Test: Skeletal Muscles - NEET PG MCQ

In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands, so that attraction cannot occur between the actin and myosin filaments to cause contraction.

Myosin is thick filament. All others in the options are thin filaments.

Once the Ca²⁺ concentration outside the reticulum has been lowered sufficiently, chemical interaction between myosin and actin ceases and the muscle relaxes.

The total length of each myosin filament is uniform, almost exactly 1.6 micrometers.

Fine, irregular contractions of individual fibers is know as fibrillations.
This is the classic picture of a lower motor neuron lesion. If the motor nerve regenerates, the fibrillations disappear. Usually, the contractions are not visible grossly, and they should not be confused with fasciculations, which are jerky, visible contractions of groups of muscle fibers that occur as a result of pathologic discharge of spinal motor neurons.

The extraocular and hand muscle has a ratio of 3–6 (total 3000 motor units) whereas gastrocnemius and back muscle (concern with posture) has a innervation ratio of 2000 (total 600 units).

Changes in muscle strength at the onset of contraction due to repeated stimuli are known as staircase effect (Treppe). Although all the possible causes of the staircase effect are not known, primarily by increasing calcium ions in the cytosol because of the release of more and more ions from the sarcoplasmic reticulum with each successive muscle action potential and failure of the sarcoplasm to recapture the ions immediately.

Sarcomere is the length between two Z lines. In resting condition, this length is 2–2.2 μm.

During muscle contraction, two Z lines come closer; the length of the ‘A band’ remains constant whereas the length of H and I band decreases.
The M line becomes more prominent.

Action potential (AP) follow all or, none law. So, amplitude (strength) of AP normally does not change. Even if it changes theoretically, it will not alter the strength of contraction of muscle.

Summation of contraction depends on both ARP and RRP. Summation of skeletal muscle fiber contractions is possible because the absolute refractory period of the sarcolemma is considerably less than the duration of raised cytoplasmic Ca²⁺ concentration and subsequent tension generation. If a second stimulus is applied to the muscle before it has fully relaxed from the first, the response to the second stimulus will add to the residual response of the first stimulus (Summation).
Summation leads to increase in height and duration of contraction.

Ionic basis of tetanus: The Ca²⁺ ions released in the cytoplasm during single muscle twitch are removed quickly and relaxation occurs. When the muscle is stimulated in rapid succession, there occurs a progressive accumulation of Ca²⁺ ions in the cytoplasm. The longer stay and increased amount of Ca²⁺ ions in the cytoplasm increase the tension developed rises to tetanic levels.

Actin, myosin, tropomyosin are present in both skeletal muscle and smooth muscle. But troponin is absent in smooth muscle, in place of that CALMODULIN is present in smooth muscle.

Troponin C contains the binding sites for the 4 molecule of Ca²⁺ that helps to initiate contraction.

Increasing frequency of motor unit activation leads to summation of contractions, which increase force of contraction. More motor unit activation or, larger motor unit activation means large number of muscle fiber will contract simultaneously, so force of contraction will be more.
Amplitude of AP has no effect on force of contraction.