Neurons in AP Psychology

A hand on a hot pan: following one signal through three neurons

Grab a hot pan handle and your hand jerks back before you consciously feel pain. That's a reflex arc, and it shows how neurons are built and how they cooperate. Heat receptors in your skin activate a sensory neuron, which carries the message from the body toward the spinal cord. In the spinal cord, an interneuron relays it to a motor neuron, which carries a command out to the arm muscles.

The interneuron sits entirely in the spinal cord, which is part of the central nervous system. The sensory neuron carries input from the periphery into the spinal cord. The motor neuron's cell body is in the spinal cord, and its axon extends out to the muscle.

Each of those neurons has the same basic parts. Dendrites receive incoming signals, and the cell body (soma) combines them. If the combined input is strong enough, the neuron sends an action potential down its axon. Many axons are wrapped in a myelin sheath, a fatty insulation made by glial cells that speeds the signal. At the axon terminals, neurotransmitters are released into the synapse, the tiny gap before the next cell.

This is the starting point of Topic 1.3, The Neuron and Neural Firing, in Unit 1: Biological Bases of Behavior. That unit asks how biology produces behavior and thought, and neurons are the smallest unit of that explanation.

One firing, start to finish: resting potential to reuptake

Neural transmission usually follows an orderly sequence, and the course expects you to know it in order.

• At resting potential, the inside of the neuron is negatively charged compared with the outside (about -70 millivolts).
• Excitatory input pushes the charge up toward threshold, the level needed to fire.
• At threshold, positive ions rush in, the neuron depolarizes, and an action potential travels down the axon.
• The all-or-none principle means the neuron either fires completely or doesn't fire at all.
• During the refractory period right after firing, the neuron can't fire again until it resets.
• Neurotransmitters cross the synapse and bind to receptors on the next cell. Leftover molecules are pulled back into the sending neuron through reuptake.

The common wrong prediction is that a stronger stimulus makes a bigger action potential. It doesn't. A hotter pan makes neurons fire more often, and more neurons fire, but each action potential is the same size.

The message a neurotransmitter sends is generally excitatory (firing becomes more likely) or inhibitory (firing becomes less likely). Glutamate is the main excitatory example and GABA is the main inhibitory one. The course limits neurotransmitters to acetylcholine, GABA, glutamate, dopamine, norepinephrine, serotonin, substance P, and endorphins.

When the sequence breaks: MS, myasthenia gravis, and drug effects

The course pairs neurons with two disorders. In multiple sclerosis, the myelin around axons in the brain and spinal cord is damaged, so signals slow down or misfire. That can show up as weakness, numbness, or poor coordination. In myasthenia gravis, the immune system interferes with acetylcholine receptors where motor neurons meet muscles. The neurons still fire, but the muscle doesn't receive the message well, so muscles weaken and tire quickly.

Psychoactive drugs can alter neural communication, including signaling at synapses in the brain. An agonist increases a neurotransmitter's effect, and an antagonist blocks or reduces it. A reuptake inhibitor keeps the neurotransmitter in the synapse longer. SSRIs do this for serotonin, and cocaine does it for dopamine.

Neuron or glial cell: sorting the cell in the scenario

Both are common cells in the brain. A question may describe a cell's job and ask you to identify it. Neurons send messages through action potentials and neurotransmitters. Glial cells provide structure, insulation (myelin), communication support, and waste removal.

If the stem describes a cell firing or releasing a chemical messenger, it's a neuron. If it describes a cell insulating axons, holding tissue together, or clearing debris, it's glial. A related trap is mixing up neuron types. Sensory neurons carry information in, motor neurons carry commands out, and interneurons connect them inside the spinal cord and brain.

Applying neurons to multiple-choice scenarios and research summaries

A toxin that blocks acetylcholine release from axon terminals points to failed neurotransmitter release, even if the neuron's electrical activity looks normal. Scenarios like that one ask which structure or step explains an observation. Interneuron firing that jumps within milliseconds of a painful stimulus points to the fast relay step of the reflex arc.

On the Article Analysis question, a neuron study might compare dendritic spine density in animals trained on a motor skill with a control group. If researchers assign the training conditions, training is the independent variable and spine density is the dependent variable. A difference in spine density could be consistent with neural plasticity, but by itself it wouldn't demonstrate long-term potentiation. Topic 2.3 describes long-term potentiation as synapses between neurons strengthening with frequent activation, which is a biological basis of memory.

On the Evidence-Based question, a claim about a neurotransmitter or drug needs reasoning at the neuron level. A strong answer goes beyond naming the term. It explains the mechanism, such as a reuptake inhibitor leaving more serotonin in the synapse, and connects it to the specific result reported in the source.