Physiology

Like those of vertebrates, insect muscles contract according to the sliding filament theory. The arrival of an excitatory nerve impulse at a neuromuscular junction causes depolarization of the adjacent sarcolemma. A wave of depolarization spreads over the fiber and into the interior of the cell via the T system. Depolarization of the T system membranes induces a momentary increase in the permeability of the adjacent sarcoplasmic reticulum, so that calcium ions, stored in vesicles of the reticulum, are released into the sarcoplasm surrounding the myofibrils. The calcium ions activate cross-bridge formation between the actin and myosin, enabling the filaments to slide over each other so that the distance between adjacent Z lines is decreased. The net effect is for the muscle to contract. Energy derived from the hydrolysis of adenosine triphosphate (ATP) is required for contraction, though its precise function is unknown. It may be used in breaking the cross-bridges, or for the active transport of the calcium ions back into the vesicles, or for both of these processes. In

FIGURE 14.4. Polyneuronal and multiterminal innervation of an insect muscle. [After G. Hoyle, 1974, Neural control of skeletal muscle, in: The Physiology ofInsecta, 2nd ed., Vol. IV (M. Rockstein, ed.). By permission of Academic Press, Inc., and the author.]

inhibitory axon inhibitory axon

FIGURE 14.4. Polyneuronal and multiterminal innervation of an insect muscle. [After G. Hoyle, 1974, Neural control of skeletal muscle, in: The Physiology ofInsecta, 2nd ed., Vol. IV (M. Rockstein, ed.). By permission of Academic Press, Inc., and the author.]

addition to sliding over each other, both the actin and the myosin filaments may shorten (by coiling), and in some myofibrils the Z lines disintegrate to allow the A bands of adjacent sarcomeres to overlap each other, thus enabling an even greater degree of contraction to occur.

Extension (relaxation) of a muscle may result simply from the opposing elasticity of the cuticle to which the muscle is attached. More commonly, muscles occur in pairs, each member of the pair working antagonistically to the other; that is, as one muscle is stimulated to contract, its partner (unstimulated) is stretched. Normally, the previously unstimulated muscle is stimulated to begin contraction while active contraction of the partner is still occurring (cocontraction). This is thought to bring about dampening of contraction, perhaps thereby preventing damage to a vigorously contracting muscle. Also, in slow movements, it provides an insect with a means of precisely controlling such movements (Hoyle, 1974). Muscle antagonism is achieved by central inhibition, that is, at the level of interneurons within the central nervous system (Chapter 13, Section 2.3). Thus, for a given stimulus, the passage of impulses along an axon to one muscle of the pair will be permitted, and hence that muscle will contract. However, passage of impulses to the partner is inhibited and the muscle will be passively stretched. It should be emphasized that in this arrangement the axon to each muscle is excitatory. In slow walking movements, for example, alternating stimulation of each muscle is quite distinct. At higher speeds this reciprocal inhibition breaks down, and one of the muscles remains permanently in a mildly contracted state, serving as an "elastic restoring element" (Hoyle, 1974). The other muscle continues to be alternately stimulated and thus provides the driving power for the activity.

As noted earlier, commonly muscles receive two excitatory axons, one "slow," the other "fast." These terms are somewhat misleading for they do not indicate the speed at which impulses travel along the axons, but rather the speed at which a significant contraction can be observed in the muscle. Thus, an impulse traveling along a fast axon induces a strong contraction of the "all or nothing" type; that is, a further contraction cannot be initiated until the original ionic conditions have been restored. In contrast, a single impulse from a slow axon causes only a weak contraction in the muscle. However, additional impulses arriving in quick succession are additive in their effect (summation) so that, with the slow axon arrangement a graded response is possible for a particular muscle, despite the relatively few fibers it may contain. Muscles with dual innervation use only the slow axon for most requirements; the fast axon functions only when immediate and/or massive contraction is necessary. For example, the extensor tibia muscleof the hindleg of a grasshopper isordinarily controlled solely via the slow axon. For jumping, however, the fast axon is brought into play.

The function of inhibitory axons remains questionable. Electrophysiological work has shown that in normal activity the inhibitory axon is electrically silent, that is, shows no electrical activity, and is clearly being inhibited from within the central nervous system. Duringperiodsofgreat activity, impulses can sometimes beobservedpassing along the axon, perhaps to accelerate muscle relaxation, though normally the use of antagonistic muscles and central inhibition is adequate. Hoyle (1974) suggested that peripheral inhibition may be necessary at certain stages in the life cycle, such as molting, when central inhibition may not be possible.

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