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.