Chapter 10 – Sensory Physiology (BIOL 241)

Sensory Receptors

• Overall function: Transduce (= change) diverse forms of environmental energy (stimuli) into electrical signals, specifically action potentials (APs) – which is the universal “language” of the nervous system (NS). This conversion process is known as sensory transduction.

  • Perceived modality (what we experientially feel, e.g., light, sound, touch) depends not on the intrinsic nature of the physical stimulus itself, but on the specific central nervous system (CNS) pathway that is activated by the receptor. This is known as labeled line coding.

• Structural varieties: Sensory receptors exhibit diverse structures tailored to their specific functions.

  • Simple dendritic endings of sensory neurons: These are typically free nerve endings, common for pain and temperature detection (e.g., nociceptors).

  • Specialized endings of neurons: These are often encapsulated structures, such as Meissner's corpuscles (touch) or Pacinian corpuscles (vibration), which enhance their sensitivity to specific mechanical stimuli.

  • Specialized, non-neuronal receptor cells: These are epithelial cells that synapse with sensory neurons, releasing neurotransmitters to generate a signal in the afferent nerve (e.g., photoreceptors in the eye, hair cells in the ear, taste buds).

• Classification by stimulus energy (modality):

  • Chemoreceptors: Detect chemical ligands in the external or internal environment. Examples include taste receptors (gustation), smell receptors (olfaction), and receptors monitoring blood pH or O_2 levels.

  • Photoreceptors: Specialized cells that transduce light energy into electrical signals (e.g., rods and cones in the retina).

  • Thermoreceptors: Respond to changes in temperature, signaling sensations of warmth or cold.

  • Mechanoreceptors: Activated by mechanical deformation of their cell membrane. This category includes receptors for touch, pressure, vibration, stretch, as well as those involved in hearing (hair cells in the cochlea) and vestibular sense (hair cells in the inner ear).

  • Nociceptors: Polymodal receptors that specifically encode tissue-damaging or intense stimuli (mechanical, thermal, chemical), which the brain interprets as pain. They have a high threshold for activation.

  • Proprioceptors: Specialized mechanoreceptors found in muscles, tendons, and joints that relay positional information about body parts (limb position, movement, and effort) to the CNS, contributing to kinesthesia and body awareness.

• Classification by location:

  • General (cutaneous) receptors: Distributed widely across the body surface and deep tissues, near epithelial surfaces. These include receptors for touch, pressure, temperature, and pain from the skin and musculoskeletal system.

  • Special-sense receptors: Located within distinct, complex sensory organs dedicated to specific senses, such as vision (eyes), hearing and equilibrium (ears), taste (tongue), and smell (nasal cavity).

Receptor Response Dynamics

• Tonic receptors: These receptors fire at a relatively constant rate as long as a stimulus persists, signaling continuous information about the stimulus presence and intensity (e.g., nociceptors for persistent pain, some proprioceptors). They show minimal or slow adaptation.

  • Provide continuous information about the duration and magnitude of a stimulus.

  • Essential for maintaining awareness of body position or ongoing pain.

• Phasic receptors: These receptors exhibit an initial burst of action potentials upon stimulus onset, then rapidly adapt by decreasing their firing rate (↓ firing) even if the stimulus remains constant. They also often show an “off” response (another burst of APs) when the stimulus is removed (e.g., olfactory receptors, many touch mechanoreceptors like Pacinian corpuscles).

  • Enable detection of stimulus change, movement, or onset/offset, rather than the absolute level or constant presence of a stimulus.

  • Crucial for selective attention and filtering out unchanging background stimuli.

• Law of Specific Nerve Energies: Postulated by Johannes Müller, this fundamental principle states that stimulation of a given sensory fiber is always interpreted by the brain as the receptor’s adequate modality, regardless of how the fiber is stimulated.

  • Adequate stimulus: This refers to the lowest-energy form that normally and most efficiently activates that specific receptor type. For instance, photoreceptors are most sensitive to light, while mechanoreceptors are most sensitive to mechanical force.

  • Examples: Pressing your eye (mechanical stimulus) still results in the perception of light (phosphenes) due to the activation of optic nerve fibers, which are part of the visual pathway.

• Generator (receptor) potentials: These are local, graded potentials produced in the sensory receptor cell in response to a stimulus. They are the functional analog of excitatory postsynaptic potentials (EPSPs) in neurons.

