Anatomy of the Spine and PNS
The spinal cord is a continuation of the brain stem. It's long, cylindrical, and
passes through a tunnel in the vertebra called the vertebral canal. The spinal
cord has many spinal segments, which are spinal cord regions from which pairs
(one per segment) of spinal nerves arise. Like the cerebrum and cerebellum, the
spinal cord has gray and white matter, although here the white matter is on the
outside. The spinal cord carries messages between the central nervous system (CNS)
and the rest of the body, and mediates numerous spinal reflexes such as the
Keeping It Together
The spinal cord lies inside the spinal column, which is made up of 33 bones
called vertebra. The vertebra, which are stacked on top of one another are divided
into four regions:
||7 cervical or neck vertebra (labeled C1-C7)
||12 thoracic or upper back vertebra (labeled T1-T12)
and attached to the ribcage
||5 lumbar or lower back vertebra (labeled L1-L5)
||5 sacrum (labeled S1-S5) and 4 coccyx (labeled as coccyx)
In regards to the lowest region, the 5 vertebra are fused together to form the
sacrum (which is part of the pelvis), and the 4 small vertebra are fused together
to form the coccyx or tailbone.
An individual vertebra is made up of several parts. The body of the vertebra is
the primary area of weight bearing and provides a resting place for the fibrous
discs which separate each of the vertebra. The lamina covers the spinal canal,
the large hole in the center of the vertebra through which the spinal nerves
pass. The spinous process is the bone you can feel when running your hands down
your back. The paired transverse processes are oriented 90 degrees to the
spinous process and provide attachment for back muscles. There are four facet
joints associated with each vertebra. A pair that face upward and another pair
that face downward.
Between the vertebral bodies (except C1 and C2) are discs that serve as
supporting structure for the spine. These discs are oval-shaped, with a tough
outer layer or annulus that surrounds a softer material called the nucleus
pulposus. These discs act as shock absorbers for the spinal bones. Ligaments
attached to the vertebra also serve as supporting structures.
These interlock with the adjacent vertebra and provide stability to the spine.
The vertebra are separated by intervertebral discs which act as cushions between
Each disc is made up of two parts. The hard, tough outer layer called the
annulus surrounds a mushy, moist center termed the nucleus. When a disc
herniates or ruptures, the soft nucleus spurts out through a tear in the
annulus, and can compress a nerve root. The nucleus can squirt out on either
side of the disc or in some cases both sides.
In many cases, degeneration or pressure from overexertion can cause a disc
to shift or protrude and bulge, causing pressure on a nerve and resultant
pain. When this happens, the condition is called a slipped, bulging, herniated,
or ruptured disc, and it sometimes results in permanent nerve damage.
The vertebra are linked by ligaments, tendons, and muscles. Back pain can occur
when, for example, someone lifts something too heavy, causing a sprain, pull,
strain, or spasm in one of these muscles or ligaments in the back.
Enlarge by passing over or clicking
The Spinal Cord and PNS
The spinal cord is an extension of the CNS. Anatomically, it begins at the
bottom of the brain stem at the area called the medulla oblongata which is
where it joins the C1 vertebra (the highest neck bone), and it ends
approximately at the level of the L1 vertebra, which is the highest bone
of the lower back.
The spinal cord is about 18 inches (45 centimeters) in length and is basically a
cylindrical mass of nervous tissue, oval or rounded in transverse section. It has
two areas of enlargement which are found in the cervical (neck) and lumbar (lower
back) segments. It occupies the upper two-thirds of the vertebral canal.
In contrast to the cerebral hemispheres, gray matter is found in the interior,
surrounded by white matter. The white matter contains ascending and descending
tracts. Some ascend to or descend from the brain, whereas others connect cells
at various levels of the cord.
The neurons of the spinal cord include: (1) somatic motor cells, the axons of which
leave by way of ventral roots and supply skeletal muscles; (2) autonomic motor cells,
the axons of which leave by way of ventral roots and go to autonomic ganglia; (3)
transmission neurons that give rise to ascending projections to the brain and to
connections with other spinal cord levels; and (4) interneurons, which connect with
other neurons at the spinal level and are concerned with sensory and reflex mechanisms.
The spinal cord is divided into segments similar to the corresponding vertebra:
cervical, thoracic, lumbar, sacral, and coccygeal. The cord also has nerve roots
and rootlets which form branch-like appendages leading from its ventral side (the
front of the body) and from its dorsal side (the back of the body). Along the dorsal
root are the cells of the dorsal root ganglia, which are critical in the transmission
of "pain" messages from the cord to the brain. It's here where injury, damage, and
trauma become pain.
Attached to the spinal cord on each side is a series of spinal roots, termed dorsal
and ventral according to their position. Generally there are 31 pairs, which comprise
8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Corresponding dorsal
and ventral roots join to form a spinal nerve. Each spinal nerve divides into a dorsal
and a ventral ramus, and these are distributed to various parts of the body.
The 31 pairs of spinal nerves from between the vertebra, each emerge in two short
||One at the front (motor or anterior root) of the
||One at the back (sensory or posterior root) of the
The motor roots carry commands from the brain and spinal cord to other parts of
the body, particularly to skeletal muscles. The sensory roots carry information
to the brain from other parts of the body.
At the bottom of the spinal cord, called the conus medullaris, is a collection of
nerves known as the cauda equina, which is Latin for "horse's tail". Early anatomists
thought the collection of nerves looked like a horse's tail.
The spinal cord is bathed in cerebrospinal fluid (CSF) and surrounded by three
protective layers called the meninges (dura, arachnoid, and pia mater) just as
the brain is.
The spinal cord carries out sensory, integrative, and motor functions, which can be
categorized as reflex, reciprocal activity (as one activity starts, another stops),
monitoring and modulation of sensory and motor mechanisms, and transmission of
impulses to the brain.
Nerve impulses travel to and from the brain through the spinal cord to a
specific location by way of the peripheral nervous system (PNS). The PNS is a
complex system of nerves that branch off from the spinal nerve roots. These
nerves travel outside of the spinal canal to the upper extremities (arms, hands
and fingers), to the muscles of the trunk, to the lower extremities (legs, feet
and toes), and to the organs of the body.
Any interruption of spinal cord function by disease or injury at a particular
level may result in a loss of sensation and motor function below that level.
Depending on the severity of the disease or injury, the loss of function may be
Multiple sclerosis (MS) attacks on the spine has a predilection for the cervical
spinal cord (67% of cases), with preferential, eccentric involvement of the dorsal
and lateral areas of the spinal cord abutting the subarachnoid space around the
cord. Approximately 55 to 75% of patients with MS have spinal lesions at some
point during the course of the disease.