Senses - Lecture Flashcards
What effects interpretation of sensory information
Sensory signals are relayed from the receptor to a specific neuron in the CNS. Projection pathways carry information concerning specific sensations to specific destinations in the CNS
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Modality (Type of Stimulus):
- What it is: The kind of stimulus (e.g., light, sound, pressure, chemical)
- How encoded: Determined by the type of receptor activated and the labeled line—each sensory neuron carries only one type of information to a specific part of the brain
- Example: Photoreceptors → visual cortex = light; mechanoreceptors → somatosensory cortex = touch
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Location:
- What it is: Where the stimulus is coming from
- How encoded: Determined by which nerve fibers are activated and their corresponding brain region (sensory homunculus, for example)
- Receptive fields help determine resolution—smaller fields = greater localization (e.g., fingertips)
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Intensity:
- What it is: Strength of the stimulus
- How encoded: By firing frequency of the neuron and number of neurons recruited (temporal and spatial summation)
- Stronger stimuli = higher firing rate + more neurons recruited
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Duration:
- What it is: How long the stimulus lasts
- How encoded: Based on changes in firing over time
- Receptors can be:
- Phasic: Rapidly adapt (e.g., smell); fire at beginning/end
- Tonic: Slowly adapt or not at all (e.g., pain); maintain firing
Ways to classify sensory organs
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By Modality (Stimulus Type):
- Thermoreceptors – detect temperature
- Photoreceptors – detect light
- Nociceptors – detect pain
- Chemoreceptors – detect chemicals (smell, taste)
- Mechanoreceptors – detect pressure, vibration, stretch
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By Origin of Stimulus:
- Exteroceptors: Detect external stimuli (e.g., skin, eyes, ears)
- Interoceptors: Detect internal stimuli (e.g., stretch in GI tract)
- Proprioceptors: Detect body position and movement (e.g., muscles, tendons)
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By Distribution in the Body:
- General (somatosensory) senses: Widely distributed (e.g., touch, pressure, temperature, proprioception)
- Special senses: Limited to head (e.g., vision, hearing, taste, smell, equilibrium)
List Receptors of General Senses, base function and general location
General characteristic of sensory receptor
Sensory receptors transduce stimulus energy, generate receptor potentials, and may lead to conscious or unconscious sensations.
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Transduction:
- Sensory receptors act as transducers, converting one form of energy (e.g., light, heat, pressure, chemical) into electrical signals (nerve impulses)
- This is the fundamental function of all sensory receptors
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Receptor Potential:
- A local electrical change in the receptor cell membrane caused by a stimulus
- May lead to the release of neurotransmitters (in non-neuronal receptors) or trigger action potentials (in sensory neurons), which then transmit signals to the CNS
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Sensation:
- The subjective awareness of a stimulus
- Not all incoming signals reach conscious perception
- Many are filtered out by the brainstem / hypothalamus to prevent sensory overload
- Some are processed unconsciously (e.g., blood pH, body temperature)
Define receptive field
The receptive field is the area of the body that, when stimulated, activates a particular sensory neuron.
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Function:
Determines how precisely a stimulus can be located (spatial resolution) -
Size Variation:
- Small receptive fields → allow fine spatial discrimination (e.g., fingertips, lips)
- Large receptive fields → allow poor localization (e.g., back, thighs)
Describe Somatosensory Projection Pathway
]Pathway by which somesthetic sensory signals (e.g., touch, pressure, pain, temperature, proprioception) travel from receptors to the primary somatosensory cortex of the brain
- Involves Three Neurons:
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First-order neuron (afferent neuron):
- From body → enters spinal cord via dorsal root of spinal nerves
- From head → enters brainstem (pons or medulla) via cranial nerves (mainly CN V)
- Large, myelinated axons for touch, pressure, proprioception
- Small, unmyelinated axons for pain, temperature
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Second-order neuron:
- Located in spinal cord or brainstem
- Decussates (crosses midline) to the opposite side in spinal cord, medulla, or pons
- Ascends to and ends in the thalamus
- Exception: Proprioception signals end in the cerebellum
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Third-order neuron:
- From thalamus to primary somatosensory cortex in the cerebrum
Summary: Receptor → 1st-order neuron → decussation → 2nd-order neuron → thalamus → 3rd-order neuron → somatosensory cortex
Sensory Adaptation
A decrease in sensitivity to a constant stimulus over time. Can occur at the receptor level or within the central nervous system (CNS).
