Chapter 10: Sensory Physiology

Overview

This chapter explores the fundamental properties and mechanisms of sensory systems in human physiology, focusing on how sensory information is detected, processed, and interpreted by the nervous system. Topics include general sensory system properties, somatic senses, chemoreception (smell and taste), and pain pathways.

General Properties of Sensory Systems

Introduction to Sensory Systems

• Special senses: vision, hearing, taste, smell, equilibrium.

• Somatic senses: touch, temperature, pain, itch, proprioception.

• Stimulus: a physical or chemical signal detected by sensory receptors.

• Receptor: a cell or structure that detects stimuli and initiates a response.

• Integration: sensory input is processed and interpreted by the central nervous system (CNS).

• Conscious perception: occurs when sensory information reaches the cerebral cortex.

Types of Sensory Receptors

Classification and Examples

Sensory receptors are specialized to detect specific forms of energy or stimuli.

Sensory Transduction

Conversion of Stimuli into Electrical Signals

• Transduction: the process by which sensory receptors convert stimuli into graded potentials.

• Graded potentials may lead to action potentials if threshold is reached.

• Adequate stimulus: the specific type of stimulus to which a receptor is most sensitive.

• Stimulus intensity is encoded by the frequency of action potentials.

Receptive Fields

Spatial Organization of Sensory Neurons

• Receptive field: the region of sensory space in which a stimulus will modify the firing of a particular neuron.

• Smaller receptive fields allow for greater spatial resolution.

• Convergence of sensory neurons can lead to larger receptive fields and decreased sensitivity.

Central Processing of Sensory Information

Integration in the CNS

• Sensory information is routed to the brain via ascending pathways.

• Most sensory pathways project to the thalamus, which acts as a relay station.

• Perceptual threshold: the level of stimulus intensity required for conscious awareness.

• Habituation: decreased perception through inhibitory modulation.

Stimulus Properties and Coding

Modality, Location, Intensity, and Duration

• Modality: type of stimulus (e.g., light, sound, pressure).

• Location: determined by which sensory neurons are activated and their projection in the brain.

• Intensity: encoded by the frequency of action potentials ().

• Duration: encoded by the duration of action potential firing.

• Lateral inhibition: enhances contrast and spatial resolution by inhibiting neighboring neurons.

Receptor Adaptation

Response to Sustained Stimuli

• Phasic receptors: rapidly adapt to a constant stimulus and stop responding.

• Tonic receptors: adapt slowly and continue to respond as long as the stimulus is present.

Somatic Senses

Types and Pathways

• Includes touch, proprioception, temperature, pain, and itch.

• Somatosensory pathways project to the cortex and thalamus.

• Primary sensory neurons synapse with secondary sensory neurons in the CNS.

Touch and Skin Receptors

Types of Stimuli and Receptors

• Touch receptors detect physical contact and movement.

• Types include free nerve endings, Merkel receptors, Meissner's corpuscles, Pacinian corpuscles, and Ruffini endings.

• Temperature receptors are free nerve endings sensitive to changes in skin temperature.

Classes of Somatosensory Nerve Fibers

Pain and Nociception

Nociceptors and Protective Responses

• Nociceptors: free nerve endings that respond to strong stimuli that may cause tissue damage.

• Found in skin, joints, muscles, bones, and viscera.

• Initiate protective responses such as withdrawal reflexes.

• Types of fibers: Aδ (fast pain), C (slow pain).

Pain Pathways and Modulation

• Pain is a subjective perception influenced by emotional and cognitive factors.

• Referred pain: pain perceived at a location other than the site of origin.

• Gate control theory: non-painful input closes the "gates" to painful input, modulating pain perception.

• Analgesic pathways involve endogenous opioids (e.g., endorphins, enkephalins).

Chemoreception: Smell and Taste

Olfactory System

• Olfaction is one of the oldest senses.

• Olfactory epithelium contains sensory neurons that detect odorants.

• Olfactory pathways project to the olfactory cortex and limbic system.

Taste Sensations

• Taste is a combination of five basic sensations: sweet, sour, salty, bitter, umami.

• Taste receptor cells are located in taste buds on the tongue.

• Type II cells detect sweet, bitter, and umami; Type III cells detect sour.

Additional info: Figures referenced in the notes (e.g., sensory pathways, receptor adaptation, referred pain) provide visual representations of the described mechanisms and anatomical structures. For deeper understanding, students should review textbook diagrams and illustrations.