Hormonal communication

Control by nervous system is very fast but also very expensive in terms of energu required for pumping sodium and potassium ions to maintain resting potentials and in protein synthesis to make all the channels and pump proteins

Energy is also needed to maintain all the neurones and other cells in the nervous system, such as Schwann cells. A much 'cheaper' alternative is to use hormones that are secreted in tiny quantities and dispersed around the body in the blood

Hormones are ideal for controlling functions that do not need to be coordinated in a hurry

Hormones such as adrenaline, insulin, glucagon and ADH are made in endocrine glands. 

• a gland is a group of cells that produces and releases one or more substances, a process known as secretion
• endocrine glands contain secretory cells that pass their products directly into the blood
• endocrine glands don't have ducts and are so known as ductless glands

Hormones are cell signalling molecules

• some hormones, like those in excretion, are water soluble, so they cannot cross the phospholipid bilayer of cell surface membranes
• these hormones bind to receptors on their target cells that in turn activate second messengers to transfer the signal throughout the cytoplasm
• steriod hormoned on the other hand are lipid soluble, so they can pass through the phospholipid bilayer
• once they've crossed the membrane, they bind to receptor molecules inside the cytoplasm or the nucleus and activate processes such as transcription

Hormonal control of the human menstrual cycle

• in the middle of the 28 day menstrual cycle the female gamate is released in the oviduct
• if fertilisation occurs while gamete is in the oviduct, the embryo that develops needs…