Phys - Motor Reflexes

Question 1

what part of the CNS controls the placing and hopping reactions

Answer 1

cortex

Question 2

describe the placing reaction

Answer 2

example: cat putting paws in weight bearing mode when you hold is over a counter

Question 3

describe the hopping reaction

Answer 3

the hop you do when someone pushes you to the side

Question 4

what part of the CNS controls the righting reflex

Answer 4

brainstem/midbrain

Question 5

what part of the CNS controls the suckle reflex

Answer 5

brainstem/midbrain

Question 6

what part of the CNS controls the yawn reflex

Answer 6

brainstem/midbrain

Question 7

what part of the CNS controls stretch (myotatic reflex)

Answer 7

spinal cord

Question 8

what part of the CNS controls golgi tendon reflex

Answer 8

spinal cord

Question 9

compare the level of organization from the CNS between a reflex and volitional motion

Answer 9

reflex: any CNS level, does not require cortex for most
volitional: requires cortical and subcortical involvement

Question 10

compare the circuitry between reflex and volitional movement

Answer 10

reflex: fixed
volitional: variable depending on motion

Question 11

purpose of the myotatic reflex (stretch)

Answer 11

contracting of a stretched muscle to protect muscle from tearing

Question 12

where are the extrafusal and intrafusal fibers of the muscle spindle

Answer 12

intrafusal: within the capsule
extrafusal: the fibers that make up the bulk of the muscle

Question 13

describe the sensory portion of a muscle spindle

Answer 13

• not contractile
• sensitive to length
• consists of nuclear bag fiber and nuclear chain fiber

Question 14

what innervates the nuclear bag and nuclear chain of the muscle spindle (primary afferent)

Answer 14

1a fibers

Question 15

describe 1a fibers

Answer 15

large, myelinated, fast

- innervates sensory portion of muscle spindle (nuclear bag and chain)

Question 16

what are 1a fibers sensitive to

Answer 16

length of muscle and rate at which the length is changing

Question 17

what do group 2 fibers innervate

Answer 17

secondary afferent parts of muscle spindle (only the nuclear chain)

Question 18

what are group 2 fibers sensitive to

Answer 18

only the length of muscle (not the rate of change in length)

Question 19

compare group 1a fibers and group 2 fibers

Answer 19

group 1a: heavily myelinated, faster, low threshold

group 2: smaller diameter, less myelin, still fast but not as fast

Question 20

what innervates the motor portion of the muscle spindle (intrafusal contractile elements)

Answer 20

gamma motorneuron

Question 21

how does the motor portion of the muscle spindle affect the sensory portion

Answer 21

motor portion controls the length of the sensory portion (and its sensitivity)

Question 22

how does contraction of the intrafusal fibers affect length of muscle spindle and length of sensory portion

Answer 22

does not change the length of the muscle spindle but shortens the intrafusal fibers which stretches the sensory portion, increasing sensitivity of the 1a and group II fibers to stretch

Question 23

compare structures of alpha and gamma motor neurons

Answer 23

alpha: large, heavily myelinated
gamma: slightly smaller, slower, still fast though

Question 24

compare function of alpha and gamma motor neurons

Answer 24

alpha: innervates skeletal muscle (extrafusal fibers) –> leads to direct motion
gamma: innervates intrafusal fibers –> does NOT lead to direct motion

Question 25

where do the 1a fibers synapse after leaving the muscle spindle

Answer 25

directly onto alpha motor neuron innervating the stretched muscle

Question 26

after the 1a fibers synapse onto the alpha motor neurons, what occurs

Answer 26

the motoneuron causes contraction of muscle to relieve the stretch and the 1a fiber’s discharge rate returns to normal

Question 27

when the 1a fiber synapses onto the alpha motor neuron, it causes contraction of that muscle that was being stretched. Describe how this contraction also causes stretching in the antagonist muscle

Answer 27

the 1a fiber also synapses onto an interneuron that inhibits the contraction of the antagonist muscle, causing it to relax and thus stretch
(inhibition is mediated by GABA)

Question 28

what reflex protects muscles from damage due to excessive force

Answer 28

golgi tendon organ

Question 29

describe the appearance of a golgi tendon organ reflex

Answer 29

a sudden relaxation of a contracted muscle to prevent from damage

Question 30

what innervates the golgi tendon organ

Answer 30

1b fibers

Question 31

how does tension change the action potentials of the golgi tendon organ

Answer 31

actions potentials increase with tension

Question 32

describe the pathway of how the golgi tendon reflex causes sudden relaxation

Answer 32

1b fiber from the golgi tendon organ synapses onto inhibitory interneuron that releases GABA to the alpha motor neuron of the contracting muscle, causing fewer action potentials and thus less tension

Question 33

describe spinal shock

Answer 33

after transection of the spinal cord, even though neurons producing a reflex are below the transection, the reflex still fails to occur

Question 34

how are reflexes regained after loss from spinal shock

Answer 34

• axonal sprouting occurs below the level of transection

- the 5HTC receptor is self-activating so it can cause depolarizations without input from areas above the transection

Question 35

how does decerebrate posturing occur

Answer 35

loss of all structures rostral to the pons and caudal to red nucleus

Question 36

compare rigidity and spasticity

Answer 36

rigidity: resists motion in all directions, continual activation of alpha motor neurons, maintained muscle contraction
spasticity: resists motion in a given direction, continual activation of gamma motor neurons, hyperactive myotatic reflex

Question 37

what is the function of the brainstem facilitatory region

Answer 37

activates gamma motor neurons and makes the muscle spindle more sensitive and is spontaneously active

Question 38

function of brainstem inhibitory region

Answer 38

inhibits gamma motor neurons making the muscle spindle more sensitive; not spontaneously active (needs to be activated from cortical and cerebellar regions)

Question 39

why does spasticity and decerebrate posture occur

Answer 39

in decerebration with damage to the red nucleus and pons, there is damage to a brainstem inhibitory region but the brainstem facilitatory region is still intact –> stretch reflexes that fight any passive movement (spasticity)

Question 40

how does head position affect decorticate posturing

Answer 40

the degree of flexion in the arms is dependent on head positioning

Question 41

in what patients would you see unilateral decorticate posturing

Answer 41

from strokes in the internal capsule

Question 42

most common cause of decorticate posturing

Answer 42

lesion of internal capsule