Neuronal Physiology

Overview – Neuronal Physiology

Neuronal physiology underpins all nervous system communication. Neurons transmit signals through rapid electrical impulses and specialised chemical synapses, enabling responses in other neurons, muscle, or glandular tissue. This article covers the mechanics of action potentials, synaptic transmission, and post-synaptic receptor types — foundational concepts in neurology, pharmacology, and clinical neuroscience.

What is Neurotransmission?

  • Definition: Communication between a neuron and another cell (neuron, muscle, organ, etc.)
  • Occurs at: The synapse
  • Synapse Components:
    • Pre-synaptic neuron = sends signal
    • Synaptic cleft = gap between neurons
    • Post-synaptic cell = receives signal
    • Synaptic potential = depolarising trigger for neurotransmitter release

Synapse Types

Neurons can synapse in different anatomical locations for varying effects:

  1. Axo-dendritic: Axon → dendrite (most common; integrates multiple inputs)
  2. Axo-somatic: Axon → soma (modulatory influence)
  3. Axo-axonic: Axon → axon (controls neurotransmitter release)
neuronal physiology - types of synapses
Creative Commons: ‘Chemical Synapse Types’ by Casey Henley; https://openbooks.lib.msu.edu/neuroscience/chapter/synapse-structure/

Action Potential: Ionic Basis

Key Ions:

  • Na⁺ (influx): Initiates depolarisation
  • K⁺ (efflux): Restores resting potential (repolarisation + hyperpolarisation)
  • Ca²⁺ (influx): Triggers vesicle exocytosis at synapse

Phases of Action Potential:

  1. Resting State: All gated channels closed
  2. Threshold: Na⁺ channels open → depolarisation
  3. Peak: Na⁺ channels inactivate, K⁺ channels open
  4. Repolarisation: K⁺ exits → returns toward baseline
  5. Hyperpolarisation: Continued K⁺ efflux
  6. Resetting: K⁺ channels close; Na⁺ channels reset
neuronal physiology - action potentials
Güler and Linaro et al Model in an Investigation of the Neuronal Dynamics using noise Comparative Study – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Phases-of-action-potential-12_fig2_334605708 [accessed 17 Jan, 2022]

Refractory Periods

  • Absolute: No new AP possible (Na⁺ inactivated)
  • Relative: New AP possible only with larger stimulus

Conduction Speed – What Affects It?

  1. Axon diameter: Larger = faster conduction
  2. Myelination: Saltatory conduction at nodes of Ranvier accelerates signal transmission

Phases of Synaptic Transmission

  1. Arrival of AP → voltage-gated Ca²⁺ channels open at axon terminal
  2. Ca²⁺ influx → triggers neurotransmitter vesicle exocytosis
  3. Neurotransmitter release (ACh, glutamate, GABA, etc.)
  4. NT diffuses across cleft and binds post-synaptic receptors
  5. Graded potential generated:
    • Excitatory (EPSP): Depolarisation
    • Inhibitory (IPSP): Hyperpolarisation
  6. Signal summation: If threshold met at axon hillock → new AP
  7. Termination: NT degraded (e.g. AChE) or reabsorbed into pre-synaptic neuron
Phases of Neurotransmission
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
neuronal physiology - graded potentials
Characterizing Neurotransmitter Receptor Activation with a Perturbation Based Decomposition Method – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Different-cases-of-IPSP-and-EPSP-summation-12_fig4_308369018 [accessed 17 Jan, 2022]

Types of Post-Synaptic Receptors

1. Ionotropic Receptors (Ligand-Gated Ion Channels)

  • Fast, direct action
  • Binding of NT → opens ion channel → alters membrane potential
  • Excitatory: Na⁺ / Ca²⁺ influx → EPSP
  • Inhibitory: Cl⁻ influx or K⁺ efflux → IPSP

2. Metabotropic Receptors (G-Protein Coupled Receptors)

  • Slower, indirect
  • NT binding → G-protein activation → 2nd messengers (e.g. cAMP)
  • Alters ion channels, gene expression, metabolism
  • Used in modulatory pathways (e.g. dopamine, serotonin)
Ionotropic vs metabotropic receptors
https://ib.bioninja.com.au/options/option-a-neurobiology-and/a5-neuropharmacology/neuromodulators.html

Functional Outcomes of Neurotransmission

  • Direct Response:
    • NMJ → muscle contraction
    • Sympathetic input → ↑ heart rate
  • Sensory Relay:
    • Peripheral neuron → spinal cord → thalamus → cortex
  • Modulation:
    • One neuron alters strength/speed of another’s signal

Summary – Neuronal Physiology

Neuronal physiology is driven by precise control of ion flux, membrane potentials, and neurotransmitter signalling. Action potentials and synapses allow rapid, targeted communication across the nervous system. The diversity of receptor types and synapse configurations enables both fast responses and complex regulation. For related anatomical organisation, see our Nervous System page.

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