D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
MUSCLE CONTRACTION
Lecture 7: Excitation-Contraction Coupling; Whole Muscle Contraction
3.
Excitation of Skeletal Muscle: (chapter 7)
• neuromuscular junction – skeletal muscle contraction depends on signals
from motor neurons; each muscle fiber usually receives one motor neuron at a
specialized site called the motor end plate (fig. 7 – 1 & ppt. 1), a connection insulated
by a Schwann cell
- synaptic terminal: ending of nerve fiber, located in a pit in the sarcolemma
(synaptic gutter) – additional pits (subneural clefts) increase sarcolemma surface area
at this point
- neurotransmitter: acetylcholine (ACh) is stored in synaptic vesicles in the
synaptic terminal; arrival of nerve impulse triggers release of ACh into synaptic
gutter (where acetyl cholinesterase destroys ACh, milliseconds after release)
- secretion: nerve impulse triggers calcium influx (voltage-gated calcium
channels), which, in turn, triggers exocytosis of vesicles
- chemically-gated channels: channels with acetylcholine receptors,
populate subneural clefts (figs. 7 – 2, 7 – 3 & ppts. 2 & 3); channels admit mainly
sodium; sodium influx produces depolarization (end-plate potential), which
stimulates sarcolemma to fire muscle AP (similar to AP of large nerve fibers but of
longer duration)
- inhibitory drugs: curare blocks chemically gated channels; botulinum toxin
(fig. 7 - 4 & ppt. 4) blocks neurotransmitter release; both result in weakened end-plate
potentials that don’t elicit APs
Bio 659
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- stimulatory drugs: drugs that produce prolonged excitation in neuromuscular
junction; act by mimicking ACh (but are not broken down) or by inhibiting acetyl
cholinesterase (prolonging ACh action); both mechanisms produce muscle spasms
(prolonged contractions)
• excitation-contraction coupling (figs. 7 - 5, 7 - 6 & ppts. 5 & 6): sarcolemma
features transverse tubules that pass through muscle fiber (T-system)
- sarcoplasmic reticulum (SR): storage site for high calcium concentration;
arranged into longitudinal tubules and terminal cisterns, in parallel with sarcomeres of
myofibrils; terminal cisterns of neighboring segments of SR form intimate
association with T-tubule (triad)
- T-tubules convey muscle AP to triad, releasing calcium ions & ‘switching on
contractile mechanism; calcium pump returns calcium to cisterns
4.
Sources of Muscular Energy (ppts. 7 & 8):
• anaerobic: creatine phosphate (alactic), lactic acid formation & disposal
• aerobic: glycogen & myoglobin
• fast (white) & slow (red) muscle fibers
5.
Properties of Whole Muscle Contraction:
• isotonic vs. isometric (fig. 6 - 12 & ppt. 9)
• simple twitch; treppe (fig. 6 - 13 & ppts. 10 to 12)
• motor units (ppt. 13) & recruitment (multiple motor unit summation) (ppts. 14)
Bio 659
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• continuous stimulation & tetanus (frequency summation) (fig. 6 - 14 & ppt. 15)