Human Senses and Sensory Receptors Flashcards

Introduction to the Senses (Chapter 16)

• Sensory Receptors: These are specialized cells or cell processes designed to detect external or internal changes.

• Selectivity: Each type of receptor is specialized to respond best to a specific type of stimulus.

• Information Relay: Receptors stimulate neurons to send information to the Central Nervous System (CNS).

• General (Somatic) Sense Receptors: These possess simple structures and are distributed throughout the entire body.     * Somatic Sensory Receptors: Located in tactile receptors of the skin and mucous membranes, as well as proprioceptors in joints, muscles, and tendons.     * Visceral Sensory Receptors: Located within the walls of blood vessels (BVs) and viscera. They monitor stretch, chemical environment, temperature, and pain.

• Special Senses Receptors (Exteroceptors): These are complex, specialized sensory organs housed specifically in the head.     * Includes olfaction, gustation, vision, audition, and equilibrium.

Fundamental Concepts of Sensation

• Sensation: Defined as the information received from receptors.

• Perception: Defined as the conscious awareness of a sensation.

• Receptive Field: The specific area monitored by an individual receptor cell.     * Localization: Smaller receptor fields allow for more precise stimulus localization.

• Adaptation: A decreased sensitivity to a continuous stimulus. Divided into two types based on speed:     * Tonic Adaptors: Slow-adapting; provide a sustained response to a stimulus.     * Phasic Adaptors: Fast-adapting; respond rapidly to stimuli but stop firing if the stimulus remains constant.

Classification of Sensory Receptors by Stimulus Type

• Chemoreceptors: Detect chemicals dissolved in fluids. Found in both interoreceptors (monitoring internal pH and gas levels in respiratory/cardiovascular systems - tonic) and exteroreceptors (olfactory receptors - phasic; gustatory receptors - tonic and phasic).

• Nociceptors: Pain receptors that detect tissue damage.     * Structure: Free nerve endings with large receptive fields and broad sensitivity.     * Function: Typically tonic, though some possess phasic function.     * Distribution: Widely distributed throughout the body but notably absent from the brain.     * Subtypes:         * Somatic: Detect chemical, heat, or mechanical damage to the body surface or skeletal muscle.         * Visceral: Detect damage to internal organs.

• Thermoreceptors: Detect changes in temperature.     * Structure: Free nerve endings located in the dermis and hypothalamus.     * Function: Mostly phasic, with some tonic function. Sensations fade if the stimulus is continuous.     * Warm Receptors: Detect temperatures between $25-45^\circ\text{C}$ ($77-113^\circ\text{F}$). Temperatures above $45^\circ\text{C}$ trigger a burning sensation.     * Cold Receptors: Detect temperatures between $10-20^\circ\text{C}$ ($50-68^\circ\text{F}$). Temperatures below $15^\circ\text{C}$ trigger a freezing sensation. Cold receptors are $3$ to $4$ times more numerous than warm receptors.

• Mechanoreceptors: Respond to physical distortion such as touch, pressure, vibration, and stretch. Subcategories include baroreceptors, proprioceptors, tactile receptors, and inner ear hair cells.

• Photoreceptors: Located in the retina; detect changes in light intensity, color, and movement.

Detailed Mechanoreceptors

• Baroreceptors: Detect pressure changes, specifically in the aortic arch and carotid sinuses to monitor blood pressure. They are mostly tonic adaptors with some phasic function.

• Proprioceptors: Monitor body positioning, posture, and movement. They are located in muscles, tendons, and joints and are slowly adapting (tonic).     * Muscle Spindles: Detect stretch in skeletal muscle (tonic and phasic adaptors).     * Golgi Tendon Organs (GTO): Detect stretch in tendons (primarily tonic adaptors).     * Joint Kinesthetic Receptors: Detect stretch in the articular capsule (tonic and phasic adaptors).

• Tactile Receptors: Provide sensations of touch (shape/texture), pressure, and vibration. They are most abundant and diverse in the skin.     1. Free Nerve Endings: Simplest, most common. Unencapsulated tips of branching dendrites in skin and mucous membranes. Detect light touch/pressure (phasic).     2. Tactile Discs (Merkel’s Discs): Extremely sensitive tonic receptors with very small receptive fields. Consist of a Merkel cell plus unencapsulated dendritic processes from a single myelinated fiber. Located in the basal layer of the epidermis.     3. Tactile Corpuscles (Meissner’s Corpuscles): Encapsulated; intertwined dendritic endings in modified neurolemmocytes and connective tissue (CT). Phasic adaptors for light touch, fine texture, and shape. Found in dermal papillae of eyelids, lips, fingertips, palms, nipples, and genitalia.     4. Bulbous Corpuscles (Ruffini Corpuscles): Encapsulated tonic receptors in the dermis and subcutaneous layer. Detect deep pressure and skin distortion.     5. Lamellated Corpuscles (Pacinian Corpuscles): Encapsulated; single dendrite wrapped in layers of CT. Large phasic receptors for deep pressure, coarse touch, and high-frequency vibration. Found deep in dermis, subcutaneous layer, and organ walls.     6. Root Hair Plexuses: Unencapsulated dendrites at the base of hair follicles. Detect hair movement (phasic/tonic).     7. End Bulbs (Krause Bulbs): Encapsulated; detect light pressure and low-frequency vibration in the dermis and mucous membranes (oral/nasal/anal/vaginal). Typically phasic.

