The major components of a neuron are detailed in the illustration to the left. The length of the axon can be very much longer, however, than that depicted in the illustration. Also, in his video course, Biology and Human Behavior: The Neurological Origins of Individuality, 2nd edition, Robert M. Sapolsky illuminates the complexity of a single neuron. He explains that an average neuron may have 10,000 dendritic spines and 10,000 axon terminals. Communication between neurons is both chemical and electrical, so the term electrochemical is used to describe the overall process.
Neurons, neurochemicals, and neurocircuits:
Very simplistically stated, chemical molecules called neurotransmitters carry messages from one neuron to the next. The vesicles on a neuron's axon terminals release neurotransmitter molecules. From a chemical perspective, a neurotransmitter molecule "fits like a key into a lock," says Sapolsky, as it contacts a receptor on a second neuron's dentritic spines. In Brainscapes: An Introduction to What Neuroscience Has Learned about the Structure, Function, and Abilities of the Brain (1995), Richard M. Restak explains that "Receptors are large dynamic protein molecules that exist along and within the cell membrane." Receptors can, explains Restak, "increase in number and avidity for their neurotransmitter according to circumstances. Large and prolonged intakes of certain substances, for instance, lead to an increase in the number of receptors for these substances—the basis for the withdrawal response in addiction…. Later, if the addicted person stays away from the drugs the receptors eventually die off…."
The union of neurotransmitter and receptor is either inhibitory or excitatory. "Each neurotransmitter influences its own receptor independent of the action of other receptors," writes Restak. "While some neurotransmitters decrease the voltage between the inside and the outside of the nerve cell and thus stimulate the cell into action (an excitatory neurotransmitter), others increase it and thus inhibit the cell from firing (an inhibitory neurotransmitter."
Once the neurotransmitter and receptor lock together, Restak describes their action as "dynamic" and "exquisitely sensitive." There are two families of receptors. Restak explains that ion channel receptors influence the activity of ion channels for sodium, potassium, calcium, and chloride, "principally, directly, or indirectly, via biochemical intermediates." He writes: "Ion channel receptors … bring about changes in membrane permeability and thus govern the flow of ions through their channel. When a neurotransmitter reacts with its receptor, their interaction results in a change in the shape of the receptor so that ions can then flow across the membrane from the point of high concentration toward the point of lower concentration." A second family of receptors "do not contain ion channels within their structures," explains Restak. Rather, they act through intermediaries, the G proteins, located inside the nerve cell. Restak writes: "Basically, G proteins function as coupling factors that serve as links between the receptor on the outside surface of the nerve cell membrane and a vast number of interlinked cellular processes within the cell."
Sufficient neurotransmitter excitation of ion channel receptors in dendritic spines creates a wave of ionic change that prompts the neuron's axon hillock to reach its action potential. An electrical signal thereby bursts down the neuron's axon (and axonal branches called collaterals) and into axonal terminals to prompt vesicles to release a chemical neurotransmitter. In other words, the neuron "fires." As vesicles release neurotransmitter molecules, each molecule floats through the microscopic synapse with the potential to make contact with a receptor. Neurons communicating with each other in one direction develop a circuit. Often, there are reciprocating circuits between brain structures.
Brain neurons: photograph from the laboratory of Te-Won Lee, Ph.D., UCSD. Sapolsky explains that "neurotransmitters can be taken back up into the presynaptic terminal and repackaged into vesicles or an enzyme can rip up the neurotransmitter" in a process of degradation.
Local neuronal circuits in the neocortex constitute cortical regions. Not only do cortical regions connect with each other, but cortical regions and subcortical nuclei interconnect as well. We will discuss cortical-subcortical circuits in more detail in Parts 2 and 3 of CorticalBrain.com. The photograph of neurons to the right is from the laboratory of Te-Won Lee.
Neurons, projections, and pathways: