The Neuron and Neural Firing — AP Psychology Study Notes

Neurons

Neurons are the fundamental units of the nervous system that transmit information through electrical and chemical signals.

Neurons carry messages throughout the nervous system, enabling communication between the brain, spinal cord, and every part of the body.

Glial Cells

Glial cells support, insulate, nourish and protect neural tissue, regulate its chemical environment and participate in signaling. They generally do not conduct axonal action potentials in the same way neurons do; calling them biologically inactive would be misleading.

Key Functions of Glial Cells:

• Provide structural support and maintain homeostasis
• Form the myelin sheath that insulates axons and speeds neural transmission
• Clean up waste products and excess neurotransmitters
• Guide neural development and assist with nutrient transport

Remember: Without glial cells, neurons couldn't function properly. They're like the support staff behind every neural "main character".

Memory Aid: Neuron Signal Flow

Dendrites → Cell Body → Axon → Terminal Buttons → Synapse → Next Neuron

Think: RECEIVE (dendrites) → PROCESS (soma) → TRANSMIT (axon) → RELEASE (terminals)

1. Sensory Neurons (Afferent Neurons)

Carry incoming information from sensory receptors (eyes, ears, skin, muscles) to the central nervous system.

Example: Detect heat from a hot stove and send signals to the spinal cord and brain.

2. Motor Neurons (Efferent Neurons)

Carry outgoing commands from the central nervous system to muscles and glands, producing movement or secretions.

Example: Send signals from the spinal cord to hand muscles to pull away from danger.

3. Interneurons

Connect neurons within the brain and spinal cord, processing information between sensory and motor neurons.

Function: Enable complex processing, reflexes, and decision-making by relaying and integrating signals.

A spinal withdrawal reflex is a rapid, automatic response coordinated through spinal circuitry. It can begin without waiting for conscious awareness; sensory information also reaches the brain, and descending signals can modulate the response. Other reflexes can involve brainstem circuits.

Example: Touching a hot stove → pulling your hand away instantly

Reflex Arc Process:

1. Sensory Neuron: Detects stimulus (heat on skin) and sends signal to spinal cord
2. Interneuron: Processes signal in the spinal cord and relays it
3. Motor Neuron: Sends command to muscle (pull hand away)
4. Delayed Brain Awareness: You feel the pain only after reacting

Key Point: The reflex arc illustrates how the CNS and PNS work together to produce rapid, protective responses faster than conscious thought.

1. Resting Potential

At the resting potential, a neuron maintains a membrane voltage commonly illustrated as $-70\text{mV}$, relative to the outside. It remains metabolically active and can receive changing input; the value varies by cell.

The neuron is polarized—ready to fire like a charged battery, with more negative ions inside the cell membrane and more positive ions outside.

State: The neuron is waiting for sufficient stimulation to trigger an action potential.

2. Threshold

Threshold is the membrane-voltage level at which an action potential is initiated, commonly illustrated as $-50\text{mV}$. It is not a universal fixed voltage or the same thing as stimulus intensity.

If incoming signals from other neurons push the neuron's internal charge to threshold or above, the neuron will fire.

If the stimulation is below threshold, the neuron does NOT fire and returns to resting potential.

3. Action Potential

The electrical impulse that travels rapidly down the axon when the neuron fires, transmitting information from the cell body to the axon terminals.

The action potential is also called "neural firing" or a "spike".

This is the fundamental unit of communication between neurons and represents the neuron's "message".

4. Depolarization

When threshold is reached, ion channels open and positive ions (sodium ions) rush into the axon, making the inside of the neuron less negative or even positive.

During the depolarizing phase of an action potential, membrane voltage can rise from an illustrative resting value of $-70\text{mV}$ to a positive peak such as $+40\text{mV}$. Exact voltages vary; an action potential also includes repolarization and often an after-hyperpolarization.

Depolarization propagates down the axon like a wave, carrying the neural signal toward the terminal buttons.

5. All-or-None Principle

A neuron either fires completely or not at all—there is no partial firing.

Once the threshold is reached, the action potential has a fixed size and strength regardless of how strong the original stimulus was.

For a given neuron, stronger stimulation does not simply make each action potential taller. Intensity can be represented by firing rate and by recruitment of additional neurons, within biological limits.

6. Refractory Period

The brief recovery time immediately after an action potential during which the neuron cannot fire again (or requires much stronger stimulation).

