into which fibers from the arachnoid layer extend forming a spider-web-like appearance known as the arachnoid trabeculae. This important area contains the circulating cerebrospinal fluid (CSF). The CSF provides a liquid cushion to the brain and the spinal cord. In some areas, sections of the arachnoid layer extend through the dura mater into the dural venous sinuses as the arachnoid granulations, also known as arachnoid villi, where the CSF is filtered back into the blood, draining it from the nervous system.

The dura mater (tough mother) is the outermost (superficial) tough layer of dense irregular connective tissue. There are portions of the dura mater within the skull where it may separate into two layers: the meningeal layer, which is deeper, and the periosteal layer, which is the most superficial layer adjacent to the periosteum on the internal surfaces of the cranial bones. In areas of the brain where the periosteal and meningeal dura split, they produce a dural venous sinus that contains venous blood. There is a potential space between the dura mater and the skull known as the epidural space that has many large arteries that bring nourishment to the meninges. In Figure 14.3, you will notice the dura folds producing a series of sinuses. The largest is known as the superior sagittal sinus.

Figure 14.3 Dural Sinuses and Veins Blood drains from the brain through a series of sinuses that connect to the jugular veins.

Ventricles

Ventricles (Figure 14.4)  are cavities within the brain that connect with each other and with the central canal of the spinal cord. They are lined with specialized neuroglia known as ependymal cells, and cerebrospinal fluid (CSF) circulates within these spaces. There are four ventricles: two lateral ventricles, one third ventricle, and one fourth ventricle. The two large lateral ventricles are cavities found in each of the hemispheres of the cerebrum, separated by a medial partition called the septum pellucidum. The third ventricle is a narrow space in the diencephalon and it is connected to the lateral ventricles by the interventricular foramen. The fourth ventricle is a space between pons and cerebellum, connected with the third ventricle by the cerebral (mesencephalic) aqueduct, and merges with the central canal of the spinal cord.

Figure 14.4 Cerebrospinal Fluid Circulation The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses.

Cerebrospinal Fluid Circulation (CSF)

Figure 14.4 highlights the flow of CSF through the ventricles and the CNS. The CSF functions to reduce the weight of the brain, providing a liquid cushion and transporting nutrients and wastes. CSF is formed when blood plasma is filtered through capillaries and is modified by specialized ependymal cells in tufts of capillaries known as choroid plexus which are found in each of the ventricles. In comparison to blood plasma, CSF has more sodium and less potassium. To provide a medium in which nutrients are delivered to neural tissue and wastes removed, CSF is continuously being formed, circulated, and reabsorbed. After circulating through the ventricles, CSF moves into the subarachnoid space and then passes through the arachnoid villi draining into the dural venous sinuses where CSF is filtered back into the blood. CSF circulation passes from the lateral ventricles to the third ventricle and through the cerebral aqueduct into the fourth ventricle. From there, CSF continues to the spinal cord, via the central canal. With the flowing of CSF through the subarachnoid space, waste products are removed.

Clinical Application

Disease-Focus: Hydrocephalus

A blockage in the CSF circulation can cause a condition known as hydrocephalus where there is an excess of CSF in the brain or a blockage of its flow. This condition leads to elevated pressure in the brain and neurological damage or death. This disorder can be treated with surgery by placing a ventriculoperitoneal (VP) shunt to drain the CSF into the peritoneal cavity.

Illustration of hydrocephalus	Little boy with hydrocephalus and his mother.

Blood-Brain Barrier (BBB)

In the CNS, neuron function may be disrupted by waste products, hormones, or drugs that may be present in the general circulation. The blood-brain barrier protects these neurons by providing a physical barrier between the neurons and circulating blood. The BBB is formed by glial cells called astrocytes (Figure 14.5) that envelop the capillaries in the brain. The extensions of astrocytes secrete chemicals that influence the capillary endothelial cells to form tight junctions, limiting the number of substances that can pass out of the blood. Lipid-soluble compounds, such as gases, alcohol, and nicotine, can pass through the plasma membrane, therefore crossing the BBB and reaching the neurons.

Figure 14.5 Glial Cells of the CNS The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the CNS in several ways.

14.2 Brain Matter and Forebrain:

White Matter and Gray Matter Distribution and Forebrain Structure and Function

Gray and White Matter Distribution

The brain and the spinal cord are composed of gray and white matter tissue (Figure 14.6). The gray matter is made of cell bodies of neurons, dendrites, and unmyelinated axons. The gray matter contains neurons that are responsible for the processing component of the CNS. In the brain, the cerebral cortex is a superficial layer of gray matter, and clusters of cell bodies of neurons can also be found deep within the brain. These clusters are known as nuclei. In the peripheral nervous system they are known as ganglia. The white matter is composed of myelinated axons organized in bundles known as tracts (Figure 14.7). In the peripheral nervous system these are called nerves. While gray matter can be compared to the central processing unit within a computer, the white matter, consisting of myelinated nerve axons, can be thought of as wires connecting various components of the nervous system.

Figure 14.6 Gray Matter and White Matter A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain. (credit: modification of work by “Suseno”/Wikimedia Commons).

