The body must maintain a constant internal environment, through a process termed homeostasis, while also being able to respond and adapt to external events. The nervous and endocrine systems both work to bring about this adaptation, but their response patterns are different. The nervous system and the endocrine system use chemical messengers to signal cells, but the speed at which these messages are transmitted and the length of their effects differs.

Nervous System

The nervous system responds rapidly to stimuli by sending electrical action potentials along neurons, which in turn transmit these action potentials to their target cells using neurotransmitters, the chemical messenger of the nervous system. The response to stimuli by the nervous system is near instantaneous, although the effects are often short lived. An example is the recoil mechanism of an arm when touching something hot.

Endocrine System

The endocrine system relies on hormones to elicit responses from target cells. These hormones are synthesized in specialized glands at a distance from their target, and travel through the bloodstream or inter-cellular fluid. Upon reaching their target, hormones can induce cellular responses at a protein or genetic level.

This process takes significantly longer than that of the nervous system, as endocrine hormones must first be synthesized, transported to their target cell, and enter or signal the cell. However, although hormones act more slowly than a nervous impulse, their effects are typically longer lasting. 

Additionally, the target cells can respond to minute quantities of hormones and are sensitive to subtle changes in hormone concentration. For example, the growth hormones secreted by the pituitary gland are responsible for sustained growth during childhood.