Two Types of Adaptation:
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Peripheral Adaptation
- Occurs at the sensory receptor
- Receptors respond strongly at first, then reduce signaling even if the stimulus continues
- Example: Smell of perfume fades after initial exposure
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Central Adaptation
- Occurs within the CNS (e.g., reticular formation, thalamus)
- Sensory signals may still arrive, but the brain filters them out
- Leads to loss of conscious awareness of the stimulus
- Example: Stop feeling your clothes after putting them on
Describe the Types and Mechanisms of Pain
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Nociceptors detect pain through three main types of stimuli:
- Mechanical – cutting, crushing, or pressure
- Thermal – extreme heat or cold
- Chemical – released by damaged tissues
Somatic Pain (skin, muscles, joints):
- Typically triggered by:
1. Mechanical injury (cuts, pressure, trauma)
2. Thermal extremes
3. Chemical mediators released from injured cells (e.g., bradykinin, histamine, prostaglandins, serotonin)
- Usually well-localized due to dense innervation and fast pain fibers
Visceral Pain (internal organs):
- Triggered mainly by:
- Stretch or distension (e.g., bloating, cramping)
- Ischemia (reduced blood flow)
- Chemical irritation (e.g., acid, inflammatory mediators)
- Poorly localized, often felt as dull or diffuse
Projection Pathway for Pain
Pain signals ascend to the brain through a three-neuron pathway, using different tracts depending on the type and source of pain.
1. First-Order Neuron (Afferent):
- Detects pain via nociceptors in skin, muscles, joints, or organs
- Cell body in dorsal root ganglion
- Enters dorsal horn of spinal cord and synapses with second-order neuron
2. Second-Order Neuron:
- Decussates (crosses to the opposite side) in the spinal cord
- Ascends via one of the following tracts:
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Spinothalamic Tract
- Carries somatic pain (sharp, localized pain from skin, muscles, joints)
- Travels to thalamus for sensory relay to cortex
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Spinoreticular Tract
- Carries visceral pain (dull, poorly localized pain from internal organs)
- Projects to the reticular formation, then to hypothalamus and limbic system, contributing to emotional and autonomic response
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Gracile Fasciculus (in posterior column pathway):
- Normally transmits deep touch, proprioception, and vibration from lower body
- Can also carry visceral pain signals, especially from pelvic organs
- Ascends ipsilaterally to the gracile nucleus in the medulla, then crosses over
3. Third-Order Neuron:
- Arises from the thalamus (or other relay center like reticular formation)
- Projects to the primary somatosensory cortex for conscious perception of pain
Referred Pain
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Definition:
Pain that is perceived at a location other than its actual source, typically on the body surface rather than the internal organ
Mechanism:
- Visceral and somatic pain fibers often converge onto the same spinal interneurons in the dorsal horn of the spinal cord
- The brain, more familiar with somatic pain, misinterprets the signal as coming from a somatic region*(like skin or muscle)
- This occurs because the CNS cannot distinguish the true origin when both pathways share a common projection
Example – Heart Attack:
- Pain from the heart (visceral organ) is referred to the left shoulder and arm
- This is because T1–T5 spinal segments receive input from both the heart and the left upper limb, leading the brain to project the pain to the limb instead of the chest
Endogenous Opioids
Endogenous opioids are internally produced, opium-like neuropeptides that act as natural painkillers.
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Types:
- Enkephalins – two small analgesic peptides, up to 200× more potent than morphine
- Endorphins and dynorphins – larger neuropeptides with analgesic effects
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Sources:
- Secreted by the central nervous system, pituitary gland, digestive tract, and other organs
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Function:
- Act as neuromodulators
- Inhibit pain transmission
- Also produce feelings of pleasure and euphoria
Pain modulation Pathway
The brain can inhibit pain signals through a descending analgesic pathway and spinal gating mechanisms.
