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How Does the Spinal Cord Work?

To understand what can happen as the result of a spinal cord injury, it helps to know the anatomy of the
spinal cord and its normal functions.

Spine Anatomy
The soft, jelly-like spinal cord is protected by the spinal column. The spinal column is made up of 33
bones called vertebrae, each with a circular opening similar to the hole in a donut. The bones are
stacked one on top of the other and the spinal cord runs through the hollow channel created by the
holes in the stacked bones.

The vertebrae can be organized into sections, and are named and numbered from top to bottom
according to their location along the backbone:

Cervical vertebrae (1-7) located in the neck
Thoracic vertebrae (1-12) in the upper back (attached to the ribcage)
Lumbar vertebrae (1-5) in the lower back
Sacral vertebrae (1-5) in the hip area
Coccygeal vertebrae (1-4 fused) in the tailbone
Although the hard vertebrae protect the soft spinal cord from injury most of the time, the spinal column is
not all hard bone. Between the vertebrae are discs of semi-rigid cartilage, and in the narrow spaces
between them are passages through which the spinal nerves exit to the rest of the body. These are
places where the spinal cord is vulnerable to direct injury.

The spinal cord is also organized into segments and named and numbered from top to bottom. Each
segment marks where spinal nerves emerge from the cord to connect to specific regions of the body.
Locations of spinal cord segments do not correspond exactly to vertebral locations, but they are roughly
equivalent.

Cervical spinal nerves (C1 to C8) control signals to the back of the head, the neck and shoulders, the
arms and hands, and the diaphragm.
Thoracic spinal nerves (T1 to T12) control signals to the chest muscles, some muscles of the back, and
parts of the abdomen.
Lumbar spinal nerves (L1 to L5) control signals to the lower parts of the abdomen and the back, the
buttocks, some parts of the external genital organs, and parts of the leg.
Sacral spinal nerves (S1 to S5) control signals to the thighs and lower parts of the legs, the feet, most of
the external genital organs, and the area around the anus.The single coccygeal nerve carries sensory
information from the skin of the lower back.

Spinal Cord Anatomy

The spinal cord has a core of tissue containing nerve cells, surrounded by long tracts of nerve fibers
consisting of axons. The tracts extend up and down the spinal cord, carrying signals to and from the
brain. The average size of the spinal cord varies in circumference along its length from the width of a
thumb to the width of one of the smaller fingers. The spinal cord extends down through the upper two
thirds of the vertebral canal, from the base of the brain to the lower back, and is generally 15 to 17
inches long depending on an individual's height.

The interior of the spinal cord is made up of neurons, their support cells called glia, and blood vessels.
The neurons and their dendrites (branching projections that help neurons communicate with each other)
reside in an H-shaped region called "grey matter."

The H-shaped grey matter of the spinal cord contains motor neurons that control movement, smaller
interneurons that handle communication within and between the segments of the spinal cord, and cells
that receive sensory signals and then send information up to centers in the brain.

Surrounding the grey matter of neurons is white matter. Most axons are covered with an insulating
substance called myelin, which allows electrical signals to flow freely and quickly. Myelin has a whitish
appearance, which is why this outer section of the spinal cord is called "white matter."

Axons carry signals downward from the brain (along descending pathways) and upward toward the brain
(along ascending pathways) within specific tracts. Axons branch at their ends and can make connections
with many other nerve cells simultaneously. Some axons extend along the entire length of the spinal cord.

The descending motor tracts control the smooth muscles of internal organs and the striated (capable of
voluntary contractions) muscles of the arms and legs. They also help adjust the autonomic nervous
system's regulation of blood pressure, body temperature, and the response to stress. These pathways
begin with neurons in the brain that send electrical signals downward to specific levels of the spinal cord.
Neurons in these segments then send the impulses out to the rest of the body or coordinate neural
activity within the cord itself.

The ascending sensory tracts transmit sensory signals from the skin, extremities, and internal organs
that enter at specific segments of the spinal cord. Most of these signals are then relayed to the brain.
The spinal cord also contains neuronal circuits that control reflexes and repetitive movements, such as
walking, which can be activated by incoming sensory signals without input from the brain.

The circumference of the spinal cord varies depending on its location. It is larger in the cervical and
lumbar areas because these areas supply the nerves to the arms and upper body and the legs and
lower body, which require the most intense muscular control and receive the most sensory signals.

The ratio of white matter to grey matter also varies at each level of the spinal cord. In the cervical
segment, which is located in the neck, there is a large amount of white matter because at this level there
are many axons going to and from the brain and the rest of the spinal cord below. In lower segments,
such as the sacral, there is less white matter because most ascending axons have not yet entered the
cord, and most descending axons have contacted their targets along the way.

To pass between the vertebrae, the axons that link the spinal cord to the muscles and the rest of the
body are bundled into 31 pairs of spinal nerves, each pair with a sensory root and a motor root that
make connections within the grey matter. Two pairs of nerves - a sensory and motor pair on either side
of the cord - emerge from each segment of the spinal cord.

The functions of these nerves are determined by their location in the spinal cord. They control
everything from body functions such as breathing, sweating, digestion, and elimination, to gross and fine
motor skills, as well as sensations in the arms and legs.

The Nervous Systems

Together, the spinal cord and the brain make up the central nervous system (CNS).

The CNS controls most functions of the body, but it is not the only nervous system in the body. The
peripheral nervous system (PNS) includes the nerves that project to the limbs, heart, skin, and other
organs outside the brain. The PNS controls the somatic nervous system, which regulates muscle
movements and the response to sensations of touch and pain, and the autonomic nervous system, which
provides nerve input to the internal organs and generates automatic reflex responses. The autonomic
nervous system is divided into the sympathetic nervous system, which mobilizes organs and their
functions during times of stress and arousal, and the parasympathetic nervous system, which conserves
energy and resources during times of rest and relaxation.

The spinal cord acts as the primary information pathway between the brain and all the other nervous
systems of the body. It receives sensory information from the skin, joints, and muscles of the trunk, arms,
and legs, which it then relays upward to the brain. It carries messages downward from the brain to the
PNS, and contains motor neurons, which direct voluntary movements and adjust reflex movements.
Because of the central role it plays in coordinating muscle movements and interpreting sensory input,
any kind of injury to the spinal cord can cause significant problems throughout the body.