|The brain is composed of over a billion nerve cells, or neurons, that communicate with one another through specialized contacts called synapses. Typically, a single neuron receives roughly 1000 synapses from other neurons; these synapses are located almost exclusively on the neuron's dendrites, long projections that radiate out from the neuron's cell body. In turn, the neuron's axon, a long thin process that grows out from the cell body of a neuron, makes synaptic connections with 1000 other neurons. In this way, neuronal signals pass from neuron to neuron to form extensive and elaborate neural circuits.|
|Some synapses excite neurons and cause them to generate signals called action potentials, large transient voltage changes that propagates down their axons. Other synapses are inhibitory and prevent the neuron from generating action potentials. The action potential propagates down the axon to the sites where the axon has made synapses with the dendrites of other nerve cells.|
|Upon arriving at the synapse, the action potential causes the nerve terminal to release small quantities of chemicals called neurotransmitters which diffuse a short distant across the synapses and bind to specialized receptors, called neurotransmitter receptors, located on the dendrites of the receiving neurons.|
Over 300 different neurotransmitters have been identified in the brain, giving rise to an additional level of complexity in how neuronal signals travel through the nervous system. A typical neuron, however, uses only one, or a small number of neurotransmitters for synaptic communication. The types of neurotransmitters that a neuron makes depends primarily on: a) where in the brain the neuron is located; b) which neurons make synapses on it; and c) which neurons it makes synapses with. In addition, the diversity in neurotransmitter receptor types found on neurons in the brain number in the thousands. However, again, a typical neuron only expresses a relatively small numbers of different neurotransmitter receptors at synapses on its dendrites.