Veterinary Physiology: Vision and Gustation Overview

Bony Orbit

The cavity in the skull that houses and protects the eyeball; formed by multiple cranial bones.

Lacrimal Bone

A bone forming part of the bony orbit; includes the fossa for the nasolacrimal duct.

Maxilla

A cranial bone that contributes to the orbit; contains the caudal foramen of the infraorbital canal.

Infraorbital Canal

A canal in the maxilla through which the infraorbital nerve and vessels pass.

Temporal Bone

A bone forming the side of the skull; continuous with the bony orbit caudally.

Pterygopalatine Fossa

A deep cavity located caudal to the orbit; part of the orbital structure.

Nasolacrimal Duct

A duct that drains tears from the eye into the nasal cavity; associated with the lacrimal bone.

Orbit Position in Herbivores

The bony orbit is located laterally, allowing for a wide field of vision.

Orbit Position in Carnivores

The bony orbit is set forward to allow binocular vision and depth perception for hunting.

Transparent Media

The clear structures of the eye that allow light to pass through for image formation. Includes: Conjunctiva, Cornea, Aqueous humour, Lens, Vitreous humour.

Conjunctiva

A thin, transparent membrane that lines the inside of the eyelids and covers the sclera; protects and lubricates the eye.

Cornea

A transparent, avascular structure at the front of the eye that allows light entry and contributes significantly to refraction.

Lens

A transparent, elastic structure that changes shape to focus light onto the retina during accommodation.

Vitreous Humour

The gel-like substance filling the space between the lens and retina; helps maintain the eye's shape and optical clarity.

Aqueous Humour

The clear fluid found in the anterior and posterior chambers of the eye; nourishes the cornea and lens and maintains intraocular pressure.

Wall of the Eye

Composed of three main layers (tunics): Retina (nervous tunic), Uvea (vascular tunic) - includes choroid, ciliary body, and iris, Sclera (fibrous tunic).

Retina

The inner nervous layer of the eye that contains photoreceptors (rods and cones); converts light into electrical signals sent to the brain.

Uvea

The vascular middle layer of the eye, composed of: Choroid - provides oxygen/nutrients to outer retina, Ciliary Body - produces aqueous humour and contains muscles for accommodation, Iris - controls pupil diameter.

Sclera

The white, fibrous outer layer of the eye that provides protection and structure.

Refractive Indices

Measures how much light is bent (refracted) as it passes through different media: Air: 1.00, Cornea, aqueous humour, vitreous humour: 1.33, Lens: 1.42 (higher in the nucleus).

Accommodation

The process by which the lens changes shape to focus on near objects. Achieved by contraction of rectus muscle fibers in the ciliary body, increasing the optical power. Critical for maintaining clear vision as objects move closer.

Parasympathetic Innervation of the Eye

Supplied by oculomotor nerve (CN III) via the ciliary ganglion. Acts on circular muscles of the iris. Effect: Constriction of the pupil (miosis).

Sympathetic Innervation of the Eye

Supplied by the ophthalmic nerve via the cranial cervical ganglion. Acts on radial muscles of the iris. Effect: Dilation of the pupil (mydriasis).

Rod Cells

Photosensitive receptor cells in the retina specialized for night vision (scotopic vision). Highly sensitive to low light, found more at the periphery of the retina.

Cone Cells

Photoreceptor cells in the retina responsible for color vision and daytime (photopic) vision; function best in bright light.

Photoreceptors

Specialized neurons in the retina (i.e., rods and cones) that convert light into electrical signals.

Scotopic Vision

Vision under low-light conditions, primarily mediated by rod cells.

Peripheral Vision

The outer field of view, mostly supported by rod cells due to their higher density at the edges of the retina.

Rhodopsin

The light-sensitive pigment found in rod cells; made up of opsin (a protein) and retinene (vitamin A derivative).

Retinene (Retinal)

A derivative of vitamin A bound to opsin in rhodopsin; exists as cis-retinene in the dark, and converts to trans-retinene when exposed to light.

Cis-Retinene

The inactive form of retinene found in the dark; it is the light-sensitive form that converts to trans-retinene upon stimulation.

Trans-Retinene

The activated form of retinene that results from exposure to light; triggers the phototransduction cascade.

Opsin

The protein component of rhodopsin that binds retinene; its structure and association with retinene enables light detection.

Vitamin A (Retinol)

A fat-soluble vitamin (alcohol form) that is a precursor to retinene, which is crucial for vision.

Cis-Retinene (Retinal)

The light-sensitive aldehyde form of retinene derived from vitamin A; found in the dark and bound to opsin in rhodopsin.

