The sensory system and the motor system make up the central nervous system (brain and spinal cord):
- Sensory receptors in our muscles, joints, and skin receive information about the state of muscular contraction, joint position, and feelings we are experiencing. Sensory nerves continually send this information to the brain (neurons). All of the information that enters the brain is interpreted and coordinated by the brain.
- The motor system subsequently responds by teaching the muscles how to contract individually and in connection to other muscles via motor nerves (neurons).
Accurate movement necessitates the coordination of motor and sensory information. Simple actions need more coordination than complex movements. The central nervous system coordinates muscle activity so that muscles cooperate to generate desired postures and movements:
- Muscles are informed when to contract, lengthen, or remain stationary in respect to other muscles in order to complete their tasks on time.
- Muscles are informed how much to shorten, lengthen, or stay stationary in order to create the necessary movement distance.
- To achieve the right pace of movement, muscles are informed how quickly or slow to touch.
- The peripheral neural system relays messages between the central nervous system and muscles.
When muscle action is regulated by the peripheral neural system, it is much easier than when it is controlled by the central nervous system. The muscle benefits from this system because it acts as a defensive mechanism. Consider the following scenario:
- The sensory receptor within the muscle recognizes when the muscle is stretched fast. The sensory information is conveyed to the spinal cord via a sensory nerve channel. Within the spinal cord, the information is transformed into a motor reaction, which is then transferred directly to the muscle through a motor nerve route. To defend itself from further harm, the muscle contracts instantly. The stretch reflex is one of the body's self-preservation mechanisms. Gravity, pushing downwards on the body's muscles especially while lifting weights, causes muscular strains, prompting the reflex reaction.
- Sensory receptors in the form of pain or pressure receptors receive and convey information to the spinal cord via a sensory nerve route, then return the information via a motor nerve pathway if the body endures damage or sustained pressure:
- Muscle contractions are elicited in reaction to pain in order to remove the body part from the source of the discomfort and protect it from further harm.
- When the body is under pressure, muscles contract to modify the posture of the body for comfort.
All of the examples above are instinctive responses that the body creates on a peripheral level in order to defend itself from harm. All of these things cause muscular contractions to increase.
The brain communicates with the peripheral nervous system and reacts to changes in muscle contractions on a regular basis. It works to keep the balance between muscular contraction and relaxation in check so that the muscles can continue to function together in pairs and groups to create typical movement patterns.