A&P Exam 1

Chapter 15: Special Senses

Special Senses

  • Vision, taste, smell, hearing, equilibrium

  • In contrast, the special sensory receptors are distinct receptor sensory organs (highly localized in head with eyes + ears) OR in distinct epithelial structures (taste buds + olfactory epithelium)

Accessory Structures of Eye

  • Eyebrow: Short, coarse hairs that overlie supraorbital margins of the skull

    • Shade the eyes from sunlight

    • Prevent the forehead perspiration from entering the eyes

  • Eyelids(palpebrae): thin, skin covered folds that anteriorly protect the eyes

    • Palpebral fissure: eyelid slit that separates the to and bottom eyelids

    • Medial + lateral commissures: medial and lateral angles of the eye where the eyelids meet

      • Lacrimal Caruncle: fleshy elevation of the medial commissure that contains sebaceous – sweat glands; produces white, oily secretions

    • Tarsal plates: connective tissue sheets that support the eyelids internally and anchor the orbicularis and levator palpebrae muscles

  • Eyelashes: project from the free margin of each eyelid

    • Follicles innervated with nerve endings

  • Tarsal Glands: modified sebaceous glands within the tarsal plates

    • Produce oil that moistens the eye and eyelid, preventing eyelids from sticking together

  • Conjunctiva: transparent mucus membrane

    • Lines the eyelids as the palpebral conjunctiva and folds back over the anterior surface of the eyeball as the bulbar conjunctiva.

  • Lacrimal Apparatus: lacrimal gland and the ducts that drain lacrimal secretions into nasal cavity

    • Lacrimal gland secretes tears

Extrinsic eye muscles

  • 4 rectus muscles originate from common tendinous ring

    • Location and movement they promote indicated by name

    • superior, inferior, lateral, medial rectus

  • 2 oblique muscles move eye in vertical plane when eye turned medially by rectus muscles

    • Superior oblique muscle originates in common with rectus muscles

      • Rotates downward and lateral

    • Inferior oblique muscle originates from the medial orbit surface and runs laterally and obliquely 

      • Rotates up and lateral

Fibrous Layer

  • Outermost coat of eyeball

  • Dense avascular connective tissue

  • 2 Regions

    • Sclera

      • Hard posterior portion

      • Bulk of fibrous layer

      • Glistening white/opaque “white of the eye”

      • Protects shape and eyebrow anchoring site for extrinsic eye muscles

    • Cornea: 

      • Transparent

      • Bulges anteriorly from junction with sclera

      • Forms window that lets light enter the eye

Vascular Layer

  • Middle pigmented layer of eye, also called uvea

  • 3 Regions

    • Choroid

      • Posterior portion of uvea

      • Supplies blood to all layers of eyeball

      • Brown pigment absorbs light to prevent scattering of light

    • Ciliary Body

      • Choroid becomes ciliary body

      • Consists of smooth muscle bundles, ciliary muscles

      • Capillaries of ciliary processes secrete fluid for anterior segment of eyeball

      • Ciliary zonule (suspensory ligament) extends from ciliary processes to lens

    • Iris

      • Colored part of the eye that lies between cornea and lens

      • Pupil: central opening that regulates amount of light entering eye

Inner Layer

  • Innermost layer: Retina

    • Millions of photoreceptors that transduce light energy

    • Other neurons involved in processes responses to light

    • Glia

  • Retina consists of 2 layers

    • Outer pigmented layer

    • Inner neural layer

  • NOTE: 

    • Layers are close together but not fused

    • Only neural layer of retina plays a direct role in vision

Rods & Cones

  • Rods

    • Dim light, peripheral vision receptors

    • More numerous and more sensitive to light

    • No color vision or sharp images

    • Numbers greatest at periphery

  • Cones

    • Vision receptors for bright vision

    • High resolution color vision

    • Macula Lutea area at posterior pole lateral to blind spot

      • Mostly cones

    • Fovea Centralis: tiny pit in center of macula lutea that contains all cones, so is the region with best visual acuity

      • Eye movement allows us to focus in on an object

Accessory continued

  • Conjunctiva: transparent mucus membrane

    • Lines the eyelids as the palpebral conjunctival folds back over the anterior surface of the eye ball