Figure 14.7 Optic Nerve Versus Optic Tract This drawing of the connections of the eye to the brain shows the optic nerve extending from the eye to the chiasm, where the structure continues as the optic tract. The same axons extend from the eye to the brain through these two bundles of fibers, but the chiasm represents the border between peripheral and central. (OpenStax)

Cerebrum and Functional Organization of Cerebral Cortex

The cerebrum (Figure 14.8) is an area of the brain that processes our intellectual functions. This is the area that regulates the functions that allow you to see, read, understand, learn, think and apply the information covered in this book. The cerebrum is divided into two halves, known as cerebral hemispheres, that are localized on the superior area of the brain.

The cerebral hemispheres are separated by the longitudinal fissure that passes along the midsagittal plane. The corpus callosum is a white matter tract that connects the hemispheres, allowing communication between the two.

Figure 14.8 The Cerebrum The cerebrum is a large component of the CNS in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex. (OpenStax)

The cerebral hemispheres are further separated into five lobes (Figure 14.9). The names of these lobes are associated with the underlying cranial bones. Each lobe has a primary cortical region for direct input or output and associated areas (Figure 14.10). The association areas are known for integrating the information. For example, while the visual cortex receives and starts the processing of visual information, the visual association area is the one that integrates that visual information of a face that we see in front of us with the specific features associated with that face, such as the name of that person and the memories associated with that person. Below you will find a description of each of the five lobes.

Figure 14.9 Lobes of the Cerebral Cortex The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions. (OpenStax)

Figure 14.10 Types of Cortical Areas The cerebral cortex can be described as containing three types of processing regions: primary, association, and integration areas. The primary cortical areas are where sensory information is initially processed, or where motor commands emerge to go to the brain stem or spinal cord. Association areas are adjacent to primary areas and further process the modality-specific input. Multimodal integration areas are found where the modality-specific regions meet; they can process multiple modalities together or different modalities on the basis of similar functions, such as spatial processing in vision or somatosensation. (OpenStax)

The frontal lobe contains areas for motor control including speech generation, odor identification, reasoning, personality, judgment, understanding of consequences, learning complicated concepts, and more.

The functions of the prefrontal cortex are integral to the personality of an individual, because it is largely responsible for what a person intends to do and how they accomplish those plans. A famous case of damage to the prefrontal cortex is that of Phineas Gage, dating back to 1848. He was a railroad worker who had a metal spike impale his prefrontal cortex (Figure 14.11). He survived the accident, but according to second-hand accounts, his personality changed drastically. Friends described him as no longer acting like himself. Whereas he was a hardworking, amiable man before the accident, he turned into an irritable, temperamental, and lazy man after the accident. Many of the accounts of his change may have been inflated in the retelling, and some behavior was likely attributable to alcohol used as a pain medication. However, the accounts suggest that some aspects of his personality did change. Also, there is new evidence that though his life changed dramatically, he was able to become a functioning stagecoach driver, suggesting that the brain has the ability to recover even from major trauma such as this.

Figure 14.11 Phineas Gage The victim of an accident while working on a railroad in 1848, Phineas Gage had a large iron rod impaled through the prefrontal cortex of his frontal lobe. After the accident, his personality appeared to change, but he eventually learned to cope with the trauma and lived as a coach driver even after such a traumatic event. (OpenStax: credit b: John M. Harlow, MD)

Deep Dive

Focus: Criminal Justice

When a person with personality problems has committed a crime, sometimes attorneys will defend the person by arguing that this person had damage in the frontal lobe and therefore that damage caused the personality problems and could have also triggered the impulse to commit the crime. For more information: https://digitalcommons.wcl.american.edu/cgi/viewcontent.cgi?article=1277&context=aulr

The parietal lobe receives sensory information, such as touch, temperature, pain, and itch. It also associates sensory information with other information, enabling you to identify a previously encountered item, such as your favorite fruit, entirely by touch. The amount of cortex for a body part is directly related to the number of receptors and, thus, the sensitivity of that area. There are maps of the primary somatosensory cortex that show just how much cortex is allocated for each body area. Processing power devoted to certain areas of the body are depicted on a visual map of the body parts. They are distorted by their relative sensitivities, and the mapping of parts is known as a sensory homunculus, as seen in Figure 14.12. There is a similar homunculus for the motor cortex related to how much control we have of each body part. The large size of the oral region and thumbs and hands in both the sensory and motor homunculus represents the immense importance to humans of speech and our opposable thumbs for using tools. Part of the ability to understand language is also in the parietal lobe. The occipital lobe processes visual information, including giving meaning to images. For example, image shapes coming from the eyes are combined in the occipital lobe in a manner that allows you to recognize your shoes solely (no pun intended) by looking at them. The temporal lobe receives and processes sound information, has areas for recognizing faces, and is the primary receptive area for smell. The insula, previously one of the least understood brain regions, is now known to process taste, smell, sound, visceral and body surface sensations, and emotions such as empathy.

Figure 14.12 The Sensory Homunculus A cartoon representation of the sensory homunculus arranged adjacent to the cortical region in which the processing takes place. (OpenStax)

The basis for parceling out areas of the cortex and attributing them to various functions has its root in pure anatomical underpinnings. The German neurologist and histologist Korbinian Brodmann, who made a careful study of the cytoarchitecture of the cerebrum around the turn of the nineteenth century, described approximately 50 regions of the cortex that differed enough from each other to be considered separate areas (Figure 14.13). Brodmann made preparations of many different regions of the cerebral cortex to view with a microscope. He compared the size, shape, and number of neurons to find anatomical differences in the various parts of the cerebral cortex. Continued investigation into these anatomical areas over the subsequent 100 or more years has demonstrated a strong correlation between the structures and the functions attributed to those structures. For example, the first three areas in Brodmann’s list—which are in the postcentral gyrus—compose the primary somatosensory cortex. Within this area, finer separation can be made on the basis of the concept of the sensory homunculus, as well as the different submodalities of somatosensation such as touch, vibration, pain, temperature, or proprioception. Today, we more frequently refer to these regions by their function (i.e., primary sensory cortex) than by the number Brodmann assigned to them, but in some situations the use of Brodmann numbers persists.

