BMS1052 Lecture Notes - Lecture 15: Depolarization, Ryanodine Receptor, Proportional Control
30 May 2018
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Week 5. Control of movement 1
MOTOR UNITS AND EXCITATION-CONTRACTION COUPLING
• Dorsal root ganglion = cell bodies of sensory
• Ventral horn = cell bodies of alpha motor neurons
• Axons project out ventral root (mixed with sensory inputs, which enter via dorsal root).
• Muscle = many muscle fibres
• Each muscle fibre is a single, multi-nucleated cell
• Each muscle fibre is innervated by only one alpha motor neuron
• Motor unit = smallest functional unit = 1 alpha motor neuron + muscle fibre that it innervates
-small motor units eg. fingers and eyes (where more precision is required)
-large motor units eg. calf muscle
• Motor neuron pool = collection of alpha motor neurons that innervate a single muscle
-in ventral horn of spinal cord
• Lower (alpha) motor neurons:
o Neurons that directly innervate muscle
o Involved in all movements (voluntary and reflexive)
o Cell bodies are in the spinal cord
o Neurotransmitter = Ach
-> cause contraction
o alpha motor neuron; lower motor neuron; motor neuron; final common pathway –
anatomical / functional / historical reasons.
• Muscle fibres have two special properties – they can change length; they can generate force
find more resources at oneclass.com
find more resources at oneclass.com
• Controlling force generation:
I – Rate coding
II – Size principle
o If a second AP occurs before fibre relaxes,
we get a force summation
o Unfused tetanus: oscillations in summed
force generation at low firing rates
-oscillations are at a single motor unit level,
do’t see jerky usle otratios
o Fused tetanus: smooth force summation
o To generate graded forces, small motor
units (small forces) are recruited first ->
giving precise control.
Smaller neurons also innervate slow,
fatigue resistant fibres
o Followed by progressively larger motor
units (larger forces)
Includes fast fatigue resistant and
then fast fatiguing fibres
o Cell body size takes care of grading motor
recruitment automatically
o Increase in force generation is proportional
to threshold required to activated motor
unit.
o It takes longer to activate larger motor units,
ensuring a graded contraction over time as
well.
• Small vs large motor neurons:
Small motor neurons have higher membrane resistance and reach threshold more easily
Small motor neurons
Large motor neurons
o Smaller surface area
o Higher membrane resistance
o innervate slow, fatigue resistant fibres
o reach spiking threshold more easily
(V=I x R)
o larger surface area
o lower membrane resistance
o harder to reach threshold
o innervated fast fatigue resistant and then
fast fatiguing fibres
• Proportional control:
o There is an inverse relationship between the number of motor units in a muscle & their
force generating capacity.
=> many small motor units; progressively fewer large motor units
• Summation occurs because a single AP releases enough Ca to saturate troponin (all myosin
binding sites on thin filaments are initially available) -> takes time and Ca is taken up by
sarcoplasmic reticulum -> blockage of binding sites before attachment can occur. Hence with
closely spaced AP, more cross-bridge attachment occurs, allowing greater force generation.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Small motor units eg. fingers and eyes (where more precision is required) Large motor units eg. calf muscle: motor neuron pool = collection of alpha motor neurons that innervate a single muscle. Lower (alpha) motor neurons: neurons that directly innervate muscle. Involved in all movements (voluntary and reflexive: cell bodies are in the spinal cord, neurotransmitter = ach. If a second ap occurs before fibre relaxes, we get a force summation: unfused tetanus: oscillations in summed force generation at low firing rates. Oscillations are at a single motor unit level, do(cid:374)"t see jerky (cid:373)us(cid:272)le (cid:272)o(cid:374)tra(cid:272)tio(cid:374)s. Ii size principle: to generate graded forces, small motor units (small forces) are recruited first -> giving precise control. Smaller neurons also innervate slow, fatigue resistant fibres: followed by progressively larger motor, fused tetanus: smooth force summation units (larger forces) Includes fast fatigue resistant and then fast fatiguing fibres: cell body size takes care of grading motor recruitment automatically.
