The nervous system (Fig. 11-1) plays a major role in homeostasis, keeping the other body systems coordinated and regulated to achieve optimal performance. It accomplishes this goal by helping the individual respond to his or her internal and external environments.

The nervous and endocrine systems are responsible for communication and control throughout the body. There are three main neural functions:

1. Collecting information about the external and internal environment (sensing).

2. Processing this information and making decisions about action (interpreting).

3. Directing the body to put into play the decisions made (acting).

Anatomy and Physiology

Organization of the Nervous System

Nerves are composed of several neurons, or nerve cells. Although neurons cannot be seen without a microscope, some nerves are easily visible. The nerves that extend from the foot to the brain can be over 2 meters long. Neural tracts are pathways between different parts of the nervous system. An example is the pyramidal tract (corticospinal tract), which is named for its triangular shape and which joins the two hemispheres of the cerebral cortex and the spinal cord.

The nervous system is divided into two main systems. (See Figure 11-2 for a schematic of the divisions). The central nervous system (CNS) is composed of the brain and the spinal cord. It is the only site of nerve cells called interneurons, which connect sensory and motor neurons. The peripheral nervous system (PNS) is composed of the nerves that extend from the brain and spinal cord to the tissues of the body. These are organized into 12 pairs of cranial nerves and 31 pairs of spinal nerves. The PNS is further divided into sensory and motor nerves. Sensory (afferent) nerves carry impulses to the brain and spinal cord, whereas motor (efferent) nerves carry impulses away from the brain and spinal cord.

Fig. 11-2 Schematic of the nervous system into the CNS and PNS.

body = somat/o

PNS nerves are further categorized into two subsystems:

Somatic nervous system (SNS): this system is voluntary in nature. These nerves collect information from and return instructions to the skin, muscles, and joints.

Autonomic nervous system (ANS): mostly involuntary functions are controlled by this system as sensory information from the internal environment is sent to the CNS, and, in return, motor impulses from the CNS are sent to involuntary muscles: the heart, glands, and organs.

Note

This chapter includes all the anatomy necessary to assign ICD-10 nervous system codes, including detail on the basal ganglia, corpus callosum, and the choroid plexus. See Appendix F for a complete list of body parts and how they should be coded.

Be Careful!

Remember that efferent means to carry away, whereas afferent means to carry toward. In the nervous system, these terms are used to refer to away from and toward the brain.

Cells of the Nervous System

The nervous system is made up of the following two types of cells:

1. Parenchymal cells, or neurons, the cells that carry out the work of the system.

2. Stromal cells, or glia, the cells that provide a supportive function.

Neurons

The basic unit of the nervous system is the nerve cell, or neuron (Fig. 11-3). Not all neurons are the same, but all have features in common. Dendrites, projections from the cell body, receive neural impulses, also called action potentials, from a stimulus of some kind. This impulse travels along the dendrite and into the cell body, which is the control center of the cell. This cell body contains the nucleus and surrounding cytoplasm.

neuron

neur/o = nerve

-on = structure

dendrite = dendr/o

From the cell body, the neural impulse moves out along the axon, a slender, elongated projection that carries the neural impulse toward the next neuron. The terminal fibers result from the final branching of the axon and are the site of the axon terminals, which store the chemical neurotransmitters like dopamine (responsible for affecting voluntary movement regulation) and serotonin (responsible for mood, sleep, and appetite). Many axons are covered by the myelin sheath, which is a substance produced by the Schwann cells that coats the axons in the peripheral nervous system. This coating gives the cells a whitish appearance, as opposed to the gray appearance of unmyelinated axons and cell bodies. The nodes of Ranvier are areas of neurons not covered by Schwann cells. The outer cell membrane of the Schwann cell is the neurilemma. Because the CNS is mostly unmyelinated, it is associated with the phrase “using your gray matter.”

From the axon’s terminal fibers, the neurotransmitter is released from the cell to travel across the space between these terminal fibers and the dendrites of the next cell. This space is called the synapse (see Fig. 11-3). The impulse continues in this manner until its destination is reached. A nerve is a group of bundled axons in the PNS. Most nerves contain both sensory and motor fibers.

