VT 111 Lec. 5 Special Senses

The Special Senses

• Taste

• smell

• hearing

• balance (equilibrium)

• vision

Sense Organs

• Are extension of the CNS that allow it to monitor what is going on inside & outside of animal

• All have specialized sensory nerve endings (dendrites) called sensory receptors

Taste

• Gustatory sense

  • Chemical

• Receptors

  • Taste buds detect chemicals dissolved in saliva

  • Located in small bumps or papillae on tongue, palate and back of mouth (throat – pharynx); tiny rounded structures made up of gustatory & supporting cells

  • Each taste bud has 50-100 sensory receptor cells

    • Modified dendrites (hairs) of cells project up into taste pores – tiny openings on surface of taste buds

  • Molecules or ligands are spread around the mouth in saliva and come into contact with dendrites

Example of signal transduction of sweet taste

A: taste bud

B: one taste cell

C: neuron

1. glucose

2. receptor

7. NTs

Smell

• Olfactory sense

  • Chemical

•  Olfactory epithelium covers turbinates (bony structures) in nasal passages

  • Large surface area

  • Humans: 5 cm2

  • Dogs: up to 170 cm2

• Hairs of cells immersed in layer of mucus

  • Molecules are dissolved in mucus -> contact sensory processes -> nerve impulses sent to brain ->interpretation of smells

Chemoreception: Taste/Smell

Taste/smell in animals

• Fish, amphibians, and reptiles have a very acute sense of chemoreception through their external nares

• Birds have poor sense of smell

• Vomeronasal organs:

  • chemoreceptor for smell located in the mouth of animals

  • detects heavy moisture-borne odors

  • reptiles, especially snakes (their forked tongue)

  • mammals, in the dorsal lip

  • Involved in social and reproductive communication byway of phermones

Hearing and Equilibrium

• Auditory sense

  • Mechanical

• Ear anatomy

  • External

    • Funnel that collects sound wave vibrations and directs them to eardrum

  • Middle

    • amplifies and transmits vibrations from eardrum to inner ear

  • Inner

    • sensory receptors (convert mechanical vibrations to nerve impulses); as well sensors for equilibrium

Divisions of the hearing apparatus: external ear

• Extends from pinna and auditory canal to tympanic membrane

  • Conducting zone

  • Pinna

    • Acts as funnel

    • May be mobile for sound detection

  • External auditory canal

    • Soft membrane-lined tube

    • L-shaped in most domestics

    • Outer vertical portion

    • Inner horizontal portion

    • Carries sound waves to tympanic membrane

  • Tympanic membrane (eardrum)

    • Thin connective tissue membrane stretched across opening between external auditory canal to middle ear cavity    

    • Sounds waves cause sympathetic vibration

Divisions of the hearing apparatus: middle ear

• Middle ear: extends from tympanic membrane to oval window.

  • Separated from external ear by tympanic membrane; separated from inner ear by membranes that cover the windows of cochlea

  • Hollowed out area of temporal bone filled with air & lined by soft tissue membranes

  • Contains the 3 bony ossicles link tympanic membrane with cochlea of inner ear (receptors for hearing are located)

    • Outermost bone: Malleus (hammer) – attached to tympanic membrane; forms synovial joint with middle bone

    • Middle bone: Incus (anvil) forms joint with stapes

    • Stapes (stirrup) – attached to oval window of cochlea

• Space opens into pharynx via eustachian tube

  • Allows equalization of air pressure on both sides of eardrum

Divisions of the hearing apparatus: inner ear

• Inner ear: extends from oval window to include vestibulocochlear apparatus

  • Cochlea: snail-shell shaped spiral cavity in temporal bone

• Organ of Corti (organ of hearing) in cochlear duct filled with endolymph (fluid)

  • Hair cells (receptor cells of hearing) rub together to generate nerve impulses -> travel to brain = sound interpretation

  • Supporting cells

  • Tectorial membrane

    • lies on top of hair (modified dendrites) of hair cells

• U-shaped tube filled with perilymph (fluid)

  • Ascending – vestibular canal

  • Descending – tympanic canal

  • Ends: oval window and round window

Process of sound reception

• Sound waves enter outer ear; air pressure vibrates tympanic membrane

• Tympanic membrane moves bony ossicles that move oval window

• Oval window vibrates, causing pressure waves in perilymph

• Pressure waves get transferred to endolymph in cochlear duct

• These waves move the tectorial membrane bending the villi of the hair cells

Process of Hearing

• Pressure wave causes basilar membrane to vibrate up/down

• Since cilia is imbedded in tectorial membrane, cilia bend and deform the apical membrane

• Na+ channels open in receptor cells to result in generator potential

  • If enough generator potentials → AP

• Each depolarized hair cell releases neurotransmitter to excite an associated neuron

Equilibrium

• Monitored by parts of the inner ear called the vestibule and semicircular canals

  • Vestibule

    • Between cochlea and semicircular canals

    • Composed of saclike spaces: utricle and saccule

    • Continuous with cochlear duct – filled with endolymph; surrounded by perilymph

    • Inside: sensory epithelium (macula) consisting of hair & supporting cells covered by gelatinous matrix with otoliths (crystals of calcium carbonate)

