1 Typical Spinal Nerve

THE TYPICAL SPINAL NERVE

Learning Objectives

By the end of the course students will be able to:

1. Describe the spine, its curvatures and vertebral column movements

2. Be able to describe a typical spinal nerve, the somatic motor and sensory components in any portion and their distribution

3. Understand the coverings and supporting structures of the spinal cord. Give the point of the termination of the spinal cord and the dural sac.

4. Define and explain the significance of dermatomes.

Reference:  Moore, Clinically Oriented Anatomy, chapter 4.

Particularly relevant Blue Boxes in Moore:

●Compression of the Lumbar Spinal Nerve Roots, p. 505

●Lumbar Spinal Puncture p. 505

●Spinal and Epidural Blocks p. 506

●Spinal Cord Injuries p. 506

Also, check out the Back Tutorial in the On-Line Syllabus on TUSK. There is a section on the Spinal Cord which is very informative.

To access the Netter Presenter Database click here

The Nervous System

The nervous system is divided into the central nervous system (CNS) consisting of the brain and spinal cord, and the peripheral nervous system (PNS) consisting of 12 pairs of cranial nerves, 31 pairs of spinal nerves and their associated ganglia. It is divided functionally into the somatic nervous system, which controls primarily voluntary activities, and the visceral (autonomic) nervous system, which controls primarily involuntary activities. It is composed  of neurons and neuroglia, which are nonneuronal cells such as astrocytes, oligodendrocytes and microglia.

Neurons are the structural and functional units of the nervous system and are specialized for the reception, integration, transformation and transmission of information.

Neurons consist of cell bodies and their processes, dendrites and axons (see fig. ). Cell bodies are located in the gray matter of the CNS, and their collections are called ganglia in the PNS and nuclei  (Netter 117) in the CNS. Dendrites are usually short and highly branched and carry impulses toward the cell body. Axons are usually single and long and have fewer branches and carry impulses away from the cell body.

Classification of Neurons.

1. Unipolar (pseudounipolar) neurons

Have one process which divides into a central branch that functions as an axon and a peripheral branch that serves as a dendrite. They are called pseudounipolar because they were originally bipolar. The two processes fuse during development to form a single process that bifurcates at a distance from the cell body. They are the sensory neurons of the PNS and found in spinal and cranial nerve ganglia

2. Bipolar neurons

Have two processes (one dendrite and one axon); are sensory and are found in the olfactory epithelium, the retina and the inner ear

3. Multipolar neurons

Have several dendrites and one axon and are most common in the CNS (e.g., motor neurons in anterior and lateral horns of the spinal cord and autonomic ganglion cells.

The Central Nervous System (CNS)

1. Brain

The brain has a cortex, which is the outer part of the cerebral hemispheres and is composed of gray matter. This matter consists largely of the nerve cell bodies, dendrites and neuroglia. It has an interior part composed of white matter which consists largely of axons forming tracts or pathways, and ventricles, which are filled with cerebrospinal fluid (CSF).

2. Spinal Cord (Netter 160)

Is cylindrical, occupies approximately the upper two-thirds of the vertebral canal, and is enveloped by the meninges. It has cervical and lumbar enlargements for the supply of the upper and lower limbs, respectively. It has centrally located gray matter, in contrast to the cerebral hemispheres, and peripherally located white matter (see fig) . It has a conical end known as the conus medullaris and ends at the level of about L2 in the adult and at about L3 in the newborn.

Structures Associated With the Spinal Cord (Netter 160, 161)

1. Cauda equina (“horses tail”)

Is formed by the lower dorsal and ventral roots of the lumbar and sacral and spinal nerves that surround the filum terminale. It is located within the subarachnoid space (lumbar cistern) below the level of the conus medullaris.

2. Denticulate ligaments

Are lateral extensions of the pia mater, consisting of 21 pairs of toothpick-like processes. They extend laterally from the pia through the arachnoid to the dura mater between dorsal and ventral roots of the spinal nerves. They help to hold the spinal cord in position within the subarachnoid space.

