Question

44\. Following is given a randomly arranged list of events that occur at neuromuscular junction to trigger muscle contraction. (i) Receptor sites on sarcolemma (ii) Nerve impulse (iii) Release of \(\mathrm{Ca}^{+2}\) from sarcoplasmic reticulum (iv) The neurotransmitter acetylcholine is released (v) Sarcomere shortern (vi) Synaptic cleft (vii) Spread of impulses over sarcolemma on T-tubules Which of the following gives the correct sequence of these steps? (a) (ii), (iv), (i), (vi), (vii), (iii), (v) (b) (ii), (iv), (vi), (i), (vii), (iii), (v) (c) (i), (ii), (iii), (iv), (v), (vi), (vii) (d) (vii), (vi), (v), (iv), (iii), (ii), (i)

Short answer

Option (b) provides the correct sequence.

Step-by-step solution

- Identify the correct sequence of events at a neuromuscular junction

The neuromuscular junction is the site where a motor neuron interfaces with a muscle fiber and uses a sequence of events to initiate muscle contraction. We must identify the sequence that correctly follows physiological processes as they occur during the triggering of muscle contraction.

- Arrange the sequence of events logically

First, the nerve impulse (ii) prompts the release of the neurotransmitter acetylcholine (iv). This neurotransmitter diffuses across the synaptic cleft (vi) and binds to receptor sites on the sarcolemma (i). This binding leads to a spread of impulses over sarcolemma and into T-tubules (vii), which in turn triggers the release of \(\mathrm{Ca}^{+2}\) from the sarcoplasmic reticulum (iii). The increase in calcium concentration causes the sarcomeres to shorten (v), which is the actual mechanism of muscle contraction.

- Match the arranged sequence with the given options

Following the sequence we arranged in step 2, we look for an option that matches this order. The correct order is (ii), nerve impulse; (iv), release of acetylcholine; (vi), synaptic cleft; (i), receptor sites on sarcolemma; (vii), spread of impulses; (iii), release of \(\mathrm{Ca}^{+2}\); (v), sarcomere shortening. By comparing this sequence to the provided options, we find that option (b) is the correct choice.

Key concepts

Muscle Contraction

Muscle contraction is an intricate process that involves a sequence of events, leading muscle fibers to shorten and generate force. It begins with a nerve impulse from the brain or spinal cord, which travels along a motor neuron to the neuromuscular junction.

This impulse triggers the release of acetylcholine, causing an electrical charge to spread across the muscle fiber's membrane, the sarcolemma. The signal delves deeper into the muscle fiber through structures called T-tubules, reaching the sarcoplasmic reticulum.

Upon receiving the signal, the sarcoplasmic reticulum releases calcium ions, which play a crucial role in the interaction between actin and myosin filaments within the sarcomeres—the contractile units of muscle fibers. This interaction facilitates the sliding mechanism of muscle contraction, ultimately resulting in the shortening of sarcomeres and, consequently, muscle contraction.

• Nerve impulse begins contraction sequence
• Acetylcholine released at neuromuscular junction
• Electrical impulse spreads via sarcolemma
• Calcium ions released from sarcoplasmic reticulum
• Myofilaments slide, causing sarcomere and muscle to shorten

Acetylcholine

Acetylcholine (ACh) is a neurotransmitter essential for muscle contraction. It's released from the motor neuron endings at the neuromuscular junction. When a nerve impulse reaches this junction, vesicles containing ACh fuse with the neuron's membrane, releasing the neurotransmitter into the synaptic cleft.

ACh molecules cross the synaptic cleft quickly and bind to ACh receptors on the sarcolemma of the muscle fiber. This binding action causes a change in the permeability of the sarcolemma to sodium and potassium ions, generating an electrical signal that stimulates muscle contraction.

• ACh released by motor neurons
• Crosses synaptic cleft and binds to receptors
• Changes ion permeability for signal generation
• Initiates muscle fiber contraction

It is worth noting that the rapid breakdown of ACh by the enzyme acetylcholinesterase is also critical, as it ensures that muscles can quickly relax following contraction.

Sarcolemma

The sarcolemma is the cell membrane of muscle fibers that plays a vital role in muscle contraction. It's not just a passive barrier; it actively participates in the transmission of electrical signals. When acetylcholine binds to the receptors on the sarcolemma, it becomes depolarized, initiating an action potential.

This action potential swiftly travels across the sarcolemma, reaching down into the muscle fiber through T-tubules. The link between the sarcolemma and the T-tubules ensures that the action potential accesses the interior of the muscle fiber, leading to a coordinated contraction.

• Sarcolemma is the muscle fiber membrane
• Depolarizes to start action potential
• Transmits signals into muscle fiber

The sarcolemma's ability to conduct electricity is what makes the swift and precise control of muscle contraction, and relaxation possible.

Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a critical storage site for calcium ions in muscle cells, essential for the regulation of muscle contraction. It is a specialized form of the endoplasmic reticulum in skeletal and cardiac muscle cells.

When an action potential travels along the sarcolemma and into the T-tubules, it signals the sarcoplasmic reticulum to release stored calcium ions into the cytoplasm of the muscle cell. These ions bind to troponin on actin filaments, leading to a conformational change that allows myosin heads to attach to actin and initiate the power stroke that causes muscle contraction.

• SR stores calcium ions
• Releases calcium in response to action potentials
• Facilitates actin-myosin interaction for contraction

Following contraction, calcium ions are actively pumped back into the SR, reducing cytoplasmic calcium levels, which allows muscles to relax. The precise control of calcium ion release and uptake is vital to the smooth execution of muscle movement.