AP Psychology · Lesson 2

The nervous system, neurons and the brain

Know Know it

The central nervous system is the brain and spinal cord. The peripheral nervous system links it to the rest of the body. Its somatic division controls voluntary muscles, and its autonomic division runs automatic functions. The sympathetic branch arouses the body for action, and the parasympathetic branch calms it.

A neuron receives messages through its dendrites and sends them down its axon. Myelin insulates the axon and speeds the signal. When input pushes a neuron past its threshold, it fires an action potential, an electrical signal that is the same size every time. This is the all-or-none principle.

At the synapse, the axon terminal releases neurotransmitters that bind to receptors on the next cell. Leftover molecules are pulled back by reuptake. A drug that mimics or boosts a neurotransmitter is an agonist, and a drug that blocks it is an antagonist.

The brainstem controls basic survival functions such as breathing, and the cerebellum coordinates movement and balance. The thalamus relays most sensory input. The hypothalamus helps control hunger, thirst, body temperature and hormones. The amygdala is linked to fear, and the hippocampus helps form new long-term memories.

Lobes of the cerebral cortex
LobeMain jobExample area
FrontalPlanning, judgment and movementMotor cortex; Broca's area (speech production)
ParietalTouch and body positionSomatosensory cortex
TemporalHearing and language comprehensionAuditory cortex; Wernicke's area
OccipitalVisionVisual cortex

The corpus callosum connects the two hemispheres. Each hemisphere mainly controls the opposite side of the body. In most people, language depends more on the left hemisphere. The brain can rewire after injury, a property called neuroplasticity, which is strongest in childhood.

Trap Stronger is not bigger

A strong stimulus does not create a larger action potential. It makes neurons fire more often, and it can make more neurons fire. The size of each signal stays the same.

Worked example Easy

A student hears a loud crash at night, and her heart starts to pound. Which part of the nervous system is most directly producing this response?

  1. Sympathetic nervous systemAnswer
  2. Parasympathetic nervous system
  3. Somatic nervous system
  4. Central nervous system

How to solve it

  1. A pounding heart is automatic, so the response is autonomic.
  2. Arousal for action comes from the sympathetic branch.

Why each choice is right or wrong

  • A. Correct. The sympathetic branch arouses the body for action, which speeds up the heart.
  • B. Incorrect. The parasympathetic branch calms the body, so it would slow the heart, not speed it up.
  • C. Incorrect. The somatic system controls voluntary muscle movement, not heart rate.
  • D. Incorrect. The central nervous system is the brain and spinal cord, while a pounding heart is driven by an autonomic branch.

Explain Explain it

Task verbs: describe, explain

Questions about neurons usually come down to what happens at the synapse. Track the neurotransmitter: is more of it acting on the receptors, or less?

Questions about the brain usually ask you to match a symptom to a region. Name the region, then say which function in the scenario shows the damage.

Worked example Hard

A split-brain patient stares at a dot. A picture of a key flashes only in her left visual field. What will she most likely be able to do?

  1. Say the word key aloud
  2. Pick up a key with her left handAnswer
  3. Pick up a key with her right hand
  4. Say and point to the key

How to solve it

  1. The left visual field goes to the right hemisphere.
  2. The corpus callosum is cut, so the left hemisphere never sees the picture.
  3. The right hemisphere controls the left hand but usually cannot speak.

Why each choice is right or wrong

  • A. Incorrect. The image reaches only the right hemisphere, and speech depends mostly on the left, so she cannot say key aloud.
  • B. Correct. The left visual field sends input to the right hemisphere, which controls the left hand, so that hand can find the key.
  • C. Incorrect. The right hand is controlled by the left hemisphere, which never received the image.
  • D. Incorrect. She could point with her left hand, but saying the word needs the left hemisphere, which never got the image.

Explain Your turn

A drug blocks the reuptake of serotonin. Describe what happens to serotonin at the synapse, and explain how this affects the receiving neuron.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Says serotonin stays in the synapse longer or builds up there
  • Explains that receptors on the receiving neuron are activated more

Model answer

Normally the sending neuron pulls serotonin back in by reuptake. With reuptake blocked, serotonin stays in the synapse longer. It keeps binding to receptors on the receiving neuron, so the serotonin signal is stronger or lasts longer, and the drug acts as a serotonin agonist.

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