Neurons
Happiness means quiet nerves. ~ American comedian W.C. Fields
Neurons are the peons of the nervous system, which comprises the brain, peripheral nerves, and a middleman: a vertebrate’s spinal cord, or an invertebrate’s nerve cord.
Neurons come in different shapes, sizes, and electrochemical properties, with their morphology (structure) related to their specific function. Though there are numerous nerve cell types, all share the general function of signal transmission and reception.
A typical nerve cell has 3 parts: dendrite, cell body (soma), and axon.
The soma is a neuron’s central processor, with a nucleus packing the genes for the bulk of the cell’s protein production. A soma may vary from 4–100 mm in diameter depending upon neuron type.
Around the soma are dendrites, which are a multiply branched bush, acting as sensory receptors for signals from other neurons. Dendrites convey electrical signals into the soma. The dendrites have a fractal aspect to maximize surface area for reception.
The lengthy nerve fiber is the axon, which can be over 1.5 meters when running down an adult’s legs, though most are much shorter. Axons in the necks of giraffes run the entire length of the neck: several meters.
Most axons are insulated in a protective myelin sheath, made of fats and proteins. The myelin sheath is an outgrowth of a glial cell.
1-micrometer gaps in the sheath – nodes of Ranvier – allow electrical signals to jump from one node to another.
Glia attached to myelin – Schwann cells – facilitate and regulate nerve function. Each neuron is controlled by glial cells.
Within a neuron, axons carry electrical impulses anywhere from 3–320 kilometers per hour. Some inter-neural communication is electrical, but much cell-to-cell communication – from the axon of a sending cell to the dendrite of a receiving neuron – takes place chemically.
Generally, an axon terminates at synapses: specialized functional structures for transmitting signals between nerve cells, or from a neuron to another cell type. Electrical signals are translated to specific chemical signals.
Ion channels essentially make the nervous system tick. ~ American biochemist and neurologist Nina Schor
At the axon terminal, tiny bulbous vesicles release neurotransmitters that traverse a synaptic gap to deliver chemical agents to dendrites of targeted receiver neurons. Synaptic gaps are so small that they can only be seen clearly using an electron microscope.
There are many types of neurotransmitters, including dopamine, serotonin, and norepinephrine. These chemical agents can be excitatory – activating an electrochemical response in dendrite receptors, or inhibitory – tending to block response by dendrite receptors.
A typical neuron may have several thousand synapses. An adult human brain has 100–500 trillion synapses.
Nerve pathways are constantly being rewired: their synapses changing: strengthening by increasing the numbers of connections or weakening by losing synapses. Connections may also become more or less speedy. This incessant rewiring is controlled by glia as part of the process and product of life experience.
The continual rejiggering of nerve pathways demonstrates that glia are the physical repository of memory and cognition, not neurons. If nerve cells were the physical substrate of the intelligence system, such ongoing extensive rewiring would be disruptive. Instead, as glia are the physiological masters of the nervous system, the impact of neuron reassignment or death is negligible.
