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EFFECTS of stimulation on the muscle

1. Single stimulus -> simple twitch

2. Frequent stimuli -> simple twitches/tetanus (incomplete/complete)

A. Single stimulus -> Simple twitch:

Response of a muscle for a single stimulus:

1. Latent period (0.01 second)

2. Contraction period (0.04 second)

3. Relaxation period (0.05 second)

Important Points in Simple Muscle Curve:

1. Point of stimulus (PS): The time when the stimulus is applied.

2. Point of contraction (PC): The time when the muscle begins to contract.

3. Point of maximum contraction (PMC): The point up to which the muscle contracts. It also indicates the beginning of relaxation of the muscle.

4. Point of maximum relaxation (PMR): The point when the muscle relaxes completely.

Isotonic simple muscle curve:

- PS: Point of stimulus

- PC: Point of contraction

- PMC: Point of maximum contraction

- PMR: Point of maximum relaxation

- LP: Latent period (0.01 sec)

- CP: Contraction period (0.04 sec)

- RP: Relaxation period (0.05 sec)

1. Latent period:

- The time interval between the point of stimulus and the point of contraction. The muscle does not show any mechanical activity during this period.

2. Contraction period:

- The interval between the point of contraction and the point of maximum contraction. The muscle contracts during this period.

3. Relaxation period:

- The interval between the point of maximum contraction and the point of maximum relaxation. The muscle relaxes during this period.

- Contraction period is always shorter than the relaxation period because contraction is an active process and relaxation is a passive process.

B. Frequent stimuli -> Simple twitches:

- Individual muscle twitches are produced because the stimuli are far enough apart to allow complete relaxation between contractions.

C. Frequent (summated) stimuli -> Tetanus:

1. Incomplete tetanus: There is partial relaxation between contractions.

2. Complete tetanus: There is no relaxation between contractions.

Types of muscle fibers:

1. Slow red fibers (type I fibers):

- Size: Small

- Number: High

- Contraction: Slow, weak, long latent period (due to less extensive SR, slower release of Ca++, slower Ca++ pump)

- Color: Red (more capillaries, more myoglobin)

- Source of energy: Aerobic metabolism (more mitochondria, more oxidative enzymes)

- Fatigue: High resistance (slowly occurs)

- Site: Long posture muscles (e.g., back muscles)

2. Fast (pale) fibers (type II b fibers):

- Size: Large

- Number: Few

- Contraction: Fast, strong, short latent period (due to more extensive SR, faster release of Ca++, faster Ca++ pump)

- Color: Pale (less capillaries, less myoglobin)

- Source of energy: Glycolysis metabolism (less mitochondria, less oxidative enzymes)

- Fatigue: Low resistance (rapidly occurs)

- Site: Fine skilled movements (e.g., eye muscles, hand muscles)

Notes:

- Sprinter athletes: Possess a high percentage of fast fibers.

- Endurance athletes: Possess a high percentage of slow fibers.

Proprioceptors (kinesthetic receptors):

Definition: Receptors that detect and respond to movement and changes in the position of different parts of the body.

Types:

1. Muscle spindle:

- Spindle-shaped, capsulated

- Situated in skeletal muscle

- Formed by 2-12 intrafusal muscle fibers

- Stimulated by moderate stretch

- Respond by contraction of muscle (stretch reflex)

- No inhibitory interneuron

2. Golgi tendon organ:

- Capsulated

- Situated in tendon

- Formed by a group of nerve endings

- Stimulated by severe stretch

- Respond by relaxation of muscle (inverse stretch reflex)

- Inhibitory interneuron