Chapter 7: Sensory Physiology – Study Notes

Sensory Physiology

Introduction to Sensory Physiology

Sensory physiology explores how the body detects, transduces, and interprets information from the environment through specialized cells and neural pathways. This chapter covers the structure and function of sensory receptors, neural coding, and the processing of sensory information in the nervous system.

Sensory Receptors

General Organization

• Sensory System: Composed of sensory receptors, afferent nerve pathways, and brain regions that process sensory information.

• Sensation: Conscious awareness of a stimulus (e.g., feeling pain in a finger).

• Perception: Understanding the meaning of a sensation (e.g., recognizing pain is due to a cut).

• Sensory Receptors: Specialized structures at the peripheral ends of afferent neurons that generate graded potentials (receptor potentials) in response to stimuli, potentially initiating action potentials.

• Classes of Sensory Receptors: Mechanoreceptors (mechanical force), Thermoreceptors (temperature), Photoreceptors (light), Chemoreceptors (chemical changes), Nociceptors (pain).

• Adequate Stimulus: The specific type of energy to which a receptor is most sensitive, though high-intensity stimuli can activate other receptors.

Primary Sensory Coding

• Transduction: Conversion of stimulus energy into a neural signal.

• Information Coding: Conveyed by the frequency and amplitude of action potentials.

• Stimulus Characteristics: Type (modality), intensity, and location.

Stimulus Type (Modality)

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

• Each modality has specialized receptors.

Lateral Inhibition

• Lateral Inhibition: Enhances localization of a stimulus by inhibiting signals from peripheral receptors, increasing contrast between the center and periphery of a stimulated region.

• Allows precise identification of stimulus location.

Central Control of Afferent Information

• Sensory signals are extensively modified before reaching higher CNS levels.

• Modification can occur via inhibition from other neurons, descending pathways, presynaptic inhibition, or interneurons.

• Example: Pain pathways are subject to continuous inhibition, which can be increased or decreased as needed.

Neural Pathways in Sensory Systems

• Afferent Neural Pathways: Chains of three or more neurons forming ascending pathways to the CNS.

• There are specific and nonspecific ascending pathways.

• Processing continues from primary sensory areas to association areas for complex integration.

Association Cortex and Perceptual Processing

• Association cortex relies on multiple sensory inputs for processing.

• Regions close to primary sensory areas process basic functions; distant regions handle complex tasks (arousal, attention, memory, language, emotion, motivation).

• Perception is influenced by context, experience, and emotional state.

Factors Affecting Perception

• Adaptation of receptors and pathway processing

• Emotions, personality, experience

• Not all stimuli reach conscious awareness (e.g., blood pressure receptors)

• Lack of receptors for certain stimuli (e.g., radio waves)

• Damaged neural pathways, drugs, and mental illness (e.g., schizophrenia) can alter perception

Somatic Sensation

Somatic Receptors and Sensations

Somatic sensation arises from skin, muscles, bones, tendons, and joints, mediated by various somatic receptors.

• Touch and pressure

• Proprioception (body position and movement)

• Temperature

• Pain

• Itch

Types of Somatic Receptors

• Meissner's corpuscle: Rapidly adapting mechanoreceptor for touch and pressure.

• Merkel's corpuscle: Slowly adapting mechanoreceptor for touch and pressure.

• Free neuron ending: Slowly adapting; includes nociceptors, itch receptors, thermoreceptors, and mechanoreceptors.

• Pacinian corpuscle: Rapidly adapting mechanoreceptor for vibration and deep pressure.

• Ruffini corpuscle: Slowly adapting mechanoreceptor for skin stretch.

Pain

• Referred Pain: Pain perceived at a site other than the origin due to convergence of visceral and somatic afferent neurons.

• Hyperalgesia: Increased sensitivity to pain, often after injury (e.g., burns).

• Pain perception can be influenced by experience, suggestion, emotions, and other sensory inputs.

Pain Management

• Analgesia: Selective suppression of pain without affecting consciousness or other sensations.

• Methods include electrical stimulation, NSAIDs, opioids, endogenous opioid release, acupuncture, and transcutaneous electrical nerve stimulation (TENS).

Itch

• Itch is a distinct sensation with mechanisms overlapping but separate from pain pathways.

• Can be acute (e.g., mosquito bite) or chronic (e.g., eczema).

Vision

Visual Perception

• Requires the eye (focuses and detects light) and neural pathways (interpret signals).

Neural Pathways of Vision

• Light is converted to action potentials via photoreceptors, bipolar cells, and ganglion cells.