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Pathway Steps:
- Cerebral cortex and hypothalamus detect pain and activate descending signals
- Signal travels to midbrain
- Midbrain relays signal to medulla oblongata
- Medulla neurons secrete serotonin onto spinal interneurons in dorsal horn
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Spinal interneurons release enkephalins, which:
- Inhibit second-order pain neurons (postsynaptic inhibition)
- Inhibit first-order nociceptors (presynaptic inhibition)
Define Spinal Gating
Spinal gating is a mechanism by which pain signals are inhibited at the level of the spinal cord, preventing them from reaching the brain.
Mechanisms of Spinal Gating:
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Descending Analgesic Pathway (Endogenous inhibition):
- Brainstem structures (midbrain, medulla) activate inhibitory interneurons in the spinal cord
- These interneurons release enkephalins, which:
- Inhibit presynaptic nociceptors (reducing neurotransmitter release)
- Inhibit postsynaptic second-order neurons (blocking signal transmission)
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Somatic Mechanoreceptor Activation (“Gate Control” Theory):
- Rubbing, shaking, or moving the painful area activates touch receptors
- These signals stimulate inhibitory interneurons in the spinal cord
- The interneurons dampen the pain signal by suppressing second-order neurons
Lingual Papillae
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Fungiform papillae – scattered, contain taste buds
- Vallate (circumvallate) papillae – large, arranged in a V-shape at back of tongue; contain numerous taste buds
- Foliate papillae – ridges on the sides of tongue; taste buds present, especially in children
- Filiform papillae – most numerous; no taste buds; aid in texture sensing and food manipulation
Structure taste bud
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Location:
Embedded within lingual papillae (especially vallate, fungiform, and foliate)
Main Cell Types:
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Taste Cells (Gustatory Cells):
- Not neurons, but epithelial cells with microvilli (taste hairs) projecting into the taste pore
- Taste hairs detect tastants (chemicals)
- Synapse with sensory nerve fibers at their base
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Supporting Cells:
- Look similar to taste cells
- Do not have taste hairs or synaptic vesicles
- Likely provide structural support
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Basal Cells:
- Stem cells that divide to replace taste cells (lifespan: ~40–60 days)
- Also give rise to supporting cells
Additional Structures:
- Taste Pore: Opening on the epithelial surface through which taste hairs extend
- Sensory Fibers: Carry signals from taste cells to the brain via cranial nerves
Mechanisms of Taste Cell Activation
Each taste modality is triggered when a dissolved tastant binds or passes through structures on the taste cell, causing a chain reaction that ends in neurotransmitter release to a sensory neuron.
1. Salty (Na⁺):
- Tastant: Sodium ions (Na⁺) from salty substances (e.g., table salt)
- Mechanism: Na⁺ enters through ion channels
- Effect: Depolarization → Ca²⁺ influx → neurotransmitter release
2. Sour (H⁺):
- Tastant: Hydrogen ions (H⁺) from acidic foods (e.g., citrus)
- Mechanism: H⁺ enters through ion channels or blocks K⁺ channels
- Effect: Depolarization → Ca²⁺ influx → neurotransmitter release
3. Sweet:
- Tastant: Sugars (e.g., glucose, sucrose) dissolved in saliva
- Mechanism: Sugar binds to a sweet receptor (G-protein-coupled) on the taste cell
- Effect: Activates G-protein (gustducin) → cAMP cascade → closes K⁺ channels → depolarization → neurotransmitter release
4. Umami:
- Tastant: Amino acids, especially glutamate (e.g., in meats, broths)
- Mechanism: Glutamate binds to umami receptor (G-protein-coupled)
- Effect: Activates G-protein → second messenger pathway → depolarization → neurotransmitter release
5. Bitter:
- Tastant: Bitter alkaloids (e.g., caffeine, quinine)
- Mechanism: Binds to bitter receptor (G-protein-coupled)
- Effect: Activates G-protein → second messenger → Ca²⁺ released from internal stores → neurotransmitter release
Projection pathway for taste
1. Cranial Nerves Involved (based on taste region):
- Facial nerve (CN VII):
- Carries taste from anterior 2/3 of the tongue
- Glossopharyngeal nerve (CN IX):
- Carries taste from posterior 1/3 of the tongue
- Vagus nerve (CN X):
- Carries taste from epiglottis, palate, and pharynx
> Different regions of the tongue are innervated by different nerves, allowing for broad sensory input.
