3. Communication Systems Signal molecules in the plasma membrane of the signal cell interact with membrane bound receptors on the target cell. These signals are therefore restricted to cells which are in direct contact CONTACT DEPENDENT Nerve cells or neurones elicit responses by the release of a neurotransmitter at synapses. Can signal over very long distances via a network of nerve cells. Very fast signalling e.g. GABA (Gamma-Amino-Butyric-Acid – an inhibitory neurotransmitter) NEURONAL Secretion of a local mediator. This affects cells in the immediate area of the signalling cell e.g. Histamine PARACRINE Secretion of a hormone into the bloodstream for dispersal. The signalling cell and the target cell can be far apart. Very slow method e.g. Insulin, Adrenaline ENDOCRINE