Introduction

Brain Coral This brain coral appears remarkably similar to its namesake, the brain. Note the extensive ridges (gyri) and valleys (sulci) that increase the surface area of the brain to maximise space for neurons and their connections. [Jaro Nemčok, CC BY-SA 3.0, via Wikimedia Commons]

The formation of the nervous system is a miraculous event: Billions of neurons are born and navigate through the developing tissue in the embryo to form meaningful connections with each other and target organs such as muscles. These neurons function in complex ways to allow the body to sense its environment, react to its environment, and integrate information in meaningful ways. The nervous system consists of all nervous tissue components and can be divided into both structural divisions based on the location of this tissue (central or peripheral) and functional divisions based on the type of information being transported (sensory, motor, and autonomic).

Basic Structure and Function of the Nervous System

Structural Divisions of the Nervous System

The nervous system can be divided into two major regions based on the location of nervous tissue: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else (Figure 12.2). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column.

Figure 12.2 Central and Peripheral Nervous System The structures of the PNS are referred to as ganglia and nerves, which can be seen as distinct structures. The main organs of the CNS are the brain and spinal cord.

Functional Divisions of the Nervous System

The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for sensation (sensory functions) and for the response (motor functions).

Controlling the Body

The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The somatic nervous system (SNS) is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them.

The autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down.

Grey and White Matter

Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. These two regions within nervous system structures are often referred to as grey matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). Figure12.3 demonstrates the appearance of these regions in the brain and spinal cord. The colours ascribed to these regions are what would be seen in “fresh,” or unstained, nervous tissue. Grey matter is not necessarily grey. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin.

Figure 12.3 Grey Matter and White Matter A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by grey matter. Grey matter makes up the outer cortex of the brain. [modification of work by “Suseno”/Wikimedia Commons]

Terminology applied to bundles of axons differs depending on location. A bundle of axons, or fibres, found in the CNS is called a tract whereas the same thing in the PNS would be called a nerve. There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of axons. When those axons are in the PNS, the term is nerve, but if they are CNS, the term is tract. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract.

The Central Nervous System: Brain, Meninges and Ventricles

Complete the activities in the worksheet below as you work through the series of 3D digital models. The worksheet is in a downloadable format.

Meninges of the Brain and Spinal Cord

The external surface of the CNS is covered by a series of membranes composed of connective tissue called the meninges (pl; meninx, singular), which protect the brain. The meninges are named from external to internal, the dura mater, arachnoid mater and pia mater (Figure 13.17).

Figure 13.17 Meningeal Layers at the Superior Sagittal Sinus (Coronal View) The layers of the meninges in the longitudinal fissure inferior to the superior sagittal sinus are shown, with the dura mater adjacent to the internal 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.

The Ventricular System

Cerebrospinal fluid (CSF) circulates throughout and around the CNS. CSF functions to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood. It also acts as a liquid cushion for the brain and spinal cord. The ventricles are the open spaces within the brain where CSF circulates. Cerebrospinal fluid is produced within the ventricles by a type of specialised membrane called a choroid plexus (Figure 13.18). A very small amount of CSF is produced by the choroid plexuses each day, for a total of about 500 milliliters daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving.

Figure 13.18 Cerebrospinal Fluid Circulation (Midsagittal View). The choroid plexus in the ventricles produces CSF, which is circulated through the ventricular system and then enters the subarachnoid space through small apertures in the ventricle in the brainstem. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid mater protrudes through the dura mater to emerge into the superior sagittal dural sinus. Note that you will not be examined on the different parts of the ventricular system in this unit.

Cerebrospinal Fluid Circulation

After flowing through the ventricular system, CSF flows from the fourth ventricle inferiorly into the central canal of the spinal cord as well as posterolaterally through a number of small apertures that open up to the subarachnoid space. The arachnoid villi/granulations are outpocketings of the arachnoid membrane into the dural sinuses so that CSF can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the internal jugular veins to be reoxygenated by the lungs and wastes to be filtered out by the kidneys.

The Cerebrum

The iconic grey mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum (Figure 13.6). The wrinkled external portion composed of grey mattter is the cerebral cortex, and the rest of the structure is deep to that external covering. There is a large separation between the two sides of the cerebrum called the longitudinal fissure. This fissure separates the cerebrum into two distinct halves, a right and left cerebral hemisphere. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for axonal communication between the two hemispheres of the cerebral cortex.

Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The cerebrum comprises the outer grey matter that is the cortex (from the Latin word meaning “bark of a tree”) and several deep nuclei. The basal nuclei are responsible for cognitive processing, the most important function being that associated with planning movements.

Cerebral Cortex

The cerebrum is covered by a continuous layer of grey matter called the cerebral cortex. This thin, extensive region of wrinkled grey matter is responsible for the higher functions of the nervous system. A gyrus (plural = gyri) is the ridge of one of those wrinkles, and a sulcus (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex. Extensive folding in the cerebral cortex enables more grey matter to fit into this limited space. If the grey matter of the cortex were peeled off of the cerebrum and laid out flat, its surface area would be roughly equal to one square meter.

The cortex can be separated into four major regions, or lobes (Figure 13.7). These four lobes lie approximately deep to the bones of the same name. Most anteriorly is the frontal lobes, with the parietal lobes located directly posterior to the frontal lobes. The main sensation associated with the parietal lobe is somatosensation, meaning the general sensations associated with the body. All of the tactile senses are processed in this area, including touch, pressure, tickle, pain, itch and vibration, as well as more general senses of the body such as proprioception and kinesthesia, which are the senses of body position and movement, respectively. The frontal lobe is primarily associated with motor functions and is also responsible for production of language and cognitive functions that can be the basis of personality, short-term memory and consciousness. In the most posterior region of the frontal lobe we find the cell bodies of motor neurons that instruct cells in the spinal cord to move skeletal muscles. Anterior to this region are a few areas that are associated with planned movements. Laterally the temporal lobes are located. The temporal lobe is associated with primary auditory sensation, memory and smell. The posterior region of the cortex is the occipital lobe, which is responsible for the processing of visual information.

Subcortical structures

The basal nuclei are a set of nuclei (areas of grey matter in the CNS) in the cerebrum (Figure 13.9) responsible for comparing cortical processing with the general state of activity in the nervous system to influence the likelihood of movement taking place. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep the urge to jump up and scream from actually happening.

The Diencephalon

The majority of the brain, the spinal cord, and the PNS all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The diencephalon is deep to the cerebrum and constitutes the walls of the third ventricle. The diencephalon contains the thalamus centrally, and the hypothalamus anteriorly. The pituitary gland is an endocrine gland that extends inferiorly from the hypothalamus.

Brainstem

The midbrain, pons and medulla oblongata are collectively referred to as the brainstem (Figure 13.12). The structure connects the brain to the spinal cord. Posteriorly attached to the brainstem, but considered a separate region of the adult brain, is the cerebellum. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla oblongata regulate several crucial functions, including the cardiovascular and respiratory systems and rates.

The cranial nerves connect through the brainstem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending neuronal tracts between the spinal cord and brain, specifically the cerebrum, pass through the brainstem.

The Cerebellum

The cerebellum, as the name suggests, is the “little brain” and looks like a miniature version of that part of the brain (Figure 13.13). The cerebellum is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord. It accounts for approximately 10 percent of the mass of the brain.

The Central Nervous System: The Spinal Cord

Macroscopic Anatomy of Spinal Cord

The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. Immediately adjacent to the brainstem is the cervical region, followed by the thoracic, then the lumbar, and finally the sacral region. The spinal cord is not the full length of the vertebral column because the spinal cord does not grow significantly longer after the first or second year, but the skeleton continues to grow. Despite this difference in length, the nerves that emerge from the spinal cord still pass between adjacent vertebrae at their respective levels. As the vertebral column grows, these nerves grow with it and result in a long bundle of nerves that resembles a horse’s tail and is named the cauda equina.

Due to the high number of neurons required to innervate the upper and lower limbs, the spinal cord is larger in the cervical and lumbar regions to allow for more room for the cell bodies of the neurons to reside. This results in a noticeable bulging of the spinal cord as the cervical enlargement (associated with the brachial plexus and upper limb innervation) and lumbar enlargement (associated with the lumbosacral plexuses and lower limb innervation). The terminal end of the spinal cord is referred to as the conus medullaris and tapers inferiorly to a point. The conus medullaris is located typically at the vertebral level of L1/L2 in adults.

