Module 7: Sense Organs Vocabulary

Properties and Types of Sensory Receptors

• General Definitions

  • Sensory Receptor: Any structure specialized to detect a stimulus. These range from simple, bare nerve endings to complex sense organs.

  • Sense Organ: A structure that combines nervous tissue with other tissues (epithelial, muscular, or connective) that enhance its response to a certain type of stimulus. They can be microscopic (dendrite in connective tissue) or complex (eye or ear).

  • Transduction: The fundamental purpose of any sensory receptor; the conversion of one form of energy (light, sound, heat, touch, vibration) into nerve signals.

  • Transducer: Any device that converts one energy form to another (e.g., sense organ, gasoline engine, lightbulb).

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• Sensation and Perception

  • Sensation: The stage where a receptor detects the environment and exhibits a local electrical change called a receptor potential.

  • Neural Path of Sensation:

    • In touch and smell, the sensory cell is a neuron. If the receptor potential is strong enough, it fires action potentials to the CNS.

    • In taste and hearing, the sensory cell is an epithelial cell. It releases a neurotransmitter from synaptic vesicles at its base to stimulate an adjacent neuron, which then signals the CNS.

  • Perception: Conscious experience and interpretation of a stimulus. Most sensory signals are filtered out in the brainstem and never reach the cerebral cortex to produce perception (e.g., blood pH monitoring).

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• The Four Kinds of Information Transmitted

  1. Modality: The type of stimulus or perception produced (e.g., vision, hearing, taste). Determined by where the signals end in the brain (labeled line code).

  2. Location: Encoded by which neurons issue signals. Any sensory neuron detects stimuli within a receptive field.

     • Tactile Discrimination: Fingertips have small fields ($1\,mm$ or less), allowing two-point touch discrimination ($2\,mm$ apart). The back has large fields ($7\,cm$ diameter), where two points may feel like one.

  3. Intensity: Refers to stimulus strength (loudness, brightness). Encoded in three ways:

     • Rising firing frequencies of sensory nerve fibers.

     • Recruitment of greater numbers of neurons.

     • Activation of different neurons with different thresholds (weak stimuli activate sensitive neurons; strong stimuli activate less sensitive, high-threshold neurons).

  4. Duration: Encoded by changes in firing frequency over time.

     • Sensory Adaptation: If a stimulus is prolonged, firing slows and awareness fades.

     • Phasic Receptors: Generate a burst of action potentials when first stimulated, then adapt quickly (e.g., smell, hair movement, cutaneous pressure). Some fire again when the stimulus stops.

     • Tonic Receptors: Adapt slowly and generate signals steadily (e.g., proprioceptors, pain receptors).

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• Classification of Receptors

  • By Modality:

    • Thermoreceptors: Heat and cold.

    • Photoreceptors: Light (eyes).

    • Nociceptors: Pain from tissue injury or threats.

    • Chemoreceptors: Chemicals (odors, tastes, body fluid composition).

    • Mechanoreceptors: Physical deformation (vibration, touch, pressure, stretch).

  • By Origin of Stimulus:

    • Exteroceptors: External stimuli (vision, hearing, cutaneous senses).

    • Interoceptors: Internal organ stimuli (stretch, pressure, visceral pain, nausea).

    • Proprioceptors: Position and movements of the body (muscles, tendons, joint capsules).

  • By Distribution:

    • General (Somatosensory) Senses: Widely distributed (skin, muscles, joints). Includes touch, pressure, blood pressure.

    • Special Senses: Limited to the head; innervated by cranial nerves (vision, hearing, equilibrium, taste, smell).

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The General Senses

• Unencapsulated Nerve Endings

  • Dendrites with no connective tissue wrapping.

  • Free Nerve Endings: Warm receptors, cold receptors, and nociceptors. Abundant in skin and mucous membranes.

  • Tactile (Merkel) Discs: Tonic receptors for light touch, textures, edges, and shapes. Flattened endings adjacent to specialized tactile cells in the stratum basale.

