Sensory Neuron Signal Transduction: From Stimulus to CNS

The Sensory Neuron Pathway: Stimulus → Action Potential → Neurotransmission

Sensory neurons convert physical/chemical stimuli into electrical signals transmitted to the CNS.

Stepwise Process

1. Stimulus Detection: Specialized endings (mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors) transduce specific energy into receptor potential.
2. Receptor Potential (Graded): Local depolarization proportional to stimulus intensity. Not propagated.
3. Threshold & Action Potential Initiation: If receptor potential reaches threshold at trigger zone (initial segment) → voltage-gated Na⁺ channels open → rapid depolarization (overshoot to +30 mV).
4. Propagation:
   • Unmyelinated: Continuous conduction.
   • Myelinated: Saltatory conduction at Nodes of Ranvier (fast, energy-efficient).
5. Repolarization & Hyperpolarization: Na⁺ channels inactivate; voltage-gated K⁺ open → K⁺ efflux → membrane potential restores (overshoots to -80 mV briefly).
6. Refractory Periods: Absolute (no new AP) → Relative (stronger stimulus needed). Prevents back-propagation, limits frequency.
7. Axon Terminal – Synaptic Transmission:
   • AP opens voltage-gated Ca²⁺ channels → vesicle fusion → neurotransmitter release (glutamate, substance P, CGRP, ATP).
   • Diffusion across synaptic cleft → binding to receptors on 2nd-order neuron (spinal cord dorsal horn / brainstem nuclei).
   • EPSP/IPSP generation → possible new AP in postsynaptic neuron.

Modulation

• Presynaptic inhibition (GABA, opioids, cannabinoids).
• Descending inhibitory pathways (periaqueductal gray → raphe → dorsal horn).
• Neuroplasticity (central sensitization in chronic pain).

This exquisitely orchestrated sequence enables perception, protection, and adaptation.