Hyperpolarization

A change in membrane potential making the inside of the cell more negative; in photoreceptors, light causes hyperpolarization and inhibits neurotransmitter release.

Depolarization in Darkness

In the absence of light, photoreceptors are depolarized due to open cGMP-gated Na⁺/Ca²⁺ channels, leading to continuous release of neurotransmitters.

Glutamate (Rods)

The neurotransmitter released by rod cells in the dark under depolarized conditions.

Acetylcholine (Cones)

The neurotransmitter released by cone cells during dark depolarization.

cGMP (Cyclic Guanosine Monophosphate)

A second messenger that keeps ion channels open in darkness; maintains depolarization of photoreceptor cells.

Transducin

A G-protein activated by trans-retinene; it triggers the enzyme cGMP phosphodiesterase during phototransduction.

cGMP Phosphodiesterase

An enzyme activated by transducin that breaks down cGMP to GMP, causing closure of ion channels.

Ion Channels (Na⁺/Ca²⁺)

Membrane channels that allow sodium and calcium to enter the photoreceptor cell; open in darkness and closed when light triggers the phototransduction cascade.

Phototransduction Cascade

The biochemical pathway initiated when light hits the retina, involving cis-trans retinene conversion, transducin activation, cGMP breakdown, ion channel closure, hyperpolarization, and neurotransmitter inhibition.

Signal Amplification (in Rods)

A single rhodopsin molecule can activate multiple transducin proteins, allowing greater sensitivity to low light; amplifies the light signal in rod cells.

Light Desensitization in Rods

Prolonged exposure to bright light reduces rod cell sensitivity, making them less responsive—this is why rods function best in dim light.

Photopic Vision

Vision under high light intensity, primarily using cone cells; responsible for color and high visual acuity.

Fovea

A small, central pit in the retina where retinal layers are parted, allowing light to directly hit cone cells; provides the sharpest visual acuity.

Visual Acuity

The clarity or sharpness of vision; highest in cone cells because each cone synapses with a single interneuron, creating distinct, unamplified signals.

Iodopsin

The visual pigment in cone cells, made of opsin and retinene; similar to rhodopsin but adapted for bright light and color detection.

Rhodopsin vs Iodopsin

Rhodopsin = pigment in rod cells (dim light); Iodopsin = pigment in cone cells (bright light).

Cone Response to Light

Cone cells respond to light similarly to rods: cis-retinene converts to trans-retinene, triggering a phototransduction cascade—but using iodopsin as the base pigment.

Gustation

The sense of taste, involving chemoreceptors that detect specific chemical stimuli from food.

Chemoreceptors (Taste)

Sensory receptors that respond to chemical substances; located in taste buds on the tongue and surrounding areas.

Taste Modalities

The four primary taste types detected by animals: Sweet, Salty, Sour, Bitter.

Taste Buds

Structures that contain gustatory chemoreceptors; responsible for detecting taste stimuli. Primarily located on papillae on the dorsal surface of the tongue.

Papillae

Small projections on the tongue's surface that are modifications of lingual mucosa; house taste buds and assist in mechanical processing.

Gustatory Papillae

Papillae that contain taste buds and are involved in detecting taste.

Mechanical Papillae

Cornified papillae that do not contain taste buds; serve a protective role for the tongue's deeper tissues.

Lingual Mucosa

A tough mucous membrane covering the tongue, involved in protection and sensory function; supports papillae.

Extra-Lingual Taste Buds

Taste buds found outside the tongue, such as on the epiglottis and pharynx; minor but functionally relevant in taste perception.

Tongue

The primary organ of gustation, covered in lingual mucosa and papillae, which house taste buds.

Fungiform Papillae

Rounded papillae located at the tip and sides of the tongue. Each contains up to 5 taste buds on top. Cranial Nerve: Facial Nerve (CN VII) innervates these papillae.

Circumvallate Papillae

Large papillae arranged in a V-shape at the back of the tongue. Each contains up to 100 taste buds, located along the sides. Associated with von Ebner's salivary glands, which secrete fluid to wash away taste stimuli. Cranial Nerve: Glossopharyngeal Nerve (CN IX) innervates these papillae.

Foliate Papillae

Leaf-like folds located on the posterior lateral edges of the tongue. Contain up to 100 taste buds along their sides. Cranial Nerves: Innervated by both CN VII (Facial) and CN IX (Glossopharyngeal).

Cranial Nerve VII (Facial Nerve)

Innervates the fungiform and part of the foliate papillae. Responsible for taste sensation from the rostral 2/3 of the tongue.

