Synapses are the junctions between either; two neurons, a neuron and a sensory cell (e.g. a photoreceptor), or a neuron and and effector cell (e.g. muscle cell). Where information is passed from one cell (presynaptic) to the other (postsynaptic). Most synapses are chemical synapses, which means that information is transferred when vesicles containing neurotransmitters fuse with the presynaptic membrane, releasing neurotransmitters into the Synaptic Cleft, the gap between the presynaptic and postsynaptic membranes.
Neurotransmitters diffuse into the synaptic cleft and bind to receptors on the postsynaptic membrane, triggering an effect in the postsynaptic cell. Depending on the combination of the neurotransmitter released and the postsynaptic receptor it binds to, the effect will either be excitatory, leading to depolarisation of the postsynaptic cell, or inhibitory, leading to hyperpolarisation of the postsynaptic cell.
Neurons rarely receive just one synaptic input, they normally receive both excitatory input from certain types of cell and inhibitory input from other types of cell, the excitatory input drives the firing of action potentials that carry information to the next presynapse, while inhibitory inputs may serve to modulate the activity of the neuron, e.g. by reducing the frequency of action potentials.
Electrical Synapses are junctions between two cells where the cells are physically connected to each other. The cells are normally connected by gap junction proteins that create a pore (the gap junction) between the cytoplasm of each cell. This relatively large pore allows ions to flow directly from one cell to the next, conveying the electrical signal with them.
At electrical synapses, Ions diffuse from areas of high concentration to areas of lower concentration, down a concentration gradient.