Neurons, Synapses, and Signaling: Structure and Function in the Nervous System

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Neurons, Synapses, and Signaling

Introduction to Neuron Structure and Function

The nervous system is composed of specialized cells called neurons that transmit information through electrical and chemical signals. Neurons are organized to efficiently process and transfer information, reflecting their critical role in animal physiology and behavior.

Neurons consist of a cell body, dendrites (which receive signals), and an axon (which transmits signals).
Information is transmitted from a presynaptic cell to a postsynaptic cell (which may be another neuron, muscle, or gland cell).
Most neurons are supported by glia (glial cells), which provide nourishment, insulation, and structural support.

Diagram of neuron structure and synaptic transmission Micrograph of neurons and glia in nervous tissue

Stages of Information Processing

Nervous systems process information in three main stages: sensory input, integration, and motor output.

Sensory neurons detect external stimuli (e.g., light, touch, smell).
Interneurons integrate and interpret sensory information.
Motor neurons transmit signals to effectors (muscles or glands), causing a response.

Diagram of sensory input, integration, and motor output

Organization of Nervous Systems

Many animals possess a complex nervous system divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

CNS: Includes the brain and nerve cord; site of information integration.
PNS: Transmits information to and from the CNS; composed of bundled neurons called nerves.

Ion Pumps, Ion Channels, and the Resting Potential

Membrane Potential and Resting Potential

Every cell maintains a voltage difference across its plasma membrane, known as the membrane potential. In neurons, the resting potential is the membrane potential when the cell is not transmitting a signal, typically between -60 and -80 mV.

Changes in membrane potential are called action potentials.

Establishing the Resting Potential

The resting potential is established by the unequal distribution of ions across the neuronal membrane, maintained by ion pumps and channels.

Sodium-potassium pumps use ATP to maintain high K+ inside and high Na+ outside the cell.
Open K+ channels allow K+ to diffuse out, creating a negative charge inside the cell.

Diagram of ion movement across the neuronal membrane

Ion	Intracellular Concentration (mM)	Extracellular Concentration (mM)
Potassium (K+)	140	5
Sodium (Na+)	15	150
Chloride (Cl–)	10	120
Large anions (A–, e.g., proteins)	100	Not applicable

Table of ion concentrations inside and outside neurons

Modeling the Resting Potential

Artificial membranes can model the resting potential by separating chambers with different ion concentrations. At equilibrium, the electrical and chemical gradients are balanced.

Model of ion diffusion across a membrane

Action Potentials: The Signals of Neurons

Gated Ion Channels and Membrane Potential Changes

Neurons contain gated ion channels that open or close in response to stimuli, altering the membrane potential. Voltage-gated ion channels respond to changes in membrane voltage.

Diagram of gated ion channel opening and closing

Graded Potentials and Action Potentials

Graded potentials are small changes in membrane polarization, varying with stimulus strength. If depolarization reaches a threshold, a large, all-or-none action potential is triggered.

Graphs of graded and action potentials Graph of action potential triggered by depolarization

Phases of the Action Potential

An action potential consists of several phases: resting state, depolarization (rising phase), repolarization (falling phase), and undershoot. During the refractory period, a second action potential cannot be initiated due to inactivation of Na+ channels.

Diagram of action potential phases and ion channel states Diagram of ion channel states during action potential

Conduction of Action Potentials

Action potentials are generated at the axon hillock and travel in one direction toward the synaptic terminals. Inactivation of Na+ channels behind the depolarization zone prevents backward propagation.

Diagram of action potential propagation along axon

Adaptations for Rapid Conduction

The speed of action potential conduction increases with axon diameter and the presence of a myelin sheath. In vertebrates, myelin is produced by oligodendrocytes (CNS) and Schwann cells (PNS). Action potentials jump between nodes of Ranvier in a process called saltatory conduction.

Diagram of myelinated axon and nodes of Ranvier

Synaptic Transmission

Types of Synapses

Neurons communicate at synapses, which can be electrical or chemical. Most synapses in the nervous system are chemical.

At electrical synapses, current flows directly through gap junctions.
At chemical synapses, neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell.

Diagram of chemical synapse structure and neurotransmitter release

Postsynaptic Potentials

Neurotransmitter binding to ligand-gated ion channels generates postsynaptic potentials:

Excitatory postsynaptic potentials (EPSPs): Depolarizations that bring the membrane potential closer to threshold.
Inhibitory postsynaptic potentials (IPSPs): Hyperpolarizations that move the membrane potential farther from threshold.

Summation of multiple EPSPs and IPSPs determines whether an action potential is generated in the postsynaptic neuron.

Termination of Neurotransmitter Signaling

Neurotransmitter signaling is terminated by enzymatic breakdown or reuptake into the presynaptic neuron, restoring the synapse to its resting state.

Diagram of neurotransmitter breakdown and reuptake

Modulation of Synaptic Signaling

Some neurotransmitters bind to metabotropic receptors, activating signal transduction pathways and second messengers, which can amplify or modulate the postsynaptic response.

Diagram of metabotropic receptor signaling

Major Neurotransmitters

Neurotransmitters are classified by their chemical structure and function. Major classes include:

Amino acids (e.g., glutamate, GABA, glycine)
Biogenic amines (e.g., dopamine, serotonin, norepinephrine)
Neuropeptides (e.g., substance P, endorphins)
Gases (e.g., nitric oxide)

Table of major neurotransmitters and their structures

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