Figure 14.13 Brodmann’s Areas of the Cerebral Cortex Brodmann mapping of functionally distinct regions of the cortex was based on its cytoarchitecture at a microscopic level. (OpenStax)

Figure 14.14 Broca’s and Wernicke’s Areas Two important integration areas of the cerebral cortex associated with language function are Broca’s and Wernicke’s areas. The two areas are connected through the deep white matter running from the posterior temporal lobe to the frontal lobe.

The limbic system (Figure 14.15) includes part of the cerebral cortex and several nuclei and tracts that interconnect cerebral cortex regions and other brain structures. It processes emotions such as pleasure, anger, and rage, while also sparking drives for hunger and sex.

Figure 14.15 The Limbic System The limbic system  includes part of the cerebral cortex and several nuclei and tracts that interconnect cerebral cortex regions and other brain structures.

The hippocampus, a vital structure for memory, is in the limbic system. Damage to both the left and right hippocampus results in the inability to form new long-term memories. This is called anterograde amnesia which is different from forgetting old memories, which is retrograde amnesia. The hippocampus has received more attention in recent decades because studies suggested the production of new neurons in the adult hippocampus, something previously deemed impossible anywhere in the brain. Then, with further research, neuroscientists began questioning the existence of hippocampal neurogenesis. The debate has continued, with 2019 research swinging the view back in favor of neurogenesis in adult humans up through ninety years of age. The amygdaloid body, also known as the amygdala, connects to the hippocampus and regulates emotional states, specifically fear.

Basal Nuclei: Motor and Cognitive Functions

The colored areas in Figure 14.16 show the basal nuclei. The caudate, putamen, and [pb_glossary id="4361"]globus pallidus[/pb_glossary] are masses of gray matter found within the white matter of the cerebrum, inferiorly to the lateral ventricles. These structures are part of the basal nuclei. You may also find them referred to as the basal ganglia in older materials. Decades ago, most scientists agreed to change the name to the correct term of nuclei, since ganglia by definition are found outside of the central nervous system. Together, the caudate, putamen, and globus pallidus form the corpus striatum. This term describes the striated appearance of a white matter structure made up of axons that carries information from and to the cortex, known as an internal capsule. This structure passes between the nuclei of the corpus striatum. The basal nuclei process motor and cognitive information and are important in learning new motor skills.

Figure 14.16 Frontal Section of Cerebral Cortex and Basal Nuclei The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal capsule), and the globus pallidus (medial to the putamen). (OpenStax)

The neurotransmitter dopamine is involved in the regulation of motor processing within the basal nuclei. The dopamine circuitry within the basal nuclei is disturbed in Parkinson’s disease. The basal nuclei receive disrupted neuronal projections coming from dopaminergic neurons in the substantia nigra located in the midbrain. Patients with Parkinson’s disease suffer from cognitive and motor impairments, most notably a resting tremor and difficulty speaking.

Diencephalon

The diencephalon (Figure 14.17) is a region that connects the cerebrum to most of the other areas of the central nervous system. The diencephalon consists of the thalamus, the hypothalamus, and the epithalamus. The thalamus is composed of paired masses of gray matter that lie on either side of the third ventricle. The thalamus is composed of a cluster of many nuclei.

Figure 14.17 The Diencephalon The diencephalon is composed primarily of the thalamus and hypothalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached.

The thalamus functions to sort and relay sensory information that passes into the brain, except olfactory information, and mediates motor activity from the cerebrum to the cerebellum and structures of the brain stem. The hypothalamus is found anteriorly and inferiorly to the thalamus. A stalk-like structure called the infundibulum is localized inferiorly to the hypothalamus and attaches to the pituitary gland. A cluster of many nuclei makes up the hypothalamus, with each nucleus having neurons that regulate specific functions related to homeostasis. The hypothalamus controls the autonomic nervous system, regulates responses to emotional states, and controls sleep-wake cycles and the endocrine system via the connection with the pituitary gland. Other vital functions include regulation of body temperature, hunger and satiety, and water balance and thirst. The epithalamus covers the third ventricle and forms the posterior part of the roof of the diencephalon. The pineal gland is part of the epithalamus. This endocrine gland secretes melatonin, a hormone that regulates day-night cycles known as circadian rhythms.

14.3 Hindbrain, Midbrain, Structure and Function, and the Cranial Nerves

Cerebellum

Even though its name means little brain, the cerebellum (Figure 14.18) contains almost half the brain’s neurons. Many of the cerebellar neurons are dedicated to coordinating and optimizing movements, as well as maintaining posture and balance. While the preliminary motor signal to make a move, such as throwing a ball or saying a word, originates in the motor area of the cerebral cortex, that signal will loop into the cerebellum and back to the cerebral cortex. The benefit of the cerebellar input is that it orchestrates fine motor adjustments that allow movement to be smoother and more precise. It can do this because it receives sensory information about body movement and position from the visual system, the vestibular system of the inner ear, and from proprioceptive pathways traveling up from the spinal cord. There are nonmotor functions of the cerebellum, such as learning and information processing, and many sleep-related functions. Research shows cerebral cortex and cerebellar interactions are crucial for memory consolidation, and some of these interactions occur particularly during sleep.

