30Sense organs

πŸ“š
EYE
β–’ Introduction:

An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system.

β–’ Accessory structures:
Image 1
❖ Eyebrow:
❖ Eyelids/ Palpebra:
β—‰ Glands:
β—ˆ Glands:
β–  At the base of Hair follicle:
● Gland of Moll: Sweat gland
● Gland of Zeis:
⟣ Modified form of: Sebaceous gland
⟣ Inflammation of gland of moll and gland of zeis is called Stye:
β–  On the Tarsal plate:
● Tarsal glands/ Meibomian glands:
⟣ Inflammation: Chalazion
β—‰ Palpebral fissure: Opening between the upper and lower eyelid.
β—‰ Nictitating membrane/ Third eyelid:
β—ˆ Supported by Harderian gland: Frog and Birds
β—ˆ Supported by Meibomian gland: Rabbit
β—ˆ Vestigial in human: Plica semilunaris
❖ Eyelashes:
❖ Conjunctiva:
β—‰ Palpebral conjunctiva:
β—ˆ Location: Lining the eyelids/ Palpebra
β—ˆ Clinical significance: Pallor for Anaemia is seen here
β—‰ Bulbar conjunctiva:
β—ˆ Location: Lining the eyeball(sclera only)
β—ˆ Clinical significance: Jaundice for hepatitis is seen here
❖ Lacrimal gland:
Image 1
β—‰ Function: Secrete tear which contain salts and lysozyme (bactericidal) and is isotonic to blood and normal saline (0.9% NaCl solution)
β—‰ Drainage pathway:
  1. Lacrimal gland
  2. Lacrimal duct
  3. Lacrimal punctum
  4. Lacrimal canal
  5. Lacrimal sac
  6. Nasolacrimal duct
  7. Nasal cavity
β—‰ Inflammation: Dacrocystitis/ Dacroadenitis
❖ Extrinsic eye Muscles:
  1. Superior rectus
  2. Inferior rectus
  3. Lateral rectus
  4. Medial rectus
  5. Superior oblique
  6. Inferior oblique
Image 1
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❖ Muscle defect: Strabismus/ Squint eyes
❖ Nerves:
β—‰ Cranial nerves involved: 6 out of 12 cranial nerves (II, III, IV, V, VI and VII)
β—‰ Muscle control: 3 control extrinsic muscles (LR_6 SO_4 Rest_3)
β–’ Eyeball:
Image 1
❖ Tunica fibrosa:
β—‰ Cornea:
β—ˆ Location: Anterior part
β—ˆ Structure: Transparent layer made up of 5 layers
β—ˆ Epithelium: Non-keratinized stratified squamous epithelium
β—ˆ Function: Major site of refraction
β—ˆ Vascularity: Avascular layer (used in eye donation - Keratoplasty)
β—ˆ Inflammation: Keratitis
β—‰ Sclera:
β—ˆ Origin: Mesodermal
β—ˆ Composition:
β–  Frog: Cartilage
β–  Rabbit and Human: White, hard, opaque thick fibrous connective tissue
❖ Tunica vasculosa:
β—‰ Description: Highly vascularized layer
β—‰ Iris:
β—ˆ Pigmentation:
β–  Cause: Melanin pigment produced by Melanocytes
β–  Rabbit: Red eye due to Red melanin
β–  Human: Green, Blue, Brown or Black eye
β—ˆ Pupil: Aperture formed by Iris
β—ˆ Muscles:
β–  Outer Radial Muscle: Contraction causes Dilation of pupil (Mydriasis)
β–  Inner Circular muscle: Contraction causes Constriction of pupil (Miosis)
β—‰ Ciliary body:
Image 1
β—ˆ Components:
  1. Ciliary muscle
  2. Ciliary process (Produces Aqueous humor)
  3. Suspensory ligament/ Zonules of Zinn (Holds the lens)
β—ˆ Chambers:
Image 1
β–  Aqueous chamber:
● Contents: Aqueous humor
● Subdivisions:
⟣ Anterior aqueous chamber:
⟣ Posterior aqueous chamber:
● Drainage:
⟣ Canal of Schlemm:
➒ Function: Aqueous humor drains out through it
➒ Disorder: Glaucoma (Blockage causes increased intraocular pressure)
β–  Vitreous chamber:
