Your nerves somehow allow this signal to pass through and, like magic, it causes that large muscle to contract. If it's okay, we'll concentrate on it as the question directly refers to the muscular contraction. We move ahead to the motor neuron, where the brain's action potential has already reached the muscle. Motor units, which are made up of an upper and a lower motor neuron, are used to control muscles. The top motor neurons, or the neuron that transmits signals from the brain, are represented by the tracts above. After connecting to lower motor neurons, upper motor neurons link to the muscle. The basic structure of your muscles is fibers inside fibers inside fibers. Sarcomeres, the smallest unit, are made up of sections that are separated by Z-lines. We have two filaments, actin and myosin, between the Z-lines. The M-line is where Myosin is connected, while Actin is a long, thin filament attached to the Z-line. The actin will be pulled by the myosin, which will cause the Z-lines to constrict inward toward the M-line. The bigger structures will follow if many of these tiny fibers do this at once, triggering the contraction of the entire muscle. The Sliding Filament Model of contraction is used to describe this.
A single Actin and Myosin pair appears quite similar if we zoom in on it. Myosin and actin do not contact when your muscles are at rest, yet they have a strong attraction for one another (they really want to touch). If not for two proteins (tropomyosin and troponin) linked to the Actin filament, they would touch. As we are still awaiting a signal, we slightly zoom out.
An action potential is sent by the lower motor neuron, which causes Acetylcholine to be released into the synapses. This results in an inflow of sodium, which changes the voltage and spreads the signal.
The action potential is no longer in the neuron but rather within the muscle. The Sarcolemma is struck by the action potential as it travels through the muscle cells.
The Sarcolemma has tubes that penetrate the cell deeply (T-Tubules). The Action Potential is sent at the Sarcomeres via these tubes.
Calcium is continually being pumped out of the cell by the Sarcoplasmic Reticulum, which houses the sarcomeres (these pumps use ATP as energy). Additionally, it has voltage-gated calcium channels that are still closed that line it.
The Voltage Gated Calcium channels open when the T-Tubules produce an action potential, allowing calcium to flood the cell.
The two proteins that surround the actin are now activated by calcium. When calcium attaches to troponin, a shape shift occurs (as proteins do when they bind). The active strands are revealed as the troponin pulls tropomyosin in its direction.
Now that the Actin sites are revealed, the Myosin is free to bind to them. However, myosin, which used some ATP and broke it down into ADP and phosphate, is the only protein that can actually do this. This myosin, which has been "charged," extends. It remains put, clinging to ADP+Phosphate like a loaded weapon.
- The myosin unleashes its energy and rushes toward the exposed Actin now that it is primed and prepared. Once more, it alters form by dragging the acting and moving it inside.
- As soon as the bullet was shot, all of the energy necessary to divide ATP into ADP and phosphate was used up, and the split chemicals were released back into the cell (the release occurs because myosin changed it shape and in this state no longer has a strong affinity for them). The mitochondria will reuse them at this location and transform them back into ATP.
- Myosin does have a high affinity for ATP in this condition, which causes ATP to attach to it once again. Myosin is released from Actin by this binding, which results in another shape shift. Myosin is therefore brought back to its primed and prepared condition. Actin may be drawn in little further if it fires once more.
As a result, the two Z-lines are pulled toward the centre by the Myosin, and the sarcomere contracts.
Since the Sarcoplasmic Reticulum's calcium pumps are actively pumping calcium out, calcium ultimately dissociates from Troponin. Actin is rendered unavailable to myosin as a result of the protection being reset. The fun is gone now because Myosin can no longer link to the actin, and when an action potential occurs, the cycle restarts.
And that’s how a signal from the nervous system, an action potential, can cause a muscle to contract. Isn’t nature cool?