Figure 14.18 The Cerebellum The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord. (OpenStax)

Separating the cerebellar hemispheres, the vermis (worm) runs along the midline inferiorly and superiorly. Fissures separate the hemispheres into lobes: anterior lobe, posterior lobe, and flocculonodular lobe. The cerebellum is connected to the brainstem through the white matter of the cerebellar peduncles. This provides pathways for information to flow between various brain regions into and out of the cerebellum. The cerebellar nuclei are embedded in the arbor vitae (tree of life) that is the white matter inside each hemisphere. On the surface of each hemisphere, the cerebellar cortex is folded into leaf-like structures called folia (leaves).

Brainstem

The brainstem is divided by the colored areas in Figure 14.19. The brainstem has neurons that control functions that are necessary to sustain life such as breathing, heart rate, and blood vessel diameter. It also coordinates movements such as swallowing, coughing, and sneezing. Pathways of sensory and motor information pass through and sometimes make connections in various nuclei of the brainstem. Moving from superior to inferior, the brainstem consists of the midbrain, pons, and medulla (medulla oblongata). The brainstem fits in between the diencephalon and the spinal cord. Many cranial nerves originate in the brain stem and can be seen on its surface. The reticular formation is a functional unit that includes axons and neuronal cell bodies connected throughout the brainstem.

Figure 14.19 The Brain Stem The brain stem comprises three regions: the midbrain, the pons, and the medulla. (OpenStax)

Midbrain

The midbrain contains sensory and motor pathways. Running lengthwise through the middle, the cerebral aqueduct provides a passage for cerebral spinal fluid from the third to the fourth ventricle. Cerebral peduncles are white matter corticospinal tracts that connect the upper motor neurons of the primary motor cortex to the lower motor neurons in the spinal cord. Two pairs of colliculi (little hills), on the posterior surface of the midbrain, are collectively referred to as the tectum (roof). The superior colliculi send the signals to move your upper body, head, and eyes in response to visual stimuli. This is especially important in reflexive tracking of moving objects. The inferior colliculi produce a similar response to auditory stimuli and will cause reflexive turning of the head in the direction of loud noises. Together, the two pairs of colliculi are sometimes referred to as the corpora quadrigemina (four bodies). The red nuclei and substantia nigra are groups of neuronal cell bodies that help control motor activity. The degeneration of the substantia nigra’s dopaminergic neurons, as well as additional inflammatory and oxidative damage, are associated with Parkinson’s disease. The nuclei for two pairs of cranial nerves, the oculomotor nerves, and trochlear nerves, are also in the midbrain.

Pons

The word pons means bridge, and in addition to it looking like a structural bridge you would cross if traveling between the left and right sides of the cerebellum, it also provides a functional bridge. Through its connections between the cerebral cortex, cerebellum, and other brain regions, it contributes to optimizing movements and controlling breathing. The nuclei for four pairs of cranial nerves, the trigeminal nerves, abducens nerves, facial nerves, and part of the vestibulocochlear nerves, are also in the pons.

Medulla Oblongata

Continuous with the spinal cord, the medulla oblongata contains all the sensory and motor tracts that run between the spinal cord and the rest of the brain. On the anterior surface, pyramids (Figure 14.20) are the continuation of the cerebral peduncles from the midbrain and therefore contain the corticospinal tracts. The corticospinal tracts carry signals that control body movements. 90% of the axons in the pyramids cross to the opposite side in a region referred to as the decussation (crossing) of the pyramids. This is why the side of the brain that controls a movement is opposite from the side of the body that moves. Posteriorly, medullary neurons make connections to the sensory axons traveling in the dorsal columns of the spinal cord and carry the signal to the thalamus. Nuclei in the medulla control reflexes such as coughing, sneezing, swallowing, vomiting, and hiccupping. Control centers for respiration, heart rate, the force of heart contraction, and blood vessel diameter are found here. Because these are critical functions for survival, damage to the medulla is often fatal. Medullary nuclei also coordinate movement via connections to the cerebellum. The range of functions of nuclei in the medulla includes taste, hearing, and balance. Lastly, the nuclei for five pairs of cranial nerves, vestibulocochlear nerves, [pb_glossary id="4362"]glossopharyngeal nerve[/pb_glossary]s, vagus nerves, accessory nerves, and hypoglossal nerves are also in the medulla.

Figure 14.20 Corticospinal Tract The major descending tract that controls skeletal muscle movements is the corticospinal tract. 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 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. (OpenStax)

Reticular Formation

The reticular formation is a scattered group of neuronal cell bodies and axons throughout the brainstem that connect with other brain regions to create the reticular activating system (RAS). The reticular activating system (RAS), a network of connections, primarily originating in the reticular formation, contains brainstem circuits that send signals to the cerebral cortex directly, and also via the thalamus, to contribute to consciousness. Sensory signals along this pathway keep you alert and oriented to your surroundings. The RAS is activated during awake states and is inactivated as part of initiating and maintaining sleep. However, when someone is sleeping, a strong enough sensory stimulus, such as a loud noise, will awaken the person via RAS activation. People differ from one another in the threshold required to activate the RAS during sleep; thus, there are “heavy” and “light” sleepers. Signals from the eyes, ears and most of the rest of the body (e.g. temperature, touch, pain) travel through the RAS, but odors do not. This is why smoke detectors are important in sleeping areas. A person may die inhaling smoke from a fire while they are sleeping because that smoke smell will not travel through the RAS and awaken them. If a person is unable to hear a sound alarm, they may consider smoke detectors that utilize extremely bright flashing lights or strong pillow vibrations to activate RAS pathways and increase the chances of their awakening. Because the RAS also sends connections to the hippocampus to stimulate memory formation, it is important to study in well-lit environments and move around to keep yourself alert.