Specialized Mechanisms and Referred Pain

• Stereocilia and Kinocilia: Specialized mechanoreceptors for hearing and equilibrium. Located in the spiral organ, ampulla, and maculae. They respond to endolymph fluid movement. Stereocilia are phasic; Kinocilia are mostly tonic.

• Referred Pain: Inaccurate localization of sensory signals where visceral pain is perceived as originating from the skin or muscle. This occurs because many somatic and visceral sensory neurons use the same ascending tracts in the spinal cord, leaving the brain unable to determine the true source.

• Integumentary Distribution: Receptor concentration varies; higher concentration equals a smaller receptive field and better discrimination.

• Two-Point Discrimination: A test using a paper clip to determine if a subject feels one or two distinct points on various body parts (forearm, neck, finger, back of hand).

Olfaction (Smell)

• Anatomy of Olfactory Organs:     * Olfactory Epithelium: Contains olfactory receptor cells (bipolar neurons with > 20 cilia-shaped dendrites), supporting cells (sustain receptors), and basal cells (stem cells that replace receptors).     * Lamina Propria: Areolar CT layer housing blood vessels and nerves.     * Olfactory Glands: Produce thick, pigmented mucus.

• Olfactory Receptor Cells: Primary neurons with unmyelinated axons. Olfactory hairs (cilia) house chemoreceptors for specific odorants.

• Neural Pathway: Axons bundle into Olfactory Nerves (CN I), project through the cribriform plate, enter olfactory bulbs, and travel along olfactory tracts to the brain.

• Function: Chemoreceptors detect volatile molecules (odorants) dissolved in mucus. Smells are discriminated based on unique combinations of stimulated cells. They are phasic adaptors and can elicit emotional responses.

Gustation (Taste)

• Receptor Cells: Chemoreceptors located on the tongue, pharynx, larynx, and epiglottis. They undergo phasic adaptation. Numbers decrease with age.

• Lingual Papillae:     1. Vallate: Large, form a "V" at the back of the tongue; house up to $100$ taste buds each.     2. Foliate: Found on posterior lateral tongue; few taste buds after childhood.     3. Fungiform: Located on tips and sides; contain approximately $5$ taste buds each.     4. Filiform: No taste buds; provide friction for moving objects on the anterior $2/3$ of the tongue.

• Taste Sensations:     * Sweet, Salty, Sour, Bitter (highest sensitivity for unpleasant tastes), and Umami (savory).     * Pharynx contains water receptors to regulate balance and prevent over-ingestion.

• Gustatory Pathway:     1. Primary neurons from tongue (via Facial Nerve CN VII and Glossopharyngeal Nerve CN IX) synapse in the nucleus solitarius of the medulla oblongata.     2. Secondary neurons extend to the thalamus.     3. Tertiary neurons extend to the primary gustatory cortex in the insula.

• Additional Senses: Spiciness and texture are detected via the Trigeminal Nerve (V).

Vision and Eye Anatomy

• Accessory Structures: Eyebrows (sweat protection), eyelashes (debris protection), eyelids (lubrication), conjunctiva (mucous membrane), and tarsal glands.

• Lacrimal Apparatus: Lacrimal gland (tears) $\rightarrow$ tear ducts $\rightarrow$ eye $\rightarrow$ lacrimal canaliculi $\rightarrow$ lacrimal sac $\rightarrow$ nasolacrimal duct $\rightarrow$ nasal cavity.

• Eye Cavities:     * Anterior Cavity: Contains aqueous humor (nourishes lens). Divided into anterior chamber (cornea to iris) and posterior chamber (iris to lens).     * Posterior Cavity: Contains vitreous humor (gel-like; maintains eye shape and stabilizes retina).

• Eye Tunics:     1. Fibrous Tunic: Outer layer. Includes the Cornea (transparent, avascular, refracts light) and Sclera (white of the eye, protective, extrinsic muscle attachment).     2. Vascular Tunic (Uvea): Middle layer. Includes the Choroid (nourishes retina), Ciliary Body (contains ciliary muscles to change lens shape and ciliary processes to secrete aqueous humor), and Iris (contains the pupil).     3. Retina (Neural Tunic): Inner layer. Contains an outer pigmented layer (absorbs stray light) and an inner neural layer (houses photoreceptors).

• The Pupil and Muscles:     * Sphincter Pupillae: Circular fibers; constrict pupil via Parasympathetic Nervous System (PNS) and CN III.     * Dilator Pupillae: Radial fibers; dilate pupil via Sympathetic Nervous System (SNS).

