Chapter 43 Sensory Systems

Events Involved in Sensory Processing and Transduction

• Stimulus: The external/internal signal that initiates sensory reception.

• Receptors: Specialized cells that detect specific modalities (e.g., light, sound).

• Reception: The process of stimuli binding to receptors leading to stimulation.

• Energy Transduction: Conversion of physical energy into electrical signals at the receptor level.

Post-Transduction Terms:

• Transmission: Sending the sensory information to the central nervous system (afferent pathway).

• Projection: Directing the sensory information to specific brain areas for processing.

• Interpretation and Perception: The brain's integration of sensory information based on previous experiences, leading to conscious awareness.

Location of Sensory Receptors Relative to Nerve:

• Sensory receptors can be either separate cells or part of nerve endings offering varying locations. Exteroceptors respond to external stimuli, while interoceptors respond to internal stimuli.

Graded Potential Examples:

• Generator Potentials: Changes in the resting potential of sensory neurons due to stimulus detection.

• Receptor Potentials: Changes occurring in separate receptor cells leading to transmitter release to neurons.

Sensory Receptors by Location and Modality:

• Visceral Receptors: Respond to internal organ stimuli (e.g. baroreceptors).

• Somatic Receptors: External stimuli (e.g. touch receptors in the skin).

• Proprioceptors: Body position (e.g. muscle spindle fibers).

• Examples of Modalities:

  • Mechanoreceptors: Mechanical changes (e.g. touch).

  • Chemoreceptors: Chemical changes (e.g. taste).

  • Photoreceptors: Light energy (e.g. vision).

Frequency Code vs Population Code:

• Frequency Code: Determines stimulus intensity by the frequency of action potentials generated. Higher frequency indicates stronger stimuli.

• Population Code: Reflects the number of activated receptors to convey stimulus strength; more active receptors suggest a stronger stimulus.

Sensory Adaptation and Receptor Types:

• Sensory Adaptation: Decreased response of receptors to sustained stimuli over time.

• Tonic Receptors: Continuous response; slow frequency decrease.

• Phasic Receptors: Quick response; responds mainly to changes (start and end of stimulus).

• Graph: Show decreasing response of tonic vs. phasic over time.

Mechanoreceptors and Comparisons:

• Free Nerve Endings: Unspecialized, detect pain and temperature.

• Unencapsulated Receptors: Detect light touch.

• Encapsulated Receptors: Enhanced sensitivity (e.g. Meissner's corpuscles for tactile sensation).

• Ruffini Corpuscles: Sense skin stretch.

• Pacinian Corpuscles: Respond to deep pressure and vibration.

• Statocysts: Balance in invertebrates.

• Hair Cells: In vertebrates' inner ear for hearing and balance.

• Lateral Line System: Detects water movement in aquatic animals.

Human Ear Structures and Functions:

• Outer Ear: Captures sound waves (pinna).

• Middle Ear: Amplifies sound through the ossicles (malleus, incus, stapes).

• Inner Ear: Houses the cochlea (sound detection) and vestibular system (equilibrium).

• Static vs Dynamic Equilibrium: Static detects position, and dynamic detects motion.

• Sound Waves Path: Through the ear canal -> tympanic membrane -> ossicles -> cochlea.

• Diagram: Label cochlea and organ of Corti structures.

Pitch Detection:

• Detected by the location of maximum vibration along the cochlea.

• Differences in Response: Loudness correlates with amplitude of sound waves, while pitch corresponds to their frequency.

Gustation Process:

• Taste is processed via taste buds containing receptor cells for sweet, sour, salty, bitter, and umami flavors.

• Cranial Nerves: Facial nerve (VII), glossopharyngeal (IX), vagus nerve (X) facilitate taste sensation.

Olfaction Process:

• Involves olfactory receptors responding to odor molecules.

• Cranial Nerve I: Transmits signals to the brain, enabling smell localization.

Comparative Eye Structures:

• Eyespots: Basic light detection.

• Compound Eyes: Composed of many ommatidia, found in insects.

• Camera Eyes: Focus light with a single lens, typical in vertebrates.

• Direct vs Indirect Eyes: Direct – light focused on retina; indirect – multi-step focusing involving multiple structures.

Structures of the Human Eye:

• Cornea: Light entry.

• Lens: Focusing light.

• Retina: Contains rods for low light and cones for color.

• Accommodation: Changes in lens shape for focus.

• Rod Responses: Adaptation changes at low and high light levels.

• Cone Modalities: Red, green, blue color detection.

Monocular vs Binocular Vision:

• Monocular: Wider field but poor depth perception.

• Binocular: Better depth perception.

• Optic Pathways: Distinguish optic nerve (carries signals from the retina), optic tract (transmits visual information from the chiasma), and optic chiasma (crossing over point of optic nerves).