Cranial Nerves

Figure 14.21 is an inferior view of the brain showing the cranial nerves.

Figure 14.21 The Cranial Nerves The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain.

Cranial nerves are considered part of the peripheral nervous system. Instead of linking to the spinal cord as the spinal nerves do, the cranial nerves are connected directly to the brain. The twelve pairs of cranial nerves branch out from the inferior portion of the brain. They are numbered as they branch off the brain from the most anterior (CN I) to the most posterior or inferior position (CN XII). In most cases, their names reflect the major structures they control. Below is a table containing the sensory and motor functions of each cranial nerve. Note the fact that some cranial nerves have strictly a sensory function, a motor function, or both. The study of the location and function of the twelve pairs of cranial nerves can be overwhelming. So, use this mnemonic device! The bolded section with words starting with S, M, or B depicts whether the nerve has a sensory only, motor only, or both (sensory and motor function).

• CN I-olfactory nerve (on) Some

• CN II-optic nerve (occasion) Say

• CN III-oculomotor nerve (our) Marry

• CN IV-trochlear nerve (trusty) Money

• CN V-trigeminal nerve (truck) But

• CN VI-abducens nerve (acts) My

• CN VII-facial nerve (funny) Brother

• CN VIII-vestibulocochlear nerve (very) Says

• CN IX-glossopharyngeal nerve (good) Big

• CN X-vagus nerve (vehicle) Brains

• CN XI-accessory nerve (any) Matter

• CN XII-hypoglossal nerve (how) Most

Table 14.1: Cranial Nerves

14.4 Development of the Nervous System, Stroke, and Alzheimer’s Disease

Development of Nervous System

Neural tube

The different tissues of the body develop from three embryonic layers: endoderm, mesoderm, and ectoderm (Figure 14.22). The nervous system develops from a specialized region of the ectoderm, called the neuroectoderm. After this region forms the neural plate, it folds inward, creating the neural fold, which closes to create the neural tube. The layer above the tube closes and a group of cells just below that surface layer and lateral to the neural tube become the neural crest that will give rise to the peripheral nervous system. The neural tube, traveling the length of the embryo, will develop into the brain and spinal cord of the central nervous system.

Figure 14.22 Early Embryonic Development of Nervous System The neuroectoderm begins to fold inward to form the neural groove. As the two sides of the neural groove converge, they form the neural tube, which lies beneath the ectoderm. The anterior end of the neural tube will develop into the brain, and the posterior portion will become the spinal cord. The neural crest develops into peripheral structures.

Vesicles

In the development of the brain (Figure 14.23), vesicles or bulges are formed at the rostral end of the tube. There are three primary vesicles: the prosencephalon (forebrain), which is the forward-most vesicle, the mesencephalon (midbrain), and the rhombencephalon (hindbrain). As the brain continues to develop, the primary vesicles become five secondary vesicles. The prosencephalon becomes the telencephalon and the diencephalon. The telencephalon will become the cerebrum and the diencephalon will develop to the thalamus, hypothalamus, and epithalamus. The mesencephalon does not differentiate further and stays as the midbrain. The rhombencephalon will become the metencephalon (that continues to differentiate to the pons and cerebellum) and the myelencephalon (that continues to become the medulla oblongata).

Figure 14.23 Primary and Secondary Vesicle Stages of Development The embryonic brain develops complexity through enlargements of the neural tube called vesicles; (a) The primary vesicle stage has three regions, and (b) the secondary vesicle stage has five regions.

Blood Supply to the Brain

A lack of oxygen to the CNS can be devastating, and the cardiovascular system has specific regulatory reflexes to ensure that the blood supply is not interrupted. There are multiple routes for blood to get into the CNS, with specializations to protect that blood supply and to maximize the ability of the brain to get an uninterrupted perfusion.

Arterial Supply

The major artery carrying recently oxygenated blood away from the heart is the aorta. The very first branches off the aorta supply the heart with nutrients and oxygen. The next branches give rise to the common carotid arteries, which further branch into the internal carotid arteries. The external carotid arteries supply blood to the tissues on the surface of the cranium. The bases of the common carotids contain stretch receptors that immediately respond to the drop in blood pressure upon standing. The orthostatic reflex is a reaction to this change in body position, so that blood pressure is maintained against the increasing effect of gravity (orthostatic means “standing up”). Heart rate increases—a reflex of the sympathetic division of the autonomic nervous system—and this raises blood pressure.

