Neurons

NEURONS = basic units of the human "biocomputer." The Nervous System is made up of long chains of neurons. No two neurons are exactly alike in size or shape. Nerves are large bundles of neuron fibers. Nerve cells, arranged in long chains and dense networks. Alone, not very smart. Joined in vast networks, they produce intelligence and consciousness.

1. FOUR BASIC PARTS:

Dendrites = They look like roots and receive messages from other neurons.

Soma = Cell body, it also accepts incoming information, which it collects and combines. Sometimes messages cause the soma to send a nerve impulse.

Axon = long, thin fiber down which impulses are sent. Carries messages from sensory organs to the brain. Some only ca. 0.1 millimeter long. Others stretch up to a meter through the adult nervous system. Impulses travel at about 8 feet per second in small, thin axons, and up to 225 miles per hour in large axons. The larger the diameter, the faster it conducts. And, a fatty layer that covers some axons (myelin) increases the speed of nerve impulses. And, they travel faster by jumping from gap to gap if fatty layer is broken by small gaps. Like miniature cables, axons carry messages from the sensory organs to brain, from the brain to muscles or glands, or simply from one neuron to the next.

Axon terminals = the branching at the end of axons. These branches link with dendrites and somas (cell bodies) of other neurons.

2. THE NERVE IMPULSE (primarily an electrical event): Each neuron is like a tiny biological battery ready to be discharged. It takes about one-thousandth of a second for a neuron to fire an impulse and return to its resting level. Thus, a maximum of 1,000 nerve impulses per second is possible. However, firing rates of 1 per second to 300-400 per second are more typical.

Ions = electrically charged molecules found in differing numbers inside and outside each nerve cell. This causes a tiny difference in electrical charge across the cell membrane.

Cell membrane = skin.

Resting potential = the state of nerve cells of brain at any time-- Must receive electrical message to activate it. It is this resting potential electrical charge across the cell membrane which becomes altered by messages arriving from other neurons until it reaches a threshold or trigger point.

Threshold = trigger point, for firing. The threshold for human neurons averages ca. minus 50 millivolts (a millivolt is one-thousandth of a volt.). At this point a nerve impulse or action potential sweeps down the axon.

Action potential = nerve impulse, sweeps down the axon.

Ion channels = tiny tunnels pierce the axon membrane, causing the action potential. These channels are normally closed by molecular "gates." The gates pop open during an action potential, allowing sodium ions to rush into the axon--happening near the soma, first, and then as action potential moves along, the gates open in sequence down the length of the axon.

Negative after-potential = the cell briefly drops below its resting level after each nerve impulse--caused by outward flow of potassium ions that occurs while the membrane gates are open. The neuron must recharge after each nerve impulse. It does this by shifting ions back across the cell membrane until the resting potential is restored.

3. NEUROTRANSMISSION: Communication between neruons, across the synapses, is molecular--in contrast to electrical event of nerve impulse.

Neurotransmitters = Potent chemicals which are released when a nerve impulse reaches the tips of the axon terminals. May excite (move closer to firing) or inhibit (make impulse less likely) next neuron. At any instant, a neuron receives messages from hundreds of thousands of other neurons. If several "exciting" messages arrive close in time, and they are not cancelled by "inhibiting" messages, the neuron reaches its trigger point. This means that chemical messages are combined before a neuron "decides" to fire its all-or- nothing action potential. There are now some 30 known or suspected neurotransmitters in the brain. Among the more important "chemical messengers" are:

acetylcholine
dopamine
adrenaline
histamine
noradrenaline
various amino acids
serotonin (Krieger, 1983)




NOTE: Many drugs operate by imitating, duplicating, or blocking the actions of neurotransmitters.
Acetylcholine normally activates muscles. Curare attaches to receptor sites on muscles- prevents
acetylcholine from reaching the sites which means a person or animal will be paralyzed. Mescaline, similar
to noradrenaline, imitates a brain transmitter. LSD and psilocybin ("magic mushrooms") appear to act
directly on receptor sites for the transmitter serotonin (Jacobs, 1987).


Synapse = tiny gap between neurons. Neurotransmitters cross these gaps, attach to sites:

Receptor sites = on the soma and dendrites of the next neuron. Transmitter molecules attach to these special sites. They also activate receptor sites on muscles and glands.

4. NEURO REGULATORS: A new class of brain transmitters. Neuropeptides or brain peptides. They do not carry messages directly. Instead, these Chemicals regulate the activity of other neurons. Doing so, they affect memory, pain, emotion, pleasure, mood, hunger, sexual behavior, and other basic processes (Krieger, 1983).

Some neurons have specific receptor sites for opiate drugs such as morphine. Thus, started the search for natural opiate-like peptides in the brain. Found that brain produces opiate-like neural regulators called enkephalins to relieve pain and stress (Iverson, 1979). Related chemicals called endorphins are released by the pituitary gland (Thompson, 1985). Enkephalins and endorphins explain "runner's high," the placebo effect, and acupuncture--and they are released to reduce pain so that it is not too disabling.

An understanding of how peptides regulate brain activities may help explain depression, schizophrenia, drug addiction, and other puzzling problems.