2. Medulla Oblongata (Solitary Nucleus):
- All taste fibers synapse in the solitary nucleus of the medulla
- Here, taste input is initially sorted and integrated
- The medulla coordinates reflexive responses like salivation, gagging, swallowing, and vomiting
- It sends signals both upward to the thalamus and laterally to emotional centers
3. Hypothalamus & Amygdala (Limbic System):
- Receive input from the medulla
- Hypothalamus: Regulates autonomic responses like hunger, thirst, digestive activity
- Amygdala: Assigns emotional tone to tastes (e.g., pleasure, disgust, craving)
> These regions connect taste to motivation, behavior, and memory (e.g., taste aversions or cravings)
4. Thalamus (Ventral Posteromedial Nucleus):
- Acts as the relay station
- Sends taste signals to the primary gustatory cortex
5. Primary Gustatory Cortex (Insula & Lower Postcentral Gyrus):
- Located in the insula and frontal operculum
- Responsible for conscious perception of taste qualities (sweet, salty, etc.)
6. Orbitofrontal Cortex (Frontal Lobe):
- Integrates taste, smell, texture, temperature, and appearance
- Contributes to flavor perception and hedonic evaluation (“Do I like this?”)
Olfactory Epithelium
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Location:
Found in the superior region of the nasal cavity, covering the superior nasal concha, nasal septum, and cribriform plate -
Key Cell Types:
- Olfactory receptor cells – bipolar neurons with olfactory hairs (cilia) that detect odorants
- Supporting cells – columnar cells that support and nourish receptor cells
- Basal cells – stem cells that replace olfactory neurons (which live ~60 days)
- Olfactory glands (Bowman’s glands) – secrete mucus that dissolves odorants
Mechanism of Olfactory Receptor Activation
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Step 1:
Odorant molecules (chemicals in air) dissolve in mucus of the olfactory epithelium -
Step 2:
Odorant binds to a G-protein-coupled receptor (GPCR) on the olfactory cilia of receptor cells -
Step 3:
Activates G-protein → increases cAMP
→ cAMP opens Na⁺ and Ca²⁺ channels → depolarization -
Step 4:
Action potential travels along the axon of the olfactory receptor neuron, through the cribriform plate, to the olfactory bulb
Olfactory projection pathway
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Step 1:
Olfactory receptor axons pass through the cribriform plate and synapse in the olfactory bulb -
Step 2:
Signals travel via the olfactory tract directly to multiple brain regions — bypassing the thalamus initially -
Primary Targets:
- Primary olfactory cortex (temporal lobe) → conscious smell perception
- Amygdala → emotional responses (e.g., attraction, disgust)
- Hypothalamus → autonomic responses and behavioral drive
- Orbitofrontal cortex (via later thalamic relay) → integrates odor with taste to form overall flavor and value judgment
Lacrimal Apparatus and tear flow pathway
- Function: Produces and drains tears to clean, protect, and moisten the eye
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Tear Production:
- Lacrimal gland (superolateral orbit) continuously secretes tears
- Tears contain bactericidal enzymes (like lysozyme), aid in O₂/CO₂ diffusion, and flush away debris
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Tear Flow Pathway:
- Tears spread across the eye surface
- Drain through lacrimal puncta (on medial eyelids)
- Enter lacrimal canaliculi → lacrimal sac
- Flow into the nasolacrimal duct
- Exit into inferior nasal meatus, explaining runny nose during crying
Palpebrae - Function and key structures
The palpebrae maintain eye moisture and protection through blinking, structural support, and oil secretion.
