CH 16 Sensory Receptors and Their Function

• Sensory Receptors- Provide information about both external and internal environments, acting as the first line of contact in the perceptual process.

  • Respond to various stimuli, including light, sound, chemical changes, temperature, and pressure, ensuring that organisms can interact effectively with their environment.

  • Each receptor type is specialized; for instance, photoreceptors in the eyes respond best to light energy, while mechanoreceptors in the skin respond to touch or pressure.

  • Transducers: Sensory receptors act as transducers by converting different forms of stimulus energy (e.g., chemical, physical, thermal) into electrical energy (action potentials), which can be interpreted by the nervous system.

  • Have a resting membrane potential established by the differential distribution of ions, with modality-gated channels that trigger action potentials when stimulated appropriately.

  • Action potentials are generated and transmitted to the Central Nervous System (CNS) for processing and interpretation, forming the basis for perception.

General Structure of Sensory Receptors

• Signal Conveyance: Signals from sensory receptors are transmitted to the CNS via sensory neurons, which are specialized for fast and effective communication of sensory information.

• Receptive Field: The receptive field is the specific area where sensory neuron endings are distributed. Smaller receptive fields enable a higher spatial resolution, allowing for more precise localization of stimuli.

Sensory Information Provided by Sensory Receptors

• Sensation: Refers to a stimulus that we become consciously aware of; in order for a sensation to be perceived, the corresponding signals must reach the cerebral cortex, where conscious awareness occurs.

• Not all stimuli lead to sensations; many signals are processed subconsciously, such as blood pressure regulation signals that affect homeostasis without conscious awareness.

• Modality: This term relates to the type of stimulus and is characterized by a "labeled line" approach where the brain interprets signals from various sensory nerves as distinct sensations (e.g., pressure perception via touch receptors versus visual perception via optic nerve signals).

  • Location Determination: The precise location of a stimulus is determined by which receptive fields are active, as depicted in the body map located in the postcentral gyrus of the brain.

  • Intensity Measurement: The intensity of a stimulus is conveyed through the frequency of nerve signals sent to the CNS; stronger stimuli prompt a greater number of neurons to fire at increased frequencies, enabling the brain to gauge the magnitude of the stimulus.

  • Duration Measurement: Receptors adapt to continuous stimuli over time through processes of adaptation.

    • Tonic Receptors: These receptors exhibit limited adaptation and remain active, continuously signaling often critical information such as pain, maintaining awareness of potential harm.

    • Phasic Receptors: These receptors adapt quickly, only responding to new or changing stimuli, such as the sensation of pressure when a new object is placed on the skin, then diminishing their signal once the pressure becomes constant.

Sensory Receptor Classification

• Classification Criteria:

  • Distribution:

    • General sense receptors: These are widely distributed throughout the body and include:

      • Somatic sensory receptors: Tactile receptors located in the skin that respond to touch, and proprioceptors situated in muscles, joints, and tendons that give feedback on body position.

      • Visceral sensory receptors: These receptors monitor internal environments, including the stretching of organs and changes in chemical conditions, contributing to the body's homeostatic balance.

    • Special sense receptors: These complex organs are specialized for certain senses including olfaction (smell), gustation (taste), vision (sight), audition (hearing), and equilibrium (balance).

  • Stimulus Origin:

    • Exteroceptors: Detect external stimuli that arise from the environment, found on the skin and mucous membranes.

    • Interoceptors: Respond to internal stimuli, primarily monitoring the conditions within internal organs through visceral sensory receptors.

    • Proprioceptors: Provide information regarding body and limb positioning, crucial for coordinated movement and balance.

  • Modality of Stimulus:

    • Types of receptors include chemoreceptors, thermoreceptors, photoreceptors, mechanoreceptors, and nociceptors.

    • Chemoreceptors: Sensitive to chemical stimuli, they are essential in the processes of taste and smell detection.

    • Thermoreceptors: These receptors respond to temperature changes, allowing the body to maintain thermal homeostasis.

    • Photoreceptors: Specialized to detect light changes, primarily located in the retina of the eye, crucial for vision.