APK 2105C Lecture Notes - Lecture 42: Motor Neuron, Motor Unit, T-Tubule
16 Jun 2018
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Chapter 12, Lecture 3
Muscle Physiology
• Mechanics of muscle contraction
o All muscle cells respond the same way to an AP
▪ Despite variability in force and duration of whole muscle contractions
• May need small, slow type of contraction
• May need quick, strong contraction—explosive movement in
weight lifting
o The motor unit
▪ One somatic motor neuron and all the muscle fibers it innervates
• ONE somatic motor neuron
o May be attached to multiple muscle cells
• There is only one synapse on each cells
o Individual cells are only activated by a single motor neuron
• All myocytes will contract if the neuron fires every single time
▪ The size of the motor unit = how many cells it innervates
o Muscle twitch = mechanical response of a
muscle cell, motor unit, whole muscle to a
single end plate potential
▪ When all the cells contract at the
same time due to the firing of a
neuron
▪ Reproducible, all or nothing event
• If you provide the same
stimulus the exact same thing
will happen every time
▪ One end-plate potential always leads
to the same amt of Ca released every
time
▪ Latent period = when no force is
generated yet—after the stimulus
• Time delay between the AP
and initiation of contraction
• Time for ECC
o Time for AP to travel down T tubule and stimulate release
of Ca
▪ Contraction phase = quick rise in tension
• Crossbridge cycling is occurring
• Cytosolic Ca levels are rising
▪ Relaxation phase = slow decline in tension
• Cytosolic Ca is returned to the SR
• Number of crossbridges decline
▪ Not all twitches are equal
• There is variability in speed of
contraction and amt of force
generated
• Muscle may take longer to reach its
max
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