2-1. INTRODUCTION

a. The nervous system can be thought of as having three major subdivisions:

(1) The central nervous system (CNS), which consists of the brain and spinal cord.

(2) The peripheral nervous system (PNS), which consists of those nerves which pass from the CNS to the periphery of the body (the cranial nerves and the spinal nerves).

(3) The autonomic nervous system (ANS), also referred to as the “involuntary” nervous system. Outside of the CNS and considered to be part of the PNS, the autonomic nervous system is sufficiently different in function to be studied as a separate division. Body functions that are not under conscious control are regulated by the ANS.

b. Neurological conditions are disorders that involve some portion of the nervous system. These conditions may result from infections, deranged physiology, or trauma. In all cases, the normal function of the nervous system has been altered, and the patient is not in control of the alterations.

Basic Human Anatomy. Lesson 11: Nervous System Part I

2-2. THE CENTRAL NERVOUS SYSTEM

The CNS consists of the brain and the spinal cord, which share a continuous, protective, fibrous membrane cover called the meninges. The meninges consist of three separate membranes, which are separated by spaces.

a. Dura Mater.

(1) The dura mater is a tough fibrous layer that serves as the outer layer of meninges and the inner lining of the cranial bones.

(2) Between the dura mater and the next layer of meninges is a potential space called the subdural space.

b. Arachnoid Mater.

(1) The arachnoid mater is a fine, membranous layer of spider-web-like threads that extend from the dura mater, through the subarachnoid space, to the innermost layer of meninges.

(2) The subarachnoid space is filled with cerebrospinal fluid (CSF).

c. Pia Mater.

(1) The pia mater is the delicate, transparent membrane that directly covers the surface of the brain and spinal cord.

(2) The pia mater contains a network of blood vessels.

d. Cerebrospinal Fluid

(1) Cerebrospinal fluid, circulating within the network of the subarachnoid space, provides the brain and spinal cord with protection. It acts as a cushion, or shock-absorber, against injury.

(2) CSF is manufactured from blood in networks of capillaries called choroid plexuses. It circulates through the ventricles (cavities inside the brain) and subarachnoid space of the meninges.

2-3. THE BRAIN

The human brain has three major subdivisions–the brainstem, the cerebellum, and the cerebrum.

a. The Brainstem. The brainstem (figure 2-1) is the basal portion of the brain. It is continuous with the spinal cord. Exiting from the sides of the brainstem are 12 pairs of nerves known as the cranial nerves. The brainstem is divided into 3 major portions.

(1) Forebrainstem (diencephalon), consisting of:

(a) Thalamus–a sensory relay station for impulses conveyed upward from the spinal cord.

(b) Hypothalamus–concerned with regulation of autonomic functions.

(2) Midbrainstem (mesencephalon) consisting of:

(a) Corpora quadrigemina–concerned with vision and hearing.

(b) Cerebral peduncles–connecting the brainstem to the cerebrum.

(3) Hindbrainstem, consisting of:

(a) Pons–concerned with transmission of impulses between the left and right hemispheres of the cerebellum.

(b) Medulla–contains nerve centers for cardiac, vasomotor, and respiratory functions. Also influences sneezing, coughing, hiccoughing, and vomiting.

b. The Cerebellum. The cerebellum (figure 2-2) is a spherical mass of nervous tissue attached to and covering the hindbrainstem. It is attached to the brainstem by three pairs of cerebellar peduncles. The cerebellum consists of three lobes; the right hemisphere, left hemisphere, and a central portion called the vermis.

(1) The outer layer, or cortex, of the cerebellum is composed of “gray matter.” Gray matter is actually the cell bodies of neurons. Many folds (gyri) and grooves (sulci) in the surface of the cortex give it a wrinkled appearance.

(2) More centrally located within the cerebellum is the “white matter.” White matter is actually the myelin covered processes of the neurons.

