1.3 The Neuron and Neural Firing

A neuron has dendrites, branches that receive messages; a cell body (soma), which keeps the cell alive and combines incoming signals; and an axon, a long fiber that carries the signal away. Many axons are wrapped in a fatty myelin sheath, which insulates them and speeds up the signal. At the end, axon terminals release chemicals into the synapse, the tiny gap between one neuron and the next.

Glial cells (glia) are the support crew. They give neurons structure, form myelin, bring nutrients, remove waste and help regulate communication between neurons.

Three kinds of neurons work together. Sensory neurons carry information from your body to the CNS. Motor neurons carry commands from the CNS to muscles and glands. Interneurons, found in the brain and spinal cord, connect the two. In a reflex arc, such as pulling your hand off a hot pan, a sensory neuron sends the signal to the spinal cord, an interneuron passes it straight to a motor neuron, and your hand moves before your brain even registers the pain.

Neural firing follows the same orderly steps every time. Know each of these terms by name.

• Resting potential: when a neuron isn't firing, the inside of the axon is negatively charged compared with the outside, ready to fire.
• Threshold: incoming excitatory signals push the charge toward a trigger level. If the combined input reaches the threshold, the neuron fires.
• Depolarization: positively charged ions rush into the axon, so the inside briefly becomes positive. This change travels down the axon as the action potential.
• Refractory period: right after firing, the neuron needs a brief recovery time before it can fire again.
• All-or-none principle: a neuron either fires completely or not at all. A stronger stimulus doesn't make a bigger action potential; it makes neurons fire more often, and makes more neurons fire.
• Reuptake: after neurotransmitters do their job at the synapse, the sending neuron reabsorbs the leftovers.

When neural transmission is disrupted, behavior changes. In multiple sclerosis, the immune system damages myelin in the brain and spinal cord, so signals slow or misfire, causing weakness, numbness and vision problems. In myasthenia gravis, the immune system interferes with the receptors muscles use to receive acetylcholine, so muscles weaken and tire quickly. You won't be tested on the sodium-potassium pump.

Excitatory neurotransmitters make the next neuron more likely to fire; inhibitory ones make it less likely. The exam uses only the eight below, and the hormones listed after them. Hormones are chemical messengers released into the bloodstream rather than into a synapse, so they act more slowly but last longer and reach the whole body.

Psychoactive drugs change perception, mood or behavior by acting on neurotransmission. An agonist mimics or strengthens a neurotransmitter's effect: heroin and morphine fit endorphin receptors. An antagonist blocks a neurotransmitter's effect: naloxone blocks opioid receptors and can reverse an opioid overdose. A reuptake inhibitor keeps a neurotransmitter in the synapse longer: many antidepressants block serotonin reuptake, and cocaine blocks dopamine reuptake.

The course groups drugs into four types. Stimulants, such as caffeine, nicotine and cocaine, increase neural activity, alertness and heart rate. Depressants, such as alcohol, decrease neural activity, slowing reactions and impairing judgment and memory. Hallucinogens, such as marijuana and LSD, distort perception or thinking. Opioids, such as heroin, relieve pain and can dangerously slow breathing.

With repeated use, the brain adapts. Tolerance means you need more of the drug to get the same effect. Addiction is compulsive craving and use despite harm. Withdrawal is the uncomfortable, sometimes dangerous set of symptoms when someone with an addiction stops using, and the symptoms are often the opposite of the drug's effects.