• Semicircular canals

  • On other side of vestibule from cochlea

  • Canals are at right angles to one another

  • Filled with endolymph surrounded by perilymph

  • Ampulla contains receptor (crista ampullaris)

Vestibule: Utricle and Saccule

• Utricle is for detection of linear acceleration or position in a horizontal plane

• Saccule is for detection of linear acceleration or position in a vertical plane

• In the macula (sensory receptor), gravity causes the gelatinous matrix with otoliths to put constant pressure on the sensory hairs when the head is still

  • Movement bends hairs which generate nerve impulses

    • Linear motion

Semicircular canals

• Each canal has a endolymph filled membranous tube surround by perilymph

  • Continuous with other inner ear structures

  • Canals are at right angles (3)

• Near utricle end → widened area called ampulla

• Contains receptor structure

  • Crista ampullaris

    • Cone shaped area of supporting cells and hair cells with processes sticking up into gelatinous structure (cupula)

      • No otoliths → no weight

      • Functions as a float and moves with endolymph

      • During movement, inertia causes endolymph to lag behind

      • Pulls on cupula and bends hairs → nerve impulse

      • Rotary motion

      • The vestibular system senses rotary motion of the head with the semicircular canals, and linear motion and the position of the head with the vestibule

The Sense of Motion

• The head moves

• Fluid movement lags in one plane of semicircular canals

• Fluid movement pulls on cupula

• Hairs are bent

• Nerve impulse is generated

• Brain receives information about motion of the head

Vestibular Syndrome in Dogs

Common clinical signs:

• Circling or falling to one side

• Pronounced head tilt

• Nystagmus—the rapid and involuntary oscillating movement of the eyeballs.

• Facial drooping may occur if there is a tumor or inflammatory disease of the inner or middle ear

Treatment:

• Supportive care

  • IV fluids

• Anti-nausea/motion sickness medications

  • Maropitant (Cerenia), meclizine (Dramamine)

• Sedatives – if severely ataxic

• Antibiotics – if inner ear infection is present

• +/- Corticosteroids – not generally

Vision

Eye components:

• Function is to help form an accurate visual image

• Function is not to detect the image

Photoreceptors:

• Located in a single layer of cells in the retina

• Function is to detect the image

• Generate visual nerve impulses

 

Mammalian eye anatomy

Eyeball - Outer Fibrous Layer

Cornea:

• Transparent; admits light to interior of the eye

• Orderly arrangement of collagen fibers

• No blood vessels; many pain receptors

Sclera:

• The “white” of the eye

• Limbus

  • Junction of the cornea and sclera

Eyeball - Middle Vascular Layer

Choroid:

• Lies between sclera and retina

• Consists mainly of pigment and blood vessels

• Tapetum lucidum is highly reflective area in rear of eye

Iris:

• Pigmented muscular diaphragm

• Controls amount of light that enters the posterior part of the eye

• Opening in center of iris = pupil

Ciliary body:

• Ring-shaped structure behind the iris

• Contains tiny muscles that adjust shape of the lens to allow near and far vision

Eyeball - Inner Nervous Layer

Retina:

Lines the back of    the eye

One component of the fundus

Contains rods and cones, the sensory receptors for vision

 

Compartments of the Eyeball

Aqueous compartment:

• Located in front of the lens

• Subdivided by iris

  • Anterior chamber

  • Posterior chamber

• Contains clear, watery fluid = aqueous humor

  • Produced in posterior chamber by cells of ciliary body

  • Drained by canal of schlemm

Vitreous compartment:

• Contains clear, gelatinous fluid = vitreous humor

• Fills whole back of eyeball behind lens and ciliary body

The Lens of the Eye

• Soft, translucent layers  of fibers

• Elastic and biconvex

• Front surface in contact with aqueous humor; back surface in contact with vitreous humor

• Helps focus a clear image on the retina through accommodation process

Formation of a Visual Image

• 4 refractive media in the eye help form a clear upside-down image on the retina

• Cornea, aqueous humor, lens, vitreous humor

• Brain inverts image

• Conscious mind sees everything right-side up

Functional Aspects

• Accommodation: the process of focusing light on the retina for close up & distance vision. Ciliary apparatus is contractile.

