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Neurons

To understand how the brain works, even on the simplest level, we must under­stand the building block out of which the brain is constructed. That is the individual nerve cell or neuron.

Chemicals found in and around neurons are known to be involved in pleasure, pain, excitement, depression, sex, hunger, and the effects of drugs. And that is only the beginning. Ultimately, neurons are involved in everything the brain does.

Components of the Neuron

The first type of neuron studied was the motor neuron (or motoneuron). It stimulates muscle fibers to produce movement.

Because it was the first type studied, the motor neuron became the prototypical textbook neuron, used to illustrate the three basic parts of neurons: dendrites, cell body, and axon. A textbook neuron (a very simplified version of a neuron) is shown below.

What are the three basic parts of a neuron?

Simplified neuron diagramA textbook neuron

Dendrites are tubes composed of the cell's outer layer or membrane. They stretch out from the cell body and divide into many small tubes (many more than the textbook neuron shows, and in all directions).

The branching pattern is at least as complex as the branches of a tree. The dendritic structure as a whole is called the dendritic tree (the Greek word dendros means tree).

A successful neuron–one that makes a valuable contribution to the nervous system–will generally grow. It is allocated more of the chemicals called nerve growth factors.

After humans pass the age of seven, the number of neurons in the nervous system declines (see "Neural Evolution" below). However, the complexity of individual neurons continues to increase, because the dendritic trees continue to grow.

An adult human has fewer nerve cells with each passing year. The remaining neurons can become more complex. Dendritic growth continues in healthy people until their 90s.

What happens to a successful neuron?

Leading away from the cell body is a specialized fiber called the axon. The axon is a thin tube specialized for carrying messages to other cells.

Axons can stretch over long distances in the body. The longest axon in the human body stretches from the base of the spine to a muscle in the big toe. An axon on one of these motor neurons can be over a meter long.

The classic 1930s model of the neuron proved to be oversimplified in many respects. Axons are output structures of neurons in the classic model, but electron microscopes discovered in the 1960s they could receive inputs as well.

Axons carry nerve impulses to synapses (areas of near contact with other cells). Synapses are areas where commun­ication between neurons takes place.

Axons are like dendrites in several respects. Like dendrites, axons are made out of cell membrane stretched into long tubes. Like dendrites, the axons may branch into a tree-shaped structures.

What is an axon?

In the case of axons, the branching pattern is called an axonal arbor­izations (the tree metaphor again). This time the label comes from the Latin arbor rather than Greek. In fact, dendritic trees and axonal arboriza­tions look much the same and are difficult to distinguish under a microscope.

Axons–the structures that carry nerve impulses–are extremely thin, compared to their potential length. One professor in a graduate neuroscience class illustrated this by bringing a ball of string into class.

She said, "Suppose you were making a scale model of a neuron. The string represents the axon. If the thickness of the string represents the thickness of the axon, how long should the string be, in our scale model?"

The professor started pulling arm­lengths of string off the ball, then stopped and revealed the truth: "Actually, for a long axon cell, you would have to unravel about two miles of string." Appropriate gasps of amaze­ment came from the audience of attentive students...

Well, perhaps nobody gasped, but it is amazing that axons can be so very thin compared to their length. An axon propagates messages down its entire length, without loss of strength, at speeds from 2-200 mph (3-320 kph).

What was the point of the professor's demonstration with a ball of string?

Axons communicate rapidly, with great precision, over long distances in the body. This is the only way the body can commun­icate rapidly over long distances. Hormones and other influences are much slower.

Cajal's Drawings

Dendritic trees and axonal arborizations can be spectacular and complex. One of the biggest and most complex dendritic trees is that of the Purkinje cell, a type found in the cerebellum.

Purkinje cells have a huge, flat, spreading dendritic tree. The cells are arranged in layers, with axons from other cells crossing through them, forming a huge grid that evidently helps coordinate movements in space: one of the functions of the cerebellum.

Where is the Purkinje cell found?

Purkinje cell
The Purkinje cell with its huge dendritic tree

The drawing here is a classic by Santiago Ramon-y-Cajal (ray-MON ee ka-YAL), a neuroanatomist who lived from 1852 to 1934. The name Ramon-y-Cajal is composed of the father's and mother's surnames and is often shortened to "Cajal."

Cajal won the Nobel Prize in 1906 for his pioneering work on neurons. The drawing of the Purkinje cell dendritic tree was one of his favorites. Cajal said it reminded him of a grape arbor.

How did Cajal make his drawings?

Ramon-y-Cajal made drawings by injecting neurons with dye. The dye spread within the cell but could not cross the cell membrane. Neurons are naturally transparent and tightly packed together. Without a stain of some kind, one cannot see much in nervous tissue, using a light microscope.

With his dye injection technique, Cajal was able to see complex neural processes (dendritic trees and axonal arborizations) using only a light microscope. Neurons are also packed full of structures too small for scientists of Cajal's era to see, including the cell's nucleus, which regulates many aspects of the cell's activity.

The gaps between neurons are so tiny that in the 1890s there was a tremendous controversy over whether any gaps were present. Some said neurons formed a continuous network (the reticular theory), others said nerve tissue consisted of separate cells (the neuron theory). Cajal finally resolved the issue.

