The Ca++=troponin complex pulls tropomyosin away from the myosin-building sites on actin, allowing cross-bridges to form. Last of all, cross-bridge cycles continue as long as Ca++ remains attach to troponin and when ATP is available. When nerve activity ceases, impulses in the muscle fiber also stop, and Ca++ is actively transported from the cytoplasm back into the sarcoplasmic reticulum. As Ca++ is released from troponin, tropomyosin returns to its inhibitory position on the thin filament, preventing the myosin heads from binding to actin. The muscle fiber relaxes. Neurons produced electrical excitation of the muscle fibers, and electrical excitation of the muscle fibers indirectly produces sliding of the myofilaments and contraction. In the muscle fiber, electrical excitation is coupled to contraction through the action of Ca++; this process is called excitation-contraction coupling. .
If a muscle fiber is stimulated by a single impulse in a motor neuron, it will contract rapidly and then relax; this is called a twitch. But, if 2 or more impulses stimulate the muscle fiber, delivered in rapid succession, there will be insufficient time for it to relax completely between twitches, and the second twitch will begin while the muscle fiber is still contracted. As a result, the 2nd twitch will add to the first to produce a greater shortening of the muscle fiber; this is known as summation. If numerous impulses are delivered so rapidly that there is no visible relaxation between successive twitches, a smooth, sustained contraction (tetanus) is produced. Normally, skeletal muscles contract with some degree of summation because motor neurons usually generate bursts of impulses in rapid succession. One of the primary ways the nervous system controls the strength of muscles contraction is by carrying the frequency of motor neuron impulses. .
A skeletal muscle has many muscle fibers, and each is innervated by only 1 motor neuron.
Utopia is completely attainable. Utopia, derived from the Greek words meaning no place, was always thought to be a model to strive for. Unfortunately human pessimism has never allowed Utopia to become a tangible place. Strangely enough it is this human aspect that is the barrier to a perfect existe...
Both lack an external tail and both are capable of reaching an upright posture as well as bipedal locomotion, walking on two legs. Humans and apes both have an appendix, which is an appendage that it believed to be used for the digestion of rocks, sticks, and tree bark. ...
Class Metamonada consists of unicellular organisms in which the nuclei are attached to the undulipodia (a cilium or flagellum whose purpose is locomotion or feeding) by rhizoplasts (also known as nuclear connectors. ... They digest cellulose from the wood that these insects eat, and at some stages of digestion, small particles of wood are visible in their cytoplasm. ... Members of this phylum lack flagella and instead used pseudopods for locomotion; they extend their protoplasm and re-form their bodies in the direction in which they wish to move (Anonymous 1998). ...
