Zoology Notes

Structure of Skeletal Muscle

A muscle fiber (cell) has special terminology and distinguishing characteristics:

• The sarcolemma, or plasma membrane of the muscle cell, is highly invaginated by transverse tubules (T tubes) that permeate the cell.
• The sarcoplasm, or cytoplasm of the muscle cell, contains calcium‐storing sarcoplasmic reticulum, the specialized endoplasmic reticulum of a muscle cell.
• Striated muscle cells are multinucleated. The nuclei lie along the periphery of the cell, forming swellings visible through the sarcolemma.
• Nearly the entire volume of the cell is filled with numerous long myofibrils. Myofibrils consist of two types of filaments, shown in Figure 1:
• Thin filaments consist of two strands of the globular protein actin arranged in a double helix. Along the length of the helix are troponin and tropomyosin molecules that cover special binding sites on the actin.
• Thick filaments consist of groups of the filamentous protein myosin. Each myosin filament forms a protruding head at one end. An array of myosin filaments possesses protruding heads at numerous positions at both ends.Within a myofibril, actin and myosin filaments are parallel and arranged side by side. The overlapping filaments produce a repeating pattern that gives skeletal muscle its striated appearance. Each repeating unit of the pattern, called a sarcomere, is separated by a border, or Z disc (Z line), to which the actin filaments are attached. The myosin filaments, with their protruding heads, float between the actin, unattached to the Z disc.

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Ans :- Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for the hormone.

In other words, a particular cell is a target cell for a hormone if it contains functional receptors for that hormone, and cells which do not have such a receptor cannot be influenced directly by that hormone. Reception of a radio broadcast provides a good analogy. Everyone within range of a transmitter for National Public Radio is exposed to that signal (even if they don't contribute!). However, in order to be a NPR target and thus influenced directly by their broadcasts, you have to have a receiver tuned to that frequency.

Hormone receptors are found either exposed on the surface of the cell or within the cell, depending on the type of hormone. In very basic terms, binding of hormone to receptor triggers a cascade of reactions within the cell that affects function. Additional details about receptor structure and function are provided in the section on hormone mechanism of action.

A traditional part of the definition of hormones described them as being secreted into blood and affecting cells at distant sites. However, many of the hormones known to act in that manner have been shown to also affect neighboring cells or even have effects on the same cells that secreted the hormone. Nonetheless, it is useful to be able to describe how the signal is distributed for a particular hormonal pathway, and three actions are defined:

Endocrine action: the hormone is distributed in blood and binds to distant target cells.

Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.

Autocrine action: the hormone acts on the same cell that produced it.