Thursday, June 24, 2010

Spinal Cord and Nerves

The spinal cord is pretty interesting.  It is a cord that extends from the base of the brair through the vertebral canal, that of course we know by now is formed by the vertebrae.  The functions of the spinal cord include reflexes, integration and being a highway for the traveling sensory (upward, afferent) and motor (downward, efferent) neurons. 
The spinal cord begins at the foramen magnum (the largest foramen in the skull) as a continuation of the medulla oblongata (the most inferior portion of the brain).  It is approximately 16-18 inches in length and terminates near the second lumbar vertebrae.  It contains two enlargements: the cervical and lumbar.  These are the origin points for nerves to the extremeties.  More specifically, the cervical enlargement is the origin of the spinal nerves to our arms while the lumbar enlargment is the origin of the spinal nerves to our legs.
The cuada equana or "horse's tail" are the dorsal and ventral roots of the lowest spinal nerves.
The conus medularis is the tappered end of the spinal cord near lumbar vertebrae 1 and 2.
The filum terminale is an extension of the pia mater. 

Protection of the spinal cord comes from the vertebrae, epidural space filled with fat, the dura mater, the arachnoid, and the pia mater.  The dura mater is literally translated to the stong mother.  It is a dense irregular connective tube and has a subdural space filled with interstitisal fluid.  The arachnoid portion of the protection is a spider web of collagen fibers that can take compression when doing any activity.  This portion has a subarachnoid space that is filled with cerebral spinal fluid.  The pia mater translates to delicate mother.  It is a thin layer that also covers the blood vessels.  The denticulate ligaments hold the pia mater in place.

Meningitis is the inflammation of the meninges.  To determine if someone has meningitis, a spinal tap (or lumbar puncture) is done to test the cerebral spinal fluid of the subarachnoid space.  More information to come on Meningitis in my next post...

There are three areas of both gray and white matter.  The Dorsal (posterior) Horn, the Ventral (anterior) Horn and the Lateral Horn (of the autonomic nervous system).

Here are some important parts of the spinal cord, but I can't remember what each of them do right now:
Lateral white column
lateral gray horn
anterior gray horn
gray commissure
anterior white commissure
anterior white column
anterior median fissure
posterior gray horn
posterior median sulcus
posterior white column
central canal
axon of interneuron
cell body of somatic motor neuron
axons of motor neurons
axon of sensory neuron
cell body of autonomic motor neuron
cell body of sensory neuron

The tracts of the cord are the highways we mentioned earlier.  Sensory tracts ascend and motor tracts descend.  The tracts are named by the position and direction of the signal.  An example from the lecure is the Anterior Spinothalamic Tract.  The impulses travel from the cord to the thalamus and it is found in the anterior portion of the cord.  The tracts that we need to know are corticospinal (controls voluntary movements), the reticulospinal and rubrosprinal (automatic movements for tone, posture and balance), the fasciculus gracilis and cuneatus (touch, pressure, propriception) and the spinothalamic (pain, warmth, and itching).  Some tracts cross over the spinal cord to the other side before it reaches the thalumus.

Lou Gehrig's Disease is also known as Amyotrophic lateral sclerosis (ALS).  It is the degeneration of motor neurons in the cord, brainstem and cortex by free radicals.

There are 31 pairs of spinal nerve.  One half of each pair goes to either the left side or right side of the body.  The pairs are named and numbered according to the region and level of the spinal cord from which they emerge.  There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves and 1 pair of coccygeal nerves.  Roots are the two points of attachment from the spinal nerves to the spinal cord.   The roots are either called dorsal or ventral.  The easiest way to tell the two apart is by looking for the dorsal root ganglion. 

Like muscles, nerves have several connective tissue coverings.  The endonurium is the wrapping around each nerve fiber (aka axon).  The perineurium surrounds a group of nerve fibers forming a fascicle.  The epinurium covers the entire nerve and blends into the dura mater at intervertebral foramen. 

Spinal nervers branch into dorsal and ventral rami.  The dorsal rami only supply the skin and muscles of the back.  Ventral rami form plexus for the anterior trunk and limbs.  A nerve plexus is the joining of nevertal rami of spinal nerves to form nerve networks or plexuses.  They are found in the neck, arms, low back and sacral regions. There is not a plexus in the thoracic region. 
The cervical plexus are the ventral rami of spinal nerves C1-C5.  They supply part of the head, neck and shoulders.  The phrenic nerve (C3-C5) keeps the diaphragm alive.  Damage to the spinal cord above C3 causes respiratory arrest. 
The brachial plexuses are from C5-T1.  They lie deep with in the shoulders.  Musculocutaneous nerves supply the muscles of the anterior arms and skin of forearms.  The unlar and median nerves supply the muscles of forearms and hands; these nerves are on the medial side of the body.  The radial nerves supply the posterior muscles of arms and the skin of forearms and hands. Axillary nerves supply the muscles and skin of anterior, lateral and posterior arms. 
There are no plexuses in the thoracic region.
The lumbar plexus is the lumbar rami of L1-L4.  It supplies the abdominal wall, external genitals and the anterior/medial thigh.  If you injure your femoral nerve, you may lose the ability to extend your leg as well as sensation in your thigh.  If you obturator nerve is damaged, there may be paralysis of the adductors. 
Lumbrosacral plexuses extend from the lumbar region into the pelvic cavity.  Obturator nerves control motor impulses to adductors of thighs.  Femoral nerves control motor impulses to muscles of anterior thigh and sensory impulses from skin of thighs and legs.  Sciatic nerves control muscles and skin of thighs, legs and feet. 
The sacral plexus is the ventral rami L4-L5 and S1-S5.  It is anterior to the sacrum and supplies buttocks perineum and part of lower limbs.  A peroneal nerve injury causes foot drop or numbness.  A tibial nerve injury produces calcaneovalgus (the loss of function on anterior leg and dorsum of foot). 
The sciatic nerve branches include the common peroneal (or fibular) nerve and then tibial nerve behind the knee.  Sciatica pain extends from the buttocks down the leg to the foot. 

Dermatomes are the area of skin that sensory fibers of a spinal nerve innervate.  If there is a damaged region on a cord, the patterns of numbness can determine which dermatome it is.  Infusing local anesthetics or cutting roots has to be done over three adjacent spinal nerves, otherwise, some pain may still exist.  A spinal cord transection is an injury that severs the cord loss of sensation and motor control below the injury. 

Reflexes do not occur with cognitive thought.  A good reaction that can be tested is the Babinski Sign.  The Babinski sign is tested by running a dull object over the lateral edge of the foot.  The big toe will either flex or extend.  This test checks for descending motor system damage.  A positive sign is normal in infants, but indicates a problem in adults.  In adults, no Babinski sign... The toes should curl under. 

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