4.1 Sensory receptor types and transduction mechanisms

Sensory receptors are specialized cells that detect and respond to various stimuli. From mechanoreceptors sensing touch to photoreceptors capturing light, these cells form the foundation of our sensory experiences. Understanding their types and functions is key to grasping how we perceive the world around us.

Sensory transduction is the process of converting stimuli into electrical signals. This involves ion channels, receptor potentials, and action potentials. Adaptation mechanisms allow our senses to adjust to constant stimuli, ensuring we remain sensitive to changes in our environment.

Sensory Receptor Types

Mechanoreceptors and Thermoreceptors

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• Mechanoreceptors detect physical stimuli such as pressure, touch, vibration, and sound waves
  • Includes hair cells in the inner ear that detect sound waves and motion (vestibular system)
  • Pacinian corpuscles detect deep pressure and vibration (mesentery)
  • Merkel cells detect light touch and texture (fingertips)
• Thermoreceptors detect changes in temperature
  • Cold receptors increase firing rate when temperature drops below normal body temperature
  • Warm receptors increase firing rate when temperature rises above normal body temperature
  • Located in the skin, hypothalamus, and other internal organs

Chemoreceptors and Photoreceptors

• Chemoreceptors detect chemical stimuli such as taste and smell
  • Taste buds contain gustatory receptor cells that detect sweet, salty, sour, bitter, and umami (savory) tastes
  • Olfactory receptor neurons in the nasal cavity detect a wide variety of odorants
  • Central chemoreceptors in the brainstem detect changes in blood pH and CO2 levels
• Photoreceptors detect light energy and transduce it into electrical signals
  • Rods are highly sensitive to light and enable vision in dim light conditions
  • Cones are less sensitive to light but enable color vision and visual acuity
  • Located in the retina of the eye

Nociceptors

• Nociceptors detect potentially damaging stimuli and give rise to the sensation of pain
  • Respond to mechanical, thermal, and chemical stimuli that may cause tissue damage
  • Have high thresholds and continue to fire in the presence of sustained stimuli (slow adaptation)
  • Classified as Aδ fibers (fast, sharp pain) and C fibers (slow, dull pain)
  • Distributed throughout the skin, muscles, joints, and internal organs

Sensory Transduction Mechanisms

Sensory Transduction and Receptor Potential

• Sensory transduction is the process by which sensory receptors convert stimuli into electrical signals (receptor potentials)
  • Involves the opening or closing of ion channels in the receptor cell membrane
  • Leads to a graded change in the membrane potential called the receptor potential
  • The amplitude and duration of the receptor potential depend on the intensity and duration of the stimulus
• Receptor potential is a graded, localized change in the membrane potential of a sensory receptor cell
  • Can be depolarizing (excitatory) or hyperpolarizing (inhibitory) depending on the type of receptor and stimulus
  • Triggers action potentials in the sensory neuron if the depolarization reaches the threshold

Ion Channels and Action Potentials

• Ion channels are pores in the cell membrane that allow specific ions to pass through
  • Gated by specific stimuli (ligand-gated, voltage-gated, or mechanically-gated)
  • Opening or closing of ion channels leads to changes in the membrane potential
  • Examples include the cyclic nucleotide-gated (CNG) channels in photoreceptors and the transient receptor potential (TRP) channels in nociceptors
• Action potentials are all-or-none electrical signals that propagate along the axon of a neuron
  • Generated when the receptor potential reaches the threshold for activation of voltage-gated sodium channels
  • Enables long-distance transmission of sensory information to the central nervous system
  • Frequency of action potentials encodes the intensity of the stimulus

Adaptation

• Adaptation is the decrease in responsiveness of a sensory receptor to a constant stimulus over time
  • Allows the receptor to remain sensitive to changes in the stimulus rather than absolute levels
  • Phasic receptors adapt quickly and respond mainly to changes in the stimulus (Pacinian corpuscles)
  • Tonic receptors adapt slowly and continue to respond to sustained stimuli (nociceptors)
  • Involves mechanisms such as inactivation of ion channels, depletion of neurotransmitters, and inhibitory feedback from the central nervous system