The Neuron and Neural Firing

For centuries, philosophers and physicians debated whether the brain operated as a continuous network of fused tissue or as a collection of discrete units. The resolution of this debate fundamentally shaped modern psychology and neuroscience. In the late nineteenth century, advances in microscopy and histological staining techniques allowed researchers to visualize nerve tissue with unprecedented clarity, revealing that the nervous system is composed of individual cells—neurons—that communicate across tiny gaps. This discovery, known as the neuron doctrine, provided the structural foundation upon which all subsequent research into perception, cognition, emotion, and behavior has been built.

These discoveries collectively addressed a central question in psychology: how does the brain convert physical and chemical events at the cellular level into the rich tapestry of human thought, feeling, and action? Understanding the neuron and the mechanism of neural firing is essential because every behavior studied in psychology—from classical conditioning to language processing—depends on the electrochemical signaling that occurs within and between neurons. On the AP Psychology exam, this topic appears prominently in the Biological Bases of Behavior unit and frequently surfaces in free-response questions that require students to trace a psychological process back to its neural underpinnings.

Before exploring how neurons fire, it is important to establish the foundational principles that govern neural structure and communication. The nervous system relies on two broad classes of cells: neurons, which transmit electrochemical signals, and glial cells (also called neuroglia), which support, insulate, and nourish neurons. Although glial cells outnumber neurons, it is the neuron that serves as the fundamental information-processing unit. The following concepts constitute the building blocks for understanding neural communication.

A typical neuron consists of three main structural regions, each serving a distinct function in the process of receiving, integrating, and transmitting neural signals. The diagram below illustrates a multipolar neuron—the most common type in the central nervous system—with its dendrites, cell body (soma), axon, myelin sheath, and terminal buttons clearly labeled.

The dendrites are tree-like extensions that receive chemical messages from neighboring neurons and convert them into small electrical signals called graded potentials. These signals travel to the soma (cell body), which contains the nucleus and integrates excitatory and inhibitory inputs. If the net excitation at the axon hillock—the junction between the soma and the axon—exceeds the threshold of approximately −55 mV, an action potential is generated. This electrical impulse then travels down the axon, which in many neurons is wrapped in a fatty myelin sheath produced by glial cells (oligodendrocytes in the CNS, Schwann cells in the PNS). The myelin sheath is interrupted at regular intervals by gaps called nodes of Ranvier, where ion exchange occurs, enabling the action potential to jump rapidly from node to node in a process called saltatory conduction. Finally, the signal reaches the terminal buttons (also called axon terminals or synaptic knobs), which release neurotransmitters into the synaptic cleft—the microscopic gap between one neuron's terminal and the next neuron's dendrite.

Neural firing—the generation and propagation of an action potential—is a rapid, sequential process driven by the movement of sodium (Na⁺) and potassium (K⁺) ions across the neuronal membrane through voltage-gated ion channels. The process unfolds in distinct phases, each governed by changes in membrane permeability.

Phases of the Action Potential

1. Resting State (−70 mV): At rest, the neuron's interior is negatively charged relative to the exterior. The sodium-potassium pump actively transports three Na⁺ ions out of the cell for every two K⁺ ions pumped in, maintaining this electrical gradient. Voltage-gated Na⁺ and K⁺ channels are closed during this phase.

2. Depolarization: When excitatory neurotransmitters bind to receptors on the dendrites, Na⁺ channels begin to open, allowing positively charged sodium ions to rush into the cell. If this influx of positive charge raises the membrane potential from −70 mV to the threshold of approximately −55 mV, voltage-gated Na⁺ channels open rapidly along the axon. Sodium floods inward, and the interior of the neuron becomes positively charged, temporarily reaching about +30 mV. This is the depolarization phase.

3. Repolarization: Once the membrane reaches about +30 mV, the Na⁺ channels close (become inactivated), and voltage-gated K⁺ channels open. Potassium ions, now driven by both the concentration gradient and electrical repulsion, rush out of the cell, restoring the negative interior charge. This is the repolarization phase.

4. Hyperpolarization: The K⁺ channels are slow to close, so potassium continues flowing out briefly, causing the membrane potential to dip below the resting level to approximately −80 mV. This brief undershoot is the hyperpolarization (or refractory period), during which the neuron is temporarily less likely to fire again.

5. Return to Rest: The K⁺ channels finally close, and the sodium-potassium pump restores the ion distribution to its resting state of −70 mV. The neuron is now ready to fire again.

Once an action potential reaches the terminal buttons, the electrical signal must be converted into a chemical one to cross the synaptic cleft—the nanometer-wide gap between the presynaptic (sending) neuron and the postsynaptic (receiving) neuron. This process is called synaptic transmission, and it is the mechanism by which neurons influence one another's activity.

When the action potential arrives at the terminal button, voltage-gated calcium (Ca²⁺) channels open, causing calcium ions to flow into the terminal. This calcium influx triggers synaptic vesicles—tiny sacs filled with neurotransmitter molecules—to fuse with the presynaptic membrane and release their contents into the synaptic cleft through a process called exocytosis. The neurotransmitters drift across the cleft and bind to receptor sites on the postsynaptic neuron's membrane in a lock-and-key fashion: only neurotransmitters with the correct molecular shape can activate a given receptor. Depending on the neurotransmitter and receptor type, the postsynaptic response is either an excitatory postsynaptic potential (EPSP), which depolarizes the membrane and makes firing more likely, or an inhibitory postsynaptic potential (IPSP), which hyperpolarizes the membrane and makes firing less likely.

AP Psychology free-response questions frequently ask students to trace a stimulus from sensory input through neural processing to behavioral output. The following worked example demonstrates how to apply knowledge of the neuron and neural firing to a scenario—the type of structured response that earns maximum points on the exam.

Understanding how neurons communicate is essential for understanding how psychoactive drugs alter behavior. Drugs influence synaptic transmission by acting as agonists (which mimic or enhance a neurotransmitter's effect) or antagonists (which block or reduce a neurotransmitter's effect). This distinction is heavily tested on the AP Psychology exam and is essential for understanding both drug therapies for psychological disorders and the mechanisms of substance abuse.

The neuron and neural firing form the cellular foundation upon which many higher-level psychological phenomena rest. Understanding these basics enables you to grasp more complex topics that appear across multiple AP Psychology units. The table below connects neural processes to advanced topics you will encounter throughout the course.

As you progress through the course, you will encounter Hebb's rule—often summarized as "neurons that fire together wire together"—which explains how repeated co-activation of neural pathways strengthens synaptic connections through long-term potentiation (LTP). This concept bridges the biological bases unit with the learning and memory units, illustrating how the micro-level process of neural firing translates into the macro-level phenomenon of acquiring new knowledge and skills. The enduring insight is that every psychological concept—whether classical conditioning, emotional regulation, or language acquisition—has a neural substrate, and understanding the neuron gives you the vocabulary to describe that substrate on the exam.