Physiology of Muscle Cells

Excitability: responds to stimuli (e.g., nervous impulses)
Conductivity: ability to produce a local effect
Contractility: able to shorten in length
Extensibility: stretches when pulled
Elasticity: tends to return to original shape & length after contraction or extension

Voluntary striated muscle attached to one or more bones
Diameter: 100 µm in diameter and 3 cm long (500um/30 cm)
Length!!!Sk Muscle cells = muscle/myofibers
Exhibits alternating light and dark transverse bands, or striations
Composed of:
- Endomysium: surrounds each muscle fiber
- Perimysium: bundles muscle fibers together into fascicles
- Epimysium: encloses the entire muscle
Long, multinucleated, formed by fusion of myoblasts
Contains satellite cells, Sarcolemma, T-tubules, Glycogen, Myoglobin, Mitochondria, and other organelles
Sarcoplasmic reticulum surrounds myofibrils longitudinally, acts as a Ca2+ reservoir

Consist of three types of myofilaments:
1. Thick filaments: 300 myosin molecules with globular heads that form cross-bridges between thick and thin filaments
2. Thin filaments: actin strands contain binding sites for myosin heads
3. Elastic filaments (titin): attaches thick filaments to Z disc, holds thick filaments in place, and prevents muscle cell overstretching
Dystrophin: an accessory protein that links thin filaments to sarcolemma and endomysium via integral proteins

A band: Dark band formed by parallel thick filaments that partly overlap with thin filaments
H band: Lighter region in the middle of an A band that contains thick filaments only
M line: Dark line in the middle of an H band, a meshwork of proteins that anchor thick filaments
I band: Light band composed of thin filaments only, also contains elastic filaments
Z disc: Protein disc that anchors thin and elastic filaments at each end of a sarcomere

Axons (fibers) of a somatic motor neuron divide as they enter the muscle
Motor unit: one nerve fiber and all muscle fibers innervated by it
Each muscle gets at least one motor nerve that contains hundreds of motor neurons with their axons
One axon branches to many terminals, each forming a junction with one muscle fiber
Muscle fibers in a motor unit spread throughout the muscle, not clustered together

Each fiber ending forms a neuromuscular junction (synapse) with a muscle fiber at the motor end plate
Synaptic cleft: gap between the axonal ending and the motor end plate
Axonal ending has synaptic vesicles containing ACh
Sarcolemma of the motor end plate has receptors for ACh and ion channels

Nerve impulse reaches the end of the axon, opening voltage-gated Ca2+ channels
Ca2+ influx causes vesicles to fuse with the axon membrane, releasing ACh
ACh binds to receptors on the sarcolemma, triggering depolarization and an action potential
Acetylcholinesterase on the sarcolemma breaks down ACh, preventing continued muscle contraction

Recall resting membrane potential
Binding of ACh opens chemically gated ion channels, causing depolarization at the motor end plate
Voltage-gated channels on the sarcolemma open, creating an action potential that propagates along the membrane
Na+ gates close and K+ gates open, causing repolarization

Action potential moves down the T-tubule in the triad
Terminal cisternae of the SR release Ca2+ into the sarcoplasm via voltage-gated channels
Ca2+ binds to troponin, shifting tropomyosin, and exposing binding sites on actin
Myosin heads attach and pull thin filaments toward the center of the sarcomere
ATP hydrolysis activates myosin heads, and ATP binding releases them for another stroke
Ca2+ reuptake by the SR causes relaxation

Thin filaments slide past thick filaments to overlap to a greater extent than when relaxed
Nerve impulse → myosin heads attach to actin in thin filaments → thin filaments move to the center of the sarcomere
Z bands become closer together, I bands shorten, H zone disappears, and the muscle fiber shortens

Ca2+ reuptake by the SR causes relaxation
Muscle fatigue causes: consumption of glycogen and ACh, slowing of the Na+/K+ pump, lactic acid accumulation, and K+ accumulation in extracellular fluid

ATP sources:
1. Phosphorylation by myokinase and creatin phosphate (phosphagen system)
2. Glycolysis from stored glycogen (anaerobically)
3. Aerobic respiration in mitochondria
Oxygen debt: O2 reserves replacement, phosphagen system replenishment, oxidation of lactic acid, and elevated metabolic rate requirements

Each smooth muscle fiber is a spindle-shaped cell with a diameter ranging from 2 to 10 µm
Smooth muscle fibers have a single nucleus
Two types of filaments: thick myosin-containing filaments and thin actin-containing filaments
Contraction depends on a rise in free intracellular Ca2+, which binds with calmodulin to activate myosin light chain kinase, leading to myosin head phosphorylation and contraction.