The Spinal Cord
Adapted from “Human Anatomy and Physiology” by Elaine N. Marieb, pp 404-413 (1989)
About 42 cm (17 inches) long and 1.8 cm (3/4 of an inch) thick, the glistening-white spinal cord
provides a two-way conduction pathway to and from the brain (Figure 12.23). In addition, it is a major
reflex center: Spinal reflexes are initiated and completed at the spinal cord level. We will discuss reflex
functions of the cord in Chapter 13. The following section focuses on the anatomy of the cord and on
the location and naming of its ascending and descending tracts.
Enclosed within the vertebral column, the spinal cord extends from the foramen magnum of the skull
(where it is continuous with the medulla of the brain stem) to the level of the first lumbar vertebra, just
inferior to the ribs. In young children, the spinal cord is relatively longer, usually terminating at the
level of the third lumbar vertebra.
Like the brain, the spinal cord is protected by bone, cerebrospinal fluid, and meninges. The
singlelayered dura mater of the spinal cord, called the spinal dural sheath (see Figures 12.21 and
12.24), is not attached to the bony walls of the vertebral column. Between the bony vertebrae and the
dural sheath is a rather large epidural space filled with fat and areolar connective tissue, which forms a
soft padding around the spinal cord. Cerebrospinal fluid fills the subarachnoid space between the
arachnoid and pia mater meninges. Inferiorly, the meningeal coverings extend well beyond the end of
the spinal cord in the vertebral canal, approximately to the second sacral vertebra (S2). Since the spinal
cord ends at L1, there is usually no danger of damaging the spinal cord beyond L3; thus, the
subarachnoid space within the meningeal sac inferior to that point provides a nearly ideal spot for
removing cerebrospinal fluid for testing. This procedure is called a lumbar puncture or tap.
Inferiorly, the spinal cord terminates in a tapering structure called the conus medullaris (ko'-nus
meh"dyoo-layr'-is). A fibrous extension of the pia mater, the filum terminate (fi'-lum ter"-mih-nah'-le)
("terminal filament") extends downward from the conus medullaris to the posterior surface of the
coccyx, where it attaches (Figure 12.23).
In humans, 31 pairs of spinal nerves arise frog the cord by paired roots and exit from the vertebra
column via the intervertebral foramina to travel to the body regions they serve. The spinal cord is about
the width of a thumb for most of its length, but it ha; obvious enlargements in the cervical and
lumbosacral regions, where the nerves serving the upper and lower limbs issue from the cord. These
enlargements are the cervical and lumbosacral enlargements, respectively (see Figure 12.23a). Because
the cord does not react the end of the vertebral column, the lumbar and sacra spinal nerve roots angle
sharply downward and travel inferiorly through the vertebral canal for some distance before exiting
through their intervertebral for amina. The collection of nerve roots at the inferior end of the vertebral
canal is named the cauda equina (kah'-duh e-kwi'nuh) because of its resemblance to horse's tail. This
rather strange arrangement reflect the different rates of growth of the vertebral column and spinal cord.
In embryos, the spinal cord occupies the entire length of the vertebral canal. Subsequently however, the
vertebral column grows inferiorly more rapidly than the spinal cord, forcing the lower spinal nerve
roots to "chase" their exit points inferiorly through the vertebral canal.
The spinal cord is somewhat flattened from front to back, and two grooves mark its surface: the
anterior median fissure and the more shallow posterior median sulcus (see Figure 12.25b and c). These
grooves extent the length of the cord and partially divide it into right and left portions. As mentioned
earlier, the gray matter of the cord is located on the inside, the white matter outside.
Gray Matter of the Spinal Cord and Spinal Roots
As in other regions of the central nervous system, the gray matter of the cord consists of a mixture of
neuron cell bodies, their unmyelinated processes, neuroglia. and blood vessels. All neurons whose cell
bodies aye 1 in the gray matter of the cord are multipolar neurons.
As already noted, the gray matter of the cord looks like the letter H or like a butterfly in cross section
(Figure 12.25). It consists of mirror-image gray masses connected by a cross-bar of gray ratter called
the gray commissure. The two posterior projections of the gray matter are the posterior (dorsal) horns;
the anterior pair are the anterior (ventral horns. An additional pair of gray matter columns, the small
lateral horns, is seen in the thoracic and upper lumbar segments of the cord.