3. Filum terminale

Is a prolongation of the pia mater from the tip of the spinal cor at the level of L2. It lies in the midst of the cauda equina and ends at the level of S2 by attaching to the apex of the dural sac.

4. Cerebrospinal Fluid

Is contained within the subarachnoid space between the arachnoid and pia mater. It is formed by vascular choroid plexuses in the ventricles of the brain (Netter 110) . It circulates through the ventricles, enters the subarachnoid space and eventually filters into the venous system through arachnoid villi projectinig into the dural venous sinuses, particularly the superior sagittal sinus.

3. Meninges

Consist of three layers of connective tissue membranes: (pia, arachnoid and dura mater – Netter 165); figure ) that surround and protect the brain and spinal cord.

The pia mater is the innermost meningeal layer; it is closely applied to the spinal cord and thus cannot be dissected from it. It also enmeshes blood vessels on the surfaces of the spinal cord.. It has lateral extensions called denticulate ligaments between dorsal and ventral roots of spinal nerves and an inferior extension known as the filum terminale.

The arachnoid mater is a filmy, transparent, spider web-like layer connected to  the pia mater by web-like trabeculations. It forms the subarachnoid space, between the arachnoid and pia mater, that is filled with cerebrospinal fluid (CSF) and that extends to about the second sacral level. The enlarged subarachnoid space between L1 and S2 is called the lumbar cistern.

The dura mater is the tough, fibrous, outermost layer of the meninges. The subdural space is a potential space between the arachnoid and dura. It extends inferiorly to the second sacral vertebral level and contains only sufficient fluid to moisten the surfaces of the two membranes. The epidural space is external to it and contains the internal vertebral plexus and epidural fat.

Clinical Notes:

Meningitis is inflammation of the meninges by viral or bacterial infection. Nonbacterial meningitis is referred to as asceptic meningitis, whereas bacterial meningitis is referred to as purulent meningitis. Viral meningitis is milder and occurs more than bacterial meningitis. Bacterial meningitis is an extremely serious illness and may result in brain damage or death, even if treated. meningitis is also caused by fungi, chemical irritation or drug allergies and tumors. Its symptoms include fever, headache, stiff neck, brain swelling, shock, convulsions, mausea and vomiting.

Caudal (epidural) anesthesia is used to block spinal nerves in the epidural space by injection of local anesthetic agents via the sacral hiatus located between the sacral cornua. Obstetricians use this method of nerve block to relieve the pains during labor and childbirth and its advantage is that the anesthetic does not affect the infant.

Saddle block is the introduction of anesthesia into the dural sac in the region corresponding with the areas of the buttocks, perineum and medial aspects of the thighs that impinge on the saddle in riding.

Lumbar puncture is the tapping of the subarachnoid space in the lumbar region, usually between the laminae of vertebrae L3 and L4 or vertebrae L4 and L5. It allows measurement of CSF pressure and withdrawal of a sample of the fluid for microbial and chemical analysis and also allows introduction of anesthesia, drugs or radioopaque material into the subarachnoid space. Click here for a video (from the New England Journal of Medicine’s series on Clinical Videos), demonstrating the procedure of lumbar puncture,

The Peripheral Nervous System

1. Cranial Nerves

Cranial nerves  (Netter 119) consist of 12 pairs and are connected to the brain rather than the spinal cord. They have motor fibers with cell bodies located within the CNS and sensory fibers with cell bodies that form sensory ganglia located outside the CNS. They emerge from the ventral aspect of the brain (except for the trochlear nerve, CN IV). They contain all four functional components of the spinal nerves (see below).

2. Spinal Nerves (Netter 161)

Spinal nerves consist of 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. They are formed from ventral and dorsal roots (Netter 165) ; each dorsal roots has a ganglion that is within the intervertebral foramen. The dorsal roots carry sensory fibers into the cord to the sensory area of the dorsal horns. The ventral roots carry motor fibers which have arisen from cell bodies in the ventral horn, and are now passing out into the body to innervate the appropriate muscles (see figure).

Spinal nerves are divided into the ventral and dorsal primary rami; the ventral primary rami enter into the formation of plexuses (i.e. cervical, brachial and lumbosacral) (see figure) ; the dorsal primary rami innervate the skin and deep muscles of the back.

