Muscular Movement

• Muscles can only pull, never push — so they work in antagonistic pairs (e.g. bicep/tricep) across joints, connected to bone by tendons.
• A muscle fibre is a giant, multi-nucleated cell-like unit packed with myofibrils (bundles of actin and myosin) and a specialised ER called the sarcoplasmic reticulum that stores calcium ions.
• Muscle contraction = the sliding filament theory. Calcium ions expose myosin-binding sites on actin; myosin heads bind, bend (power stroke), and pull actin inward — repeating over and over using ATP.
• There are two muscle fibre types: fast twitch (powerful, anaerobic, tire quickly) and slow twitch (endurance, aerobic, tire slowly).
• The heart is myogenic — it beats by itself. A wave of excitation travels SAN → atria → AVN → Bundle of His → Purkyne fibres → ventricles.
• Cardiac output = heart rate × stroke volume. ECGs show this electrical activity as P, QRS, and T waves.
• During exercise, the medulla oblongata — via baroreceptors and chemoreceptors — speeds up heart rate and breathing rate using sympathetic (noradrenaline) and parasympathetic (acetylcholine) neurones.
• Adrenaline reinforces this "fight or flight" response, boosting heart rate and redirecting blood to muscles.
• A spirometer measures tidal volume, breathing rate, oxygen consumption, and respiratory minute ventilation — all of which increase with exercise.

Here's the key insight that unlocks this whole topic: a muscle on its own is useless. Muscles can only do one thing — contract and pull. They cannot push. So for a muscle to actually move your body, it needs something rigid to pull against. That's the skeleton's job: it's incompressible, meaning it won't squash or bend when a muscle tugs on it. Muscle + bone together = movement.

Both are strong connective tissue, but they connect completely different things:

• Tendons connect muscle → bone. They're flexible but do NOT stretch — this matters, because if a tendon stretched like elastic, all the force from your muscle contracting would be "wasted" stretching the tendon instead of moving the bone.
• Ligaments connect bone → bone. Their job is to hold the skeleton together at joints, keeping everything in place.

pairs antagonistic muscle action

The classic example is your upper arm:

highly specialised cell-like unit many nuclei

Because muscle fibres are unusual, they get their own special vocabulary for parts that are equivalent to normal cell structures: