The Central and Peripheral Nervous System
Although the brain
often gets most of the credit for being the "central processing unit" of the
body, the spinal cord also plays a crucial role in what is
known as the central nervous system (CNS). The CNS controls
most functions of the body, including movement, organ functions, and
The spinal cord descends from the brain down the middle of the back, and 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 also 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 spinal
cord injury can cause significant problems throughout the body.
The CNS communicates with the limbs, heart, skin, muscles, and other organs
of the body through a network of nerves collectivly known as the
peripheral nervous system (PNS). Most of the nerves of the PNS
connect to the spinal cord at various locations up and down the back. 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 itself divided into
the sympathetic nervous system, which mobilizes organs and their
functions during times of stress and arousal ("fight or flight"), and the
parasympathetic nervous system, which conserves energy and resources
during times of rest and relaxation ("rest and digest").
Because the soft, jelly-like spinal cord is so important for nearly every
function of the body, it is protected by the boney 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 one another, 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
The spinal cord is also cushioned by a fluid called Cerebral
Spinal Fluid (CSF), which protects the delicate nerve tissues from being
damaged by rubbing against the inside of the vertebrae.
Although the hard vertebrae and CSF 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.
Spinal Cord AnatomyLike the vertebrae, 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
- 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
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. An injury to the spinal cord may inhibit the
functioning of any nerves located below the site of injury.
Spinal Cord Physiology
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." A
number of diseases damage the myelin and the white matter, including multiple
sclerosis and postinfectious
Axons carry signals downward from the brain (along descending pathways called
efferent nerves) and upward toward the brain (along ascending pathways
called afferent nerves) 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
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.Injury
Spinal cord injuries can be caused by trauma to the spinal column
(stretching, bruising, applying pressure, severing, laceration, etc.). The
vertebral bones or intervertebral disks can shatter, causing
the spinal cord to be punctured by a sharp fragment of bone. Usually, victims of spinal cord injuries will suffer
loss of feeling in certain parts of their body. In milder cases, a victim might
only suffer loss of hand or foot function. More severe injuries may result in paraplegia, tetraplegia, or full body paralysis (called Quadriplegia) below the site of
injury to the spinal cord.
Damage to upper motor neuron axons in the spinal cord results in a
characteristic pattern of ipsilateral deficits. These include hyperreflexia, hypertonia and muscle weakness. Lower motor neuronal
damage results in its own characteristic pattern of deficits. Rather than an
entire side of deficits, there is a pattern relating to the myotome affected by the damage. Additionally, lower
motor neurons are characterized by muscle weakness, hypotonia, hyporeflexia and muscle atrophy.
Spinal shock and neurogenic shock can occur from a spinal injury. Spinal
shock is usually temporary, lasting only for 24–48 hours, and is a temporary
absence of sensory and motor functions. Neurogenic shock lasts for weeks and can
lead to a loss of muscle tone due to disuse of the muscles below the injured
The two areas of the spinal cord most commonly injured are the cervical
spine (C1-C7) and the lumbar spine (L1-L5). (The notation C1, C7, L1, L5
refer to the location of a specific vertebra in either the cervical, thoracic, or lumbar
region of the spine.)
© 2010 Vivacare. Last updated April 12, 2011.
Reference: National Institute of Neurological Disorders and Stroke
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