• Photoreceptors and bipolar cells generate graded potentials; ganglion cells initiate action potentials.

• ON- and OFF-pathways in the retina enhance contrast and image resolution.

• Some ganglion cells project to the suprachiasmatic nucleus (biological clock), thalamus, and midbrain for various visual functions.

Color Vision

• Color perception depends on the wavelength of light reflected, absorbed, or transmitted by objects.

• Three types of cones: L (red), M (green), S (blue).

• Color discrimination is best in bright light; rods function in dim light but do not distinguish color.

Color Blindness

• Most common is red-green color blindness (X-linked, affects mainly men).

• Results from mutations in cone pigment genes.

Eye Movements

• Six skeletal muscles control eye movement: saccades (fast, jerky) and slow tracking movements.

• Slow movements are involved in tracking objects and compensating for head movement (vestibular input).

Common Eye Diseases

• Cataract: Clouding of the lens due to protein accumulation; common after age 65.

• Glaucoma: Increased intraocular pressure damages retinal cells; major cause of irreversible blindness.

• Macular Degeneration: Damage to the macula lutea, leading to central vision loss; age-related form (AMD) is common in the elderly.

Audition (Hearing)

Mechanisms of Hearing

• Hearing is based on the transmission of sound waves through air, detected by the external, middle, and inner ear, and processed by the brain.

• Sound requires a medium; cannot occur in a vacuum.

Neural Pathways in Hearing

• Cochlear nerve fibers synapse in the brainstem, then ascend through the thalamus to the auditory cortex in the temporal lobe.

Vestibular System

Vestibular Information and Pathways

• Vestibular information is used to control eye muscles, maintain posture and balance, and provide awareness of body position and acceleration.

• Vestibular afferent neurons transmit information through the brainstem and thalamus to the parietal lobe.

• Vestibular input is integrated with proprioceptive information.

Chemical Senses

Overview

• Chemical senses include taste (gustation) and smell (olfaction).

• Chemoreceptors detect specific chemicals in the environment.

Taste (Gustation)

• Taste buds (about 10,000) are the sense organs for taste, located in the mouth and throat.

• Taste receptor cells have microvilli to increase surface area and contain proteins for transduction.

• Basal cells at the base of taste buds replace damaged receptor cells.

• Food molecules must be dissolved in liquid to be detected.

Types of Taste Receptors

• Sweet

• Sour

• Salty

• Bitter

• Umami (savory, associated with glutamate and amino acids)

Signaling Mechanisms

• Salty: Detected by sodium influx.

• Sour: Detected by hydrogen ions blocking potassium efflux.

• Sweet: Detected by glucose binding to G-protein-coupled receptors.

• Bitter: Often associated with plant alkaloids; activates G-protein-mediated pathways.

• Umami: Detected via G-protein-coupled receptor mechanisms.

Smell (Olfaction)

• Olfaction uses chemoreceptors in the olfactory epithelium of the nasal cavity.

• Olfactory receptor neurons are bipolar and are replaced every ~2 months.

• Cilia on these neurons contain receptor proteins for odor molecules.

• Axons form the olfactory nerve (cranial nerve I).

Mechanism of Olfaction

• Odorant molecules dissolve in mucus and bind to receptors on cilia.

• Binding activates a G-protein pathway, increasing cAMP and opening cation channels, depolarizing the cell.

• Humans can identify at least 10,000 odors with about 400 receptor types.

Factors Affecting Smell

• Attentiveness, hunger, gender (women generally more sensitive), smoking (decreases sensitivity), age (sensitivity decreases), and mucosal health.

• Some individuals have anosmia (complete loss of smell) due to genetic defects.

Clinical Application: Vestibular Dysfunction

Case Study: A 65-year-old man with dizziness, especially after rapid head movements, was diagnosed with benign paroxysmal positional vertigo (BPPV), likely due to displaced otoliths in the semicircular canals. The vestibular apparatus is crucial for balance, posture, and spatial orientation by detecting head movement and position.

Summary Table: General Principles of Sensory Stimulus Processing

Principle	Description
Transduction	Conversion of stimulus energy into electrical signals
Modality	Type of stimulus detected (e.g., light, sound, pressure)
Intensity	Encoded by frequency of action potentials
Location	Determined by receptive field and lateral inhibition
Adaptation	Decrease in response to a constant stimulus
Perception	Interpretation of sensory signals by the brain
Additional info:	Table entries inferred from standard sensory physiology principles