Types of Refractory Periods:

• Absolute Refractory Period: The neuron cannot fire under any circumstances
• Relative Refractory Period: The neuron can fire, but only with a stronger-than-normal stimulus

Purpose: Ensures the action potential travels in one direction and prevents signals from piling up. The neuron resets to resting potential during this time.

Additional Process: Repolarization

After depolarization, repolarization involves sodium-channel inactivation and increased outward potassium current. A brief after-hyperpolarization may occur. Ion gradients are maintained over time; repolarization is not simply sodium flowing back out through the same channels.

Worked voltage difference

This optional arithmetic example compares two membrane potentials; it is not a full action-potential equation.

The change is positive because the inside becomes less negative. Millivolts measure voltage; this calculation does not calculate firing frequency or signal speed.

Step-by-Step Neural Firing

How Synaptic Transmission Works

1. Action potential arrives at the axon terminal (terminal button)
2. Neurotransmitters are released from vesicles into the synapse (synaptic cleft)
3. Neurotransmitters cross the gap and bind to receptor sites on the receiving neuron's dendrites
4. Binding causes changes in the receiving neuron (excitatory or inhibitory effects)
5. Signal termination: Depending on the transmitter, reuptake into neurons or glia, enzymatic breakdown and diffusion help clear transmitter and limit signaling.

Key Point: The synapse is where neurons communicate chemically. Neurotransmitters are the chemical messengers that carry signals across this gap.

Reuptake

Reuptake is the process by which neurotransmitters are reabsorbed back into the presynaptic neuron (the sending neuron) after they have transmitted their signal.

This cleanup process stops the signal and prepares the neuron for the next transmission, preventing continuous stimulation.

AP Exam Alert

Only 8 neurotransmitters are tested on the AP Psychology exam. Focus on these and their primary functions.

AP Exam Note

Scope: Focus on the listed hormones and their behavioral roles. Detailed endocrine-gland information is outside this topic’s current AP scope except the pituitary gland, which is included with brain structures.

How Drugs Affect Neurotransmitters

Drugs affect neurotransmitters in three main ways:

Tolerance, Addiction, and Withdrawal

Multiple Sclerosis (MS)

The immune system damages myelin (the insulating sheath on axons), disrupting action potential transmission.

Result: Slower or failed neural signals causing numbness, weakness, vision problems, and cognitive difficulties.

Myasthenia Gravis

The immune system blocks acetylcholine receptors, preventing neurotransmitter binding.

Result: Weakened muscle control, especially in eyes, face, and limbs.

Multiple Choice Tips

• Know neuron parts and their functions (dendrites, soma, axon, myelin, terminals)
• Understand the firing process: resting potential → threshold → action potential → refractory period
• Master the all-or-none principle: neurons fire completely or not at all
• Memorize all 8 neurotransmitters and their primary functions
• Distinguish between excitatory and inhibitory neurotransmitters
• Know drug mechanisms: agonist, antagonist, reuptake inhibitor

Free Response Question (FRQ) Tips

• Use precise terminology: action potential, threshold, depolarization, reuptake, synapse
• Explain processes step-by-step: trace neural firing from dendrites to axon terminals
• Connect neurotransmitters to behaviors: dopamine and reward, serotonin and mood, GABA and anxiety
• Apply drug effects: explain how agonists/antagonists change neural communication
• Use examples: reflex arc, MS disrupting myelin, SSRIs blocking serotonin reuptake

The Big Picture

Neurons are the basic units of the nervous system that transmit information through electrical signals (action potentials) down the axon and chemical signals (neurotransmitters) across the synapse. Neural firing follows the all-or-none principle, and neurotransmitters can be excitatory or inhibitory. Psychoactive drugs alter behavior by acting as agonists, antagonists, or reuptake inhibitors.

Essential Concepts

• Neurons vs. glial cells
• Neuron structure (dendrites, soma, axon)
• Myelin sheath function
• Resting potential ($-70\text{mV}$)
• Threshold and action potential
• All-or-none principle
• Depolarization process
• Refractory period
• Synapse and neurotransmitters
• Excitatory vs. inhibitory
• 8 neurotransmitters (exam focus)
• Agonist, antagonist, reuptake
• Drug classes (stimulants, depressants)
• Tolerance, addiction, withdrawal