The cell membrane of a muscle cell is called the sarcolemma.
Sarcolemma
Has a unique feature: it has holes in it. These 'holes' lead into tubes called transverse tubules (or T-Tubules for short), Golgi apparatus, abundant myofibrils and myoglobin and mitochondria.
These tubules pass down into the muscle cell and go around the myofibrils. However, these tubules do not open into the interior of the muscle cell; they pass completely through and open somewhere else on the sarcolemma (i.e., these tubules are not used to get things into and out of the muscle cell), the impulses in muscle cells is to bring about contraction and forms a physical barrier against the external environment. The functions of T-Tubules are to conduct impulses from the surface of the cell (sarcolemma)
down into the cell and, specifically, to another structure in the cell called the sarcoplasmic reticulum (modified endoplasmic reticulum) and activate the SR. The SR forms a network around the myofibrils, storing and providing the Ca that is required for muscle contraction. Sarcoplasmic reticulum is very abundant in skeletal muscle cells . There are "pumps" (active transport) for calcium so that calcium is constantly being "pumped" into the SR from the sarcoplasm.
As a result, in a relaxed muscle, there is a very high concentration of calcium in the SR and a very low concentration in the sarcoplasm (and, therefore, among the myofibrils & myofilaments). The membrane has special openings, or "gates", for calcium.
In a relaxed muscle, these gates are closed and calcium cannot pass through the membrane. So, the calcium remains in the SR. However, if an impulse travels along the membrane of the SR, the calcium "gates" open &, therefore, calcium diffuses rapidly out of the SR & into the sarcoplasm where the myofibrils & myofilaments are located. A muscle fiber is excited via a motor nerve that generates an action potential that spreads along the surface membrane (sarcolemma) and the transverse tubular system into the deeper parts of the muscle fiber.
marto answered the question on
April 16, 2019 at 08:18