Define motor unit and muscle twitch, and describe the events occurring during the three phases of a muscle twitch.
- Explain how smooth, graded contractions of a skeletal muscle are produced.
- Differentiate between isometric and isotonic contractions.
The Correct Answer and Explanation is::
1. Define motor unit and muscle twitch, and describe the events occurring during the three phases of a muscle twitch.
Motor Unit:
A motor unit consists of a single motor neuron and all the muscle fibers it innervates. When the motor neuron fires an action potential, all the muscle fibers in that motor unit contract simultaneously. The size of a motor unit varies depending on the muscle’s function — fine control muscles (like those controlling the eye) have small motor units, while large muscles involved in powerful movements (like the quadriceps) have large motor units.
Muscle Twitch:
A muscle twitch is a single, brief contraction and relaxation cycle of a muscle fiber in response to a single stimulus (action potential). It represents the simplest form of muscle contraction.
Phases of a Muscle Twitch:
A muscle twitch has three distinct phases:
- Latent Period:
This is the brief delay between the stimulus and the onset of contraction. During this time, the action potential is spreading across the sarcolemma, and calcium ions are being released from the sarcoplasmic reticulum, but no visible contraction occurs yet. - Contraction Phase:
During this phase, calcium ions bind to troponin, causing tropomyosin to move and expose the binding sites on actin filaments. Cross-bridge cycling between actin and myosin occurs, causing the muscle fibers to shorten and generate tension. - Relaxation Phase:
Calcium ions are actively pumped back into the sarcoplasmic reticulum, causing the cross-bridges to detach, and the muscle fiber returns to its resting length. Tension decreases during this phase.
2. Explain how smooth, graded contractions of a skeletal muscle are produced.
Skeletal muscle contractions are graded, meaning they can vary in strength and do not simply contract at full force or not at all. This smooth control of muscle tension is achieved primarily by two mechanisms:
- Motor Unit Recruitment:
By activating more motor units progressively (from small to large), the muscle increases the number of contracting fibers, thereby producing greater tension. This allows for fine control of muscle force. - Frequency of Stimulation (Wave Summation):
If a muscle fiber is stimulated repeatedly before it has completely relaxed, the second contraction will be stronger than the first, due to residual calcium in the cytoplasm. This process is called temporal summation or wave summation. When stimuli are frequent enough, the contractions fuse into a sustained contraction called tetanus, producing maximum force smoothly without individual twitches.
Together, these mechanisms allow the nervous system to control muscle force precisely, enabling smooth, coordinated movements instead of jerky, all-or-nothing contractions.
3. Differentiate between isometric and isotonic contractions.
| Aspect | Isometric Contraction | Isotonic Contraction |
|---|---|---|
| Definition | Muscle generates tension without changing length | Muscle changes length while maintaining constant tension |
| Muscle Length | Remains the same | Shortens (concentric) or lengthens (eccentric) |
| Example | Holding a heavy object steady without moving it | Lifting a weight (concentric) or lowering it slowly (eccentric) |
| Function | Stabilizes joints and maintains posture | Produces movement of body parts |
Explanation:
In isometric contractions, the muscle tension increases to resist a load, but the muscle fibers do not shorten because the force generated is equal to the resistance. These contractions are important for maintaining posture or holding objects steady.
In isotonic contractions, the muscle tension remains fairly constant while the muscle changes length. There are two types:
- Concentric: The muscle shortens as it overcomes resistance (e.g., lifting a dumbbell).
- Eccentric: The muscle lengthens while controlling the movement (e.g., slowly lowering the dumbbell).
Both types of contractions are essential for different types of movements and muscle control.
