sensory quiz

Sensory Physiology - Touch, Taste, and Smell

Process of Sensation

• Sensation: the conscious or subconscious awareness of changes in the external or internal environment
• Perception: the conscious interpretation of sensations, primarily a function of the cerebral cortex
• Each unique type of sensation (e.g., touch, pain, vision, or hearing) is called a sensory modality
• Two types of sensory modalities: general senses (somatic and visceral) and special senses (smell, taste, vision, hearing, and equilibrium)

Sensory Receptors

• Classified based on microscopic structure, location, and type of stimulus detected
• Types of sensory receptors:
  • Free nerve endings (detect pain, temperature, touch, and itch sensations)
  • Encapsulated nerve endings (detect pressure, vibration, and some touch sensations)
  • Separate cells (specialized cells that synapse with sensory neurons, e.g., olfactory sensory neurons)

Location of Receptors

• Exteroceptors: detect external stimuli, providing information about the external environment (e.g., hearing, vision, smell, taste, touch, pressure, vibration, temperature, and pain)
• Interoceptors or visceroceptors: detect internal stimuli, monitoring conditions within the body (e.g., blood pressure, stretch, chemicals, nausea, hunger, and temperature)
• Proprioceptors: detect changes in muscle length, tension, and joint position

Type of Stimulus Detected

• Mechanical energy (sound waves, pressure, vibration, and proprioception)
• Electromagnetic energy (light and heat)
• Chemical energy (taste, smell, and chemoreceptors)

Adaptation

• A characteristic of most sensory receptors, where the receptor potential decreases in amplitude during a maintained stimulus
• Causes the frequency of nerve impulses in the sensory neuron to decrease
• Perception of a sensation may fade or disappear even though the stimulus persists

Somatic Sensations

• Arise from stimulation of sensory receptors embedded in the skin or subcutaneous tissue
• Include tactile sensations (touch, pressure, vibration, itch, and tickle) and proprioceptive sensations (position, movement, and tension)

Tactile Sensations

• Include touch, pressure, vibration, itch, and tickle
• Mediated by different types of encapsulated mechanoreceptors (e.g., Meissner corpuscles, Pacinian corpuscles, and Merkel discs) and free nerve endings
• Adaptation rates vary among receptors (rapidly adapting, slowly adapting, and non-adapting)

Proprioception

• Allows recognition of body parts and their position in space
• Includes kinesthesia (perception of body movements) and weight discrimination
• Mediated by proprioceptors embedded in muscles, tendons, and joints

Olfactory System (Smell)

• Detects chemical molecules in the air
• Olfactory epithelium contains olfactory sensory neurons (OSNs) with olfactory receptors
• Each OSN expresses only one type of olfactory receptor
• Binding of an odorant molecule to an olfactory receptor protein activates a G-protein signaling pathway, leading to a graded potential in the OSN

Gustatory System (Taste)

• Detects chemical molecules in food and drinks

• Taste buds contain gustatory epithelial cells with gustatory microvilli (gustatory hairs) that project to the external surface through the taste pore

• Five primary tastes: salty, sour, sweet, bitter, and umami

• Complex flavors are combinations of the five primary tastes### Gustatory System

• Dendrites contact many gustatory epithelial cells in several taste buds.

• Basal epithelial cells, which are stem cells, are found at the periphery of the taste bud near the connective tissue layer and produce new cells.

Physiology of Gustation

• Chemicals that stimulate gustatory epithelial cells are known as tastants.
• Na+ enters gustatory epithelial cells via Na+ channels, causing depolarization and release of neurotransmitter.
• G proteins activate enzymes that produce the second messenger inositol trisphosphate (IP3).
• IP3 causes depolarization of the gustatory epithelial cell and release of neurotransmitter.
• H+ in sour tastants flow into gustatory epithelial cells via H+ channels, causing depolarization and release of neurotransmitter.
• Tastants dissolve in saliva and contact the plasma membranes of the gustatory microvilli, which are the sites of taste transduction.
• This results in a depolarizing receptor potential, stimulating exocytosis of neurotransmitter, and triggers graded potentials and nerve impulses in the first-order sensory neurons that synapse with gustatory receptor cells.

Gustatory Epithelial Cells

• Individual gustatory epithelial cells respond to only one type of tastant.
• Each gustatory epithelial cell is “tuned” to detect a specific primary taste.
• Each taste bud contains gustatory epithelial cells for each type of tastant, allowing all primary tastes to be detected in all parts of the tongue.
• Different tastes arise from activation of different combinations of gustatory epithelial cells.
• The threshold for bitter substances, such as quinine, is lowest (high sensitivity) and may have a protective function.
• Complete adaptation to a specific taste can occur in 1–5 minutes of continuous stimulation.