Musculoskeletal system

What the exam asks

Expect a sarcomere diagram, a length-tension curve, or a limb drawn with distances marked on it. The commonest items are: say which band changes and which does not; read the optimal length off a length-tension curve and explain a point on either limb; predict what a blocked pump or a depleted ATP store does; and balance torques about a joint to find a muscle force. The trap that catches most candidates is answering length-tension with effort rather than geometry. Tension does not fall at long lengths because the muscle is tired or stretched thin, it falls because there is less filament overlap, so there are fewer cross-bridges available to pull. Every point on that curve is a statement about how much of the thin filament the thick filament can currently reach. The second trap is on the lever: torque needs the perpendicular distance from the pivot to the line of the force, not the length of the bone, and that distance changes as the joint angle changes.

Muscle and bone are examined as a machine: a force generator whose output depends on its length, and a set of rigid bars pivoting about joints. The biology half asks how a nerve signal becomes force. The physics half asks what the skeleton then does with that force. Both halves are handed to you in the stem, so what you carry in is the reasoning that connects them.

The force generator works by sliding, not shortening. Thick and thin filaments each keep their own length and slide past one another, so a sarcomere shortens because its ends are drawn in, not because anything in it contracted. Everything follows from that: which bands narrow, why tension depends on length, and why a muscle stretched too far cannot pull.

Calcium is the switch and ATP is the currency, and the exam wants you to know that both directions cost. Calcium arriving uncovers the binding sites on the thin filament and lets cross-bridges cycle; calcium being pumped back out of the cytosol is what ends the contraction, and that pump burns ATP. Relaxation is active. A muscle that runs out of ATP does not go floppy, it seizes.

The skeleton half is torque, and it is one equation. Force times perpendicular distance to the pivot, balanced on both sides. Muscles insert close to the joint, so their moment arm is short, so their force must be large. That is not a design fault. It is the price paid for speed and range at the far end of the limb.

What to hold

• Filaments slide past each other and never change length. The sarcomere shortens because the Z lines are pulled towards the centre.
• The A band is the length of the thick filament, so it cannot narrow during contraction. The I band and the H zone both narrow, because both are defined by regions where filaments do not yet overlap.
• Calcium binds troponin, which moves tropomyosin off the myosin-binding sites on actin. Without calcium the sites are covered and no cross-bridge can form, however much ATP is present.
• ATP binding to myosin is what releases it from actin; hydrolysis then re-cocks the head. This is why ATP depletion locks the muscle in a bound, stiff state rather than a relaxed one.
• Relaxation is active: calcium has to be pumped back into the sarcoplasmic reticulum against its gradient, and that costs ATP.
• Active tension tracks the overlap between thick and thin filaments, which is why there is an optimal length and why tension falls off on both sides of it.
• Stretched past optimal, fewer cross-bridges can form because there is less overlap. Compressed below it, thin filaments from opposite ends foul each other and the thick filaments run into the Z lines.
• Passive tension comes from elastic elements and rises steeply as the muscle is stretched, so total tension at long lengths is not a measure of what the cross-bridges are doing.
• A single fibre's action potential is all-or-none, so whole-muscle force is graded by firing frequency and by recruiting more motor units, never by a larger signal.
• Shortening speed is set by the myosin ATPase isoform; fatigue resistance is set by the metabolic machinery. They are separate properties, which is why a fibre can be both fast and fatigue-resistant.
• Most limb joints are third-class levers, with the muscle inserting between the joint and the load, so the muscle force always exceeds the load it moves.
• A short moment arm buys speed and range: a small shortening at the insertion swings the far end of the limb a long way, quickly.