Anatomy of the Spinal Cord
The spinal cord
is the inferior extension of the brain that extends down into the
vertebral column. The two major functions of the spinal cord are
**control reflex activities
**transmit information by way of the ascending and descending
tracts to the brain from peripheral nerves
The spine
is separated into five sections. The first 8 vertebrae are called
the cervical vertebrae. This section has a
slight bulge because of the nerves that emerge from it. The next
12 vertebrae are called the thoracic vertebrae,
and the next 5 are called the lumbar vertebra.
The lumbar section is also enlarged, due to the nerves that
emerge there. The next section is called the sacrum,
and consists of 5 sacral vertebrae that are fused together into
on plate. The final 4 vertebrae make up to coccyx,
or the tailbone. These are also fused together.
The spinal
cord exits the skull through the foramen magnum,
a hole in the occipital lobe of the skull. The spinal cord has an
average length of 45 centimeters, although that number varies
depending on torso length. The diameter of the spinal cord in
about that of a finger. The cord tapers to a smaller point from
top to bottom, or from the cranial end to the caudal
end. At the caudal end, the spinal cord narrows into a
sharp point called the conus medullaris, from
which extends a thin filament called the filium terminale.
There are
31 pairs of spinal nerves in total. The nerves in the lower
region of the spinal are called the cauda equina,
because of their resemblance to a horse's tail.
The spinal
cord can be divided into regions by various fissures. The anterior
median fissure and the posterior fissure
divide the spinal cord longitudinally. Through the very center of
the spinal cord is a canal called the central canal,
which houses the CSF.
The gray
matter of the spinal cord consists of masses of cell
bodies, dendrites, efferent neurons, and unmyelinated axons, as
well as blood vessels and glial cells. The gray matter creates a
rough "H" shape. Each half of the gray matter section
has an anterior and posterior column,
and is connected in the middle by a bar called the gray
commisure.
The white
matter of the spinal cord consists of myelinated axons
arranged into bundles called tracts. These tracts pass up and
down from the brain. In each half of the cord, the white matter
is divided into three columns called funiculi:
the anterior, posterior, and lateral
funiculi. Each funiculus is divided into fasciculi,
or tracts, which are bundles of myelinated
axons.
Tracts of the Spinal Cord
Ascending
tracts carry impulses up towards the brain.
Descending
tracts carry impulses back from the brain
Some
examples of tracts are:
Tract |
Direction |
Function |
Spinothalamic | Ascending | Convey information regarding touch, temperature, and pain |
Lateral Corticospinal Anterior Corticospinal | Descending | Convey impulses directing precise, skilled, voluntary movement to nerves that go to skeletal muscles |
Tectospinal | Descending | Posture, reflexes |
Fasciculus Cuneatus | Ascending | Skin, joints, muscles, pressure, etc, from the waist and up |
Fasciculus Gracilis | Ascending | Skin, joints, muscles, pressure, etc, from the waist and down |
Reflexes
A reflex arc is
the neural pathway that links the receptor, the central
nervous system, and the effector.
A reflex action is predictable, automatic, and specific. Internal
as well as external stimuli have reflex responses. The
homeostatic reflex response to high blood pressure is just as
much a reflex as the physical drawback from a painful touch. All
reflexes, even the simplest, involve reception, transmission,
integration, and response.
Certain components are present in all reflexes
* receptor
to receive the stimulus
** sensory
(afferent) neuron to transmit impulse from the receptor to the
CNS
*** reflex
center in the CNS for integration, including an association
neuron
** motor
(efferent) neuron to transmit impulse from the CNS to the
effector
* effector
to carry out the response
A spinal reflex,
or a simple reflex, is carried
out solely by the spinal cord, without the participation of the
brain. Other reflex actions only involve the part of the brain
that doesn't involve conscious thought. Still other reflex
actions are subject to conscious control. Reflexes that involve
the brain are higher reflexes.
A simple reflex DOES NOT involve tracts, while a higher reflex
DOES.
A stretch reflex
is monosynaptic, meaning there is only one
junction between neurons. This type of reflex only requires two
neurons, and the best example of such a reflex is the patellar
reflex. The tendon is tapped, and impulses are
sent through sensory neurons to
the spinal cord, where they synapse with motor neurons
and then transport impulses to the motor units of the leg
muscles. The leg, as the response of the reflex, kicks out
suddenly.
The patellar reflex consists of
*a receptor that receives the stimulus
*a sensory neuron that transmits the impulse to the spinal cord
*the CNS, where integration of the impulse takes place
*a motor neuron to transmit the decision to the effector'
*an effector, a muscle that carries out the response
(in a muscular reflex such
as this, the muscles that extend the leg as a response are
allowed to do so by the inhibition of their antagonistic muscles.
The muscles required in the reflex carry out their actions, and
the muscles that do the opposite action are inhibited. This
stimulation of once muscle and inhibition of another is called reciprocal
inhibition)
The flexor
reflex is polysynaptic, meaning
there are multiple junctions between neurons. Three
neurons are required for this type of reflex. An example is
yanking your hand away from a hot stove. The receptors in the
hand sense the heat, and transfer the impulse along a sensory
neuron, which goes to the CNS. In the spinal
cord, the sensory neuron synapses with an association
neuron, which identifies the problem and
solution. The association neuron synapses with the motor
neuron, which goes to the arm muscles to yank
the hand away.
This reflex uses the five
basic parts of a reflex, with an association neuron being the
extra step in the passage through the CNS.
(In a reflex where one limb
is stimulated, the opposite limb extends as well, in a crossed
extensor reflex that keeps the
body balanced. It results from the contraction of muscles in the
opposite limb, and protects the body from further harm.)
Some common reflexes are...
Reflex |
Details |
Babinski sign |
When the lateral aspect of the foot sole is stroked, baby less than 1 ½ years of age will extend the big toe and abduct the others. An adult will curl under, or flex all the toes. If an adult exhibits the reaction of a baby, it can indicate a problem in the corticospinal tract |
Patellar (knee jerk) | When the patellar ligament is tapped, the leg ought to extend. If it doesnt, it can indicate damaged nerves or reflex centers |
Achilles (ankle jerk) | When the calcaneal (Achilles) tendon is tapped, the foot should extend. If it doesnt, it may indicate damaged nerve cells in the spinal cord. |
Abdominal |
When the lateral portion of the abdomen is stroked, the muscles should contract. If they dont, it may indicate problems with the corticospinal tract or reflex centers, or multiple sclerosis |
Pupil |
When a bright light is shone
into the eye, the pupil of the eyes constricts. If
it doesnt, a lack of oxygen may be a problem, or
lesions in the pons.
|
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