spinal cord begins below the medulla and ends just above the
small of the back at the conus medularis. The area
within the vertebral column beyond the end of the spinal
cord is called the cauda equina.
The spinal cord is protected by the vertebrae and
the meninges. The dura mater, arachnoid mater and pia mater
of the spinal cord are continuous with those of the brain.
Cerebrospinal fluid is in the subarachnoid
that lies between the arachnoid and pia mater and in the
central canal, a space in the middle of
the gray matter of the cord. It provides a hydraulic cushion
for the spinal cord.
(sensory) and ventral (motor) horn cells
the cord is viewed in a cross-section, its gray matter is
"H" shaped or, as described by Bhatnagar, 2002,
butterfly shaped. It has two ventral and two dorsal horns.
The white matter surrounding the cell bodies of the cord is
made up of ascending and descending fibers. Motor tracts are
found on the ventral and lateral aspects of the cord while
sensory tracts run along its dorsal area.
lower motor neurons are located on the ventral aspect of the
cord. They are either alpha or gamma cells.
Alpha cells are the principle lower motor
neurons of the spinal cord and form the main portion of the
final common pathway. They conduct rapid motor impulses,
with each alpha cell innervating approximately 200 muscle
Gamma neurons are also part of the final
common pathway according to some sources but they are only
half as numerous as alpha cells. Gamma cells conduct slow
motor impulses. Their major function is to stretch muscle
Interneurons connect the anterior and
posterior horns of the gray matter and are involved in the
reflex arc. They work within the same segment of the spinal
cord, with a segment being defined as the horizontal section
of the cord that gives rise to one pair of spinal nerves.
Neurons travel between segments, sending
projections up to the brain stem and cerebellum. They
project in an ascending, not descending manner.
neurons are found throughout the central nervous system.
They are much more numerous than motor neurons; the ratio
between the two types of cells is 30:1.
The main function
of the association neurons in the spinal cord is that of
inhibitory control. They also interconnect other cells with
including Bhatnager and Andy, (1995), do not distinguish
between interneurons and internuncial neurons. Even if these
two types of association neurons are grouped together, they
should definitely be distinguished from the spinal nerves
which are lower motor neurons, forming a final common
pathway for information descending from the brain.
thirty-one pairs of spinal
nerves. These nerves are mixed, having both
a sensory and a motor aspect. Their motor fibers begin on
the ventral part of the spinal cord at the
anterior horns of the gray matter. The
roots of their sensory fibers are located on the dorsal side
of the spinal cord in the posterior root
ganglia. When the motor and sensory fibers
exit the spinal column through the intervertebral foramina
and pass through the meninges, they join together to form
the spinal nerves.
receive only contralateral innervation from first order
Eight pairs of spinal nerves are
located in the uppermost, cervical region of the cord:
are found in the thoracic region.
are in the lumbar area.
are in the sacral area.
is found in the most inferior, coccygeal region.
second order lower motor neurons, the spinal nerves, form
part of the final common pathway for information traveling
from the central nervous system to the periphery. The spinal
nerves provide innervation to body areas below the neck
while cranial nerves (also second order neurons) carry
impulses only to the head and neck, except for the vagus.
(You will understand shortly that cranial nerves can be
sensory, motor or both).
the sensory and motor fibers of the spinal nerves form a
This type of reflexive behavior occurs when a message from
afferent fibers causes a motor reaction before
going to the brain. For example, if you touch a hot
burner on the stove, sensory information about the
temperature of the burner travels along spinal nerves to
your spinal cord and are carried directly to their motor
nuclei by interneurons; the motor command goes out along the
axons of the lower motor neuron causing you to move your
hand away from the stove. As messages do not have to travel
up to the brain to be processed, reactions mediated by this
reflex arc can occur very rapidly. Of course you WILL feel
pain shortly thereafter (milliseconds) as the information
gets to the parietal lobe via the thalamus
The Autonomic (self regulating)
autonomic nervous system is involved in the control of the
heart, glands and smooth muscles of the body and plays a
major role in regulating unconscious, vegetative functions.
It works together with the endocrine system to control the
secretion of hormones and is itself controlled by the
fibers make up the bulk of the autonomic system, some
anatomists consider it to be purely motoric although it does
include some afferent axons that carry information from the
autonomic nervous system is considered to be one of the
three main divisions of the human nervous system in its own
right, parts of both the central nervous systems and the
peripheral nervous systems play a role in its functions.
nervous system has two components, the sympathetic system and the
system. These two aspects have antagonistic
system prepares the body for fight or
flight reactions. Action of this system results in
accelerated heart rate, increased blood pressure and blood
flow away from the periphery and digestive system toward the
brain, heart and skeletal muscles. It also causes
adrenaline to be released,
temporarily increasing physical strength.
system brings the body back to a state of
equilibrium. It slows heart rate and decreases the release
of hormones into the blood stream. The activity of the
parasympathetic system causes more localized reactions than
does the sympathetic system as much of its output is to
nervous system consists of four chains of nuclei or ganglia,
two of which are located on either side of the spinal cord.
The outer chains of nuclei form the parasympathetic division of the system
while those closest to the spinal cord make up its
rami of the autonomic nervous system are the axons of
pre-ganglionic and ganglionic fibers. Most of the axons of pre-ganglionic fibers are myelinated. Their cell bodies are found in the gray matter of the brain stem and spinal cord. Their axons synapse with neurons within the two ganglionic chains.
of the autonomic nervous system are neurons located in some
of the cranial nerves of the brain stem and in some of the
spinal nerves that project to the ganglionic chains of the
autonomic nervous system. The autonomic nervous system is
closely connected with the central and peripheral nervous
originate within the ganglia. They project to
are neurons that are located in the target organs and
muscles of the autonomic nervous system.
It can be said
that the motor pathways of the autonomic nervous system are
made up of its pre-ganglionic and ganglionic cells.
The fibers of the
ganglionic chain of the parasympathetic system are not as
well-defined as those of the sympathetic chain. All
pre-ganglionic neurons of the sympathetic system synapse with
the sympathetic chain. This is not true of the
parasympathetic pre-ganglionic cells, however. Some of them
synapse with the chain, but others go directly to end organs