Overview of Muscles
To better understand what happens in muscles that are affected by spasticity, it is helpful to first understand some key facts about muscles and the nerves that control their movements.
- More than 600 muscles, along with the bones, make up the musculoskeletal system.
- The bones provide the structure or support for the body, and the muscles provide the ability to move.
- Muscles are connected to bones by tough cords of tissue called tendons.
- Most muscles reach from one bone to another and usually cross a joint.
- The muscles cause the bones to move in relationship to each other.
- Most muscles of the musculoskeletal system work in pairs—called agonists and antagonists. During a movement, the muscle responsible for moving the body part contracts or shortens; this muscle is called the agonist. The antagonist muscle acts against or in opposition to the agonist muscle, stretching when the agonist contracts. The antagonist muscle is responsible for moving the body part back to its original position.
- A muscle acts as the agonist in one action and as an antagonist in the opposite action. For example, when bending the elbow and raising the hand toward the shoulder, the biceps muscle contracts and is the agonist; the triceps muscle stretches and is the antagonist. When the movement is reversed and the elbow is extended, the triceps muscle contracts (is the agonist) and the biceps muscle lengthens (is the antagonist).
- Muscles that commonly work in as agonist-antagonist pairs are listed below.
||Part of the body
||Chest and back
|Anterior deltoids-posterior deltoids
||Front and back of the shoulder
||Upper back and shoulders
|Abdominus rectus-spinal erectors
||Abdomen and lower back
|Left and right external obliques
||Left and right side of the abdomen
||Front and back of the thigh
||Shin and calf
||Top and underside of upper arm
- The movement of muscles is coordinated and controlled by the nervous system. A part of the brain called the motor cortex sends messages or signals through the spinal cord to the nerves in the body that then "tell" the muscle to move. The motor cortex on the right side of the brain controls the left side of the body, while the left side of the brain controls the right side of the body. The primary motor cortex is responsible for starting movements. The motor association cortex coordinates complex movements. An example of a complex movement would be controlling the amount of pressure in the muscles in the hand when a person picks up a glass so that the glass is not shattered or dropped. An area deep within the brain called the thalamus relays and processes the information between the other areas of the brain and the spinal cord.
- Three types of nerve cells or neurons are important in regulating the signals between the muscles and tendons and the brain and spinal cord. These include sensory neurons, motor neurons, and interneurons.
- When muscles receive a signal (are stimulated), they contract. This signal may be a message that the muscle receives from the brain in response to a person's desire to move; this is a voluntary stimulus. The signal may be a reflex, or an involuntary stimulus. A reflex occurs, for example, when the doctor taps the patient's knee with a rubber hammer and the lower leg jerks upward.
- The movement of muscles, and therefore the body, is also controlled by two types of reflexes called stretch and tendon reflexes. The stretch reflex involves two kinds of nerve cells, one that is part of the sensory system (a sensory neuron) and one that is part of the movement system (a motor neuron). When a muscle is stretched, the stretch reflex is stimulated. The opposite muscle in the pair contracts and stimulates the tendon reflex.
- Muscles contain receptors, or areas that receive messages from the nervous system These receptors are called muscle spindles. It is these receptors that "sense" the amount of stretch in a muscle. The receptors send a signal through the spinal cord to the motor neuron in the muscle. This signal causes the muscle to reverse a stretch by contracting or shortening.
- When a person's arm or leg is relaxed and the limb is moved around by someone else, which is called passive range of motion, the underlying structure—the placement and interaction of the bones, tendons, and muscles—should determine any resistance to the movement. Stiffness or contraction of the muscle should not be a determining factor.