THE BRAIN FROM TOP TO BOTTOM

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Anatomy by Level of Organization

Function by Level of Organization

We Are All Complex Networks

Axons Play Unexpected Role in Processing Information

The Variety and Structural Complexity of Neurons

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The cell body of a neuron is very small (from a few hundredths to a few tenths of a millimetre in diameter). The axons of some neurons, such as those in the cortex that send signals to other nearby neurons, are scarcely any longer than a hair is thick.

But the axons of other neurons, such as the motor neurons in the spinal cord that control the feet, can sometimes measure over a
metre in length!

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NEURONS

The human nervous system is composed of two types of cells: glial cells and neurons. Neurons are the ones that make us so clever.

Like all the cells in the human body, every neuron has a membrane that surrounds its cytoplasm and a nucleus that contains its genes. Neurons also have small organelles that let them produce energy and manufacture proteins.

But because the neurons’ main job is to transmit information, they also have two types of highly specialized extensions that distinguish them from other cells.

Dendrites, with their tree-like branching structure, gather information and relay it to each neuron’s cell body.

Axons are generally very long, and each neuron has only one. This axon carries information away from the neuron’s cell body toward other neurons, with which it makes connections called synapses. Axons can also directly stimulate other types of cells, such as muscle and gland cells.

Fewer Glial Cells Than You Might Think?

GLIAL CELLS

The human nervous system is composed of two types of cells: glial cells and neurons.

Compared with neurons, glial cells play a less obvious role in intercellular communication, so you tend to hear about them less often. And when you do hear about them, it is often for the wrong reason, such as the outdated idea that while the brain contains about 100 billion neurons, it also contains 5 to 50 times as many glial cells. These latter figures are still cited frequently in neuroscience textbooks, but the consensus figure was actually revised downward early in the first decade of the 21st century, when several studies suggested that the ratio of glial cells to neurons is actually much lower: about 1 to 1 (in other word, that the brain contains just about 1 billion glial cells and 1 billion neurons).

But that does not mean they are not essential. In fact, they are so essential that without them, the neurons could not work properly.

The glial cells provide the neurons with nourishment, physical support, and protection. Glial cells also dispose of the waste materials generated when neurons die, and accelerate neural conduction by acting as an insulating sheath around certain axons.

Each of these functions is performed by a different specialized type of glial cell.