Summary on Muscle Tissue

Summary

Muscular tissue is used for movement, stability of the body, control of body passages and openings, and heat production. Muscle cells are able to carry out excit-ability, conductivity, contractility, extensibility, and elasticity. A muscle fiber is a long, slender cell with multiple nuclei just inside the sarcolemma. The sar-coplasm is occupied mainly by myofibrils, which are thread-like bundles of protein filaments. A muscle fiber is a long, slender cell with multiple nuclei just inside the sarcolemma. The sarcoplasm is occupied mainly by myofibrils, which are thread-like bundles of protein filaments. Each myofibril is a bundle of protein myofilaments. The thick filaments are made up mostly of myosin and the thin filaments mostly of actin.

Nerves and muscle fibers meet at a complex of synapses called a neuromuscular junction. Each tip of a nerve fiber ends in the synaptic knob. A narrow gap, the synaptic cleft, separates the synaptic knob from the sarcolemma. The synaptic knob contains synaptic ves-icles. An enzyme called acetylcholinesterase, found in the synaptic cleft and as part of the sarcolemma, breaks down ACh to terminate stimulation of the muscle fiber.

Smooth muscle fibers are much smaller than skeletal muscle fibers. Smooth muscle does not have the striations that skeletal muscle has. A different type of myosin exists in smooth muscle. There are two types of smooth muscle: multiunit (found in the eyes and blood vessels) and visceral (found in hollow organs). The wave-like motion of many tubular organs, known as peristalsis, is caused by visceral smooth muscles. Smooth muscle, unlike skeletal muscle, is affected by the neurotransmitter called norepinephrine. Smooth muscle takes approximately 30 times longer than skel-etal muscle to contract and relax, but uses much less energy. Smooth muscle contraction is regulated by nerves, hormones, and local chemical changes.