Spinal Cord Injury: Treatments and Rehabilitation (cont.)
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.
Next: What Happens When the Spinal Cord Is Injured? »
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