2.3 Neurophysiology: action potentials and synaptic transmission

Neurons communicate through electrical signals and chemical messengers. Action potentials, triggered by ion movements, travel along neurons. At synapses, neurotransmitters are released, binding to receptors on the receiving neuron. This process forms the basis of information transfer in the nervous system.

Synapses come in electrical and chemical varieties, each with unique properties. Synaptic plasticity allows connections to strengthen or weaken over time, enabling learning and memory formation. This adaptability is crucial for the brain's ability to process and store information.

Neuronal Communication

Ionic basis of neuronal potentials

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• Resting membrane potential maintained by unequal ion distribution across neuronal membrane
  • Sodium-potassium pump and selective ion channel permeability establish resting potential
  • Interior of neuron more negative than exterior at rest, typically around -70 mV
• Action potential generation triggered when membrane potential reaches threshold value (~-55 mV)
  • Voltage-gated sodium channels open causing rapid Na+ influx and membrane depolarization towards Na+ equilibrium potential
  • Delayed opening of voltage-gated potassium channels allows K+ efflux, repolarizing and hyperpolarizing membrane before returning to resting state
• Refractory periods limit action potential frequency
  • Absolute refractory period: inactivated Na+ channels prevent another action potential
  • Relative refractory period: increased threshold for action potential generation due to lingering K+ channel activation

Process of synaptic transmission

1. Synaptic vesicle fusion
   • Action potential arrives at presynaptic terminal, opening voltage-gated calcium channels and allowing Ca2+ influx
   • Increased intracellular Ca2+ triggers fusion of synaptic vesicles with presynaptic membrane, releasing neurotransmitters into synaptic cleft
2. Neurotransmitter binding and receptor activation
   • Released neurotransmitters diffuse across synaptic cleft and bind to specific postsynaptic receptors
   • Ligand-gated ion channels open, changing postsynaptic membrane potential through ion flow
   • Metabotropic receptors activate intracellular signaling cascades modulating neuronal activity
3. Neurotransmitter clearance
   • Neurotransmitters removed from synaptic cleft by presynaptic reuptake, enzymatic degradation, or diffusion away from synapse

Synaptic Diversity and Plasticity

Electrical vs chemical synapses

• Electrical synapses formed by gap junctions allow direct, bidirectional transmission of electrical signals between neurons
  • Enable rapid, synchronous activity in neuronal networks (retina, inferior olive)
• Chemical synapses use neurotransmitters for unidirectional communication from presynaptic to postsynaptic neuron
  • Slower than electrical synapses due to multi-step transmission process
  • Allow signal amplification, integration, and modulation
  • Predominant synapse type in central nervous system enabling diverse signaling and plasticity

Synaptic plasticity and learning

• Short-term synaptic plasticity influenced by presynaptic Ca2+ dynamics and vesicle availability
  • Facilitation: enhanced neurotransmitter release with repeated stimulation
  • Depression: decreased neurotransmitter release with repeated stimulation
• Long-term synaptic plasticity involves persistent changes in synaptic strength
  • Long-term potentiation (LTP): increased synaptic strength induced by high-frequency stimulation or coincident pre- and postsynaptic activity
    • Requires NMDA receptor activation, Ca2+ influx, and protein synthesis
  • Long-term depression (LTD): decreased synaptic strength induced by low-frequency stimulation or specific timing of pre- and postsynaptic activity
    • Involves NMDA receptor activation, Ca2+ influx, and protein phosphatases
• Synaptic plasticity enables experience-dependent modification of neural circuits, underlying learning and memory formation
  • Hebbian plasticity: "neurons that fire together, wire together"
  • LTP and LTD serve as cellular correlates of learning and memory
  • Strengthening and weakening of specific neuronal connections through synaptic plasticity allows for adaptive changes in behavior and cognition (skill acquisition, spatial navigation)