Movement, Locomotion and Types of Muscles

Not every movement changes the organism's location

Movement is one of the most recognisable properties of living organisms. Cytoplasm streams inside many cells; cilia beat at the surface of an epithelial cell; and a hand changes position when its muscles contract. Locomotion is a special case of movement in which the organism changes place. Walking, swimming and flying are obvious examples, whereas moving an eyelid or shifting the jaw is not locomotion. All locomotion involves movement, but many movements occur without a change of place.

One structure can perform several tasks. A ciliated Paramecium swims using its cilia, yet ciliary motion also guides food towards its oral region. Hydra uses tentacles for prey capture and can employ body and tentacle movement during locomotion. Human limbs support walking but also reposition the body while seated. Therefore classification should follow the outcome of the movement, not an assumption that a body part is exclusively a locomotory organ. Animals seek food, shelter, mates, favourable conditions and escape from threats through locomotion.

Amoeboid movement follows dynamic changes in cell shape

Some leucocytes and macrophages move by projecting temporary pseudopodia. Actin-based microfilaments help rearrange the cytoskeleton, allowing the cell membrane to protrude and the rest of the cell to follow. This resembles the way Amoeba changes shape, although animal immune cells have specialised signalling and adhesion machinery. Amoeboid movement can help immune cells migrate into tissues, encounter pathogens and participate in defence. It is different from a whole limb moving because the same cell continually remodels its outline.

Cilia and flagella produce repeated bending

Cilia are membrane-covered projections built around microtubule machinery. Coordinated ciliary beating can propel unicellular organisms or move material along the surface of a stationary tissue. Human respiratory epithelium uses cilia to help move mucus and trapped dust away from deeper air passages. In parts of the female reproductive tract, cilia contribute to movement of the ovum. Eukaryotic flagella are commonly longer and fewer; the flagellum of a sperm cell generates bending waves for locomotion. Cilia and flagella differ in appearance but use related microtubule-based motor mechanisms.

Muscular movement generates force through shortening

Muscles convert chemical energy into mechanical force. A muscle may shorten while pulling a bone, sustain tension while holding a posture, or resist a load while being lengthened. The textbook identifies four important properties. Excitability is the ability to respond to a stimulus, contractility is the ability to generate tension, extensibility allows stretching, and elasticity supports recovery after deformation. These properties help explain why muscles can both create movement and stabilise a joint.

Human skeletal muscles arise primarily from mesoderm during development and constitute a large fraction of adult body mass. The textbook gives approximately 40–50%, but actual values depend on sex, age, body size and training. Muscle activity is integrated with skeletal levers and motor commands from the nervous system. When a person reaches towards an object, several muscles act in a coordinated sequence while other muscles control posture and prevent unwanted movement.

Three muscle classes differ in appearance and regulation

Skeletal muscle fibres are elongated, usually multinucleate and visibly striated because thin and thick filaments form regular repeating units. These muscles attach to the skeleton directly or through tendons and are under voluntary somatic motor control. Smooth or visceral muscle occurs in the walls of organs such as the digestive tract and reproductive passages. Its cells are usually spindle-shaped, lack visible striations and are regulated involuntarily by autonomic, local and hormonal signals. Cardiac muscle forms the contractile heart wall: its cells are striated, branched and joined by intercalated discs, and it works involuntarily.

It is misleading to claim that involuntary muscle has no neural influence. Smooth muscle can receive autonomic nerve input, and cardiac rhythm is modified by autonomic signalling even though heart muscle possesses specialised intrinsic pacemaking activity. Likewise, striation does not automatically imply voluntary control: cardiac muscle is striated but involuntary. Use three independent features when classifying muscle—location, microscopic pattern and type of regulation—rather than relying on only one label.

Quick Quiz

Model Answers

1. Movement includes local changes such as eyelid closure or cytoplasmic streaming; locomotion changes an organism's position, as during human walking or swimming by Paramecium.

2. Amoeboid movement uses pseudopodia and actin-driven changes of cell shape; ciliary movement uses coordinated bending of microtubule-based projections; muscular movement uses contraction of specialised muscle tissue to produce force.

3. Skeletal: striated, voluntary, attached to skeletal structures; smooth: nonstriated, involuntary, visceral walls; cardiac: striated branched cells, involuntary heart wall.

4. Excitability is response to stimuli; contractility is force generation; extensibility is ability to stretch; elasticity is tendency to recover shape after deformation.

5. Cardiac muscle has branching connected cells, intrinsic rhythm and involuntary control, unlike long multinucleate skeletal fibres controlled by somatic motor neurons.