  • Hair Receptors (Root Hair Plexuses): Dendrites coiled around a hair follicle; respond to hair movement. Phasic (adapt quickly).

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• Encapsulated Nerve Endings

  • Nerve fibers wrapped in glial cells or connective tissue, mostly mechanoreceptors.

  • Tactile (Meissner) Corpuscles: Phasic; light touch and texture. Located in dermal papillae of hairless areas (fingertips, lips, eyelids).

  • End Bulbs (Krause): Similar to tactile corpuscles but found in mucous membranes.

  • Bulbous (Ruffini) Corpuscles: Tonic; heavy touch, pressure, skin stretching, and joint movements. Located in dermis and joint capsules.

  • Lamellar (Pacinian) Corpuscles: Phasic; deep pressure, stretch, tickle, and vibration. Large ovoids ($1$ to $2\,mm$ long) with concentric layers of fibroblasts and Schwann cells. Found in periosteum, pancreas, and deep dermis.

  • Muscle Spindles and Tendon Organs: Discussed in relation to muscle reflexes.

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• Somatosensory Projection Pathways

  • Three-Neuron System:

    1. First-order Neurons: From receptor to spinal cord or brainstem. Large/myelinated for touch/proprioception; small/mostly unmyelinated for heat/cold.

    2. Second-order Neurons: Decussate (cross over) to the contralateral thalamus (or cerebellum for some proprioception).

    3. Third-order Neurons: From thalamus to the primary somatosensory cortex.

  • Somatosensory Signals from the Head: Travel via cranial nerves (mostly Trigeminal, $CN\,V$) to the pons and medulla.

  • Signals Below the Head: Enter posterior horn of spinal cord; ascend via spinothalamic and other tracts.

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• Pain

  • Function: Adaptive reflex to avoid damage. Deadened by peripheral neuropathy in diseases like leprosy and diabetes mellitus.

  • Categories:

    1. Nociceptive Pain: From tissue injury (inflammation). Responsive to anti-inflammatories.

       • Visceral Pain: Internal organs; diffuse, dull, hard to locate. Causes: stretch, chemical irritation, ischemia.

       • Deep Somatic Pain: Bones, joints, muscles (e.g., arthritis, sprains).

       • Superficial Somatic Pain: Skin (cuts, burns).

    2. Neuropathic Pain: From injury to nerves, spinal cord, or brain (stabbing/burning feeling). Includes phantom limb pain and fibromyalgia.

  • Nerve Fiber Types in Pain:

    • Fast Pain: Myelinated $A-\delta$ fibers ($12$ to $30\,m/s$). Sharp, localized, immediate.

    • Slow Pain: Unmyelinated type $C$ fibers ($0.5$ to $2.0\,m/s$). Dull, aching, burning, delayed.

  • Pathways for Pain:

    • Head: via $CN\,V, VII, IX, X$ to medulla; then to thalamus.

    • Below Neck: via Spinothalamic tract (conscious pain), Spinoreticular tract (relayed to hypothalamus/limbic system for emotional/behavioral reactions), and Gracile fasciculus (visceral pain).

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• Referred Pain and Modulation

  • Referred Pain: Misinterpretation of visceral pain as coming from the skin due to neural convergence in the CNS (e.g., heart attack pain radiating to left arm via $T1$ to $T5$ segments).

  • Endogenous Opioids: Neuromodulators like enkephalins, endorphins, and dynorphins. Secreted by the CNS and pituitary in stress/exercise.

  • Spinal Gating Mechanisms:

    1. Descending Analgesic Fibers: Arise in midbrain (central gray) and medulla; travel via reticulospinal tract. They secrete serotonin to stimulate interneurons that release enkephalins, blocking the transmission of substance P (postsynaptic and presynaptic inhibition).

