Synaptic Transmission: Electrical and Chemical Synapses in the Nervous System

Chapter Outline and Introduction

This chapter explores the mechanisms by which neurons communicate at synapses, focusing on both electrical and chemical synapses. It also covers the structure, synthesis, and degradation of neurotransmitters, and the integration of neural signals.

• Electrical Synapses

• Chemical Synapses

• Neural Integration

• Presynaptic Modulation

• Neurotransmitters: Structure, Synthesis, and Degradation

Key foundational topics to master before this chapter include: Gated ion channels, Membrane potential, Graded potentials, Action potentials, Catecholamine synthesis, and Protein synthesis.

Electrical Synapses

Definition and Mechanism

Electrical synapses are specialized connections between neurons or glial cells that allow direct passage of ions and small molecules through gap junctions. This enables rapid, bidirectional communication.

• Gap junctions are protein channels that connect the cytoplasm of adjacent cells, allowing ions to flow directly between them.

• Electrical signals generated in one cell can be transmitted almost instantaneously to an adjacent cell.

• Most electrical synapses are bidirectional, but some allow current to flow in only one direction.

• Electrical synapses are found in the retina, certain areas of the brain, and are important for synchronizing activity in groups of neurons.

Example: Electrical synapses in the heart and some brain regions help synchronize the activity of large groups of cells.

Chemical Synapses

Definition and Mechanism

Chemical synapses are the most common type of synapse in the nervous system. They involve the release of neurotransmitters from a presynaptic neuron into a synaptic cleft, which then bind to receptors on a postsynaptic cell to transmit the signal.

• The presynaptic neuron releases neurotransmitters stored in synaptic vesicles.

• The synaptic cleft is a narrow space (about 30–50 nm wide) between the presynaptic and postsynaptic neurons.

• The postsynaptic neuron contains receptors that bind neurotransmitters, leading to changes in membrane potential.

• Signal transmission at chemical synapses is typically unidirectional.

Functional Anatomy of Chemical Synapses

• Axodendritic synapses: Presynaptic axon terminal synapses with a dendrite of the postsynaptic neuron (most common).

• Axosomatic synapses: Presynaptic axon terminal synapses with the soma (cell body) of the postsynaptic neuron.

• Axoaxonic synapses: Presynaptic axon terminal synapses with another axon terminal, modulating neurotransmitter release.

Example: Axoaxonic synapses can regulate the amount of neurotransmitter released by the presynaptic neuron, affecting signal strength.

Neurotransmitter Release and Synaptic Transmission

• Neurotransmitters are synthesized in the neuron and stored in synaptic vesicles.

• Arrival of an action potential at the axon terminal opens voltage-gated calcium channels.

• Calcium influx triggers exocytosis of synaptic vesicles, releasing neurotransmitter into the synaptic cleft.

• Neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane.

• The effect on the postsynaptic cell depends on the type of receptor activated (ionotropic or metabotropic).

Equation:

$\text{Neurotransmitter Release} \propto [\text{Ca}^{2+}]_{\text{in}}$

Example: The amount of neurotransmitter released increases with higher intracellular calcium concentration.

Termination of Neurotransmitter Action

• Neurotransmitters are removed from the synaptic cleft by:

  • Enzymatic degradation (e.g., acetylcholinesterase breaks down acetylcholine)

  • Reuptake into the presynaptic neuron or nearby glial cells

  • Diffusion away from the synaptic cleft

• Termination is rapid, usually within milliseconds, to ensure precise signaling.

Signal Transduction Mechanisms at Chemical Synapses

Types of Postsynaptic Receptors

• Ionotropic receptors: Ligand-gated ion channels that mediate fast synaptic responses. Neurotransmitter binding directly opens the channel, causing rapid changes in membrane potential.

• Metabotropic receptors: G protein-coupled receptors that mediate slower, longer-lasting responses by activating second messenger pathways.

Example: Glutamate binding to an AMPA receptor (ionotropic) causes a rapid excitatory postsynaptic potential (EPSP), while binding to a metabotropic glutamate receptor triggers slower modulatory effects.

Excitatory and Inhibitory Synapses

• Excitatory synapses: Neurotransmitter binding causes depolarization of the postsynaptic membrane, increasing the likelihood of an action potential (e.g., glutamate acting on AMPA receptors).

• Inhibitory synapses: Neurotransmitter binding causes hyperpolarization, decreasing the likelihood of an action potential (e.g., GABA acting on GABAA receptors).

Equation:

$\text{EPSP}: \text{Depolarization} \rightarrow \text{Increased probability of action potential}$ $\text{IPSP}: \text{Hyperpolarization} \rightarrow \text{Decreased probability of action potential}$

Summary Table: Electrical vs. Chemical Synapses

Feature	Electrical Synapse	Chemical Synapse
Structure	Gap junctions	Synaptic cleft, vesicles, receptors
Signal Transmission	Direct ion flow	Neurotransmitter release
Speed	Very fast (almost instantaneous)	Slower (milliseconds)
Directionality	Usually bidirectional	Unidirectional
Modulation	Limited	Highly modifiable
Prevalence	Rare in adult CNS	Most common in CNS

Key Terms and Definitions

• Synapse: The junction between two neurons where communication occurs.

• Presynaptic neuron: The neuron sending the signal.

• Postsynaptic neuron: The neuron receiving the signal.

• Neurotransmitter: Chemical messenger released by neurons to transmit signals across a synapse.

• Synaptic cleft: The small gap between presynaptic and postsynaptic neurons at a chemical synapse.

• Excitatory postsynaptic potential (EPSP): A depolarizing graded potential in the postsynaptic neuron.

• Inhibitory postsynaptic potential (IPSP): A hyperpolarizing graded potential in the postsynaptic neuron.

Additional info:

• Some content was inferred and expanded for clarity, such as the detailed mechanisms of neurotransmitter release and receptor types.

• Specific neurotransmitters and their synthesis/degradation will be covered in later sections of the chapter.