The posterior horns contain association neurons The anterior horns contain nerve cell bodies of somatic
motor neurons, which send their axons out via the ventral root of the spinal cord (see Figure 12.25c) In
ultimately reach the skeletal muscles (effector organs. The amount of ventral gray matter present at a
given level of the spinal cord reflects the amount of skeletal muscle innervated at that particular level.
Thus, the anterior horns are largest in the limb-innervatingcervical and lumbosacral regions of the cord
and are responsible for the enlargements seen in those regions. The symptoms of poliomyelitis (po"-leo-mI"-uhli'-tis) (polio = gray matter; myelitis = inflammation of the spinal cord) result from the
destruction of anterior horn motor neurons by the poliovirus. Initial symptoms include fever, headache,
muscle pain and weakness, and loss of certain somatic reflexes. However, as the disease progresses,
the patient develops paralysis and atrophy of the muscles served. The victim may die from paralysis of
the respiratory muscles or from cardiac arrest if neurons in the medulla oblongata are destroyed. In
most cases, the poliovirus enters the body in feces-contaminated water (such as might occur in a public
swimming pool), and the incidence of the disease has traditionally been highest in children and during
the summer months. Fortunately, the Salk and Sabin polio vaccines have nearly eliminated this disease.
The lateral horn neurons are autonomic (sympathetic division) motor neurons that serve visceral
organs. Their axons leave the cord via the ventral root along with those of the somatic motor neurons.
Since the ventral roots contain both somatic and autonomic efferents, they serve both motor divisions
of the peripheral nervous system.
Afferent fibers carrying impulses from peripheral sensory receptors form the dorsal roots of the spinal
cord (see Figure 12.25c). The nerve cell bodies of the associated sensory neurons are found in an
enlarged region of the dorsal root called the dorsal root ganglion or spinal ganglion. After entering the
cord, their axons may take a number of routes. For example, some enter the posterior white matter of
the cord directly and travel to synapse at higher cord or brain levels, while others synapse with neurons
in the spinal cord gray matter at the level they enter.
The dorsal and ventral roots are very short and fuse laterally to form the spinal nerves. The spinal
nerves, which are part of the peripheral nervous system contain both sensory and motor neurons.
Ascending (Sensory) Pathways and Tracts
The ascending pathways conduct afferent impulses that enter the spinal cord upward via chains of two
or three successive neurons (first-, second-, and third-order neurons) to various parts of the brain. Most
of this information results from stimulation of touch, pressure, temperature, and pain receptors in the
skin and from stimulation of proprioceptors, which report on the degree of stretch in the muscles,
tendons, and joints. In general, this information is conveyed along six main pathways on each side of
the spinal cord.
Recall that our ability to identify and appreciate the kind of sensation being transmitted-that is, whether
it is touch, temperature, pain, and so on depends on the specific location of the target neurons in the
cerebral sensory cortex, with which the ascending pathway fibers synapse, and not on the nature of the
message, which is always action potentials. Each sensory nerve fiber is analogous to a "labeled line" in
a telephone system and is identified by the brain as associated with a particular sensory modality.
Receiving information via a certain line always tells the brain "who" is calling, whether it is a taste bud
or a pressure receptor. Furthermore, when a sensory neuron is excited, the brain interprets its activity as
a specific sensory modality, regardless of how the receptor is activated. If we press a Pacinian receptor
of the index fingertip, jolt it with an electrical shock, or electrically stimulate the area of the
somatosensory cortex that recognizes it, the result will be the same. We will perceive deep touch or
pressure and will interpret it as coming from the index fingertip. This phenomenon, by which the brain
refers sensations to their usual point of stimulation, is called projection.
Descending (Motor) Pathways and Tracts
The descending tracts that deliver efferent impulses from the brain to the spinal cord and are the major
motor pathways concerned with movement.
The cerebellum influences motor activity by acting through relays on the motor cortex. The cerebellum
also interacts with the basal nuclei and with lower brain stem motor centers such as the red nucleus of
the midbrain and various reticular nuclei. These nuclei, in turn, influence lower motor neurons.