CH 48- Neurons, Synapses, and Signaling: Structure and Function in the Nervous System

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 supported by glial cells, which provide nourishment, insulation, and structural support. The organization and structure of neurons are closely related to their function in information transfer.

• Dendrites: Receive incoming signals from other neurons.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Axon: Conducts electrical impulses away from the cell body toward synaptic terminals.

• Synaptic Terminals: Release neurotransmitters to communicate with postsynaptic cells.

• Synapse: The junction where information is transmitted from one cell to another.

Types of Neurons and Nervous System Organization

Neurons are classified based on their function in the nervous system. The nervous system processes information in three main stages: sensory input, integration, and motor output.

• Sensory Neurons: Transmit information about external stimuli (e.g., light, touch, smell).

• Interneurons: Integrate and interpret sensory input within the central nervous system (CNS).

• Motor Neurons: Transmit signals from the CNS to muscles or glands, causing a response.

In animals with complex nervous systems:

• Central Nervous System (CNS): Site of integration (brain and nerve cord).

• Peripheral Nervous System (PNS): Carries information into and out of the CNS; neurons in the PNS are bundled into nerves.

Ion Basis of the Resting Membrane Potential

Ion Concentrations and Membrane Potential

Every cell maintains a voltage across its plasma membrane, known as the membrane potential. In neurons, the resting potential is typically between -60 and -80 mV. This potential is established by differences in ion concentrations inside and outside the cell, maintained by ion pumps and channels.

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

• Sodium-Potassium Pump: Uses ATP to maintain high K+ inside and high Na+ outside the cell.

• Ion Channels: Allow selective movement of ions, converting chemical potential to electrical potential.

Modeling the Resting Potential

The resting potential can be modeled using an artificial membrane separating two chambers with different ion concentrations. Potassium ions (K+) diffuse down their gradient, creating a negative charge inside the cell. At equilibrium, the electrical and chemical gradients are balanced.

The equilibrium potential for an ion can be calculated using the Nernst equation:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

For K+ and Na+ in neurons:

• $E_K \approx -90$ mV

• $E_{Na} \approx +62$ mV

Action Potentials: The Signals of Neurons

Generation and Properties of Action Potentials

Neurons contain gated ion channels that open or close in response to stimuli, leading to changes in membrane potential. Voltage-gated ion channels are crucial for the generation of action potentials.

• Graded Potentials: Small changes in polarization; magnitude varies with stimulus strength.

• Action Potential: A large, all-or-none change in membrane voltage triggered when depolarization reaches threshold.

Action potentials have a constant magnitude and transmit signals over long distances. They are generated at the axon hillock and travel in one direction toward the synaptic terminals.

Phases of the Action Potential

The action potential consists of several phases:

• Resting State: Most voltage-gated Na+ and K+ channels are closed.

• Depolarization: Na+ channels open, Na+ enters the cell.

• Rising Phase: Rapid influx of Na+ causes membrane potential to become positive.

• Falling Phase: Na+ channels inactivate, K+ channels open, K+ exits the cell.

• Undershoot: Membrane potential temporarily becomes more negative than resting potential.

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

Conduction of Action Potentials

Action potentials propagate along the axon by depolarizing adjacent regions of the membrane. In vertebrates, myelin sheaths produced by glial cells (oligodendrocytes in the CNS, Schwann cells in the PNS) insulate axons and increase conduction speed. Action potentials jump between nodes of Ranvier in a process called saltatory conduction.

Synaptic Transmission

Types of Synapses

Neurons communicate with other cells at synapses, which can be electrical or chemical:

• Electrical Synapses: Direct flow of current through gap junctions.

• Chemical Synapses: Neurotransmitters carry information across the synaptic cleft (most common type).

At chemical synapses, the presynaptic neuron releases neurotransmitters stored in synaptic vesicles. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic cell.

Postsynaptic Potentials and Summation

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.

Multiple EPSPs and IPSPs can combine through summation to determine whether an action potential is generated in the postsynaptic neuron.

Termination and Modulation of Neurotransmitter Signaling

Neurotransmitter signaling is terminated by:

• Enzymatic breakdown of neurotransmitters in the synaptic cleft.

• Reuptake of neurotransmitters by the presynaptic neuron.

Some synapses use metabotropic receptors, where neurotransmitter binding activates a signal transduction pathway involving second messengers, leading to amplification of the signal.

Major Neurotransmitters

Neurotransmitters are classified into several groups based on their structure and function:

Neurotransmitter	Structure
Acetylcholine	See image for structure
Amino Acids	Glutamate, GABA, Glycine
Biogenic Amines	Norepinephrine, Dopamine, Serotonin
Neuropeptides	Substance P, Met-enkephalin
Gases	Nitric oxide (NO)

Example: Glutamate is the main excitatory neurotransmitter in the CNS, while GABA is the main inhibitory neurotransmitter.