Class 15: Communication in the Nervous System

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Last updated: September 30 , 2025

Class 15: Communication in the Nervous System

Each of the questions below relate directly to the question of how the nervous system actually communicates when it is working.

Why does cocaine (and "speed" [= methamphetamine]) make some people feel very euphoric and some people very crazy?

Why do some runners report that they feel extraordinarily good after a very long run?

Why do soldiers in war who have been wounded sometimes not actually realize they've been hurt?

What is multiple sclerosis? What is Parkinson's disease?

During the 1980s, the Iraqi Army under the command of Saddam Hussein used nerve gas regularly in suppressing civil war and against the Kurds and the Iranian Army. Similarly the Syrian Army used nerve gas (a chemical weapon) against women and children during the civil war between 2013 and 2018. Why do nerve weapons kill many of the people who breathed the gas?

How are eating disorders possibly related to some types of suicidal acts?

1. Nervous Tissue: The Basic Hardware

The basic "nerve cell" is call a neuron

Dendrites: receive signals from other neurons. Often a neuron has many dendrites (= branches of a tree)

Soma (= cell body): produces some neurotransmitters. In the center is the nucleus with the cell's DNA.

Axon

For all neurons, there is only one axon which sends a signal to other neurons beginning at the axonal hillock and ending at the terminal buttons. Terminal buttons synapse (connect to) the dendrites of other neurons.

The axon is covered with insulation called myelin

Outside the brain and spinal cord, the myelin is produced by Schwann cells wrapping themselves around the axon. This covering is usually called its myelin sheath.

The Schwann cells are separated by very small gaps which expose the membrane of the axon itself. Those gaps are called Nodes of Ranvier.

Inside the brain and spinal cord, glial cells (see below) wrap their arms around the neuron with a myelin covering.

In the brain/spinal cord there are about 86 billion neurons (about 60-70 billion in the cerebellum alone)

Glia (="Glue")

Cells in the brain which provide structural support for the neurons in the brain. They tend to hold these neurons in their proper place.

They also nourish and decontaminate (remove waste from) neurons.

Recent evidence shows that they have a clear role in modulating how neurons transmit and receive signals

Other recent research suggests glial cell dysfunction is associated with major mental illnesses (e.g., schizophrenia & depression) and glial cell deterioration may contribute to Alzheimer's disease.

FALSE/OLD BELIEF: 10 glial cells for every neuron

TRUE/NEW RESEARCH: Roughly 1 glial cell for every neuron (ca. 84 billion glial cells)

Most brain tumors arise from glial cells, NOT from neural cells.

How does the nervous system transmit signals or information? It uses two methods:

Electrical signals along the neuron's axon, and

Chemical signals transmitted from a neuron's terminal button at the end of the axon across a tiny gap (= the synapse) to receptors on the dendrites of other neurons

2. The Neural Impulse: Using Energy to Send Information

The Neuron at Rest = A Tiny Battery

Resting potential = "a neuron's stable, negative charge when the cell is inactive"

Both the outside and the inside of the neuron is filled with liquid in which there are three major ions, that is, atoms with an electrical charge. They are

Sodium = Na+ (a positively charged ion)

Potassium = K+ (a positively charged ion)

Chlorine = Cl- (called "cloride" - a negatively charged ion)

Note, though, that the concentration of these ions is different between the inside and the outside of the neuron's membrane.

The membrane of the axon contains multiple "gates" or channels which are actually proteins that can allow charged atoms and molecules either to go outside the membrane or inside the membrane.

One protein is a sodium gate (Na+ gate) which, when opened, lets sodium ions (Na+) flow from outside the neuron's membrane to inside the neuron's membrane

One protein is a potassium gate (K+ gate) which, when opened, lets potassium ions (K+) flow from inside the neuron's membrane to outside the neuron's membrane

The fluid outside the axon's membrane is more highly charged (contains more positive ions) than the fluid on the inside of the membrane. This imbalance means that the inside charge of the membrane is -70 mV (millivolts, i.e., -70/1000 volt). This is the neuron's "resting potential" (that is, its charge when it isn't doing anything). It also means that the fluid outside the membrane is 70 mV more positive (i.e., +70 mV).

Although the membrane tends to leak charged ions, there is another protein which is called the Sodium-Potassium Ion Pump which continually maintains the resting charge or resting potential of -70mV.

