Answer
At a chemical synapse, the presynaptic neuron releases neurotransmitter into the synaptic cleft.The neurotransmitter molecules cross the synaptic cleft and are bound by receptors on the postsynaptic membrane. The message transmitted in this way can travel in one direction only, from presynaptic neuron to post synaptic cell. However, there are cell to cell connections that permit electrical messages to travel in both directions. A case in point are gap junctions that exist in smooth and cardiac muscles. Gap junctions permit the conduction of electrical messages in either direction between adjacent cells.
Work Step by Step
Gap junctions are cylindrical connections formed by protein molecules (connexins) between adjacent cells; they are found in smooth muscle and cardiac muscle --but not in striated skeletal muscle. These junctions permit the intercellular exchange of ions and small molecules between neighboring cells. Consequently, an electrical current may pass from cell to cell in both directions.
Gap junctions facilitate electrical coupling of cells; in addition they aid hormonal action by allowing the passage of Ca++ ions and cAMP second messenger between cells.