(3) The cerebellum is concerned with coordination of nerve impulses to the voluntary muscles and with the equilibrium of the body.

c. The Cerebrum. The cerebrum (figure 2-2) is the largest part of the brain. The outer layer of the cerebrum is called the cerebral cortex. It is composed of the cell bodies of neurons and is often referred to as the “gray matter.” Many folds (gyri) and grooves (sulci) increase the surface area of this layer. Beneath the cortex lies the “white matter,” actually the myelin covered processes of the neurons. The cerebrum is divided into right and left hemispheres by a longitudinal fissure. A band of nerve fibers called the corpus callosum connects the hemispheres and provides for communication between them. Each cerebral hemisphere is further divided into lobes, which are named after the overlying cranial bones (frontal, parietal, temporal, and occipital).

(1) Frontal lobe.

(a) The frontal lobe is located beneath the frontal bones of the skull. It is divided from the parietal lobes by the central sulcus and from the temporal lobes by the lateral fissure.

(b) The functions of the frontal lobe include personality, behavior, intellectual functioning, creative thinking, morals, ethics, and level of consciousness.

(c) The precentral gyrus, within the frontal lobe and directly in front of the central sulcus, initiates and controls voluntary muscle movements (motor function). Impulses originating here travel along motor pathways through the spinal cord and stimulate skeletal muscles on the opposite side of the body. (This is important to remember when correlating clinical symptoms with associated cerebral damage.) The percentral gyrus also contains Broca’s speech center, which is involved in the motor activities necessary for speech.

(2) Parietal lobes.

(a) The parietal lobes are located beneath the parietal bones of the skull. They are posterior to the central sulcus and superior to the lateral fissure.

(b) The functions of the parietal lobes include comprehension of written and spoken language, discrimination of fine touch, and stereogenesis. Sterogenesis is the ability to recognize, by touch alone, the size, shape, texture, and consistency of objects.

(c) The postcentral gyrus, within the parietal lobes and directly behind the central sulcus, controls and interprets sensations from the opposite side of the body. These sensations include pain, heat, cold, and pressure.

(3) Temporal lobes.

(a) The temporal lobes are located beneath the temporal bones of the skull and inferior to the lateral fissure.

(b) The functions of the temporal lobes include taste, smell, balance, and hearing. (The perception of sound as well as the interpretation of sound as words.)

(c) The area of the brain concerned with the comprehension of both written and spoken language (Wernicke’s area) is located in both the parietal and temporal lobes.

(4) Occipital lobe.

(a) The occipital lobe is located beneath the occipital bone of the skull, at the posterior of the cerebrum behind the parietal lobes.

(b) The occipital lobe receives and interprets visual stimuli.

Basic Human Anatomy. Lesson 11: Nervous System Part II

2-4. THE SPINAL CORD

a. The spinal cord, located within the vertebral canal of the spine, is continuous with the brainstem. (Together, the brain and spinal cord are referred to as the neuraxis.) The spinal cord extends from the foramen magnum of the skull to the level of the first lumbar vertebrae, at which point it tapers to fine threads of tissue.

b. The spinal cord has two enlargements along its length that are due to an increase in the mass of nervous tissue required to serve the limbs.

(1) The cervical enlargement is associated with the nerves of the upper extremities.

(2) The lumbosacral enlargement is associated with the nerves of the lower extremities.

c. The spinal cord is composed of a central mass of gray matter (cell bodies of neurons) surrounded by white matter (myelinated processes of the neurons). The areas of gray and white matter are referred to as columns.

d. In a cross-section of the spinal cord (figure 2-3), the gray matter appears to be H-shaped. Each arm of the H is called a horn, resulting in two anterior horns and two posterior horns. The connecting middle portion is called the gray commissure.

e. A very narrow canal, called the central canal, is located in the center of the spinal cord. This central canal is continuous with the fourth ventricle of the brain and contains CSF.

f. The processes of the neurons that compose the surrounding white matter are grouped into pathways called fiber tracts.

(1) Tracts conducting impulses from the brain are called motor tracts.

(2) Tracts conducting impulses to the brain are called sensory tracts.

(3) At some specific point along the neuraxis, these pathways cross to the opposite side of the cord and continue their path. (Each crossing is called a decussation.) Thus, the right cerebral hemisphere of the brain communicates with the left half of the body, and the left cerebral hemisphere communicates with the right half of the body.

g. Thirty-one pairs of spinal nerves exit from the spinal cord.

