OBJECTIVES FOR FUNCTIONAL ANATOMY OF THE SPINAL CORD©

Blair H. Turner, Ph.D.

Define and explain. Where appropriate, be able to identify on a drawing:

For each nerve and tract, know its:

Location of cell body
Site of termination
Side of termination
Does axon cross, or not?
Function
Lesion deficit

  Functional Anatomy Of The Spinal Cord

I. INTRODUCTION

The spinal cord retains much of the character of the original neural tube in that it exhibits ventricular (ependymal), mantle (spinal gray) and marginal (spinal white matter) layers. The cord is segmentally organized in that it is connected to the periphery by paired cervical, thoracic, lumbar, sacral and coccygeal spinal nerves. The cord is surrounded by the vertebral column and the three meninges: pia mater, arachnoid mater and dura mater. It exhibits two obvious enlargements or swellings -the cervical and lumbar enlargements. These swellings comprise those portions of the spinal cord which innervate the upper and lower limbs, respectively. Caudal to the lumbar enlargement, the cord narrows to a cone shape. This is termed the conus medullaris. A slender filament called the filum terminale extends downward from the conus to attach to the posterior surface of the coccyx. There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal) each composed of dorsal and ventral root filaments. Since the spinal cord is shorter than the vertebral column, certain spinal nerves have to descend to reach their respective intervertebral foramina. These spinal nerves form a bundle which resembles a horse's tail and is appropriately termed the cauda equina.

 II. SPINAL NERVES

A. Afferent (sensory) division. The afferent and efferent axons of mixed spinal nerves separate into two divisions as the the spinal cord is reached. The afferent axons form the dorsal roots, whose cell bodies are contained in the dorsal root ganglia. This dorsal root consists of sensory fibers derived from both cutaneous (skin) and deep (muscles and tendons) origins in the periphery. The myelinated fibers may be roughly divided into three main groups: large (12 - 20µ )medium (6 - 12µ) and small (1 - 6µ) diameter. The largest of these fibers (17 - 20µ) are derived exclusively from muscle nerves and carry information from sensory receptors located on muscles and called muscle spindles. Sensory axons from muscle participate in monosynaptic reflexes (described below). The majority of axons in dorsal root are non-myelinated (C-fibers) many being well under 1µ in diameter and not visible with the light microscope. The dorsal root contains a dorsal (sensory) root ganglion that is located within the intervertebral foramen. This ganglion contains the cell bodies of the sensory neurons. Dorsal roots fibers are distributed in varying numbers to most regions of the spinal gray on the side of entry. In addition, some large fibers ascend the cord in the ipsilateral dorsal white columns to reach the medulla.

B. Efferent (motor) division, The ventral root consists of motor fibers which innervate the striated and smooth muscles of the body. The cell bodies of the axons projecting to the somatic musculature are located in the ventral horns. The cell bodies of the axons projecting to smooth and cardiac muscle are located in the lateral horns of the spinal cord, present at T1-L2 levels. These are the preganglionic sympathetic axons, which terminate in the paravertebral ganglia and the superior cervical ganglion. The ventral or motor root consists of the motor or efferent nerve fibers whose cell bodies are located within the spinal cord gray. The motor fibers transmit impulses from the spinal cord via motor roots and spinal nerves to the muscles and glands of the body. The ventral roots contain the axons of alpha motor neurons, which supply extrafusal striated muscle. The diameters of these myelinated axons range from 12-20µ. Ventral roots of thoracic and lumbar segments also contain preganglionic, sympathetic axons - those at sacral levels carry preganglionic, parasympathetic fibers.

III. INTERNAL STRUCTURE OF THE SPINAL CORD

In cross section, the spinal cord is composed of:

A. Gray Matter. This is the central region composed primarily of nerve cell bodies. They are unmyelinated and appear gray in fresh tissue. The spinal gray matter is H-shaped consisting of paired dorsal and ventral horns, and, at thoracic levels, the intermediate horn.

The dorsal horns (alar plate derivatives) contain neurons which receive incoming sensory fibers in dorsal roots and modify the information brought to them, and either send fibers to local spinal motoneurons for spinal reflex mechanisms or project ascending fibers to higher centers of the nervous system.

Intermediate (or lateral) horns of the spinal cord are found at thoracic and upper lumbar levels. They contain the preganglionic neurons of the sympathetic system. The preganglionic neurons of the parasympathetic system, found in the sacral spinal cord, are not located in a prominent lateral horn.

