Nervous control is directed to a specific target by the axon of a neuron. All-or-none electrical signals (action potentials) can travel long distances along axons. These signals go to particular locations, such as a small set of skeletal muscle fibers (a structural organization called a motor unit). At the junction between the axon terminals and the cell that is innervated (called a synapse), neurotransmitters are released. These chemicals pass from the axon terminal to alter the activity of the post-synaptic cell. Sometimes, they act on neighboring cells, but the nervous system is very careful to mop up any neurotransmitters that go astray.
Endocrine control involves chemicals that are released from endocrine cells into the surrounding interstitial fluids. These may have local effects, either on the cell that secreted the chemical (autocrine) or nearby cells (paracrine). Hormones are chemicals that enter the blood stream to travel all over the body. Since hormones must travel through blood to targets, responses to hormones are slow and sustained. Specificity is due to specific target cell receptors. Hormones bind only to their own receptors and these specific receptors must be on the target cell in order for it to respond. Hormones are good for controlling growth, reproduction, and metabolism.
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