Glia

The supportive, or stromal, glia are also called neuroglia (Fig. 11-4). They accomplish their supportive function by physically holding the neurons together and also protecting them. There are different kinds of neuroglia, including astrocytes, ependymal cells, oligodendroglia, microglia, and Schwann cells. Astrocytes connect neurons and blood vessels and form a structure called the blood-brain barrier (BBB), which prevents or slows the passage of some drugs and disease-causing organisms to the CNS. Ependymal cells line the ventricles of the brain and produce cerebrospinal fluid. Oligodendroglia cover the axons of neurons in the central nervous system, forming their myelin sheath. Microglia perform an active protective function by engulfing and ingesting infectious organisms.

Fig. 11-4 Types of neuroglia. A, Astrocyte. B, Microglia. C, Ependymal cells. D, Oligodendrocyte. E, Schwann cell.

Be Careful!

The abbreviation BBB can stand for either blood-brain barrier or bundle branch block, a cardiac condition.

oligodendroglia

olig/o = scanty

dendr/o = dendrite

Exercise 1:

Organization of the Nervous System

Fill in the blanks.

1. The two main divisions of the nervous system are the _________________ and the _________________.

Underline the correct answer.

2. Sensory neurons (transmit, receive) information (to, from) the CNS.

3. Motor neurons, also called (efferent, afferent) neurons, transmit information (to, from) the CNS.

4. The (somatic, autonomic) nervous system is voluntary in nature, whereas the (somatic, autonomic) nervous system is largely involuntary.

Exercise 2:

Cells of the Nervous System

Match the terms with their word parts.

6. perineural ________________________________________________________________________________________

7. oligodendritic ____________________________________________________________________________________

8. microglial ________________________________________________________________________________________

The Central Nervous System

brain = encephal/o

cerebrum = cerebr/o

cerebellum = cerebell/o

cortex = cortic/o

lobe = lob/o

As stated previously, the CNS is composed of the brain and the spinal cord.

Brain (Encephalon)

The brain is one of the most complex organs of the body. It is divided into four parts: the cerebrum, the diencephalon, the brainstem, and the cerebellum (Fig. 11-5).

Cerebrum

The largest portion of the brain, the cerebrum, is divided into two halves, or hemispheres (Fig. 11-6). It is responsible for thinking, reasoning, and memory. The surfaces of the hemispheres are covered with gray matter and are called the cerebral cortex. Arranged into folds, the valleys are referred to as sulci (sing. sulcus), and the ridges are called the gyri (sing. gyrus). The cerebrum is further divided into sections called lobes, each of which has its own functions:

1. The frontal lobe contains the function of speech and the motor area that controls voluntary movement on the contralateral side of the body. It also is in charge of personality, emotions, and problem solving.

2. The temporal lobes contain the auditory and olfactory areas, and is where sequencing and memory occur.

3. The parietal lobes control the sensations of touch and taste, and also is control of spatial perception.

4. The occipital lobe is responsible for vision.

5. The insular lobes, located under the frontal and temporal lobes, are responsible for empathy, interceptive (internal sensing) awareness, and cognition. The term insula is derived from the Latin word meaning “island,” probably because it is not obviously continuous with the other lobes. Synonyms for insular lobe are the island of Reil and insular cortex.

The corpus callosum is the thick band of nerve fibers that joins the two hemispheres of the cerebrum.

The basal ganglia, located under the cerebrum, are a collection of structures responsible for a number of different functions. The corpus striatum functions as a reception area that actuates the globus pallidum to perform an inhibitory action on motor function areas. The substantia nigra has a function in the release of dopamine. The subthalamic nucleus influences the globus pallidum after receiving stimulus from either the cerebral cortex or the striatum. The claustrum is a thin layer of gray matter with poorly understood functioning, although it is considered to be part of the basal ganglia. Collectively, damage to the basal ganglia plays a role in several neurological disorders, including Parkinson’s disease and Huntington’s disease.

globus pallidum = pallid/o

Be Careful!

Basal ganglia is a misnomer because ganglia only appear in the PNS. Basal nuclei is the more technically correct name, although basal ganglia is more commonly used. Collections of cell bodies in the CNS are termed nuclei, whereas in the PNS, they are called ganglia. ICD-10 uses the term basal ganglia.

