Neuronal Physiology Study Notes

Overview of Neuronal Physiology

• Focus on electrotonic potentials

Postsynaptic Potentials

• Definition: Change in membrane potential as a result of neurotransmitter release.
• Types:
  • General Term: Postsynaptic potential
  • Specific Terms: End-plate potential (in skeletal muscle), receptor potential (in response to sensory stimuli like pressure/pain)
• Distinction from Action Potentials:
  • Postsynaptic potentials are different from action potentials and can react differently depending on neurotransmitter type.

Types of Neurotransmitters

• Excitatory Neurotransmitters: Result in Excitatory Postsynaptic Potentials (EPSPs)
• Inhibitory Neurotransmitters: Result in Inhibitory Postsynaptic Potentials (IPSPs)
• Characteristics of EPSPs and IPSPs:
  • EPSP: Depolarizing stimulus
  • IPSP: Hyperpolarization of the membrane

Structure of Neurons

• Major Components:
  • Cell body: Contains dendrites and axon hillock
  • Axon: Conducts action potentials
• Zones of Neurons:
  • Input Zone: Receives input through dendrites and cell body
  • Integrative Zone: Axon hillock where the decision to fire an action potential is made
  • Conductive Zone: Axon conducts the action potential down to synaptic terminals

Neuronal Functionality

• Neurons as complex computation devices that integrate signals:
  • Input Zone: Receives synaptic input from various sources.
  • Balancing act of excitatory (EPSPs) and inhibitory inputs (IPSPs):
  • If excitatory inputs are strong enough to reach axon hillock, action potential fires.
  • If inhibitory inputs are stronger, they prevent firing.
• Electrotonic potentials: Local potentials that can be graded and summed up (both spatial and temporal summation).

Characteristics of Electrotonic Potentials

• Definition: Local potentials that originate from synapses
• Graded Nature: Size of response is proportional to the strength of the stimulus.
• No Refractory Period: Unlike action potentials, electrotonic potentials do not involve voltage-gated sodium channels.
• Passive Propagation: Similar to how electricity travels down a copper wire; diminishes with distance.

Comparison: Electrotonic Potentials vs. Action Potentials

• Electrotonic potentials:
  • Graded responses proportional to stimulus strength
  • Not propagated but decay over distance
• Action potentials:
  • All-or-none events with consistent amplitude
  • Dependent on sufficient stimulus strength to reach threshold

Recording Examples

• If a stimulus is recorded at various points down the axon, it will appear strong initially but very weak or nonexistent further away:
  • Depends on distance from the stimulus site.

EPSPs: Mechanism

• Presynaptic neurons generate action potentials that cause EPSPs in postsynaptic neurons.
• If EPSPs are weak, they may not be sufficient to generate an action potential.
• Temporal Summation: Multiple EPSPs generated from the same synapse in quick succession increase the chances of firing an action potential.
• Spatial Summation: EPSPs from different presynaptic neurons converge to create a stronger overall response.

IPSPs: Mechanism

• Hyperpolarization occurs, making it harder to initiate action potential.
• An action potential can be generated from a presynaptic neuron while the postsynaptic neuron experiences hyperpolarization.

Summary of Integration Zones in Neurons

• Diagrams illustrating input, integrative, and conduction zones show the progression from local potentials (EPSPs and IPSPs) to action potentials.
• Demonstrates how multiple synaptic inputs contribute to decision-making in neuron firing.

Implications for Neuronal Communication

• Understanding the dynamics of EPSPs and IPSPs provides insight into neuronal processing and decision-making.
• The balance between excitatory and inhibitory inputs determines neural network functionality and behavior.