Muscle and nervous tissues

Muscle tissue: built to shorten

Muscle cells contain long protein filaments, mainly actin and myosin, arranged so that they can slide past one another and pull the cell shorter. This shortening is contraction, and it is an active process that uses ATP. Because each muscle cell is attached at its ends to something — bone, another muscle cell or the wall of a hollow organ — contraction produces movement, maintains posture or moves material along a passage. Muscle tissue is also a major source of body heat, because contraction releases energy as heat. Three types exist. Skeletal muscle is attached to bone and produces body movement; cardiac muscle forms the heart wall and pumps blood; smooth muscle lines hollow organs and moves their contents. The three differ in appearance, in control and in endurance, but all share the same basic ability to shorten.

For example: When you lift a cup, the biceps shortens, pulling on a tendon attached to bone. Nothing is pushed; the movement comes from the muscle getting shorter.

Skeletal muscle

Skeletal muscle is the type you can see and usually think of first. Its cells are long, cylindrical and have many nuclei, and under the microscope they show alternating light and dark bands, so they are described as striated. Skeletal muscle is voluntary: you decide when to move it, though it also takes part in reflexes and in maintaining posture. It contracts quickly and powerfully but tires when it works hard for a long time. Each muscle contains bundles of fibres, and each fibre receives a nerve ending, so the nervous system can activate the muscle in graded steps: a few fibres for a light task, many for a strong one. This precise control is what allows fine movements such as writing and powerful movements such as jumping.

For example: Writing uses small, precise contractions in the hand, while jumping uses many fibres at once in the legs — the same tissue, activated differently.

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Cardiac and smooth muscle

Cardiac muscle forms the wall of the heart. Its cells are striated like skeletal muscle but are shorter, branched and joined end to end by special junctions called intercalated discs, which let the electrical signal spread quickly from cell to cell so the heart contracts as a unit. Cardiac muscle is involuntary: we do not consciously control the heartbeat, and the tissue has its own rhythm, modified by nerves and hormones. It is highly resistant to fatigue because the heart must work continuously for a lifetime. Smooth muscle is found in the walls of hollow organs such as the stomach, intestines, bladder, uterus and blood vessels. Its cells are spindle-shaped with a single nucleus and have no visible striations. Smooth muscle is also involuntary; it contracts slowly and can sustain contraction for long periods, moving food, urine or blood, and it can stretch and still work, which is essential in a bladder or a uterus.

For example: Your stomach churns food and your arteries adjust their width without any conscious decision, because both contain smooth muscle.

Common misconception: Involuntary does not mean uncontrolled. Cardiac and smooth muscle are regulated by nerves, hormones and local signals; they simply are not directed by conscious will.

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Nervous tissue: cells that signal

Nervous tissue is built to generate and conduct electrical signals quickly. It contains two broad cell groups. Neurons are the signalling cells; they have a cell body containing the nucleus, dendrites that receive signals from other cells, and usually a single long axon that carries the signal away to its target. Some axons are wrapped in a fatty insulating layer called myelin, which speeds conduction. Neuroglia, or glial cells, are the supporting cells: they protect and nourish neurons, remove waste and debris, form myelin and help maintain the environment around the neurons. Nervous tissue is found in the brain, spinal cord and nerves, and it controls and coordinates almost every other tissue by sending rapid instructions and receiving feedback. Unlike epithelium, nervous tissue has very little matrix, but it is not a simple sheet: its cells are organised into circuits.

For example: When you touch something hot, sensory neurons carry the signal to the spinal cord, where a circuit triggers motor neurons that make skeletal muscle pull your hand away.

Movement and control together

Muscle and nervous tissue depend on each other. Muscle contraction is triggered by signals from motor neurons; without them, even healthy muscle cannot act. Nervous tissue, in turn, depends on muscle for output: it decides what movement should happen, but the muscle carries it out. Both depend on ion gradients and ATP, which is why conditions that disturb ion balance or energy supply affect movement and signalling early. The two families also share an electrical mechanism based on the movement of ions across cell membranes, studied in sections 2.2 and 2.3. Recognising this shared basis helps explain why disorders of electrolytes can cause both muscle weakness and abnormal nerve function at the same time, and why fatigue has effects on both systems.

For example: A low potassium level can cause both muscle weakness and disturbed heart rhythm, because muscle and nervous tissues both rely on ion gradients.