The ventral horns contain neurons which may receive fibers directly from dorsal roots or adjacent regions of the spinal cord, or receive long descending fibers from the cortex and brainstem. The axons of the cells of the ventral horn (spinal motoneurons) exit the central nervous system and end on muscle fibers, causing them to contract. The configuration of the ventral horn varies greatly at different levels. In general, the motoneurons supplying muscles of the trunk (axial musculature) are located in the ventromedial portion of the ventral horn. In the cervical and lumbar enlargement of the cord, the motoneurons supplying muscles of the extremities appear and are located in the ventrolateral ventral horns which are so prominent at these levels.

B. White Matter

White matter occupies the periphery of the spinal cord, and is composed largely of ascending and descending myelinated axons of nerve cells with supporting neuroglia. Myelin, the fatty insulating coat of axons, appears white in fresh tissue. These ascending and descending axons are organized into bundles (known as tracts) which occupy specific regions within the white matter. The ascending tracts convey sensory data rostrally to the brainstem, while the descending conduct impulses originating primarily but not exclusively within motor regions of the brainstem and the neocortex of the cerebral hemispheres. Such impulses terminate upon spinal motor and interneurons. Neuronal connections in the spinal cord also form the basis of important spinal reflexes.

IV. ASCENDING TRACTS

All incoming information to the spinal cord from the peripheral sensors enters the dorsal root. Thus, destruction of the dorsal root will produce anesthesia in the associated dermatome. These incoming axons are the first order relays in a somatic neural system composed of a chain of axons. Some of these first order relays, upon entering the spinal cord, ascend to higher centers without synapsing in the spinal cord. The major example of this are the dorsal columns, which carry fine touch and pressure information from the skin up to higher centers. Other first order relays do synapse in the spinal cord, specifically, in the gray matter of the dorsal horn. The major example of this is the spinothalamic system, which relays pain, temperature, and coarse touch sensations from the skin to higher centers in the central nervous system. In this case, first order fibers in the dorsal root carrying this type of information enter the spinal cord and synapse on cell bodies of the dorsal horn. These cell bodies then project their axons to the opposite side of the spinal cord, where they ascend to higher centers. These crossing neurons are known collectively as the spinothalamic tract, and they are the second order relays of the spinothalamic system. Destruction of the spinothalamic tract causes an inability to perceive pain and temperature sensations on the skin of the opposite of the body.

V. DESCENDING TRACTS

All outgoing information from the entire central nervous system that causes behavior must do so via the ventral roots of the spinal cord, or the motor components of cranial nerves. For this reason, these axons are know collectively as the final common pathway. They are also known as lower motor neurons, because they are the lowest link in a hierarchical chain of motor pathways that begins in the cerebral hemispheres. Destruction of this pathway, therefore, will result in a flaccid paralysis of the associated muscles. Muscle atonia, a-reflexia, and atrophy are the result. Descending pathways from many locations in the central nervous system project down onto the nerve cell bodies of the final common pathways. They are known, collectively, as upper motor neurons. The corticospinal tract is the best example of such a pathway. Its cell body is located in the precentral gyrus of the frontal lobe, and its axon descends to the spinal cord, synapsing (i.e., terminating) on the cell bodies of the lower motor neurons in the ventral horns of all levels. Destruction of this tract produces a spastic paralysis (hypotonia and hyper-reflexia of stretch reflexes) and an inability to execute precise movements.    

DEFINING REFLEXES

A reflex is a movement or activity of the body performed automatically and without conscious volition in consequence of a nervous impulse transmitted to the central nervous system by afferent axons from a receptor to a nerve center (such as the spinal cord or brainstem), where relay neurons activate effectors (such as muscles or glands) via efferent axons. Reflexes may be:

1. Purely somatic (such as the knee-jerk reflex). In this case, only the afferents and efferents of the somatic nervous system are involved, controlling the levers of bone and striated muscle.

2. Purely autonomic (such as the salivatory reflex). In this case, only afferents and efferents of the autonomic nervous system are involved, controlling the secretions of glands.

3. Mixed somatic and autonomic (such as the respiratory response to raised or lowered levels of blood gases). In these cases, both autonomic and somatic nervous systems control the response.

Each reflex presented in this course may be defined in the following three ways. You will be tested on your precise knowledge of each.

A. Verbal Definition. This is a description in writing of all the anatomical components of the reflex, and the function of each component. Such a description includes:

• The type of stimulus which elicits the reflex.

• The cell type or organ that turns the physical stimulus into neural excitation.

• The name of the nerves relaying this afferent excitation toward the central nervous system, and the location of their cell bodies and terminations.

• The name, location and termination of the neurons in the next relay of this excitation.

• The name, location and termination of the neurons in the final, efferent relay of excitation.

