what causes calcium to be released in sliding filament action for muscle contraction?
The release of calcium in muscle contraction is a crucial step in the sliding filament theory. It occurs due to an electrochemical signal known as an action potential, which travels down the motor neurons connected to the muscle fibers. When an action potential reaches the neuromuscular junction (the connection between the neuron and the muscle fiber), it triggers the release of a neurotransmitter called acetylcholine.
To understand this process, you'll need to know how muscles and neurons work together. Here's how you can break it down:
1. Motor Neurons: Nerves that connect the brain or spinal cord to muscle fibers. They control muscle contractions by transmitting electrical signals.
2. Neuromuscular Junction: This is the connection point where the motor neuron meets the muscle fiber. It consists of the end of the motor neuron (called the presynaptic terminal) and the muscle fiber membrane (called the postsynaptic membrane).
3. Action Potential: When the motor neuron receives a signal to initiate muscle contraction, it generates an electrical impulse known as an action potential. This electrical signal travels down the motor neuron.
4. Acetylcholine Release: When the action potential reaches the presynaptic terminal, it triggers the release of acetylcholine, a neurotransmitter stored in vesicles within the terminal.
5. Binding of Acetylcholine: Acetylcholine diffuses across the synaptic cleft (a small gap between the presynaptic terminal and the postsynaptic membrane) and binds to specific receptors on the postsynaptic membrane.
6. Opening of Ion Channels: The binding of acetylcholine to its receptors on the postsynaptic membrane causes the ion channels to open. Specifically, it leads to the opening of sodium (Na+) channels, resulting in an influx of sodium ions.
7. Generation of Muscle Cell Action Potential: The influx of sodium ions depolarizes the muscle fiber's membrane, generating a muscle cell action potential that propagates along the entire muscle fiber.
8. Transverse Tubules: Muscle fibers have intions called transverse tubules (T-tubules), which are extensions of the cell membrane that penetrate into the muscle fiber.
9. Sarcoplasmic Reticulum: The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells. It surrounds the myofibrils (bundles of contractile proteins) and stores calcium ions (Ca2+).
10. Calcium Release: When the muscle cell action potential reaches the T-tubules, it triggers the release of stored calcium ions from the sarcoplasmic reticulum into the surrounding cytoplasm of the muscle fiber.
11. Initiation of Contraction: The released calcium ions then bind to a protein called troponin, which causes a conformational change in another protein called tropomyosin. This change exposes binding sites on the actin filaments.
12. Sliding Filament Action: The exposed binding sites on actin filament allow myosin heads (proteins on thick filaments) to bind to actin. This forms cross-bridges, and the myosin heads undergo a series of cyclic movements, pulling the actin filaments towards the center of the sarcomere (the basic unit of muscle contraction).
In summary, the release of calcium ions upon stimulation of the motor neuron and subsequent binding to the appropriate proteins is necessary for muscle contraction to occur.