Motor Reflexes

Question 1

Difference between monosynaptic and polysynaptic reflexes

Answer 1

Monosynaptic = afferent fiber synapses directly onto efferent fiber; usually excitatory

Polysynaptic = afferent fiber synapses onto interneuron (either excitatory or inhibitory)

Question 2

Differences between reflex activity and volitional movement

Answer 2

Reflexes: precision motions in response to afferent stimuli mediated at all levels of CNS; rapid initiation, many elicited even during unconsciousness

Volitional movement: originates in cortical areas; longer onset latency d/t higher processing, requires conscious awareness

Question 3

Difference between the 2 types of lower motor neurons found in motor neuron pools of ventral horn

Answer 3

Alpha-motor neurons: innervate extrafusal muscle fibers (fibers that result in contraction)

Gamma-motor neurons: innervate intrafusal muscle fibers, a component of the muscle spindle

Question 4

What are muscle spindles?

Answer 4

4-8 specialized intrafusal muscle fibers surrounded by a capsule of CT that are highly specialized mechanosensory organs found in most skeletal muscles and provide proprioceptive info about muscle LENGTH

Question 5

2 types of intrafusal fibers found in muscle spindles

Answer 5

Nuclear bag fibers — sensory bag-like region that contains many nuclei (can be static or dynamic)

Nuclear chain fibers — central portion, containing chain-like array of nuclei

Question 6

2 types of sensory afferents found coiled around central part of muscle spindle

Answer 6

Group Ia afferents (annulospiral endings)

Group II afferents (flowerspray endings)

Question 7

2 types of y-motor neuron efferent fibers found in motor component that stimulates contraction of intrafusal fiber in muscle spindle

Which part of the muscle spindle do they innervate?

Answer 7

Dynamic y-motor neuron (innervates dynamic nuclear bag fibers)

Static y-motor neuron (innervates static nuclear bag fibers)

Question 8

Difference between group Ia afferents and group II afferents in terms of muscle spindle changes that they are sensitive to

Answer 8

Group Ia —sensitive to length of muscle and how fast the length is changing (this is the primary afferent)

Group II — sensitive ONLY to the length of stretch (this is the secondary afferent)

Question 9

T/F: within the spinal cord, y-motor neurons form an excitatory synapse directly on the homonymous muscle’s alpha motor neuron in the ventral horn

Answer 9

True; this is an unusual monosynaptic connection. Most sensory afferents do not directly contact a lower motor neuron

Note that they also form an excitatory synapse on GABAergic inhibitory interneurons that inhibit the antagonistic muscle (reciprocal innervation)

Question 10

The stretch reflex appears as a contraction of a stretched muscle, initiated by muscle spindles. Why do we need this?

Answer 10

Protects the muscle from tearing due to over-stretch

Question 11

Sequence of events in stretch reflex

Answer 11

1. Muscle stretch
2. Muscle spindle stretch
3. Increase discharge of spindle Ia afferents
4. Excitation of alpha-motor neurons
5. Muscle contraction increases to oppose stretch

Question 12

Other names for stretch reflex

Answer 12

Deep tendon reflex

Myotactic reflex

Question 13

The ____ of the myotatic reflex refers to the amount of muscle force generated in response to a given stretch of the intrafusal fibers

Answer 13

Gain

Question 14

The gain of the myotatic reflex refers to the amount of muscle force generated in response to a given stretch of the intrafusal fibers.

What happens when the gain is high vs. low?

Answer 14

If the gain is high, a small stretch produces large increase in alpha-motor neurons firing, leading to increased tension in the extrafusal fibers

If the gain is low, a greater stretch will be required to generate the same amount of tension in the extrafusal fibers

Question 15

What structure participates in an inverse myotatic reflex with autogenic inhibition, appearing as a sudden relaxation of a contracted muscle?

Answer 15

Golgi tendon organs (GTOs)

Question 16

Describe GTOs

Answer 16

Innervate tendons at junction with muscle

A bare nerve ending with many branches

Question 17

Tension in the tendon increases APs in the GTO. What type of afferent fiber is connected to GTOs?

Answer 17

Ib afferent fiber

Question 18

The inverse myotatic reflex appears as a sudden relaxation of a contracted muscle, why do we need this?

Answer 18

Protects the muscle from damage due to excessive force

Question 19

Order of events in inverse myotatic reflex with GTOs

Answer 19

1. Muscle contracts putting massive strain on tendon
2. Increased tension increases APs in Ib afferent fiber
3. Ib afferent synapses on inhibitory interneuron in SC
4. Inhibition of a-motor neuron
5. Muscle abruptly relaxes

Question 20

Is the GTO reflex monosynaptic or polysynaptic? What about the muscle spindle reflex for muscle stretch?

Answer 20

GTO reflex is polysynaptic (segmental) reflex

Muscle spindle reflex is monosynaptic, segmental reflex

Question 21

During the GTO muscle reflex, the Ib afferent synapses on an inhibitory interneuron in the SC. What else does it synapse on?

Answer 21

The alpha-motor neuron to the antagonist muscle, further opposing muscle activity

Question 22

What is the flexor reflex

Answer 22

Reflexive withdrawal from noxious stimuli

Question 23

The flexor reflex stimulates pain-sensing fibers and synapses on excitatory _______, which synapse on _________ that innervate flexor muscles required to withdraw from the stimulus

Inhibitory interneurons synapse on alpha-motor neurons that innervate antagonistic ____ muscles that would impede withdrawal from the stimulus

Simultaneously, flexion of the stimulate limb is accompanied by opposite reaction from the contralateral limb. This means that _____ muscles are stimulated while _____ muscles are inhibited in the contralateral limb

Answer 23

Interneurons; alpha-motor neurons

Extensor

Extensor; flexor [this is the crossed-extension reflex]

Question 24

An acute spinal cord injury resulting in loss of reflexes and sensation below the level of the injury, requiring a very gradual recovery partly due to loss of excitatory bias from CNS

Answer 24

Spinal shock

Question 25

____ = increased muscle tone, hyperactive stretch reflexes, and clonus

Answer 25

Spasticity

Question 26

Difference between spasticity and rigidity

Answer 26

Spasticity is velocity dependent while rigidity is not

Question 27

What type of rigidity is an ominous sign of severe brain damage, particularly above the superior colliculus? How does it present?

Answer 27

Decorticate rigidity

Arms flexed or bent inward on chest, hands clenched into fists, legs extended, feet turned inward

Question 28

How is decorticate rigidity dependent on head position?

Answer 28

If head is in neutral, affected leg is extended and affected arm is somewhat flexed

If you rotate the head in either direction, increases extensor tonus on that side and flexion of opposite arm

Question 29

What type of rigidity is an ominous sign of severe brain damage, particularly rostral to the pons? How does it present?

Answer 29

Decerebrate rigidity

Extensor rigidity and spasticity: head is arched back, arms extened at sides, legs extened, arms and legs internally rotated

Question 30

Decerebrate rigidity results in rigidity due to an increase in ________ excitability and spasticity due to an increase in ______ firing

Answer 30

Alpha-motor neuron

Gamma-motor neuron

[overall loss of descending suppression yet excitatory region remains active]