Nem’s Notes…
Phase 2 Year 3
NEUROSCIENCE 6 (page 1 of 2)
Neuromuscular Function
Neuromuscular The neuromuscular junction is the term used to describe the synapses between the
Junction
axons of motor neurons and skeletal muscle. Any action potentials produced in the
axon will induce action potentials in the muscle and cause contraction. Structural
specialisations allow this to occur easily and include:
(a) a large number of active zones on the presynaptic terminal
(b) folds in the motor-end plate (postsynaptic membrane) increasing surface
area which are packed with neurotransmitter receptors.
The active zones are precisely aligned to the folds in the motor-end plate.
Motor Unit
A motor unit is composed of an:
(a) alpha motor neuron and
(b) the muscle fibres that it innervates
Muscle is made up of a mix of two different types of motor unit, fast and slow.
Fibre Type
Fatigability
Mitochondria
Metabolism
Contraction
α-Motor Fibre
Conduction
Action Potentials
Trophism
Fast
Rapidly Fatigued
Fibres have Few Mitochondria
Anaerobic
Powerful, Rapid
Large Diameter
Fast Axons
High Frequency
Slow
Slowly Fatigued
Many Mitochondria
Aerobic
Slower Contraction
Small Diameter
Slow Axons
Low Frequency
Initially during development a muscle fibre may receive input from several different
motor neurons although eventually each fibre receives input from only one alpha
motor neuron due to synpaptic elimination.
This process is regulated by electrical activity in the muscle. Stimulation of the muscle
causes elimination of all but one input although the exact mechanism is not clearly
understood. Experience and training of muscle innervation play a role in determining
its eventual unit type.
The alpha motor neuron type that innervates a particular muscle determines the
muscle fibre type (eg small motor fibres give rise to slow units, large to fast). If a motor
neuron is surgically replaced with the other type, the muscle adopts different
characteristics.
Stretch Reflex
The stretch reflex is an example of a monosynaptic reflex since the afferent neuron
from muscle spindle stretch receptors synapse directly to the efferent spinal motor
neuron.
DRAW DIAGRAM Fig 13.7 p366 Neuroscience
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Nem’s Notes…
Phase 2 Year 3
NEUROSCIENCE 6 (page 2 of 2)
Neuromuscular Function
Flexor Reflex
The flexor reflex is an example of a polysynaptic reflex. This is a complex reflex arc
which is used to withdraw limbs from aversive stimuli. A single nociceptive input
causes activation of excitatory interneurons which cause contraction of all flexor
muscles in the affected limb.
At the same time a crossed extensor reflex occurs in the contralateral limb causing
contraction of all the extensor muscles. This provides postural support during the
reflex.
DRAW DIAGRAM Fig 4.9 p54 CRASH COURSE
Pathological
Reflexes
Hyperreflexia is an abnormal exaggerated reflex response to a stimulus. It is often a
sign of upper motor neuron dysfunction. Upper motor neurons are those from the
cortex or brainstem connecting to the interneurons in the anterior horn of the spinal
cord.
Hyporeflexia is an absence of, or diminished reflex to, a stimulus and can be due to
muscle fibre denervation and subsequent muscle atrophy, myopathies or lower motor
neuron dysfunction. Lower motor neurons are those from the spine to the muscle.
Trauma to the spinal cord can also affect reflexes due to disruption of the spinal arc.
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