Photoreception and Visual Pathway

• Rods: Approximately $125$ million per eye. Highly sensitive, black-and-white vision, effective in dim light. Highest density in the peripheral retina. Multiple rods converge on single ganglion cells (less precise).

• Cones: Approximately $6$ million per eye. Require intense light for color vision. Three types: Blue ($26\%$), Green ($10\%$), and Red ($74\%$). Highest density in the fovea centralis (area of sharpest vision). One-to-one relationship with bipolar/ganglion cells for high acuity.

• Visual Pathway: Photoreceptors $\rightarrow$ Bipolar cells $\rightarrow$ Ganglion cells (axons form Optic Nerve CN II) $\rightarrow$ Optic Chiasm (decussation of half of fibers) $\rightarrow$ Optic Tracts $\rightarrow$ Lateral geniculate bodies (thalamus) $\rightarrow$ Visual cortex (occipital lobe).

• Accommodation:     * Near Vision: Ciliary muscle contracts, suspensory ligaments loosen, lens becomes rounded.     * Distant Vision: Ciliary muscle relaxes, suspensory ligaments tighten, lens flattens.

• Vision Disorders:     * Emmetropia: Normal vision.     * Myopia (Nearsighted): Image focused in front of retina; corrected with concave (diverging) lens. Affects approximately $1/3$ of the population.     * Hyperopia (Farsighted): Image focused behind retina; corrected with convex (converging) lens. Affects approximately $1/4$ of the population.     * Color-Blindness: Nonfunctional cones, most commonly red-green (missing red cones). Inherited in $10\%$ of males and $0.67\%$ of females.

Audition (Hearing) and Ear Anatomy

• Regions of the Ear:     1. External Ear: Auricle (collects waves), External auditory canal (with ceruminous glands and hairs), and Tympanic Membrane (vibrates in response to sound).     2. Middle Ear: Air-filled tympanic cavity containing the Auditory Tube (equalizes pressure) and the three Auditory Ossicles (Malleus, Incus, Stapes). Muscles include the Tensor Tympani (CN V) and Stapedius (CN VII).     3. Inner Ear: Contains the Bony Labyrinth (filled with perilymph) and Membranous Labyrinth (filled with endolymph, high in $K^+$).

• Cochlea Structure:     * Scala Vestibuli: Upper; sound enters via oval window.     * Cochlear Duct: Middle; contains the Spiral Organ (organ of Corti).     * Scala Tympani: Lower; sound exits via round window.

• Spiral Organ: Hair cells sit on the Basilar Membrane and their stereocilia are embedded in the Tectorial Membrane. Sound causes basilar membrane vibration, distorting hair cells against the tectorial membrane, triggering CN VIII.

• Pitch and Volume:     * Pitch (Frequency): Measured in Hertz ($Hz$). Humans detect $20-20,000\,\text{Hz}$ (most sensitive to $1500-4000\,\text{Hz}$). High frequency vibrates the stiff base near the oval window; low frequency vibrates the apex.     * Volume: Depends on wave amplitude; larger movement of the membrane stimulates more hair cells.

Equilibrium and Balance

• Semicircular Ducts: Detect angular acceleration (head rotation) in three planes:     * Anterior: Nodding ("yes").     * Posterior: Tilting (ear to shoulder).     * Horizontal: Shaking head ("no").     * Mechanism: The Ampulla houses the Crista Ampullaris. Hair cells are embedded in a gelatinous Cupula. Endolymph movement bends the cupula and hair cells.

• Utricle and Saccule: Detect static equilibrium and linear acceleration.     * Receptors: Housed in Maculae.     * Utricle: Horizontal movement.     * Saccule: Vertical movement.     * Mechanism: Hair cells are in the Otolithic Membrane, topped with Otoliths (calcium carbonate "ear stones"). When the head tilts, gravity pulls the otoliths, bending the membrane and hair cells.

• Integration: Information is carried by the vestibular branch of CN VIII and is assisted by vision and proprioceptors.

Laboratory Activities and Clinical Tests

• Proprioception Activity: Close eyes and attempt to touch fingertip to nose and elbow; standing on one foot with eyes open vs. closed (noting benefit of visual and tactile cues).

• Afterimage: Staring at a red parrot image for $20$ seconds then looking at white space to see the complementary color due to local adaptation of color receptors.

• Blind Spot Test: Closing one eye and moving an image toward the face until a specific point disappears from the optic disc.

• Rinne Test: Using a tuning fork on the mastoid process vs. air conduction near the ear canal to test for conductive deafness.

• Weber Test: Placing a tuning fork on the midsagittal line of the head to check for equal hearing in both ears.

• Orienting to Sound: Determining sound origin with head fixed vs. moving and with one ear closed.

• Inherited Taste: Testing ability to taste Sodium Benzoate and PTC.