(46) relaxation
Steps of Muscle Relaxation
Overview of Muscle Contraction and Relaxation
Muscle fibers can contract when a neuron stimulates them, resulting in the generation of an action potential and contraction of the muscle.
The process of relaxation is crucial for returning both neuron and muscle to a resting state.
Role of Acetylcholinesterase (AChE)
Acetylcholinesterase is an enzyme responsible for breaking down acetylcholine (ACh) in the synapse.
When stimulation from the neuron ceases, AChE works to clear ACh from the synaptic cleft.
AChE is represented metaphorically as a
Pac-Manthat gobbles up ACh to stop further muscle stimulation.
The removal of ACh is vital to allow the muscle fiber to cease contraction and return to relaxation.
Neuronal Resting State
For the neuron to return to its resting state:
Calcium ions must be pumped out of the neuron.
This process utilizes active transport mechanisms which require ATP (energy).
Muscle Fiber Relaxation
With ACh levels reduced, the muscle fiber stops generating its own action potential.
Sodium ions will also be pumped out of the muscle fiber, which is crucial in halting the electrical activity in the fiber.
It is noted that potassium ions also play a role in maintaining the electrical conditions necessary for muscle and nerve cell function, but will be discussed in further detail in future lessons.
Calcium and the Sarcoplasmic Reticulum (SR)
Following the cessation of action potential:
The sarcoplasmic reticulum starts pumping calcium ions back into storage.
Both the neuron and muscle fiber work to expel calcium ions, with the muscle fiber pumping calcium back into the SR using ATP.
Effect on Troponin-Tropomyosin Complex
The decrease in calcium levels causes troponin-tropomyosin (TT) proteins to cover the active sites on actin filaments.
This prevents myosin heads from binding to actin, effectively stopping the contraction.
A metaphor of a tug-of-war game is used to describe how myosin heads let go of the actin when they can no longer grab onto the active sites.
Return to Original Length
As the SR continues to pump calcium back into storage, the muscle fiber returns to its original length.
The muscle relaxes, and the sarcomeres cease contracting, leading to muscle relaxation.
Steps of Muscle Relaxation
Overview of Muscle Contraction and Relaxation
Muscles are like stretchy rubber bands. They can get shorter and tighter when a message from the brain tells them to move. This message makes a tiny spark called an action potential that helps the muscle contract.
After the muscle has contracted, it needs to relax so it can get back to its normal state.
Role of Acetylcholinesterase (AChE)
There is a special helper called Acetylcholinesterase, or AChE for short, that acts like a clean-up crew. It breaks down a chemical called acetylcholine (ACh) that helps tell the muscles to contract.
When the brain’s message stops, AChE comes in to clear away ACh. It’s like a little Pac-Man that eats up ACh so that the muscles can stop moving.
Neuronal Resting State
For the brain message system (the neuron) to get back to normal, it needs to remove tiny particles called calcium ions.
This needs energy (like a battery) to work.
Muscle Fiber Relaxation
When there’s less ACh, the muscle stops getting those contraction messages and no longer has action potentials happening.
Another tiny particle, sodium, gets pumped out of the muscle, which helps it stop working. Potassium ions also help but we will learn about them later.
Calcium and the Sarcoplasmic Reticulum (SR)
After the action potential stops, a part of the muscle called the sarcoplasmic reticulum gets to work putting away the calcium ions back into storage.
Both the neuron and muscle fiber work together to remove calcium, using energy to pump it back in the SAR.
Effect on Troponin-Tropomyosin Complex
With less calcium, a combination of proteins called troponin-tropomyosin cover the spots on actin where myosin heads usually grab on.
This is like a game of tug-of-war where if the rope is let go, no one can pull anymore. Myosin heads can let go of actin when they can’t grab on.
Return to Original Length
As the calcium is stored away, the muscle can relax completely and go back to its normal size and shape, like a rubber band that goes back to being stretchy!