15.3: Summary

The nervous system has two types of cells: neurons, which transmit nerve impulses, and glial cells, which support the function of neurons. A neuron has many branched extensions called dendrites, which receive impulses from other neurons.

The typical neuron also has a long extension called an axon, which culminates in another branched structure called the axon endings. The axon acts as the transmission line of the neuron.

A neuron’s cell membrane is polarized by positive ions on one side and negative ions on the other. Nerve impulses travel along this membrane. When a neuron is stimulated, the polarity is temporarily reversed (depolarized) at points along the membrane, creating an impulse that travels down the axon like a wave.

Neurons aren’t physically connected. The gap between them is called a synapse. Impulses are carried across the synapse by chemical substances called neurotransmitters. When an impulse reaches an axon ending, it releases a neurotransmitter that travels across the synapse to a receptor on an adjacent neuron. A whole area of research is devoted to studying the interactions at these synapses.

Synapses also occur at the juncture of neurons and muscle. To stimulate a muscle contraction, the neuron releases the neurotransmitter acetylcholine, which attaches to receptors on the muscle to cause contraction. Acetylcholine is quickly cleared from the receptor through enzyme action, allowing the muscle to relax.

Stimulants increase the sensitivity of neurons. They increase alertness, improve mood, and often reduce appetite. Many are addictive, and large amounts can lead to irritability. Caffeine is a commonplace example of a stimulant.

At the other extreme are opioids (e.g., fentanyl) that are depressants and powerful painkillers. They can produce euphoria, and are highly addictive. We are in the midst of an opioid epidemic in the U.S.

Inhibitors make neurons less sensitive to stimulation and inhibit transmission of nerve impulses. Their general effects are relaxation and less anxiety, e.g., barbiturates and alcohol.

The developing nervous system is particularly sensitive to alcohol. Pregnant women are advised to completely abstain from alcohol. Alcohol passes freely across the placenta—the blood-alcohol level is the same in mother and fetus.

A severe outcome is Fetal Alcohol Syndrome, which includes both physical and mental disabilities caused by interference with fetal brain and physical development due to alcohol consumption by the mother during pregnancy.

Fetal Alcohol Spectrum Disorder describes the wide range of conditions that can occur in someone whose mother consumed alcohol during pregnancy. Many of these conditions are relatively mild and not immediately obvious. An estimated 10% of pregnant women drink during pregnancy.

Neurons are constantly active, requiring constant supplies of oxygen and glucose. They also need vitamins and minerals to make ATP and neurotransmitters. Although it’s well established that nutritional deficiencies can cause brain or nerve dysfunction, there’s very little indication that large doses of particular nutrients can enhance brain function. The same could be said for altering undesirable behavior through diet.

The brain continues to grow, and neurons continue to divide through infancy. Malnourishment during this period may hamper brain growth and permanently reduce the number of neurons. If malnourishment occurs only after this period of cell division, the neurons may not grow to full size, but the ill effects appear to be reversible if proper nourishment is restored.

Lead is a toxin with widespread effects. It’s chemically similar to the essential minerals calcium, iron, and zinc. Much of its damage comes from its ability to displace these essential minerals, thereby interfering with the function of the many enzymes for which these minerals act as cofactors.

Children absorb lead much more easily than adults. Lead has been shown to delay the mental and physical development of fetuses and young children. High lead levels have been linked to poorer psychomotor skills and lower scores on intelligence tests.

Serotonin is a neurotransmitter made from the essential amino acid tryptophan. It has a calming effect. When lab rats are made deficient in serotonin or tryptophan, they become irritable and develop insomnia. Eating a protein-rich meal doesn’t increase serotonin, because protein has relatively more of the amino acids that compete with tryptophan for entry into the brain.

Carbohydrates are more effective in increasing serotonin, since the insulin made in response to glucose causes cells throughout the body to take in more of the competing amino acids, making it easier for tryptophan to enter the brain. This may explain the calming effect of carbohydrate-rich foods.

Tryptophan supplements were widely promoted, and many people took them. Because tryptophan is classified as a dietary supplement rather than a drug, it hasn’t been rigorously tested for safety, purity, or effectiveness. This lack of testing is good reason to be cautious about supplements. Some are often self-prescribed in large doses, and side effects haven’t been determined in controlled studies.