Last updated: Feb 12, 2019
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?
- When the Syrian Army used nerve gas (a chemical weapon) against women and children during the current civil war, why did it 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
- In the brain/spinal cord there are about 86 billion neurons (about 60-70 billion in the cerebellum alone)
- 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
- Particular attention is now being paid to how the neuron & glial cell interact with each other.
- Glial cells appear to regulate aspects of neurotransmitter production
- 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.
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"
- 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. The difference is not very important.)
- 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
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
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
• 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)
- Produces only excitatory PSPs
- 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
- Candace Pert & Solomon Synder 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