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 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 receptor 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 respond to mechanical pressure if intense enough.

Primary Sensory Coding

Sensory coding is the conversion of stimulus energy into a signal that conveys relevant information to the CNS. Information is encoded by the frequency and amplitude of electrical signals.

• Key Characteristics: Type of input (modality), intensity, and location.

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

• Receptor Specialization: Each modality has a specialized 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.

• Function: Enables precise localization of a stimulus.

• Mechanism: Afferent neurons at the edge of a stimulus are strongly inhibited compared to those at the center.

• Result: Enhances contrast between center and periphery, improving localization.

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.

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

• Example: Descending pathways can directly inhibit afferent neuron terminals or act via interneurons.

Neural Pathways in Sensory Systems

Afferent neural pathways consist of chains of three or more neurons connected by synapses, forming ascending pathways to the CNS. There are specific and nonspecific ascending pathways.

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

Association Cortex and Perceptual Processing

The association cortex relies on various sensory inputs to process information. Regions close to primary sensory areas serve basic functions, while distant regions contribute to arousal, attention, memory, language, emotion, and motivation.

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

Factors Affecting Perception

• Sensory receptor adaptation and afferent pathway processing

• Emotions, personality, and 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) can alter perception

Somatic Sensation

Somatic Receptors

Somatic sensation arises from skin, skeletal 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, and 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 that can be altered by past experiences, emotions, and simultaneous activation of other sensory modalities.

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

• Hyperalgesia: Increased sensitivity to painful stimuli, common after severe injuries.

Example: Heart pain is often felt in the left arm due to referred pain.

Pain Management

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

• Electrical stimulation of CNS areas

• Pharmacological agents (NSAIDs, opioids)

• Endogenous opioids released from inhibitory pathways

• Acupuncture (linked to endogenous opioid activation)

• Transcutaneous electrical nerve stimulation (TENS)

Itch

Itch is a distinct sensation with mechanisms overlapping but separate from pain pathways. It can be acute or persistent, originating from skin receptors or abnormal CNS function.

Vision

Visual Perception

Visual perception requires the eye to focus images and respond to 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 through photoreceptors, bipolar cells, and ganglion cells. Photoreceptors and bipolar cells undergo graded responses; ganglion cells initiate action potentials.

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

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

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

Visual Pathways Beyond Cortex

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

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

Color Vision

Color vision depends on the activation of cone photoreceptor cells, each sensitive to different wavelengths.

• Three Types of Cones: L (red), M (green), S (blue)

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

• Intensity: In bright light, cones allow color discrimination; in dim light, rods dominate and only shades of gray are perceived.

Color Blindness

Color blindness results from mutations in cone pigments, most commonly red-green color blindness (X-linked recessive).

• Prevalence: Mainly affects men (1 in 12)

• Mechanism: Lack or abnormal form of red or green cone pigments

Eye Movement

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

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

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

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; major cause of irreversible blindness

• Macular Degeneration: Impairment of macula lutea; loss of central vision, especially age-related (AMD)

Audition (Hearing)

Mechanisms of Hearing

Hearing is based on the physics of sound and the physiology of the ear. Sound energy is transmitted by vibration of molecules in a medium (usually air).

• Sound Transmission: Requires a medium; no sound in a vacuum

• Ear Anatomy: External, middle, and inner ear structures

Neural Pathways in Hearing

• Cochlear nerve fibers synapse with interneurons in the brainstem

• Multineuron pathway transmits information through the thalamus to the auditory cortex in the temporal lobe

Vestibular Information and Pathways

Vestibular Function

The vestibular system provides information for controlling eye muscles, maintaining posture and balance, and awareness of body position and acceleration.

• Vestibular nerve fibers transmit information through the brainstem and thalamus to the parietal lobe

• Integration with proprioceptive sensory information

Chemical Senses

Taste (Gustation)

Taste is detected by chemoreceptors in taste buds, which are small groups of receptor cells in the mouth and throat.

• Taste Buds: About 10,000 in the mouth and throat; 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 receptor cells

Types of Taste Receptors

• Sweet

• Sour

• Salty

• Bitter

• Umami (savoriness; glutamate and similar 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: Associated with poisonous substances; activates G-protein-mediated pathways

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

Smell (Olfaction)

Smell is a chemical sense using chemoreceptors. Olfactory receptor neurons are located in the olfactory epithelium in the upper nasal cavity.

• Olfactory Receptor Neurons: Bipolar neurons; replaced every two months by stem cells

• Cilia: Contain receptor proteins for odor molecules

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

Mechanism of Smell

• Odorant molecules diffuse into the nose, dissolve in mucus, and bind to specific receptors on cilia

• Activation of G-protein pathway increases cAMP, opens cation channels, and depolarizes the cell

• Humans can identify at least 10,000 odorants with about 400 types of olfactory receptors

Factors Affecting Sense of Smell

• Attentiveness

• Hunger (greater sensitivity when hungry)

• Gender (women generally have keener sensitivity)

• Smoking (decreases sensitivity)

• Age (ability decreases with age)

• State of olfactory mucosa (congestion reduces sensitivity)

• Genetic defects (anosmia: total lack of smell)

Clinical Case Study: Vestibular Apparatus and Balance

An active 65-year-old man experienced dizzy spells, especially after rapid head movements. Diagnosis: benign paroxysmal positional vertigo (BPPV), involving disruption of vestibular apparatus function. Loose otoliths may float into semicircular canals, interrupting normal fluid movement.

• Role of Vestibular Apparatus: Maintains balance by detecting head position and movement, providing spatial orientation, and controlling eye movements and posture.

Summary Table: General Principles of Sensory Stimulus Processing

Principle	Description
Modality	Type of stimulus detected (e.g., light, sound, pressure)
Intensity	Strength of stimulus, encoded by frequency of action potentials
Location	Site of stimulus, determined by receptive fields and lateral inhibition
Duration	Time course of stimulus, affected by adaptation of receptors
Perception	Interpretation of stimulus by the brain, influenced by context and experience

Summary Table: Types of Somatic Receptors

Receptor	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

Key Equations

• Frequency Coding: The frequency of action potentials encodes stimulus intensity. $\text{Stimulus Intensity} \propto \text{Action Potential Frequency}$

• Receptor Potential: Graded potential generated by sensory receptor in response to stimulus. $\text{Receptor Potential} = \text{Change in Membrane Potential}$

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