Diencephalon

The diencephalon is composed of the thalamus and the structure inferior to it, the hypothalamus. The thalamus is responsible for relaying sensory information (with the exception of smell) and translating it into sensations of pain, temperature, and touch. The epithalamus, which includes the pineal gland, is named for its location above the thalamus. It connects the limbic system to the rest of the brain. The limbic system is responsible for motivated behavior and arousal and influences the endocrine and autonomic motor systems. The medial and lateral geniculate nuclei are sensory relays within the thalamus that receive and send visual and auditory information. Together they are termed the metathalamus. The pulvinar are nuclei located in the medial posterior thalamus with functions involving visual attention.

The hypothalamus activates, integrates, and controls the peripheral autonomic nervous system, along with many functions such as body temperature, sleep, and appetite. The mammillary bodies are small, rounded structures that participate in the ability to recognize people, places, and objects that are stored in memory.

Brainstem

The brainstem connects the cerebral hemispheres to the spinal cord. It is composed of three main parts: the midbrain, pons, and medulla oblongata. The midbrain connects the pons and cerebellum with the hemispheres of the cerebrum. It is the site of reflex centers for eye and head movements in response to visual and auditory stimuli. The superior and inferior colliculi of the midbrain are composed of sensory visual and auditory nerve fibers. The cerebral peduncles of the midbrain are motor nerve fibers that connect to the spinal cord and cerebellum. The pons serves as a bridge between the medulla oblongata and the cerebrum. The pneumotaxic and apneustic centers are networks of neurons that regulate respiration. The anterior part of the pons is the basis pontis, which has a role in motor function. The locus ceruleus (Latin for blue spot) functions in the physiological response to stress. The superior olivary nucleus is involved in hearing. Finally, the lowest part of the brainstem, the medulla oblongata, regulates heart rate, blood pressure, and breathing.

Be Careful!

Myel/o can mean bone marrow or spinal cord; memorization and context will be the student’s only methods to determine which is which.

Cerebellum

Located inferior to the occipital lobe of the cerebrum, the cerebellum coordinates voluntary movement but is involuntary in its function. For example, walking is a voluntary movement. The coordination needed for the muscles and other body parts to walk smoothly is involuntary and is controlled by the cerebellum.

Spinal Cord

spinal cord = cord/o, chord/o, myel/o

The spinal cord extends from the medulla oblongata to the first lumbar vertebra (Fig. 11-7). It then extends into a structure called the cauda equina (which means “horse’s tail” in Latin). The conus medullaris is the end of the spinal cord, whereas the cauda equina is the collection of nerve roots that extends from it.

Fig. 11-7 The spinal cord with an inset of a cervical segment showing emerging cervical nerves.

meninges = mening/o, meningi/o

The spinal cord is protected by the bony vertebrae surrounding the spinal, or vertebral, canal and the coverings unique to the CNS, called meninges. The spinal cord is composed of gray matter, the cell bodies of motor neurons, and white matter, the myelin-covered axons or nerve fibers that extend from the nerve cell bodies. The 31 pairs of spinal nerves emerge from the spinal cord at the nerve roots.

nerve root = rhiz/o, radicul/o

dura mater = dur/o

Meninges

The meninges act as protective coverings for the CNS and are composed of three layers separated by spaces (Fig. 11-8). The dura mater is the tough, fibrous outer covering of the meninges. Its literal meaning is “hard mother.” The falx cerebri is the sickle-shaped fold between the hemispheres. The dura mater is classified as cranial dura mater and spinal dura mater. The tentorium cerebella is an extension of the dura mater that separates the cerebellum from the lower part of the occipital lobes. The diaphragm sellae is a small dural fold that covers the sella turcica; it has a small opening for the infundibulum of the pituitary. The space between the dura mater and arachnoid membrane is called the subdural space. If in the skull, it is the cranial subdural space; if in the spine, it is referred to as the spinal subdural space. Likewise, the subarachnoid space is further categorized as cranial subarachnoid and spinal subarachnoid spaces. The next layer is the arachnoid membrane, a thin, delicate membrane that takes its name from its spidery appearance. The subarachnoid space is the space between the arachnoid membrane and the pia mater. It contains cerebrospinal fluid (CSF), a clear fluid that protects the brain and spinal cord and removes waste products and monitors for internal changes. CSF is also present in cavities in the brain called ventricles. Finally, the pia mater is the thin, vascular membrane that is the innermost of the three meninges; its literal meaning is “soft mother.” The arachnoid and pia mater are referred to as leptomeninges for their slender appearance as opposed to the thick, tough nature of the dura mater.

