HOW DO MUSCLES CREATE MOVEMENT?


What are the parts of a muscle? How do these parts work together to produce movement?

Electronic signals are sent through the neurons of the brain to the sight of desired movement. When they reach this spot the signals close an electric pathway and tell to muscle to expand or contract.

The contractile proteins of muscle are actin and myosin. The thick myosin filaments have tiny heads protruding out of them, which touch the actin filaments. As the brain transmits a signal for a muscle to contract, the heads of the myosin pull the actin closer, which also pulls the z-lines closer. A group of sarcomeres make up the myofibril, which is the main unit of muscle fiber.

The action from the heads of the myosin shortens the sarcomeres and as thousands of these sarcomeres are contracting, the entire muscle is shortened and it pulls the bone in the desired direction. The myosin needs ATP and calcium in order to fuel the reactions.

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1. The thin filaments of actin are separated:
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2. As the muscle is contracting, the thick myosin filaments remain in position while the thin actin filaments slide toward one another. This movement is what brings the z-lines closer:
external image contaction.jpg


3. At the muscle's maximal contracted state, the actin filaments are overlapped and the process is complete:
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Each skeletal muscle fiber connects to a fiber from a nerve cell called a motor neuron. This nerve fiber extends outward from the brain or spinal cord, and a muscle fiber contracts only when a motor neuron stimulates it.

The connection between the motor neuron and muscle fiber is called a neuromuscular junction. Here, the muscle fiber membrane in specialized to form a motor end plate. In this region of the muscle fiber, nuclei and mitochondria are abundant, and the cell membrane (sarcolemma) is extensively folded.

The end of the motor neuron branches and projects into recesses of the muscle fiber membrane. The cytoplasm at the distal ends of these motor neuron fibers is rich in mitochondria and contains many tiny vesicles that store chemicals called neurotransmitters.

When a nerve impulse traveling from the brain or spinal cord reaches the end of a motor neuron fiber, some of the vesicles release of a neurotransmitter into the gap between the neuron and the motor end plate of the muscle fiber. This action stimulates the muscle fiber to contract.

At the origin muscle attach to the immovable end. At the insertion the muscle attach to the movable end. When the muscle contracts the insertion is pulled towards the origin, this causes movement of a muscle. Flextion and extension are terms used to describe the change in the angle between bones and the joints.

Skeletal muscles work together to make muscle move. Prime movers are responsible for a a main body movement, and the synergists assist the prime mover. Antagonists act against the prime mover to make the muscle move in the opposite direction.

Contraction
1. The distal end of a motor neuron releases acetylcholine.
2. Acetylcholine diffuses across the gap at the neuromuscular junction.
3. The muscle fiber membrane is stimulated, and a muscle impulse travels deep into the fiber through the transverse tubules and reaches the sarcoplasmic.
4. Calcium ions diffuse from the sarcoplasmic reticulum into the sarcoplasm and bind to troponin molecules.
5. Troponin and tropomyosin interact to expose binding sites on actin.
6. Actin and myosin filaments form linkages.
7. Myosin cross-bridges pull actin filaments inward.
8. Muscle fiber shortens as a contraction occurs.