The Action Potential => Movement of Ions In/Out of Neuron

Action potential = "a very brief shift in a neuron's electrical charge that travels along an axon"

The axonal hillock (the place where the axon leaves the body of the neuron) may become stimulated to send a signal. At that time (when that spot has moved from -70 mV to -40 mV), this change of voltage causes a sequence of openings and closings of the gates or channels in the membrane. -40mV is called the "threshold of excitation"

First, positive sodium ions (Na+) to rush into the membrane and the voltage goes up from -70 to about +40 to +50 mV (depolarization)

Secondly, when the potential reaches about +40 to +50mV, positive potassium ions (K+) rush out of the membrane and the voltage drops from about +50 mV to about -90 mV (repolarization). When the voltage inside drops below -70 mV (the resting potential), the inside has become hyperpolarized (= hyperpolarization).

Finally, for about 1-2 mSec (1/1000 of a second), the Sodium-Potassium Ion Pump (Na-K Pump) restores the balance (that is, moves the inside of the neuron from hyperpolarization to its resting potential). During this time the neuron cannot fire another signal. This is called the absolute refractory period.

Here is an excellent YouTube video (#013 A Review of the Action Potential from Interactive Biology) that reviews what I've described above. (Note: In this video, the commentator talks about the depolarization going to +40 mV. That specific number is at the low end of what is reported in research studies. I more often see the figure of +50 mV. He also uses the value of -55mV as the threshold of excitation, that is, when the action potential begins; that is a higher number than I usually see. These specific differences are not very important.)

All-or-None Law

An axon either fires or it doesn't fire. This is called the "All or None Law." There is no such thing as a partial action potential.

When the positive sodium ions (NA+) flow into the membrane, they trigger the gates nearby to open. Like the fall of one domino causing the next domino to fall, each gate opening triggers the next gate's opening.

3. The Synapse: Where Neurons Meet

Synaptic Cleft: the space or gap between the presynaptic and postsynaptic neuron
Synaptic Vesicles: a sac containing neurotransmitter molecules
Neurotransmitters: Chemical Molecules (9 standard and 40+ other)

Receptor Sites: neurotransmitter molecules attach to these & cause changes on the postsynaptic membrane

a postsynaptic potential (PSP) = "a voltage change at a receptor site on a postsynaptic cell membrane"

Postsynaptic potentials (PSPs): either an increase of positive voltage (= excitatory PSP) or increase of negative voltage (= inhibitory PSP)
PSPs make a neuron either more or less likely to fire

Reuptake: neurotransmitter molecules are absorbed back via reuptake channels in the presynaptic membrane.

Integrating Signals

Behavior arises from multiple simultaneously-integrated signals (patterns of activity across neural networks) NOT by the firing of a single neuron
Integration results from the overall and combined effects of both excitatory and inhibitory PSPs as their relative strengths change over time.
Neural networks are sculpted by elimination of old synapses (synaptic pruning)

4. Neurotransmitters and Behavior

A. Acetylcholine (Ach)

•    Motor neurons and muscles
•    Attention, arousal & memory
•    Can be mimicked by Nicotine (AGONIST = molecule which fills slot & causes neuron to fire)
•    Too little Ach in Alzheimer's disease

B. Monoamines

1. Dopamine (DA)

Motor control: loss of DA = Parkinson's Disease

Both an increase in pleasurable emotions and a sense of wanting/urgency (salience) in the "reward pathway" in the medial forebrain bundle

Too much activity at DA receptors => associated with SCZ (schizophrenia)

Cocaine & amphetamines elevate activity at DA sites. High levels of DA associated with addictive disorders

2. Norepinepherine (NE)

•    Modulation of mood and arousal
•    Cocaine & amphetamines also raise activity at NE sites
•    Lowered levels may be related to depression

3. Serotonin (5-HT)

•    Regulation of sleep & wakefulness, eating, aggression
•    Low levels associated with depression, obsessive-compulsive disorder (OCD), eating disorders, and aggressive tendencies
•    Prozac and other antidepressant drugs affect serotonin circuits

C. GABA & Glutamate (both amino acids)

1. GABA (gama-aminobutyric acid)

Produces only inhibitory PSPs

Serves to inhibit central nervous system

Regulates anxiety (too little GABA = increased anxiety)

2. Glutamate

Produces only excitatory PSPs

The major excitatory neurotransmitter in the brain.

Key component to memory via LTP (long-term potentiation) of sites that become permanently more excitable

Some sense that glutamate may also be involved in SCZ

D. Endorphins

Candace Pert & Solomon Snyder discovered endorphins

How does morphine (derived from opium) work?

Binds to receptor sites in brain and rest of body

Why? Body produces naturally-occurring opiate molecules

Endorphins = body own molecules - similar in effect & structure to opiates. These provide pleasure and relief from pain