Muscle contraction
Overview.
Muscle contraction.
Overview
The brain controls the skeletal muscles, in the body.
Skeletal muscles move the skeleton, which enables the body to move.
Neurons in the motor cortex, in the brain, controls the skeletal muscles.
These neurons send signals to muscles, via the nerves.
These nerves meet the muscles, at the neuromuscular junction.
This is discussed in the module neuromuscular junction.
Neurotransmitters transmit the message from the nerve ending, to the muscle.
This results in a series of actions, in the muscle.
Eventually, the muscle contracts.
Fibres in the muscle structure, are designed to contract and stretch.
Muscle Anatomy is discussed in the module, Muscle Anatomy.
Muscle contraction.
The action potential that originates in the motor neuron,
reaches the axon terminal.
Acetylcholine transmits a message to the motor end plate,
of the muscle cell.
The nerve ending of the motor neuron, branches out.
The same nerve can connect to many neuromuscular junctions.
If the signal strength is sufficient, an action potential is generated,
in the muscle membrane.
This membrane has traverse tubules, or T-tubules which extend into the muscle fibre.
T-tubules carry electrical current from the surface to the interior.
This helps all parts of the muscle cell to get the signal and contract, at the same time.
The main agent that induces muscle contraction, is calcium.
The myofibrils are surrounded by sarcoplasmic reticulum.
Sarcoplasmic reticulum is complex network of interconnected hollow tubes.
They contain a fluid rich in calcium ions.
One of its functions is to store and release calcium ions.
The action potential in the membrane causes some calcium channels to open.
This calcium binds to the sarcoplasmic reticulum, causing more calcium to be released.
The calcium diffuses through the sarcoplasm, and reaches the sarcomeres in the myofibrils.
The muscle cell itself has a intricate and interesting structure.
Muscle cells are long fibres.
Muscle fibres have many smaller bundles, inside it, called myofibrils.
These myofibrils have distinct longitudinal subunits called sarcomeres.
Sarcomere are elastic.
They can contract or extend.
The sarcomere are attached end to end, in a junction called the Z-line.
The Z-line is a protein, which has a zig zag appearance, which gives it, its name.
A single muscle fibre in the biceps, may contain 100 thousand sarcomeres.
The sarcomere are the basic contracting units of a muscle fibre.
Within the sarcomere there are many sub units called filaments.
There are two types of filaments.
There are thin filaments called actin.
There are thick filaments called myosin.
Within the sarcomere the actin and myosin filaments, are arranged in parallel.
They are arranged alternately.
Between every two actin filaments, there is a myosin filament.
Cross bridges extend from myosin and contact the actin filament.
The actin filaments are attached, to the Z-lines, at both ends of the sarcomere.
The myosin filament is located in the centre, between the actin filaments.
The myosin and actin filaments can slide along each other.
The myosin filament can pull the actin filaments adjacent to it,
so as to bring them closer together.
This causes the sarcomere unit to contract.
When all the sarcomere units contracts, the whole muscle fibre contracts.
When all the muscle fibres in the fascicles contract, the fascicles will contract.
When all the fascicles in the muscle contract, the whole muscle contracts.
Actin filaments are like a string of pearls.
Myosin filaments have a knob like head.
These heads make contact with the actin filaments, to enable it to pull them.
The contraction process is a cycle of many steps.
Calcium initiates the contraction cycle, in the sarcomere.
The actin filament, is a protein,
which looks like two intertwined double chain of pearl strings.
There are many binding sites, in the actin string.
The binding sites, are where the myosin head, binds to the actin.
A protein called tropomyosin, runs along the actin fibre.
Another protein, called troponin, is located at some junction points,
in the actin fibre.
The troponin and tropomyosin complex, regulates the muscle contraction.
In the resting state, the tropomyosin blocks the binding sites.
In this condition, the myosin heads cannot bind with the actin.
There is no contraction, of the sarcomere.
When calcium is released, it binds to the troponin.
This causes a conformational change in the protein.
Troponin is attached to tropomyosin.
Tropomyosin slides off, along the actin fibre.
This exposes the binding site.
Now the sarcomere is ready for contraction.
The myosin filament has many small heads.
There are mini hockey sticks, attached to each other.
When the myosin binding head, is exposed,
the myosin heads binds with the actin filament.
The head acts like a cross bridge, between the myosin and actin filaments.
Now the two filaments, are in contract with each other, at each cross bridge.
The next step is called the power stroke.
The myosin heads pull the actin filaments, towards the centre of sarcomere.
We can imagine, many myosin heads walking its way along the actin fibre.
The actin fibre is located on both sides of the myosin fibre.
The actin fibres on both the sides are pulled together, towards the centre.
The actin filaments are attached to the Z line.
This causes the whole sarcomere to contract.
There are many sarcomeres in a myofibril.
There are many myofibrils in a muscle cell.
The contraction in the sarcomere, causes the muscle cell,
and eventually the whole muscle to contract.
In the next step, ATP binds to the myosin head.
This causes the myosin head, to be released from the binding site.
The myosin head is now ready, to bind at another site.
The repeated binding and pulling action, of the myosin head,
causes the required amount of contraction in the sarcomere.
There are about 500 cross bridges in each myosin filament.
They attach and detach from the binding site, at the rate of about 5 times a second.
The process repeats itself, as long as the calcium concentration is high,
and enough ATP energy is available.
The signal for muscle contraction, comes from the calcium ions.
To contract a muscle energy is required.
The energy comes from the ATP molecules.
ATP is synthesised in the mitochondria, present in the muscle cells.
ATP is the driving force that enables the contraction process.
Energy is derived from the hydrolysis of ATP by water.
ATP is converted to ADP and inorganic phosphate.
The inorganic phosphate can transfer energy to another molecule.
In the case of muscles, energy is transferred to the myosin head.
When the stimulus for the contraction ends,
calcium returns to the sarcoplasmic reticulum.
The active transport pumps, located in the sarcomere,
transfers the calcium ions from outside,
to the inside of the sarcoplasmic reticulum.
Calcium is stored inside the sarcoplasmic reticulum for future use.
When calcium is pulled back, the calcium concentration comes down.
The troponin, tropomyosin complex, again comes to action.
The calcium ion bound to troponin comes off.
The tropomyosin slides back, and blocks the binding sites, in actin.
Myosin can no longer bind with actin.
The muscle then comes to a resting state.