Muscle anatomy
Overview.
Muscle structure.
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.
This is discussed in the module muscle contraction.
Fibres in the muscle structure, are designed to contract and stretch.
Muscle structure.
Muscle cells are bundled together, in units, called fascicles.
A muscle like the biceps, will have many such fascicles.
Unlike other cells muscles cells are thread like fibres.
The fibres can be short, or as long as 35 cm..
The cross section of a muscle, however is very small.
It is in the range of 10 to 100 microns.
Overall muscle cells are the largest cells in the body.
The cells are so large, that a single muscle cell will have multiple nuclei.
This enables the muscle cell, to synthesise proteins, locally where it is required.
There could be about 30 nuclei in 1 mm of a muscle cell.
In typical cells, the nuclei is in the centre of the cell.
The muscle cells, the nuclei is located just under the muscle membrane.
Muscle cells do not grow in numbers.
They grow in size.
This is called hypertrophy.
The fluid inside the muscle cell is called a sarcoplasm.
This is equivalent to the cytoplasm in other cells.
The cytoplasm can have many mitochondria organelles.
The mitochondria provides ATP energy to the muscle cell.
The sarcoplasm also has glycogen, myoglobin, oxygen etc..
Myoglobin is the protein which binds oxygen to iron.
This helps to store some oxygen in the cell.
The muscle cell itself has a intricate and interesting structure.
Muscle cells are long fibres.
Inside the muscle cell, there are smaller bundles, 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.
A single muscle fibre in the biceps, may contain 100 thousand sarcomeres.
The sarcomere are the basic contracting units of a muscle fibre.
Inside the sarcomere there are many filaments, which are proteins.
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.
Each muscle fibre is surrounded by a plasma 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 myofibrils are surrounded by sarcoplasmic reticulum.
Sarcoplasmic reticulum is complex network of interconnected hollow tubes.
They contain a fluid rich in calcium ions.
It stores and releases calcium ions.
The release of the calcium ions, causes the myosin fibres to pull together.
This is what causes the muscle to contract.
The signal from the motor neuron, eventually results in the release of calcium ions,
from the sarcoplasmic reticulum, which causes the muscle to contract.
When contraction is complete, the calcium ions return to the sarcoplasm reticulum.
This causes the muscle to relax.