A sensory receptor senses the change and sends action potentials by its afferent sensory neuron to the integration center in the brain or spinal cord. The sensory receptor is the endocrine cell or gland , there is no afferent path, the integration center is the endocrine cell or gland, the efferent path is the secretion of the hormone into the blood , the effectors are most of the cells in the body, and response is either to change the rate of an enzymatic reaction , or control transport of certain molecules across cell membranes, or to control gene expression and the making of new proteins . The term hormone means to excite or arouse . Peptide hormones are composed of a few amino acids up to large proteins. They are synthesized as preprohormones near the ribosomes and as they move through the rough endoplasmic reticulum and the Golgi the signal sequence is removed making them prohormones .
Peptide hormones are water-soluble and dissolve easily in the extracellular fluid.
They have short half-lives and need continuous secretion for long duration of action. They bind to surface membrane receptors that utilize various second messenger systems to cause their responses. Steroid hormones are all derived from cholesterol . They are secreted from cells after being synthesized by smooth endoplasmic reticulum and they diffuse to the cell membranes and out of the cells as needed. They are lipid soluble so they need protein carriers to be carried in the blood.