28: Motor Reflexes Flashcards
Four characteristics of reflexes
Involuntary, fast, short-acting, precise
Three spinal reflexes
- Myotatic
- Golgi tendon reflex
- Crossed extensor reflex
Five brainstem/midbrain reflexes
- Vestibular
- Righting reflex
- Suckle reflex
- Yawning
- Eye/head movements
Two cortical reflexes
- Placing reaction
2. Hopping reaction
Purpose of the Golgi tendon reflex
Protect muscle from damage due to excess force
Purpose of myotatic reflex
Protect muscle from tearing due to stretch
Golgi tendon and myotatic reflexes: poly vs monosynaptic
Golgi tenon: polysnaptic
Myotatic: monosynaptic
Golgi tenon organs
Bare nerve endings with many branches innervating tendons
Conduction velocity of 1a vs 2a fibers
1a: higher conduction velocity than 2a
Spinal shock
With transection of the SC, all reflexes are abolished, even if circuit is still intact
How to recover reflexes after spinal shock
- Axonal sprouting below transection
2. Expression of self-activating receptor phenotypes (ex: 5HTC)
Brainstem facilitatory vs inhibitory region
Facilitatory region: activates gamma motor neurons -> muscle spindle more sensitive
Inhibitory region: inhibits gamma motor neurons -> muscle spindle less sensitive
Which region is spontaneously active vs needs activation from cortical region: brainstem facilitatory vs inhibitory regions
Facilitatory region: spontaneously active
Inhibitory region: requires activation from cortical regions
What happens to brainstem inhibitory region with loss of cortex?
Inhibitory region cant be activated, so facilitatory region dominates, causing spasticity
Decerebrate posturing
Contraction of all anti-gravity/extensor muscles
Decorticate posturing
Flexion of UE joints + extension of LE joints
What causes decerebrate posturing?
Loss of all structures rostral to pons
Rigidity vs spasticity in decerebrate posturing
Rigidity: resists motion in all directions due to contractions
Spasticity: resists motion in a given direction due to hyperactive myotatic reflex
What causes spasticity in decerebrate posturing
Continual activation of gamma motor neurons via brainstem
What causes flexion of UE joints in decorticate posturing vs extension of LE joints
UE flexion: disinhibition of red nucleus
LE extension: disinhibition of reticuospinal + vestibulospinal pathways
Which one relies on head positioning, decerebrate or decorticate posturing?
Decorticate
Unilateral decorticate posturing
Common with strokes that are in the vicinity of the internal capsule
