Problem 157 - Objective Biology for NEET

Question

Assertion: Muscle as a whole doesn't obey all or none law. Reason: Each muscle fibre contracts maximally whenever it contracts.

Step-by-Step Solution

Answer

Yes, the assertion 'Muscle as a whole doesn't obey all or none law' and the reason 'Each muscle fibre contracts maximally whenever it contracts' are both correct, and the reason successfully explains the assertion. The key concept is that the behavior of individual muscle fibers (each obeying the 'all or none' law) adds up to give a wide range of contraction strengths for the muscle as a whole.

Step 1: Understanding the Assertion

Start by comprehending the assertion: 'Muscle as a whole doesn't obey all or none law.' This can be interpreted that not all muscle contractions are of the same magnitude or strength but instead, they can vary in strength.

Step 2: Understanding the Reason

In the second step, consider the reason given: 'Each muscle fibre contracts maximally whenever it contracts.' Each muscle fiber does indeed follow the all or none law wherein when a stimulus reaches a particular threshold, the fiber contracts completely.

Step 3: Comparing Assertion and Reason

The final step requires comparing the reason and assertion. Individual muscle fibers follow the all-or-none principle, meaning that they contract fully when triggered. However, a whole muscle does not act similarly - its strength of contraction can vary. It is because it is made up of many different fibers, which can be recruited in different numbers to achieve different strengths of contraction. Therefore, the statement 'Muscle as a whole doesn't obey all or none law' is correct, and the reason 'Each muscle fibre contracts maximally whenever it contracts' is a valid explanation for why this is the case.

Key Concepts

All or None Law

In the realm of muscle physiology, the "all or none law" plays a significant role in understanding how muscles contract. This principle pertains specifically to the behavior of individual muscle fibers. Each muscle fiber operates on the basis that once a specific threshold stimulus is reached, there is a full and complete contraction.
This means that a muscle fiber does not contract partially or weakly. Instead, it contracts all the way. Whether the stimulus is minimal or maximal, as soon as it crosses the threshold, the fiber will contract fully. This principle is crucial because it ensures that any given muscle fiber within a muscle group will behave predictably in response to a nerve impulse. However, this law applies only to individual fibers, not entire muscles. Each fiber acts independently, deciding on full contraction based on the threshold stimulus, while the overall muscle might respond differently to stimuli, depending on how many fibers get activated.

Muscle Fibers

Muscle fibers are the building blocks of muscle tissue, and they exhibit unique and fascinating characteristics. They are long, cylindrical cells that have the ability to contract and produce force.
It is these fibers that are responsible for our body movements. Each muscle is composed of numerous muscle fibers, and the physical properties, such as the number and type of fibers, influence both muscle efficiency and strength. Muscle fibers respond to stimuli by following the all or none law on an individual level, as mentioned above. There are different types of muscle fibers:

• **Type I fibers:** Also known as slow-twitch fibers, they are more efficient at using oxygen over long periods. They are great for endurance activities.
• **Type II fibers:** Known as fast-twitch fibers, these are better suited for short bursts of speed and strength. They fatigue quicker than type I fibers.

The type and amount of fibers recruited during muscle activity depend on the task's nature, allowing muscles to respond aptly to varying demands.

Strength of Contraction

The strength of a muscle contraction is not just a matter of how hard one "tries" to contract a muscle. It involves the complex coordination of activating multiple muscle fibers and synchronizing them in a way that peaks their output. That is why a muscle does not obey the all or none law as a whole.
Instead, the overall strength of a muscle contraction varies, depending on how many muscle fibers are activated and which types. There are factors that influence the strength of a contraction:

• **Number of activated fibers:** More activated fibers lead to a stronger contraction.
• **Type of muscle fibers:** Fast-twitch fibers provide more strength but for a shorter duration, while slow-twitch fibers provide endurance but less force.
• **Frequency of stimulation:** More frequent stimulation can lead to a stronger contraction, a phenomenon known as summation.

This flexibility in muscle contraction allows the body to adapt seamlessly to demands requiring different levels of strength and endurance, whether lifting a heavy object or running a marathon.