Question

1. Explain the steps involved in muscle contraction starting from stimulation of the sciatic nerve (somatic motor neuron) to contraction of the muscle fibers within the gastrocnemius. Be sure to include any movements of ions, neurotransmitters, myofilaments and other relevant structures/particles in your answer. (3 pts) 2. Why do we see a graded response (tension increasing) in single muscle twitch when increasing the voltage (strength) of the stimulus that is applied? Was there a point at which maximal tension was reached no matter how high you increased the stimulus? Explain why you think this happened in terms of motor units and recruitment. (2 pt) Tension (percent of maximum) 3. Describe what is occurring in the graph to the right at the level of the sarcomere (100 is optimal length of the sarcomere). Label the following on the graph to the right: fully slack, moderately slack, normal tension, stretched, overstretched. Explain why each length of the sarcomere produces a certain tension. (4 pts) 40 60 80 100 4. Why is there a latent period in the muscle contraction Decreased in an isometric contraction? Does the duration of the length latent period change with increased stimulus strength? Why or why not? (2 pts) 120 140 160 Increased length

Answer 1

Ans) 1) During skeletal muscle contraction, the thick filament slides over the thin filament by a repeated binding and releases myosin along the filament. This whole process occurs in a sequential manner.
Step 1:
Muscle contraction is initiated by signals that travel along the axon and reach the neuromuscular junction or motor end plate. The neuromuscular junction is a junction between a neuron and the sarcolemma of the muscle fibre. As a result, acetylcholine (a neurotransmitter) is released into the synaptic cleft by generating an action potential in the sarcolemma.
Step 2:
The generation of this action potential releases calcium ions from the sarcoplasmic reticulum in the sarcoplasm.
Step 3:
The increased calcium ions in the sarcoplasm lead to the activation of actin sites. Calcium ions bind to the troponin on actin filaments and remove the tropomyosin, wrapped around actin filaments. Hence, active actin sites are exposed and this allows myosin heads to attach to this site.
Step 4:
In this stage, the myosin head attaches to the exposed site of actin and forms cross bridges by utilizing energy from ATP hydrolysis. The actin filaments are pulled. As a result, the H-zone reduces. It is at this stage that the contraction of the muscle occurs.
Step 5:
After muscle contraction, the myosin head pulls the actin filament and releases ADP along with inorganic phosphate. ATP molecules bind and detach myosin and the cross bridges are broken.
Step 6:
This process of formation and breaking down of cross bridges continues until there is a drop in the stimulus, which causes an increase in calcium. As a result, the concentration of calcium ions decreases, thereby masking the actin filaments and leading to muscle relaxation.

2) A graded muscle response allows variation in muscle tension. Summation occurs as successive stimuli are added together to produce a stronger muscle contraction. ... Increasing the number of motor neurons involved increases the amount of motor units activated in a muscle, which is called recruitment.

- Muscles exhibit graded contractions in two ways: Quantal Summation or Recruitment - this refers to increasing the number of cells contracting. This causes the contractions to build on one another producing a wave pattern or, if the stimuli are high frequency, a sustained contraction called tetany or tetanus.

3) The active length-tension relationship is determined by overlapping of actin and myosin filaments. An increase in preload provides better overlapping of actin and myosin filaments, allowing the muscle cell to generate more contractile force.

- The cross-bridges can only form where thin and thick filaments already overlap, so that the length of the sarcomere has a direct influence on the force generated when the sarcomere shortens. This is called the length-tension relationship.

4) The latent period, or lag phase, the contraction phase, and the relaxation phase. The latent period is a short delay (1-2 msec) from the time when the action potential reaches the muscle until tension can be observed in the muscle. . Even when a muscle is at rest, there is random firing of motor units.