Study Notes on Sensory Receptors and Properties

16.1a General Properties of Receptors

  • The fundamental purpose of any sensory receptor is transduction.

    • Transduction: The conversion of one form of energy to another—light, sound, heat, touch, vibration, or other forms of stimulus energy into nerve signals.

    • Any device that converts one energy form to another is a transducer (e.g., sense organ, gasoline engine, lightbulb).

  • Two Stages in Sensory Process:

    1. Sensation:

    • Sensory receptors detect environmental stimuli and generate a small local electrical change called a receptor potential.

    • In senses like touch and smell, the sensory cell is a neuron. If the receptor potential is sufficiently strong, the neuron fires action potentials, creating a nerve signal to the CNS.

    • For senses like taste and hearing, the sensory cell is an epithelial cell (not a neuron), but it releases neurotransmitters to stimulate adjacent neurons, which in turn send signals to the CNS.

    1. Perception:

    • Occurs after sensory signals reach the brain, leading to conscious experience and interpretation of stimuli.

    • Not all sensations result in perception; many signals are filtered out in the brainstem before reaching the cerebral cortex, preventing overload from unimportant stimuli.

    • Some nerve signals relate to functions that do not require conscious awareness, such as monitoring blood pH and body temperature.

16.1b Transmission of Sensory Information

  • Sensory receptors transmit four types of information:

    1. Modality:

    • Refers to the type of stimulus or corresponding perception (e.g., vision, hearing, taste).

    • Different modalities are identified by the specific pathways in the brain where the sensory signals end.

    • Example: If auditory signals were rerouted to the visual cortex, they could be perceived as light rather than sound.

    1. Location:

    • Encoded by specific neurons sending signals to the brain.

    • Each sensory neuron detects stimuli within an area known as its receptive field.

    • For example, a sensory neuron in the sense of touch may cover an area of skin up to 7 cm in diameter, leading to ambiguity in localization.

    • In parts of the body that require fine discrimination (like fingertips), the receptive field may be as small as 1 mm in diameter, enabling better localization of touch.

    1. Intensity:

    • Indicates the strength of the stimulus (e.g., loud vs. soft sounds, bright vs. dim lights).

    • Encoded in three ways:
      a. As stimulus intensity increases, the firing frequency of sensory nerve fibers also increases.
      b. More intense stimuli recruit a greater number of neurons to fire.
      c. Weak stimuli only activate the most sensitive neurons, while stronger stimuli can activate less sensitive neurons with higher thresholds.

    1. Duration:

    • Encoded by changes in the firing frequency over time, with receptors exhibiting sensory adaptation.

    • Sensory adaptation: Prolonged stimulus results in slower firing rates and reduced perception of the stimulus.

    • Phasic receptors: Generate bursts of action potentials then adapt quickly to stimuli (e.g., smell, touch).

    • Tonic receptors: Adapt slowly and respond consistently (e.g., proprioceptors, pain receptors) to maintain awareness.

Receptive Field Examples

  • Diagram illustrating receptive fields:

    1. A neuron with a large receptive field (7 cm diameter, e.g., back skin) responds to touches within this field as one point of contact.

    2. Neurons with smaller receptive fields (1 mm diameter, e.g., fingertips) can differentiate closer touch points (2 mm apart as separate touches).

Classification of Receptors

  • By Modality of Stimulus:

    1. Chemoreceptors: Respond to chemicals (odors, tastes, body fluid composition).

    2. Mechanoreceptors: Respond to physical deformation caused by touch, pressure, stretch, vibration (e.g., hearing and balance organs, skin receptors).

    3. Thermoreceptors: Respond to heat and cold (nerve endings in the skin).

    4. Photoreceptors: Respond to light (found in the eyes).

    5. Nociceptors: Pain receptors that respond to tissue injury or damage threats; abundant in skin and mucous membranes.

  • By Origin of Stimulus:

    1. Exteroceptors: Sense external stimuli (vision, hearing, taste, smell, cutaneous senses).

    2. Interoceptors: Detect internal stimuli (organs; sensations of stretch, pressure, visceral pain).

    3. Proprioceptors: Sense body position and movements, located in muscles, tendons, and joint capsules.

  • By Distribution in Body:

    1. General (somatosensory) senses: Use widely distributed receptors (skin, muscles, tendons, etc.) for sensations like touch, pressure, pain.

    2. Special senses: Limited to the head, innervated by cranial nerves, and involve complex sense organs (vision, hearing, taste, smell).