unit 21 review
Special senses allow us to perceive the world around us. This unit focuses on chemical senses: taste and smell. We'll explore how taste buds and olfactory receptors detect dissolved chemicals, transmit signals to the brain, and contribute to our perception of flavors and odors.
The gustatory and olfactory systems work together to enhance our eating experience and warn us of potential dangers. We'll examine the anatomy of taste buds and olfactory epithelium, the types of taste receptors, and how the brain processes chemical sensory information.
Key Concepts
- Sensory systems detect and process information from the environment and the body's internal state
- Five main senses: vision, hearing, taste, smell, and touch
- Sensory receptors transduce stimuli into electrical signals that are transmitted to the brain
- Sensory pathways consist of receptors, sensory neurons, and specific brain regions that process sensory information
- Chemical senses (taste and smell) detect dissolved chemicals in the mouth and nose
- Taste buds contain gustatory receptor cells that detect sweet, salty, sour, bitter, and umami tastes
- Olfactory receptors in the nasal cavity detect a wide variety of odors
- Sensory adaptation occurs when receptors become less responsive to a constant stimulus over time
Sensory System Basics
- Sensory receptors are specialized cells or neurons that detect specific types of stimuli (mechanical, chemical, or electromagnetic)
- Receptors transduce stimuli into receptor potentials, which are graded electrical signals
- Receptor potentials trigger action potentials in sensory neurons when the threshold is reached
- Sensory neurons transmit action potentials to the central nervous system (brain and spinal cord)
- Sensory information is processed and integrated in specific regions of the brain
- Perception is the conscious awareness and interpretation of sensory information
- Sensory adaptation allows organisms to maintain sensitivity to changes in stimuli while ignoring constant background stimuli
- Sensory thresholds determine the minimum intensity of a stimulus required to trigger a response
Taste: Gustatory System
- Taste buds are the sensory organs for gustation (taste) located on the tongue, soft palate, and epiglottis
- Each taste bud contains 50-100 gustatory receptor cells that detect dissolved chemicals (tastants)
- Five basic taste qualities: sweet, salty, sour, bitter, and umami (savory)
- Gustatory receptor cells are modified epithelial cells with microvilli that extend into the taste pore
- Tastants bind to specific receptors on the microvilli, triggering a receptor potential
- Receptor potentials cause neurotransmitter release, activating sensory neurons that synapse with the gustatory receptor cells
- Sensory information is transmitted via the facial, glossopharyngeal, and vagus nerves to the gustatory cortex in the brain
- Taste preferences and aversions can be innate or learned, and are influenced by factors such as genetics, culture, and experience
Smell: Olfactory System
- Olfactory receptors are located in the olfactory epithelium of the nasal cavity
- Olfactory receptor neurons (ORNs) are bipolar neurons with cilia that extend into the mucus layer
- Odorant molecules dissolve in the mucus and bind to specific receptors on the cilia
- Humans have approximately 400 different types of olfactory receptors, allowing detection of a wide range of odors
- Binding of odorants triggers a receptor potential in the ORNs, leading to action potential generation
- Axons of ORNs project to the olfactory bulb, forming glomeruli where they synapse with mitral and tufted cells
- Mitral and tufted cells send axons to the olfactory cortex and other brain regions for processing and perception
- Olfactory adaptation occurs rapidly, allowing detection of new odors in the environment
Chemical Receptors
- Chemical receptors detect dissolved chemicals in the environment or within the body
- Chemoreceptors are found in the gustatory and olfactory systems, as well as in various internal organs
- Taste receptors (gustatory receptor cells) and olfactory receptors (olfactory receptor neurons) are examples of chemoreceptors
- Internal chemoreceptors monitor blood pH, carbon dioxide levels, and oxygen levels to regulate breathing and maintain homeostasis
- Carotid bodies and aortic bodies are peripheral chemoreceptors that detect changes in blood gas levels and pH
- Central chemoreceptors in the medulla oblongata detect changes in cerebrospinal fluid pH
- Chemoreceptors in the hypothalamus monitor blood osmolarity and regulate thirst and water balance
- Enterochromaffin cells in the gastrointestinal tract detect chemical changes and release hormones that regulate digestion and satiety
Sensory Pathways
- Sensory pathways consist of receptors, sensory neurons, and specific brain regions that process sensory information
- Gustatory pathway: taste receptors → sensory neurons (facial, glossopharyngeal, vagus nerves) → gustatory cortex
- Olfactory pathway: olfactory receptors → olfactory bulb → olfactory cortex and other brain regions
- Somatosensory pathway: touch, pressure, temperature, and pain receptors → sensory neurons → thalamus → somatosensory cortex
- Visual pathway: photoreceptors (rods and cones) → bipolar cells → ganglion cells → thalamus → visual cortex
- Auditory pathway: hair cells in the cochlea → spiral ganglion neurons → cochlear nuclei → inferior colliculus → thalamus → auditory cortex
- Vestibular pathway: hair cells in the semicircular canals and otolith organs → vestibular ganglion neurons → vestibular nuclei → thalamus → vestibular cortex
- Sensory information is processed and integrated at multiple levels along the pathways, allowing for reflex responses and conscious perception
Clinical Applications
- Ageusia (loss of taste) and hypogeusia (reduced taste sensitivity) can result from various factors, including medication side effects, nutrient deficiencies, and nerve damage
- Anosmia (loss of smell) and hyposmia (reduced smell sensitivity) can be caused by upper respiratory infections, head trauma, neurodegenerative diseases, and certain genetic disorders
- Dysgeusia (distortion of taste) and phantosmia (phantom smells) are often associated with underlying medical conditions or medication side effects
- Burning mouth syndrome is characterized by a burning sensation in the mouth without an identifiable cause
- Sjogren's syndrome is an autoimmune disorder that affects the salivary and lacrimal glands, leading to dry mouth and dry eyes, which can impact taste and olfactory functions
- Zinc deficiency can cause taste disturbances and impaired wound healing
- Exposure to certain chemicals, such as heavy metals and pesticides, can damage taste and olfactory receptors
- Disorders affecting the central nervous system, such as Parkinson's disease, Alzheimer's disease, and multiple sclerosis, can alter taste and smell perception
Review Questions
- What are the five basic taste qualities detected by gustatory receptor cells?
- How do olfactory receptor neurons transduce odor stimuli into electrical signals?
- What is the role of the olfactory bulb in processing olfactory information?
- Describe the gustatory pathway from taste receptors to the gustatory cortex.
- What are the differences between peripheral and central chemoreceptors?
- How does sensory adaptation contribute to the detection of changes in stimuli?
- What are some common causes of ageusia and anosmia?
- Explain the concept of sensory thresholds and their importance in sensory perception.