Neuro 55: Muscle tone and reflexes Flashcards
1
Q
Stretch reflex: what is it? what is the stimulus/response?
A
- stimulus = stretch of a muscle
- response = contraction of the same muscle
- aims to keep a constant muscle length & resists passive stretch
- AKA deep tendon reflexes
2
Q
Stretch reflex circuit
A
- afferent = IA fibers from the spindle synapses onto the mn
- efferent = axon of the alpha mn to the same muscle
- monosynaptic reflex
- short latency reflex
- IA also synapses onto an inhibitory interneuron (neither motor neuron or sensory) that synapses onto a mn to the antagonistic muscle = reciprocal innervation
3
Q
Reciprocal innervation
A
- when the IA afferent will also synapse on an interneuron that will then synapse on the mn of the antagonistic muscle
- allows for coordination
4
Q
What controls the strength of the stretch reflex?
A
- affected by the rest of the motor system
- abnormalities in muscle tone is caused by problems in the control of the stretch reflex and these abnormalities are seen in many diseases/disorders
5
Q
Increased excitability of the stretch reflex
A
- hypertonia
- causes rigidity
- usually due to damage in the motor system
6
Q
Decreased excitability of the stretch reflex
A
- hypotonia & flaccidity
- can be caused by some developmental disorders
7
Q
Decerebrate rigidity
A
- experimentally cut at the midbrain
- see increased tone in extensors (the antigravity muscles) of all 4 limbs
- the rigidity is mediated by the stretch reflex
- when the dorsal roots are cut the rigidity goes away b/c removing the sensory input from the body
- less common clinically, but can occur b/c or hemorrhage in the midbrain, etc.
8
Q
Decorticate rigidity
A
- more common clinically
- increased muscle tone in the FLEXORS of the ARMS and the EXTENSORS of the LEGS
- different effects on the flexors and the extensors
- can be caused by stroke
9
Q
Spacticity
A
- increased muscle tone (rigidity) –> velocity dependent (varies with how fast the limb is stretched)
- hyperactive stretch reflexes
- clonus
- clasp knife reflex
- the cause is thought to be that the pain fibers in the muscle inhibits the stretch reflex and causes the rigidity
- can be seen incerebral palsy, MS, stroke, etc.
10
Q
Cerebral Palsy
A
- can have spasticity due to damage to the motor system at or prior to the birth
- spasticity will effect their appearance, posture, and ability to move
- are in danger of forming contractures or having joint problems, like arthritis
11
Q
Tx for rigidity in CP
A
- dorsal rhizotomy = cut the afferents to the stretch reflex
- injections of baclofen into the spinal cord to inhibit the stretch reflex
12
Q
Rigidity in Parkinson’s disease
A
- differs from spasticity
- rigidity in PD effects both the flexors and extensors so the “cogwheel rigidity” is present
13
Q
Withdrawl reflex: what is it? what is the stimulus? what is the response?
A
- AKA flexion-crossed extension
- this is a protective reflex
- stimulus = noxious, damaging or painful stimulus to the skin –> but can occur in the absence of conscious pain too (ex. transected spinal cord)! or can occur to other non-painful stimuli (ex. crawly spider/insect feeling!)
- response = contraction of flexors and relaxation of extensors at every joint in the effected limb to remove it from the stimulus
- this is a spinal reflex
- reflex also occur on the opposite side to maintain posture
- this reflex can be overriden too –> ex. supress the reflex long enough to put the hot dish down safely
14
Q
Double reciprocal innervation
A
- occurs during the withdrawl reflex
- the contralateral limb’s extensors are stimulated to maintain posture
- get excitation of the extensors and inhibition of the flexors
15
Q
Spinal shock
A
- complete absence of reflexes that immediately follows a transection of the spinal cord
- reflexes gradually return
- the more complex the CNS is, the longer the recovery period is
- can recover to hyperreflexia
- mechanism not really understood