Cross-sectional anatomy of spinal cord

As shown in Figure 13.14, the grey matter of the spinal cord in cross-section is subdivided into regions that are referred to as horns. The posterior (dorsal) horn is responsible for sensory processing. The anterior (ventral) horn sends out motor signals to the skeletal muscles. The dorsal and ventral horns are largest in the cervical and lumbosacral regions of the spinal cord where additional neurons are needed to innervate the upper and lower limbs. The lateral horn, which is only found in the thoracic and upper lumbar regions, is the central component of the sympathetic division of the autonomic nervous system.

Figure 13.14 Cross-section of Spinal Cord The cross-section of a thoracic spinal cord segment shows the dorsal, ventral and lateral horns of grey matter. LM ×40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

 Just as the grey matter is separated into horns, the white matter of the spinal cord is separated into columns. Ascending tracts of nervous system fibres in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain.

An example of a descending tract is the corticospinal tract responsible for the conscious or voluntary movements of skeletal muscles. Any motor command from the primary motor cortex is sent down the axons of motor neurons to reach either the cranial motor nuclei or the ventral horn of the spinal cord. The axons of the corticospinal tract are largely contralateral, meaning that they cross the midline of the brainstem or spinal cord and synapse on the opposite side of the body. Therefore, the right motor cortex of the cerebrum controls muscles on the left side of the body, and vice versa.

An example of an ascending tract is the spinothalamic tract responsible for pain and temperature sensations. The spinothalamic tract begins with neurons in a dorsal root ganglion that have nerve endings in the skin. These neurons extend their axons to the dorsal horn, where they synapse with a second neuron. The name “spinothalamic” comes from this second neuron, which has its cell body in the spinal cord gray matter and connects to the thalamus. Axons from these second neurons then decussate (cross) within the spinal cord and ascend to the brain and enter the contralateral thalamus. The neurons in the thalamus then project their axons to the cerebral cortex of the parietal lobe for somatosensory perception.

The Peripheral Nervous System

Nerves

Bundles of axons in the PNS are referred to as nerves (referred to as tracts in the CNS). Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.

Cranial Nerves

The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck. A number of cranial nerves are also responsible for parasympathetic innervation of visceral organs such as the organs of the thoracic and abdominal cavities. There are twelve cranial nerves, which are designated CNI through CNXII for “Cranial Nerve,” using Roman numerals 1 through 12 (Figure 13.23). They can be classified as sensory nerves, motor nerves, or a combination of both (mixed nerves). The names of the cranial nerves are listed in Table 13.3 along with a brief description of their function.

The olfactory nerve and optic nerve are responsible for the sense of smell and vision, respectively. The oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trigeminal nerve is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve is responsible for the muscles involved in facial expression, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve is responsible for the senses of hearing and balance. The vagus nerve is responsible for contributing to homeostatic control of the organs of the thoracic and superior abdominal cavities.

Figure 13.23 The Cranial Nerves The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain.

Three of these cranial nerves also contain autonomic parasympathetic fibres. The oculomotor, facial and vagus nerves contain these parasympathetic fibres. The oculomotor fibres initiate pupillary constriction and change the shape of the lens of the eye, whereas the facial fibres initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and superior abdominal cavities to slow down heart rate and breathing, and activate digestive function.

Another important aspect of the cranial nerves is the functional role each nerve plays. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIII) nerves. The oculomotor (III) nerve is purely motor. The trigeminal (CNV), facial (CNVII), and vagus (CNX) nerves all contain mixed fibres, that is a combination of motor and sensory fibres.

Table 13.3 Cranial Nerves Note that only the cranial nerves that you need to learn in this unit are indicated in this table.

Spinal Nerves

The nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons. There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges – cervical, thoracic, lumbar and sacral. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level.

Spinal nerves extend laterally away from the vertebral column to innervate the periphery. Axons from different spinal nerves will come together into a peripheral nerve. Most nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganisation of the axons in the ventral spinal rami to follow different courses. This occurs at four places along the length of the vertebral column, each identified as a network of nerve fibres called a nerve plexus. Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level (Figure 13.24).

Figure 13.24 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 thoracic diaphragm. The brachial plexus supplies nerves to the upper limb. The lumbar plexus supplies nerves to the anterior parts of the lower limb. The sacral plexus supplies nerves to the posterior parts of the lower limb.