    • This anterior surface is called the bulbar conjunctiva, which covers only the white eye

    • Its very thin with blood vessels present

    • Conjunctival sac is a split like spaced that occurs between the conjunctiva covered eyeball and eyelids

      • Functions as a lubricant that produces mucus and prevents drying

Lacrimal Apparatus

  • Consists of lacrimal gland and ducts that drain into nasal cavity

  • Lacrimal gland is located in orbit above lateral end of eye and secretes lacrimal secretion (tears), a dilute saline solution containing mucus, antibodies, and antibacterial lysozyme

  • Blinking spreads tears toward medial commissure, where they enter paired lacrimal canaliculi via lacrimal puncta

  • Tears then drain into lacrimal sac and nasolacrimal duct, which empties into nasal cavity

Retina

Pigmented Layer

  • Single cell thick lining

    • Next to choroid and extends anteriorly to cover ciliary body and posterior face of iris

  • Pigment cells absorb light and prevent it from scattering in the eye

    • Also act as phagocytes participating in photoreceptor cell renewal

      • Store vitamin A needed by photoreceptor cells

Neural layer

  • Transparent layer that runs anteriorly to margin of ciliary body

    • Anterior end has serrated edges called ora serrata

  • Composed of 3 main types of neurons

    • Photoreceptors, bipolar cells, ganglion cells

  • Ganglion cell axons exit eye as optic nerve 

  • Optic Disc

    • Site where optic nerve leaves eye

    • Lacks photoreceptors, so referred to as the blind spot

    • Retina has quarter binion photoreceptor sites that are one of two types:

      • Rods

      • Cones

  • Amacrine

  • Horizontal

Optic disc

  • Site where optic nerve leaves eye

  • Lacks photoreceptors, so referred to as the blind spot

Macula lutea

  • Lateral to the blind spot of each eye is this oval region

  • Contains mostly cones

Fovea centralis

  • Pit in center of macula

  • Best visual activity 

  • Contains only cones

  • Eye movement allows you to focus in on objects so the fovea can pick it up

Chambers and fluids

  • Posterior Seg: Vitreous humor

    • Transmits light

    • Supplies lens

    • Contributes to intraocular pressure

  • Iris divides Anterior Segment into Anterior Chamber (between cornea and iris) and Posterior Chamber (between iris and lens)

  • Anterior Segment filled with Aqueous Humor

    • Forms and drains content

    • Supplies nutrients and oxygen to leans and cornea

    • Carries metabolic waste

Lens

  • Biconvex, transparent, flexible, and avascular

  • Changes in shape to precisely focus light on retina

  • 2 Regions

    • Lens epithelium: anterior region of cuboidal cells that differentiate into lens fiber cells

    • Lens fibers: form bulk of lens and are filled with transparent protein crystallin

  • Lens fibers are more continually added, so lens becomes more dense, convex, and less elastic with age

  • Clouding of Lens:

    • Consequence of aging, diabetes mellitus, heavy smoking, frequent exposure to intense sunlight

    • Some congenital 

    • Crystallin proteins clump

    • Vitamin C increases cataract formation

    • Lens can be replaced surgically with artificial lens

Focusing

Distant

  • Eyes are best adapted for distance

  • Far point of vision: distance where no change in lens shape is needed to focus (20 ft for emmetropic eye)

  • Distance vision:

    • Ciliary muscles are completely relaxed

    • Causes pull on ciliary zonules

    • Stretches lens flat

Close

  • Light from close objects (less than 6 m) diverages as it approaches eyes and comes to a focal point farther from the lens

  • Restoring focus requires 3 simultaneous processes:

    • Accommodation of the lenses

    • Constriction of the pupils

    • Convergence of the eyeballs

  • This is induced by blurring of retinal image

Myopic (nearsightedness)

  • Occurs when distant objects focus in front of the retina rather than on it.

