Question

for the biceps and triceps muscle

How is the mechanical behavior of a single muscle fiber in tension different

from the whole muscle? What is the functional value?

Answer 1

MUSCLE TENSION

Most important characteristic of muscle is it's ability to develop tension & to exert force on bony lever.

Tension can be either active or passive; total tension is the sum of both active & passive tension.

Passive tension

Refers to tension developed in parallel elastic component.

Also known as non-contractile tension.

Created by lengthening the muscle beyond the slack length of the tissues.

Parallel muscle component may add to active tension by muscle when lengthened; or it may become slack & doesn’t contribute to total muscle tension

Active tension

Refers to tension developed by contractile element of muscle. So called as contractile tension.

It is initiated by cross bridge formation & movement of thin & thick filaments.

The amount of tension generated depends on neural factors (frequency, number & size of motor unit) & mechanical properties (isometric length tension relationship & force velocity relationship) of muscle.

Isometric length tension relationship o There is direct relationship between isometric tension development & length of sarcomeres in muscle fiber.

At optimal length - maximal isometric tension (due to position of thin & thick filaments forming maximum number of cross bridges in sarcomere)

Lengthening or shortening beyond optimal length cause reduced amount of active tension, (fewer cross bridges formation)

As muscle elongates, passive elastic tension increases.

This passive tension is added to active tension resulting in total tension.

When the muscle is shortened - sarcomere s at shorter length - reduces distance between Z discs - inter-digitation of thin & thick filaments - interferes with formation of cross bridges.

This is applicable only in isometric contraction.

During dynamic contraction length tension relationship must be combined with force velocity relationship to determine the effect that both length & velocity have on muscle tension.

As muscle elongates, passive elastic tension increases.

This passive tension is added to active tension resulting in total tension.

When the muscle is shortened - sarcomere s at shorter length - reduces distance between Z discs - inter-digitation of thin & thick filaments - interferes with formation of cross bridges.

This is applicable only in isometric contraction.

During dynamic contraction length tension relationship must be combined with force velocity relationship to determine the effect that both length & velocity have on muscle

Force - velocity relationship o Another factor affecting tension development is speed of shortening of myofilaments. (Rate at which myofilaments slide & form & re-form cross bridges)

Speed of shortening depends on type & length of muscle fiber. Force velocity relationship describes the relation between velocity of muscle contraction & the force produced, (concentric & eccentric muscle contraction)

Anterior muscles (flexors), posterior (extensors), lateral (abductors), medial (adductors).

Contracting your biceps exhibits flexion, i.e. it brings your forearm closer to your upper arm and decreasing the angle between the two. So, your biceps is described as a "flexor" muscle. In the illustration below, the image on the right shows the biceps flexing. The opposing muscle of a flexor is called the "extensor" muscle. Your triceps is an extensor. When you contract your triceps your arm straightens and the angle between the forearm and the upper arm increases.

Events of muscle stimulation

· Conduction of nerve impulse from CNS to neuromuscular junction.

· Depolarization of sarcolemma - influx of Na+ ions & efflux of k+ ions.

· Release of Ca2+ ions from sarcoplasmic reticulum.

· Diffusion of Ca2+ ions to actin filament.

Molecular changes during muscle contraction

· Binding Ca2+ions to troponin.

· Troponin - Ca2+complex removes tropomyosin blockage of actin sites.

· Heads of myosin - ATP complex form Crossbridges to actin filament.

· Hydrolysis of ATP induces conformational changes in the heads of myosin.

· 1 mole of Actin + 3 moles of myosin Actomyosin

Molecular changes during muscle relaxation

· Ca2+ ions sequestered from actin filament by sacroplasmic reticulum.

· Ca2+ returns to sacroplasmic reticulum.

· Ca2+ released from troponin - Ca2+complex. Troponin permits tropomyosin return to blocking position.

· Myosin-action cross-bridges break.

· ATP - myosin Complex reformed in heads of thick filament.