PHY2011 Lecture Notes - Lecture 27: Myocyte, Neural Coding, Skeletal Muscle
20 Jun 2018
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Week 10 L2 (T5BL1) – Motor units and muscle sensors (movement
control)
Locomotion includes:
oPropulsion
oEquilibrium control
oSteering
oCompensation for predicted and unpredicted perturbations
oCombination with other movements
Movement requires:
oPlanning (of movement)
oInitiation (of movement)
oCoordination (of multiple muscles in space and time)
oRefinement (of movement using sensory feedback)
oOptimisation (and learning of repeated movements)
oFeedback loops further modulate the flow
Spinal cord contains the machinery to move limbs in a rhythmic order -brain not required
oBrain required for:
Maintenance of balance and posture (Cerebellum)
Flexibility of movement/preventing rigidity (Thalamus and Midbrain)
Action sequencing and selecting (Basal ganglia and Thalamus)
Preparing, planning and sensory guidance (Supplementary and pre-
supplementary cortex)
Output for fine control and intentional movements (Motor and Pre-motor
cortex)
Using sensory information for guidance (Posterior parietal cortex)
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Muscle fibres and grading force:
Skeletal muscle needs to be told to be active -smooth muscle spontaneously active
Αα motor neurons control skeletal muscles -Lower Motor Neuron (LMN)
oLargest diameter and myelinated
oFrom spinal cord/brainstem to Neuromuscular Junction (NMJ) on muscle
oReceive input from higher brain regions via the Upper Motor Neurons (UMN)
These neurons bring input, they don’t go to a muscle (Named 150 years ago)
Each motor neuron runs to only one muscle, but doesn’t contact all muscle fibres
oMotor unit = α motor neuron plus all the muscle fibres it contacts
oMotor Neuron Pool = collection of motor units contacting a single muscle
oInnervation ratio = fibres per motor neuron
oMotor unit size varies based on role of muscle
Small = fine detail (Slow oxidative)
Intermediate = in between (Fast oxidative, glycolytic, fatigue resistant)
Large = lots of force (Fast, glycolytic, fatiguable)
Small Intermediate Large
S FR FF
SOG FOG FG
2g 10g 50g
Hours <1hr 1-2 mins
G depends on glycogen storage
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Grading the force produced by muscles
o1. Recruitment – number of active motor neurons/number of active muscle fibres
o=Modulation of number of active muscle fibres
Neurons activated in systematic order
Based on size principle
- Small activated then intermediate then large
- Small units are easier to recruit because they are less “leaky”
oHave a larger change (EPSP) in membrane potential with the
same amount of stimulus
EPSPs need to sum to reach threshold, therefore less are needed if small
- Temporal summation
oIf EPSPs are spread out in time, then they can sum enough
to reach the thresholds of small neurons, but not big
neurons
o if the rate can be varied (increased), then can control
which neurons are activated (small then inter then large)
oNote that small LMNs reach max quicker than Inter and
large b/c they are less leaky, and current can take effects
quicker than in bigger ones.
o2. Rate coding -increasing the amount of force produced by each individual muscle
fibre
o= Modulation of the rate of Actions Potentials (firing rate) in individual motor
neurons
Summation of muscle contractions (Not of APs -APs can’t sum-refractory
periods)
It takes 50msec for a muscle contraction to reach its peak & no refractory
period
- if second twitch produced before end of the first, they can sum
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