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Biology 219 – Human Physiology
Clemens
Muscle Physiology 2
Text: Ch. 12
E. Activation of Contraction in Skeletal Muscle
1. Neuromuscular Junction - synapse between motor neuron and muscle fiber
motor neuron AP → ACh release → synaptic transmission → EPP → muscle AP
2. Muscle Action Potential
- formed along the sarcolemma, similar to axon membrane
3. Excitation-Contraction Coupling
“Calcium is the key.”
a) T tubules - conduct APs deep into the muscle fiber
- DHP receptors function as voltage sensors, activated by the muscle AP
b) sarcoplasmic reticulum (SR) - stores Ca2+ and releases it to myofilaments;
- ryanodine receptors (RyR) in the SR membrane are Ca2+ channels
RyR of the SR interact with DHP receptors of the T tubule
Ca2+ released through RyR channels activates muscle contraction
- Ca2+ pumps in the SR membrane actively transport Ca2+ back into the SR
Sequence of Events in Excitation-Contraction Coupling
1) Muscle AP travels down the T-tubules.
2) DHP receptors on the T-tubule are activated.
3) Activated DHP receptor opens RyR Ca2+ channels.
4) Ca2+ diffuses out of the SR into the cytosol and to the thin filaments.
5) Ca2+ binds to troponin, which moves tropomyosin off the myosin binding site.
6) Myosin binds to actin → sarcomere contracts (crossbridge cycle)
When APs stop, Ca2+ is pumped back into SR → sarcomeres relax
F. Control of Whole Muscle Contraction
motor unit = one motor neuron + all the muscle fibers it stimulates
small motor units (1:10) → fine control, less force per unit
large motor units (1:2000) → coarse control, greater force per unit
recruitment - increase in number of active motor units
- increased CNS stimulation activates more motor neurons with higher thresholds
↑ motor unit recruitment → more muscle fibers activated → ↑ force of contraction
H. Length-Tension Relationship
sarcomere length = 2.0 - 2.2 µm (“resting length”)
- maximal number of crossbridges → maximal tension
length > 2.2 µm - overlap decreases → tension decreases
length < 2.0 µm - interference between filaments
→ tension decreases
Tension (% )
G. Muscle Twitch and Summation
1. Twitch
- single contraction relaxation cycle, evoked by a single muscle AP
a. latent period
b. contraction phase
c. relaxation phase
2. Summation - two or more closely spaced APs → ↑ force
3. Tetanus
- high frequency of APs → maximum, sustained force
Sarcomere Length (µm)
- skeletal muscle mostly operates at the top part of the L-T curve
- cardiac muscle operates at shorter than optimal length, so ↑ stretch → ↑ force
Biology 219 – Human Physiology
Clemens
I. Muscle Metabolism
1. Functions of ATP in muscle contraction:
 detaches the actin-myosin crossbridges
 energizes the myosin head
 powers the Ca2+ pump in the SR
2. Sources of ATP
a. intracellular ATP stores
b. phosphocreatine + ADP → creatine + ATP
c. glycolytic (anaerobic) metabolism
glycogen → glucose-6-P→ lactic acid, yields 3 ATP per glucose unit
d. oxidative (aerobic) metabolism
glycogen → glucose-6-P → CO2 + H2O, yields ~32 ATP per glucose
triglycerides → fatty acids → CO2 + H2O, yields >100 ATP per fatty acid
3. Muscle fiber types
a. slow-twitch oxidative (SO) (“red muscle”)
- slow myosin ATPase, small diameter → low power
- abundant mitochondria and myoglobin
- high triglyceride content
- highly aerobic → slow fatiguing
b. fast-twitch oxidative-glycolytic (FOG)
- fast myosin ATPase, intermediate diameter, intermediate power
- can utilize both aerobic and anaerobic metabolism
c. fast-twitch glycolytic (FG) (“white muscle”)
- fast myosin ATPase, large diameter → high power
- high glycogen content
- mostly anaerobic → fast fatiguing
J. Smooth Muscle Physiology
1. General Features
- no sarcomeres, oblique arrangement of thick and thin filaments
- contraction results from crossbridge formation between thick and thin filaments
- contraction activated by action potentials, graded potentials, or chemical signals
- spontaneous contraction (pacemaker activity) in some smooth muscle cells
Activation of Contraction in Smooth Muscle
1) Depolarization of smooth muscle cell (AP or GP)
2) Ca2+ enters cytosol from ECF (through voltage-gated Ca2+ channels)
and/or from sarcoplasmic reticulum (through RyR Ca2+ channels)
3) Ca2+ binds to calmodulin → Ca-calmodulin
4) Ca-calmodulin complex activates myosin light chain kinase (MLCK)
5) MLCK catalyzes phosphorylation of myosin light chain
6) phosphorylation of myosin light chain → crossbridge formation → contraction
When myosin light chain is dephosphorylated, crossbridge activity stops → relaxation
2. Types of Smooth Muscle
a. single-unit smooth muscle
- extensive connections between cells via gap junctions
- fewer innervation points (via varicosities of autonomic motor neurons)
b. multi-unit smooth muscle
- multiple innervation points, little or no connection between cells