  • They can see close objects but far ones are blurred

  • Results from an eyeball that is too long

  • Corrected with concave lens

Hyperopia (farsightedness)

  • Eyeball is too short, so focal point is behind retina

  • Corrected with a convex lens

Functional anatomy of photoreceptors

  • Consists of cell body, synaptic terminal, and two segments:

    • Outer segment: light-receiving region

    • Inner segment: joins cell body

  • Cell body is connected to synaptic terminal via inner fibers

  • Plasma membrane of outer segment folds back to form many discs

  • Photoreceptors are vulnerable to damage

Formation and breakdown of rhodopsin

  1. Pigment Synthesis

    1. Rhodopsin forms and accumulates in the dark 

    2. Vitamin A is oxidized/usineruzed ti the 11-cis-retinal form and then combined with opsin to form rhodopsin

  2. Pigment Bleaching

    1. When rhodopsin absorbs light, retinal changes shape to its all-trans retinal isomer

    2. This allows surrounding protein to relax and uncoil to its light activated form

    3. Rhodopsin breaks down into retinal and opsin

  3. Pigment Regeneration

    1. Enzymes in pigmented layer slowly convert all-trans retinal to its 11-Cis retinal form after it detaches from opsin

    2. Requires ATP

Signal Transmission

In the dark

  1. cGMP-gated channels open, allowing cation influx. Photoreceptors depolarizes

  2. Voltage gated Ca channels open in synaptic terminals

  3. Neurotransmitters is released continuously

  4. Neurotransmitter causes IPSP’s in bipolar cells. Hyperpolarization results

  5. Hyperpolarization closes voltage gated Ca channels, inhibiting neurotransmitter release

  6. No ESPS’s occur in ganglion cell

  7. No action potentials occur along optic nerve

In the light

  1. cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes

  2. voltage-gated Ca2+ channels close in synaptic terminals

  3. No neurotransmitter is released.

  4. Lack of IPSPs in bipolar cells results in depolarization.

  5. Depolarization opens voltage-gated Ca2+ channels; a neurotransmitter is released.

  6. EPSPs occur in ganglion cells. 

  7. Action potentials propagate along the optic nerve.

Light adaptation

  • Occurs when we move from darkness into bright light

  • We are momentarily dazzled because the sensitivity of the retina is still “set” for dim light.

    • Both rods and cones are strongly stimulated and large amounts of visual pigments break down, producing a flood of signals.

  • The rod system turns off—all of the transducins migrate to the inner segment, uncoupling rhodopsin from the rest of the transduction cascade.

  • Visual acuity and color vision continue to improve over the next 5-10 minutes.

Dark adaptation

  • Occurs when we go from a well-lit area into a dark one

  • Initially, we see nothing but velvety blackness because 

    • Our cones stop functioning in low-intensity light

    • The bright light bleached our rod pigments, and the rods are still turned off. 

  • Once we are in the dark, rhodopsin accumulates, transducin returns to the outer segment, and retinal sensitivity increases.

Visual pathway

  • Retinal ganglion axons exit at optic chiasma via optic nerves

    • Medial fibers cross over and lateral fibers DO NOT

  • Most synapse with lateral geniculate nuclei of THALAMUS

  • Thalamic axons project through internal capsule to form OPTIC RADIATION in cerebral  white matter

  • Fibers project to primary visual cortex of occipital lobe

  • Function: conscious perception of visual images

Depth Perception

  • Each eye has a 170 degree visual field

  • Depth perception comes from the fusion of the 2 fields

  • Allows for accurate object location

  • Requires both eyes, losing 1 eye upset depth perception

  • Many animals have panoramic instead of depth perception, which is less visual field overlapping due to more laterally placed eyes

Gustatory

Location and structure of taste buds

  • Most of our taste buds are located in papillae

  • 3 Kinds of papillae:

    • Fungiform papillae: over entire tongue surface

      • 1-5 taste buds each

      • House the most taste buds

    • Vallate Papillae:

      • Form inverted V on back tongue

      • Largest and least number (8-12)

    • Foliate Papillae: 

      • On side walls of tongue

      • Contain many taste buds during childhood but fewer with age

Basic Taste

  • Sweet: sugars, saccharin, alcohol, some amino acids, some lead salts

  • Sour: hydrogen ions in solution

  • Salty: metal ions (inorganic metals); sodium chloride tastes the saltiest

  • Bitter: alkaloids such as quinine and nicotine, caffeine, and nonalkaloids like aspirin