Cranial Nerve IX (Glossopharyngeal Nerve)

Innervates the circumvallate and posterior foliate papillae. Responsible for taste sensation from the caudal 1/3 of the tongue.

Umami

A fifth basic taste modality, in addition to sweet, sour, salty, and bitter. Named from the Japanese word for 'delicious'. Detects amino acids and glutamate (commonly found in meat, cheese, and broth). Contributes to the savory flavor in food.

Filiform Papillae

The smallest and most numerous mechanical papillae; cover most of the tongue and provide a rough texture for grooming and food manipulation.

Conical Papillae

Larger but less numerous mechanical papillae; abundant in the ox and cat, giving their tongues a rough texture.

Marginal Papillae

Temporary mechanical papillae found in newborn carnivores; help with suckling during early life.

Gustatory Cells

Specialized chemoreceptor cells found in the center of the taste bud; detect specific taste stimuli and initiate action potentials.

Supporting (Sustentacular) Cells

Non-sensory cells that form the outer layer of the taste bud; provide structural support and maintain the taste bud's environment.

Basal Cells

Undifferentiated cells at the base of the taste bud; regenerate into supporting or gustatory cells to maintain taste function.

Taste Receptor

A specialized chemoreceptor responsible for detecting specific taste compounds, such as salt, sweet, sour, bitter, or umami.

Taste Transduction

The process by which a taste receptor converts a chemical stimulus into an electrical signal (action potential), which is then sent to the brain for interpretation.

Facial Nerve (CN VII)

Carries taste action potentials from the rostral two-thirds of the tongue to the brain.

Glossopharyngeal Nerve (CN IX)

Transmits taste signals from the caudal third of the tongue to the brain.

Vagus Nerve (CN X)

Conveys some taste information from the back of the oral cavity to the brain.

Salt Taste Transduction

Sodium ions (Na⁺) enter taste bud cells through ion channels on their membranes. This causes depolarization of the cell, which opens voltage-gated calcium (Ca²⁺) channels, allowing Ca²⁺ influx. The increase in intracellular calcium triggers neurotransmitter release, sending the taste signal to sensory nerves.

Sweet Taste Transduction

Sweet compounds (e.g., saccharides) activate G-protein-coupled receptors (GPCRs) on taste cells. This activates gustducin, a G-protein, which then stimulates adenylate cyclase inside the cell. Adenylate cyclase increases cAMP concentration, causing closure of potassium (K⁺) channels. Closure of K⁺ channels leads to depolarization of the taste cell, triggering neurotransmitter release.

Bitter Taste Transduction

Bitter compounds also activate GPCRs on taste receptor cells. Activated GPCR releases gustducin, which splits into three subunits. One subunit activates phosphodiesterase, an enzyme that produces a secondary messenger. The secondary messenger causes closure of potassium (K⁺) channels and stimulates the endoplasmic reticulum to release calcium ions (Ca²⁺). The increase in Ca²⁺ and closure of K⁺ channels cause depolarization. Depolarization leads to neurotransmitter release, transmitting the bitter taste signal.

Sour Taste

Indicates the presence of acidic compounds, primarily detected by hydrogen ions (H⁺).

Sour Taste Receptor Type 1

An ion channel that allows H⁺ ions to flow into the taste receptor cell, contributing to depolarization.

Sour Taste Receptor Type 2

A potassium (K⁺) ion channel that normally allows K⁺ to escape the cell. Hydrogen ions block this channel, trapping K⁺ inside, which contributes to depolarization.

Sour Taste Receptor Type 3

A protein channel that opens to allow sodium ions (Na⁺) into the cell when H⁺ ions bind to it. This Na⁺ influx triggers the opening of voltage-regulated calcium (Ca²⁺) channels, further promoting depolarization.

Sour Taste Transduction Mechanism

These receptors work together to depolarize the taste cell, causing release of neurotransmitters that signal sour taste to the brain.

Striated Muscle

Includes skeletal and cardiac muscles; characterized by visible striations due to the orderly arrangement of thick (myosin) and thin (actin) filaments in sarcomeres.

Smooth Muscle

Muscle without striations; lacks sarcomeres and has no orderly overlap of thick and thin filaments.

Skeletal Muscle Fiber Structure

Single muscle cells called muscle fibers. Each fiber surrounded by endomysium (connective tissue). Groups of fibers form fasciculi, surrounded by perimysium. Fasciculi bundles form the whole muscle.

Myofibrils & Sarcomeres

Skeletal muscle fibers and cardiac muscle cells contain myofibrils composed of sarcomeres arranged in series, responsible for striated appearance and contraction.