Skeletal Muscle Contractions – Characteristics

Skeletal Muscle Contractions – Characteristics

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SKELETAL MUSCLE CONTRACTIONS -CHARACTERISTICS

Characteristics include:

• All or none law.
• Stimulus-response relationship.
• Frequency of stimulus.
• Motor unit recruitment.
• Starling’s Law.
• Isometric contractions
• Muscle Tone.
• Denervation hypersensitivity.

1. “All or none law”:

• Motor unit is “Motor neuron collectively with all its peripheral branches & innervated muscle fibers”.
• Motor unit obey all or none law.
• Ie., On stimulation, either all motor unit fibers will contract maximally or not contract at all.
• This depends upon stimulus intensity (threshold/subthreshold).

Thus,

• Motor unit is “Unit of contraction”.
• Nerve fiber is “Unit of activation”.

Concentration of motor unit:

• 3-6 fibers in a motor unit:
• Seen with muscles for fine, graded & precise movement.
• Eg: Fingers & eye.

600-1000 muscle fibers per unit:

• Seen with muscles for gross movements.
• Eg: Leg.

2. Stimulus-Response relationship:

• Contractile response depends on stimulus strength.
• With increasing stimulus strength → Number of contracting motor units increases.
• Hence, 
• Increased no. of contracting motor units → Stronger muscle contraction.

3. Frequency of stimulus:

• Increased frequency of stimulation → Increases contraction strength & frequency of activation of motor units.

This explains, 

Simple muscle twitch:

• Contractile response of a skeletal muscle to a single brief stimulus.

Tetanus:

• Due to summation of twitches.

Partial/Incomplete tetanus:

• State of repetitive muscle contractions separated by partial relaxation, at slower frequency of stimulation.

Complete tetanus:

• State of sustained muscle contraction, at higher frequency of stimulation.

Post-tetanic potentiation:

• Phenomenon of “Repetitive stimulation enhancing force development due to increased intracellular Ca2+.

Mechanism behind:

• Ca2+concentration in sarcoplasm determines muscle tension to produce tetanus.

During single twitch:

• Ca2+ released into sarcoplasm insufficient for tetanic tension.

On rapid & successive muscle stimulation:

• With each stimulus Ca2+ efflux into sarcoplasm.
• Thus, progressive sarcoplasmic Ca2+ accumulation.

On maximum sarcoplasmic Ca2+ levels:

• Muscle tension created.

For tetanic response:

• Tetanic tension is about four times twitch tension.

4. Motor unit recruitment:

• Start of muscle contraction – Smallest motor units contract first.
• On insufficient power generation, larger motor units are recruited.
• “Henneman principle”/”Size principle”
• Order of recruitment from smaller to larger motor unit.
• Increases contraction strength 

5. Starlings law:

Explains that, 

• There is an optimal length at which force generated by muscle is maximal.
• Ie., Upto a certain limit,
• Greater the initial length/length at relaxed state ——> Greater is contraction force.

6. Isotonic Vs isometric contraction

Isotonic contraction:

• Contraction with change of length at constant tension.
• Tension is equal to weight lifted during muscle contraction.

Isometric contraction:

• Contraction with increased tension & constant length.
• Generates more contraction force.

Exercise:

• Gym exercises are isotonic type.
• Involves change in muscle length with constant tension.
• Requires greater energy than isometric contraction.
• Thus, isotonic exercise best increases muscle strength.

7. Tonus:

• State of muscles in partial contraction.
• Defined as “Muscle resistance to passive stretch”.
• Involves γ-motor neuron activity for contraction.
• Assessed by observing resistance offered by muscle to passive stretch.

8. Denervation hypersensitivity:

• Destruction of skeletal muscle nerve supply causes –
• Abnormal muscle excitability.
• Increased sensitivity to circulating acetylcholine.

“Fibrillation”:

• Fine irregular contraction of individual fiber.
• Classically in lower motor neuron lesion.
• Not visible grossly.
• On motor nerve regeneration, fibrillation disappears.

Fasiculations – 

• Jerky, visible contractions.
• Occurs with group of muscle fibers/complete motor unit supplied by motor neuron.

9. Muscle fiber type:

Type I/slow motor units:

• Have early recruitment.

Type II motor units:

• Type IIa/“Fast-Fatigue Resistant” (FR) motor units.
• Type IIb/“Fast-Fatigable” motor units.

Factors increasing force of muscle contraction:

• Increased number of motor units.
• Increasing frequency of stimulus (Tetanic stimulus).
• Increasing stimulus strength.
• Appropriate initial length.
• Larger motor unit recruitment (Henneman principle).
• Type II/fast unit.
• Isometric contraction.