Question

Put the following statement in proper order to describe muscle contraction. 1\. Signal sent by CNS via motor neuron. 2\. Generation of action potential in the sarcolemma. 3\. Release of \(\mathrm{Ca}^{+2}\) from sarcoplasmic reticulum. 4\. The neurotransmitter acetycholine released motor endplate. 5\. Sarcomere shorterns. (a) \(1 \rightarrow 2 \rightarrow 4 \rightarrow 3 \rightarrow 5\) (b) \(1 \rightarrow 4 \rightarrow 2 \rightarrow 3 \rightarrow 5\) (c) \(1 \rightarrow 4 \rightarrow 3 \rightarrow 2 \rightarrow 5\) (d) \(5 \rightarrow 4 \rightarrow 3 \rightarrow 2 \rightarrow 1\)

Short answer

The correct order of steps for muscle contraction is option (b): \(1 \rightarrow 4 \rightarrow 2 \rightarrow 3 \rightarrow 5\). This sequence is: Signal sent by CNS via motor neuron, the neurotransmitter acetycholine released at motor endplate, generation of action potential in the sarcolemma, release of \(\mathrm{Ca}^{+2}\) from sarcoplasmic reticulum, and finally, the sarcomere shortens.

Step-by-step solution

Look at all the options

First, you should understand each individual statement given. Then, analyze all given options and look for a logical sequence of statements. Read carefully through options (a) to (d) and try to establish a logical sequence based on your understanding of muscle contraction process.

Rule out incorrect orders

Next, you would want to rule out any options that have incorrect orders. Having knowledge of muscle contraction, you know that the contraction is initiated by a signal from the Central Nervous System (CNS), so any option that doesn't start with this step (Statement 1) can be ruled out. Therefore, option (d) can be eliminated as it starts with the sarcomere shortening, which is the final stage of the muscle contraction.

Select the correct order

Now you should select the option that represents the correct sequence of a muscle contraction process. By understanding that the neurotransmitter acetylcholine is released at the motor endplate as a result of the initiation signal from the CNS (Statement 4), then Generation of action potential in the sarcolemma comes as a result of the released acetylcholine (Statement 2), followed by the release of \(\mathrm{Ca}^{+2}\) from the sarcoplasmic reticulum (Statement 3) and finally, the sarcomere shortens (Statement 5). Therefore, option (b), which presents this order: \(1 \rightarrow 4 \rightarrow 2 \rightarrow 3 \rightarrow 5\), is the correct sequence.

Key concepts

Central Nervous System

The central nervous system, or CNS, acts as the brain's messenger to the muscles. It consists of the brain and spinal cord, working together to process information and send signals to different parts of the body.
The CNS initiates the muscle contraction process by sending a signal through a motor neuron. This electrical signal, known as a nerve impulse, travels down the spinal cord and into the peripheral nervous system, where it ultimately reaches the muscle.
In muscle contraction, this signal from the CNS is the very first step, setting off a chain of events that lead to the muscle fibers contracting and thus causing movement. Understanding how the CNS works in conjunction with other bodily systems gives insight into how our bodies control voluntary movements.

Acetylcholine

Acetylcholine is a chemical messenger known as a neurotransmitter that plays a crucial role in muscle contraction. It is released from the motor neuron at a region called the motor endplate, located near the muscle fiber.
When the nerve impulse reaches the end of the neuron, acetylcholine is released into the synaptic cleft, the space between the neuron and muscle fiber. This neurotransmitter then binds to receptors on the muscle surface, known as the sarcolemma, which stimulates the muscle to contract.
Without the presence of acetylcholine, the signal from the CNS could not convert into a response in the muscle. This highlights the importance of acetylcholine in transferring the message from the nervous system to the muscular system effectively.

Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a specialized type of smooth endoplasmic reticulum found in muscle cells. Its main function is to store and release calcium ions ( Ca^{+2} ). These ions are vital for muscle contraction.
Once the action potential travels along the sarcolemma and reaches the SR, it triggers the release of the stored calcium ions into the muscle cytoplasm.
The presence of calcium in the cytoplasm allows binding with proteins on the muscle's actin filaments, facilitating the cross-bridge cycle, which ultimately leads to muscle contraction. Without the efficient storage and release of calcium by the SR, the muscle fibers would not be able to contract properly.

Sacrolemma

The sarcolemma is the cell membrane that surrounds each muscle fiber. It acts as a barrier and communicates the signal for contraction from the motor neuron to the inside of the muscle cell.
Upon receiving the neurotransmitter acetylcholine, the sarcolemma generates an action potential—a wave of electrical change that spreads across the muscle fiber.
This change is essential as it triggers the muscle contraction processes inside the fiber, leading to the release of calcium from the sarcoplasmic reticulum. The sarcolemma's ability to effectively transmit electrical signals ensures the muscle responds rapidly and correctly to the CNS.

Sarcomere

Sarcomeres are the basic units of a muscle's striated muscle tissue and are responsible for the muscle's contractile properties. They are composed of repeating patterns of actin and myosin filaments that slide past each other during muscle contraction.
The shortening of the sarcomere leads to the shortening of the entire muscle fiber, which results in movement.
During contraction, the presence of calcium ( Ca^{+2} ) allows myosin heads to bind to actin filaments, creating cross-bridges which pull the actin filaments toward the center of the sarcomere. This pulling action shortens the sarcomere and produces force. Understanding sarcomeres is crucial to comprehending how muscles contract and generate movement.