14. Motor Control Reflexes Flashcards
What are two cortical reflexes?
placing reaction
hopping reaction
What are 3 spinal reflexes?
stretch (myotatic)
golgi tendon reflex
crossed extensor
What are 4 brainstem/midbrain reflexes?
vestibular
righting reflex (keeps from falling over)
suckle
yawn
What are the main purposes of a reflex? 3
protection
corrective actions
quick response
What are 4 characterisitcs of a motor controlled reflex?
involuntary
fast
short-acting
precise
What is needed for reflexes to be effective?
precision and SPEED
What is the main difference of vilitional motion compared to reflexes?
Volitional requires cortical and subcortical involvement and is at a slower speed
Myotatic/stretch reflex is shortening of a stretched muscle to protect muscles from tearing. What are two characteristics?
initiated by muscle spindle
monosynaptic (aff synapses on efferent)
Muscle spindle is within and parallel skeletal muscle in a fusiform capsule, and has what 2 components?
afferent (sensory) and efferent (contractile) components
The fibers outside the fusiform capsule are called extrafusal fibers (actin/myosin) and inside are called?
intrafusal fibers
The intrafusal fibers have motor (lateral) and sensory (medial) fibers. The sensory portion of intrafusal fibers/muscle spindle is not contractile and has what twoparts?
nuclear bag fiber
Nuclear chain fiber
both have different afferents, sensitive to length
The primary afferent that innervates both bag and chain, sensitive to static/dynamic or length and how fast its changing is?
1a fiber large and myelinated
The 1a fiber has a slow basal firing rate to manage stretch. What occurs during stretch?
firing rate increases in the 1a fiber, and even fasting during a FAST stretch
The secondary afferent is smaller and myelinated. What does it innervated and what is it sensitve to?
Group 2/II fiber innervates nuclear chain fiber
sensitive to length only
What is the difference between primary and secondary afferents?
primary are 1a fibers, innervates both, and detects length and change in length
secondary are 2 fibers, innervates chain only and detects length (static) only
The intrafusal contractile elements are innervated by ____ which controls what?
gamma motoneuron (not alpha) control length of sensory portion
By contracting the intrafusal muscles, we stretch the sensory portion which leads to what?
increases the sensitivity of the Ia and II fibers to stretch
What is the main difference between alpha and gamma motoneurons?
Alpha is innervation to extrafusal leading to motion
Gamma is innervation to intrafusal, causing contraction
The 1a (and 2) afferent fibers goes to SC via dorsal root and synapse directly onto alpha motoneuron, using what NT and causing what?
using EAA, causing contraction of the extrafusal muscles via Alpha motoneuron, allowing discharge firing rate to go back to normal
When the stretch reflex is elicited, what happens to the antagonist muscle?
it gets stretched
To allow the antagonistic muscle to relax/stretch during myotatic reflex, the 1a afferent fiber also synapses on an interneuron of the antagonist M which does what?
releases GABA, decreasing APs, inhibiting the Alpha motoneuron of the antagonist M
Golgi tendon reflex AKA inverse myotatic, appears during sudden relaxation of a contracted muscle to protect?
muscle from damage due to excessive force
What is characteristic of the golgi tendon reflex? (2)
initiated by golgi tendon organ GTO
polysynaptic - many synapses
GTOs innervate tendons via bare nerve endings. APs increase with tension, which is relayed via what to the SC?
1b fiber (slightly smaller and slower)
What is the pathway for the GTO reflex?
- 1b afferent fiber from GTO in tendon goes to SC
- synapses on interneuron of contraction muscle
- Interneuron releases GABA, causing fewer APs and less tension in the muscle
What is the main difference between myotatic and golgi tendon reflexes?
myotatic prevents stretch of muscle
GTO prevents damage from over contraction
Most of input from the brain is inhibitory to reflexes, but certain regions provide ?
excitatory input
How do we know that the brain has some things to do with lower reflexes?
if there is a transection at C1, the reflex at L4/5 no longer works
Spinal shock (after complete transection) may take months to years until reflexes come back. What are the two reasons they may come back?
- axonal sprouting below level transection
2. expression of receptor phenotypes that are self-activating (such as 5HTC receptor)
Decerebrate posturing results from loss of all structures that are ?
rostral to the pons and CAUDAL to the red nucleus
Rigidity is resisting motion in all directions due to maintained contraction/activation of alpha motoneurons. What is activating these motoneurons?
the brainstem-we dont know why
Spasticityis resistin motion in a given direction due to hyperactive myotatic reflex/continual activation of gamma-motoneurons. Why?
brainstem contracting intrafusal M, lengthening nuclear bag/chain fibers (become extremely sensitive)
What is the region of the brain that is spontaneously active and activates gamma-motoneurons making muscle spindles more sensitive?
brainstem facilitatiry region
What is the region of the brain that inhibits gamma motorneurons making the muscle spindle fibers less sensitive. The region requires activation from cortical regions….
Brainstem inhibitory region
So when there is brain damage that knocks out the brainstem inhibitory region, this leads to what and why?
leads to spasticity because the brainstem facilitory region is activating gamma motoneruons = stretch reflexes that fight ANY passive motion
Main difference between rigidity and spasticity?
rigid: contraction muscles
spas: excessive stretch d/t hyper myotatic
What can be seen with decorticate posturing?
flexion of upper limbs (disinhibition red nucleus)
extension of lower limbs w internal rotation (disinhibition of reticulospinal and vestibulospinal)
dependent on head postioin
What is the most common site for a stroke at which there is unilateral decorticate posture afterwards?
internal capsule stroke (loss cortical input)
What is contraction of all anti-gravity muscles (arms in extension), loss of input from all structures rostral to the pons (caudal to red nucleus), severe brain injury?
decerebrate posturing
