THE BRAIN FROM TOP TO BOTTOM

Anatomy by Level of Organization

Function by Level of Organization

The next time you take a walk in the woods in the wintertime, take a good look at the trees and notice how similar they are in structure to neurons. The trees’ branches are like dendrites, and the buds on these branches are like dendritic spines. The trees’ trunks are like axons, and their roots are like the axons’ numerous terminal buttons.

From there it should be just a small leap to imagine that you are taking a wintertime stroll right through the middle of your own brain.

NEURONS

Neurons form networks through which nerve impulses travel. From each neuron’s dendrites to the sometimes very distant tip of its axon, these impulses propagate through the neural membrane in the form of electricity.

But because the neurons communicate with one another without touching one another, they use special molecules called neurotransmitters to pass nerve impulses from one neuron to the next.

This chemical transmission of nerve impulses causes the axon and the dendrites to develop specialized structures that facilitate it.

Thus, dendrites have thousands of “spines” sticking up out of their surface.

The bulb-like terminal buttons of the axons, which secrete the neurotransmitters, are positioned opposite these spines.

But the form of these component structures of the synapse varies tremendously, as does the overall form of the neurons themselves.

A neuron’s connections from and to other neurons.

GLIAL CELLS

Various types of glial cells, each in its own way, help the neurons of the central nervous system to function properly.

Star-shaped astrocytes provide neurons with mechanical support, supply them with nutrients, and maintain the equilibrium of the extracellular environment. They also digest and eliminate all kinds of debris.
The microglial cells constitute the first line of defence against foreign invaders. They are the macrophages of the brain.

The oligodendrocytes form the myelin sheath that surrounds the axons of many neurons. The very special way in which the oligodendrocytes wrap around the axons accelerates neural conduction.

The glial cells that perform these same functions in the peripheral nervous system have different names. The glial cells that provide mechanical support are called satellite cells, while those that manufacture myelin are called Schwann cells.

But of all the glial cells, the astrocytes are probably the ones whose functions are the most complex.