Wednesday, July 9, 2008

A short introduction to the neuron

Doing my part to contribute to basic science blog posts on the Web.

This blog is, as stated, mostly a neuroscience blog. In this post, I'll be teaching those of you who don't know, perhaps, a friggin' thing I'm talking about about the basic component of the nervous system.

The neuron is the basic building block of the nervous system. There are several different shapes of neurons, which I'll describe later, but here's a basic neuron:

(Carlson, Niel. A. (1992). Foundations of Physiological Psychology. Needham Heights, Massachusetts: Simon & Schuster. pp. 36)

There are some parts of the neuron that aren't labeled here, including the nodes of Ranvier, but this is a basic neuron. A basic description of the parts -

Dendrites - These receive synaptic signals and contain receptors on them for neurotransmitters.
Soma - The cell body contains the nucleus and all that other cellular shit.
Axon - The tunnel which electric and chemical signals travel down.
Myelin sheath - The insulating cover for the axon. Gets eaten in people with MS.
Nodes of Ranvier - The axon between the myelin sheaths. Electrical signals travel in saltatory ('jumping') motion between the nodes.
Terminal button - These send synaptic signals.

The most important part of a neuron is its cell membrane. This is what receives electrical and chemical signals. Here is a diagram of a neuron membrane:


There are ion pumps in the cell walls. There are three ions which are important to the neuron: sodium, potassium, and calcium. These make your electrical impulses work. Gradients of charge across a cell produce potentials, which are differences in the voltage across a cell membrane and which drive electrical charges, known as action potentials, which do the work of your nervous system. Here is an action potential:


Action potentials always begin with a stimulus and depolarization - once the voltage across the membrane approaches a threshold (in this case a small negative voltage), the action potential is triggered. Action potentials are an all-or-nothing action, kind of like an orgasm, where the voltage becomes positive then drops down again and undershoots a tad.

There's an equation which relates the charge in a cell given the concentrations of ions, which is called the Nernst equation (which relates to a lot of other cells, but is used in neurons) and something one of my professors called the Extremely Important Equation.

The Nernst equation:

E = \frac{R T}{z F} \log\frac{[\mbox{ion outside cell}]}{[\mbox{ion inside cell}]}
where E = equilibrium potential, RT/F = 59.1 mV, z = the number of electrons transferred, and the words in brackets should be self-explanatory.
The Extremely Important Equation, which I think is called the Goldman equation:

E_{m} = \frac{P_{K^+}} {P_{tot}} E_{K^+} + \frac{P_{Na^+}} {P_{tot}} E_{Na^+} + \frac{P_{Cl^-}} {P_{tot}} E_{Cl^-}
where Em = equilibrium potential of the membrane, E(ion) = equilibrium potential of an ion, P(ion) = the permeability of the ion in arbitrary units, usually siemens for conductance, and Ptot = the total permeability of all permeant ions.

And here's some basic physics equations:

Here's some different types of neurons, categorized by function:

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