Motor Control: reflexes Flashcards
In order to be effective, reflexes must be what?
precise
how can you tell the difference between a reflex and volitional movement?
speed (a reflex is much faster than the fastest volitional movement)
what is the purpose of a reflex?
direct and rapid response to sensory stimuli
what is the purpose of a volitional movement?
response to stimuli; but also need and desire
what is the initiation of a reflex?
sensory input
what is the initiation of a volitional movement?
higher cognition; sensory input
what is the appearance of the myotatic (stretch) reflex?
contraction (shortening) of a stretched muscle *most prominent during a passive stretch
what is the purpose of the myotatic (stretch) reflex?
to protect the muscle from tearing due to stretch (passive)
what is the myotatic (stretch) reflex initiated by?
muscle spindle
is the myotatic (stretch) reflex monosynaptic or polysynaptic?
monosynaptic
what does it mean to say that the myotatic (stretch) reflex is monosynaptic?
the sensory axon arising from the muscle spindle synapses directly onto the alpha-motor neuron
where is the muscle spindle found?
within the skeletal muscle, embedded in a fusiform capsule
what is the sensory element that detects muscle stretch?
the muscle spindle
what two sensors make up the sensory portion of the muscle spindle?
nuclear bag fiber and nuclear chain fiber
what is the primary afferent of the muscle spindle that innervates both the nuclear bag fiber and the nuclear chain fiber?
the Ia fiber
the Ia fiber changes its firing rate to what two things?
the length of the muscle and how fast the length is changing
What does the secondary afferent of the muscle spindle innervate?
only the nuclear chain fiber
what is the secondary afferent fiber?
group II fiber
what is the group II fiber sensitive to/ what will it change its firing rate to?
sensitive only to the length of the muscle (not to the speed it took to get there)
What is the motor portion of the muscle spindle (the intrafusal fibers) innervated by?
gamma motor neuron
what is the purpose of the motor portion of the muscle spindle?
it controls the length of the sensory portion of the muscle spindle
What happens when you contract the intrafusal muscles (the motor portion) of the muscle spindle?
it is going to lengthen/ stretch the sensory portion
What happens when the sensory portion of the muscle spindle is stretched?
it is going to be more sensitive to a superimposed stretch (increases the sensitivity of the Ia and II fibers to stretch)
What innervates the extrafusal fibers?
an alpha-motor neuron
What controls sensitivity of the muscle spindle?
the gamma motor neuron
Which motor neuron activity directly leads to motion?
alpha-motor neuron
within the spinal cord, the Ia afferent from the muscle spindle synapses where?
directly onto the alpha-motor neuron innervating the stretched muscle
what is the NT released when the Ia afferent synapses onto the alpha motor neuron?
EAA
what does the released EAA cause the alpha motor neuron to do?
contract the stretched muscle
What does contraction of the stretched muscle do?
it relieves the stretch, returning the Ia discharge rate back to normal
What happens to the antagonist muscle during the myotatic (stretched) muscle reflex?
it is actually getting stretched
Besides the alpha motor neuron, what else does the Ia afferent fiber branch to synpase onto?
an interneuron
Is the interneuron excitatory or inhibitory and what does it release?
inhibitory and releases GABA or glycine
what does the GABA/glycine have its inhibitory effects on?
the alpha motor neuron of the antagonist muscle
What happens to the antagonist muscle when exposed to GABA/glycine?
it will have fewer action potentials and therefore relax
what does the relaxation of the antagonist muscle allow it to do?
it allows it to stretch and go along with the reflex
what is the appearance of the golgi tendon reflex?
sudden (abrupt) relaxation of a contracted muscle
what is the purpose of the golgi tendon reflex?
to protect the muscle from damage due to excessive force
what is the golgi tendon reflex initiated by?
the golgi tendon organ
is the golgi tendon reflex monosynpatic or polysynaptic?
polysynaptic
What innervates tendons?
golgi tendon organ
What is the golgi tendon organ?
it is a bare nerve ending with multiple branches
As contraction of a muscle increases, what happens to the action potentials in the golgi tendon organ that is innervating the tendon of that muscle?
they increase as tension increases
what carries the information from the tendon to the spinal cord?
Ib fiber
what is the Ib afferent from the golgi tendon going to synapse onto?
an interneuron
what does the Ib afferent release when it synapses onto the interneuron?
EAA
is the interneuron excitatory or inhibitory?
inhibitory
what does the inhibitory interneuron release when it synapses onto the alpha motor neuron of the contracted muscle?
GABA
What happens when the alpha-motor neuron is inhibited by the activation of the spinal interneuron?
abrupt relaxation of the contracted muscle- returning the golgi tendon organ discharge rate back to normal
What is spinal shock?
loss of reflex below the level of the transection (lesion of the spinal cord)
recovery of reflexes are believed to be result from what?
axonal sprouting below the level of the transection and expression of receptor phenotypes that are self activating
is the brainstem facilitatory region spontaneously active or does it require activation?
it is spontaneously active
Normal reflexes depend on the balance of what two regions?
the brainstem facilitatory region and the brainstem inhibitory region
what does the brainstem inhibitory region require activation from?
cortical regions
in a patient with damage to the cortex, is the brainstem facilitatory region or the brainstem inhibitory region most affected?
the brainstem inhibitory region
what would uncontrolled brainstem facilitatory region result in?
hyperactive reflexes as well as stretch reflexes that fight any passive motion aka spasticity
What causes decerebrate posturing?
it results from loss of all structures rostral to the pons
What is true about all of the muscles in a patient with decerebrate posturing?
all of their muscles are extended
What is rigidity?
it is maintained muscle contraction; continual activation of the alpha-motor neurons
You have to apply what to see spasticity?
a passive motion
What is spasticity?
myotatic reflex is hyperactive and you have continual activation of the gamma motor neurons
what is the cause of rigidity?
there is a loss of cortical influence that inhibits a medullary input to the alpha-motor neurons
What does decorticate posturing look like?
flexion of the upper limb joints and extension of the lower limbs
what causes the flexion of the upper limb joints as seen in decorticate posturing?
disinhibition of the red nucleus and its control of the UE flexors
what causes the extension of the lower limbs seen in decorticate posturing?
disinhibition of reticulospinal and vestibulospinal pathways
What is decorticate posturing dependent on?
head position
What would cause unilateral decorticate posturing?
strokes in the vicinity of the internal capsule
