Neuromuscular control Flashcards
What is an alpha motor neuron?
LMN of brainstem and spinal cord
What muscles do alpha motor neurons innervate?
Extrafusal muscles - skeletal muscles that cause contraction
activation causes contraction
Motor neuron pool
contains all the alpha motor neurons innervating a single muscle
What is a motor unit?
A single motor neuron and all the muscle fibres it innervates ~ 600
stimulation of one unit causes contraction of all muscle fibres
What are the three types of motor units?
Slow (Type 1),
Fast & Fatigue Resistant (2a),
Fast & Fatiguable (2b)
What are the structural properties of slow type compared to others?
Small cell body diameter, small dendrites, thin axons, slowest velocity
How are motor units types classified?
- Tension generated
- Contraction speed
- Fatiguability
Which muscle type generates the greatest force
fast fatiguable 2b, low duration
What are the two mechanisms to regulate muscle force?
Recruitment
Rate coding
What is recruitment?
units are recruited in order
First small units (slow), and then more units as demand increases. This allows fine control
What is rate coding?
units fire at a range of frq
as demand increases, motor units fire more.
slow units fire until too fast to recover (summation) and then faster units take over
What are neurotrophic factors?
Growth factors that travel down motor neurons to supply muscle fibres that are innervated
What is the function of neurotrophic factors?
Promote growth of neurons after injury and prevent death
What is it called when muscle fibres are swapped with innervation type and what effect does it cause?
Cross innervation - slow muscle types can become fast and vice versa
This is because muscle fibre characteristics are dependent on the nerve that innervates them
Explain plasticity of muscle fibres and 3 conditions they may occur in
Muscle fibres changing properties under certain conditions
Training - fatiguable to fatigue resistant
Spinal cord injury/ severe deconditioning - slow to fast (less use of postural muscles)
Ageing - preferential loss of fast type
What is a reflex?
Automatic response to stimulus -
involuntary coordinated pattern of muscle contraction due to peripheral stimuli
Difference between reflex and voluntary movement
Once released, cannot be stopped
What is the descending (supraspinal) control of reflexes with an example?
Reflex strength can be controlled by higher motor areas - Jendrassik manoeuvre - clenched teeth, making fist, pulling apart fingers exacerbate knee jerk reflex
Explain how descending (supraspinal) control of reflexes occurs
Both inhibitory and excitatory regulation on stretch reflex in higher CNS centres - inhibitory control usually, but decerebration in the Jendrassik manoeuvre allows excitatory control
Brain normally dampens down reflexes, can be diminished if the brain is disconnected e.g. via stroke. Damage to motor cortex can cause positive signs
What is hyperreflexia and how does it occur?
Overactive reflexes, loss of descending inhibition from upper motor neuron lesion
What is clonus and how does it occur?
Involuntary oscillation of muscle contraction, loss of descending inhibition from upper motor lesion
What is Babinski’s sign and how does it occur?
Blunt instrument on sole stimulates big toe - adults - downward curl, infant - upward curl. Abnormal (positive sign) - adult - upwards curl // upper motor neuron lesion
What is hyporeflexia and how does it occur?
Below normal reflexes, lower motor neuron disease