  • Relaxation of ciliary muscle moves lens posteriorly, increasing tension on suspensory ligaments → pulls lens flat for long distance vision

  • Contraction of ciliary muscle causes lens to move forward, loosening tension on ligaments → lens forms a ball for close up vision*

    • eyestrain

• Humans have very strong powers of accommodation

• Carnivores have mediocre close up vision

• Herbivores have poor close up vision

Extraocular Structures

Conjunctiva:

• Thin, moist, transparent membrane

• Covers front portion of eyeball

  • Bulbar conjunctiva

  • Bulbar = globe

• Lines interior surfaces of eyelids

  • Palpebral conjunctiva

  • Palpebral = eyelids

Conjunctival Sac:

• Space between bulbar and palpebral portions of conjunctiva

Eyelids:

• Upper and lower folds of skin, lined by conjunctiva

Lateral and Medial Canthus:

• Corners where the eyelids come together

 

Extraocular Structures continued…

Tarsal Glands = Meibomian Glands:

• Their tiny openings are found along eyelid margin

• Produce waxy substance to prevent tears from overflowing onto the face

Eyelashes:

• Prominent on upper lid of most animals

Third Eyelid = Nictitating Membrane:

• T-shaped plate of cartilage covered by conjunctiva

• Located medially between eyelids and eyeball

• No muscle attachment; passive movements

• Lymph nodules and accessory lacrimal gland on ocular surface

Extraocular Structures continued…

Lacrimal Apparatus:

• Structures that produce and secrete tears and drain tears away from the surface of the eye

• Lacrimal glands are the primary source of tears

Tear Drainage System:

• Lacrimal puncta

• Lacrimal sacs

• Nasolacrimal duct

Eye Muscles:

• Small, skeletal muscles

• Attach to sclera

• Capable of wide range of movements

Tears

• Liquid film that moistens and protects the surface of the eye

3 main layers of tears:

• Inner mucous layer – from cells in conjunctiva

  • Contains antibacterial substances

• Middle tear layer – from lacrimal glands and accessory lacrimal glands of the third eyelid

  • Keeps the cornea moist

• Outer oily layer – from tarsal or meibomian glands

  • Reduces evaporation of underlying tear layer

  • Prevents tears from flowing over the lid margin

Unique Structures in Animals

Nictating membrane: found in all domestic mammals

• 3rd eyelid situated ventromedially, between eyelids and globe

• “flips”  across the eye to protect it

• Consists of a T-shaped plate of cartilage covered by conjunctiva

• Contains lymph nodules and a tear gland

• Passive movement

Tapetum lucidum:

• mirror like pigmentation of choroid

• Increases light gathering ability of retina

• Improves night vision

• Found in domestic animals

• Except: swine and humans

Retina

• Lines most of the vitreous compartment

Made up of layers of neurons:

• Pigment layer (outer layer)

• Photoreceptors (layer)

  • Rods

    • More sensitive to light

  • Cones

    • Sensitive to color and detail

    • Fovea centralis (primates) - lots of cones to perceive great detail

• Neurons relay nerve impulses from photoreceptors to to optic nerve

  • Bipolar (layer) cells

  • Ganglion (layer) cells

• Nerve fiber layer (inner layer)

  • Nerve fibers converge at optic disc → leave eye to form optic nerve

    • “Blind spot” – no photoreceptors

Rods & Cones

Rods:

• High sensitivity

• Better in low light

• B/W vision

• Low visual discrimination

• Highest concentration in peripheral area of retina

Cones:

• Low sensitivity

• Color vision

• High visual discrimination

• Primates: High concentration in central area of retina called the fovea centralis

  • Clearest area of vision

  • Reading

Extraocular Eye Muscles

• Lateral Rectus

• Dorsal Rectus

• Medial Rectus

• Ventral Rectus

• Dorsal Oblique

• Ventral Oblique

General Types of Stimuli

• Mechanical

  • Touch, hearing, balance

• Thermal

  • Hot, cold

• Electromagnetic

  • Vision

• Chemical

  • Taste, smell

Visceral Sensations

• Miscellaneous category of interior body sensations

• Vague and poorly localized

  • Hunger

  • Thirst

  • Visceral stretch

    • GI tract

    • Urinary system

Touch and Pressure

• Tactile Sense

  • Sensation of something in contact with the surface of the body

• Pressure

  • Sensation of something pressing on the body surface

• Different touch and pressure receptors produce sensations of light contact, deep pressure, vibration, or hair movement

Temperature

• 2 types of temperature receptors

  • Superficial

    • Receptors in the skin

    • Detect changes in skin temperature

• Central

  • Receptors in hypothalamus

  • Monitor temperature of the blood

Pain

• Pain Receptors = Nociceptors

  • Widely distributed inside and on surface of the body

  • Not present in the brain

  • May be simple nerve endings or more specialized structures to detect mechanical forces, temperature, etc.

  • Purpose: protect body from damage

Pain: 4 processes contribute to sensory pain

• Transduction –conversion of stimulus to nerve impulse occurs at sensory nerve ending

• Transmission – impulse moves up the sensory nerve fibers to spinal cord

• Modulation – occurs at spinal cord, can amplify or suppress impulses thru synapses between neurons in dorsal horn. Can be influenced by drugs

• Perception – conscious perception occurs in cerebral cortex

Pain Classifications

• Superficial—affecting skin (& subcutaneous areas) and body surface

• Deep—involving muscles & joints

• Visceral – relating to internal organs

• Acute – sharp and intense

• Chronic – dull and aching

Proprioception

• Ability to sense where your limbs/body parts are

• Operates primarily at subconscious level

• Stretch receptors located in muscles, tendons, ligaments and joints capsules keep CNS informed about movement of limbs, etc

• Evaluation: curling animal’s foot and seeing how long it takes them to correct the position