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Function:
- Protect the eye from injury, debris, and excessive light
- Blink reflex spreads tears to moisten cornea and clear particles
- Help with sleep by closing eye and blocking stimuli
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Key Structures:
- Orbicularis oculi muscle: enables blinking and closing the eyelids
- Tarsal plate: stiff connective tissue that gives eyelid shape
- Tarsal glands (Meibomian): secrete oily substance to slow tear evaporation
- Eyelashes: trap and deflect airborne particles
Conjunctiva – Structure and Function
- Transparent mucous membrane that lines the inner surface of the eyelids and covers the anterior eyeball (excluding the cornea)
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Functions:
- Lubricates the eye by producing mucus and tears (minor contribution)
- Protects eye by trapping debris and pathogens
- Provides a vascular supply to the sclera and heals rapidly when damaged
- Contains abundant sensory innervation
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Clinical Note:
- Inflammation due to infection or irritation leads to conjunctivitis (pink eye)
> Summary: The conjunctiva is a mucous membrane that protects and nourishes the anterior eye surface and eyelids.
Extrinsic Eye Muscles – Actions, Innervation, and Deficits
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Superior oblique
- Nerve: Trochlear (IV)
- Movement: Internal rotation, slight depression
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Superior rectus
- Nerve: Oculomotor (III)
- Movement: Rolls eye up
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Lateral rectus
- Nerve: Abducens (VI)
- Movement: Rolls eye laterally
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Medial rectus
- Nerve: Oculomotor (III)
- Movement: Rolls eye medially
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Inferior oblique
- Nerve: Oculomotor (III)
- Movement: External rotation, slight elevation
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Inferior rectus
- Nerve: Oculomotor (III)
- Movement: Rolls eye down
intrinsic eye muscles
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Ciliary muscle
- Function: Changes lens shape for focusing (accommodation)
- Innervation: Parasympathetic fibers of Oculomotor nerve (CN III) via ciliary ganglion
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Sphincter pupillae (circular muscle)
- Function: Constricts pupil (miosis) in bright light
- Innervation: Parasympathetic fibers from CN III via ciliary ganglion
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Dilator pupillae (radial muscle)
- Function: Dilates pupil (mydriasis) in low light or sympathetic activation
- Innervation: Sympathetic fibers from superior cervical ganglion
Emmetropia
- State of relaxed vision when viewing distant objects (≥ 20 ft / ~6 m)
- Ciliary muscles relaxed → suspensory ligaments taut → lens thin
- Light focuses directly on retina without accommodation
Convergence
- Inward movement of both eyes to focus on near object
- Ensures light from object hits same spot on both retinas
- Uses medial rectus muscles (CN III – Oculomotor nerve)
Lens accommodation
- Adjustment of lens curvature to focus on close objects
- Ciliary muscles contract → suspensory ligaments relax → lens thickens
- Controlled by parasympathetic fibers of CN III via ciliary ganglion
Pupillary miosis
- Constriction of pupil to improve near focus and reduce spherical aberration
- Circular muscle of iris contracts
- Controlled by parasympathetic fibers from CN III via ciliary ganglion
Near response
- Group of adjustments allowing the eyes to focus on nearby objects
- Includes three coordinated actions:
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Convergence of eyes
- Eyes turn medially to align visual axes with the object
- Ensures image is focused on the same part of each retina
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Constriction of pupil
- Reduces entry of peripheral light rays
- Minimizes spherical aberration (blurry edges), improving image clarity
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Accommodation of lens
- Ciliary muscle contracts → suspensory ligaments relax → lens becomes rounder
- Increases refraction to focus light on retina
Visual Accommodation
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Myopia (nearsightedness)
- Eye is too long or cornea too curved
- Light focuses in front of retina → distant objects appear blurry
- Corrected with concave (diverging) lenses
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Hyperopia (farsightedness)
- Eye is too short or cornea too flat
- Light focuses behind retina → near objects appear blurry
- Corrected with convex (converging) lenses
Aqueous Humor: Flow and Function
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Production:
