Motor Reflexes

Question 1

What are examples of a cortical reflexes?

Answer 1

Placing reaction and hopping reaction

Question 2

What are examples of spinal cord reflexes?

Answer 2

Stretch/myotactic, golgi tendon reflex and crossed extensor

Question 3

What are examples of a brainstem/midbrain reflex?

Answer 3

Vestibular, righting reflex, yawn and eye/head movements

Last three can still occur in anencephalic infants

Question 4

What is the purpose of a reflex?

Answer 4

Protection, correct action without conscious thought, the need to respond quickly
Infants - need to do something but don’t have voluntary ability

Question 5

What are some characteristics of reflexes?

Answer 5

Involuntary, fast/quick, short acting, precise

Question 6

Describe the precision of reflexes

Answer 6

Required in order to be effective
Can make reflexes look like volitional movement
Difference between the two is reflexes are faster than volitional movement

Question 7

Compare the level of organization between a reflex and volitional motion

Answer 7

Reflex: any CNS level and doesn’t require presence of cortex for most
Volitional motion: requires cortical and subcortical involvement

Question 8

Compare the purpose for a reflex compared to volitional movement

Answer 8

Reflex: direct and rapid response to sensory stimuli

Volitional motion: response to stimuli, need, desire

Question 9

Compare the initiation of a reflex to volitional motion

Answer 9

Reflex: sensory input
Volitional: high cognition, sensory input

Question 10

Compare the circuitry of a reflex to volitional motion

Answer 10

Reflex: fixed
Volitional: variable depending on motion

Question 11

Compare the speed of a reflex to volitional motion

Answer 11

Reflex: faster than volitional
Volitional: variable depending on need

Question 12

What is the specificity for a reflex and volitional motion?

Answer 12

Both have high specificity

Question 13

What is the appearance of a myotactic (stretch) reflex?

Answer 13

Contraction/shortening of a stretched muscle back to its normal length

Question 14

What is the purpose of the myotactic reflex?

Answer 14

Protect muscles from tearing due to stretch

Question 15

What are the characteristics of a myotactic reflex?

Answer 15

Monosynaptic segmental reflex
Requires passive stretch of the muscle
Initiated by muscle spindle

Question 16

Describe a muscle spindle

Answer 16

Found within skeletal muscle embedded in a fusiform capsule
In parallel to the muscle fibers
Contains both afferent (sensory) and efferent (contractile) components

Question 17

The capsule of the muscle spindle is fusiform, the fibers within the capsule are called what?

Answer 17

Intrafusal fibers

Question 18

Describe characteristics of the sensory component of the muscle spindle

Answer 18

Is not contractile
Is the portion sensitive to length
Is actually two sensors with different afferents including nuclear bag fiber and nuclear chain fiber
Innervated by primary and secondary afferents

Question 19

What are the primary afferents to the sensory component of the muscle spindle?

Answer 19

Via Ia fiber which innervates both the nuclear bag and nuclear chain
Large myelinated Ia fiber (high conduction velocity, low threshold)
Sensitive to both the length of the muscle and how fast the length is changing

Question 20

Describe the secondary afferents to the sensory component of the muscle spindle

Answer 20

Via smaller myelinated group II fiber which innervates the nuclear chain fiber only
Sensitive to only the length of the muscle

Question 21

Within the SC the Ia afferent from the muscle spindle synapse directly onto what?

Answer 21

The alpha motor neuron innervating the stretched muscle
The motor neuron is excited by the activation of the Ia afferents -> contraction relieves the stretch returning the Ia discharge rate back to normal

Question 22

What occurs in the antagonist muscle during the myotactic reflex?

