Synapses are the regions between the terminal of an axon and the receptors on other cells. Neurotransmitters can bring the messages across the synapses. Synapses can be open or enclosed, large or small. Synaptic inputs can affect the neuron activities. The cells releasing neurotransmitters are presynaptic cells, and those binding to the neurotransmitters with membrane receptors are postsynaptic cells. Chemical communications can also happen outside synapses, e.g., receptors in the autonomic nervous system can be activated by substances in the blood.
Synapses have the feature of plasticity, i.e., they can be removed, generated, and changed. Such plasticity allows the functional adaptation and the development of the nervous system, including the functions of learning, memory, and recovery from damages. In fact, plasticity is the feature of the whole nervous system, just with variances in the extent. For example, the cerebral cortex has been found to be more plastic than the spinal cord.
The neurons are connected with each other with the small openings in their membranes, which are called gap junctions. Ions and electric currents can pass between the neurons via the gap junctions and have influences in the alterations of membrane potentials on connected neurons. The gap junction may also be involved in the chemical synaptic communications. Such coordination is critical for the cerebral cortex activities and may be involved in the neuronal basis of attention.
The other type of cells in the nervous system is glia. The glial cells play the role of functional partners of neurons. In addition to the nutritive and protective functions, they can also regulate the activities and communications of neurons. Different subtypes of glia have different functions. Some can protect against chemical alterations, some can change the features of neuronal signaling, and some are involved in immune regulations.