Spinal cord and nerves

Lecture Notes

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I Spinal cord cross section (apply reflex arc to anatomical structures)

A. Spinal nerves consist of myelinated axons of sensory and motor neurons entering/exiting the cord
1. Posterior nerve root contains sensory neuron axons with their cell bodies in posterior root ganglion. root conveys sensory info to association neurons (gray matter)
2. Anterior root contains motor axons . This root convey motor info from association neurons (gray matter) to effector cells.

B. Gray matter is unmyelinated cell bodies, axons and dendrites
1. Anterior gray horns contains motor cell bodies

2. Posterior gray horns contains association neuron cell bodies (control center


C. White matter (spinal tracts) are columns of myelinated neurons
1. Ascending tracts - neurons convey sensory information up cord to brain

2. Descending tracts - neurons convey motor information from brain down cord
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Draw and label a cross section of the spinal cord. Over this drawing, diagram the components of a reflex arc. Then add ascending and descending neurons.


II. gross anatomy

A. Spinal nerve pairs a. Some form network (plexus) before going to/coming from areas they service (except thoracic nerves)
 referred pain =visceral pain same paths as somatic pain therefore  feel visceral referred to pain on skin
b. Dermatomes -skin map representing spinal nerve functions
B. Cervical & lumbar enlargements to incorporate increased neural pathways from/to appendages

C. Cord ends at about L2 then finishes as separate nerves (= cauda equina)

D . Protective layers and spaces of spinal cord.
1 . Epidural space -between bone and outer spinal covering,  filled with fat
2 . Dura mater meninge - tough connective tissue layer
3 . Subdural space - between dura mater and next layer
4 . Arachnoid mater meninge - spider webby connective tissue layer
5 . Subarachnoid space - between space and next layer contains cerebrospinal fluid
6 . Pia mater meninge - transparent thin connective tissue layer closest to white matter.
 vascular layer
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Draw and label the protectice layers of a spinal cord.

III. CSF
cerebrospinal fluid, produced in ventricles of brain, bathes spinal cord in oxygenated, nutrient-rich fluid. Flows down subarachnoid space and through central canal.

IV. Cross section of nerve

A. Myelin sheath around axon

B. Connective layers for protection

1. Endoneurium
2. Perineurium
3. Epineurium

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Draw and label the connective layers of a peripheral nerve.
V. Spinal somatic reflexes (very fast responses used to control skeletal muscle position)

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A. Stretch reflex, e.g., knee jerk (=patellar reflex) 1. Stretch receptors in muscles (muscle spindles) are stimulated when muscle is stretched. Input goes via sensory then connects with motor that contracts same muscle that is stretched

2. Sensory neuron also synapses with inhibitory association neurons that synapse with motor neurons to antagonistic muscles on same side (ipsilateral) . Therefore antagonistic muscles cannot contract to reduce prime muscle contraction that reduces stretching.

When action potentials, (i.e., from sensory) stimulate contraction of one muscle and inhibit contraction of its antagonists, this is called reciprocal innervation.

3. Total result - muscle cannot be overstretched. Stretch reflex causes contraction as response to muscle over-lengthening.
 
 

B. Tendon reflex 1. As tendon undergoes more tension Golgi tendon organ increase action potentials through sensory neurons to inhibitory association neuron which causes muscle to relax

2. It also interacts with stimulatory neuron for antagonistic muscle which contracts thereby taking tension of prime mover (another example of reciprocal innervation, also ipsilateral)

3. Result - tendon reflex cause muscle relaxing as response to muscle over-contraction

 

Stretch & tendon work against each other to keep muscles in tone and maintain posture

 

C. Flexor reflex (withdrawal) 1. Stimulus (pain) travels up sensory neuron and synapses with association neuron to stimulate flexor to contract and move away from pain source

2. Signal also goes through another association neuron that ascends/descends to another segment on same side (ipsilateral) that also stimulates another flexor from another level of the spinal cord (multisegmented) therefore  one sensory neuron stimulates several flexor muscles

3. Sensory input also travels to inhibitory association neuron that inhibits extensor so flexion is easier and faster (reciprocal innervation)

4. Result - rapid reflexive withdrawal from pain
 

D. Crossed extensor
 sensory impulse enters one side and exits other via association neuron which stimulates extensors (on opposite side where flexors are stimulated and extensors are inhibited -contralateral) therefore withdraw leg away (flex)  while other leg's extensors contract to maintain balance
(multisegmental, contralateral reciprocal innervation)

Diagram each spinal somatic reflex and describe its stimulus and the response..


VI. Spinal autonomic reflexes (rapid reflexive responses at spinal level)

A. Micturition reflex
 increasing urine volume stimulates contraction of bladder muscles

B. Erection & ejaculation/lubrication
reproductive reflexes

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Professor Thomas M. Lancraft
Human Anatomy and Physiology Courses
at St. Petersburg College
St. Petersburg/Gibbs Campus

11/2006