Answer 22

Stretches during this reflex
Ia afferent branches and synapses onto an interneuron of the same segment which is inhibitory (GABA or glycine) —> causes ipsp’s in the antagonist muscle (relaxation) while the agonist contracts

Question 23

Describe alpha motor neurons

Answer 23

Large heavily myelinated fibers
Innervate skeletal muscle via NMJ (extrafusal fibers)
Responsible for activating muscle
Activity directly leads to motion

Question 24

Describe gamma motor neurons

Answer 24

Slightly smaller and slower than alpha but still fast overall
Innervates the contractile component (intrafusal fibers) of the muscle spindle via NMJ
Activity causes contraction but does not directly lead to motion
Controls sensitivity of muscle spindle

Question 25

What is the appearance of the golgi tendon reflex?

Answer 25

Sudden/abrupt relaxation of contracted muscles

Question 26

What is the purpose of the golgi tendon reflex?

Answer 26

Protect muscle from damage due to excessive force

Question 27

What are some characteristics for the golgi tendon reflex?

Answer 27

Requires active contraction of muscle Initiated by the golgi tendon organ Polysynaptic segmental reflex

Question 28

Describe the golgi tendon organ

Answer 28

Innervates tendons Bare nerve ending with lots of branches APs increase with tendon (generated by contraction of muscle) Ib fiber taking into the spinal cord (slightly smaller diameter than Ia, slight decrease in myelin, slower conduction velocity but higher threshold)

Question 29

Describe the pathway for the golgi tendon reflex

Answer 29

Ib afferent from golgi tendon release EAA —> synapses on GABA-nergic interneuron -> synapse with alpha motor neuron of contracting muscles -> motor neuron is inhibited by the activation of the interneuron -> abrupt relaxation of the muscle occurs returning the golgi tendon organ discharge rate back to normal

Question 30

Describe central modification

Answer 30

The higher centers of the brain are often seen to be inhibitory to the reflexes but certain regions do provide an excitatory input + = alpha motor neuron - = nociceptors and mechanoreceptors

Question 31

Describe the brainstem facilitatory region

Answer 31

Is spontaneously active | By activating the gamma motor neurons the muscle spindle becomes more sensitive

Question 32

Describe the brainstem inhibitory region

Answer 32

Requires activation from cortical regions | Inhibits gamma motor neurons making the muscle spindle less sensitive

Question 33

Describe spasticity

Answer 33

With the loss of the cortex, the brainstem inhibitory regions are not activated leaving the brainstem facilitatory region to dominate The end result of the uncontrolled brainstem facilitatory region is stretch reflexes that fight any passive motion (spasticity)

Question 34

When a spinal transection occurs, even though the neurons producing the reflex are below the level of the transection (and therefore intact) what happens to the reflex?

Answer 34

The reflex fails to occur (spinal shock)

Question 35

What causes spinal shock?

Answer 35

Hyperpolarization of spinal neurons due to loss of excitatory input from cortex

Question 36

Recovery of reflexes can sometimes occur after a spinal transection and this is believed to result from what?

Answer 36

Axonal sprouting below the level of the transection | Expression of receptor phenotypes that are self activating (5HTC receptor)

Question 37

Decerebate posturing is the result from what?

Answer 37

Loss of all structures rostral to the pons (caudal to red nucleus)

Question 38

Describe the rigidity that occurs in decerebate posturing

Answer 38

Resists motion in all directions Results from maintained muscle contraction Continual activation of alpha motor neurons By the brainstem

Question 39

Describe spasticity during decerebate posturing

Answer 39

Resists motion in a given direction Myotactic reflex hyperactive Continual activation of gamma motor neurons via brainstem which contracts intrafusal muscle and lengthens muscle bag and chain

Question 40

Describe decorticate posturing

Answer 40

Flexion of the upper limb joints (disinhibition of red nucleus and its control of UE flexors) Extension of the lower limbs (disinhibition of reticulospinal and vestibulospinal pathways) Dependent on head position (release of postural reflexes)

Question 41

Describe unilateral decorticate posturing

Answer 41

Results from stroke in the vicinity of the internal capsule | A relatively common site for a stroke