Histology: Muscle Tissue & Nervous Tissue

Muscle tissue converts chemical energy into movement through contracting proteins, and nervous tissue carries the electrical signals that command it. Together they form the body's action system.

It matters because every heartbeat, breath, step and thought runs on these two tissues - and their diseases, from heart failure to paralysis to neuropathy, are among medicine's greatest challenges.

Key Concepts

• Skeletal muscle: striated, voluntary, multinucleated fibres; attached to bone by tendons.
• Cardiac muscle: striated but branching, one central nucleus per cell, joined by intercalated discs; involuntary and fatigue-resistant.
• Smooth muscle: spindle-shaped cells, no striations, involuntary; walls of the gut, blood vessels, airways and uterus.
• Sarcomere: the contractile unit between two Z-discs, about 2.2 micrometres at rest; myosin pulls actin to shorten it.
• Neuron: dendrites receive, the cell body integrates, the axon transmits, the synapse passes the signal on.
• Myelin sheath: insulation by Schwann cells (peripheral) or oligodendrocytes (central); speeds conduction up to about 120 m/s.
• Glia: support cells - astrocytes, oligodendrocytes, microglia and Schwann cells - that nourish, insulate and defend neurons.
• Resting potential: about -70 mV inside the neuron; the action potential peaks near +30 mV.

How It Works

• Contraction: a nerve impulse releases calcium inside the muscle fibre; myosin heads grab actin filaments and pull; millions of sarcomeres shortening together move the limb.
• Signalling: the action potential races down the axon, triggers neurotransmitter release (for example acetylcholine) at the synapse, and the next cell fires.
• Cardiac rhythm: intercalated discs let the electrical wave sweep the heart as one unit, so chambers contract in sequence.
• Smooth muscle tone: slow, sustained contraction maintains blood pressure and moves food through the gut without conscious effort.

Worked Example

Identify the muscle on a slide: branching cells joined end-to-end, each with a single central nucleus and faint cross-striations, with dark step-like lines where cells meet.

1. Step 1: striations rule out smooth muscle; branching rules out skeletal muscle.
2. Step 2: the dark step-like lines are intercalated discs - the signature of cardiac muscle.
3. Step 3: conclusion - cardiac muscle, built to contract rhythmically as one electrical unit for an entire lifetime.
4. Clinical link: cardiac cells barely regenerate, so a heart attack's dead tissue becomes scar - which is why rapid treatment matters.

Common Misconceptions

• Bigger muscles mean more muscle cells. Mostly hypertrophy - existing fibres enlarge - rather than new cells.
• Neurons never regenerate. Central nervous system regeneration is very limited, but peripheral nerves can regrow slowly after injury.
• Smooth muscle is only in the gut. Also in blood vessels, airways, the uterus and the iris of the eye.

Real-World Applications

• Cardiology: heart attacks, arrhythmias and heart failure are cardiac-muscle diseases.
• Neurology: multiple sclerosis attacks myelin; motor neuron disease destroys the signalling cells.
• EMG and nerve-conduction studies diagnose muscle and nerve disorders electrically.
• Anaesthesia uses neuromuscular blockers that interrupt the nerve-muscle signal.

Muscle pulls, nerve commands. Master their three-plus-one cell types - skeletal, cardiac, smooth, and the neuron with its glia - and the whole moving, sensing body starts to make sense.