Page 110 - 84 human physiolofy part-1

Chapter 6

neuromuscular junction. There is one neuromuscular junction for each fiber.
• The acetylcholine diffuses across the cleft and binds to nicotinic receptors on the motor end
plate, opening channels in the membrane for sodium and potassium. Sodium rushes in, and
potassium rushes out. However, because sodium is more permeable, the muscle fiber membrane
becomes more positively charged, triggering an action potential.
• The action potential on the muscle fiber causes the sarcoplasmic reticulum to release
calcium ions(Ca++).
• The calcium binds to the troponin present on the thin filaments of the myofibrils. The
troponin then allosterically modulates the tropomyosin. Normally the tropomyosin physically
obstructs binding sites for cross-bridge; once calcium binds to the troponin, the troponin forces
the tropomyosin to move out of the way, unblocking the binding sites.
• The cross-bridge (which is already in a ready-state) binds to the newly uncovered binding
sites. It then delivers a power stroke.
• ATP binds the cross-bridge, forcing it to conform in such a way as to break the actin-
myosin bond. Another ATP is split to energize the cross bridge again.
• Steps 7 and 8 repeat as long as calcium is present on thin filament.
• Throughout this process, the calcium is actively pumped back into the sarcoplasmic
reticulum. When no longer present on the thin filament, the tropomyosin changes back to its
previous state, so as to block the binding sites again. The cross-bridge then ceases binding to the
thin filament, and the contractions cease as well.
• Muscle contraction remains as long as Ca++ is abundant in sarcoplasm.

Types of Contractions:

• Isometric contraction--muscle does not shorten during contraction and does not require the
sliding of myofibrils but muscles are stiff.
• Isotonic contraction--inertia is used to move or work. More energy is used by the muscle
and contraction lasts longer than isometric contraction.
• Twitch--exciting the nerve to a muscle or by passing electrical stimulus through muscle
itself. Some fibers contract quickly while others contract slowly.

The Efficiency of Muscle Contraction:

• Only about 20% of input energy converts into muscular work. The rest of the energy is
heat.
• 50% of energy from food is used in ATP formation.
• If a muscle contraction is slow or without movement, energy is lost as maintenance heat.
• If muscle contraction is rapid, energy is used to reduce friction.

Summation of Muscle Contraction: It is the adding together of individual muscle twitches to make
strong muscle movements.

• Multiple motor unit summation--increasing number of motor units contracting
simultaneously.
• Wave summation--increasing rapidity of contraction of individual motor units.
• Tetanization--higher frequency successive contractions fuse together and cannot be
distinguished from one another.

110 | Human Physiology