Lecture 4 (Asc): Spinal cord Org and Reflexes

Question 1

How many pairs of spinal nerves?

Answer 1

31 pairs

Question 2

How many pairs of each level of spinal nerves?

Answer 2

8 cervical (neck and arms)
12 thoracic (thorax)
5 lumbar (abdomen)
5 sacral (pelvis)
1 coccygeal (legs)

Question 3

define DERMATOME

Answer 3

area of skin innervated by one segment of the spinal cord (iow, baby one pair of spinal nerves)

dermatomes of adjacent spinal cord segments overlap

Question 4

Afferent fibers…

Answer 4

arrive
sensation

in posterior or dorsal root

general somatic afferent: proprioceptive/exteroceptive
general visceral afferent: interoceptive

Question 5

Efferent fibers…

Answer 5

exit
anterior or ventral root, leaves spinal cord
motor component

Question 6

what are the three choices of nerves carrying SENSORY information into the spinal cord?

Answer 6

1. become first part off reflex loop, and usu induces at same level
2. enter grey matter (dorsal horn) and connect directly/indirecrtly to neurons which project to higher centers
3. enter spinal cord but do not make connection, and ascend with neurons in the brainstem

Question 7

what is the MUSCLE SPINDLE?

Answer 7

• small connective tissue capsule
• attached to connective tissue septa of the muscle (parallel with extrafusal)
• stretching the spindle initiates AP in afferent fibers from endings in the central region of the spindle

Question 8

describe Lower motor neurons

Answer 8

cell bodies in ventral horn of grey matter/

they only contract

Question 9

describe knee jerk reflex

Answer 9

hammer taps and stretches muscle spindle, relays message through sensory neuron

sensory neuron synapses with

1. extensor motor neuron
2. interneuron that inhibits flexor motor neuron

leg extends

this is UNILATERAL motion

Question 10

describe ARM stretch reflex

Answer 10

this is where the extensor is relaxed and the flexor is activated (like holding a glass of water)

Question 11

describe withdrawal reflex

Answer 11

ipsilateral: nociceptive sensory neuron synapses with three interneurons.
1. activate flexor
2.inhibit extensor
3. crosses and ascends to perceive pain
(flexor reflex)

for balance, interneuron synapses with contralateral to
1. activate extensor
2. relax flexor
(crossed extension reflex)

Question 12

describe the Golgi tendon organ

Answer 12

• sensitive to muscle contraction
• begins a reflex when stretched
• reflex promotes less contraction (negative feedback)

Question 13

what is autogenic inhibition? (Golgi)

Answer 13

autogenic inhibition

• relaxation of muscle in response to relatively high tension on the tendon
• rapid and discrete

Question 14

How does contraction of intrafusal muscles regulate muscle spindle responsiveness?

Answer 14

gamma motor neurons contract the intrafusal muscles to maintain sensitivity

muscle spindle afferents monitor stretch of membrane

Question 15

How is contraction of gamma motor neurons controlled?

Answer 15

alpha-gamma coactivation

gamma is controlled by descending fiber inputs