Reflexes

Question 1

what are somatic reflexes?

Answer 1

automatic stereotypic motor responses by voluntary muscles to adequate sensory stimuli

Question 2

what classifies a somatic reflex?

Answer 2

1: # of synapses:
- monosynaptic
- polysynaptic

2: Level of neuraxis involved:
- intra segmental (1 level of SC (few segments))
- inter segmental (many spinal segments)
- supra segmental (above SC - reflex has to go up to the brain and then all the way back down again before the body reacts)

Question 3

what is the 1 monosynaptic reflex?

Answer 3

stretch reflex

Question 4

what triggers a stretch reflex?

Answer 4

by any stretching of the tendon (manually moving the joint or using a reflex hammer)

Question 5

what is the stretch reflex also known as?

Answer 5

myotatic reflex

Question 6

what is a stretch reflex & reciprocal inhibition?

Answer 6

a segmental reflex

stretching a single muscle spindle induces contraction limited to the part of the muscle containing the spindle (inhibitory interneuron–> alpha motor neuron to antagonist muscle)

Question 7

explain stretching the biceps as a stretch reflex?

Answer 7

stretch biceps tendon; impulse goes up Ia afferent

Ia goes to motor neuron to biceps to excite muscle

collateral comes off Ia afferent in SC to Ia inhibitory interneuron

alpha motor neuron goes to triceps inhibited (Ia reciprocal inhibition)

Question 8

describe spasticity during stretch reflex

Answer 8

increase in tonic reflex

**when working with a patient with spasticity– move slowly!!!

Question 9

tonic vibration reflex:

Answer 9

put vibrator on bicep; vibraion causes ms to contract within 15-20 seconds. remove vibrator, ms relaxes after a few seconds

Question 10

what is the flexor reflex?

Answer 10

mainly a cutaneous reflex
Polysynaptic
Intersegmental

protects from noxious stimuli- “withdrawal reflex”

the stronger the stimulus the more motor neurons recruited (more muscles) to withdraw limb from stimulus (usually a flexion motion); antagonist is also inhibited (mostly extensors)

pain receptor sends up stimulus (group III afferents) to dorsalateral fasiculus (lamina 1-3), goes to interneuron (all excitatory) then to alpha motor neuron of flexor muscles

II, III and IV sensory fibers (III and IV- pain)

Question 11

what is the crossed extensor reflex?

Answer 11

contralateral system during flexor/withdrawal reflex
(so we don’t fall)

works for both upper and lower body; but we can modulate the stimulus especially for the upper body (in a SC injury, you would always see the flexor and crossed extensor reflexes bc they can’t modulate the signal)

inhibitory interneuron at end of chain to inhibit alpha motor neuron of antagonist

Intersegmental reflex

goes up and down SC to respective level
Crosses over through the anterior white commissure

Question 12

what is the alpha-gamma loop?

Answer 12

?

Question 13

what is an example of a stretch reflex?

Answer 13

deep tendon reflex

Question 14

what is a DTR?

Answer 14

a fractional manifestation of the stretch reflex

tested by tapping the tendon of a muscle

this stretching stimulates the sensory endings on the intrafusal fibers, particularly the primary afferents (Ia). –> Ia monosynaptically stimuate the alpha motoneurons that innervate that muscle and polysynaptically inhibit the alpha motoneurons of the antagonist muscle –> consequently the agoinst muscle suddenly contracts and the antagonist relax

Question 15

what are other names for autogenic inhibition?

Answer 15

GTO reflex

non-reciprocal inhibition

Question 16

what is autogenic inhibition?

Answer 16

inhibition is happening in the agonist muscle instead of the antagonist muscle

protective mechanism (example: protect against muscle tear when lifting a heavy load

Question 17

how does autogenic inhibition work?

Answer 17

rigorous firing of the afferent fibers innervates GTOs –> GTO receptors respond primarily to muscle contraction (stimulus needs to be strong enough to trigger Ib inhibitory interneurons)

GTO Ib afferent fibers project to the SC and polysynaptically inhibit the alpha motoneuron innervating the agonist (homologous) muscle and sends off collateral to facilitate motoneurons of the antagonist muscle

GTOs provide continuous and accurate info about muscle force, which both segmental and supraspinal pathways use to control movement

*joint afferents and cutaneous afferents also connect with Ib inhibitory interneuron to inhibit homologous muscle

Question 18

what is another name for recurrent inhibition?

Answer 18

renshaw cell inhibition

Question 19

what is a renshaw cell?

Answer 19

located in the ventral horn only

= an inhibitory neuron
self regulating

when inhibited, it cancels out and actually facilitates= Disinhibition (very common in NS)

Renshaw cells are controlled by descending pathways (without this control, a person can develop spasticity)

Question 20

how does recurrent inhibition work?

Answer 20

alpha motor neuron has collateral that goes to Renshaw cell and facilitates it. Renshaw cell inhibits the alpha motor neuron that started firing in the first place –>inhibiting itself. (inhibit doesn’t mean “stop”, but slowing it down – keeps system from taking off, so you have some control. NEGATIVE FEEDBACK LOOP

renshaw cell also sends off fiber to Ia inhibitory interneuron and inhibits that (disinhibits)
Ia inhibitory interneuron sends fiber to motor neuron of antagonist. because it has been disinhibited, it facilitates that antagonist motor neuron and causes it to fire

Question 21

what is pre-synaptic inhibition?

Answer 21

a proprioceptive reflex of Ia fibers

over-riding mechanism

Question 22

how does pre-synaptic inhibition work?

Answer 22

Ia’s from periphery give off a collateral to a Ia facilitory interneuron (reciprocal interneuron) which sends a fiber back to motor neuron synapse; decrease potential needed to cause an action potential= less neurotransmitter gets released= inhibitory affect

when Ia afferent comes up to SC and synapses with a motor neuron, a neurotransmitter is released (ACh) and must be a certain amount to get motor neuron to reach threshold. Fiber from presynaptic interneuron decreases the amount of Ach released from the Ia afferent fiber

descending sytems are controlling interneuron to help regulate/inhibit itself
AP is tiny by the time it gets to the synapse

Ia afferent splits, creates pre-synaptic interneuron and reconnects to form motor neuron; causes an inhibition effect by decreasing the amount of neurotransmitter available to neuron–> doesn’t reach threshold (causes a depolarizing effect)

Question 23

what is a central pattern generator?

Answer 23

neural circuits that produce self-sustaining patterns of behavior in the absence of sensory input

generates a rhythmic pattern of motor activity

responsible for the basic pattern however usually modified by sensory info and other CNS centers

controlled by higher centers but can by modulated by afferent input (curb, broken glass, etc)

in brain stem and SC

Question 24

what are some examples of central pattern generators?

Answer 24

respiration, walking, swimming, etc

Question 25

what are “half centers”?

Answer 25

proposed by Graham Brown in 1911

flexor and extensor muscles were controlled by 2 systems of neurons termed half centers

Question 26

what is proprioceptive modification and stance regulation?

Answer 26

afferents from flexor stretch receptors initiate swing phase

sensory nerves are coming in and modulating the CPG system

Question 27

what are functions of cutaneous effects?

Answer 27

detection of external objects

adjusts stepping movements to avoid them

Question 28

what are some examples of cutaneous effects?

Answer 28

mechanical stim to dorsal foot during swing phase of gait causes excitation of flexors MNs and inhibition to extensor MNs –>leading to rapid flexion of foot away from the stimulus and elevation of the leg

identical stimulus given during stance phase however causes excitation of extensors that reinforces ongoing extensor activity

called phase-dependent reflex reversal **