Autonomic Nervous System

The structures and function of the autonomic nervous system have been referred to throughout earlier sections of this chapter. To recap, the autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis.

There are two subdivisions of the autonomic nervous system, namely the sympathetic nervous system and the parasympathetic nervous system. Cardiac muscle, smooth muscle of tubular organs and most glands are innervated by both subdivisions. For each tissue or gland one subdivision of the ANS increases its activity (excitatory function) whereas the other subdivision of the ANS decreases its activity (inhibitory function). The activity of cardiac muscle is increased by sympathetic innervation and decreased by parasympathetic innervation. The parasympathetic system increases the activity of smooth muscle of the intestinal wall whereas the sympathetic system decreases its activity to redirect blood flow towards the skeletal muscles. Sweat glands, vessels and suprarenal/adrenal glands are innervated solely by the sympathetic nervous system. The combined functions of each subdivision of the ANS regulate the body in response to contrasting stimuli. The parasympathetic nervous system function is referred to as rest and digest, in contrast to the sympathetic nervous system function being referred to as fight or flight.

Key Terms

arachnoid granulation outpocket of the arachnoid membrane into the dural sinuses that allows for reabsorption of CSF into the blood; granulation refers to the hardened adult structure compared to arachnoid villi which refer to the soft structure found in children

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

ascending spinal tract central nervous system fibres carrying sensory information from the spinal cord or periphery to the brain

basal nuclei nuclei of the cerebrum (with a few components in the superior brainstem and diencephalon) that are responsible for assessing cortical movement commands and comparing them with the general state of the individual; largely related to motor functions, as evidenced through the symptoms of Parkinson’s and Huntington’s diseases

brachial plexus nerve plexus associated with the inferior cervical spinal nerves and first thoracic spinal nerve

brainstem region of the adult brain that includes the midbrain, pons, and medulla oblongata

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

cerebellum region of the adult brain connected primarily to the pons and is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord

cerebral cortex external grey matter covering the cerebrum, marked by ridges and valleys known as gyri and sulci, respectively

cerebral hemisphere one half of the bilaterally symmetrical cerebrum

cerebrum region of the adult brain responsible for higher neurological functions such as memory, emotion, and consciousness

cervical plexus nerve plexus associated with the superior cervical spinal nerves

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

corpus callosum large white matter structure that allows axons of neurons to cross between the right and left cerebral hemispheres

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

descending tract central nervous system fibres carrying motor commands from the brain to the spinal cord or periphery

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

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 sinus any of the venous structures surrounding the brain, enclosed within the dura mater, which drain blood from the CNS to the common venous return of the jugular veins

extraocular muscles six skeletal muscles that control eye movement within the orbit

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

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

foramen magnum large opening in the occipital bone of the skull through which the spinal cord emerges

frontal lobe most anterior region of the cerebral cortex responsible for voluntary motor movement, personality, planning and intelligence

gyrus ridge formed by convolutions on the surface of the cerebrum

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

kinesthesia general sensory perception of movement of the body

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

longitudinal fissure large separation along the midline between the two cerebral hemispheres

lumbar plexus nerve plexus associated with the lumbar spinal nerves

meninges protective outer coverings of the CNS composed of connective tissue

midbrain middle region of the adult brainstem

nerve plexus network of nerves in the PNS

occipital lobe region of the cerebral cortex responsible for vision perception

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

olfaction special sense responsible for smell, which has a unique, direct connection to the cerebrum

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 responsible for somatosensation

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

phrenic nerve systemic nerve from the cervical plexus that innervates the thoracic diaphragm

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

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

sacral plexus nerve plexus associated with the inferior lumbar and sacral spinal nerves

sciatic nerve systemic nerve from the sacral plexus and extends across the hip joint and gluteal region into the posterior femoral region

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

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

sulcus groove formed by convolutions in the surface of the cerebral cortex

superior sagittal sinus dural sinus that runs along the superior border of the longitudinal fissure and drains blood from the majority of the external cerebrum

sympathetic 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

temporal lobe region of the cerebral cortex responsible for processing of sound, memory, smell and emotion

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

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

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

ventral horn gray matter of the spinal cord containing multipolar motor neurons, sometimes referred to as the anterior horn

ventricles spaces for cerebrospinal fluid to circulate through the brain

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