Chapter 07 - Sensory Physiology: Mini-Textbook Study Notes

Sensory Physiology

Sensory Receptors

The sensory system is responsible for detecting and processing stimuli from the environment. Sensory receptors are specialized cells or structures that respond to specific types of stimuli and initiate neural signals.

• Sensory System: Composed of sensory receptors, 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: Located at the peripheral ends of afferent neurons; generate graded potentials called receptor potentials that can initiate action potentials.

• Classes of Sensory Receptors: Mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors, and nociceptors.

• Adequate Stimulus: The type of energy to which a receptor responds best; highly specific but can respond to other energies at high intensity.

Example: Photoreceptors in the eye respond best to light, but can also be activated by mechanical pressure at high intensity.

Primary Sensory Coding

Sensory coding is the process by which stimulus energy is converted into neural signals that convey information to the CNS.

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

• Information Conveyed: By the frequency and amplitude of electrical signals.

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

Stimulus Type (Modality)

Each sensory modality corresponds to a specific type of stimulus, such as heat, cold, pressure, sound, or light.

• Stimulus Modality: The type of input (e.g., heat, sound).

• Specialized Receptors: Each modality has a specialized receptor.

Lateral Inhibition

Lateral inhibition is a mechanism that sharpens sensory perception by inhibiting signals from the edges of a stimulus.

• Lateral Inhibition: Enhances localization of a stimulus by inhibiting peripheral signals.

• Effect: Increases contrast between center and periphery, improving acuity.

Central Control of Afferent Information

Sensory signals are extensively modified before reaching higher CNS levels.

• Modification: Inhibition from collaterals, descending pathways, presynaptic inhibition, or interneurons.

• Pain Pathways: Afferent input is continuously inhibited, allowing modulation of pain signals.

Neural Pathways in Sensory Systems

Sensory information is transmitted via ascending pathways formed by chains of neurons.

• Afferent Neural Pathways: Chains of three or more neurons connected by synapses.

• Specific and Nonspecific Pathways: Transmit sensory information to the CNS.

• Processing: Continues from primary cortical areas to association cortex for complex integration.

Association Cortex and Perceptual Processing

The association cortex integrates sensory information for perception, emotion, and motivation.

• Association Cortex: Relies on primary sensory areas for processing.

• Functions: Basic sensory processing, arousal, attention, memory, language, emotion, motivation.

• Example: Hearing a growling dog may elicit different emotional responses depending on visual confirmation.

Factors Affecting Perception

• Sensory receptor adaptation and pathway processing

• Emotions, personality, experience

• Not all stimuli produce conscious sensations (e.g., blood pressure stretch receptors)

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

• Damaged neural pathways

• Drugs and mental illness (e.g., schizophrenia)

General Principles of Sensory Stimulus Processing
Stimulus detection, coding, adaptation, and central processing
Modulation by inhibitory and excitatory pathways
Integration in association cortex
Influence of psychological and physiological factors

Somatic Sensation

Somatic Receptors

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

• Somatic Sensation: Touch, pressure, proprioception, temperature, pain, itch.

• Somatic Receptors: Mechanoreceptors, nociceptors, thermoreceptors.

Type	Adaptation	Function
Meissner's corpuscle	Rapid	Touch, pressure
Merkel's corpuscle	Slow	Touch, pressure
Free neuron ending	Slow	Pain, itch, temperature, mechanoreception
Pacinian corpuscle	Rapid	Vibration, deep pressure
Ruffini corpuscle	Slow	Skin stretch

Pain

Pain is a complex sensation influenced by physiological and psychological factors.

• Referred Pain: Pain felt at a site other than the origin (e.g., heart pain felt in left arm).

• Hyperalgesia: Increased sensitivity to pain, often after injury.

• Modulation: Past experiences, emotions, and other sensory modalities can alter pain perception.

Pain Management

Pain can be selectively suppressed without affecting consciousness or other sensations.

• Analgesia: Selective suppression of pain.

• Mechanisms: Electrical stimulation, NSAIDs, opioids, endogenous opioids, acupuncture, TENS (Transcutaneous Electrical Nerve Stimulation).

Itch

Itch is a distinct sensation with mechanisms overlapping pain pathways.

• Origin: Can result from abnormal CNS function or stimulation of skin receptors.

• Types: Acute (e.g., mosquito bite) or persistent (e.g., eczema).

Vision

Visual Perception

Vision requires the eye to focus light and neural pathways to interpret signals.

• Eye: Organ that focuses and responds to light.

• Neural Pathways: Transform visual images into graded and action potentials.

Neural Pathways of Vision

Light signals are converted into action potentials via photoreceptors, bipolar cells, and ganglion cells.

• Photoreceptors: Undergo graded responses; lack voltage-gated channels for action potentials.