- Secreted by ciliary body epithelium into the posterior chamber (behind iris)
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Flow pathway:
- Posterior chamber
- Through the pupil
- Into the anterior chamber
- Drains into the canal of Schlemm (scleral venous sinus)
- Enters venous circulation
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Function:
- Nourishes lens and cornea
- Maintains intraocular pressure
- Removes metabolic waste
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Clinical Note:
- Impaired drainage leads to increased pressure → glaucoma
Posterior Cavity and Vitreous Humor
- Vitreous humor: Gelatinous substance in the posterior cavity between lens and retina
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Functions:
- Maintains intraocular pressure
- Supports the shape of the eye
- Presses retina against choroid, allowing nutrient diffusion
- Stabilizes position of lens and retina
- Aqueous humor flows across its surface toward retina
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Clinical note:
- Loss of pressure (e.g., with liquefaction of vitreous) can lead to retinal detachment
Tunics of Eye
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Fibrous Tunic
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Sclera
- Provides structural support and protection
- Serves as attachment site for extrinsic eye muscles
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Cornea
- Transparent and curved to refract (bend) light into the eye
- Allows light to enter the anterior eye
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Sclera
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Vascular Tunic (Uvea)
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Choroid
- Highly vascularized; supplies nutrients and oxygen to retina
- Pigmented to absorb stray light and prevent reflection inside eye
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Ciliary Body
- Contains ciliary muscles that control the shape of the lens (accommodation)
- Secretes and reabsorbs aqueous humor into the anterior cavity
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Iris
- Colored part of the eye
- Contains smooth muscles that adjust pupil diameter to regulate light entry
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Choroid
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Neural Tunic (Retina)
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Pigmented Layer
- Absorbs stray light and prevents image distortion
- Supports and nourishes photoreceptors
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Neural Layer
- Contains photoreceptors (rods and cones) that detect light
- Converts light into neural signals sent via the optic nerve
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Pigmented Layer
Retina structure
The retina (neural tunic) has two layers:
- Pigmented epithelium: Outer layer that absorbs stray light, supports photoreceptor metabolism, and continually regenerates photopigments.
- Neural layer: Inner layer containing photoreceptors and neurons for visual signal processing.
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Photoreceptors:
- Rods: Specialized for low-light (scotopic) vision; do not detect color.
- Cones: Specialized for color vision and sharp detail in bright light.
- Each has an outer segment with light-sensitive discs and an inner segment with mitochondria and organelles.
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Synaptic pathway:
- Rods/cones detect light and generate receptor potentials (not action potentials).
- Signal passes to bipolar cells (first-order neurons).
- Then to ganglion cells, which generate action potentials that travel via the optic nerve.
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Support and anchoring:
- Retina is firmly attached only at the optic disc (posteriorly) and ora serrata (anteriorly).
- Aqueous humor and vitreous body help press retina against the choroid to maintain contact.
Distribution of rods and cones in the retina
- Macula lutea: Central region of retina with high cone concentration for detailed, color vision.
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Fovea centralis (center of macula):
- Contains only cones (100% cones).
- Site of greatest visual acuity and sharpest image formation.
- Best color and detail vision occurs when an object is focused here.
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Optic disc:
- Contains no rods or cones.
- Location where ganglion cell axons exit the eye to form the optic nerve.
- Known as the blind spot — no vision occurs if an image lands here.
- The brain “fills in” missing information, and constant small eye movements help prevent gaps in perception.
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Peripheral retina:
- Contains more rods than cones.
- Specialized for low-light vision and motion detection, but lower resolution.