  • Umami: amino acids glutamine and aspartate (beef, cheese taste, monosodium glutamate)

Physiology

  • For a chemical to be tasted it must dissolve in saliva, diffuse into a taste pane and contact the gustatory hairs

  • Gustatory cells contain neurotransmitters

  • When tastant binds to receptors, it induces a graded depolarizing potential causing neurotransmitters to release

  • Different thresholds for activation

Gustatory Pathway

  • .(VII) (IX) (X) →

  • Solitary nucleus (medulla) →

  • Thalamus →

  • Gustatory cortex

Olfactory

Structure and receptors

  • Olfactory epithelium: smell organ made of pseudostratified epithelium

    • Located in the roof of the nasal cavity

    • Covers the superior nasal conchae

  • Olfactory sensory neurons: bipolar neurons located within the olfactory epithelium

    • Surrounded and cushioned by supporting cells

    • Contain thin optical dendrites that terminate in a Knob

      • Olfactory cilia radiate from knob, which increase receptive area and are covered by a mucus that acts as a solvent to capture and dissolve airborne odorants.

    • Contain unmyelinated areas that gather into fascicles and form filaments of the olfactory nerve (cranial nerve I)

  • Olfactory stem cells: lie at base of epithelium

Physiology

  • To smell a substance it must be volatile

    • Gaseous state

    • Able to be dissolved in olfactory epithelium fluid

  • Activation of sensory neurons

    • Dissolved odorants blind to receptor proteins in olfactory cilia

    • Open cation channels and generate receptor potential

    • AP is conducted to 1st relay station in the olfactory bulb

  • Smell Transduction

    • Odorant binds to receptor and activated G-protein

    • G protein activation causes cAMP synthesis

    • cAMP opens Na and Ca channels

    • Na influx causes depolarization and impulse transmission

    • Ca influx causes decreased response to sustained stimulus (olfactory adaptation)

Pathway

Olfactory Sensory Neurons and Olfactory Bulbs

  • Axons of olfactory sensory neurons form the olfactory nerves.

  • These nerves synapse in the olfactory bulbs (distal ends of the olfactory tracts).

  • Mitral cells (second-order sensory neurons) are located in the olfactory bulbs within structures called glomeruli ("little balls").

    • Each glomerulus receives input from neurons with the same type of receptor.

    • Each glomerulus represents a single aspect of an odor (like one note in a chord).

    • Different odors activate unique subsets of glomeruli (forming different "chords").

Signal Processing in the Olfactory Bulbs

  • Mitral cells refine, amplify, and relay olfactory signals.

  • Granule cells in the olfactory bulbs inhibit mitral cells, ensuring only highly excitatory impulses are transmitted.

Olfactory Pathways

  1. Olfactory Tracts:

    • Composed mainly of mitral cell axons.

    • Carry impulses from the olfactory bulbs to the piriform lobe of the olfactory cortex.

  2. Two Major Pathways from the Olfactory Cortex:

    • Conscious Interpretation Pathway:

      • Information travels to the frontal lobe (above the orbit).

      • Responsible for conscious interpretation and identification of smells.

      • Only some information passes through the thalamus.

    • Emotional Response Pathway:

      • Information flows to the hypothalamus, amygdaloid body, and other limbic system regions.

      • Elicits emotional responses to odors:

        • Danger-associated smells (e.g., smoke, gas, skunk) trigger the sympathetic fight-or-flight response.

        • Appetizing odors stimulate salivation and digestive activity.

        • Unpleasant odors can trigger protective reflexes (e.g., sneezing, choking).

Key Concepts

  • Glomeruli: Represent specific aspects of odors; different odors activate different subsets.

  • Mitral cells: Refine and relay olfactory signals.

  • Granule cells: Inhibit mitral cells to filter out weak signals.

  • Olfactory pathways:

    • Conscious pathway (frontal lobe) for smell identification.

    • Emotional pathway (limbic system) for emotional and physiological responses.