The internal carotid artery enters the cranium through the carotid canal in the temporal bone. A second set of vessels that supply the CNS are the vertebral arteries, which are protected as they pass through the neck region by the transverse foramina of the cervical vertebrae. The vertebral arteries enter the cranium through the foramen magnum of the occipital bone. Branches off the left and right vertebral arteries merge into the anterior spinal artery supplying the anterior aspect of the spinal cord, found along the anterior median fissure. The two vertebral arteries then merge into the basilar artery, which gives rise to branches to the brain stem and cerebellum. The left and right internal carotid arteries and branches of the basilar artery all become the circle of Willis, a confluence of arteries that can maintain perfusion of the brain even if narrowing or a blockage limits flow through one part (Figure 14.24).

Figure 14.24 Circle of Willis The blood supply to the brain enters through the internal carotid arteries and the vertebral arteries, eventually giving rise to the circle of Willis.

Venous Return

After passing through the CNS, blood returns to the circulation through a series of dural sinuses and veins (Figure 14.25). The superior sagittal sinus runs in the groove of the longitudinal fissure, where it absorbs CSF from the meninges. The superior sagittal sinus drains to the confluence of sinuses, along with the occipital sinuses and straight sinus, to then drain into the transverse sinuses. The transverse sinuses connect to the sigmoid sinuses, which then connect to the jugular veins. From there, the blood continues toward the heart to be pumped to the lungs for reoxygenation.

Figure 14.25 Dural Sinuses and Veins Blood drains from the brain through a series of sinuses that connect to the jugular veins. (OpenStax)

Disorders

A stroke (Figure 14.26) or cerebrovascular accident (CVA) can be caused by a block in or rupture of an arterial blood vessel, which decreases the blood supply to the brain causing the neurons to die in the area normally supplied by the blood vessels. Because stroke is the third leading cause of death in the U.S. this is a very important clinical topic. Because the brain is so important, blood flow to different regions has alternate routes called collateral circulation. This can be seen in the image of the circle of Willis described previously. Because of major blood sources to the brain, the internal carotid arteries and the basilar artery all feed into the circle, and a blockage of any artery can be made up by increasing blood flow in the others. The results of a stroke are associated with the location of the damaged areas in the brain and the severity. Hemorrhagic strokes, as illustrated in Figure 14.26, are associated with a worse outcome. Common symptoms are weakness or paralysis causing difficulty moving and or speaking, dizziness, headache, and blurring of vision. A less severe version of a stroke is called a transient ischemic attack (TIA) and has some of the same symptoms but no permanent damage. TIAs are often an ominous sign of an impending stroke and should never be ignored. Family history and aging are some of the risk factors along with smoking, obesity, and cholesterol. As with heart disease, which is the number one cause of death in the U.S., diabetes is a major risk factor. This is especially important in populations where diabetes is more common, such as Hawaiʻi.

Figure 14.26 Hemorrhagic Stroke (a) A hemorrhage into the tissue of the cerebrum results in a large accumulation of blood with an additional edema in the adjacent tissue. The hemorrhagic area causes the entire brain to be disfigured as suggested here by the lateral ventricles being squeezed into the opposite hemisphere. (b) A CT scan shows an intraparenchymal hemorrhage within the parietal lobe. (OpenStax: credit b: James Heilman)

Alzheimer’s disease (AD) is a progressive disease that, while starting earlier in life, does not typically become noticeable until a person reaches their sixties. The symptoms include: progressive loss of intellectual functions, deterioration of language and visuospatial skills, and, most notably, memory impairment. The causes are still being investigated, but a combination of genetic and environmental factors seems to play a role in this disease. Postmortem investigation has shown that the cerebral cortical neurons of patients with this disease contain an accumulation of protein fibers that form neurofibrillary tangles, and that deposits of an abnormal protein, called amyloid, are present around the cerebral arterioles. The most common treatments are medications that inhibit the enzyme acetylcholinesterase to increase the available acetylcholine, which is important for memory recall.

Chapter Summary

Key Terms

abducens nerve

sixth cranial nerve; responsible for contraction of one of the extraocular muscles

amygdala

nucleus deep in the temporal lobe of the cerebrum that is related to memory and emotional behavior

anterograde amnesia

type of memory loss that occurs when a person can’t form new memories

aphasia

loss of ability to understand or express speech, caused by brain damage

arachnoid granulation

outpocket of the arachnoid membrane into the dural sinuses that allows for reabsorption of CSF into the blood

arachnoid mater

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

arachnoid trabeculae

filaments between the arachnoid and pia mater within the subarachnoid space

arbor vitae

the cerebellar white matter, so called for its branched, tree-like appearance

astrocytes

glial cells that envelop the capillaries in the brain

basal nuclei

nuclei of the cerebrum (with a few components in the upper brain stem and diencephalon) that are responsible for assessing cortical movement commands and comparing them with the general state of the individual through broad modulatory activity of dopamine neurons; largely related to motor functions, as evidenced through the symptoms of Parkinson’s and Huntington’s diseases

basilar artery

blood vessel from the merged vertebral arteries that runs along the dorsal surface of the brain stem

blood-brain barrier

physical barrier between the neurons and circulating blood

brain stem

region of the adult brain that includes the midbrain, pons, and medulla oblongata and develops from the mesencephalon, metencephalon, and myelencephalon of the embryonic brain