ventricle = ventricul/o

slim, delicate = lept/o

There are four ventricles in the brain: two lateral ventricles (left and right), the third ventricle, and the fourth ventricle. Each is connected by foramina (openings). The lateral ventricles are connected by the foramen of Monro and the third and fourth by the aqueduct of Sylvius. The choroid plexus is the membrane within the ventricles of the brain and spinal cord that is lined with ependymal cells that produce CSF. Cisterns are dilations of the subarachnoid space that contain CSF. The cisterna magna is between the dorsal surface of the medulla oblongata and the cerebellum and receives its CSF from the fourth ventricle.

magna = large

The Peripheral Nervous System

The peripheral nervous system is divided into:

• 12 pairs of cranial nerves that conduct impulses between the brain and head, neck, thoracic, and abdominal areas, and

• 31 pairs of spinal nerves that closely mimic the organization of the vertebrae and provide innervations to the rest of the body.

The peripheral nerves are a combination of afferent (sensory) nerves and efferent (motor) nerves. The motor nerves are either voluntary or involuntary. The autonomic nervous system (ANS) consists of nerves that regulate involuntary function such as cardiac or smooth muscle. The ANS is further divided into the sympathetic and parasympathetic nervous systems, two opposing mechanisms that provide balance in the body:

• The sympathetic nervous system is capable of producing a “fight-or-flight” response. This is the part of the nervous system that helps the individual respond to perceived stress. The heart rate and blood pressure increase, digestive processes slow, and sweat and adrenal glands increase their secretions.

• The parasympathetic nervous system tends to do the opposite of the sympathetic nervous system—slowing the heart rate, lowering blood pressure, increasing digestive functions, and decreasing adrenal and sweat gland activity. This is sometimes called the “rest and digest” system. An example of a sensory response follows:

“Eight-year-old Joey is hungry. He decides to sneak some cookies before dinner. Afraid his mother will see him, he surreptitiously takes a handful into the hall closet and shuts the door. As he begins to eat, the closet door flies open. Joey’s heart begins to race as he whips the cookies out of sight. When he sees it’s only his sister, he relaxes and offers her a cookie as a bribe not to tell on him.”

Joey’s afferent (sensory) somatic neurons carried the message to his brain that he was hungry. This message was interpreted by his brain as a concern, and the response was to sneak cookies from the jar and hide himself as he ate them. When the closet door flew open, his sensory neurons perceived a danger and triggered a sympathetic “fight-or-flight” response, which raised his heart rate and blood pressure and stimulated his sweat glands. When the intruder was perceived to be harmless, his parasympathetic nervous system took over and reduced his heart rate, bringing it back to normal. The same afferent fibers perceived the intruder in two different ways, with two different sets of autonomic motor responses (sympathetic and parasympathetic).

Cranial Nerves

Cranial nerves are named by their number and also their function or distribution. See Figure 11-9 and the table on the next page.

The Cranial Nerves

Number	Name	Origin of Sensory Fibers	Effector Innervated by Motor Fibers
I	Olfactory	Olfactory epithelium of nose (smell)	None
II	Optic	Retina of eye (vision)	None
III	Oculomotor	Proprioceptors* of eyeball muscles	Muscles that move eyeball; muscles that change shape of lens; muscles that constrict pupil
IV	Trochlear	Proprioceptors* of eyeball muscles	Muscles that move eyeball
V	Trigeminal	Teeth and skin of face	Some muscles used in chewing
VI	Abducens	Proprioceptors* of eyeball muscles	Muscles that move eyeball
VII	Facial	Taste buds of anterior part of tongue	Muscles used for facial expression; submaxillary and sublingual salivary glands
VIII	Vestibulocochlear (auditory)		None
	Vestibular branch	Semicircular canals of inner ear (senses of movement, balance, and rotation)
	Cochlear branch	Cochlea of inner ear (hearing)
IX	Glossopharyngeal	Taste buds of posterior third of tongue and lining of pharynx	Parotid salivary gland; muscles of pharynx used in swallowing
X	Vagus	Nerve endings in many of the internal organs (e.g., lungs, stomach, aorta, larynx)	Parasympathetic fibers to heart, stomach, small intestine, larynx, esophagus, and other organs
XI	Spinal accessory	Muscles of shoulder	Muscles of neck and shoulder
XII	Hypoglossal	Muscles of tongue	Muscles of tongue