Ear hearing and balance

3 Areas

  • External Ear: hearing

  • Middle Ear: hearing

  • Internal Ear: hearing and equilibrium

External

  • Auricle

    • Shell shaped projection surrounding opening of external acoustic meatus

    • Composed of elastic cartilage and occasional hair

    • Helix = thicker rim

    • Lobule = fleshy dangling earlobe with no cartilage

    • Function: funnel sound waves into the external acoustic meatus

  • External Acoustic Meatus

    • Short curved, tube extending from auricle to the eardrum

    • Frame = elastic cartilage and remainder carved into temporal bone

    • Entirely lined with shin bearing hairs, sebaceous glands, and ceruminous glands

      • Ceruminous glands: modified apocrine secrete cerumen (wax) to trap foreign stuff and repel bugs

  • Tympanic Membrane

    • Sound waves entering EAM hit this

    • Thin translucent connective tissue membrane covered externally by skin and mucosa internally

    • Flat cone shape, apex protrudes medially

    • Sound waves = eardrum vibrates

    • Function: Transfer sound energy to the tiny bones of middle ear and sets them vibrating

Middle

  • Also called the tympanic cavity

  • Small air filled mucosa-lined cavity in the petrous part of the temporal bone

  • Has two openings, the superior oval window and inferior round window

  • The tympanic cavity arches upward as the epitympanic recess acts as the roof

  • The mastoid antrum allows communication with mastoid ear cells housed in the mastoid process

  • The anterior wall contains the opening of the pharyngotympanic tube, which is an opening that runs down to link the middle ear with the nasopharynx

  • The tube is flattened and closed, and it only opens when swallowing or yawning takes place to equalize pressure in the middle ear with external pressure.

  • This allows for the ear the eardrum to vibrate freely and prevent distortion of sound

  • Auditory ossicles

    • Malleus

    • Incus

    • Stapes

    • Ossicles transmit the vibratory motion of the eardrum to the oval window, which sets the fluids of the internal ear into motion and excite hearing receptors

  • During loud sound, stapedius and tensor tympani contract reflexively to limit the ossicle vibration and minimize damage to hearing receptors

Internal

  • Also referred to as the labyrinth

  • Located in temporal bone behind eye socket

Labyrinth

  • 2 major divisions

  • Bony labyrinth: System of tortuous chambers worming through the bone

    • Vestibule

    • Semicircular canals

    • Cochlea

    • Filled with perilymph fluid, similar to CSF

  • Membranous labyrinth: Continuous series of membranous sacs and ducts contained within the bony labyrinth, filled with potassium rich endolymph

Vestibule

Central egg-shaped cavity of the bony labyrinth.

Contains two membranous sacs:

  1. Utricle: Detects horizontal acceleration (e.g., moving forward/backward).

    1. Continuous with semicircular canal

  2. Saccule: Detects vertical acceleration (e.g., moving up/down).

    1. Continuous with cochlear duct

Houses maculae, which are sensory receptors for static equilibrium (head position relative to gravity) and linear acceleration.

Cochlea

Spiral-shaped, snail-like structure in the bony labyrinth.

Divided into three chambers:

  1. Scala vestibuli: Filled with perilymph; connected to the oval window.

  2. Scala media (cochlear duct): Filled with endolymph; contains the organ of Corti (hearing receptor).

  3. Scala tympani: Filled with perilymph; terminates at the round window.

Basilar membrane supports the organ of Corti, which contains hair cells (sensory receptors for hearing).

Semicircular canals

  • Three canals oriented in different planes: anterior, posterior, and lateral.

  • Detect rotational (angular) acceleration (head rotation).

  • Each canal has an ampulla, which contains crista ampullaris (sensory receptors for dynamic equilibrium).

Transmission of sound to internal ear

  1. Sound waves vibrate the tympanic membrane

  2. Auditory ossicles vibrate and pressure is amplified

  3. Pressure waves created by the stapes pushing on the Oval Window move through fluid in the scala vestibuli

  4. Sounds w/ frequencies below the hearing range travel through the helicotrema and do not excite hair cells

  5. Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells

Resonance of basilar membrane

  • Soundwaves make the basilar membrane vibrate

  • Max displacement of membrane occurs when the membrane’s fibers are tuned to a specific frequency

    • Called resonance

  • Fibers cover the width of the membrane

    • Fibers near oval window are shorter and resonate to higher frequencies

    • Fibers near cochlear are long and resonate to lower frequencies

Sound transduction

Inner hair cells are responsible for sending auditory signals to the brain.

