S

Synapses and Their Functions

Module Overview

  • Focus: Synapses in neurobiology

  • Instructor: Dr. Yasemin Salgirli Demirbas, DVM, PhD

Lab Announcement

  • Next lab: Online (last online lab)

  • Schedule adjusted to finish all labs by semester end

Synapse Definition

  • Specialized junction between two neurons

  • Presynaptic neuron influences postsynaptic neuron

  • Includes parts of both neurons and extracellular space

Synaptic Cleft

  • Also known as synaptic gap (0.02 nm)

  • Prevents direct current propagation; uses neurotransmitters for signal transmission

Anatomical Types of Synapses

  • Categories:

    • Axodendritic: axon to dendrite

    • Axosomatic: axon to soma

    • Axoaxonic: axon to axon

    • Dendrodendritic: dendrite to dendrite

    • Dendrosomatic: dendrites to soma

Functional Anatomy

  • Types of Synapses:

    • Electrical: Fast signal transmission via gap junctions

    • Chemical: Majority in human CNS, neurotransmitter-mediated

Synaptic Transmission Process

  1. Action potential opens voltage-gated calcium channels in presynaptic neuron.

  2. Calcium influx facilitates synaptic vesicle fusion.

  3. Neurotransmitter released, diffuses across cleft.

  4. Binds to postsynaptic receptor, opening ion channels.

  5. Signal terminated by breakdown, reuptake, or diffusion.

Neurotransmitter Types

  • Cotransmitters: multiple neurotransmitters may be released.

Receptor Types

  • Ionotropic Receptors: Directly gated ion channels; quick response (e.g., ACh, glutamate).

  • Metabotropic Receptors: Indirectly gated; slower, long-lasting effects via second messengers.

Autoreceptors

  • Respond to neurotransmitters released by the same neuron; regulate internal neurotransmitter processes.

Nonsynaptic Communication

  • Chemical release acts over larger distances.

  • Involves neuromodulators and hormones; broader signaling beyond synapses.

Electrical Synapses

  • Rapid signal transmission; allow for synchronized neuronal activity.

  • Definition blur between presynaptic and postsynaptic due to bidirectional current flow.

Synaptic Termination

  • Must clear neurotransmitters to reset postsynaptic potential. Possible methods:

    • Enzymatic breakdown

    • Reuptake into presynaptic neuron

    • Diffusion away

  • Disruption can affect neurological function.

Excitatory vs Inhibitory Synapses

  • EPSP: Depolarization that brings neuron closer to firing threshold.

  • IPSP: Hyperpolarization that decreases firing potential.

Summation and Integration

  • EPSPs can sum to reach action potential threshold.

  • IPSPs and EPSPs can cancel each other out for balance.