Chapter 07 - Sensory Physiology: Comprehensive Study Notes

Sensory Physiology

Sensory Receptors

The sensory system is responsible for detecting and processing stimuli from the environment, enabling perception and interpretation. Sensory receptors are specialized structures at the peripheral ends of afferent neurons that respond to specific types of stimuli.

• 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: Generate graded potentials called receptor potentials in response to stimuli, which 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; specificity is high but can respond to other energies at high intensity.

Example: Photoreceptors in the eye respond best to light but may respond to mechanical pressure if intense enough.

Primary Sensory Coding

Primary sensory coding refers to the conversion of stimulus energy into signals that convey relevant sensory information to the CNS.

• Coding: Conversion of stimulus energy into neural signals.

• Information Conveyance: By frequency and amplitude of electrical signals.

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

Stimulus Type (Modality)

Stimulus modality refers to the type of input, such as heat, cold, pressure, sound, or light. Each modality has specialized receptors.

• Modality: Type of stimulus (e.g., heat, pressure).

• Specialized Receptors: Each modality is detected by a specific receptor.

Lateral Inhibition

Lateral inhibition is a process that sharpens sensory perception by inhibiting signals from the edges of a stimulus, enhancing contrast and localization.

• Lateral Inhibition: Enables precise localization of stimulus.

• Edge Inhibition: Afferent neurons at the edge are strongly inhibited compared to those at the center.

• Contrast Enhancement: Increases ability to localize sensory input.

Central Control of Afferent Information

Sensory signals are extensively modified before reaching higher CNS levels, through inhibition and descending pathways.

• 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

Afferent pathways are formed by chains of neurons connected by synapses, transmitting sensory information to the CNS.

• Afferent Pathways: Chains of three or more neurons.

• Specific and Nonspecific Pathways: Transmit sensory information.

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

Association Cortex and Perceptual Processing

The association cortex processes sensory information, contributing to arousal, attention, memory, language, emotion, and motivation.

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

• Complex Integration: Regions farther from primary areas process information with more complexity.

• 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 sensation (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)

Somatic Sensation

Somatic Receptors and Sensations

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

• Touch and pressure

• Awareness of 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, mechanoreceptors.

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

• Ruffini corpuscle: Slowly adapting mechanoreceptor for skin stretch.

Pain

Pain is a complex sensation influenced by multiple factors and can be referred or sensitized.

• 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: Pain can be altered by experience, emotions, and activation of other sensory modalities.

Pain Management

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

• Electrical stimulation of CNS areas

• Pharmacological agents (NSAIDs, opioids)

• Endogenous opioids

• Acupuncture (linked to endogenous opioid pathways)

• Transcutaneous electrical nerve stimulation (TENS)

Itch

Itch is a distinct sensation with mechanisms overlapping pain pathways.

• Can originate from abnormal CNS function or skin receptor stimulation

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

Vision

Visual Perception

Visual perception requires the eye to focus images 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 through photoreceptors, bipolar cells, and ganglion cells.

• Photoreceptors and Bipolar Cells: Undergo graded responses.

• Ganglion Cells: First to initiate action potentials.

• ON- and OFF-pathways: Photoreceptors depolarized in absence of light; glutamate released onto bipolar cells.

ON-Pathway

• Bipolar cells spontaneously depolarize without input.

• Glutamate receptors are inhibitory.

• Light decreases glutamate, depolarizes ON-bipolar cells, increases action potentials in ganglion cells.

OFF-Pathway

• Bipolar cells hyperpolarize without input.

• Glutamate receptors are excitatory.

• Action potentials generated in absence of light; inhibited when light strikes photoreceptors.

Coexistence of ON- and OFF-pathways: Improves image resolution and contrast.

Other Visual Pathways

• Ganglion cells with opsin-like pigment project to suprachiasmatic nucleus (biological clock).

• Other pathways project to midbrain and cerebellum for eye movement coordination.

Color Vision

Color vision depends on the activation of cone photoreceptors sensitive to different wavelengths.

• Color Perception: Related to wavelengths reflected, absorbed, or transmitted by objects.

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

• Intensity: Discrimination depends on light intensity; rods dominate in dim light, leading to grayscale vision.

Color Blindness

• Most common: Red-green color blindness (X-linked recessive, mainly in men).

• Results from mutations in cone pigment genes.

Eye Movement

• Six skeletal muscles control eye movement.

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

• Slow Movements: Track objects and compensate for head movement (vestibular system involvement).

Common Diseases of the Eye

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

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

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

Audition (Hearing)

Mechanisms of Hearing

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

• Sound energy transmitted by vibration of molecules in air, liquid, or solid.

• No sound in a vacuum.

Neural Pathways in Hearing

• Cochlear nerve fibers synapse in the brainstem.

• Multineuron pathway transmits information through thalamus to auditory cortex.

Vestibular Information and Pathways

Vestibular Function

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

• Controls eye muscles

• Maintains upright posture and balance

• Provides awareness of body position and acceleration

• Vestibular nerve fibers transmit information to parietal lobe vestibular centers

• Integrated with proprioceptive sensory information

Chemical Senses

Taste (Gustation)

Taste is detected by chemoreceptors in taste buds, which are found in the mouth and throat.

• About 10,000 taste buds, each with receptor cells arranged around a taste pore.

• Microvilli increase surface area and contain proteins for transduction.

• Basal cells replace damaged taste receptor cells.

• Food molecules must be dissolved in liquid to contact taste receptor cells.

Types of Taste Receptors

• Sweet

• Sour

• Salty

• Bitter

• Umami (savoriness, glutamate)

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: Associated with poisonous substances; activates G-protein-mediated pathways.

• Umami: Depolarizes via G-protein-coupled receptor mechanism.

Smell (Olfaction)

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

• Olfactory receptor neurons are bipolar and replaced every two months.

• Cilia contain receptor proteins for odor molecules.

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

Detection and Signaling

• Odorant molecules must diffuse, dissolve in mucus, and bind to cilia receptors.

• Activation increases cAMP, opens cation channels, depolarizes cell.

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

Factors Affecting 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: Vestibular Apparatus and Balance

The vestibular apparatus is crucial for maintaining balance and spatial orientation. Disruption, such as in benign paroxysmal positional vertigo (BPPV), can cause dizziness and imbalance. Loose otoliths may interfere with normal fluid movement in semicircular canals, affecting vestibular function.

• Role: Maintains balance, posture, and spatial orientation by detecting head movement and position.

• Integration: Vestibular information is combined with proprioceptive input for coordinated movement and balance.

Table: Summary of General Principles of Sensory Stimulus Processing

Principle	Description
Stimulus Modality	Type of stimulus detected (e.g., light, sound, pressure)
Stimulus Intensity	Strength of stimulus, encoded by frequency of action potentials
Stimulus Location	Determined by receptive fields and lateral inhibition
Adaptation	Decrease in receptor response with sustained stimulation
Central Processing	Modification and integration of sensory signals in CNS
Perception	Interpretation of sensory information in association cortex
Additional info:	Factors such as emotion, experience, and mental state can alter perception