ABSTRACT
The main type of cell forming nervous tissues is the neuron (nerve cell). The myelin sheath surrounding many axons permits faster conduction of the action potential, which 'jumps' from one node of Ranvier to the next. The presynaptic neuron contains synaptic vesicles which contain a chemical neurotransmitter. The balance of excitatory and inhibitory influences determines whether or not the neuron is sufficiently depolarized to start an action potential travelling along the postsynaptic neuron. The rapid onset of neural responses results from the fast transmission of action potentials. The existence of so many different neurotransmitters permits synaptic transmission to be very specific. Synapses and neural circuits are often referred to by a name derived from the transmitter they use. In general, a drug that enhances the action of a neurotransmitter is called an agonist, while one that opposes its action is an antagonist.