Non Receptor cells of the retina
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Bipolar cells
- Function: First-order neurons; receive input from rods and cones
- Synapse on: Ganglion cells
- Note: High convergence with rods (many rods → one bipolar cell); little to no convergence with cones (1:1 ratio enhances visual acuity)
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Ganglion cells
- Function: Second-order neurons; generate action potentials and transmit signals to brain
- Axons form: Optic nerve (CN II)
- Note: More convergence from rod pathways than cone pathways
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Horizontal cells
- Function: Connect photoreceptors laterally
- Role: Modulate input to bipolar cells; help enhance contrast and spatial resolution
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Amacrine cells
- Function: Connect bipolar cells and ganglion cells laterally
- Role: Involved in contrast enhancement, motion detection, and adapting to light intensity changes
Lens – Role in Vision
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Function:
- Transmits light from the pupil to the retina
- Focuses inverted image onto photoreceptors of retina
- Changes shape for accommodation (focusing near vs far)
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Structure:
- Made of concentric layers of transparent cells
- Suspended by suspensory ligaments behind iris
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Accommodation:
- Ciliary muscles contract → lens rounds → near vision
- Ciliary muscles relax → lens flattens → distance vision
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Pathway of light:
- Passes through cornea → anterior cavity → pupil → lens
- Lens focuses light on retina → photoreceptors depolarize → signal sent via CN II (optic nerve)
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Photoreceptor activation:
- Cones need bright light → color vision
- Rods activate in low light → grayscale night vision (cones deactivate)
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Clinical note:
- Cataract = clouding of lens → blurred vision
- Caused by aging, UV, diabetes, smoking
Structure of Rods and Cones
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Pigment Epithelium:
- Located behind photoreceptors
- Absorbs stray light to prevent reflection
- Engulfs and recycles shed outer segment discs
- Stores vitamin A needed for retinal regeneration
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Outer Segment:
- Contains membrane discs stacked in cylindrical shape
- Discs contain visual pigments:
- Rods: rhodopsin (opsin + retinal)
- Cones: photopsin (opsin + retinal)
- Retinal undergoes cis → trans isomerization upon light exposure (bleaching)
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Inner Segment:
- Contains nucleus and organelles (mitochondria, Golgi apparatus)
- Responsible for ATP production and protein synthesis (including photopigments)
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Synaptic Terminal:
- Forms synapse with bipolar cells (first-order neurons)
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Photopigments:
- Rhodopsin (rods): absorbs at ~500 nm; highly sensitive to dim light
- Photopsin (cones): three variants absorbing short (420 nm), medium (531 nm), and long (558 nm) wavelengths for color vision
Bleaching
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Bleaching is the light-induced breakdown of a photopigment after it absorbs a photon.
- Specifically, retinal changes from the cis to trans form, causing it to dissociate from opsin.
- This reaction renders the opsin temporarily nonfunctional for detecting more light.
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Why It’s Important:
- Bleaching is necessary to trigger a visual signal (hyperpolarization of photoreceptor → signal to bipolar cells).
- However, once bleached, the pigment cannot respond to light again until retinal is converted back to the cis form and rebinds to opsin — this is the recovery or regeneration phase.
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Physiological Significance:
- Prevents continuous stimulation — each pigment must reset before detecting light again.
- Explains why vision adapts slowly in darkness (rods must regenerate rhodopsin).
- Explains color fade in bright light (cones bleach faster than they regenerate).
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Time to Regenerate:
- Cones: recover in 90 seconds → why bright-light vision returns quickly
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Rods: take 5 to 10 minutes or longer → explains slow dark adaptation
- Rods are highly sensitive but recover slowly after bleaching
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Overall Role in Vision:
- Ensures visual sensitivity adapts to changing light conditions
- Underlies light and dark adaptation and protects photoreceptors from overstimulation
Only a fraction of photopigments are bleached at any given time. While some photoreceptors are being stimulated and bleached, others are recovering and being recharged by the retinal pigment epithelium (RPE). The RPE absorbs stray light and recycles retinal from the trans → cis form. Rods and cones don’t all fire at once — you have millions of them, and they operate in overlapping fields, so even if one is in the middle of bleaching and recovery, nearby ones are active and feeding your brain the visual signal. Meanwhile, your eyes constantly move (microsaccades) — this prevents any single photoreceptor from being locked into continuous activation or bleaching.
