Definition:
The chemical messengers that are secreted from one part of the tissues/organs and enter into the circulation to carry out their action at another organ /tissues are called as hormones.
Classification:
There are many ways to classify hormones. However, recent method of classification, according to their site of synthesis and action, divides hormones into four types:
- Endocrine hormones: These are classical hormones, which are secreted by the endocrine glands and transported by blood towards the site of action. Examples: insulin, glucagon etc.
- Neurocrine hormones: These are also called as neurotransmitter or neuro-hormones which are released from the nerve endings and carried to the target site. Generally, neurohormones are released at synapses. Examples : Epinephrines, Norepinephrines etc.
- Paracrine hormones: These are the chemical messengers, which diffuse from the control cell to the target cells through the interstitial fluid.
- Autocrine hormones: These are the chemical messengers which act on the cell which synthesizes them. Examples: