AP Psychology 1.3: The Neuron and Neural Firing

Two common kinds of cells make up your nervous system. Neurons are the cells that carry information. Glial cells support them: they give the system its structure, insulate neurons, help them communicate and carry away waste.

Branching dendrites take in messages from other neurons. The cell body keeps the neuron alive and combines the messages coming in. The axon, a long fiber, carries the neuron's own message away from the cell body. Many axons are wrapped in a myelin sheath, a fatty layer that insulates the axon and speeds the message along. At the far end, the terminal buttons release chemical messengers into the synapse, the tiny gap between one neuron and the next cell.

Neurons also differ in the job they do. Sensory neurons carry messages from the body in toward the brain and spinal cord. Interneurons, inside the brain and spinal cord, pass messages from one neuron to another. Motor neurons carry commands out to the muscles.

The hot pan used all three, in a reflex arc. A sensory neuron carried the message from your fingers to your spinal cord, an interneuron there passed it straight to a motor neuron, and your arm muscles pulled back. The central and peripheral nervous systems worked together, and your brain was not needed for the first move.

A neuron at rest still holds a slight negative charge inside, compared with the outside, a state called the resting potential. Messages arriving at the dendrites nudge that charge up or down. If they push it to a level called the threshold, the neuron fires: gates in the axon open, positively charged particles flow in, and the inside briefly turns positive. That change is depolarization, and it races down the axon as an action potential, the neural impulse.

Firing follows the all-or-none principle. Once the threshold is reached, the neuron fires at full strength, and below it, the neuron does not fire at all. A stronger stimulus does not make a bigger action potential. It makes the neuron fire more often.

Right after it fires, the neuron goes through a refractory period, a brief pause in which it cannot fire again while it returns to its resting potential.

When the action potential reaches the terminal buttons, they release chemical messengers into the synapse, and the next cell picks up the message. The sending neuron then draws the leftover messengers back inside, which is reuptake.

Two disorders disrupt this process. In multiple sclerosis, the immune system damages the myelin sheath in the brain and spinal cord, so messages slow or fail. In myasthenia gravis, the immune system attacks the places on muscles where acetylcholine, one of those chemical messengers, attaches, so muscles weaken and tire quickly.

The chemicals released into the synapse are neurotransmitters. Each one sends either an excitatory message, making the next neuron more likely to fire, or an inhibitory one, making it less likely. What a neurotransmitter does can depend on where in the nervous system it is released. You need eight:

• Glutamate: the brain's main excitatory neurotransmitter, important in learning and memory.
• GABA: the brain's main inhibitory neurotransmitter, which calms neural activity.
• Acetylcholine: carries the message from motor neurons to muscles, and helps with learning and memory.
• Dopamine: movement, and the feeling of reward that makes you want to repeat something.
• Serotonin: mood, hunger and sleep.
• Norepinephrine: alertness and arousal.
• Endorphins: the body's own pain relievers, which also bring pleasure.
• Substance P: carries pain messages.

Hormones do similar work outside the nervous system. Cells release them into the bloodstream, so they travel farther and more slowly, and their effects last longer. You need five:

• Adrenaline: readies the body for action, with a pounding heart in fear or excitement.
• Ghrelin: released when the stomach is empty, and signals hunger.
• Leptin: released by fat cells, and signals that the body has enough energy stored, which lowers hunger.
• Melatonin: rises in the evening as it gets dark, and brings on sleepiness.
• Oxytocin: bonding and trust, such as between a parent and a newborn.

A psychoactive drug changes behavior and mental processes by changing how neurotransmitters work. The course names three kinds of action:

• An agonist encourages firing. Many agonists copy a neurotransmitter, attaching where it attaches and sending the same message.
• An antagonist discourages firing. Many antagonists sit in those same places without sending the message, blocking the neurotransmitter out.
• A reuptake inhibitor blocks reuptake, so the neurotransmitter stays in the synapse longer and keeps acting.

These kinds overlap: a reuptake inhibitor strengthens a message, so some books count it as an agonist. Where both names are offered, reuptake inhibitor is the more exact one.

Drugs have psychological effects, on mood, alertness, perception and thinking, and physiological effects on the body. Caffeine, for example, makes you more alert (psychological) and raises your heart rate (physiological). Four groups are named for what they typically do:

• Stimulants, such as caffeine and cocaine, typically increase neural activity.
• Depressants, such as alcohol, typically decrease neural activity.
• Hallucinogens, such as marijuana, typically distort perception, thinking or both.
• Opioids, such as heroin, typically act as pain relievers.

With repeated use comes tolerance: the same dose has less effect, so a person needs more for the effect one dose used to give. Addiction is a compulsive need for a drug that continues despite the harm it causes. Once a person is addicted, stopping can bring withdrawal, the distressing symptoms that follow when the drug is no longer taken.