In order for a skeletal muscle to contract, your brain sends a signal, from an upper motor neuron down the spinal cord where it synapses with the cell bodies of lower motor neurons located in the anterior horn of the spinal cord.
From here, the signal travels through the lower motor neuron’s axon and until it reaches the axon terminal which is next to a muscle fiber.
At the site where an axon terminal meets the muscle fiber, called the neuromuscular junction, it releases small membrane-enclosed synaptic vesicles filled with acetylcholine.
Acetylcholine is a neurotransmitter that tells the muscle to contract.
Now before we continue with the actual events that happen during the contraction, let’s focus on one muscle cell a myocyte and its functional units called sarcomeres.
A myocyte is a long cylindrical cell with multiple nuclei located just below the sarcolemma, which is the cell membrane.
The sarcolemma is unique because it makes these tiny tunnels called T-tubules that project downwards from the surface towards the center of the muscle fiber.
The cytoplasm of a myocyte is called sarcoplasm, and the myocyte has a special type of smooth endoplasmic reticulum which is called sarcoplasmic reticulum.
The sarcoplasmic reticulum stores lots of calcium and runs parallel to the T tubules.
Now, the sarcoplasm is filled with stacks of long filaments called myofibrils and each myofibril consists of contractile proteins and regulatory proteins.
Contractile proteins include thick myosin and thin actin filaments.
The thick myosin filament is made up of hundreds of myosin proteins, and each myosin protein has a tail and two myosin heads - it looks a bit like two golf clubs with their handles twisted around one another.
Multiple myosin proteins join their tails together to form the central part of the thick filament.
In comparison, the thin actin filaments are made up of small, globular proteins called G-actin.
Each G-actin has an active site where the myosin head binds to it during contraction.
These G-actin proteins forms a filament that looks like a long helix structure -- like a pearl necklace that’s gently twisted. This entire filament is called F-actin.
F-actin is associated with two regulatory proteins - tropomyosin and troponin.
Tropomyosin is a string-like protein that wraps around F-actin, covering its active sites so that the myosin heads can’t bind to it.
Troponin proteins are smaller and are made up of three subunits - there’s a T subunit that binds to tropomyosin, an I subunit that binds to F-actin, and a C subunit that binds to calcium ions.
Together, the F-actin, the troponin and tropomyosin make a complete thin filament.
As it turns out, these thick and thin filaments don’t extend through the entire length of the myocyte, but instead, they’re arranged in short units called sarcomeres.
When we look at sarcomeres with an electron microscope, the thick myosin filaments look dark, while the thin actin filaments look light which gives the muscle fiber a striped appearance.
Alright, now let’s zoom in and relate these bands to a structure of one sarcomere. At the center of the sarcomere is the M line made of myomesin proteins, where the thick filaments attach.
At the borders of the sarcomere are the two Z-discs made of alpha actin proteins, where the thin filaments attach.
For every thick filament, there are two thin filaments-one above and one below and the two types of filaments overlap.