Synapses: Mechanisms of Synaptic Transmission and Integration

Basic Cellular Physiology & the Anatomy and Physiology of the Nervous System

Overview

This section introduces the fundamental concepts of neuronal structure and function, focusing on synaptic transmission and integration. Understanding these mechanisms is essential for comprehending how the nervous system communicates and processes information.

Neurons: Structure and Function

Regions of the Neuron

• Dendrites: Receive incoming signals from other neurons.

• Cell Body (Soma): Integrates incoming signals and contains the nucleus.

• Axon: Propagates action potentials away from the cell body toward other neurons or effectors.

• Axon Terminals: Transmit signals to other cells via synapses.

Each region is specialized for a particular function: reception (dendrites), integration (soma), propagation (axon), and transmission (axon terminals).

Electrical Activity of Neurons

Resting Membrane Potential

• The resting membrane potential is the voltage difference across the neuronal membrane at rest, typically around -70 mV.

• It is established by differences in ion concentration (mainly Na+ and K+) and selective membrane permeability.

• Key equation:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for K+)

• Graded potentials: Small, localized changes in membrane potential that decay with distance.

• Action potentials: Large, rapid, all-or-none electrical signals that travel long distances along the axon.

Synapses

Definition and Types

• A synapse is a junction that allows a neuron to communicate with another cell (neuron, muscle fiber, or gland).

• Presynaptic neuron: Sends the signal toward the synapse.

• Postsynaptic cell: Receives the signal.

• Types of synaptic connections include axodendritic, axosomatic, and axoaxonic synapses.

Chemical vs. Electrical Synapses

Feature	Chemical Synapse	Electrical Synapse
Transmission	Neurotransmitter release across synaptic cleft	Direct ion flow via gap junctions
Directionality	Unidirectional	Uni- or bidirectional
Speed	Slower (synaptic delay ~1-5 ms)	Faster (minimal delay)
Prevalence	Most common in adult CNS	Common in embryonic NS, heart, smooth muscle

Mechanism of Chemical Synaptic Transmission

1. Action potential arrives at the axon terminal.

2. Voltage-gated Ca2+ channels open, allowing Ca2+ influx.

3. Ca2+ triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter (NT) by exocytosis.

4. NT diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.

5. Binding opens ion channels, generating a graded postsynaptic potential.

6. NT effects are terminated by reuptake, diffusion, or enzymatic degradation.

Postsynaptic Potentials

Excitatory Postsynaptic Potentials (EPSPs)

• Binding of NT (e.g., glutamate) opens non-selective cation channels.

• Na+ influx exceeds K+ efflux, causing depolarization.

• If the depolarization reaches threshold, an action potential is triggered.

• EPSPs are typically found on dendritic spines.

Inhibitory Postsynaptic Potentials (IPSPs)

• Binding of NT (e.g., GABA, glycine) opens Cl- or K+ channels.

• Results in hyperpolarization, moving the membrane potential further from threshold.

• Reduces the likelihood of action potential generation.

Synaptic Integration

Summation

• Neurons receive multiple EPSPs and IPSPs simultaneously.

• Temporal summation: Multiple signals from one presynaptic neuron in rapid succession.

• Spatial summation: Simultaneous signals from multiple presynaptic neurons.

• The axon initial segment (trigger zone) has a lower threshold for action potential generation due to a high density of voltage-gated sodium channels.

Regulation of Synaptic Strength

• High-frequency stimulation increases Ca2+ in the presynaptic terminal, enhancing NT release (facilitation).

• Presynaptic inhibition or facilitation can modulate NT release.

• Postsynaptic receptor sensitivity can change (desensitization or upregulation).

• Drugs can affect synaptic transmission by altering NT synthesis, release, reuptake, or degradation (e.g., selective serotonin reuptake inhibitors).

Developmental Aspects of Neurons

• The neural tube forms the CNS; the neural crest forms the PNS and other structures.

• Neuroblasts proliferate, migrate, and connect to targets via axon outgrowth and synapse formation.

• Growth cones at axon tips sense environmental cues for guidance.

• The chemoaffinity hypothesis suggests axons find their targets using molecular cues (e.g., nerve growth factor).

Example: Synaptic Plasticity

• Changes in synaptic strength underlie learning and memory.

• Dendritic spines can change shape and number in response to activity.

Additional info: These notes synthesize the provided lecture objectives and readings, expanding on the mechanisms and significance of synaptic transmission and integration in the nervous system.