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

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Neurons, Synapses, and Signaling

Neuron Organization and Structure Reflect Function in Information Transfer

Neurons are specialized cells of the nervous system responsible for transmitting information throughout the body. Their structure is closely related to their function, with distinct regions for receiving, integrating, and transmitting signals.

Cell Body (Soma): Contains the nucleus and most organelles.
Dendrites: Highly branched extensions that receive signals from other neurons.
Axon: A long extension that transmits signals to other cells, often ending in multiple synaptic terminals.
Axon Hillock: The cone-shaped base of the axon where action potentials are typically initiated.
Synapse: The junction between the axon terminal of one neuron and the target cell (another neuron, muscle, or gland cell).
Neurotransmitters: Chemical messengers released from synaptic terminals to transmit signals across the synapse.

Diagram of neuron structure and synaptic transmission

Glial Cells: Non-neuronal cells that nourish, insulate, and support neurons. Types include astrocytes, oligodendrocytes, and Schwann cells.

Micrograph showing neuron and neuroglial cells

Synaptic Structure: Electron micrographs reveal synaptic vesicles in the presynaptic terminal, the synaptic cleft, and the postsynaptic cell membrane.

Electron micrograph of synapse showing synaptic vesicles and postsynaptic cell

Glia in Nervous Tissue: Glial cells form networks that support and interact with neurons.

Fluorescent image showing glia and neuron cell bodies

Introduction to Information Processing

Nervous systems process information in three main stages:

Sensory Input: Sensors detect external and internal stimuli and transmit information via sensory neurons.
Integration: Interneurons in the brain or ganglia process and integrate sensory input.
Motor Output: Motor neurons carry signals from the CNS to effectors (muscles or glands) to elicit a response.

Diagram of sensory input, integration, and motor output

The nervous system is divided into the central nervous system (CNS)—brain and spinal cord—and the peripheral nervous system (PNS), which carries information to and from the CNS. Bundles of PNS neurons form nerves.

Neurons are classified by function and structure:

Sensory Neurons: Transmit sensory information to the CNS.
Interneurons: Integrate information within the CNS.
Motor Neurons: Transmit signals from the CNS to effectors.

Diagram of sensory neuron, interneuron, and motor neuron

Ion Pumps and Ion Channels Establish the Resting Membrane Potential of a Neuron

All cells maintain a voltage difference across their plasma membrane, known as the membrane potential. In neurons, the resting potential is typically around -70 mV, with the inside of the cell more negative than the outside.

Sodium-Potassium Pump: Uses ATP to maintain high K+ inside and high Na+ outside the cell.
Ion Channels: Selectively allow ions to diffuse across the membrane, converting chemical potential to electrical potential.

Diagram of sodium-potassium pump and ion channels in neuron membrane

The Nernst equation calculates the equilibrium potential for a particular ion:

$E_{ion} = 62\ \mathrm{mV} \times \log \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

Diagram showing equilibrium potentials for K+ and Na+

Generalized Nernst equation:

$V_m = \frac{61}{z} \times \log_{10} \frac{[C]_0}{[C]_i}$

Nernst equation for membrane potential

Action Potentials: Signals Conducted by Axons

Neurons communicate via changes in membrane potential. Gated ion channels open or close in response to stimuli, leading to graded potentials or action potentials.

Depolarization: Membrane potential becomes less negative (e.g., Na+ influx).
Hyperpolarization: Membrane potential becomes more negative (e.g., K+ efflux).

Intracellular recording of membrane potential

Graded Potentials: Small changes in membrane potential; magnitude varies with stimulus strength.

Action Potentials: Large, all-or-none changes in membrane potential that propagate along the axon.

Graphs of graded potentials and action potential

Generation of Action Potentials

Action potentials occur in several stages:

Resting State: Most voltage-gated Na+ and K+ channels are closed.
Depolarization: Voltage-gated Na+ channels open, Na+ enters the cell.
Rising Phase: Membrane potential rapidly increases as more Na+ channels open.
Falling Phase: Na+ channels inactivate, K+ channels open, K+ exits the cell.
Undershoot: K+ permeability is temporarily higher than at rest, then returns to baseline.

Voltage-gated ion channels during action potential Phases of the action potential

During the refractory period, a second action potential cannot be initiated due to inactivation of Na+ channels.

Conduction of Action Potentials

Action potentials are typically initiated at the axon hillock and propagate unidirectionally toward synaptic terminals. Inactivated Na+ channels prevent backward propagation.

Conduction of action potentials along the axon

Evolutionary Adaptation of Axon Structure

The speed of action potential conduction increases with axon diameter and the presence of a myelin sheath. Myelin is produced by oligodendrocytes (CNS) and Schwann cells (PNS).

Myelinated axon and Schwann cell structure

In myelinated axons, action potentials jump between nodes of Ranvier in a process called saltatory conduction.

Saltatory conduction in myelinated axon

Neurons Communicate with Each Other at Synapses

Neurons communicate at synapses, which can be electrical (via gap junctions) or chemical (via neurotransmitters). Most synapses in the nervous system are chemical.

Diagram of chemical synapse structure

At chemical synapses:

The presynaptic neuron releases neurotransmitter from synaptic vesicles into the synaptic cleft.
The neurotransmitter binds to receptors on the postsynaptic cell, causing ion channels to open and generating a postsynaptic potential.

Synaptic vesicle release and neurotransmitter action

Generation and Summation of Postsynaptic Potentials

Postsynaptic potentials can be:

Excitatory (EPSPs): Depolarize the postsynaptic membrane, bringing it closer to threshold.
Inhibitory (IPSPs): Hyperpolarize the membrane, moving it further from threshold.

Ligand-gated ion channels and postsynaptic potentials

Summation of EPSPs and IPSPs determines whether the postsynaptic neuron will fire an action potential:

Temporal Summation: Multiple EPSPs from a single synapse in rapid succession add together.
Spatial Summation: EPSPs from multiple synapses on the same neuron combine.

Summation of postsynaptic potentials

IPSPs can counteract EPSPs, and the net effect at the axon hillock determines action potential generation.

Summary Table: Ion Concentrations Inside and Outside of Mammalian Neurons

Ion	Intracellular Concentration (mM)	Extracellular Concentration (mM)
Potassium (K+)	140	5
Sodium (Na+)	15	150
Chloride (Cl−)	10	120
Large anions (A−)	100	Not applicable

Additional info: This table highlights the ionic gradients maintained by neurons, which are essential for generating membrane potentials and action potentials.