Muscle and motility

**Muscles pull on a skeleton** — the muscle contracts and the skeleton acts as a system of levers.

A muscle can **only contract (pull)** — it cannot push or lengthen itself, so it moves a bone in one direction only.

Two muscles on **opposite sides of a joint** with **opposite effects** — one bends the joint, its partner straightens it.

The **biceps** contracts to **bend (flex)** the elbow; the **triceps** contracts to **straighten (extend)** it.

A **tendon** joins **muscle to bone**; a **ligament** joins **bone to bone**.

Whole **muscle** → muscle **fibre** (cell) → **myofibril** → **sarcomere** (the contractile unit).

The **functional contractile unit** of striated muscle — the region **between two adjacent Z-discs**.

**Actin** is the **thin** filament; it is **anchored to the Z-discs** and projects inward.

**Myosin** is the **thick** filament (with protruding **heads**); it sits in the **centre**, held in register by the **M-line**.

A giant **elastic** protein: it **anchors myosin to the Z-disc**, keeps it centred, and provides **recoil / elasticity** (springs the sarcomere back, resists overstretching).

The region containing **actin only** (no myosin) — it appears **light** and spans a Z-disc.

**A band** = the **full length of the myosin** filament (dark, includes the overlap). **H zone** = the **central, myosin-only** part of the A band (no actin overlap).

The **central line** that **links the myosin filaments** and holds them in register.

During contraction the **actin and myosin filaments slide past each other** so they **overlap more**; the sarcomere shortens but the **filaments do not shorten**.

**No** — they keep the **same length** and simply **slide** to overlap more. Only the **overlap** and the **sarcomere length** change.

They are **pulled closer together**, because the actin filaments (anchored to them) slide inward, so the **sarcomere shortens**.

It gets **shorter** — the I band is the **actin-only** region, and more overlap leaves less actin uncovered.

It gets **shorter** — the H zone is the **myosin-only** region, and the actin tips slide inward to cover more of the myosin.

The **A band equals the length of the myosin filaments**, and **myosin does not shorten** — so the A band is **unchanged**. This shows the filaments slide, not shrink.

Sarcomeres are joined **end to end (in series)**; when many shorten **together**, their shortenings **add up** along the fibre, so the **whole muscle** shortens.

**Tropomyosin** — a thread held in place by **troponin** — lies over and **covers** the binding sites.

It **holds tropomyosin** in the blocking position and has a **binding site for Ca²⁺**; when Ca²⁺ binds, it moves tropomyosin away.

It is the **switch**: released from the sarcoplasmic reticulum, it **binds troponin**, which **exposes** the myosin-binding sites.

One ATP **binds the myosin head to detach** it from actin; that same ATP is then **hydrolysed to re-cock** the head.

The myosin head **pivoting to pull the actin filament toward the centre** of the sarcomere, shortening it.

**Ca²⁺ is pumped back** into the sarcoplasmic reticulum, so **tropomyosin re-blocks** the binding sites and no new cross-bridges form.

With **no ATP**, the myosin heads **cannot detach** from actin, so the cross-bridges stay **locked**.

A muscle can only **pull (contract)**, not push — so a **second muscle** pulling the opposite way is needed to **reverse** the movement.

One muscle **contracts (pulls)** while its **antagonist relaxes** (and is stretched); they **swap** to reverse the movement.

**Biceps flexes** (bends) the elbow; **triceps extends** (straightens) it. They are an **antagonistic pair**.

**Cartilage** reduces friction on the bone ends; **synovial fluid** lubricates; **ligaments** hold bones together and **stabilise** the joint.

**Hinge** (elbow, knee) moves in **one plane**; **ball-and-socket** (hip, shoulder) moves in **many directions** and can rotate.

Bones are **levers** that turn about a **joint (pivot)**, so a muscle's pull is converted into a **larger, controlled movement** of the limb.

A **tendon** joins **muscle to bone**; a **ligament** joins **bone to bone**.