Learning Objective

Learners will be able to describe the structure, function, and physiological behavior of neuronal synapses, differentiate excitatory and inhibitory synaptic potentials, and explain how graded potentials integrate at the axon hillock to generate an action potential.

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Synapses are specialized junctions where neurons communicate with one another. These connections determine how signals are processed, integrated, and transmitted across neural circuits. Understanding synaptic physiology is essential for interpreting neuronal signaling, clinical neurology, and pharmacological interventions.

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Types of Synapses

Chemical Synapses

• Use neurotransmitters
• Most common type
• Synaptic delay (≈0.5 ms)
• Can be excitatory (EPSP) or inhibitory (IPSP)

Electrical Synapses

• Gap junctions
• Very fast, bidirectional
• Found in smooth muscle, cardiac muscle, and some brain circuits

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Activity

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Synaptic Potentials

Excitatory Postsynaptic Potential (EPSP)

• Caused by the opening of ligand-gated Na⁺ or Ca²⁺ channels
• Membrane becomes less negative → depolarization
• Increases the likelihood of an action potential

Inhibitory Postsynaptic Potential (IPSP)

• Caused by the opening of Cl⁻ or K⁺ channels
• Membrane becomes more negative → hyperpolarization
• Decreases the chance of an action potential

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Synapse Location and Signal Integration

• Synapses occur on dendrites and the cell body (soma)
• Potentials generated spread passively toward the axon hillock
• Axon hillock has the highest density of voltage-gated Na⁺ channels
  • The threshold is lowest here
  • The action potential typically begins here

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Forms of Integration

Spatial Summation

• Multiple synapses fire simultaneously.

Temporal Summation

• Repeated firing from the same synapse in rapid succession.

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Summary Table

Synaptic Properties Overview

Synapse Locations & Functions