The Spinal Cord and Spinal Nerves


  • The CNS is wrapped by layers called MENINGES
  • EPIDURAL SPACE: space just outside of the dura mater and within the vertebral foramen that houses the spinal cord and its meninges
  • DURA MATER: outermost of the meninges, it is made of collagen fibers and is therefore a mechanically protective covering
  • SUBDURAL SPACE: not usually a visible space - the arachnoid is in reality inflated right up against the dura mater.
  • SUBDURAL HEMATOMA: blood fills the subdural space and puts pressure on the spinal cord
  • ARACHNOID: a filmy, lightweight layer that, like a spiderweb, has microscopic extensions which gently hold the spinal cord in place
  • SUBARACHNOID SPACE: space between the arachnoid and the pia mater that is full of cerebrospinal fluid (CSF) in which the spinal cord floats - the subarachnoid space is a sort of hydraulic shock absorber for the CNS
  • PIA MATER: the layer that closely wraps the spinal cord (there is no space between the pia and the spinal cord)- contains blood vessels and supplies circulation to the CNS

Meninges can develop tumors called MENINGIOMAS
     - These are always benign tumors, but they can be dangerous or fatal if they start to displace other tissues and disrupt the spinal cord
  • HEMANGIOMA: an abnormal growth of blood vessels that would, if on the skin, is called a Port Wine Stain. May also be found growing in the vessels of the pia mater,and can be dangerous

  • SPINAL NERVES: run in/out of the spinal cord and extend to/from the body
     - Segmental - each nerve controls a segment of the body, or DERMATOME (a “slice” of skin)
     - Near the spinal cord, they branch to form a dorsal root and a ventral root, and then branch more to form rootlets which connect to the spinal cord
     - DERMATOMAL DISTRIBUTIONS: signs that is restricted to particular dermatomes in the skin
- Inflammation and pain in a particular dermatome is characteristic of shingles
- Numbness in a particular dermatome is a clue to a specific disc being herniated
  • CAUDA EQUINA: spinal nerves with roots that are found at the end of the spinal cord, and that extend downward and out through the sacral foramina
     - The nerves are “hanging” in an extension of the subarachnoid space, and are surrounded by CSF
  • SPINAL TAP: a lumbar puncture to draw CSF or to check spinal cord pressure in the safest region possible, the cauda equina

SPINAL CORD

  • GANGLION: cluster of cell bodies outside of the CNS
dorsal vs. ventral can always be distinguished by the “swelling” that is the dorsal root ganglion on the dorsal root just before the dorsal and the ventral roots meet to become the spinal nerve
  • WHITE MATTER: myelinated axons
  • GRAY MATTER: cell bodies

Three classes of neurons make up a REFLEX ARC:
     1. AFFERENT (SENSORY) NEURONS: can generate action potentials and carry a sensation to the CNS
            - bring impulses INTO the CNS
            - All afferent neurons enter the dorsal root
            - These are BIPOLAR type neurons, which have their cell body located in the dorsal root ganglion, and two axons which extend in opposite directions (one out to the extremity and the other to the CNS)
     2. EFFERENT (MOTOR) NEURONS: carry information/commands from the CNS out to the periphery
            - Cell bodies located in the ventral gray horns
            - All efferent neurons leave the CNS through the ventral root
     3. INTERNEURONS: distribute nerve impulses WITHIN the CNS
            - The most complicated type of all neurons; involved in processing
            - Cell bodies located in the dorsal gray horn and axons extend within the spinal
  cord
Reflexes are “hard-wired” as a consequence of the wiring patterns of efferent neurons,interneurons, and afferent neurons.
  • REFLEX ARCS are involved in fast, unconscious, involuntary reactions.
There is more gray matter in areas of the spinal cord where lots of reflexes are occurring
     - Cervical (arms), lumbar (legs), and sacral (bladder, rectum, sex organs) regions of the spinal cord
Syphilis and polio are two diseases that destroy opposite ends of a reflex arc (for instance, one destroys the sensory half, but leaves the motor half unaffected and vice versa). Multiple sclerosis is a disease that destroys the interneurons and thus the connection between the two halves of the arc.


  • TRACT: a group or "bundle" of axons traveling together
     - ASCENDING TRACTS: sensory
            1. DORSAL COLUMNS: the biggest ascending tracts that bring in information from the outside world
                 - Sensory information about the skin and body position
                 - If damaged, there would be a sensory deficit
            2. ANTEROLATERAL SPINOTHALAMIC TRACTS: send information about pain and temperature sensation to the thalamus
     - DESCENDING TRACTS: motor
            1. PYRAMIDAL TRACTS: voluntary muscle movement
            2. EXTRAPYRAMIDAL TRACTS: involuntary muscle movement
RAMUS: a branch off of a spinal nerve (plural = rami)

NERVES:
PHRENIC NERVE: controls the diaphragm and breathing
     - Out of C3, C4, and C5 - this is important because if the neck is broken below C5 then breathing is still possible
     - Right and left phrenic nerves run down between the heart and lungs to the diaphragm
     - breathing pacemaker: electrodes connect to the phrenic nerve
     - Hiccups: unexplained bursts of action potentials down the phrenic nerve cause spasms of the diaphragm
BRACHIAL PLEXUS: controls the arms
     - Out of C5, C6, C7, C8 and T1
     - Extensions of the brachial plexus:
            1. MUSCULOCUTANEOUS NERVE: controls the coracobrachialis, biceps
     brachi, and brachialis muscles
            2. ULNAR NERVE: runs down and behind the medial malleolus of the humerus to the little finger and part of the ring finger
            3. MEDIAN NERVE: controls lots of the flexor muscles of the hand
                 - comes off of the brachial plexus and runs down the middle of the ventral surface of the forearm
                 - carpal tunnel inflammation will put pressure on the median nerve and will cause loss of sensation in most of the hand and will also cause pain
            4. RADIAL NERVE: controls lots of the extensor muscles of the hand
                 - runs down behind the arm and down the extensor surface of the forearm
LUMBOSACRAL PLEXUS: out of L2, L3, L4, L5, S1, S2, S3, S4, and S5
     - Branches to form three nerves:
            1. FEMORAL NERVE: controls the quadriceps
            2. OBTURATOR NERVE: runs through the obturator foramen and controls the adductor muscles
            3. SCIATIC NERVE: the biggest, longest and fattest nerve in the body
                 - controls the butt, hamstrings, and calves
                 - becomes the POPLITEAL NERVE and then the TIBIAL and FIBULAR (Or PERONEAL) NERVES
                 - SCIATICA: pain and/or paralysis down the leg due to inflammation of or lack of circulation to the sciatic nerve


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