Properties of the Motor Unit Flashcards
Define ‘motor unit’
A single (lower) motor neurone and all the muscle fibres it innervates
Define synergist and anatgonist
Synergist - working together
Antagonist - opposing actions
How many motor neurones innervate each muscle fibre? What is the exception to this?
one
during development
How many muscle fibres are contacted by a single motor neurone?
many but depend on different parts of the body
fine control = few muscle fibres
Lower motor neurones in the ventral horn are distributed in a predictable way depending on their function. Describer the position of mn’s of axial muscles compared to distal. And describe the position of mn’s of flexors compared to extensors
mn to axial muscles lie medical to those to distal muscles
mn to flexors lie dorsal to those controlling extensors
Describe what happens when the T tubule meets the SR and what purpose this has
a specialised coupling of the proteins in the 2 membranes:
- Voltage sensitive protein in T tubule
- Ca2+ channel in SR
- leads to Ca2+ influx when depolarised
Describe some differences between slow and fast twitch fibres, including the type of metabolism
Slow - aerobic metabolism
- many mitochondria, myoglobin, capillaries
- produce less force
- glycogen stores
Fast - anaerobic metabolism
- different form of myosin to produce more force
How many types of muscle fibres can be each motor unit?
How many types of muscle fibres can be in a muscle?
One only in motor unit
Usually a mixture in a mixture
Describe the differences between motor neurones of fast units and those of slow units
Fast - bigger, faster conducting, generate high frequency bursts of AP’s
Slow - smaller, slower conducting, generate steady, low frequency bursts
What determines the muscle type?
The alpha/lower motor neurone type determines the muscle type
3 ways to increase force of muscle contraction
Name one way that is a common misunderstanding
- Increasing firing of alpha moter neurones (ceiling effect)
- Recruit additional synergistic motor units (smallest recruited first)
- Promote changed in the structure of the muscle fibres themselves (cross bridges
What are muscle spindles?
What do they act as?
Specialsied muscle fibres within a fibrous capsule
Type 1a sensory axons
Act as proprioceptors
What is a myotatic reflex?
when muscle is pulled upon it tends to contract
Describe a myotatic reflex arc
muscle spindle stretched, sensory afferent fibre (cell body in dorsal root ganglion) synpases in ventral root. Alpha motor neuron causes muscle to contract
Describe the effects of upper motor neurone lesions and lower motor neurone lesions
Upper - weakness - spasticity - hyperreflexia Lower - weakness AND atrophy - fasciculations - hypotonia - hyporeflexia