  • Graded amplitude: The magnitude of the generator potential is directly proportional to the intensity of the stimulus. A stronger stimulus produces a larger generator potential.

  • When threshold is reached: If the generator potential is strong enough to reach the threshold at the trigger zone (often the first node of Ranvier in a sensory neuron's axon), it initiates action potentials. The frequency of these action potentials encodes the strength and duration of the stimulus (frequency coding).

  • Phasic receptors: In these receptors, the generator potential amplitude quickly diminishes or adapts even during a constant stimulus, leading to rapid adaptation of the AP firing frequency.

  • Tonic receptors: In contrast, the generator potential is maintained throughout the duration of the stimulus, leading to sustained AP firing.

Special Senses – Taste (Gustation)

• Receptor location & type: Taste receptors are specialized epithelial cells, not neurons, found within taste buds.

  • Taste buds: Approximately 50!$–$!100 modified epithelial cells are clustered within each taste bud, which are primarily located on the papillae of the tongue (fungiform, circumvallate, foliate papillae). A smaller number are found on the palate, pharynx, and epiglottis.

  • Each bud is capable of transducing all five universally accepted basic tastant categories: sweet, sour, salty, bitter, and umami (a savory taste evoked by amino acids like glutamate).

• Signal mechanisms: The mechanisms by which tastants cause depolarization in taste receptor cells vary by category:

  • Salty & sour: Tastants directly enter or block ion channels on the receptor cell membrane, leading to depolarization. For salty, Na^+ ions from salts enter through special EnaC channels. For sour, H^+ ions from acids enter through proton channels or block K^+ channels, causing depolarization.

  • Sweet & bitter: These tastants bind to specific G-protein-coupled receptors (GPCRs) on the membrane surface, initiating intracellular signaling cascades (↑ second messengers like cAMP or IP_3) that ultimately lead to channel modulation and depolarization.

• Exteroceptor vs interoceptor roles:

  • External chemical monitoring: The primary role is flavor perception from ingested food and drinks, helping to identify nutritious (sweet/umami) and potentially harmful (bitter/sour) substances.

  • Internal (e.g., gut chemoreceptors): Specialized chemoreceptors exist in the gastrointestinal tract, monitoring blood or chyme chemistry. These help regulate digestion and nutrient absorption, influencing satiety and metabolic processes.

Special Senses – Smell (Olfaction)

• Olfactory epithelium at superior nasal cavity contains three major cell types crucial for olfaction:

  1. Receptor cells: These are bipolar neurons, unique in being continually replaced throughout life. Their apical dendrites extend into the mucus layer with numerous cilia that contain odorant receptors. Their axons form fascicles that constitute the olfactory (I) nerve, projecting directly to the olfactory bulb in the brain.

  2. Supporting cells (Sustentacular cells): These provide physical and metabolic support to the olfactory receptor neurons, analogous to glial cells. They also house detoxifying enzymes (e.g., cytochrome P450) that metabolize odorants, preventing their accumulation and prolonging the life of receptor cells.

  3. Basal (stem) cells: These are undifferentiated stem cells located at the base of the epithelium. They continuously divide and differentiate into new olfactory receptor neurons, replacing worn-out or damaged ones approximately every 1!$–$!2 ext{ months}. This neuronal regeneration is unique in the adult nervous system.

• Transduction: The process by which chemical odorants are converted into electrical signals is complex.

  • Odorants bind to a large family of GPCRs: The human genome dedicates a significant proportion (over 400 genes) to encoding olfactory receptors, making it one of the largest gene families in vertebrates. Each olfactory neuron typically expresses only one type of odorant receptor.

  • G-protein activation: Upon odorant binding, the GPCR activates a G-protein (specifically G{olf}). This leads to the activation of adenylate cyclase, which increases intracellular cyclic AMP (cAMP) (or phospholipase C, increasing IP3).

  • Cation channels open: The elevated cAMP (or IP_3)) directly or indirectly binds to and opens cyclic nucleotide-gated (CNG) cation channels (e.g., Na^+ and Ca^{2+} channels) on the cilia membrane, leading to an influx of positive ions.

  • Receptor depolarization: This cation influx causes a generator potential (depolarization) in the olfactory receptor neuron. If the depolarization reaches threshold, it triggers action potentials that propagate along the axon to the olfactory bulb.

  • Highly phasic: Olfactory receptors exhibit rapid adaptation to constant odors, meaning their firing rate quickly decreases even with continuous exposure. This enables detection of new or changing odors and prevents sensory overload.

