Neuromuscular Control Flashcards
How does the grey and white matter look on a cross section of the spinal cord?
White matter on the outside
Butterfly shape of grey matter on the inside
What is the alpha motor neuron? Where is it located?
Occupy the anterior / ventral horn of the grey matter of the spinal cord
They are the lower motor neurons of the brainstem and spinal cord
Innervate the extrafusal muscle fibres (muscle fibres with contractal element in them) - activation = muscle contraction
What are intrafusal muscle fibres?
Contain sensory organs - responds to stretch and tension within the muscle
Communicates sensory info about the status of the muscle - brings about reflex activity
What is a motor neuron pool?
Contains all the alpha motor neurons innervating a single muscle
What is a motor unit?
How does this develop before birth?
A single motor neuron and all the muscle fibres it innervates
Smallest functional unit required to produce force
Before birth, there may be an overlap in the muscle fibres that receive innervation from multiple neurons, but one neuron wins eventually
So a muscle fibre is only innervated by one motor neuron
But one motor neuron may innervate many muscle fibres (avg. 600)
What are the 3 types of motor unit?
How are they distributed in muscles?
Slow (S, type 1) - smallest diameter cell bodies, small dendritic trees, thinnest axons, slowest conduction velocity
Fast, fatigue resistant (FR, type IIA) - larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity
Fast, fatiguable (FF, type IIB) - larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity
Randomly distributed, not congregated in one area
For the 3 different types of motor units, what is the: force generated, speed of contraction, and fatiguability?
Type I (S) = least force, continues to generate it’s own max force for over an hour
Type IIA (FR) = more force, continues to generate max force but dies after 10 minutes
Type IIB (FF) = greatest force, loses ability to generate it’s max force quickly
So how does the function of the muscle relate to motor unit?
Function = dependent on the type of motor unit that makes up most of the muscle e.g. for standing and posture = mainly S
What are the 2 main mechanisms by the CNS that can regulate muscle force? And how do they work?
Recruitment = turn on number of neurons required for muscle fibre contraction, so more force required = more neurons turned on. Governed by the ‘size principle’ = slow units recruited first, fast units recruited afterwards
Rate coding = motor neurons fire at range of frequencies, increased firing rate = increased force production by unit
What is meant by muscle wastage? How does it occur?
i.e. if a muscle still has it’s blood supply, but not it’s nerve supply, the muscles can no longer be contracted
What are neurotrophic factors?
Neurotrophic factors - growth factor, prevents neuronal death, promotes growth of neurons after injury
What are motor unit and fibre characteristics dependent on?
The nerve that innervates them
e.g. if the innervation to slow and fast muscles are swapped, the properties of the muscle also swap due to the swapped innervation
Is there plasticity of the different motor units? Which are common / uncommon?
Fibre types can change properties under many different conditions Type IIB (FF) to IIA (FR) most common following training Type I to II - only in cases of injury / severe deconditioning (e.g. in spaceflights)
Ageing is associated with which type of motor unit loss?
Loss of types I and II fibres, but preferential loss of II as larger propertion of type I fibres = slower contraction time
What is a reflex?
An involuntary, automatic response to a stimulus without reaching the level of consciousness
Magnitude and timing depends on intensity and onset of stimulus