Introduction to Nervous System Anatomy Revised by Michael Peters, PhD

OBJECTIVES:

Following this lecture and having studied this syllabus material, the student should:

1. be able to define the major divisions of the Nervous System; 2. understand which aspects of the Nervous System will be studied in the Gross Anatomy

Course; 3. be able to discuss the generalized concepts of spinal versus cranial nerves; 4. be able to diagram and discuss the gross structure of a spinal nerve in detail; 5. understand the concept and some details of the Autonomic Nervous System; 6. be able to discuss the functional components of spinal nerves; 7. understand the dermatome concept; 8. understand the concepts of segmental and peripheral nerve innervation of skeletal

muscles; and 9. be able to answer Head to Toe Questions in Gross Anatomy #201­215.

SELF­STUDY RESOURCES:

• CBIL Network & Lab Computers: Head to Toe Questions in Gross Anatomy (#201­215) • Reading: Essential Clinical Anatomy (3 rd Ed.) (ECA): pp. 30­43 • Grants Atlas of Anatomy (12 th Ed.): Figs. 4.41­4.57

LECTURE OVERVIEW

I. Major divisions of the nervous system include:

A. Central nervous system (CNS) ­ an anatomical (i.e. structural) classification

1. Brain 2. Spinal cord

B. Peripheral nervous system (PNS) ­ an anatomical (i.e. structural) classification

1. Cranial nerves ­ arise from the brain 2. Spinal nerves ­ arise from the spinal cord

C. Somatic nervous system (SNS) ­ a functional classification.

Parts of this system begin in the CNS and enter the PNS to supply the body wall.

D. Autonomic nervous system (ANS) ­ a functional classification.

Parts of this system begin in the CNS and enter the PNS to supply the viscera.

1. Parasympathetic division 2. Sympathetic division

E. Detailed study of the CNS lies within the discipline of Neuroanatomy, which will be taught in the spring during the Neurosciences course. In the Gross Anatomy Course, you will study the PNS and peripheral portions of the SNS and ANS in detail. Ultimately, you will learn the innervation of skeletal muscles in detail, sensory innervation of the skin in detail, and the general patterns of motor and sensory innervation of the thoracic and abdominopelvic viscera.

II. Cranial nerves

Fig 1

A. Twelve pairs of cranial nerves arise from various parts of the brain and have both numbers designated by Roman numerals and names.

1. Most of the cranial nerves will not be studied until you begin the Head and Neck portion of the course, when they will be studied in detail.

B. Cranial nerve XI, the spinal accessory nerve, has been introduced this week in regard to the trapezius muscle, which is a muscle that it innervates.

C. Cranial nerve X, the vagus nerve, will be discussed during the study of the parasympathetic system, the Thorax, the Abdomen, and the Head and Neck.

III. Spinal nerves

A. There are 31 pairs of spinal nerves that arise from the spinal cord. Each pair of spinal nerves arises from a particular spinal cord segment and is named for that spinal cord segment, including: 8 cervical pairs, 12 thoracic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 coccygeal pair. (Grants Atlas, 12 th Ed., Fig. 4.50)

1. There are 8 cervical nerve pairs and only 7 cervical vertebrae. Therefore, the first cervical nerves exit the vertebral canal between the skull and the C1 vertebra and the eighth (last) cervical nerves pass through the intervertebral foramina between the C7 and T1 vertebrae. Thus, the C8 spinal nerves lie above the T1 vertebra and the T1 spinal nerves lie below the T1 vertebra.

2. From the T1 pair downward, each pair of nerves is named according to the vertebra that lies above it. For example, the L2 spinal nerves pass through the intervertebral foramina below the L2 vertebra.

B. Gross structure of Spinal nerves (Grants Atlas of Anatomy, 12 th Ed., Fig.4.49)

Fig 2

1. The Dorsal (posterior) root (sensory or afferent root) of a spinal nerve arises from the posterolateral aspect of the spinal cord, swells as it forms the dorsal root ganglion (spinal ganglion), which contains cell bodies of the pseudounipolar neurons that comprise the dorsal root, and joins the ventral root to form the spinal nerve.

2. The Ventral (anterior) root (motor or efferent root) of a spinal nerve arises from the anterolateral aspect of the spinal cord, contains axons arising from stellate cell bodies located in the ventral gray horn or the intermediolateral cell column of the spinal cord, and joins the dorsal root to form the spinal nerve.

3. The Spinal nerve itself occupies the space within an intervertebral foramen and, from the point of its formation by the joining of the dorsal and ventral roots to the point of its branching into dorsal and ventral branches (i.e. rami), is only about one centimeter long.

4. The Dorsal (posterior) ramus (ramus=branch and should not be confused with root) is the smaller branch of the spinal nerve, which passes posteriorly through the paravertebral musculature and itself splits into a medial and a lateral branch, both of which innervate the Intrinsic back muscles, but only one of which continues to the skin.

a. As a rule, each dorsal ramus supplies a specific segment of muscle and skin and does not merge with other dorsal rami to form nerve plexuses.

b. The bilateral cutaneous area supplied by dorsal rami runs from the vertex of the skull down to the lower gluteal region and includes about two­thirds of the skin of the posterior half of the body.