Vestibular System (Equilibrium)

• Anatomy: The vestibular system is crucial for maintaining balance, posture, and spatial orientation.

  • Vestibular apparatus + cochlea = inner ear: Both structures are housed within the temporal bone.

  • Vestibular apparatus: This consists of two main components: the otolith organs (utricle and saccule) and the three semicircular canals (SCCs).

  • Structures lie within the membranous labyrinth: This is a series of interconnected sacs and ducts filled with endolymph (a K^+ rich fluid, similar to intracellular fluid), which is suspended within the bony labyrinth, a cavity filled with perilymph (similar to CSF).

• Common receptor design: The fundamental sensory receptor throughout the vestibular and auditory systems is the hair cell.

  • Hair cells: Each hair cell possesses 20!$–$!50 stereo-cilia (graded in height, actually microvilli) and a single, taller kinocilium (a true cilium, typically the tallest). These hair bundles are embedded in a gelatinous membrane.

  • Direction-sensitive: The mechanical bending of the stereocilia toward the kinocilium opens mechanically-gated K^+ channels, leading to K^+ influx from the endolymph and depolarization of the hair cell. This depolarization causes neurotransmitter (NT) release onto afferent neurons, thereby increasing the firing rate of action potentials along the vestibular division of the VIII cranial nerve. Conversely, bending away from the kinocilium closes these channels, leading to hyperpolarization (↓ APs) and reduced NT release. The precise orientation of the kinocilium relative to the stereocilia determines the direction of maximal sensitivity.

• Otolith organs (utricle & saccule): These detect linear acceleration (changes in velocity in a straight line) and static head tilt relative to gravity.

  • Macula: This is the sensory epithelium within each otolith organ, containing hair cells whose bundles are embedded in a gelatinous otolithic membrane. This membrane is studded with calcium carbonate crystals called otoliths (or “ear stones”) that add inertia and weight.

  • Utricle: The macula in the utricle is primarily oriented horizontally. It detects horizontal acceleration (e.g., when a car starts moving forward, the otoliths lag, pushing hair bundles backward, signaling acceleration) and head tilts that are off-vertical (e.g., tilting the head sideways).

  • Saccule: The macula in the saccule is oriented vertically. It detects vertical acceleration (e.g., when an elevator descends, the otoliths are pulled upward relative to the hair cells, signaling downward acceleration) and head tilts in the sagittal plane (e.g., bending the head forward/backward).

• Semicircular canals (SCCs): There are three mutually perpendicular ducts (anterior, posterior, and horizontal) in each ear. They are specialized to encode rotational (angular) acceleration of the head.

  • Enlarged base (ampulla): At the base of each SCC is an enlargement called the ampulla, which houses the crista ampullaris, the sensory organ.

  • Crista ampullaris: Hair bundles of the hair cells within the crista ampullaris project into a gelatinous, dome-shaped structure called the cupula, which spans the lumen of the ampulla.

  • Endolymph inertia: When the head rotates, the bony labyrinth and the SCCs move with it. However, the endolymph fluid inside the SCCs, due to its inertia, lags behind and flows in the opposite direction of the head rotation. This endolymph movement deflects the cupula, bending the hair bundles and causing either depolarization or hyperpolarization of the hair cells, signaling the direction and magnitude of the angular acceleration.

Auditory System (Hearing)

• Basic physics: Sound is created by vibrations that produce pressure waves.

  • Sound waves: These travel omnidirectionally through a medium (e.g., air, water). They are characterized by two main properties: frequency (f) and intensity (amplitude). Frequency is measured in Hertz (Hz = cycles per second, s^{-1}).

  • Pitch: This is the perceptual correlate of sound frequency. Higher frequency sounds are perceived as higher pitch; lower frequency sounds as lower pitch.

  • Loudness: This is the perceptual correlate of wave amplitude (intensity). Greater amplitude waves are perceived as louder sounds. Intensity is typically measured in decibels (dB), a logarithmic scale that reflects the vast range of human hearing sensitivity.

• Ear regions: The ear is anatomically divided into three main parts:

  • Outer ear: Composed of the pinna (auricle) and the external auditory canal. Its primary function is to funnel sound waves to the tympanic membrane (eardrum) and provide directional cues.