Broca’s area

region of the frontal lobe associated with the motor commands necessary for speech production and located only in the cerebral hemisphere responsible for language production, which is the left side in approximately 95 percent of the population

caudate

nucleus deep in the cerebrum that is part of the basal nuclei; along with the putamen, it is part of the striatum

central canal

hollow space within the spinal cord that is the remnant of the center of the neural tube

cerebellar cortex

receives information from most parts of the body

cerebellar hemispheres

two divisions of the cerebellum on either side of the vermis

cerebellar nuclei

deep cerebellar nuclei

cerebellum

region of the adult brain connected primarily to the pons that developed from the metencephalon (along with the pons) and is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord

cerebral aqueduct

connection of the ventricular system between the third and fourth ventricles located in the midbrain

cerebral cortex

outer gray matter covering the forebrain, marked by wrinkles and folds known as gyri and sulci

cerebral hemisphere

one half of the bilaterally symmetrical cerebrum

cerebral peduncles

white matter corticospinal tracts that connect the upper motor neurons of the primary motor cortex to the lower motor neurons in the spinal cord

cerebrospinal fluid

clear liquid produced within spaces in the brain called ventricles

cerebrum

region of the adult brain that develops from the telencephalon and is responsible for higher neurological functions such as memory, emotion, and consciousness

choroid plexus

specialized structures containing ependymal cells lining blood capillaries that filter blood to produce CSF in the four ventricles of the brain

circle of Willis

unique anatomical arrangement of blood vessels around the base of the brain that maintains perfusion of blood into the brain even if one component of the structure is blocked or narrowed

corpora quadrigemina

superior and inferior colliculi that are parts of the visual and the auditory pathways respectively

corpus callosum

large white matter structure that connects the right and left cerebral hemispheres

corpus striatum

major input site of the basal ganglia (or basal nuclei）

cranial nerve

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

decussation

when fibers cross from one side of a structure to the other side of a structure

diencephalon

region of the adult brain that retains its name from embryonic development and includes the thalamus and hypothalamus

dopamine

a type of monoamine neurotransmitter

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

dural venous sinus

a group of sinuses or blood channels that drains venous blood circulating from the cranial cavity

ectoderm

outermost layer of cells or tissue of an embryo in early development, or the parts derived from this, which include the epidermis and nerve tissue

endoderm

innermost embryonic germ layer from which most of the digestive system and lower respiratory system derive

epidural space

area between the dura mater and the vertebral wall, containing fat and small blood vessels

epithalamus

region of the diecephalon containing the pineal gland

facial nerve

seventh cranial nerve; responsible for contraction of the facial muscles and for part of the sense of taste, as well as causing saliva production

flocculonodular lobe

area of the cerebellum that receives vestibular and visual information

folia

gyrus of the cerebellar cortex

foramen magnum

large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral arteries enter the cranium

forebrain

anterior region of the adult brain that develops from the prosencephalon and includes the cerebrum and diencephalon

fourth ventricle

the portion of the ventricular system that is in the region of the brain stem and opens into the subarachnoid space through the median and lateral apertures

frontal lobe

region of the cerebral cortex directly beneath the frontal bone of the cranium

globus pallidus

nuclei deep in the cerebrum that are part of the basal nuclei and can be divided into the internal and external segments

glossopharyngeal nerve

ninth cranial nerve; responsible for contraction of muscles in the tongue and throat and for part of the sense of taste, as well as causing saliva production

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

gyrus

ridge formed by convolutions on the surface of the cerebrum or cerebellum

hindbrain

posterior region of the adult brain that develops from the rhombencephalon and includes the pons, medulla oblongata, and cerebellum

hydrocephalus

blockage in the CSF circulation

hippocampus

gray matter deep in the temporal lobe that is very important for long-term memory formation

hypoglossal nerve

twelfth cranial nerve; responsible for contraction of muscles of the tongue

hypothalamus

major region of the diencephalon that is responsible for coordinating autonomic and endocrine control of homeostasis

infundibulum

hollow stalk which connects the hypothalamus and the posterior pituitary gland

internal capsule

two-way tract for the transmission of information to and from the cerebral cortex

internal carotid artery

branch from the common carotid artery that enters the cranium and supplies blood to the brain

insula

brain region that process taste, smell, sound, visceral and body surface sensations, and emotions such as empathy

jugular veins

blood vessels that return “used” blood from the head and neck

lateral apertures

pair of openings from the fourth ventricle to the subarachnoid space on either side and between the medulla and cerebellum

lateral sulcus

surface landmark of the cerebral cortex that marks the boundary between the temporal lobe and the frontal and parietal lobes

lateral ventricles

portions of the ventricular system that are in the region of the cerebrum

limbic system

structures at the edge (limit) of the boundary between the forebrain and hindbrain that are most associated with emotional behavior and memory formation

longitudinal fissure

large separation along the midline between the two cerebral hemispheres

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

medulla oblongata

lowest portion of the brainstem

melatonin

amino acid–derived hormone that is secreted in response to low light and causes drowsiness

meninges

protective outer coverings of the CNS composed of connective tissue

meningitis

inflammation of the meninges

meningeal layer

dense fibrous membrane that passes through the foramen magnum and is continuous with the dura mater of the spinal cord

mesencephalon

primary vesicle of the embryonic brain that does not significantly change through the rest of embryonic development and becomes the midbrain

mesoderm

middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals

metencephalon

secondary vesicle of the embryonic brain that develops into the pons and the cerebellum

midbrain

middle region of the adult brain that develops from the mesencephalon

myelencephalon

secondary vesicle of the embryonic brain that develops into the medulla

nerve

cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system

neural crest

tissue that detaches from the edges of the neural groove and migrates through the embryo to develop into peripheral structures of both nervous and non-nervous tissues

