What Actually Happens Inside a Muscle When You Lift a Bag

Picking up a school bag feels effortless, but it needs nerve signals, calcium ions, ATP and countless protein filaments sliding past one another inside your muscles.

This part covers types of movement, types of muscle, the structure of a myofibril, and how muscles contract.

Movement is any change in position of the body or its parts, while locomotion is voluntary movement that carries the whole organism from place to place; human cells show amoeboid and ciliary movement, and the body moves using muscles.

Movement versus locomotion:

- Movement — a change in position, such as blinking or moving the tongue
- Locomotion — voluntary movement that changes location, such as walking, running, swimming or flying
- All locomotions are movements, but not all movements are locomotions

Types of movement in humans:

- Amoeboid — by pseudopodia formed by streaming protoplasm, with microfilaments involved; shown by macrophages and leucocytes
- Ciliary — by coordinated beating of cilia; in the trachea it removes dust particles, and in the female reproductive tract it moves the ovum
- Muscular — by contraction of muscles in the limbs, jaws, tongue and other organs

An everyday example. A child waving from a bus window shows movement; the same child walking to school shows locomotion.

The substance. Locomotion needs muscles, bones and nerves working together, which is why muscle structure comes next.

Skeletal muscle is attached to bones, striated and voluntary; visceral muscle lines hollow internal organs, is smooth and involuntary; and cardiac muscle forms the heart, is striated, branched and involuntary.

Skeletal muscle:

- Location — attached to bones
- Structure — striated, with a banded appearance
- Control — voluntary, under nervous control

Visceral (smooth) muscle:

- Location — inner walls of hollow organs such as the alimentary canal and reproductive tract
- Structure — non-striated, smooth
- Control — involuntary

Cardiac muscle:

- Location — heart
- Structure — striated and branched; cells joined by intercalated discs so they contract as a unit
- Control — involuntary

An everyday example. You can decide to raise your arm, but you cannot decide to speed up digestion after a big meal — skeletal versus visceral muscle.

The substance. Striated does not mean voluntary — cardiac muscle is striated yet involuntary.

A myofibril is a chain of sarcomeres, each running between two Z lines; the dark A band contains thick myosin filaments, the light I band contains only thin actin filaments, and the H zone is the middle of the A band where actin does not overlap myosin.

Organisation. Each muscle fibre is a syncytium bounded by the sarcolemma, packed with parallel myofibrils; its sarcoplasmic reticulum stores Ca${}^{2+}$.

Bands of a myofibril:

- I band (isotropic) — light; actin only; bisected by the elastic Z line
- A band (anisotropic) — dark; contains myosin, with actin overlapping its ends
- H zone — the central part of the A band not overlapped by thin filaments
- Sarcomere — the portion between two successive Z lines; the functional unit of contraction

Actin (thin) filament:

- Two F actins wound helically, each a polymer of G actin
- Tropomyosin — two filaments running along the F actin
- Troponin — a complex protein at intervals on tropomyosin; at rest, it masks the myosin-binding sites on actin

Myosin (thick) filament:

- A polymer of meromyosins
- Each has a globular head with a short arm (heavy meromyosin) and a tail (light meromyosin)
- The head is an active ATPase with binding sites for ATP and actin, forming a cross arm

An everyday example. Two combs pushed into each other's teeth resemble the interlocking thin and thick filaments of a sarcomere.

The substance. The A band never changes length during contraction — only the I band and H zone shorten.

Muscle contracts when a nerve signal releases calcium ions, which make troponin uncover binding sites on actin; ATP-energised myosin heads bind actin and pull the thin filaments towards the centre of the sarcomere, so the filaments slide past each other.

Step by step:

- A signal from the CNS travels down a motor neuron; the neuron and the muscle fibres it controls form a motor unit
- At the neuromuscular junction, the neurotransmitter acetylcholine is released, generating an action potential in the sarcolemma
- This releases **Ca${}^{2+}$ from the sarcoplasmic reticulum** into the sarcoplasm
- Ca${}^{2+}$ binds troponin, unmasking the active sites on actin
- The myosin head, energised by ATP hydrolysis, binds actin to form a cross bridge
- The cross bridge pulls actin towards the centre of the A band, drawing the Z lines inwards and shortening the sarcomere
- The head releases ADP and phosphate, binds fresh ATP, detaches, and the cycle repeats

What changes:

- I bands shorten; the H zone narrows or disappears
- A band length stays the same

An everyday example. A tug-of-war team pulling the rope hand over hand is like myosin heads repeatedly grabbing, pulling and releasing actin.

The substance. The filaments themselves do not shorten — they slide over one another.