    2. Mechanoreceptor Stimulation: Rubbing an injury stimulates large type $A$ myelinated fibers that trigger enkephalin-secreting interneurons to block pain.

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The Chemical Senses

• Gustation (Taste)

  • Lingual Papillae (Bumps on Tongue):

    1. Filiform: Spikes; no taste buds. Sense food texture (mouthfeel).

    2. Foliate: Parallel ridges on sides; most buds degenerate by age $2$ to $3$.

    3. Fungiform: Mushroom-shaped; concentrated at tip and sides.

    4. Vallate: V-shaped at the rear; only $7$ to $12$ papillae but contain half of all taste buds ($250$ each).

  • Taste Bud Structure: Garlic-bulb shaped; $50$ to $100$ cells.

    • Taste Cells: Epithelial cells with taste hairs (microvilli) in a taste pore. Synapse with sensory nerves. Life span: $9$ to $15\,days$.

    • Basal Cells: Stem cells.

    • Supporting Cells: No sensory role.

  • The Five Primary Tastes:

    1. Salty: Metal ions ($Na^+, K^+$). Vital for electrolytes.

    2. Sweet: Sugars/carbohydrates (high caloric value).

    3. Umami: Savory/meaty; amino acids (aspartic/glutamic). Motivate protein intake.

    4. Sour: Acids (citrus).

    5. Bitter: Alkaloids (nicotine, caffeine). Lowest threshold for protection against toxins.

  • Physiology of Taste:

    • Sugars/Alkaloids/Glutamate bind to surface receptors (G proteins/second messengers).

    • Sodium/Acids depolarize the cell directly.

  • Projection: $CN\,VII$ (anterior $2/3$ tongue), $CN\,IX$ (posterior $1/3$), $CN\,X$ (palate/pharynx). To solitary nucleus in medulla, then to hypothalamus/amygdala (reflexes) and thalamus/primary gustatory cortex (insula).

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• Olfaction (Smell)

  • Olfactory Mucosa: Patch in the roof of nasal cavity ($5\,cm^2$); $10$ to $20\,million$ olfactory cells.

  • Olfactory Cells: Neurons (bowling-pin shaped) with $10$ to $20$ olfactory hairs (immobile cilia). Only neurons exposed to environment; life span $60\,days$. Axons form the Olfactory nerve ($CN\,I$).

  • Physiology: Odorant binds to receptor $\rightarrow$ G protein activation $\rightarrow$ cAMP second-messenger system $\rightarrow$ ion channels open ($Na^+$ or $Ca^{2+}$) $\rightarrow$ depolarization/action potential.

  • Projection Pathway: Axons pass through cribriform plate to olfactory bulbs. Synapse with mitral and tufted cells in glomeruli (each dedicated to one odor type). Most fibers lead to primary olfactory cortex (temporal lobe). Extrathalamic pathway (bypasses thalamus initially).

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Hearing and Equilibrium

• The Nature of Sound

  • Pitch: Perception of "high" or "low." Determined by frequency ($Hz$). Human range: $20$ to $20,000\,Hz$. Most sensitive: $1,500$ to $5,000\,Hz$.

  • Loudness: Perception of amplitude (intensity). Measured in decibels ($dB$). $0\,dB$ is human threshold. Every $10\,dB$ step is a $10\times$ increase in intensity. Threshold of pain: $120$ to $140\,dB$.

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• Anatomy of the Ear

  • Outer Ear: Auricle (pinna) and Auditory canal ($3\,cm$ s-shaped passage). Contains ceruminous glands (cerumen/earwax contains lysozyme and low pH).

  • Middle Ear:

    • Tympanic membrane: Eardrum ($1\,cm$ diameter).

    • Auditory Tube: (Eustachian tube) Equalizes pressure; pathway for infection (Otitis media).

    • Auditory Ossicles: Malleus, Incus, Stapes (smallest bone). Stapes base sits in oval window.

    • Middle-Ear Muscles: Stapedius and Tensor tympani (Tympanic reflex).

