Chapter 07 - Sensory Physiology: Comprehensive 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 at the peripheral ends of afferent neurons that respond to specific types of stimuli, generating graded potentials called receptor potentials. These can initiate action potentials that travel to the central nervous system (CNS).

• 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).

• Perception: Understanding the meaning of a sensation (e.g., recognizing pain as caused by a cut).

• Sensory Receptors: Located at the peripheral ends of afferent neurons; generate receptor potentials.

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

• Adequate Stimulus: The type of energy to which a receptor responds best, though high-intensity stimuli can activate other receptors.

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

Primary Sensory Coding

Sensory coding is the conversion of stimulus energy into signals that convey relevant information to the CNS. The information is encoded by the frequency and amplitude of electrical signals.

• Coding: Conversion of stimulus energy into neural signals.

• Information Conveyed: By frequency and amplitude of action potentials.

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

Stimulus Type (Modality)

Each sensory modality (e.g., heat, cold, pressure, sound, light) is detected by specialized receptors. The modality refers to the type of stimulus.

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

• Specialized Receptors: Each modality has 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 the brain’s ability to localize input.

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, or interneurons.

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

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

Neural Pathways in Sensory Systems

Afferent neural pathways consist of chains of neurons connected by synapses, forming ascending pathways to the CNS. Both specific and nonspecific pathways transmit sensory information.

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

• Ascending Pathways: Specific and nonspecific pathways.

• 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. Perception is influenced by many factors.

• Association Cortex: Integrates sensory information for perception.

• Factors Affecting Perception: Adaptation, emotions, personality, experience, lack of receptors, damaged pathways, drugs, mental illness.

Somatic Sensation

Somatic Receptors

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

• Sensations: Touch, pressure, proprioception, 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 physiological and psychological factors. Referred pain is pain felt at a site other than the origin.

• Referred Pain: Pain perceived at a location different from the source (e.g., heart pain felt in the 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 (analgesia) without affecting consciousness or other sensations.

• Analgesia: Selective suppression of pain.

• Mechanisms: Electrical stimulation, pharmacological agents (NSAIDs, opioids), endogenous opioids, acupuncture, transcutaneous electrical nerve stimulation (TENS).

Itch

Itch is a distinct sensation with mechanisms overlapping but separate from pain pathways. It can be acute or chronic and is often associated with skin conditions.

• Distinct Pathways: Itch signaling overlaps with nociceptor mechanisms.

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

Vision

Visual Perception

Vision requires the eye to focus light and neural pathways to interpret signals. The retina contains photoreceptors that transduce light into electrical signals.

• Eye: Organ that focuses and responds to light.

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

Neural Pathways of Vision

Light signals are converted into action potentials through interactions between photoreceptors, bipolar cells, and ganglion cells. ON- and OFF-pathways enhance image resolution.

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

• Ganglion Cells: First cells to initiate action potentials.

• ON-Pathway: Bipolar cells depolarize in absence of input; glutamate receptors are inhibitory.

• OFF-Pathway: Bipolar cells hyperpolarize in absence of input; glutamate receptors are excitatory.

• Image Resolution: Coexistence of ON- and OFF-pathways improves contrast perception.

Other Visual Pathways

Some ganglion cells project to brain regions other than the visual cortex, such as the suprachiasmatic nucleus (biological clock), thalamus, and midbrain for eye movement coordination.

• Suprachiasmatic Nucleus: Receives information for circadian rhythm.

• Thalamus and Midbrain: Coordinate eye and head movements, gaze fixation, and pupil size.

Color Vision

Color vision depends on the activation of cone photoreceptors sensitive to different wavelengths. Human retinas have three types of cones: L (red), M (green), and S (blue).

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

• Cones: Three types, each sensitive to a range of wavelengths.

• Intensity: In bright light, cones allow color discrimination; in dim light, rods dominate and color vision is lost.

Color Blindness

Color blindness results from mutations in cone pigments, most commonly red-green color blindness, which is X-linked and affects men more frequently.

