Question

• Be able to define the following terms: axial skeleton, appendicular skeleton, sarcoplasmic reticulum, acetylcholine, motor unit, locomotion, peristalsis

• Know the structure and components of muscle fibers (myofibrils, sarcomeres, actin, myosin)

• From Figure 39.4 – Know how myosin and actin interact to produce muscle contractions, including how and where myosin heads attach to actin

Answer 1

1- Terms-

Axial skeleton- axial skeleton occupies the body's main axis of the body. It includes four structure skull in the head, vertebral column in the neck and trunk ,sternum and ribs.

Appendicular skeleton- appendicular skeleton is placed inside axes. It comprises pectoral and pelvic girdles and limb bones.

Sarcoplasmic reticulum- striated muscle fibre bounded by sarcolemma that is plasma membrane and contain in its sarcoplasm(cytoplasm) having well-developed endoplasmic reticulum that is specialized for calcium storage and is called sarcoplasmic reticulum.

Acetylcholine- acetylcholine is a neurotransmitter which generate an action potential from one nerve cell to other and also in muscle.

Motor unit- it is unit of motor neuron and all muscle fibers where motor neurone innervates the muscle fibers.

Locomotion-locomotion is the movement of an animal as a whole from one place to another.

Peristalsis- peristalsis is progression of coordinated contraction of involuntary circular muscles which is preceded by a simultaneous contraction of a longitudinal muscle and relaxation of the circular muscle in the lining of gut.

2.

Myofibril- It is muscle fibre which is arranged in parallel row and form the dark and light line. These lines are found in alternate order.

Sarcomere- The part of muscle fibre between two successive Z line function as contractile unit termed sarcomere.

Actin - It is secondary myofilament. Actin is a thin filament of polymerized protein made up of 2 'F'(fibrous) strands helically wound to each other and each 'F' is a polymer of monomeric 'G' actin. Two filament of tropomyosin protein run along the length of 'F' actin. Are complex troponin protein of three globular peptides is distributed at regular intervals on tropomyosin.

Myosin- It is primary myofilament formed of protein myosin. Each myosin filament is a polymerized protein made of many monomeric protein called meromyosin. Each meromyosin has two important parts a globular head with the short arm and tail, the former is term heavy meromyosin and the latter called light meromyosin.

3. Initiation of muscle contraction start by sending the signal to the central nervous system via a motor neuron. Motor unit is formed by composition of motor neuron along with the muscle fibre. The junction called neuromuscular junction or motor end plate is junction between motor neuron and sarcolemma of muscle fibre. A neural signal reaching this junction release a neurotransmitter acetylcholine which generates an action potential in sarcolemma. This action potential spread throughout the muscle fibre and causes the release of calcium ions into the sarcoplasm . This result in increase in calcium level. Now there is binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin. The myosin head now bind to the exposed active site on actin to form a cross bridge. This process is occur by utilizing energy from ATP hydrolysis. Attached actin filament pulled towards the centre of 'A' band . The 'Z' line attached to these actin are also pulled inward thereby causing a shortening of the sarcomere i.e.. contraction. During shortening of the muscle the 'I' band get reduced whereas the 'A' band retain the length.