- Properties of Skeletal
1) Skeletal muscles are made up of a number of muscle fibres.
2) When these muscle fibres get stimulated,
they show different types of responses.
- Threshold Stimulus:
1) It is
the minimum intensity of stimulus which is necessary for the contraction
of a muscle fibre.
- All or None Response:
1) When the stimulus is equal or more than that of the threshold value,
the muscle fibre contracts.
2) It contracts with a maximum force and does not depend on the strength
of the stimulus.
3) On the other hand, it does not contract if the strength of the
stimulus is below the threshold value.
- Single Twitch: When a single stimulus is
applied to a muscle fibre, it gives a single
contraction, also known as a single twitch or muscle twitch.
- Tetanus: When a number of stimuli
are given to a muscle fibre in succession, it
remains in a state of sustained contraction which is known as tetanus.
1) The muscle fibre cannot remain in the state
of tetanus indefinitely so if the stimulus continues, there is a decline
in the force of contraction and ultimately it comes to zero.
2) The muscle fibre then fails to respond to
further stimuli and get fatigue.
3) When the muscle fibre does not get enough
oxygen it starts respiring anaerobically.
4) The muscle fatigue occurs due to accumulation of lactic acid which is
produced as a result of anaerobic respiration.
5) The muscle fibre starts working again after
6) Muscle fatigue can occur after a strenous
- Structure of
1) A striated muscle is made up of a number of muscle fibres.
2) Each muscle fibre is made up of a number of
3) The myofibrils consists of alternate dark
and light bands.
4) The dark bands are called A bands and the
light bands, I bands.
5) These bands contain thin actin filaments
and thick myosin filaments.
6) A dark line called Z line is present at the centre of I band and a
lighter region called H band is present at the centre of A band.
7) Sarcomere is the area which is
present between the two Z lines.
8) Sarcomere is the structural and functional
contractile unit of a myofibril.
- Sliding Filament Theory of
Muscle Contraction: This theory was proposed by two groups which were headed by
H.E. Huxley and A. F. Huxley in 1954.
These scientists proposed this theory on the basis of the following
During muscle contraction, length of A
band remains constant.
The length of I band decreases.
The H zone becomes shorter and ultimately disappears.
A thicker line called as M line appears in a fully contracted
The length of sarcomere also decreases
due to decrease in the length of I band.
- Red Muscle Fibres:
1) These fibres are dark red in colour
due to the presence of myoglobin. They are
thin and slow acting.
2) They possess a large number of mitochondria.
3) They are specialised for sustained work
over a long period of time but at a slow rate.
4) They do not get fatigue easily.
- White Muscle Fibres:
1) These fibres are white/light in colour due to absence or presence of little amount
2) They possess less number of mitochondria.
3) They are thicker and fast acting.
4) They are specialised for fast and strenous physical work but for a short duration.
5) They get fatigue easily.