  • Inner Ear: Bony/Membranous labyrinths. Filled with perilymph (between) and endolymph (within).

    • Cochlea: Snails-like spiral ($2.5$ coils). Modiolus (bony axis).

    • Cochlear Chambers: Scala vestibuli (superior), Scala tympani (inferior), Cochlear duct (middle). Scala media contains the spiral organ (Acoustic organ).

    • Hair Cells: Inner Hair Cells (IHCs; $3,500$; provide $90\text{-}95\%$ of hearing) and Outer Hair Cells (OHCs; $20,000$; tuning).

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• Physiology of Hearing

  • Ossicle Function: Concentrate energy from the large eardrum to the small oval window ($18\text{-}fold$ area difference) to overcome inertia of fluid.

  • Stimulation: Basilar membrane vibrates $\rightarrow$ IHCs bob up/down. Stereocilia bend against the stationary tectorial membrane. Tip links pull open mechanically gated K+ channels. Endolymph potential ($+80\,mV$) vs. hair cell ($-40\,mV$) drives $K^+$ influx, causing depolarization/neurotransmitter release.

  • Sensory Coding:

    • Loudness: High firing frequency and recruitment of more hair cells.

    • Pitch: Based on which part of the basilar membrane vibrates. Proximal end (base) is narrow/stiff for high frequencies; Distal end (apex) is wide/flexible for low frequencies.

  • Auditory Projection: $CN\,VIII \rightarrow$ cochlear nuclei (medulla) $\rightarrow$ superior olivary nucleus (pons) $\rightarrow$ inferior colliculi (midbrain; startle reflex) $\rightarrow$ thalamus $\rightarrow$ primary auditory cortex. Involves four neurons.

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• Equilibrium

  • Vestibular Apparatus: Three semicircular ducts and two chambers (saccule and utricle).

  • Static Equilibrium/Linear Acceleration: Sensed by Maculae.

    • Structure: Hair cells with stereocilia and a kinocilium in a gelatinous otolithic membrane weighted with otoliths (calcium carbonate granules).

    • Saccule: Vertical macula; vertical acceleration (elevators).

    • Utricle: Horizontal macula; head tilt and horizontal acceleration.

  • Dynamic/Angular Acceleration: Sensed by Semicircular Ducts.

    • Structure: Dilated ampulla containing crista ampullaris. Hair cells embedded in a gelatinous cupula.

    • Action: Head rotation causes endolymph to lag and push the cupula, bending hairs.

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Vision

• Light and Accessory Structures

  • Visible Spectrum: $400$ to $700\,nm$ (violet to red).

  • Eyelids (Palpebrae): Contain tarsal glands (oil) and orbicularis oculi muscle.

  • Conjunctiva: Transparent mucous membrane; highly vascular.

  • Lacrimal Apparatus: Gland (superolateral) $\rightarrow$ across eye $\rightarrow$ lacrimal punctum $\rightarrow$ nasolacrimal duct $\rightarrow$ nasal cavity.

  • Extrinsic Muscles: Superior, inferior, medial, lateral rectus; Superior oblique ($CN\,IV$); Lateral rectus ($CN\,VI$); others ($CN\,III$).

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• Anatomy of the Eye

  • Tunics:

    1. Fibrous Layer: Sclera (white) and Cornea (transparent, refracts light).

    2. Vascular Layer (Uvea): Choroid (pigmented), Ciliary body (muscular ring/secretes aqueous humor), and Iris (pupil diameter).

    3. Inner Layer: Retina and optic nerve.

  • Optical Components: Cornea, Aqueous humor (reabsorbed by scleral venous sinus), Lens (accommodation), and Vitreous body (jelly, maintains pressure).

  • Retina: Attached at optic disc and ora serrata. Features the macula lutea and central fovea centralis (sharpest vision). Optic disc is the blind spot.

