Neuronal Signaling and Neurotransmission: Structure and Function in the Nervous System

The Nervous System: Neuronal Signaling and Neurotransmission

Introduction

The nervous system is composed of specialized cells called neurons that communicate via electrical and chemical signals. This guide covers the mechanisms of action potential propagation, synaptic transmission, and the roles of major neurotransmitters in vertebrate and invertebrate nervous systems.

Neuronal Signaling

Refractory Period

After an action potential, neurons experience a refractory period during which a second action potential cannot be initiated. This is due to the temporary inactivation of voltage-gated Na+ channels.

• Absolute refractory period: No new action potential can be generated.

• Relative refractory period: A stronger-than-normal stimulus may initiate another action potential.

Conduction of Action Potentials

Action potentials are electrical impulses that travel along the axon by depolarizing adjacent regions of the membrane.

• Initiated at the axon hillock.

• Regenerated along the axon, allowing long-distance signaling.

• Inactivated Na+ channels behind the depolarization zone prevent backward propagation, ensuring unidirectional travel toward synaptic terminals.

Conduction Speed

The speed of action potential propagation depends on axon diameter and the presence of a myelin sheath.

• Axon diameter: Larger diameter increases speed.

• Myelin sheath: Insulates axons in vertebrates, increasing conduction speed.

Saltatory Conduction and Nodes of Ranvier

Myelinated axons have gaps called nodes of Ranvier where voltage-gated Na+ channels are concentrated.

• Action potentials 'jump' from node to node in a process called saltatory conduction.

• Only nodes of Ranvier can generate action potentials.

Synaptic Transmission

Types of Synapses

• Electrical synapses: Direct flow of electrical current between neurons.

• Chemical synapses: Use neurotransmitters to transmit signals across the synaptic cleft; most synapses are chemical.

Chemical Synapses: Mechanism

1. Arrival of action potential at the synaptic terminal depolarizes the membrane, opening voltage-gated Ca2+ channels.

2. Ca2+ influx triggers vesicle fusion and neurotransmitter release.

3. Neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels on the postsynaptic membrane, opening them.

Generation of Postsynaptic Potentials

• Binding of neurotransmitter to ligand-gated ion channels causes a change in membrane potential, known as a postsynaptic potential.

• Postsynaptic potentials can be excitatory (EPSP) or inhibitory (IPSP).

Summation of Postsynaptic Potentials

• Temporal summation: Multiple EPSPs from the same synapse in rapid succession add together.

• Spatial summation: EPSPs from different synapses on the same neuron add together.

• Combined summation can reach the threshold potential, triggering an action potential.

Termination of Neurotransmission

• Neurotransmitters are removed from the synaptic cleft by diffusion, uptake by surrounding cells, or enzymatic degradation.

Neurotransmitters

Overview

• The same neurotransmitter can have different effects depending on the target cell type.

• Five major classes: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases.

Major Neurotransmitters: Structures

Neurotransmitter	Structure
Acetylcholine	H3C–O–C(=O)–CH2–CH2–N(CH3)3
Glutamate	H2N–CH–CH2–COOH–COOH
GABA (gamma-aminobutyric acid)	H2N–CH2–CH2–CH2–COOH
Glycine	H2N–CH2–COOH
Norepinephrine	HO–C6H4–CH(OH)–CH2–NH2
Dopamine	HO–C6H4–CH2–CH2–NH2
Serotonin	HO–C8H6–CH2–NH2

Acetylcholine

• Common neurotransmitter in vertebrates and invertebrates.

• Functions: muscle stimulation, memory formation, learning.

• Receptors: ligand-gated and metabotropic.

• Toxins such as nerve gas (sarin) and botulinum toxin disrupt acetylcholine signaling.

Biogenic Amines

• Derived from amino acids (e.g., serotonin from tryptophan, dopamine from tyrosine).

• Regulate sleep, mood, attention, learning, and memory.

• Psychoactive drugs (LSD, mescaline) affect biogenic amine receptors.

• Parkinson's disease: associated with dopamine deficiency.

• Depression: treated with drugs that block serotonin reuptake (e.g., Prozac).

Functional Regions of the Brain

Brain Regions and Functions

• Frontal lobe: Decision making, planning, motor control.

• Parietal lobe: Sensory integration, somatosensory cortex.

• Temporal lobe: Auditory cortex, language comprehension.

• Occipital lobe: Visual processing.

• Cerebellum: Coordination and balance.

Neuronal Pathways

• Cholinergic neurons: Found in regions such as the basal forebrain, hippocampus, and neocortex; involved in learning and memory.

• Dopaminergic neurons: Located in areas like the substantia nigra and ventral tegmental area; involved in movement, motivation, and reward.

Summary Table: Major Neurotransmitters and Functions

Class	Example	Main Function
Acetylcholine	Acetylcholine	Muscle stimulation, learning, memory
Biogenic Amines	Dopamine, Serotonin, Norepinephrine	Mood, attention, movement, sleep
Amino Acids	Glutamate, GABA, Glycine	Excitatory/inhibitory signaling
Neuropeptides	Endorphins	Pain modulation
Gases	Nitric oxide	Local signaling

Key Equations and Concepts

• Threshold potential: The membrane potential at which an action potential is triggered.

• Action potential propagation: $V_{m} = V_{rest} + ext{EPSP} + ext{IPSP}$

• Summation: $ext{Total EPSP} = ext{EPSP}_{temporal} + ext{EPSP}_{spatial}$

Additional info:

• Neurotransmitter imbalances are linked to neurological and psychological disorders.

• Neurons use different neurotransmitters for diverse functions and signaling specificity.