neural plate

plate of ectoderm along the dorsal midline of the early vertebrate embryo that gives rise to the neural tube and neural crests　 

neural fold

elevated edge of the neural groove

neural groove

region of the neural plate that folds into the dorsal surface of the embryo and closes off to become the neural tube

neural plate

thickened layer of neuroepithelium that runs longitudinally along the dorsal surface of an embryo and gives rise to nervous system tissue

neural tube

precursor to structures of the central nervous system, formed by the invagination and separation of neuroepithelium

nuclei

cluster of neurons in the central nervous system, located deep within the cerebral hemispheres and brainstem

occipital lobe

region of the cerebral cortex directly beneath the occipital bone of the cranium

occipital sinuses

dural sinuses along the edge of the occipital lobes of the cerebrum

oculomotor nerve

third cranial nerve; responsible for contraction of four of the extraocular muscles, the muscle in the upper eyelid, and pupillary constriction

olfactory nerve

first cranial nerve; responsible for the sense of smell

optic nerve

second cranial nerve; responsible for visual sensation

parietal lobe

region of the cerebral cortex directly beneath the parietal bone of the cranium

periosteal layer

a layer of periosteum that covers the inner surface of the skull

peripheral nervous system

refers to parts of the nervous system outside the brain and spinal cord

pia mater

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

pineal gland

endocrine gland that secretes melatonin, which is important in regulating the sleep-wake cycle

pyramids

fiber bundles in the medulla that appear triangular and contain motor fibers, the majority of which are part of the corticospinal tract

pituitary gland

bean-sized organ suspended from the hypothalamus that produces, stores, and secretes hormones in response to hypothalamic stimulation (also called hypophysis)

plexus

network of nerves or nervous tissue

pons

part of the brainstem, a structure that links the brain to the spinal cord

primary vesicle

initial enlargements of the anterior neural tube during embryonic development that develop into the forebrain, midbrain, and hindbrain

proprioception

general sensory perceptions providing information about location and movement of body parts; the “sense of the self”

prosencephalon

primary vesicle of the embryonic brain that develops into the forebrain, which includes the cerebrum and diencephalon

putamen

nucleus deep in the cerebrum that is part of the basal nuclei; along with the caudate, it is part of the striatum

reticular formation

diffuse region of gray matter throughout the brain stem that regulates sleep, wakefulness, and states of consciousness

retrograde amnesia

amnesia where a person can’t recall memories that were formed before the event that caused the amnesia

rhombencephalon

primary vesicle of the embryonic brain that develops into the hindbrain, which includes the pons, cerebellum, and medulla

secondary vesicle

five vesicles that develop from primary vesicles, continuing the process of differentiation of the embryonic brain

septum pellucidum

thin membrane located at the midline of the brain between the two cerebral hemispheres

sigmoid sinuses

dural sinuses that drain directly into the jugular veins

somatosensation

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

spinal nerve

one of 31 nerves connected to the spinal cord

straight sinus

dural sinus that drains blood from the deep center of the brain to collect with the other sinuses

striatum

the caudate and putamen collectively, as part of the basal nuclei, which receive input from the cerebral cortex

subarachnoid space

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

superior sagittal sinus

dural sinus that runs along the top of the longitudinal fissure and drains blood from the majority of the outer cerebrum

tectum

region of the midbrain, thought of as the roof of the cerebral aqueduct, which is subdivided into the inferior and superior colliculi

telencephalon

secondary vesicle of the embryonic brain that develops into the cerebrum

temporal lobe

region of the cerebral cortex directly beneath the temporal bone of the cranium

thalamus

major region of the diencephalon that is responsible for relaying information between the cerebrum and the hindbrain, spinal cord, and periphery

third ventricle

portion of the ventricular system that is in the region of the diencephalon

tract

bundle of axons in the central nervous system having the same function and point of origin

transverse sinuses

dural sinuses that drain along either side of the occipital–cerebellar space

trigeminal nerve

fifth cranial nerve; responsible for cutaneous sensation of the face and contraction of the muscles of mastication

trochlear nerve

fourth cranial nerve; responsible for contraction of one of the extraocular muscles

vagus nerve

tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal cavities

ventricles

remnants of the hollow center of the neural tube that are spaces for cerebrospinal fluid to circulate through the brain

vermis

unpaired medial structure that separates the cerebellar hemispheres

vertebral arteries

arteries that ascend along either side of the vertebral column through the transverse foramina of the cervical vertebrae and enter the cranium through the foramen magnum

vestibulocochlear nerve

eighth cranial nerve; responsible for the sensations of hearing and balance

Wernicke’s area

region of the brain that contains motor neurons involved in the comprehension of speech

white matter

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

Sources

• Anatomy and Physiology Lab I by Daniel McNabney and DeLoris Hesse

• OpenStax Anatomy and Physiology (http://openstax.org/details/books/anatomy-and-physiology)
• Anatomy & Physiology Lab Homework by Laird C. Sheldahl (CC SA 4.0)
• Windward Community College Histology Guide & Anatomy and Physiology 2 Lab Manual
• https://www.health.harvard.edu/nutrition/why-nutritionists-are-crazy-about-nuts
• https://commons.wikimedia.org/wiki/File:Human_brain_NIH.png
• https://commons.wikimedia.org/wiki/File:Blausen_0614_LimbicSystem.png