2-5. THE PERIPHERAL NERVOUS SYSTEM

a. Connecting the CNS to all parts of the body are nerves. A nerve is a collection of neuron processes, grouped together, and located outside of the CNS. (Neuron processes, grouped together, and inside the CNS are the fiber tracts of the spinal cord.) Nerves outside the CNS are referred to as peripheral nerves, or the PNS. These nerves connect the CNS to the periphery of the body.

b. The PNS is made up of a large number of nerves arranged in pairs. Each pair includes one nerve for the left side of the body and one nerve for the right side.

(1) Peripheral nerves connected to the brainstem are called cranial nerves. They are numbered from I through XII and have individual names.

(2) Peripheral nerves connected to the spine are called spinal nerves. They are identified by a letter, representing the corresponding region of the vertebral column, and a number representing the sequence within the region. For example, L-5 is the fifth spinal nerve in the lumbar region.

2-6. THE CRANIAL NERVES

a. Olfactory Nerve (I).

(1) Sensory nerve.

(2) Transmits smell impulses from receptors in the nasal mucosa to the brain.

b. Optic Nerve (II).

(1) Sensory nerve.

(2) Transmits visual impulses from the eye to the brain.

c. Oculomotor Nerve (III).

(1) Motor nerve.

(2) Contracts the eyeball muscles.

d. Trochlear Nerve (IV).

(1) Motor nerve.

(2) Contracts the eyeball muscles.

e. Trigeminal Nerve (V).

(1) Mixed nerve.

(2) Transmits pain, touch, and temperature impulses from the face and head to the brain (sensory function).

(3) Contracts the muscles of chewing (motor function).

f. Abducens Nerve (VI).

(1) Motor nerve.

(2) Contracts eyeball muscles.

g. Facial Nerve (VII).

(1) Mixed nerve.

(2) Transmits taste impulses from the tongue to the brain (sensory function).

(3) Contracts the muscles of facial expression and stimulates secretion of salivary and lacrimal glands (motor function).

h. Vestibulocochlear Nerve (VIII).

(1) Sensory nerve.

(2) Transmits hearing and balance impulses from the inner ear to the brain.

i. Glossopharyngeal Nerve (IX).

(1) Mixed nerve.

(2) Transmits taste impulses and general sensations from the tongue and pharynx (sensory function) to the brain.

(3) Contracts the swallowing muscles in the pharynx and stimulates secretions of the salivary glands.

j. Vagus Nerve (X).

(1) Mixed nerve.

(2) Transmits sensory impulses from the viscera (heart, smooth muscles, abdominal organs), pharynx, and larynx to the brain.

(3) Secrets digestive juices, contracts the swallowing muscles of the pharynx and larynx, slows down the heart rate, and modifies muscular contraction of smooth muscles.

k. Spinal Accessory Nerve (XI).

(1) Mixed nerve.

(2) Transmits sensory impulses from the pharynx and larynx to the brain.

(3) Contracts the muscles of the pharynx, larynx, and the neck.

l. Hypoglossal Nerve (XII).

(1) Motor nerve.

(2) Contracts the muscles of the tongue.

2-7. THE SPINAL NERVES

a. There are 31 pairs of spinal nerves, identified as follows:

(1) Cervical nerves (8) (C-1 through C-8).

(2) Thoracic nerves (12) (T-1 through T-12).

(3) Lumbar nerves (5) (L-1 through L-5).

(4) Sacral nerves (5) (S-1 through S-5).

(5) Coccygeal nerve (1).

b. In the human body, every spinal nerve has essentially the same structure and components. By learning the anatomy of one spinal nerve, you can understand the anatomy of all spinal nerves. Like a tree, a typical spinal nerve has roots, a trunk, and branches (rami) (figure 2-4).

(1) Coming off of the posterior and anterior sides of the spinal cord are the posterior (sensory) and anterior (motor) roots of the spinal nerve. An enlargement on the posterior root is the posterior root ganglion. (A ganglion is a collection of neuron cell bodies, together, outside the CNS.)

(2) Laterally, the posterior and anterior roots of the spinal nerve join to form the spinal nerve trunk. The spinal nerve trunk of each spinal nerve is located in the appropriate intervertebral foramen of the vertebral column. (An intervertebral foramen is a passage found on both sides of a vertebrae. It is formed by the columnar alignment of the vertebrae.)

