01 / 24

The Neuron and
Neural Firing

Explore how neurons communicate through electrical and chemical signals: from resting potential to the action potential and synaptic transmission.

AP Psychology

What Is a Neuron?

A neuron is a specialized cell that transmits information throughout the nervous system using electrical impulses and chemical signals. Neurons are the basic building blocks of the brain, spinal cord, and peripheral nerves.

100 billion
Approximate number of neurons in the human brain
Electrochemical
Neurons use both electrical and chemical signals
Communication
Neurons form networks that underlie all thought and behavior

Parts of the Neuron

Dendrites Soma Nucleus Myelin Sheath Axon Axon Terminals Nodes of Ranvier

Three Types of Neurons

Sensory (Afferent)
Carry information from sensory receptors to the brain and spinal cord
Interneurons
Connect sensory and motor neurons within the central nervous system
Motor (Efferent)
Carry signals from the brain and spinal cord to muscles and glands

The Resting Potential

When a neuron is not firing, the inside of the cell is negatively charged compared to the outside. This electrical difference is called the resting potential, approximately \(-70 \text{ mV}\).

Cell Membrane Outside: Positive (+)
\(Na^+\)
\(Na^+\)
\(Na^+\)
 Inside: Negative (−)
\(K^+\)
\(Proteins^-\)
\(K^+\)
Resting: \(-70 \text{ mV}\)

The Action Potential

When a neuron receives enough stimulation to reach the threshold (about \(-55 \text{ mV}\)), an action potential fires: a rapid electrical impulse travels down the axon.

Voltage (mV) Time (ms)
\(-70\)
\(-55\)
 0
\(+40\)
 Threshold Resting Depolarization Repolarization Refractory
\(+40 \text{ mV}\)

Phases of Neural Firing

1
Resting state: The neuron sits at \(-70 \text{ mV}\). \(Na^+\) channels are closed, \(K^+\) leak channels maintain the negative charge.
2
Depolarization: A stimulus opens \(Na^+\) channels. \(Na^+\) rushes in, making the inside positive (up to \(+40 \text{ mV}\)).
3
Repolarization: \(Na^+\) channels close. \(K^+\) channels open and \(K^+\) flows out, restoring the negative charge.
4
Refractory period: The neuron briefly dips below \(-70 \text{ mV}\) (hyperpolarization), then the pump resets the ion balance.

The All-or-None Law

A neuron either fires with full intensity or does not fire at all. There is no "partial" action potential. This is like pulling the trigger on a gun: the bullet fires at full force or not at all.

Threshold
The minimum stimulation needed to trigger an action potential (about \(-55 \text{ mV}\))
Intensity Coding
Stronger stimuli cause neurons to fire more frequently, not more strongly

Myelin and Speed

Myelin Node Node Node Saltatory Conduction

The Synapse

Presynaptic Neuron Vesicles Synaptic Cleft (Gap) Postsynaptic Neuron Receptor Sites Lock & Key Mechanism

Excitatory vs. Inhibitory Signals

Excitatory (EPSP) Inhibitory (IPSP)
Effect on Charge Makes inside more positive Makes inside more negative
Direction Toward threshold Away from threshold
Result Increases firing chance Decreases firing chance
Example NT Glutamate GABA

Cleaning Up the Synapse

After neurotransmitters do their job, they must be removed from the synaptic cleft. There are three main mechanisms:

Reuptake
The sending neuron reabsorbs excess neurotransmitters (like a vacuum)
Enzymatic Breakdown
Enzymes in the cleft destroy the neurotransmitter molecules
Diffusion
Neurotransmitters drift away from the cleft naturally

Key Neurotransmitters

Neurotransmitter Function Linked To
Acetylcholine (ACh) Muscle action, learning, memory Alzheimer's (deficit)
Dopamine Movement, pleasure, reward Schizophrenia (excess), Parkinson's (deficit)
Serotonin Mood, sleep, appetite Depression (deficit)
GABA Major inhibitory NT Anxiety disorders (deficit)
Glutamate Major excitatory NT Seizures (excess), memory
Endorphins Pain control, pleasure Runner's high, morphine mimics

How to Trace a Neural Signal

1
Stimulus received: Dendrites pick up signals (chemical or electrical) from other neurons or sensory receptors.
2
Summation at soma: The cell body adds up all excitatory and inhibitory inputs.
3
Threshold check: If the sum reaches \(-55 \text{ mV}\) at the axon hillock, an action potential fires.
4
Signal travels: The action potential races down the axon (jumping node-to-node if myelinated).
5
Neurotransmitter release: At the axon terminal, vesicles release neurotransmitters into the synaptic cleft.
6
Binding and response: Neurotransmitters bind to receptors on the next neuron, exciting or inhibiting it.

Example: Touching a Hot Stove

Pain Receptor Sensory Neuron Inter- neuron Motor Neuron Arm Reflex Arc Pathway Signal reaches the spinal cord (not the brain) for fastest response

Example: How Drugs Affect Synapses

Agonists
Mimic or enhance a neurotransmitter's effect. Example: morphine mimics endorphins at receptor sites.
Antagonists
Block a neurotransmitter's effect by occupying its receptor. Example: curare blocks ACh, causing paralysis.
Agonist Receptor Drug Activates Antagonist Receptor Block NT Blocks

Common Mistakes

✕ Neurons touch each other
Neurons do not physically touch. There is always a synaptic gap between them where neurotransmitters cross.
✕ Stronger stimulus = stronger signal
The all-or-none law means each action potential is the same strength. Intensity is coded by firing rate, not amplitude.
✕ Confusing agonist and antagonist
Agonists activate (think "agree"). Antagonists block (think "anti"). The AP exam frequently tests this distinction.

Summary of Key Concepts

Concept Definition Key Detail
Resting Potential Neuron at \(-70 \text{ mV}\) \(Na^+/K^+\) pump maintains it
Action Potential Electrical impulse down axon All-or-none; threshold \(-55 \text{ mV}\)
Myelin Sheath Fatty insulation on axon Saltatory conduction
Synapse Gap between neurons Lock-and-key NT binding
Reuptake NT reabsorbed by sender SSRIs block this process

Real-World Connection: Multiple Sclerosis

Multiple sclerosis (MS) is a disease in which the immune system attacks and destroys the myelin sheath. Without myelin, neural signals slow down or stop entirely.

Symptoms
Numbness, muscle weakness, vision problems, loss of coordination
Why It Matters
Demonstrates that myelin is essential for normal neural communication

Question 1 of 5

Which part of the neuron receives incoming signals from other neurons?

Question 2 of 5

During an action potential, depolarization occurs when:

Question 3 of 5

The all-or-none law states that:

Question 4 of 5

A drug that blocks the receptor sites for a neurotransmitter, preventing it from having its normal effect, is called:

Question 5 of 5

The process by which the sending neuron reabsorbs excess neurotransmitters from the synaptic cleft is called: