Lec 11

Lecture Overview

• Topic: Synapses and Neurotransmitters

• Focus: Understanding neuronal communication, synapse structure, neurotransmitter functions, and their physiological effects.

Neuron Review

• Key Structures in Neurons:

  • Presynaptic Axon: Sends impulses to the synapse.

  • Dendrites: Receive signals at the receiving end of the neuron.

  • Axon Hillock: Initiates action potentials based on synaptic input.

  • Axon Initial Segment: Site where action potentials are generated.

  • Node of Ranvier: Gaps between myelin sheath that facilitate rapid signal transmission.

  • Synaptic Potentials:

    • Excitatory Postsynaptic Potentials (EPSPs) increase likelihood of action potential.

    • Inhibitory Postsynaptic Potentials (IPSPs) decrease likelihood.

  • Neurotransmitter Release: Involves telodendria that transmit impulses across the synaptic cleft.

Synapse Structure and Function

• Definition:

  • Junction between two neurons or between neuron and effector (muscle/gland).

• Components:

  • Presynaptic Neuron: Sends neurotransmitters into synaptic cleft.

  • Postsynaptic Neuron: Receives neurotransmitters.

  • Synaptic Cleft: Gap between presynaptic and postsynaptic neurons.

Types of Synapses

• Chemical Synapses:

  • Mechanism: Release of neurotransmitters from presynaptic neuron, binding to receptors on postsynaptic neuron.

• Electrical Synapses:

  • Mechanism: Direct ion flow between cells via gap junctions.

Chemical Synapse Anatomy

• Key Features:

  • Axon Terminal: Contains synaptic vesicles storing neurotransmitters.

  • Voltage-Gated Ca2+ Channels: Open upon action potential arrival, allowing Ca2+ influx, triggering neurotransmitter release.

  • Synaptic Vesicles: Contain neurotransmitters for exocytosis.

  • Receptors: Located on postsynaptic neurons, bind neurotransmitters.

  • Reuptake Mechanisms: Remove neurotransmitters from the synaptic cleft after action.

Action Potential Consequences

• Phases of Response:

  • Presynaptic Activation: Causes opening of voltage-gated Ca2+ channels.

  • Neurotransmitter Release Process:

    1. Vesicles fuse with plasma membrane.

    2. Released neurotransmitters bind to receptors on postsynaptic neuron.

  • Resulting Ionic Changes:

    • EPSP: Inward Na+ movement causes depolarization.

    • IPSP: Inward Cl- movement/hyperpolarizes.

Synaptic Integration

• Definition: Summation of multiple synaptic inputs influencing overall neuron output.

• Types of Summation:

  • Temporal Summation: Same synapse activated multiple times in rapid succession.

  • Spatial Summation: Simultaneous inputs from multiple synapses.

Presynaptic Modulation

• Mechanisms:

  • Facilitation Increases: Enhances likelihood of neurotransmitter release.

  • Inhibition Decreases: Prevents neurotransmitter release.

Neurotransmitter Effects on Postsynaptic Cells

• Inhibitory/Excitatory Effects:

  • Binding of neurotransmitters can lead to different postsynaptic responses based on receptor types and ion channels.

• Neurotransmitter Types:

  • Acetylcholine (ACh): Major in PNS, involved in muscle activation, breaking down by Acetylcholinesterase.

  • Biogenic Amines: Includes dopamine, norepinephrine, serotonin; regulate mood and response systems.

  • Amino Acids: e.g., Glutamate (excitatory) and GABA (inhibitory); important in overall neural function.

  • Neuropeptides: Include endogenous opioids, which manage pain and stress responses.

Implications of Neurotransmitter Dysregulation

• Clinical Importance:

  • Recognizing changes in neurotransmitter functions is key in diseases like Alzheimer’s, Parkinson's, and depression.

  • Investigating receptor sensitivity and modulation can aid in treatment strategies (e.g., MAO inhibitors for depression).

Summary Questions

• Describe the communication at synapses and compare EPSP vs. IPSP.

• Explain synaptic integration and the significance of neuronal summation.

• Identify neurotransmitter roles and implications for conditions such as Alzheimer’s and depression.