Phys - Motor Reflexes Flashcards
what part of the CNS controls the placing and hopping reactions
cortex
describe the placing reaction
example: cat putting paws in weight bearing mode when you hold is over a counter
describe the hopping reaction
the hop you do when someone pushes you to the side
what part of the CNS controls the righting reflex
brainstem/midbrain
what part of the CNS controls the suckle reflex
brainstem/midbrain
what part of the CNS controls the yawn reflex
brainstem/midbrain
what part of the CNS controls stretch (myotatic reflex)
spinal cord
what part of the CNS controls golgi tendon reflex
spinal cord
compare the level of organization from the CNS between a reflex and volitional motion
reflex: any CNS level, does not require cortex for most
volitional: requires cortical and subcortical involvement
compare the circuitry between reflex and volitional movement
reflex: fixed
volitional: variable depending on motion
purpose of the myotatic reflex (stretch)
contracting of a stretched muscle to protect muscle from tearing
where are the extrafusal and intrafusal fibers of the muscle spindle
intrafusal: within the capsule
extrafusal: the fibers that make up the bulk of the muscle
describe the sensory portion of a muscle spindle
- not contractile
- sensitive to length
- consists of nuclear bag fiber and nuclear chain fiber
what innervates the nuclear bag and nuclear chain of the muscle spindle (primary afferent)
1a fibers
describe 1a fibers
large, myelinated, fast
- innervates sensory portion of muscle spindle (nuclear bag and chain)
what are 1a fibers sensitive to
length of muscle and rate at which the length is changing
what do group 2 fibers innervate
secondary afferent parts of muscle spindle (only the nuclear chain)
what are group 2 fibers sensitive to
only the length of muscle (not the rate of change in length)
compare group 1a fibers and group 2 fibers
group 1a: heavily myelinated, faster, low threshold
group 2: smaller diameter, less myelin, still fast but not as fast
what innervates the motor portion of the muscle spindle (intrafusal contractile elements)
gamma motorneuron
how does the motor portion of the muscle spindle affect the sensory portion
motor portion controls the length of the sensory portion (and its sensitivity)
how does contraction of the intrafusal fibers affect length of muscle spindle and length of sensory portion
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
compare structures of alpha and gamma motor neurons
alpha: large, heavily myelinated
gamma: slightly smaller, slower, still fast though
compare function of alpha and gamma motor neurons
alpha: innervates skeletal muscle (extrafusal fibers) –> leads to direct motion
gamma: innervates intrafusal fibers –> does NOT lead to direct motion
where do the 1a fibers synapse after leaving the muscle spindle
directly onto alpha motor neuron innervating the stretched muscle
after the 1a fibers synapse onto the alpha motor neurons, what occurs
the motoneuron causes contraction of muscle to relieve the stretch and the 1a fiber’s discharge rate returns to normal
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
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)
what reflex protects muscles from damage due to excessive force
golgi tendon organ
describe the appearance of a golgi tendon organ reflex
a sudden relaxation of a contracted muscle to prevent from damage
what innervates the golgi tendon organ
1b fibers
how does tension change the action potentials of the golgi tendon organ
actions potentials increase with tension
describe the pathway of how the golgi tendon reflex causes sudden relaxation
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
describe spinal shock
after transection of the spinal cord, even though neurons producing a reflex are below the transection, the reflex still fails to occur
how are reflexes regained after loss from spinal shock
- 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
how does decerebrate posturing occur
loss of all structures rostral to the pons and caudal to red nucleus
compare rigidity and spasticity
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
what is the function of the brainstem facilitatory region
activates gamma motor neurons and makes the muscle spindle more sensitive and is spontaneously active
function of brainstem inhibitory region
inhibits gamma motor neurons making the muscle spindle more sensitive; not spontaneously active (needs to be activated from cortical and cerebellar regions)
why does spasticity and decerebrate posture occur
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)
how does head position affect decorticate posturing
the degree of flexion in the arms is dependent on head positioning
in what patients would you see unilateral decorticate posturing
from strokes in the internal capsule
most common cause of decorticate posturing
lesion of internal capsule
