motor control: reflexes

Question 1

what is the purpose of a reflex

Answer 1

protection
corrective action w/o conscious thought
infants – need to feed, not voluntary

Question 2

characteristics of reflexes

Answer 2

involuntary
rapid
short acting
precise – must be in order to be effective

Question 3

purpose of myotatic reflex

Answer 3

prevent tearing of muscle due to stretch

Question 4

what initiates the myotatic (stretch) relfex

Answer 4

muscle spindle

monosynaptic, segmental reflex

Question 5

charcteristics of the sensory portion of a muscle spindle

Answer 5

not contractile
sensitive to length
composed of 2 fiber types(both innervated by 1a fiber)
    nuclear bag fibers
    nuclear chain fibers

Question 6

the 1a fiber (primary afferent) innervating both the nuclear bag and chain is sensitive to

Answer 6

length of m

how fast length is changing

Question 7

what type of fiber is the secondary afferent of the muscle spindle
what does it innervate
what is it sensitive to

Answer 7

group II fiber
innervates chain fiber only
sensitive only to length of m

Question 8

motor portion of the intrafusal fibers are innervated by

Answer 8

gamma motorneuron (both bag and chain)
control length of sensory portion
controls sensitivity of sensory portion of intrafusal fibers

Question 9

in a myotatic reflex, why is the antagonist m also stretched

Answer 9

primary afferent fibers (1a fibers) synapse on alpha motor neurons of agonist to contract, but also an interneuron that synapses with an alpha neuron for the antagonist m preventing its contraction

Question 10

purpose of golgi tendon reflex

Answer 10

protect muscles from damage due to excessive force

Question 11

what initiates the golgi tendon reflex

Answer 11

golgi tendon organ

polysynaptic, segmental reflex

Question 12

what type of fiber innervates the golgi tendon organ

Answer 12

type 1b fiber

APs increase w/ tension

Question 13

how does relaxation of contracting m occur in the golgi tendon reflex

Answer 13

type 1b fibers synapse w/ inhibitory interneurons which synapse w/ alpha motorneurons of contracting muscle
–> fewer APs/less tension in m

Question 14

what stimulates the myotatic reflex vs golgi tendon reflex

Answer 14

myotatic - passive stretch of a m.

golgi - active contraction of a m.

Question 15

decerebrate posture –

spasticity vs rigidity

Answer 15

spasticity - resists motion in a given direction
continual activation of gamma motorneurons
rigidity - resists motion in all directions
continual activation of alpha motorneurons

Question 16

brainstem fascilitory region

Answer 16

activates gamma motorneurons to make muscle spindle more sensitive to stretch
spontaneously active

Question 17

brainstem inhibitory region

Answer 17

inhibits gamma motorneurons making muscle spindle less sensitive to stretch
needs cortical regions to activate

Question 18

spasticity reflex

Answer 18

with loss of cortex, brainstem (-) region is not activated leading to unopposed brainstem (+) region activation –> spasticity and decerebrate posture?

Question 19

spinal shock

Answer 19

transection of SC
all reflexes gone
due to hyperpolarization of neurons from lack of
excitatory input from cortex

Question 20

decorticate posture

motor control - reflexes ppt

Answer 20

flex UE, extend LE
can be bilat or unilat
loss of cortical inputs - most common by lesion of IC

Question 21

decerebrate posture

motor control - reflexes ppt

Answer 21

contract all anti-gravity m. – ext. UE and LE
lesion caudal to red nucleus, but rostral to
reticulospinal and vestibulospinal nuclei
usually indicative of severe brain injury