Functional Components in Peripheral Nerves

       1. General somatic afferent (GSA) fibers

Transmit pain, temperature, touch and proprioception from the body to the CNS

2. General somatic efferent (GSE)

Carry motor impulses to skeletal muscles of the body

3. General visceral afferent (GVA) fibers

Convey sensory impulses from visceral organs to the CNS

4. General visceral efferent (GVE) fibers (autonomic nerves)

Transmit motor impulses to smooth muscle, cardiac muscle and glands

5. Special somatic afferent (SSA) fibers

Convey special sensory impulses of vision, hearing and balance to the CNS

6. Special visceral afferent (SVA)

Transmit smell and taste sensations to the CNS

7. Special visceral efferent (SVE) fibers

Conduct motor impulses to the muscles of the head and neck, such as muscles for  mastication, facial expression and elevation and movement of the pharynx and larynx

Dermatome, Myotome and Sclerotome

A dermatome (Netter 162) is an area of skin innervated by sensory fibers derived from a particular spinal nerve or segment of the spinal cord. Knowledge of the segmental innervation is useful clinically to produce a region of anesthesia or to determine which nerve has been damaged.

A myotome is a group of muscles innervated by motor fibers derived from a single spinal nerve segment.

A sclerotome is the area of a bone innervated from a single spinal segment

INNERVATION OF SKELETAL MUSCLE (Moore pp 46 – 56)

The innervation of all skeletal muscle — save that of the head and neck — is by nerve fibers that arise from the spinal cord, pass through various rami, plexes, and peripheral nerves, and reach their target muscle. In the Neuroscience course, you will consider in detail the motor pathways from brain to spinal cord; here we will just briefly consider the anatomy of the spinal cord and peripheral nerves that are the final links in this chain.

As described above, the dorsal roots of spinal nerves carry sensory fibers into the cord to the sensory area of the dorsal horns. The ventral roots carry motor fibers which have arisen from cell bodies in the ventral horn, and are now passing out into the body to innervate the appropriate muscles.

Spinal nerves are divided into the ventral and dorsal primary rami; the ventral primary rami enter into the formation of plexuses (i.e. cervical, brachial and lumbosacral); the dorsal primary rami innervate the skin and deep muscles of the back.

Together, the dorsal and ventral roots combine to form a (MIXED) SPINAL NERVE, so called because it has both sensory (afferent) and motor (efferent) fibers. Note that while the motor fibers arise from cell bodies within the ventral horn of the spinal cord, the cell bodies of the sensory fibers are found in the DORSAL ROOT GANGLION.

In the upper limb, for example, there is a very clear division — formed, in the body, by tough sheets of connective tissue –between the muscles which flex and the muscles which extend the joints of the limb. The importance of this is that the innervation of these two groups is divided as well; and to repeat, the flexors receive preaxial nerves, and the extensors receive postaxial nerves.

All these terms have reference to the embryologic origin of the limbs. In the embryo, the limbs are not in their adult position: rather both the upper and lower limbs come from LIMB BUDS in which the extensor muscles are dorsal and the flexor muscles are ventral. The limb buds contain a skeletal axis, and two groups of muscles: an anterior flexor group and a posterior extensor group. Note, then, how the ventral ramus sends a postaxial nerve into the posterior compartment, and a preaxial nerve into the anterior compartment.

Therefore, in the adult in the anatomical position:

UPPER LIMB:

Anterior — muscles are flexors, preaxial innervation.

Posterior — muscles are extensors, postaxial innervation.

LOWER LIMB:

Anterior — muscles are extensors, postaxial innervation.

Posterior — muscles are flexors, preaxial innervation.

This concept will gain its full meaning upon actual study of the limbs; at which point it would be appropriate to review this section.

Thus, in the fetus, the limbs are rotated from the adult position in a ‘bear climbing a tree’ manner; with all the flexor surfaces facing medially you will recognize that this is approximately the adult position of the upper limb, but that the lower limb has to undergo considerable rotation to end up in the adult position (with the flexor aspect of the limb facing posteriorly rather than anteriorly.)