Muscle Physiology: The Big Picture

So how does a muscle contract?

In order to answer this question we must first examine what tells a muscle to contract. Let’s say that I am sitting here writing and want to pick up a cup of coffee. In order to do so I must send a command to the muscles in my arm. The command comes from a thought generated in my nervous system. The command travels from my brain to my spinal cord to a nerve that attaches to a muscle in my arm. The command tells my muscle to contract and my arm dutifully responds by moving closer to the coffee. Muscles are made of protein. If we were to examine a skeletal muscle under a microscope we would see that it is composed of tiny protein fibers or filaments. When a muscle receives a command from the nervous system to contract the protein filaments slide past each other. In fact one of the filaments connects to the other and drags it along. Think of thousands of overlapping filaments sliding past each other as the muscle contracts. The command to contract must somehow get from the outside of the muscle to the inside. Tiny messengers called neurotransmitters bring the message from the nerve to the muscle. Other chemical messengers that tell the protein filaments to contract then pass on the message. Muscles need energy to contract. Muscles must have some sort of power source in order to power the sliding filaments. The energy comes from ATP. ATP connects to one type of filament and extracts the energy so that it can pull the other filament along.

Skeletal Muscle Contractions

Steps of a skeletal muscle contraction:

1. An action potential reaches the axon of the motor neuron.
2. The action potential activates voltage gated calcium ion channels on the axon, and calcium rushes in.
3. The calcium causes acetylcholine vesicles in the axon to fuse with the membrane, releasing the acetylcholine into the cleft between the axon and the motor end plate of the muscle fiber.
4. The skeletal muscle fiber is excited by large mylenated nerve fibers which attach to the neuromuscular junction. There is one neuromuscular junction for each fiber.
5. The acetylcholine diffuses across the cleft and binds to nicotinic receptors on the motor end plate, opening channels in the membrane for sodium and potassium. Sodium rushes in, and potassium rushes out. However, because sodium is more permeable, the muscle fiber membrane becomes more positively charged, triggering an action potential.
6. The action potential on the muscle fiber causes the sarcoplasmic reticulum to release calcium ions(Ca++).
7. The calcium binds to the troponin present on the thin filaments of the myofibrils. The troponin then allosterically modulates the tropomyosin. Normally the tropomyosin physically obstructs binding sites for cross-bridge; once calcium binds to the troponin, the troponin forces the tropomyosin to move out of the way, unblocking the binding sites.
8. The cross-bridge (which is already in a ready-state) binds to the newly uncovered binding sites. It then delivers a power stroke.
9. ATP binds the cross-bridge, forcing it to conform in such a way as to break the actin-myosin bond. Another ATP is split to energize the cross bridge again.
10. Steps 7 and 8 repeat as long as calcium is present on thin filament.
11. Throughout this process, the calcium is actively pumped back into the sarcoplasmic reticulum. When no longer present on the thin filament, the tropomyosin changes back to its previous state, so as to block the binding sites again. The cross-bridge then ceases binding to the thin filament, and the contractions cease as well.
12. Muscle contraction remains as long as Ca⁺² is abundant in sarcoplasm.

Types of Contractions

• Isometric contraction: muscle does not shorten during contraction and does not require the sliding of myofibrils but muscles are stiff.
• Isotonic contraction: inertia is used to move or work. More energy is used by the muscle and contraction lasts longer than isometric contraction. Isotonic muscle contraction is divided into two categories: concentric, where the muscle fibers shorten as the muscle contracts (ie. biceps brachialis on the up phase of a biceps curl); and eccentric, where the muscle fibers lengthen as they contract (ie. biceps brachialis on the down phase of a biceps curl).
• Twitch: exciting the nerve to a muscle or by passing electrical stimulus through muscle itself. Some fibers contract quickly while others contract slowly.
• Tonic: maintaining postural tone against the force of gravity.