Notes on Synaptic Transmission

Dendrites and Soma

• Dendrites receive signals from other neurons.
• The soma (cell body) contains the nucleus and organelles necessary for the neuron's functions.

Overview of Synaptic Transmission

Learning Outcomes

1. Explain events of signal transmission at electrical and chemical synapses.
2. Discuss how excitatory and inhibitory neurotransmitters modulate synaptic information flow.
3. Differentiate spatial and temporal summation.

Synapses

• Connections between neurons where an action potential in one neuron induces an action potential in another across a synapse.
• Synapses can adapt and change in response to neuron firing.
• Types of synapses:
  1. Electrical Synapses
  2. Chemical Synapses

Electrical Synapses

• Transmit action potentials through direct flow of electrical currents via gap junctions.
• Allow bidirectional information transfer between neurons.
• Key Features:
  • Speed: Very fast, synchronizing groups of neurons (important in smooth and cardiac muscle).
  • Lack of Modulation: Less flexible than chemical synapses and lack versatility in signaling.

Gap Junctions

• Intercellular channels made of connexins that facilitate the passage of ions and signaling molecules between cells, allowing membrane potential changes to pass.

Chemical Synapses

• More abundant in the human body compared to electrical synapses.
• One-way transfer of information through synaptic clefts.
  • Presynaptic Neuron: Releases neurotransmitters at axon terminal.
  • Postsynaptic Neuron: Receives neurotransmitters.
• Mechanism: Release, diffusion, and reception of neurotransmitters.
• Key Features:
  • Slower but More Selective: Allows modifications, amplifications, and inhibitions of signals over time.
  • Signal Conversion: Converts signals from electrical to chemical back to electrical.

Electrical vs. Chemical Synapses

Types of Chemical Synapses

1. Axodendritic: Axon to dendrite.
2. Axosomatic: Axon to soma (cell body).
3. Axoaxonic: Axon to axon.

Signal Transmission at Chemical Synapses

• When an action potential arrives at the presynaptic axon terminal:
  1. Activates voltage-gated Ca²⁺ channels.
  2. Ca²⁺ influx triggers synaptic vesicles to release neurotransmitters into the synaptic cleft.
• Neurotransmitters bind to receptors on the postsynaptic neuron, causing either depolarization or hyperpolarization.

Postsynaptic Potentials (PSPs)

• Excitatory and inhibitory postsynaptic potentials can occur at synapses:
  • EPSPs (Excitatory Postsynaptic Potentials): Depolarizing interactions that bring the membrane potential closer to threshold for firing an action potential.
  • IPSPs (Inhibitory Postsynaptic Potentials): Hyperpolarizing interactions that decrease the likelihood of firing an action potential.

Summation of PSPs

• Spatial Summation: Multiple presynaptic neurons stimulate a postsynaptic neuron simultaneously.
• Temporal Summation: High-frequency stimulation from a single presynaptic neuron over time.

Termination of Synaptic Signals

• Chemical signals need to be turned off to reset the postsynaptic neuron. Mechanisms include:
  • Reuptake of neurotransmitters.
  • Enzymatic degradation.
  • Diffusion away from the synaptic cleft.
  • Glial cell uptake.

Effects of Failures in Signal Termination

• Disruption of signal termination can lead to physiological issues:
  • Insecticides inhibit enzymes that break down neurotransmitters (e.g., acetylcholine).
  • Cocaine blocks neurotransmitter reuptake, leading to accumulation and abnormal signaling effects.