HOW CONTRACTIONS WORK
The muscles are in fact just a tool that allows contractions to occur; it's actually the brain which gives the order and chooses to contract the muscle. When you decide to do an action, you tell your brain to send a signal to your muscle. The motor cortex is the area of the brain that receives this order. It receives the information from several regions of the brain that give the sense of movement, speed, etc. The cortex analyses this information and translates it into a nerve impulse which is sent to a first neuron. The electrical signal generated by the nerve impulse moves through the projection of this neuron.
To reach the muscle, the nerve impulse must pass through several stages. So, in the upper part of the spinal cord, the impulse is passed to a second neuron called the motor neuron, which joins up with the muscle, causing the nerve impulse to stimulate it. This motor neuron is divided into several nerve endings and each of these endings is in contact with the muscle fibre. The combination of the motor neuron and the fibres is called the motor unit.
The meeting point between the motor neuron and the fibres is called the neuromuscular junction. It is here that the electrical signal triggers the release of chemical molecules, the famous neurotransmitters. A series of electrical signals are then produced, allowing the released molecules to move to the filaments so that they contract. This in turn happens in numerous muscle fibres throughout the muscle.
And there you have it, the muscle works!