• The name of the effector organ (muscle or gland) that is the motor response evoked by the initial stimulus.

B. Graphic Definition.

C. Table Definition.

To facilitate understanding and learning of the reflexes taught in this course, you should classify the parts of any reflex in tabular form, as shown in the discussion of the stretch reflex that follows.
 

A MONOSYNAPTIC STRETCH REFLEX A Somatic Reflex

A. Verbal Definition. The simplest of all behaviors is the monosynaptic reflex, which involves the direct, serial linkage of two sets of neurons, the sensory neurons of the dorsal root with the motor neurons of the ventral root. All such reflexes are ipsilateral only, meaning that sensory activation of a sense receptor on one side is always followed by contraction of that same muscle. The knee jerk reflex is an example. Tapping the patella stretches the golgi tendon organ of the quadriceps femoris, which in turn pulls on the muscle, stretching it and the sensory receptors located on the muscle fibers, the muscle spindles. Stretching the muscle spindle depolarizes the sensory neurons which innervate it. This causes an action potential in these afferent neurons which travels down the nerve, enters the dorsal root, then the dorsal horn of the spinal cord, finally ending motor neuron cell bodies in ventral horn. These cells now are activated, sending action potentials out along their axons, which leave the spinal cord via the ventral root, then along the nerve which then terminates on special effector organs (motor endplates) of the muscle which was originally tapped. Neurochemical events occurring when the action potential of the motor axons contacts the motor endplate initiates a series of ionic events in the quadriceps which results in shortening (i.e., contraction) of the muscle fibers. This contraction is transmitted to the tendon of the quadriceps, which is attached to the tibia, causing it to extend.

B. Graphic Definition.

C. Table Definition.
 

A POLYSYNAPTIC STRETCH REFLEX

A Somatic Reflex

The following is an example of a polysynaptic stretch reflex that occurs simultaneously with the monosynaptic reflex defined previously. Its function is to inhibit the muscle opposed to the muscle excited in the monosynaptic reflex so that the excited muscle can contract and thereby move the joint.

A. Verbal Definition. Muscles are attached to bone on opposite sides of a joint, such that one will cause extension of a limb, its opposite flexion. For a monosynaptic reflex to occur, the opposing muscle of the limb must simultaneously be relaxed. This is accomplished via a three-neuron relay which inhibits the opposing muscles (the flexors) so that extension of the limb may occur. Relay #1: These are the same sensory neurons activated in the monosynaptic reflex. But besides ending on motor neurons in the ventral horn of the same spinal level, axonal branches (known as collaterals) ascend or descend in the spinal cord for several segmental levels before ending on Relay #2, interneurons in the spinal gray matter. These interneurons are inhibitory, projecting locally onto Relay #3, motor neurons in the ventral horn which innervate the muscle antagonistic to the quadriceps. This inhibition of the flexor muscles is necessary for extension of the limb to occur.

B. Graphic Definition.

C. Table Definition.

Upper Motor Neurons

In normal, everyday behavior, reflexes rarely operate in isolation from the rest of the central nervous system. Rather, they are continuously modulated by these higher centers in order to take into account all the present environmental circumstances. These upper motor neurons are distinguished from the lower motor neurons which form the efferent relay of the reflex by the fact that all their parts-dendrites, cell bodies, axons and axon terminals lie completely within the central nervous system. The classification of "upper motor neuron" includes many different nuclei, with different functions, within the brain. What they all share in common are projections down upon the lower motor neurons of the ventral horns of the spinal cord.

The corticospinal tract is one of the most commonly illustrated example of an upper motor neuron. The cell bodies is located in the precentral gyrus of the cerebral hemispheres. Their axons are very long, leaving the precentral gyrus and coursing down through all levels of the central nervous system and into the spinal cord. Collectively, all these axons terminate on lower motor neurons of the ventral horn at all spinal levels. The linkage between the upper and lower motor neurons established in the ventral horn enables all higher centers of the brain to coordinate the contraction and relaxation of muscles so that complex behaviors may be produced.

Destruction of the corticospinal tract, whether through disease or physical injury, produces a distinctive set of deficits.

1. Loss of the ability to produce precise movements.

2. Paresis (incomplete loss of muscle power) or paralysis .

3. Initial loss of muscle tone, followed in time by increased tone in antigravity muscles (spasticity).

4. Hyperactive deep tendon reflexes.

5. The Babinski sign (dorsiflexion and fanning of the toes upon stroking the sole of the foot).

6. Loss of abdominal and cremasteric reflexes

Copyright©1996-99 B. Turner. All rights reserved.