• Bipolar Cells: ON-pathway cells depolarize spontaneously; OFF-pathway cells hyperpolarize spontaneously.

• Ganglion Cells: First cells to initiate action potentials.

• ON/OFF Pathways: Improve image resolution by enhancing contrast.

Color Vision

Color vision is determined by the activation of cone photoreceptors sensitive to different wavelengths.

• Cones: Three types—L (red), M (green), S (blue).

• Color Perception: Depends on wavelength and intensity of light.

• Rods: Sensitive in dim light; do not distinguish color.

Color Blindness

Color blindness results from mutations in cone pigments, most commonly affecting red-green discrimination.

• Red-Green Color Blindness: Most common, affects 1 in 12 men.

• Genetics: Recessive mutation on X chromosome.

Eye Movement

Six skeletal muscles control eye movement, enabling fast (saccades) and slow tracking.

• Saccades: Rapid, jerking movements for searching visual field.

• Slow Movements: Track moving objects and compensate for head movement.

• Vestibular System: Provides information for compensatory movements.

Common Diseases of the Eye

• Cataract: Opacity/clouding of the lens, common after age 65.

• Glaucoma: Damage to retinal cells due to increased intraocular pressure; major cause of irreversible blindness.

• Macular Degeneration: Impairment of macula lutea, loss of central vision; age-related form (AMD) common in elderly.

Audition (Hearing)

Mechanisms of Hearing

Hearing is based on the physics of sound and the physiology of the ear.

• Sound Transmission: Vibration of molecules in air, liquid, or solid.

• Ear Structure: External, middle, and inner ear; cochlea; auditory nerve.

• Hair Cells: Mechanoreceptors in cochlea transduce sound vibrations into neural signals.

Decibel Levels of Common Sounds	Effects
30 dB (whisper)	Minimal effect
60 dB (normal conversation)	Safe
90 dB (lawn mower)	Prolonged exposure may cause hearing loss
120 dB (rock concert)	Pain threshold
140 dB (jet engine)	Immediate hearing damage

Neural Pathways in Hearing

• Cochlear Nerve: Synapses with interneurons in the brainstem.

• Pathway: Information transmitted through thalamus to auditory cortex.

Vestibular Information and Pathways

Vestibular System

The vestibular system provides information for balance, posture, and spatial orientation.

• Functions: Control eye muscles, maintain posture, provide awareness of position and acceleration.

• Vestibular Nerve Fibers: Transmit information to vestibular centers in parietal lobe.

• Integration: Vestibular information is integrated with proprioceptive input.

Chemical Senses

Taste (Gustation)

Taste is mediated by chemoreceptors in taste buds located in the mouth and throat.

• Taste Buds: About 10,000, arranged around a taste pore.

• Microvilli: Increase surface area, contain proteins for transduction.

• Basal Cells: Replace damaged taste receptor cells.

• Requirement: Food molecules must be dissolved in liquid to contact taste receptors.

Taste Type	Detection Mechanism
Sweet	Glucose binding to G-protein-coupled receptors
Sour	Hydrogen ions blocking potassium efflux
Salty	Sodium influx
Bitter	G-protein mediated second-messenger pathways
Umami	G-protein-coupled receptor mechanism (glutamate)

Smell (Olfaction)

Smell is a chemical sense using chemoreceptors in the olfactory epithelium.

• Olfactory Receptor Neurons: Located in upper nasal cavity; bipolar neurons replaced every two months.

• Cilia: Contain receptor proteins for odor molecules.

• Olfactory Nerve: Cranial nerve I; axons form the nerve.

Sense of Smell

• Detection: Odorant molecules diffuse, dissolve in mucus, bind to receptors.

• Transduction: G-protein pathway increases cAMP, opens cation channels, depolarizes cell.

• Discrimination: Humans can identify at least 10,000 odorants with about 400 receptor types.

Factors Affecting Sense of Smell

• Attentiveness

• Hunger (greater sensitivity when hungry)

• Gender (women generally have keener sensitivity)

• Smoking (decreases sensitivity)

• Age (decreases with age)

• State of olfactory mucosa (congestion reduces sensitivity)

• Genetic defects (anosmia: inability to smell)

Clinical Case Study: Benign Paroxysmal Positional Vertigo (BPPV)

BPPV is a disorder of the vestibular apparatus, often caused by loose otoliths disrupting fluid movement in semicircular canals. The vestibular apparatus is essential for maintaining balance, posture, and spatial orientation by detecting head movements and acceleration.

• Symptoms: Dizziness, headaches, nausea, vomiting, triggered by head movement.

• Diagnosis: Based on symptoms and exclusion of other causes.

• Role of Vestibular Apparatus: Maintains balance by detecting changes in head position and movement.

Additional info: Academic context was added to clarify mechanisms, receptor types, and clinical relevance. Definitions and examples were expanded for completeness.