38.4 Muscle Contraction and Locomotion

Muscle Tissue Types and Functions

Muscles convert chemical energy into mechanical force, making them responsible for every movement your body produces. The three types of muscle tissue differ in structure, control, and location, and those differences directly reflect the jobs they perform.

Types of muscle tissue

Skeletal muscle attaches to bones via tendons and produces voluntary movements like walking, running, and maintaining posture. Under a microscope, it has a striped (striated) appearance because its contractile proteins are organized into repeating units called sarcomeres.

Cardiac muscle is found only in the heart and is also striated, but it's involuntary. Cardiac cells connect to each other through intercalated discs, which contain gap junctions that allow electrical signals to pass rapidly from cell to cell. This is what keeps the heart beating in a coordinated rhythm.

Smooth muscle lines the walls of hollow organs like blood vessels, the intestines, and the bladder. It lacks striations, is involuntary, and consists of spindle-shaped cells. Smooth muscle controls processes like constricting blood vessels and moving food through the digestive tract.

Muscle Contraction Mechanism

Sliding filament model of contraction

The sliding filament model explains how muscles generate force at the molecular level. Contraction happens not because filaments themselves shorten, but because two sets of filaments slide past each other, pulling the ends of each sarcomere closer together.

A sarcomere is the basic functional unit of a muscle fiber. Each muscle fiber is made up of many myofibrils, and each myofibril is a chain of sarcomeres arranged end to end. Within a sarcomere, there are two key protein filaments:

• Thick filaments made of myosin, a motor protein with protruding heads
• Thin filaments made of actin, along with the regulatory proteins troponin and tropomyosin

Here's how the cross-bridge cycle works:

1. At rest, tropomyosin blocks the myosin-binding sites on actin, preventing contraction.
2. When calcium ions ($Ca^{2+}$) are released, they bind to troponin. This causes a conformational change that shifts tropomyosin out of the way, exposing the binding sites on actin.
3. Myosin heads (already energized from hydrolyzing ATP) bind to the exposed sites on actin, forming a cross-bridge.
4. The myosin head pivots, pulling the thin filament toward the center of the sarcomere. This is the power stroke.
5. A new molecule of ATP binds to the myosin head, causing it to detach from actin.
6. Myosin hydrolyzes the ATP, re-cocking the head into its high-energy position, ready to bind again.

This cycle repeats as long as $Ca^{2+}$ and ATP are available. Without ATP, myosin cannot release from actin, which is why muscles become rigid after death (rigor mortis).

Excitation-contraction coupling

Excitation-contraction coupling is the process that links a nerve signal to the mechanical event of contraction. It bridges the gap between the nervous system and the sliding filament mechanism.

1. A motor neuron releases acetylcholine (ACh) into the synaptic cleft at the neuromuscular junction. ACh binds to receptors on the muscle cell membrane (the sarcolemma), triggering depolarization.
2. The depolarization wave spreads along the sarcolemma and dives into T-tubules, which are deep infoldings of the sarcolemma that penetrate into the interior of the muscle fiber.
3. Depolarization of the T-tubules activates voltage-gated $Ca^{2+}$ channels called dihydropyridine (DHP) receptors. These physically interact with ryanodine receptors on the sarcoplasmic reticulum (SR), causing the ryanodine receptors to open.
4. $Ca^{2+}$ floods out of the SR into the sarcoplasm, binds to troponin, and the cross-bridge cycle begins.
5. For relaxation, $Ca^{2+}$-ATPase pumps actively transport $Ca^{2+}$ back into the SR. As sarcoplasmic $Ca^{2+}$ concentration drops, tropomyosin re-covers the binding sites on actin, and contraction stops.

Types of muscle contractions

Not all contractions produce the same kind of movement:

• Isotonic contraction: The muscle changes length while maintaining roughly constant tension. Lifting a dumbbell (concentric, muscle shortens) or lowering it slowly (eccentric, muscle lengthens under load) are both isotonic.
• Isometric contraction: The muscle generates tension without changing length. Pushing against a wall or holding a plank position are examples. The cross-bridge cycle still occurs, but the force produced isn't enough to move the load.

Myoglobin is an oxygen-binding protein found in muscle tissue that acts as a local oxygen reserve. It stores $O_2$ and releases it during sustained contractions when blood supply alone can't keep up with demand. Muscles with high myoglobin content appear darker (think dark meat vs. white meat in poultry).