Topic 1.3 Notes – The Neuron and Neural Firing

What Neurons Do

A neuron is a specialized cell that receives, processes, and transmits information. Glial cells are the support team. They insulate neurons, help them communicate, give structure, and remove waste.

Behavior and mental processes come from neural networks, not one neuron working alone. That matters on the AP exam because questions often connect a tiny cell-level change to a larger behavior.

Here’s the basic structure of a neuron and the direction a signal usually travels.

Neuron structure

The usual flow looks like this:

1. Dendrites receive incoming messages.
2. The soma or cell body combines excitatory and inhibitory input.
3. The axon carries the electrical impulse away from the soma.
4. Myelin insulates the axon and speeds transmission.
5. Axon terminals release neurotransmitters.
6. The synapse or synaptic gap is where the message crosses to the next neuron, muscle, or gland.

Two disorders help show why structure matters:

• Multiple sclerosis damages myelin, so signals travel poorly.
• Myasthenia gravis disrupts acetylcholine receptors at the neuromuscular junction, causing muscle weakness.

Types of Neurons and the Reflex Arc

Three neuron types work together in one pathway:

Sensory neurons

They carry incoming information from receptors to the CNS or central nervous system.

Interneurons

They process and connect information within the CNS.

Motor neurons

They carry outgoing commands to muscles and glands.

A reflex arc shows this teamwork in action. In a withdrawal reflex, the signal travels from a skin receptor through a sensory neuron to the spinal cord, where an interneuron passes it to a motor neuron.

Reflex arc

If you touch a hot stove, this happens:

1. Heat activates skin receptors.
2. A sensory neuron carries the signal to the spinal cord.
3. An interneuron relays it in the spinal cord.
4. A motor neuron sends a command to the muscles.
5. Your hand pulls away.

The key idea is speed. A reflex is automatic and rapid. The spinal cord can trigger the response before you consciously feel the pain, though the brain still gets the pain message.

How Neural Firing and Synaptic Transmission Work

Inside one neuron, communication is electrical. Between neurons, it is chemical.

The firing sequence goes in order:

1. Resting potential means the neuron is polarized and ready.
2. Excitatory input makes firing more likely. Inhibitory input makes it less likely.
3. Threshold is the minimum stimulation needed to fire.
4. Reaching threshold causes depolarization and an action potential down the axon.
5. All-or-none means the neuron fires completely or not at all.
6. The refractory period is the brief recovery time before it can fire again.

A common trap is this. Stronger stimulation does not make a bigger action potential. It causes more frequent firing or activates more neurons. Also, AP Psych does not test sodium-potassium pump details.

Across the synapse:

1. Action potential reaches axon terminals.
2. Neurotransmitters are released.
3. They bind to matching receptors on the next cell.
4. The effect can be excitatory or inhibitory.
5. Reuptake pulls neurotransmitters back into the sending neuron.

Neurotransmitters and Hormones

Know these neurotransmitters cold:

• Dopamine: reward, motivation, attention, learning, movement
• Serotonin: mood, sleep, appetite
• Norepinephrine: alertness, arousal
• Glutamate: main excitatory neurotransmitter; learning and memory
• GABA: main inhibitory neurotransmitter; calms neural activity
• Acetylcholine: muscle contraction, learning, memory
• Endorphins: pain relief, pleasure/euphoria
• Substance P: pain transmission

High-value contrasts:

• Glutamate vs GABA = excitation vs inhibition
• Endorphins vs substance P = reducing pain vs sending pain signals

Hormones are chemical messengers that travel through the bloodstream, so they usually act slower but longer than neurotransmitters.

• Adrenaline: fight-or-flight
• Leptin: satiety
• Ghrelin: hunger
• Melatonin: sleep-wake cycle
• Oxytocin: bonding, trust, childbirth, milk release

How Psychoactive Drugs Change Communication

Psychoactive drugs change brain activity, behavior, and mental processes by altering neural communication.

Main mechanisms:

• Agonists mimic or increase neurotransmitter action
• Antagonists block neurotransmitter action
• Reuptake inhibitors keep neurotransmitters in the synapse longer

Named examples you should recognize:

• Heroin is an opioid agonist
• Naloxone is an opioid antagonist
• SSRIs block serotonin reuptake
• Cocaine is a stimulant and also blocks reuptake, including dopamine

Major drug classes:

• Stimulants increase neural activity and arousal, like caffeine and cocaine
• Depressants decrease CNS activity, like alcohol
• Hallucinogens distort perception or cognition, like marijuana in AP classification
• Opioids relieve pain and may cause euphoria, like heroin

Repeated use can lead to:

• Tolerance: less effect from the same dose
• Addiction: compulsive use despite harm
• Withdrawal: distress when use stops after the body adapts