5. The Ventral (anterior) ramus (ramus=branch and should not be confused with root) is the larger branch of the spinal nerve, which passes anteriorly, giving off muscular branches, and typically gives off lateral and anterior cutaneous branches.

a. Certain of the ventral rami (unlike dorsal rami) interchange branches with one another, forming complicated somatic nerve plexuses known as cervical, brachial, lumbar, and sacral plexuses.

b. The ventral rami of the thoracic spinal nerves (except T1 and to a lesser extent T2) do not contribute to plexuses; thus, the thoracic spinal nerves are often described as "typical" spinal nerves.

c. Part of the ventral ramus of spinal nerve T1 and the ventral rami of T2­T11 are also classified as intercostal nerves, since they lie between ribs, and the ventral ramus of T12 is also known as the subcostal nerve, since it lies below the last (12th) rib.

Fig 3

d. The lateral cutaneous branch of a "typical" ventral ramus itself divides into anterior and posterior branches at the midaxillary line; these supply the skin areas in which they lie.

e. The small anterior cutaneous branch of a "typical" ventral ramus itself divides into medial and lateral branches just lateral to the anterior midline; these supply the skin areas in which they lie.

f. The ventral rami of spinal nerves are connected to the sympathetic trunk by one or two rami communicantes (singular, ramus communicans).

IV. Autonomic nervous system (ANS) (Grants Atlas of Anatomy, 12 th Ed., Fig. 4.51)

A. The ANS is a functional rather than a distinctly structural classification.

1. Some of the neurons (preganglionic) of the ANS lie within the CNS and send their fibers through parts of the PNS, such as the ventral roots of spinal nerves or the motor parts of cranial nerves.

2. Other parts of these same neurons and other neurons (postganglionic) form relatively distinct parts of the PNS.

B. A consistent feature of the ANS is the two­neuron connection, which transmits an impulse from the CNS to an end organ and has an autonomic ganglion interposed between the two neurons.

Fig 4

1. The Preganglionic neuron has its cell body located within the CNS, sends its axon through the ventral root of a spinal nerve or the motor portion of a cranial nerve, and forms a synapse with a postganglionic neuron in an autonomic ganglion.

2. The Postganglionic neuron has its cell body in an autonomic ganglion and sends its axon to an end organ.

C. The ANS supplies smooth muscle, cardiac muscle, and glands.

1. The structures supplied by the ANS are those over which one ordinarily has no voluntary control.

2. Certain organs, such as the bladder and bowel, are supplied by the ANS, but voluntary control can be superimposed on the autonomic control.

D. The two divisions of the ANS are the:

1. Parasympathetic division (or the “craniosacral” system): This system consists of preganglionic neurons that arise from the brainstem and run in cranial nerves III, VII, IX, and X (“cranio”) as well as preganglionic neurons that arise from spinal cord segments S2­S4 and run through the ventral roots of these spinal nerves (“sacral”) and the postganglionic neurons with which these preganglionic neurons synapse.

Fig 5

2. Sympathetic division (or the “thoracolumbar” system): This system consists of preganglionic neurons that arise from spinal cord segments T1­L2/L3 and run through the ventral roots of these spinal nerves and the postganglionic neurons with which these preganglionic neurons synapse.

a. Sympathetic chains

i. These are a bilateral series of interconnected ganglia running from the skull to the coccyx that also connect (via rami communicantes) to ventral rami of spinal nerves.

ii. White rami communicantes (input to the chains) convey preganglionic fibers from the ventral rami of spinal nerves T1­L2/L3 to the ganglia of the sympathetic chains.

iii. Gray rami communicantes (output from the chains) convey postganglionic fibers from the ganglia of the sympathetic chains back to the ventral rami of all the spinal nerves.

Fig 6

b. Splanchnic nerves of the sympathetic system

i. They convey preganglionic fibers from the sympathetic chains to sympathetic ganglia located outside the chain (collateral ganglia, usually located along the aorta).

ii. They provide innervation to abdominal and pelvic organs (viscera).

E. In general, the parasympathetic and sympathetic divisions of the ANS produce antagonistic functions.

1. The functions of the parasympathetic system will be discussed throughout this course as the particular organs it innervates are discussed; however, in general it tends to promote quiet and orderly processes of the body (i.e. it maintains homeostasis).

2. The functions of the sympathetic system will be discussed throughout this course; however, in general it is the system which interrupts homeostasis to prepare the body for a "fight or flight" reaction. It is especially important to note that activation of sympathetic nerves causes constriction of most blood vessels, consequently raising blood pressure.

V. Functional components (i.e. fiber types) of spinal nerves. There are four functional components in each spinal nerve (cranial nerves may carry one or more of seven components; that is, those in spinal nerves plus three others).

Fig 7

A. General somatic afferent (GSA) fibers (axons are referred to as fibers)

1. Fibers of the GSA type carry sensory impulses to the spinal cord from end organs located in body wall structures such as skin, muscle, and bone.

a. Afferent fibers carry impulses into the spinal cord; hence, afferent means sensory.

b. Somatic refers to the body wall.

