October 2026 - The Muscular System Study Guide

The three muscle types

Two properties tell them apart: whether you control them, and whether they appear striped under a microscope.

Muscles pull, so they work in pairs

A contracting muscle shortens and pulls the bone it is attached to. Nothing in muscle can push, so returning a limb to its original position requires a second muscle pulling the other way. These opposing pairs are called antagonistic.

At the elbow, the biceps contracts to bend the arm while the triceps relaxes; to straighten it, the triceps contracts and the biceps relaxes. The muscle doing the work is the agonist, and its opposite number is the antagonist — the roles swap depending on the direction of movement.

How a contraction happens

Enough of the mechanism to answer what the exam asks, without the full biochemistry.

• A motor neuron delivers a signal at the neuromuscular junction.
• Calcium is released inside the muscle fibre.
• Protein filaments — actin and myosin — slide past one another, shortening the fibre.
• ATP supplies the energy. Without it, the filaments cannot release, which is why muscle stiffens after death.

Attachment and the wider role

• Skeletal muscle attaches to bone by tendons, which transmit the pull.
• The origin is the attachment that stays relatively fixed; the insertion is the one that moves.
• Muscle also generates heat — shivering is muscular contraction used for warmth.
• Smooth muscle does slow, sustained work: moving food along the gut by peristalsis, and narrowing or widening blood vessels.

Identifying muscle type from a description

Terms to know

Skeletal muscle

Voluntary, striated muscle attached to bone.

Cardiac muscle

Involuntary, striated muscle found only in the heart.

Smooth muscle

Involuntary, non-striated muscle in organs and vessels.

Striated

Showing a striped appearance under a microscope.

Antagonistic pair

Two muscles that pull a joint in opposite directions.

Agonist

The muscle producing a given movement.

Antagonist

The muscle that opposes it and relaxes during that movement.

Origin and insertion

The fixed attachment and the moving attachment of a muscle.

Neuromuscular junction

Where a motor neuron meets a muscle fibre.

What the HESI asks most

The same core ideas appear in different wording. If you can answer these without stopping to think, you have what this section requires.

Key points

• Skeletal: voluntary, striated. Cardiac: involuntary, striated, heart only. Smooth: involuntary, non-striated.
• Muscles pull and never push, so they are arranged in antagonistic pairs.
• The agonist contracts while the antagonist relaxes, and the roles reverse.
• Tendons attach skeletal muscle to bone.
• Contraction is driven by calcium and powered by ATP.