Cell Communication.
Cells in multicellular organisms
need to communicate with one another to coordinate their function and control
their growth. The human body has several means of transmitting information
between cells. These mechanisms include direct communication between adjacent
cells through gap junctions, autocrine and paracrine signaling, and endocrine
or synaptic signaling. Autocrine signaling occurs when a cell releases a
chemical into the extracellular fluid that affects its own activity (Fig. 4.9).
With paracrine signaling, enzymes rapidly metabolize the chemical
mediators, and therefore they act mainly on nearby cells. Endocrine
signaling relies on hormones carried in the
bloodstream to cells throughout the body.
Synaptic signaling occurs
in the nervous system, where neurotransmitters act only on adjacent nerve cells
through special contact areas called synapses. In some parts of the
body, the same chemical messenger can function as a neurotransmitter, a
paracrine mediator, and a hormone secreted by neurons into the bloodstream.