● Contents: Vitreous humor
● Hyaloid canal/ Cloquet's canal:
⟣ Location: From blind spot to Central point of lens
⟣ Function: Provides nourishment to the developing lens
β—‰ Choroid:
β—ˆ Description: Pigmented vascular layer which prevents the internal reflection of light inside the eyeball
β—ˆ Tapetum lucidum: Reflecting layer between retina and choroid found in carnivores (cat, dog, lion, etc.) that causes eyes to glow at night
❖ Tunica nervosa/ Retina:
Image 1
β—‰ Origin: Ectodermal
β—‰ Landmark: Orra serrata
β—‰ Layers:
β—ˆ Pigmented layer (Outer): Retinal pigment epithelium
β—ˆ Nervous tissue layer/ Neural layer (Inner):
β–  Components:
  1. Rods and cones (photoreceptors)
  2. Outer limiting membrane
  3. Outer nuclear layer
  4. Outer plexiform layer
  5. Inner nuclear layer (Bipolar cell, Horizonal cell, Amacrine cell, Muller cell)
  6. Inner plexiform layer
  7. Ganglion cell layer
  8. Nerve fiber layer
  9. Inner limiting membrane
Image 1
β–  Photoreceptors:
● Rod cells:
⟣ Number: 120 million
⟣ Function: Dim light vision
⟣ Pigment: Rhodopsin (Visual purple)
⟣ Specialization: Only rods in Nocturnal animals (Owl, rats, bats)
β–’ Disorders of eye:
Disorder
Defects
Cause
Correction
Myopia/ Short sightdness
Can see near object only/ Can't see far object
Decrease in focal length/ Increased eyeball size
Concave lens
Hypermetropia/ Long sightedness
Can see far object only/ Can't see near object
Increased focal length/ Decreased eyeball size
Convex lens
Astigmatism
Blurred vision
Irregular curvature of Cornea
Cylindrical lens
Presbyopia
Blurred vision
Loss of accomodating power in Old age
Bifocal lens
Cataract
Increased opacity of lens
Glaucoma
Increased intraocular pressure
Blockage of Canal of Schlemm
Trachoma (IOM 2004)
Inflammation of Conjunctiva
Chlamydia trachomatis
Xerophthalmia/ Dry eye
Keratinisation of conjunctiva and cornea/ Decreased tear production
Vitamin A deficiency
Photophobia
Difficulty seeing in bright light
πŸ“š
EAR
Image 1
β–’ External Ear:
❖ Distribution: Well developed in Mammals only (absent in whale, seal, ornithorinchus, etc.)
❖ Pinna:
β—‰ Components:
β—ˆ Helix: Elastic cartilage (includes Antihelix)
β—ˆ Lobule: Auricular muscles (vestigial in human, functional in Rabbit; includes Antilobule)
❖ External auditory canal:
β—‰ Length: 24 mm
β—‰ Parts:
β—ˆ Outer cartilaginous part:
β–  Structures:
  1. Hair follicles
  2. Ceruminous gland (secretes Cerumen/earwax)
  3. Sebaceous glands
β—ˆ Inner bony part:
β–’ Middle Ear:
❖ Tympanic bulla: Part of temporal bone surrounding the tympanic cavity
❖ Tympanic cavity: Air filled cavity of middle ear
❖ Tympanic membrane/ Eardrum/ Myringo:
β—‰ Origin: All three germ layers
β—‰ Inflammation: Myringoplasty
❖ Ear ossicles:
Image 1
Ear ossicles
Shapes
Joints (Synovial joints)
Muscles attached
Malleus
Hammer
Hinge joint
Tensor tympani
Incus
Anvil
Ball and Socket joint
Stapes
Stirrup
Stapedius (Smallest muscle)
β—‰ Eustachian/ Auditory/ Pharyngo-tympanic tube:
β—ˆ Function: Connects middle ear and nasopharynx (opens during swallowing and yawning)
β—ˆ Clinical note: More prone to Otitis media in infants due to shorter and more horizontal tube
Image 1
β–’ Internal Ear:

