S. D. V. Stephens

notes / Basic Neuroscience and Geometry of Neuroscience / chapter 1: Neurons and Signalling / From my notebook

N5 Stains, Microscopes, and the Neuron Doctrine

The structure of this book is a bottom-up approach, i.e., we start with the cells, move on to the tissue, then organs, etc.

The brain is by far the most complex and sophisticated structure in the human body (and perhaps the known universe).

The nervous system consists of two cell types (which may be broken down into smaller subtypes) called neurons and glia. Neurons are known to be the main cells responsible for the functions of the brain, while, loosely and not entirely accurately, the glia (glial cells) are the glue holding the neurons together.

Histology

Formaldehyde is used to harden the jelly-like brain so that it can be observed in thin slices cut with a microtome.

  • Microtome: a device that uses steel, glass, or diamond knives to cut microscopically thin slices of tissue. Now we also use femtosecond lasers and other techniques.
  • Nissl bodies: dyed clumps of material surrounding the nuclei of neurons.
  • Nissl stain: stains/dyes used on neurons and glia to differentiate the cytoarchitecture of the cells/tissue.
  • Cytoarchitecture: the structure and arrangement of cells.
  • Histology: the study of microscopic structures in tissue.
  • Golgi stain: a silver chromate solution used to dye a small percentage of neurons accurately.
  • Neurohistology: exactly how it sounds.

“The gain in the brain is mainly in stain.”

The parts of a neuron

The neuron is made up of a central body and the long tubes which flow from the central body. The central body is called the cell body, the soma (plural somata), and the perikaryon (plural perikarya) (lots of names for one thing). The tubes are called neurites and are divided into two types: axons and dendrites.

Note. Design superior neuron cells? Via bioengineering.

Axons can be very long (up to 1 m or more) and are uniform. There is usually one axon per cell body. Dendrites are rarely longer than 2 mm, taper off into a point, and are large in number.

Axons were believed to be the “wires” that carry the output of the neuron/cell body.

Sketch of a neuron: a round soma with a nucleus, branching dendrites above and around it, and a single axon running down whose side branches leave at roughly right angles, with a note that axon branches tend to favor right angles.

Beside the sketch is a pasted reproduction of one of Cajal’s drawings: a slice of Golgi-stained tissue showing several neurons with their branching dendrites and long axons running mostly horizontally across it.

Dendrites were then believed to be receiving input, acting as the antennae.

The neuron doctrine

Definition N5.1 (Neuron doctrine). The neuron doctrine: cell theory also applies to neurons (contact, not continuity).

Santiago Ramón y Cajal and Golgi argued over this (with Cajal being right). It was not until the 1950s and the advent of the electron microscope that this was resolved.

Scales and microscopes

In meters:

100 µm is the near-limit of the naked eye.

Neurons are 4–20 µm.

Light microscopes have a limit of 0.1 µm, but the space between neurons is 20 nm (0.02 µm).

The electron microscope uses an electron beam and has a limit of 0.1 nm. The structure of the brain at this size is called the ultrastructure.

Laser microscopy can go to 20 nm across and works on non-fixed, living tissue.