The Muscular System 227
- A layer of areolar tissue called the fascia is on top of
the epimysium.^
- Under a microscope, skeletal muscle cells have
cross-striations due to the overlap of the dark bands
of the thick protein myosin (called A bands) and the
light bands of the thin protein actin (called I bands).
8.^ In the middle of an I band is a Z line.^
9.^ In the middle of an A band is an H line or zone.^ - The area between two adjacent Z lines is called a
sarcomere.^
- Electron microscopy reveals that the muscle fibrils of
actin and myosin that make up a muscle cell are
surrounded by a sarcotubular system composed of T
tubules and an irregular curtain called the sarco-
plasmic reticulum.^
- The function of the T tubules is the rapid transmis-
sion of a nerve impulse to all the fibrils in a cell
while the sarcoplasmic reticulum stores calcium ions.
The Physiology of Muscle
Contraction
- All of the muscle cells or fibers innervated by the
same motor neuron are called a motor unit.^
- Muscle cells have four properties: excitability by a
stimulus; conductivity of that stimulus through their
cytoplasm; contractility, which is the reaction to the
stimulus; and elasticity, which allows the cell to re-
turn to its original shape after contraction.^
- Muscle contraction is caused by the interaction of
three factors: neuroelectrical, chemical, and energy
sources.
Neuroelectrical Factors
1.^ Muscle cells have positively charged sodium ions^
(Na^1 ) in greater concentration outside the cell than
inside.^
2.^ Muscle cells have positively charged potassium ions^
(K^1 ) in greater concentration inside the cell than
outside.^
- The outside of a muscle cell is positively charged
electrically and the inside is negatively charged.
This electrical distribution is known as the resting
potential of the cell membrane.
4. When a motor neuron innervates the muscle cell,
acetylcholine is secreted from the axon terminals into
the neuromuscular junction. This causes so-dium ions
to rush inside the cell membrane, creat-ing an
electrical potential (changing the inside from negative
to positive).^ - Potassium ions move outside the cell membrane to try
to restore the resting potential but cannot do so
because so many sodium ions are rushing in.^ - The influx of positive sodium ions causes the T tu-
bules to transmit the stimulus deep into the muscle
cell, creating an action potential.^ - The action potential causes the sarcoplasmic
reticulum- to release calcium ions into the fluids
surrounding- the myofibrils of actin and myosin.^ - Troponin and tropomyosin (inhibitor substance)
have kept the actin and myosin filaments separate-^
but the calcium ions inhibit the action of the
troponin- and tropomyosin.^ - The calcium causes the myosin to become activated
myosin. The activated myosin now links up with the
actin filaments.
Chemical Factors- The cross-bridges or heads of myosin filaments have
ATP. When the cross-bridges link with the actin, the
breakdown of the ATP releases energy that is used to
pull the actin filaments in among the myosin
filaments. The area between two Z lines gets smaller,
whereas the A band remains the same. This is
contraction at the molecular level.^ - Meanwhile, the sodium-potassium pump has op-
erated. It has pumped out the sodium ions that
initially rushed in and pulled back in the potassium
ions that had rushed out, restoring the muscle cell’s
resting potential. The calcium ions get reabsorbed by
the sarcoplasmic reticulum, causing the action
potential to cease and restoring the resting poten-tial.
The muscle cell now relaxes as contraction ceases. - The whole process of contraction occurs in 1/40 of a
second.
Energy Sources- ATP is the energy source for muscle contraction: actin
1 myosin 1 ATP → actomyosin 1 ADP 1 PO 4 1 the
energy of contraction.