Santiago Ramon-y-Cajal spent much of his professional life from about 1889 onward trying to persuade other scientists to drop the reticular theory in favor of the neuron theory. Two years before his death, when he was in his 80s, Cajal marshaled all the evidence in favor of the neuron theory one last time.

What was Cajal's proof that neurons were separate?

By then (1932) most people realized Cajal was right all along. Neurons were individual cells, separate and distinct. Cajal knew this because of his dye-injection technique. The dye did not spread between cells, so he knew there was some barrier or gap between them.

How Neurons Communicate

The word nerve is used in everyday conversation. Technically, a nerve is a bundle of axons outside the central nervous system (that is, outside the brain and spinal cord). A bundle of axons wholly contained within the central nervous system is called a tract or pathway.

What is a nerve? What is a tract or pathway?

Neurons communicate, among other ways, by sending signals called nerve impulses. These impulses travel through the membrane tubes of the dendrites and axons.

Each axon may branch into a whole tree, and nerve impulses go down each branch when an axon divides. Therefore, a single neuron may send signals to thousands of other neurons, giving it potentially great influence on a whole population of cells.

Meanwhile, the dendrites and cell body (and often the axon) of a neuron may receive nerve impulses from thousands of other neurons. So the nervous system is one big network of neurons, with each cell having inputs and outputs that connect it to hundreds or thousands of other nerve cells.

The output from an axon arrives at an area called a synapse (SIN-apse in the American pronunciation, SINE-apse in the British pronunciation). At a synapse, two neurons are separated by a tiny gap called the synaptic cleft.

When a nerve impulse reaches the end of an axon, it stimulates chemicals called transmitters or neurotransmitters to flow rapidly across the synaptic cleft. Each neuron might stimulate thousands of other neurons this way.

What is a synapse?

When transmitters flow across a chemical synapse, they have one of two effects on the post-synaptic neuron (the neuron that comes after the synapse). They either excite it (make it more likely to fire a nerve impulse itself) or inhibit it (make it less likely to fire a nerve impulse itself).

Each neuron responds to many such inputs. Based on the pattern of activity, a neuron may be pushed over its threshold to fire an impulse itself, passing information on to more neurons.

Each neuron is like a pattern recognizer, responding to a complex pattern of inputs. One part of the pattern is spatial (the configuration of synapses firing at a particular time and where they are located on a neuron). The other part of the incoming pattern is temporal (the timing of inputs).

Both spatial and temporal variations can influence the post-synaptic neuron (the one receiving the inputs). Some of the incoming synapses are excitatory, some are inhibitory.

The excitatory synapses make the post-synaptic neuron more likely to fire. Inhibitory synapses, when activated, make the receiving neuron less likely to fire. Therefore, patterns triggering a post-synaptic neuron can be highly complex.

How is each neuron like a pattern recognizer?

That is the sort of talent we need, in neurons, to explain human cognition. Our cognitive world is full of pattern recog­nition. Some neurons fire when you smell garlic, others fire when you see a familiar face, and so forth.

Neural Evolution

Cell death is actually a normal and necessary part of development in the nervous system. In some parts of the brain, more than half of the neurons generated during embryonic devel­opment die before a baby is even born.

To survive, a neuron requires a survival signal to turn off a built-in suicide program. The suicide program within each cell is called programmed cell death or apoptosis.

It is said there are two ways for a cell to die: necrosis or apoptosis (Edmonds, 2010). Necrosis is a "rather messy affair" where a cell dies and rots and causes inflammation. "Apoptosis, on the other hand, is rather civil." Cells are disassembled and their components are recycled.

One neuron's loss is another neuron's gain. Neighboring neurons compete to fill the space left by a cell that dies. Cell death may stimulate the growth of neighboring cells.

What is apoptosis? How does the brain arrive at its adult form?

In children below the age of 7, new cells are added faster than existing ones are killed. The number of neurons in the human brain peaks around age 7.

After that, the total number of neurons decreases gradually for the remainder of a person's life. Remaining neurons can grow larger and more complex.

neural evolution
Neurons become less numerous but more complex.

The number of neurons peaks in childhood, although individual neurons (such as the one high­lighted in black here) may persist from infancy to old age. Survivors grow more complex even as the total number of neurons diminishes.

Neurons grow if they receive stimulation with neurotrophic (nerve-growing) factors. The first to be discovered was named simply "Nerve Growth Factor" (NGF). As it turns out, there are others.

Neurons compete for growth factors. Those receiving NGFs grow, while others shrivel and die. In a properly functioning nervous system, neurotrophins are chan­neled to neurons making useful contributions to the organism.

What role does NGF play in the competition among neurons?

This process goes on throughout life: neurons that make a successful contribution to ongoing activity continue to receive NGFs and they continue to survive and grow. That is one reason for the slogan "Use it or lose it," applied to learning and also to the effects of old age.

Many people in their 90s continue to do work they love, and they often remain mentally sharp. "Whatever you continue to do, you can keep doing" is the rule. Skills that are actively used remain supported by healthy neurons, even in old people.

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Reference:

Edmonds, M. (2010) What is apoptosis? How Stuff Works. Retrieved from: http://science.howstuffworks.com/life/cellular-microscopic/apoptosis.htm


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