Reflexes Flashcards
What reflexes are cortical?
placing reaction
hopping reaction
What reflexes are spinal?
stretch (myotatic)
golgi tendon reflex
crossed extensor
What reflexes are brainstem/midbrain?
vestibular
righting reflex
suckle
yawn
eye/head movements
What can make reflexes look like volitional movement?
the preciseness of reflexes
but reflexes are faster
What is the difference in level of organization for reflexes and volitional mvt?
reflexes at any CNS level
volitional requires cortical and subcortical involvement
What is the differency in circuitry btw reflexes and volitional mvt?
reflexes = fixed
volitional = variable depending on motions
What is a muscle spindle
within skeletal muscle, embedded in fusiform capsule
parallel to muscle fibers
has both afferent and efferent components
What are the parts of intrafusal fibers?
sensory: nuclear bag fiber, nuclear chain fiber; not contractile - sensitive to length
motor portion
What does the 1a fiber do?
innervates both the nuclear bag and nuclear chain
large, myelinated
sensitive to both length and how fast length is changing
How does the firing of the 1a fiber change with stretch?
increases in frequency
gets even faster when speed of stretch increases
What is the secondary afferent nerve?
group 2 fiber = smaller, myelinated
innervates only the nuclear chain fiber
sensitive only to length
What innervates the motor portion of the intrafusal contractile element?
gamma motor neuron
controls the length of the sensory portion
How does is the sensitivity of the intrafusal fiber complex increased?
contractile intrafusal fibers contract –> stretches the sensory portion –> increases sensitivity of 1a and II fibers
In myotactic reflex, what does the 1a afferent fiber synapse on and what NT is used?
alpha motor neuron
EAA
How is antagonist muscle relaxed during a reflex?
1a afferent synapse on an interneuron –> releases inhibitory GABA on alpha motoneuron –> relaxes antagonist muscle
What does a golgi tendon reflex look like?
sudden relaxation of a contracted muscle –> protects from damage due to excessive force
How does a golgi tendon work?
bare nerve ending w/ lots of branches –> AP increases w/ tension –> 1b fiber to SC interneuron –> releases GABA on alpha motoneuron –> relaxes contracting muscle
*high threshold = need a lot of gaba to overcome muscle contracting
What turns the golgi tendon discharge rate back to normal?
abrupt relaxation of a muscle
How do higher centers of the brain affect reflexes?
often seen to be inhibitory
certain regions can be excitatory
What occurs in spinal shock?
transection of the spinal cord –> even though neurons producing reflexes below level of transection are intact, reflex fails to occur
recovery later bc of axonal sprouting and expression of receptors that are self-activating
What is decerebrate posturing?
results from loss of all structures rostral to pons, but caudal to the red nucleus
see regidity bc of continual activation of alpha-motoneurons (not clear why)
spacicity bc of activation of gamma-motoneurons
extended arms and legs
Why does cortical damage cause spasticity?
brainstem facilitory region activates gamma motoneurons –> makes m spindle more sensitive –> spontaneously active
need cortical activation –> brainstem inhibitory region –> stops reflexes
w/ loss of corex –> inhibitory region not activated –> + region dominates –> spasticity
Why does flexion of upper limb occur in decorticate posturing?
disinhibition of red nucleus and its control of UE flexors
Why does extension of the lower limbs occur in decorticate posturing?
disinhibition of reticulospinal and vestibulospinal pathways
What can cause unilateral decorticate posturing?
stroke in vecinity of internal capsule affecting one side