(3) Where the spinal nerve trunk emerges laterally from the intervertebral foramen, the trunk divides into two major branches. These branches are called the anterior (ventral) and posterior (dorsal) primary rami (ramus, singular). The posterior primary rami go to the back. The anterior primary rami go the sides and front of the body and also to the upper and lower extremities.

c. A nerve has been defined as a collection of neuron processes. These processes may belong to different types of neurons–afferent (sensory), efferent (motor), or the visceral motor neurons of the autonomic nervous system. (The ANS will be discussed separately.)

(1) An afferent neuron carries sensory information from the periphery to the CNS.

(2) An efferent neuron carries motor commands from the CNS to the periphery of the body.

2-8. NERVE ACTION

a. A stimulus acts upon a sensory receptor. The information is carried by an afferent (sensory) neuron through the merging branches of the spinal nerve that has been affected. The information is carried through the posterior root ganglion and posterior root to the spinal cord. Once the information reaches the spinal cord, it ascends the appropriate fiber tract to the designated area of the brain.

b. Motor information (commands) from the brain will descend along the appropriate fiber tract within the spinal cord until the appropriate spinal nerve is innervated. The efferent (motor) neurons carry the command from the spinal cord to the effector organ.

2-9. REFLEX ARC

The simplest reaction of the human nervous system is the reflex. A reflex is an automatic reaction to a stimulus. The pathway from the receptor organ to the reacting muscle is called a reflex arc (figure 2-5). The pathway of a reflex arc contains five components.

a. The stimulus is received by a receptor organ specific to that stimulus.

b. The information is transmitted to the CNS by the afferent neuron of the appropriate peripheral nerve.

c. Within the spinal cord, the afferent neuron synapses with a special connecting neuron called the internuncial neuron (or interneuron).

d. In turn, the internuncial neuron synapses with the efferent neuron’s cell body. The axon of the efferent neuron carries the information to the effector organ.

e. The effector organ receives the command to act.

2-10. THE AUTONOMIC NERVOUS SYSTEM

a. The ANS is the portion of the nervous system concerned with innervation of smooth muscle, cardiac muscle and the glands. The ANS regulates visceral activities such as:

(1) Respiration.

(2) Gastrointestinal motility.

(3) Glandular secretion.

(4) Contraction of smooth muscles.

(5) Constriction and dilation of the pupils.

(6) Constriction and dilation of the blood vessels.

(7) Rate and force of cardiac muscle contraction.

b. It has always been thought that autonomic control is an “automatic” function and not of conscious control. However, recent research indicates that conscious control is possible with proper training. A classic example is the control of headache and hypertension by utilizing biofeedback techniques.

c. In the autonomic nervous system, there are two neurons, one following the other, that connect the CNS with the viscera of the body.

(1) The first neuron extends from the CNS to a ganglion, and is therefore called the preganglionic neuron. The cell body of the second neuron is located within this ganglion.

(2) The processes of the second neuron extend from the ganglion to the viscera. The second neuron is called the postganglionic neuron.

d. The ANS is organized into two major divisions:

(1) The sympathetic nervous system, also called the thoraco-lumbar outflow because the associated neurons originate in the thoracic and lumbar regions of the spinal cord.

(2) The parasympathetic nervous system, also called the cranio-sacral outflow because the associated neurons originate in the brainstem and the sacral region of the spinal cord.

e. Under ordinary circumstances, the sympathetic and parasympathetic nervous systems have opposite effects on bodily activities. That is, one system will stimulate action and the other will inhibit action. The interplay maintains bodily function in a state of equilibrium known as homeostasis.

f. The “fight or flight” response is produced by the sympathetic nervous system when conditions of stress or threat prevail.

(1) The sympathetic nervous system activates energy producing structures and helps the body expend effort and energy wisely.

(a) Heart rate increases.

(b) Pupils dilate.

(c) Gastric motility decreases.

(d) Urinary and anal sphincters close

(e) Blood sugar level rises.

(f) Adrenalin (epinephrine) is released by the adrenal medulla.

(2) Later, when the stress or threat has been eliminated, equilibrium is restored by the parasympathetic nervous system.