Series elastic component of a muscle
The components such as sarcolemma sheath at the end of the muscle fibers, the tendon and the hinged arms of the cross bridges of the myosin filaments are known as series elastic components of the muscle. stretch slightly as tension increases. Consequently, the contractile unit must shorten an extra 3-5% to make up for the stretch of these elements. During isometric contraction, these series elastic components develop greater tension that opposes the contraction of the myofibrils. Hence the contraction becomes zero, but the tension is very high.
In isotonic contractions, the contractile elements shorten and stretch the series elastic components. This causes the tensionto rise just to exceed the force of contraction due to the effect of the weight. Thereafter the tension in the muscle remains constant.
Types of muscles
Based on the duration of contraction, the muscle fibers are classified as slow (tonic) and fast (twitch) fibers. Every muscle in the body is composed of slow and fast muscle fibers.
| Slow Muscle fibers | Fast Muscle fibers |
| Smaller fibres | Much larger fibres |
| Innervated by small nerve fibres | Innervated by comparatively large nerve fibres |
| Have extensive blood supply, hence referred as “Red muscle”, shows prolonged performance of work (Long contraction time). | Have less blood supply, hence called as “White muscle“, shows quick and repetitive contractions (Short contraction time). |
| Increased number of sarcosomes | Fewer sarcosomes |
| Muscle metabolism by aerobic or oxidation of glucose, fatty acids and amino acids, causes the release of 38 | Glycolysis or anaerobic type of metabolism liberating 2 molecules of lactic acid and 2 molecules of ATP. |
| Large amount of myoglobin in sarcoplasm. | Lack of myoglobin. |
| Less sarcoplamic reticulum | Extensive sarcoplasmic reticulum |
| Less Ca++ ion release | Rapid Ca++ ion release |
| Has large motor units (More muscle fibres/ neuron), no fine degree ofcontrol. | Has few motor units (less muscle fibres/ neuron), higher degree of fine control. |
| Adapted for prolonged, continued muscle activity – like support of the body against gravity | Adapted for rapid and powerful muscle contractions – like jumping, running |
Based on energy source and rate of contraction, skeletal muscle fibers can be classified into slow-twitch (type-I) /oxidative and fast-twitch (type-II) /glycolytic fibers. Type-I fibers are always oxidative and slow muscle fibers. Type-II is either oxidative or glycolytic and rate of contraction varies from slow to fast and superfast.
The motor units of the muscle
Each motor neuron leaves the ventral horn of the spinal cord and innervates many muscle fibres. All the muscle fibres in a muscle that are innervated by a single motor neuron (all the muscle fibres supplied by a single motor neuron) are called as motor unit.
The ratio of motor nerve to the number of muscle fibres innervated in a given skeletal muscle is called innervation ratio. The number of muscle fibres per motor unit differs as per the function for which the muscle is used in the body.
Small muscles which act rapidly and which require finer control of movement have an innervation ratio of 1:2 to 1:4. Large muscles that do not require finer control have an innervation ratio of 1:800 or more.