• Red-Green Color Blindness: Lack or abnormality of red or green cone pigments.

• Genetics: Recessive mutation on X chromosome.

Eye Movement

Six skeletal muscles control eye movement, performing fast (saccades) and slow movements for tracking and compensation.

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

• Slow Movements: Track objects and compensate for head movement; controlled by vestibular system and visual feedback.

Common Diseases of the Eye

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

• Glaucoma: Increased intraocular pressure damages retinal cells; caused by poor drainage of aqueous humor; 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 energy is transmitted by vibrations in a medium, usually air.

• Sound Transmission: Requires a medium; cannot occur in a vacuum.

• Ear Anatomy: External, middle, and inner ear structures transmit and amplify sound.

Neural Pathways in Hearing

Cochlear nerve fibers synapse with interneurons in the brainstem, then transmit information through the thalamus to the auditory cortex.

• Cochlear Nerve: Synapses in brainstem.

• Auditory Cortex: Located in temporal lobe; receives processed sound information.

Vestibular Information and Pathways

Vestibular Function

The vestibular system provides information for balance, spatial orientation, and control of eye movements. Vestibular nerve fibers transmit information to the parietal lobe.

• Functions: Control eye muscles, maintain posture and balance, provide spatial awareness.

• Integration: Vestibular information is integrated with proprioceptive input.

Chemical Senses

Taste (Gustation)

Taste is detected by chemoreceptors in taste buds, which are groups of receptor cells in the mouth and throat. Food molecules must be dissolved to interact with taste receptors.

• Taste Buds: About 10,000 in mouth and throat; contain receptor cells and basal cells.

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

• Basal Cells: Replace damaged taste receptor cells.

Types of Taste Receptors

• Sweet

• Sour

• Salty

• Bitter

• Umami: Associated with glutamate and amino acids; conveys savoriness.

Signaling Mechanisms of Taste

Each taste modality uses a distinct signaling mechanism:

• 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 regularly.

• Olfactory Epithelium: Located in upper nasal cavity; contains receptor neurons.

• Cilia: Contain receptor proteins for odor molecules.

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

Sense of Smell

Odorant molecules must diffuse, dissolve in mucus, and bind to specific receptors. Activation of G-protein pathways increases cAMP, opening cation channels and depolarizing the cell.

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

Factors Affecting Sense of Smell

• Attentiveness

• Hunger: Sensitivity increases when hungry.

• Gender: Women generally have keener olfactory sensitivity.

• Smoking: Decreases sensitivity.

• Age: Ability decreases with age; many elderly lose sense of smell.

• Mucosa State: Congestion reduces sensitivity.

• Anosmia: Genetic defects can cause total loss of 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. The vestibular system detects head movements and position, integrating this information with visual and proprioceptive inputs to maintain equilibrium.

Table 7.1: Summary of General Principles of Sensory Stimulus Processing

Principle	Description
Stimulus Modality	Type of stimulus detected (e.g., light, sound, pressure)
Stimulus Intensity	Encoded by frequency of action potentials
Stimulus Location	Determined by receptive field and lateral inhibition
Adaptation	Decrease in response to constant stimulus
Perception	Influenced by CNS processing, emotions, experience
Additional info:	Table entries inferred from context and standard sensory physiology principles

Table 7.2: Decibel Levels of Common Sounds and Their Effects

Sound Source	Decibel Level (dB)	Effect
Whisper	30	Minimal effect
Normal Conversation	60	No damage
Heavy Traffic	85	Possible hearing damage with prolonged exposure
Rock Concert	120	Pain threshold; risk of hearing loss
Jet Engine	140	Immediate hearing damage
Additional info:	Table entries inferred from standard decibel levels and effects

Key Equations

• Frequency Coding: $\text{Frequency of action potentials} \propto \text{Stimulus intensity}$

• Receptor Potential: $\text{Receptor potential} = \text{Change in membrane potential in response to stimulus}$