2. Cell bodies of GSA fibers are located in dorsal root ganglia and the fibers travel in the dorsal roots, spinal nerves, ventral rami, and dorsal rami.

3. Information carried in GSA fibers is interpreted at the conscious level as detailed and precisely localized sensation.

4. The types of sensation carried in GSA fibers include:

a. Pain (somatic pain) and temperature b. Light touch c. Pressure d. Two­point discrimination (detecting distance between points touched) e. Stereognosis (determining the form and nature of objects by touch) f. Proprioception (realizing the position of joints and limbs without sight)

Fig 8

B. General visceral afferent (GVA) fibers (axons are referred to as fibers)

1. Fibers of the GVA type carry sensory information to the spinal cord from end organs located in blood vessels of the body wall and in the internal organs (viscera).

2. GVA fibers have their cell bodies located in dorsal root ganglia and are found in dorsal roots, spinal nerves, ventral rami, dorsal rami, white rami communicantes, sympathetic trunks, and splanchnic nerves. That is, most GVA fibers follow sympathetic routes, but do not synapse as they pass through the sympathetic ganglia and, of course, carry impulses in an afferent direction.

3. Some GVA fibers have their cell bodies in the dorsal root ganglia of spinal cord segments S2, S3, and S4 or in the sensory ganglia of the vagus nerve. Such GVA fibers follow parasympathetic routes, but do not synapse as they pass through the parasympathetic ganglia and, of course, carry impulses in an afferent direction.

4. Sensory information carried in GVA fibers does not necessarily reach conscious levels.

5. Visceral pain from internal organs is carried in the GVA fibers that follow sympathetic routes. Visceral pain reaches conscious levels, but is a diffuse, poorly localized sensation.

C. General somatic efferent (GSE) fibers (axons are referred to as fibers)

Fig 9

1. Fibers of the GSE type carry impulses from the spinal cord to skeletal (voluntary) muscle.

a. Efferent fibers carry impulses away from the spinal cord; hence efferent means motor.

2. Cell bodies of GSE fibers are located in the ventral gray horn of the spinal cord and the fibers traverse the ventral roots, spinal nerves, ventral rami, and dorsal rami.

D. General visceral efferent (GVE) fibers (axons are referred to as fibers)

1. Fibers of the GVE type are the axons of autonomic motor neurons.

2. Preganglionic fibers of the GVE type carry impulses from the spinal cord to autonomic ganglia and postganglionic fibers of the GVE type then carry impulses to smooth muscle, glands, or arrector pili muscles of the hair follicles.

3. Preganglionic fibers of the GVE type have their cell bodies located in the intermediolateral gray cell column of the spinal cord. (Cranial nerve nuclei house cell bodies of preganglionic GVE fibers that are from the cranial part of the parasympathetic system.)

Fig 10

4. Preganglionic fibers of the GVE type traverse the ventral roots, spinal nerves, and ventral rami. If the preganglionic fibers are sympathetic, they leave the ventral rami via white rami communicantes to enter a sympathetic chain, may synapse at the level of entry, may run up or down the chain to synapse at a different level, or may enter thoracic, lumbar, or sacral splanchnic nerves. If the preganglionic fibers are parasympathetic (and from the sacral part of that system), they leave the ventral rami of S2­S4 as pelvic splanchnic nerves.

5. Postganglionic sympathetic fibers are also GVE fibers. If they arise in a sympathetic chain ganglion they enter a gray ramus communicans at the same

level to join the ventral ramus at that level (and then might backtrack to enter a dorsal ramus). If they arise in a collateral sympathetic ganglion (i.e. outside of the sympathetic chain), they then join a periarterial nerve plexus and follow the branches of that artery to their end organs.

6. Postganglionic parasympathetic fibers are also GVE fibers. They are generally very short, as their cell bodies are located in parasympathetic ganglia near or within the organ that they innervate.

VI. Dermatomes

A. A dermatome is an area of skin supplied by a single spinal (segmental) nerve.

1. On the trunk the segmental arrangement of dermatomes is easily recognized.

2. Because of migrations of nerves during limb development, the patterns of dermatomes on the limbs are more complicated.

3. Because of overlap, complete loss of sensation in a single dermatome would result only if three consecutive spinal nerves were cut.

B. The patterns of dermatomes will be studied throughout the course.

Fig 11

VII. Segmental innervation of muscles

A. Each skeletal muscle is innervated by somatic motor fibers from specific spinal nerves.

1. Because of the formation of somatic nerve plexuses, the segmental innervation of muscles cannot be recognized by dissection.

2. The segmental innervation of muscles has been discerned by clinical studies and this information is important to the anatomist and neuroscientist.

B. The specific named peripheral nerves that innervate muscles can be determined by dissection.

1. We will learn in detail the names of the peripheral nerves that innervate each skeletal muscle as the course progresses.

2. We will learn selected segmental (spinal) innervations of muscles as the course progresses.

The Netter images in this syllabus are used as follows:

The Netter Presenter Image © 2001, Icon Learning Systems. All rights reserved.

Clinical Correlate: Shingles (Herpes zoster)

From the Merck Manual – 2008.