13. Motor Control Reflexes Flashcards
What are the types of motor control reflexes? What do they control?
Cortical, Brainstem/midbrain, spinal reflexes
What is the purpose of a reflex?
Protection + Correction action w/o constant thought + need to respond quickly + infants (have need to feed but don’t have voluntary ability)
What is the difference bw reflex activity and volitional movement?
SPEED!
Reflex is FASTER than fastest volitional movement
What are the cortical responses? Are they voluntary?
Placing reaction- place in weight bearing position
Hopping reaction- hop in direction pushed
NOT voluntary
What are the Brainstem/midbrain?
Vestibular
Righting reflex- head goes back up to keep you from falling when you stumble
Suckle
Yawn
Eye/head movement to turn to sound
What are the spinal reflexes?
Ones you typically think of! DTRs
Stretch (myotatic)
Golgi tendon reflex
Crossed extensor
What are some characteristics of reflexes?
Involuntary, fast/quick, short-acting, precise
What is the myotatic reflex? What is the appearance and purpsose?
Contraction (shortening) of stretched muscle
Protects muscle from tearing due to stretch
Initiated by muscle spindle
Monosynaptic, segmental reflex (directly synapses on efferent; info goes clearly up to brain!)
What are intrafusal fibers?
Fibers within skeletal muscle that are embedded in fusiform capsule (encapsulated)
Parallel to muscle fibers
Contain both afferent and efferent components in the muscle spindle
What is the bulk of the muscle fibers of myotatic reflex called?
Extrafusal fibers- everything outside intrafusal basically
What are the characteristics of the sensory portion of the muscle spindle of the myotatic reflex?
Not contractile, portion sensitive to length
Actually TWO sensors with different afferents
What are the two sensors of the sensory portion of the myotatic reflex?
Nuclear bag fiber
Nuclear chain fiber
What innervates the nuclear bag and nuclear chain of the sensory system of the myotatic reflex?
large, myelinated 1a fiber (primary afferent)
What is the primary afferent fiber sensitive to?
BOTH length of muscle (static) and how fast length changing (dynamic)
How does the myotatic reflex work?
at resting length, it fires a certain discharge; always have some sort of activity of 1a fiber and when stretch muscle, see length increases and number AP increase in proportion to change of length and rate of change
How is the secondary afferent different than the primary afferent fiber?
Secondary afferent is a smaller, less myelinated group 2 fiber that ONLY innervates the nuclear chain fiber
It is sensitive ONLY to length of muscle (static only, no dynamic component)
What are the characteristics of the motor portion of the myotatic reflex?
same histology as skeletal muscle
consists of intrafusal contractile elements
innervated by GAMMA motoneuron
control LENGTH of sensory portion
What innervates the intrafusal muscle fibers of myotatic reflex? what happens when they are contracted?
gamma-motoneuron
when contracted, they stretch the sensory portion of the myotatic reflex–> renders sensory portion more sensitive to a superimposed stretch
How do gamma motoneurons control sensitivity?
even though overall length of muscle spindle remains same, contraction of intrafusal fibers stretches sensory portion and increases sensitivity of 1a and 2 fibers to stretch
use both nuclear bag and chain!
Within the spinal cord, where does the 1a afferent from muscle spindle synapse? What happens when it synapses?
directly onto the alpha-motoneuron innervating the stretched muscle!
NT released here as EAAs!!
How does the myotatic reflex go back to normal?
after motoneuron is excited by activation of 1a afferent, contraction relieves the stretch, returning the 1alpha discharge rate back to normal
What happens to the antagonist muscle in the myotatic reflex?
it gets stretched!!
if don’t do anything, get counter stretch reflex in it and so need to relax that muscle (do it by reciprocal inhibition)
How are interneurons coming into play in the myotatic reflex?
the 1a afferent can send its normal signaling to the alpha motoneuron to make action potentials but also can send a branch to another neuron in the psinal cord (interneuron), which then synapses on different alpha motoneurons–> innervate antagonis m.
excitation–> activates interneuron which releases INHIBITION (mediated by GABA)–>relaxation and lengthening of antagonist m.
What is the golgi tendon reflex in appearance, purpose, and characteristics?
INVERSE MYOTATIC/AUTOGENIC inhibition
appearance: sudden (abrupt) relaxation of a contracted muscle
purpose: protect muscle from damage due to an excessive force
characteristics: initiated by Golgi tendon organ; polysynaptic, segmental reflex!!
What is the role of golgi tendon organs?
innervate tendon
bare nerve ending with LOTS of branches
APs INCREASE with tension/force
What is the innervation of golgi tendon organs? what does it lead to?
1b fibers to spinal cord (smaller and slower fibers)
1b afferent from golgi tendon will synapse on interneuron (inhibitory releasing GABA)–> inhibit alpha motoneuron of contracting muscle
voluntary telling alpha motoneuron that need to contract muscle
but this is HIGH threshold! (need enough Cl- and GABA to counter Na thats entering)
This leads to fewer APs and less tension in muscle
What is the overall concept of golgi tendon organ reflex?
motoneuron is inhibited by activation of spinal interneuron–> abrupt relaxation of muscle occurs, returning golgi tendon organ discharge rate back to normal
What is central modification? why is it important?
higher centers of brain are often seen to be inhibitory to reflexes, but certain regions do provide an excitatory input!
(nociceptors/mehanoreceptors = -
alpha motoneurons= +
What is spinal shock? why might it occur?
an example of excitatory influence
even though the neurons producing the reflex are BELOW level of transection, the reflex fails to occur!! (so even if circuitry intact)
HOW: by cutting spinal cord, weve lost all normal tone coming down from brain; alpha motoneurons super hyperpolarized and EPSP still happening but cant get up to threshold
What is recovery of reflexes believed to result from in spinal shock?
- axonal sprouting BELOW level of transection- spread branches to INNERVATE eachother
- expression of receptor phenotypes that are self-activating (5HTC reeptor); helps reestablish circuits
What is decerebrate posturing?
results from loss of ALL structures ROSTRAL to pons (caudal to red nucleus)
get two distinct components: RIGIDITY and PLASTICITY
What is rigidity?
resists motion in ALL directions
results from maintained muscle contraction
continual activation of alpha-motoneurons (by brainstem)
maybe bc of anti-gravity muscles (havent been bipedal long enough)
What is spasticity?
resist motion in GIVEN direction; stretch reflexes that fight ANY passive motion
myotatic reflex is HYPERactive
continual activation of GAMMA-motoneurons
contrasts intrafusal muscle
lengthens nuclear bag/chain fibers
brainstem involved
In what ways can the activity of gamma neurons be controlled?
BRAINSTEM FACILITATORY region- by activating gamma motoneurons, makes muscle spindle MORE sensitive; region is also spontaneously active
BRAINSTEM INHIBITORY region- inhibits gamma motoneurons, making muscle spindles LESS sensitive; requires activation from cortical regions
What happens to spasticity if lose cortex?
brainstem (-) region is NOT activate, leaving only facilitatory/+ region to dominate; get hyperactive reflexes –> spasticity
(basically when damage rostral/above red nucleus)
end result = stretch reflexes that fight any passive motion
What does decorticate posturing look like and what is dependent on?
flexion of upper limb joints (disinhibition of red nucleus and its control of UE flexors)
Extension of lower limbs (disinhibition of reticulospinal and vestibulospinal pathways)
dependent on head position (release of postural reflexes)
What are you likely to see in decorticate posturing? what does it result from usually?
see patients with unilateral decorticate posturing
results from stroke in vicinity of internal capsule
relatively common site for stroke (internal capsule)
*red nucleus intact ?
