Question

Which of the following statements about the mechanism of muscle contraction are correct? (i) Acetylcholine is released when the neural signal reaches the motor end plate. (ii) Muscle contraction is initiated by a signal sent by CNS via a sensory neuron. (iii) During muscle contraction, isotropic band gets elongated. (iv) Repeated activation of the muscles can lead to lactic acid accumulation. (a) (i) and (iv) (b) (i) and (iii) (c) (ii) and (iii) (d) (i), (ii) and (iii)

Short answer

(i) and (iv) are correct.

Step-by-step solution

Understanding Acetylcholine Release

Acetylcholine is a neurotransmitter released into the synaptic cleft when a motor neuron impulse reaches the motor end plate. It is essential for the transmission of the neural signal to the muscle fiber, thus initiating muscle contraction. Statement (i) is correct.

Recognizing the Source of the Signal for Muscle Contraction

Muscle contraction is initiated by a signal from the central nervous system (CNS), but it is sent via a motor neuron, not a sensory neuron as stated in (ii). Sensory neurons carry information from the body to the CNS, not initiate muscle contraction. Therefore, statement (ii) is incorrect.

Analyzing Changes in Muscle Bands During Contraction

During muscle contraction, the I (isotropic) band actually shortens, not elongates. This occurs because the actin filaments slide over the myosin, shortening the sarcomere, the functional unit of muscle fibers. Statement (iii) is incorrect.

Examining the Effect of Repeated Muscle Activation

Repeated or sustained muscle activation, especially in the absence of sufficient oxygen (anaerobic respiration), can lead to the accumulation of lactic acid in the muscles. This can cause muscle fatigue. Statement (iv) is correct.

Key concepts

Acetylcholine Release

The communication between your nerves and muscles is the cornerstone of how you move your body. When your brain decides it's time to take action, it sends out a signal that ultimately causes muscles to contract. This is where acetylcholine enters the picture.

Acetylcholine is a neurotransmitter, a chemical messenger, which is released by motor neurons when they receive a nerve impulse. This release occurs specifically at the motor end plate – a specialized region where a motor neuron interfaces with a muscle cell. As soon as the acetylcholine is released it crosses the tiny gap, known as the synaptic cleft, and binds to receptors on the muscle cell's surface. This binding event leads to a series of reactions inside the muscle cell, ultimately resulting in contraction. Therefore, acetylcholine plays an indispensable role in converting a neural signal into a mechanical action – the contraction of your muscles.

Signal from CNS

Our bodies are like intricate networks, constantly sending signals from one place to another. The central nervous system, or CNS, is effectively the control centre of this network. It comprises the brain and spinal cord and is responsible for processing sensory information and governing responses.

When the CNS decides that a muscle needs to contract – whether it’s to walk, type, or lift weights – it sends an electric-like signal down the motor neurons. These are the neurons that directly interact with muscle cells. Contrary to what some believe, sensory neurons are not directly involved in this process; instead, they are responsible for transmitting information from the body to the CNS. Once the signal reaches a muscle, thanks to the motor neuron, it triggers the release of acetylcholine and kicks off muscle contraction.

Muscle Band Changes

Muscles are not only the engines for your movement but are also fascinating in their design. A muscle fiber contains myofilaments arranged in repeating units called sarcomeres. Within each sarcomere, there are two types of bands that are important for muscle contraction: the isotropic (I) band and anisotropic (A) band.

When a muscle contracts, the thin actin filaments within the sarcomere slide closer together, pulled by the thicker myosin filaments. This process is wonderfully orchestrated by the interaction of these proteins and is powered by ATP, the cell's energy molecule. During this contraction, it's the I bands – which are light bands composed of actin – that shorten, not elongate. This brings the Z lines closer together, effectively shortening the sarcomere and leading to muscle shortening or contraction. These precise alterations in the muscle bands enable us to perform everything from grand movements to delicate tasks.

Lactic Acid Accumulation

Even the strongest muscles have their limits. When we tax our muscles, either through prolonged activity or intense workouts, they sometimes can't get enough oxygen to sustain the energy they need via aerobic respiration. In this situation, they switch to anaerobic respiration – a method of producing energy without oxygen.

During anaerobic respiration, glucose is broken down and one of the byproducts is lactic acid. When lactic acid accumulates in the muscles faster than it can be removed, it can lead to muscle fatigue and soreness. This scenario is common during exercises like sprinting or heavy lifting, which can lead to that burn you feel, indicating that you’re pushing your muscles hard. It might be uncomfortable, but it's a sign that you're reaching the intensity needed for your muscles to get stronger.