Description of figures


Figure 01.  Types of neurons in the central nervous system.  This figure shows examples of three types of neurons in the human brain.  The key point made by this figure is that the shape of neurons, specifically the shape of their dendrites, varies markedly across the brain.

On the left is an unusual type, found in the cerebellum.  It's dendrites (labeled "d") make up an extensive tree, with many fine branches.

In the middle is a neuron in the cerebral cortex, with one major dendrite that ascends toward the surface of the cortex and many smaller dendrites that form a symmetrical tree around the cell body.

On the right is a type of cell found a part of the brain called the thalamus, with dendrites that extend in two directons away from the cell body.



Figure 02.  This key figure shows the main functional regions of a typical neuron.

The dendrites and cell body of a neuron are its reception zones.  They are the sites at which synapses formed by axons from other neurons typically occur.  Neurons usually receive a few thousand such synapses.

Integration of synaptic inputs occurs at the axon hillock-initial segment region of a neuron.  This region is rich in voltage-sensitive sodium channels, which open when a sufficient input from excitatory synapses reaches them.  Their opening produces an action potential. 

The axon is a region that propogates the action potentials.  The number of such action potentials, and perhaps the specific pattern, is the code a neuron uses to influence other neurons (that is why the region is referred to as "encoding").

The axon terminals are the outputs sites, at which the activity of one neuron is transferred to other neurons by way of synapses.


Figure 03.  This complete schema of a neuron shows its internal makeup and the relationship of one neurons to its neighbors.  

Key organgelles in a neuron are stacks of rough endoplasmic reticulum (referred to as Nissl bodies) and a robust Golgi complex.  These are the cardinal signs of a cell that is actively synthesizing proteins and packaging them for some purpose.

Microtubules are the organelles used to transport packaged proteins down the axon and dendrites.

Neurofilaments are the major cyoskeletal organelle.  These establish and maintain a neuron's complex shape.



Figure 04.  A simpler cartoon of a neuron again accents the Nissl bodies and Golgi complex, the microtubules and the neurofilaments.  Insulating myelin is added to the figure, as is the special regions of the axon where the action potential is initiated (the initial segment) and the regions where it is periodically regenerated (the nodes of Ranvier).  

Don't be confused by a few simplifications to the drawing:

1) Most axons have several hundred nodes of Ranvier, not just two.  That's because most axons are many times longer than the diameter of the cell's dendritic field.

2) Axons end in thousands of terminals, not just one big one.

3) Myelin is made by different types of glial cells in the central and peripheral nervous system.  In the CNS, myelin is made by oligodendrocytes.  In the PNS it is made by Schwann cells (so this cell is from the PNS).  The difference appears critical for the ability of PNS axons to regenerate when they are cut.


Figure 05.  A light micrograph showing dark blue regions in the cell body of a neuron.  These are Nissl bodies.

Figure 06.  In this light micrograph, a collection of neurons in the spinal cord that send signals to muscles cells is shown.  The cell bodies and dendrites are the easiest parts of the neurons to see.

Figure 07.  A collection of neurons in a part of the brain called the hippocampus is shown in this light micrograph.  In the bright blue region... a layer of cell bodies... are the stained cell bodies of a few neurons.  Out of those cell bodies grow one set of ascending dendrites and a second set of descending dendrites.

Figure 08.  This drawing shows dendritic spines in a neuron from a young animal (A) and from an old animal (B).  Loss of spines is typical in aging, and represents one way in which neurons progressively lose the ability to adapt to changes.  In C is shown dendritic spines at higher magnification.


Figures 9-13 show various aspects of neuroglial cells

Figure 9 illustrates the close relationship between astrocytes and the brain vasculature.  Astrocyte processes contact 80% of the capillary surface and help to move nutrients from the capillary to neurons.

Figure 10 is an electron micrograph of a oligodendrocyte.  The nucleus of the oligodendrocyte is labeled N.  Around it is the dark cytoplasm of this cell.  Growing from the body of the cell are two dark arms, which end as very dark rings around pale cores.  The rings are myelin, made by the successive wrapping of oligodendrocyte arms.  The pale core regions are the axons of neurons.  Within the axons you can see relatively large, dark regions...these are the mitochondria... and large numbers of cross-sectioned microtubules and neurofilaments.

Figure 11 is a light micrograph of what myelin looks like when it is stained for phospholipids.  You should compare this figure and Figure 10.

Figure 12 shows a myelinated axon at very high magnification.  The principal point of the figure is to show how myelin is composed of layers, much like the rings of a tree.  In the axon, itself, are a few tube-like organelles... the microtubules... and many more thread-like organelles... the neurofilaments.

Figure 13 is a light micrograph from human brain stained for an immune-system protein.  The figure shows microglia because these are the cells of the CNS that express immunoproteins and that otherwise function as the immune-competent cells of the brain and spinal cord.