  • Middle ear: An air-filled cavity located between the tympanic membrane and the inner ear (cochlea). It houses the three smallest bones in the body, collectively called the ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). These ossicles form a lever system that transmits and amplifies pressure waves from the large tympanic membrane to the much smaller oval window of the cochlea, overcoming the impedance mismatch between air and the fluid-filled inner ear.

  • Inner ear: Contains both the cochlea (responsible for hearing) and the vestibular apparatus (responsible for equilibrium).

• Detailed cochlear mechanics, organ of Corti, and auditory pathways were referenced (slides 10-50) but not elaborated in the transcript – review textbook for full sequence from basilar-membrane vibration to auditory cortex mapping: Briefly, sound waves entering the cochlea cause vibrations of the basilar membrane. Hair cells in the organ of Corti, sitting on the basilar membrane, are stimulated by these vibrations based on their frequency (place coding). This mechanical stimulation opens ion channels in the hair cells, leading to depolarization and neurotransmitter release. The ensuing electrical signals are then transmitted via the cochlear division of the VIII nerve to various brainstem nuclei, thalamus, and finally to the primary auditory cortex, where sound is perceived and processed.

Vision

• Electromagnetic window: The human eye is equipped to transduce photons within a very narrow portion of the electromagnetic spectrum, known as the visible light spectrum, typically ranging from approximately 400 nm (violet) to 700 nm (red) wavelengths, into electrical signals (APs).

• Iris & pupil control: The iris is a pigmented muscular diaphragm that controls the amount of light entering the eye through the pupil (the opening in the center of the iris).

  • Circular (constrictor) muscles (Sphincter pupillae): These muscles are arranged concentrically around the pupil. They contract under parasympathetic input (via the oculomotor nerve, CN III), causing pupil constriction (miosis), which reduces the amount of light entering the eye and increases the depth of field.

  • Radial (dilator) muscles (Dilator pupillae): These muscles are arranged radially. They contract under sympathetic input, causing pupil dilation (mydriasis), which allows more light to enter the eye in dim conditions.

• Accommodation (focusing power of lens): The eye's ability to change the focal length of the lens to keep objects at various distances in sharp focus on the retina.

  • Ciliary muscle contraction state: The ciliary muscle, a smooth muscle ring, alters tension on the suspensory ligaments (zonules) that connect to the lens capsule. This changes the curvature and thus the refractive power of the elastic lens.

  • Distant vision (greater than 20 ft or 6 meters): The ciliary muscle is relaxed. This increases tension on the suspensory ligaments, which pull the lens flatter and thinner (least convex or least refractive power). Light rays from distant objects are nearly parallel and require less refraction to focus on the retina.

  • Near vision: The ciliary muscle contracts. This reduces the diameter of the ring, thereby relaxing the tension on the suspensory ligaments. The elastic lens becomes thicker and more convex (more spherical, increasing its refractive power) to converge diverging light rays from close objects onto the retina. This process involves the near triad: pupillary constriction, lens accommodation, and convergence of the eyes.

• Visual acuity & common refractive errors:

  • Acuity: Refers to the resolving power of the eye, specifically the ability to discern fine details or separate two closely spaced points. It's often measured with Snellen eye charts (e.g., 20/20 vision).

  • Myopia (nearsightedness): This condition occurs when the eyeball is too long, or the cornea/lens has too much refractive power, causing light from distant objects to focus in front of the retina. Distant objects appear blurry. It is corrected with concave (diverging) lenses, which spread out the light rays before they enter the eye.

  • Hyperopia (farsightedness): This occurs when the eyeball is too short, or the cornea/lens has too little refractive power, causing light from distant objects (or objects at all distances without accommodation) to focus theoretically behind the retina. Near objects appear blurry. It is corrected with convex (converging) lenses, which converge the light rays before they enter the eye.

  • Astigmatism: This is a refractive error caused by an asymmetric curvature of the cornea or, less commonly, the lens. This uneven curvature causes light rays to be refracted differently along different meridians, leading to multiple focal points and blurred or distorted vision at all distances. It is corrected with cylindrical lenses.

• Retina architecture: The retina is a complex, multilayered neural tissue lining the back of the eye, backed by the pigmented epithelium (RPE).

  • Light path: Light must pass through several transparent neural layers before reaching the photoreceptors. The typical path is: Ganglion cell layer → Amacrine cells → Bipolar cell layer → Horizontal cells → Photoreceptor layer (rods & cones). This arrangement is