Week 5 ELM 11: Synaptic Integration

Synaptic Integration

Simple Neural Networks

• Convergence:

  • Example: Motor neuron. A single motor neuron receives input from multiple sources.

• Divergence:

  • Example: Pain receptor. A single pain receptor can send signals to multiple areas of the brain.

Decisions, Decisions, Decisions!

• Neurons receive thousands of inputs, both excitatory and inhibitory.

• The neuron integrates these inputs to determine its response.

Excitatory Input

• Excitatory input leads to an influx of Na^+ ions.

• This results in an excitatory postsynaptic potential (EPSP), which is a small depolarization.

Membrane Potential

• Resting potential: -70 mV

• Threshold: -55 mV

• Action potential peak: +40 mV

• Depolarization: The membrane potential becomes more positive.

• Repolarization: The membrane potential returns to its resting state.

• Refractory period: A period after an action potential during which it is more difficult or impossible to trigger another action potential.

Factors Influencing Action Potential Firing

1. Net sign of combined input: Neurons integrate both EPSPs and IPSPs to determine their response.

2. Strength of synaptic input: Different inputs can have varying strengths.

3. Location of synapse: Synapses closer to the hillock/initial segment have a greater influence.

4. Firing frequency of the presynaptic neuron: Higher firing rates can lead to greater postsynaptic effects.

Summation

• The process by which postsynaptic potentials (PSPs) are combined to produce a larger postsynaptic potential.

Summation: Location and Timing

• Spatial Summation:

  • PSPs from multiple synapses are added together.

  • The closer the synapse is to the axon hillock/initial segment, the greater its influence.

• Temporal Summation:

  • Multiple PSPs from a single synapse are added together.

  • The more frequently the inputs fire, the greater the summation.

Electrical Anatomy of a Neuron

• Dendrites: Current attenuates (decreases) over time and distance.

• Axon: Current does not attenuate.

Cable Theory

• Describes how voltage changes as it travels down a neuron.

• \lambda (length constant): The distance over which the voltage drops to 37% of its original value. The formula is: V = V_0 e^{-x/\lambda} where:

  • V is the voltage at distance x.

  • V_0 is the original voltage at the source.

  • x is the distance from the source.

• At distance \lambda, voltage is 37% of original.

PSPs and Attenuation

• PSPs travel passively to the hillock/initial segment and attenuate as they travel.

• Synapses closer to the hillock/initial segment have more influence because they attenuate less.

Hillock

• High density of voltage-sensitive sodium channels (VSSC).

• The location where action potentials are initiated.

• A weak signal far from the hillock may not trigger an action potential.

• A strong signal near the hillock is more likely to trigger an action potential.

Spatial Summation and Threshold

• Spatial summation of PSPs allows multiple inputs to reach threshold and trigger an action potential.

Temporal Summation

• For PSPs to be additive, new action potentials (APs) must arrive before previous PSPs decay.

Temporal Summation of PSPs

• Presynaptic neuron firing causes postsynaptic potentials.

• If the presynaptic neuron fires repeatedly in rapid succession, the postsynaptic potentials summate.

Chemical Synapse

• Action potential arrives at the presynaptic terminal.

• Calcium (Ca^{2+}) influx.

• Vesicles containing neurotransmitter fuse with the presynaptic membrane.

• Neurotransmitter is released into the synaptic cleft.

• Neurotransmitter binds to postsynaptic receptors (ligand-gated channels).

• Ions (e.g., Na^+) flow through the channels, causing an EPSP.

• Uptake systems or enzymes remove/break down the neurotransmitter.

Spatial and Temporal Summation of PSPs

• Spatial summation: PSPs from multiple synapses.

• Temporal summation: PSPs from a single synapse over time.

Shunting Inhibition

• A single inhibitory synapse, located close to the soma, can effectively shut off all other inputs to that dendrite.

• Location is key for this type of inhibition.

ACh and M-type K+ Channel

• Acetylcholine (ACh) can activate M-type potassium (K^+$) channels.

• Activation of these channels can hyperpolarize the neuron, inhibiting it.

Information Encoding

• Focuses on how neurons convey information about stimuli.

Stimulus Size and Membrane Potential

• Neurons use an AM (amplitude modulation) to FM (frequency modulation) conversion.

• The size of the stimulus is converted into the frequency and pattern of action potentials.

Neurons Use an FM Code

• Both the frequency and pattern of action potentials convey information.

• The same number of action potentials can have different effects on the postsynaptic cell depending on their timing and pattern.

FM Encoding

• Absolute Refractory Period: Cannot produce another action potential.

• Relative Refractory Period: The cell is less excitable, requiring a larger stimulus to fire.

• Refractory period is due to inactivation of Na^+ current and activation of K^+$$ current.

FM Encoding Details

• The rate of firing is determined by the relative refractory period (RRP).

• Threshold is at rest.

FM Modulation

• The pattern of action potentials can be modulated to convey different information.

• Threshold at rest.

Reward Pathway

• Components:

  • Ventral tegmental area (VTA)

  • Nucleus accumbens

  • Ventral pallidum

• Dopamine (DA) Release:

  • Tonic (1-8 Hz): Baseline level of DA release.

  • Phasic (15-20 Hz): Burst of DA release.

• High DA release = reward.

• Fast/high drug concentrations are more rewarding.

• Expected reward = tonic activity.

• Better than expected reward = phasic activity.

• Worse than expected reward = pause in activity.

Reasons for Encoding

• To prevent firing at low stimulation levels (reduce noise).

• To allow patterns of activity to convey information.