These electrical signals pass through the midbrain to the spinal cord along so-called upper motor neurons . Within the spinal cord, they connect to lower motor neurons, which convey the signals out of the spinal cord to the surface of the muscle involved. Electrical stimulation of the muscle causes contraction. The force of contraction pulling on the skeleton causes movement of the limb. Damage to the upper motor neurons or their connection to lower motor neurons is the cause of most spasticity.
Muscles are present in opposing groups, as illustrated by the biceps and triceps of the upper arm.
Contraction of the biceps bends the elbow, while contraction of the triceps straightens it. When a muscle is being intentionally stimulated to contract, it is called an agonist . Opposing muscles are called antagonists . For instance, when you flex your biceps, it is the agonist, while the triceps is the antagonist. Note that any given muscle will be an agonist in one situation, but an antagonist in another.