Skeletal Muscle Contraction & Rheobase | UPSC Mains ZOOLOGY-PAPER-II 2024

Explain the mechanism of contraction in skeletal muscle. What do you mean by Rheobase and Chronaxie?

How to Approach

This question requires a detailed understanding of muscle physiology, specifically the sliding filament theory of muscle contraction. The second part asks for definitions of Rheobase and Chronaxie, which are electrophysiological properties of muscle fibers. The answer should be structured to first explain the mechanism of contraction, detailing the roles of actin, myosin, calcium, and ATP. Then, it should clearly define Rheobase and Chronaxie, explaining their significance in understanding muscle excitability. Diagrams would be beneficial but are not possible in this text-based format.

Skeletal muscle contraction is fundamental to movement, posture, and various physiological processes. This intricate process relies on the coordinated interaction of proteins within muscle fibers, converting chemical energy into mechanical force. The process is governed by the sliding filament theory, where actin and myosin filaments interact to shorten the sarcomere, the basic contractile unit of muscle. Understanding the nuances of this process, alongside concepts like Rheobase and Chronaxie, is crucial for comprehending neuromuscular function and diagnosing related disorders. Rheobase and Chronaxie are important parameters used in electrophysiology to characterize the excitability of muscle fibers.

Mechanism of Contraction in Skeletal Muscle

The mechanism of skeletal muscle contraction can be divided into several key steps:

1. Neuromuscular Junction & Action Potential

A motor neuron releases acetylcholine (ACh) at the neuromuscular junction.
ACh binds to receptors on the muscle fiber membrane (sarcolemma), initiating an action potential.
The action potential propagates along the sarcolemma and down the T-tubules.

2. Calcium Release

The action potential triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum (SR).
Ca²⁺ binds to troponin, a protein complex on the actin filament.

3. Cross-Bridge Formation & Sliding Filament Mechanism

Troponin undergoes a conformational change, shifting tropomyosin (another protein on actin) and exposing myosin-binding sites.
Myosin heads, already energized by ATP hydrolysis, bind to these exposed sites on actin, forming cross-bridges.
The myosin head pivots, pulling the actin filament towards the center of the sarcomere (power stroke). This requires energy from ATP.
ATP binds to the myosin head, causing it to detach from actin.
The myosin head re-energizes by hydrolyzing ATP, returning to its high-energy conformation, ready to bind again.
This cycle of attachment, pivoting, detachment, and re-energizing repeats, causing the actin and myosin filaments to slide past each other, shortening the sarcomere.

4. Muscle Relaxation

The motor neuron stops releasing ACh.
ACh is broken down by acetylcholinesterase.
Ca²⁺ is actively transported back into the SR by Ca²⁺-ATPase pumps, reducing Ca²⁺ concentration in the sarcoplasm.
Troponin and tropomyosin return to their original positions, blocking myosin-binding sites on actin.
Cross-bridge formation ceases, and the muscle relaxes.

Rheobase and Chronaxie

These terms relate to the electrical properties of excitable tissues, including muscle.

Rheobase

Rheobase is the minimum current strength required to elicit an action potential in an excitable cell. It represents the threshold current needed to depolarize the cell membrane sufficiently to trigger the opening of voltage-gated ion channels and initiate an action potential. It is typically measured in milliamperes (mA).

Chronaxie

Chronaxie is the duration of a current pulse, in milliseconds, required to elicit a minimal response (usually a muscle twitch) when the current strength is twice the rheobase. It is a measure of the excitability of a muscle fiber. A shorter chronaxie indicates a more excitable muscle fiber, meaning it responds quickly to stimulation. Conversely, a longer chronaxie indicates lower excitability.

The relationship between Rheobase and Chronaxie is described by the Weiss equation: Chronaxie = k * Rheobase, where k is a constant.

Parameter	Definition	Units	Significance
Rheobase	Minimum current to elicit an action potential	mA	Indicates threshold for excitation
Chronaxie	Pulse duration for minimal response at 2x Rheobase	ms	Indicates excitability of the muscle fiber

Additional Resources

Key Definitions

Sarcomere

The basic contractile unit of a muscle fiber, defined as the segment between two successive Z-lines.

Acetylcholinesterase

An enzyme that rapidly breaks down acetylcholine, terminating its action at the neuromuscular junction.

Key Statistics

Approximately 40% of the human body mass is skeletal muscle.

Source: Guyton and Hall Textbook of Medical Physiology (Knowledge cutoff 2023)

Skeletal muscle accounts for approximately 85% of the body’s total heat production.

Source: Marieb & Hoehn, Human Anatomy & Physiology (Knowledge cutoff 2023)

Examples

Muscle Cramps

Muscle cramps often occur due to electrolyte imbalances (e.g., potassium, calcium, magnesium) which disrupt the normal muscle contraction and relaxation cycle, leading to sustained involuntary contractions.

Frequently Asked Questions

▶What is the role of ATP in muscle contraction?

ATP provides the energy for both the power stroke of the myosin head and the detachment of myosin from actin. It is also required for the active transport of calcium ions back into the sarcoplasmic reticulum during muscle relaxation.