Today we talk about the building blocks of brains and nervous systems: Neurons. The title of today’s post is actually the name of an excellent textbook – originally by Kuffler and Nicholls, but now by Nicholls, Martin, Wallace and Fuchs. The text is one of the basics for undergraduate and graduate students alike, but is also readable by the educated public.
Yup, neurons. Not nerves. Neurons are brain cells and the cells that make up nuclei, ganglia, fibers, tracts, roots, and even nerves. But please, since this is a guide to readers and writers, do not make the mistake of calling brain cells “nerves”. A neuron is a singular cell that has several very important properties which will be described below. All brain tissue is made up of neurons, but not all brain tissue contains nerves. A nerve is a bundle of neurons – in particular the portion of the neuron called an “axon”, and it specifically connects the brain and spinal cord with sites in the rest of the body – such as muscles and sensory organs.
The special properties of neurons are: (1) ability to separate chemical ions to produce an electrical charge gradient across its membrane, (2) the ability to “gate” diffusion of those ions across the cell membrane so that ionic charge can be built up, then discharged selectively, (3) the ability to perform this gating in response to an input stimulus (electrical, chemical, mechanical), and (4) the ability to transfer a signal to adjacent cells via direct electrical contact, or by release of a chemical that will replicate the electrochemical process in the next neuron in a sequence.
Figure at the right is an excellent diagram of a model neuron. [For the record, I got it from this site, and it says "Copyright 2001, Therese Winslow."] Note the section marked “dendrites” – this is the region that collects inputs from hundreds and thousands of other (input) neurons. The soma contains the metabolic “machinery” of the neuron; the axon is the conductive channel that transfer a signal at distances of microns to meters; and the synaptic terminal is the output zone of the neuron. These terminals can connect again to tens, hundreds or even thousands of “downstream” neurons. By the way – a “nerve” is a bunch of axons, all running in the same direction, with the cell bodies back in the brain (or spinal cord), and the synaptic terminals out in the body somewhere. There are also nerves running in reverse, with the cell bodies and dendrites out in the muscles, fingertips, etc, and the synaptic terminals in spinal cord and brain.
I could go on at length about how the process works (in fact, I teach exactly this subject to graduate students) but instead I want to describe how different specializations of these 4 characteristics produces the variety of neurons that serve specific functions in the brain and nervous system.
(A) If the trigger to gate the ions and release the stored electrical potential is chemical, and the synaptic terminal releases chemicals called neurochemicals onto other neurons – that is a common output or projection neuron.
(B) If the synaptic terminal is on a muscle cell, it is a motor neuron.
(C) If the trigger to release the electrical potential is light, it is a retinal (visual) neuron.
(D) … if mechanical, it may be a tactile (touch) or pain neuron or a “proprioceptor” (joint and muscle position) neuron.
(E) … if vibratory, it may be an auditory (hearing) neuron.
(F) … if complex chemicals, it may be a taste or olfaction (smell) neuron.
(G) If the synaptic terminal release chemicals into the bloodstream instead of onto another neuron, it is a “neurosecretory: neuron such as those that produce hormones.
There are other specialized cell types in the brain which provide support, metabolism, chemical synthesis, insulation, waste removal, etc. These are generally called “glia” but also have more specific names based on shape and function – such as “astrocytes” and “oligodendrocytes.”
The signals transmitted by neurons can increase or decrease the activity of the targets. It is commonly *mis-stated* that these are excitatory or inhibitory neurons. The truth is that the combination of neurotransmitter, the specific receiving molecule (receptor) on the target neurons, and the position of the receptors on the target neurons all combine to determine whether a particular synapse of a neuron is excitatory or inhibitory. Thus a *lot* of different functions can be performed by the same basic building blocks with only minor variations.
In fact, put enough neurons together, and you can even build … a brain!