Key Concepts

1. The myoneural junction is a specialized synapse between the motor axon and a skeletal muscle fiber. A motor nerve and all of the muscle fibers it innervates is called a motor unit.

2. Neuromuscular transmission involves presynaptic transmitter release, diffusion of transmitter across the synaptic cleft, and binding to postsynaptic receptors.

3. The immediate postsynaptic electrical response to transmitter molecule binding is a local depolarization called the endplate potential, which is graded according to the relative number of channels that have been opened by the transmitter binding.

4. The endplate potential is localized to the endplate region and is not propagated. It causes current to flow into the muscle fiber at the endplate; the resulting outward current across adjacent areas of membrane leads to their depolarization and the generation of propagated nerve-like action potentials in the muscle cell membrane.

5. A twitch is a single muscle contraction, produced in response to a single action potential in the muscle cell membrane. A tetanus is a larger muscle contraction that results from repetitive stimulation (multiple action potentials) of the cell membrane. Its force represents the temporal summation of many twitch contractions.

6. Isometric contraction results when an activated muscle is prevented from shortening and force is produced without movement.

7. Isotonic contraction results when an activated muscle shortens against an external force (or load). The external load determines the force that the muscle will develop, and the developed force determines the velocity of shortening.

8. The length-tension curve describes the effect of the resting length of a muscle on the isometric force it can develop. This relationship, which passes through a maximum at the normal length of the muscle in the body, is determined largely by the molecular and cellular ultrastructure of the muscle.

9. The force-velocity curve describes the inverse relationship between the isotonic force and the shortening velocity in a fully activated muscle.

10. The power output of an isotonically contracting skeletal muscle is determined by the velocity of shortening, which is determined by the size of the load; it is maximal at approximately one-third of the maximal isometric force.

11. All muscles are arranged so that they may be extended by the action of antagonistic muscles or by an external force such as gravity. Muscles do not forcibly reextend themselves after shortening.

12. The control of skeletal muscle contraction is exercised through the thin filaments and is termed actin-linked. Smooth muscle contraction is controlled primarily via the thick filaments and is termed myosin-linked.

13. The links between cellular excitation and mechanical contraction in smooth muscle are varied and complex. In most of the pathways, the cellular concentration of free calcium ions is an important link in the process of activation and contraction.

14. The primary step in the regulation of smooth muscle contraction is the phosphorylation of the regulatory light chains of the myosin molecule, which is then free to interact with actin. Relaxation involves phosphatase-mediated dephosphorylation of the light chains.

15. The contractions of smooth muscle are considerably slower than those of skeletal muscle, but are much more economical in their use of cellular energy. A crossbridge mechanism called the "latch state" enables some smooth muscles to maintain contraction for extremely long periods of time.

16. Smooth muscle tissues, especially those in the walls of distensible organs, can operate over a wide range of lengths.

Chapter 8 dealt with the mechanics and activation of the internal cellular processes that produce muscle contraction. This chapter treats muscles as organized tissues, beginning with the events leading to membrane activation by nerve stimulation and continuing with the outward mechanical expression of internal processes.

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