Problem 156 - Objective Biology for NEET

Question

Assertion: Muscle contraction force increases with rise in strength of stimulus. Reason: This is due to increased contraction of individual muscle fibres with increase in stimulus strength.

Step-by-Step Solution

Answer

The assertion and reason are correct, and they align with each other. This is because the strength of muscle contraction relies on the activation of more motor units, which occurs with stronger stimuli.

Step 1: Understand the Assertion

The assertion states that the force of muscle contraction increases with rise in strength of stimulus. This pertains to the basic principle of muscle physiology where a stronger stimulus leads to a more forceful muscle contraction.

Step 2: Understand the Reason

The reason provided claims that the rise in contraction force is due to increased contraction of individual muscle fibres with increase in stimulus strength. This is a concept known as 'motor unit recruitment'. A motor unit consists of a motor neuron and all of the muscle fibers it innervates. When a stronger stimulus is applied, more motor units (and thus more muscle fibers) are activated.

Step 3: Evaluate Assertion and Reason

The assertion and reason are both scientifically sound. The force of muscle contraction indeed increases with stronger stimuli, and this is due to the recruitment of more motor units. Thus, the assertion and the reason correspond with each other.

Key Concepts

Stimulus Strength

Muscle contraction is deeply influenced by the strength of the stimulus applied. Simply put, a stimulus is an external force that prompts an action in the body, such as a muscle contraction. The stimulus needs to be of adequate strength to cause muscle fibers to contract effectively.

In muscle physiology, the relationship between stimulus strength and contraction force is quite direct. As the strength of the stimulus increases, it triggers more robust contractions within the muscle.

• A weak stimulus might not excite enough fibers, resulting in a feeble contraction.
• Conversely, a strong stimulus will activate more muscle fibers, generating a stronger contraction.

The key takeaway is that the stimulus needs to be sufficiently strong to ensure all of the necessary fibers are recruited, contributing to the overall force of the contraction.

Motor Unit Recruitment

Motor unit recruitment is a fundamental concept that explains how muscles adjust to the demands of varying stimulus strengths. A motor unit consists of a motor neuron and the muscle fibers it controls. When your body needs to generate more force, it recruits more motor units.

This process is akin to rallying groups of workers (motor units) to complete a task. The more workers you have, the stronger and more efficient the task is carried out.

• With a weak stimulus, only a few motor units are needed, hence fewer muscle fibers contract.
• For a stronger stimulus, more motor units are called into action, enhancing the contraction force.

Intriguingly, not all motor units are alike. Some control only a few slow fibers for small, precise actions, while others command many fast fibers necessary for quick, powerful movements. The body's elegant recruitment strategy ensures muscles respond appropriately to varying needs.

Muscle Physiology

Muscle physiology is the study of how muscles function, including the processes behind muscle contraction and force production. To understand muscle contraction, one must grasp the basic structure of muscle tissue.

Muscles are made up of many fibers, and each fiber contains myofibrils packed with sarcomeres, the functional units of a muscle. When a stimulus reaches the muscle, it triggers an electrical signal, causing the sarcomeres to shorten and the muscle to contract.

• Stimuli must be adequately strong to cause all sarcomeres within a muscle fiber to contract efficiently.
• This contraction mechanism is driven by the sliding filament model, where myosin heads pull actin filaments closer.

Thus, the strength of muscle contraction is intimately connected to muscle physiology, stimulus strength, and motor unit recruitment. Together, these factors ensure that muscles respond powerfully and efficiently to various stimuli.