Color vision
based on the stimulation of three types of cone photoreceptors, each containing a different type of photopsin that absorbs light at different wavelengths:
- Short-wavelength (S) cones: peak at 420 nm (blue light)
- Medium-wavelength (M) cones: peak at 531 nm (green light)
- Long-wavelength (L) cones: peak at 558 nm (red light)
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Color perception arises from the relative activation of these three cone types; the brain interprets the ratio of activity to generate the perceived hue.
- Example: Equal activation of red + green cones → perception of yellow.
- Blue-green light (~500 nm) → activates both blue and green cones to varying degrees.
- Rods do not contribute to color vision; they are active in low light and peak at ~500 nm.
- Light must fall within 400–750 nm to be detected:
- <400 nm: damaging to cells (UV)
- > 750 nm: not energetic enough (infrared)
color blindness
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Color blindness is a genetic or acquired condition in which one or more types of cone photoreceptors are missing or nonfunctional.
- Most common form: red-green color blindness
- Caused by absence or malfunction of L (red) or M (green) cones
- Individuals have difficulty distinguishing reds from greens
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Blue-yellow color blindness (rare):
- Caused by lack of S (blue) cones
- Impairs blue-yellow discrimination
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Total color blindness (monochromacy):
- Extremely rare
- No functional cones → vision relies on rods only → black-and-white vision
- Most common form: red-green color blindness
Dual Vision system
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Rods and cones provide complementary functions—you can’t have a cell that is both highly sensitive and high resolution.
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Rods
- High sensitivity to light (activated by very little light)
- Enable night vision (scotopic vision)
- Located mostly in peripheral retina
- Exhibit high convergence: multiple rods synapse on fewer bipolar/ganglion cells → increased sensitivity but low resolution
- Monochrome vision (no color)
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Cones
- Require bright light to activate
- Densely packed in fovea centralis
- Exhibit low convergence: one-to-one or few-to-one signaling → high resolution, precise spatial detail
- Enable color vision and sharp focus (photopic vision)
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Why both?
- Rods give us sensitivity to detect light in dim conditions
- Cones give us acuity and color, but only when light is sufficient
- A single type of cell cannot do both due to trade-off between sensitivity and spatial precision, so a dual system is essential
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Rods
Light and Dark Adaptation
Light adaptation (e.g., stepping into sunlight):
- Pupils constrict to reduce light entry
- Photopigments (esp. rhodopsin) bleach faster than they can regenerate
- Retinas overstimulated, causing discomfort and temporary reduced acuity
- Color vision and sharpness reduced for ~5–10 minutes until cones adjust
Dark adaptation (e.g., lights off):
- Pupils dilate to allow more light in
- Rhodopsin in rods regenerates slowly (~20–30 minutes)
- Rods gradually take over from cones, restoring night vision
Stereoscopic Vision
The ability to perceive depth and three-dimensional structure by comparing slightly different views from the left and right eyes.
- Mechanism:
- Each eye views the same object from a slightly different angle (binocular disparity).
- The brain uses this disparity to compute depth and relative distance.
Visual Pathway
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1. Retina:
- Photoreceptors (rods & cones) → activate bipolar cells (1st-order neurons) → stimulate ganglion cells (2nd-order neurons).
- Axons of ganglion cells form the optic nerve (CN II).
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2. Optic Nerve → Optic Chiasm:
- Right and left optic nerves converge at the optic chiasm.
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Partial decussation:
- Nasal retinal fibers (receiving lateral visual field) cross to the contralateral side.
- Temporal retinal fibers (receiving medial visual field) remain uncrossed.
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3. Optic Tracts:
- Each optic tract carries information from the contralateral visual field.
- (e.g., left optic tract = right visual field from both eyes)
- Project to the lateral geniculate nucleus (LGN) of the thalamus.
- Each optic tract carries information from the contralateral visual field.
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4. Thalamus to Cortex:
- Third-order neurons from the LGN → travel via optic radiations → to primary visual cortex (V1) in the occipital lobe.
- Interprets basic visual information (e.g., orientation, movement).
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Other Targets:
- Superior colliculi: Reflexive eye/head movement
- Pretectal nuclei: Pupillary light reflex
- Suprachiasmatic nucleus: Circadian rhythm regulation