Table 1: Vestibular Apparatus and Cochlea

Feature
Semicircular canal
Vestibule
Cochlea
Bony labyrinth
Semicircular canal (SCC)
β—‰ Contains: Perilymph
β—‰ Function:
  • β€’
    Dynamic equilibrium
Vestibule
β—‰ Contains: Perilymph
β—‰ Function:
  • β€’
    Static equilibrium
Cochlea
β—‰ Contains: Perilymph
β—‰ Function:
  • β€’
    Hearing
β—‰ Compartments:
  1. Scala vestibuli
    1. Reissner's membrane
  2. Scala tympani
    1. Scala vestibuli and scala tympani meet at helicotrema
Membranous labyrinth
Semicircular duct (SCD)
β—‰ Contains: Endolymph
Utricle and Saccule
β—‰ Contains: Endolymph
Cochlear duct
β—‰ Contains: Endolymph
β—‰ Parts:
  1. Scala media / Cochlear duct
  2. Basilar membrane
    1. Fused to form Organ of Corti
  3. Tectorial membrane
    1. Stereocilia are embedded in it
Sensory cell
Cristae ampullaris
β—‰ Components:
  1. Hair cell / Sensory cell
    1. Stereocilia
  2. Supporting cells
Macula
β—‰ Components:
  1. Hair cell / Sensory cell
    1. Stereocilia (40–80 per cell)
    2. Kinocilium (1 per cell)
  2. Supporting cells
Organ of Corti / Spiral organ
β—‰ Components:
  1. Hair cell / Sensory cell
    1. Stereocilia (40–80 per cell)
  2. Supporting cells
    1. Pillar cell
    2. Boettcher cell
    3. Claudius cell
    4. Deiters' cell
    5. Hensen's cell
Membrane
Cupula
Otolithic membrane
β—‰ Contains:
  • β€’
    \(CaCO_3\) crystals called Otoliths / Statoliths / Otoconia / Statoconia / Ear dust
Tectorial membrane
Image 1
❖ Vestibular apparatus:
β—‰ Bony labyrinth:
β—ˆ Contents: Perilymph is the fluid present
β—ˆ Components:
β–  Semicircular canal (SCC): Dynamic equilibrium (Rotational motion) . Three semicircular structure perpendicular to each other
β–  Vestibule: Static equilibrium (motion in a linear acceleration)
β–  Cochlea: Hearing
β—‰ Membranous labyrinth:
β—ˆ Contents: Endolymph
β—ˆ Components:
β–  Semicircular duct (SCD):
β–  Utricle and Sacule:
β–  Cochlear duct:
Image 1
● Structure:
⟣ Scala vestibule:
➒ Boundary: Reissner's membrane
⟣ Scala media/ Cochlear duct:
➒ Basilar membrane: Fused to form Organ of Corti
➒ Tectorial membrane: Stereocilia are embedded in it
⟣ Scala tympani:
➒ Connection: Meets Scala vestibule at Helicotrema
β—‰ Sensory structures:
Structure
Sensory cells
Supporting cells
Membrane
Cristae ampullaris
Hair cell with Stereocilia
Supporting cells
Cupula
Macula
Hair cell with Stereocilia and Kinocillium
Supporting cells
Otolithic membrane (contains CaCO3 crystals)
Organ of Corti
Hair cell with Stereocilia
Pillar cell, Boettcher cell, Claudius cell, Deiter's cell, Hensen's cell
Tectorial membrane
Image 1
πŸ“š
NOSE
β–’ Membrane: Schneiderian membrane
β–’ Cells:
  1. Bipolar olfactory nerve cells
  2. Supporting cells
  3. Basal cells
β–’ Disorder: Anosmia (loss of sense of smell)
πŸ“š
Key
πŸ“š
SKIN
β–’ Layers of epidermis:
❖ Epidermis:
β—‰ Layers:
Layer
Presence
Cell Features
Other features
Stratum corneum
Keratin
Composed of hardened, flattened and cornified cells
Nail is derived from this layer
Stratum lucidium
Eleidin
Nuclei absent but cell outlines distinct
present in only thick skin of palm and soles
Stratum granulosum
Keratohyalin, Abundant granules
Stratum spinosum
Provides firmness and rigidity
Stratum germinativum/Malpighian layer/stratum basale
Single layer of columnar cells on basement membrane
Innermost layer, constantly produces new cells
β—‰ Features:
β—ˆ Vascularity: No blood vessels but has nerve fibres
β—ˆ Pigmentation:
β–  Melanocytes: Contain melanin pigment
β–  Skin color factors: Combined effect of melanin, carotene and haemoglobin
β–  Disorders:
● Albinism: Absence of melanin (recessive genetic character)
β—ˆ Derivatives: Claws present on digits are derivatives of the epidermis
❖ Dermis:
β—‰ Origin: Mesodermal
β—‰ Thickness: Two or three times as thick as epidermis
β—‰ Composition:
  1. Fibrous connective tissue
  2. Unstriped muscle fibres
  3. Blood capillaries
  4. Nerve fibres
  5. Fat cells
β—‰ Structures:
  1. Hair follicles
  2. Sweat glands
  3. Sebaceous glands
β–’ Skin derivatives:
❖ Types:
  1. Hair
  2. Claws
  3. Nails
  4. Hoofs
  5. Cutaneous glands
❖ Hair:
β—‰ Distribution: Found only in mammals
β—‰ Muscles: Arrector pili muscles (unstriped) attached to hair follicle and stratum germinativum
β—‰ Color: Black or brown due to melanin in cortex
β–’ Cutaneous glands:
❖ Sweat glands:
β—‰ Location: Around lips (mostly merocrine), axillary regions (apocrine)
❖ Sebaceous gland:
β—‰ Product: Sebum
β—‰ Type: Holocrine
❖ Modified glands:
Gland
Type
Location
Note
Meibomian glands
Modified sebaceous gland
Dermis of eye-lid
Near edge and close to eye-lashes
Zeis gland
Modified sebaceous gland
Dermis of eye-lid
Opens into hair follicles of eye lashes
Ceruminous or Wax gland
Modified sweat gland
External auditory meatus
Mammary glands
Modified sweat glands
Produce milk in female mammal
Perineal glands
Modified sweat gland
Associated with reproductive organs
β–’ Receptors:
❖ Types:
  1. Pain receptors
    2. β—ˆ Tactile receptors:
    1. Meisseners corpuscles
    2. Merckel's disc
    3. Pacinian corpuscles
    4. Genital corpuscles
    5. Vibrioreceptors on vibrissae
    β—ˆ Thermoreceptors:
      ● End bulbs of Krause (Frigidoreceptor): Cold receptor
      ● End bulbs of Ruffini (Caloreceptor): Heat receptors