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Principles (Coordination)

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Effectors in the body such as muscles and glands may get their signals to contract or secrete substances in two ways: electrically by nerve impulses or chemically by hormones.


Nerve cells have electrical impulses pass along them which results in their secretion of a neurotransmitter onto the target cells which respond quickly, locally and in a short-lived way.


Hormones on the other hand circulate in the bloodstream, reaching distant target cells and exerting a long-lasting but slow effect.



Above is a synapse and below you can see an example of the interaction between multiple hormones across a big distance (brain- and ovary-secreted hormones exerting both local and general effects).



Histamine is released at sites of allergy or injury as part of the immune response to enable the pathogen-removing activity of specialised blood cells. Its effects are localised and include vasodilation, pain following insect bites, hay fever symptoms and motion sickness.


Similarly, prostaglandins act locally to induce inflammation of affected tissues, as well as some other slightly different effects such as fever and inducing labour.


In flowering plants, a hormone called IAA (indoleacetic acid) controls tropism i.e. how the plant responds to different environmental stimuli. At the same time, the growth factors which stimulate plant growth diffuse to other tissues and regulate the direction of growth based on external influences such as nutrient concentration and light.

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