Neuromuscular control Flashcards
what are the lower motor neurons of brainstem and spinal cord?
alpha motor neuron (antra/anterior horn cells)
what do alpha motor neurons do?
innervate extrafusal muscles fibres of skeletal muscles
what does activation of alpha motor neurons do?
causes muscle contraction
what is a motor neuron pool?
all alpha motor neurons that innervate a single muscle
what is a motor unit?
single motor neuron together with all the muscle fibers that it innervates
smallest functional unit with which to produce a force
how many fibres does a motor neuron supply?
600 muscle fibres
what does stimulation of one motor unit cause?
contraction of all muscle fibres in that unit
can there be cross motor neuron stimulation?
no- none of the muscle fibres innervated by 1 motor neurone will be innervated by a different motor neuron also
what are the types of motor unit?
Slow, (S, type 1)
Fast, fatigue resistant (FR, type II)
fast, fatigable (FF, type III)
what are the differences in diameter cell bodies for each type motor unit?
Slow= smallest
FF= largest
what are the differences in dendritic tree sizes between motor unit?
S= small
FF= largest
what are the differences in axon thickness between difference types motor unit?
S= thinnest
FF= thickest
what is the difference in conduction velocity in the different types of motor unit?
S= slowest
FF= fastest
what is the response to repeated stimulation at level that evokes maximum tension for different types of motor unit?
what muscle fibres are used for postural stability fibres?
slow muscle fibres
what are the mechanisms of regulation of muscle force?
recruitment
rate coding
what is recruitment of muscle fibres?
- Motor units can be recruited (not random)
- Depends on ‘size principle’
- Smaller units recruited first (slow twitch units)
- As more force required more units recruited
- Allows fine control where low force required
what is the response of each motor unit type for a single motor neuron action potential?
what is rate coding for regulation of muscle force?
- Motor units can fire at range of frequencies
- Slow units = lower frequency
- As firing rate increases force produced by unit increases
- Summation occurs when units fire at freq too fast to allow muscle to relax between action potentials
- During voluntary contraction all motor units start at firing rate of 8
what growth factor prevent neuronal death?
neurotrophic factors
what is neurotrophic factor?
- Type of growth factor
- Prevent neuronal death
- Promote growth of neurons after injury
- Motor unit and fibre characteristics dependant on nerve which innervates them
- If fast twitch muscle and slow muscle are cross innervated, the slow becomes fast and the fast becomes slow.
- Motor neuron has effect on properties of muscle fibre it innervates
what can happen to fibre types under specific conditions?
can change properties
- Type IIB to IIA most common following training
- Type I to II possible in severe deconditioning or spinal cord injury
- Microgravity during spaceflights results in shift from slow to fast muscle fibre types
- Aging associated with loss type I and II
- Faster loss type II
- Larger proportion type I fibres left in ages muscle (slower contraction times)
what is a reflex?
- Automatic response to stimulus that involves nerve impulse passing from receptor to nerve centre to effector without reaching level consciousness
- Involuntary coordinated pattern of contraction and relaxation elicited by peripheral stimuli
- usually one muscle contract, one inhibited
what is the magnitude and timing of a reflex determined by?
intensity and onset of stimulus
how do reflexes differ from voluntary contraction?
cannot be stopped once released
what can reflexes be influenced by?
- Jendrassik manoeuvre (Reduce inhibition brain or upper CNS exert on reflexes)
- Larger patellar tendon reflex
- Clenching teeth or pulling against locked fingers
what is the level of supraspinal control of reflexes?
higher centres of CNS exert inhibitory and excitatory regulation upon stretch reflex
- Decerebration (separate cortex from lower brainstem and spinal cord regions) reveals the excitatory control from supraspinal areas (greater reflex response and elevated level of tonic contraction before relaxation
what can occur to reflexes during brain stem
rigidity and spasticity from brain damage giving over-active or tonic stretch reflex
what is hyper-reflexia?
overactive reflexes due to loss of descending inhibition
what is hyper-reflexia associated with?
upper motor neuron lesions (stroke)- loss voluntary movement in UMNL
what is clonus?
involuntary and rhythmic muscle contraction from manual stretch
what is babinski sign?
- Particularly evident in corticospinal tract lesions
- Stimulate sole foot with blunt instrument- the big toe movement:
- Curl downwards= normal
- Curl upwards= abnormal in adults (+ve Babinski sign) (note normal in infants as corticospinal area not yet developed) and toes flex
what is hyporeflexia?
below normal/ ansent reflexes
what is hyporeflexia associated with?
lower motor neuron lesions