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• Formation of an Image

  • Refraction: Light bends when passing between media of different indices ($n$). Air ($n=1.00$), Cornea ($n=1.38$), Lens ($n=1.40$). The cornea refracts light more than the lens.

  • Near Response:

    1. Convergence: Eyes orient medially.

    2. Constriction (Miosis): Pupil narrows to reduce spherical aberration.

    3. Accommodation: Ciliary muscle contracts, suspensory ligament relaxes, lens becomes more convex to increase refraction.

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• Sensory Transduction in the Retina

  • Photoreceptor Cells:

    • Rods: Stack of discs with rhodopsin (opsin + retinene/vitamin A). Responsible for night (scotopic) and monochromatic vision.

    • Cones: Tapered; contain photopsin. Responsible for day (photopic) and color (trichromatic) vision.

  • Mechanism: In the dark, retinal is cis-retinal. Light changes it to trans-retinal (bleaching). This stops the dark current (steady glutamate release). Bipolar cells are inhibited by glutamate; when light hit rods, glutamate stops, and bipolar cells fire, stimulating ganglion cells.

  • The Dual Visual System (Duplicity Theory):

    • Scotopic (Night): High convergence (up to $600$ rods per bipolar), high spatial summation/sensitivity, low resolution.

    • Photopic (Day): Foveal cones have 1:1 ratio with bipolars; no convergence, high resolution (acuity), low sensitivity.

  • Color Vision: Based on three cone types: Short-wavelength ($S$, $420\,nm$), Medium ($M$, $531\,nm$), and Long ($L$, $558\,nm$).

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• Visual Projections

  • Hemidecussation: Half of the fibers from each optic nerve cross at the optic chiasm. The right hemisphere sees the left visual field.

  • Targets: Thalamus (lateral geniculate nucleus) $\rightarrow$ Primary visual cortex (occipital lobe). Some fibers go to superior colliculi (visual reflexes).

  • Streams: Ventral stream (temporal lobe; object recognition/"what") and Dorsal stream (parietal lobe; spatial relationships/"where").

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Questions & Discussion

• If the receptive field in the back is $7\,cm$ in diameter, is it ever possible for two touches $1\,cm$ apart to be felt separately?

  • No. If both points fall within the same receptive field, they will stimulate only one neuron, and the brain will sense it as a single point of contact.

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• Cold and warm receptors—are they phasic or tonic?

  • They are phasic receptors. You adapt to hot bathwater or a cold lake over time, becoming more comfortable as the firing of the neurons slows down.

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• Why would high air pressure in the middle ear reduce the movements of the basilar membrane?

  • Excessive pressure pushes the tympanic membrane and muffles vibrations, reducing the energy transferred via the ossicles to the inner ear fluids.

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• Functional difference between the three rows of outer hair cells vs. one row of inner hair cells?

  • Inner hair cells provide almost all sensory input for hearing. Outer hair cells receive motor input and shorten to tense the basilar membrane, sharpening the frequency response (cochlear tuning).

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• Effects of an Ophthalmoscope Examination:

  • Allows direct view of the fundus (rear) of the eye (macula lutea, fovea, optic disc) and noninvasive check on cardiovascular health via retinal blood vessels.

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• What happens to a person's vision if the right optic radiation is destroyed?

  • The person would lose the left visual field in both eyes (blindness in that field), but visual reflexes (via the superior colliculus/midbrain) would likely remain intact as those fibers branch off before the radiation.

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• Medical History of Anesthesia:

  • Early methods involved alcohol, opium, or choking. Michael Faraday suggested ether; Sir Humphry Davy suggested nitrous oxide.

  • Crawford W. Long (1842): First to use ether for tumor removal; results not published immediately.

  • William Morton (1846): Publicly demonstrated ether at Massachusetts General Hospital; eventually went broke trying to patent it.

  • Modern Anesthetics: Isoflurane, Diazepam (activates GABA), and Local anesthetics (Procaine/Novocaine, which block $Na^+$ channels).