Stereocilia (hair cell microvilli) pivot in response to basilar membrane vibrations, opening mechanically gated ion channels.

  • Depolarization: K+ and Ca2+ enter, causing neurotransmitter release and increased action potentials in cochlear nerve fibers.

  • Hyperpolarization: Ion channels close, reducing neurotransmitter release.

Outer hair cells amplify basilar membrane motion (cochlear tuning) and protect inner hair cells from damage via efferent feedback.

Auditory Pathway

  1. Transmits auditory information from cochlear receptors (inner hair cells)

  2. To axons ascend in the lateral lemniscus (fiber tract)

  3. To the inferior colliculus (auditory reflex center)

  4. Projects to the medial geniculate nucleus of the thalamus

  5. Then project to the primary auditory cortex, which provides conscious awareness of sound

Auditory apparatus

  • Perception of pitch

    • Sound waves of different frequencies activate hair cells in different positions along length of basilar membrane

    • Impulse from specific hair cells are interpreted as specific piutches

  • Detection of loudness

    • Louder sounds cause larger movements of the tympanic membrane, auditory ossicles, and oval window, and pressure waves of greater amplitude in the fluids of the cochlea

    • Larger waves then cause larger movements in the basilar membrane, larger deflections of hairs in the hair cells, and larger graded potentials

    • More neurotransmitters released and generate more action potentials

  • Location of Sound

    • Several brain stem nuclei localize a sound’s source in space by 2 cues 

      • Relative intensity

      • Relative timing

Vestibular Apparatus

  • Definition: The vestibular apparatus is responsible for maintaining equilibrium and balance. It consists of the semicircular canals, vestibule (utricle and saccule), and associated receptors.

  • Function: It detects head movements and sends signals to the brain to initiate reflexes for maintaining balance.

Maculae

  • Anatomy

    • Located in the utricle and saccule of the vestibule.

    • Contain hair cells with stereocilia and a kinocilium.

    • Hair cells are embedded in an otolith membrane studded with calcium carbonate crystals (otoliths).

    • Utricle maculae are horizontal and respond to horizontal movements; saccule maculae are vertical and respond to vertical movements.

  • Activating receptors

    • Linear acceleration or deceleration causes the otolith membrane to slide, bending the hair cells.

    • Bending toward the kinocilium depolarizes hair cells, increasing neurotransmitter release and nerve impulses.

    • Bending away from the kinocilium hyperpolarizes hair cells, decreasing neurotransmitter release and nerve impulses.

Cristae Ampullares

  • Anatomy

    • Located in the ampullae of the semicircular canals.

    • Contain hair cells with stereocilia and a kinocilium embedded in a gel-like ampullary cupula.

    • Detect rotational (angular) movements of the head.

  • Activating receptors

    • Rotational acceleration or deceleration causes endolymph to move, bending the cupula and hair cells.

    • Bending in one direction depolarizes hair cells, increasing nerve impulses.

    • Bending in the opposite direction hyperpolarizes hair cells, decreasing nerve impulses.

Equilibrium pathway to brain

  • Signals from the vestibular apparatus travel to the vestibular nuclei in the brainstem and the cerebellum.

  • The vestibular nuclei integrate inputs from the eyes, somatic receptors, and vestibular apparatus to initiate reflexes for balance.

  • The cerebellum coordinates fine motor control and posture.

  • Reflexes include the vestibulo-ocular reflex (eye movements to stabilize vision) and adjustments to neck, limb, and trunk muscles.

Deafness

Conduction Deafness:

  • Caused by obstruction (e.g., earwax, perforated eardrum) or ossicle issues (e.g., otosclerosis).

  • Sound conduction to the inner ear is impaired.

Sensorineural Deafness:

  • Caused by damage to hair cells, cochlear nerve, or auditory cortex.

  • Results from aging, loud noise exposure, or diseases.

Tinnitus

  • Definition: Ringing, buzzing, or clicking in the ears without external stimuli.

  • Causes: Cochlear nerve degeneration, inflammation, or side effects of medications (e.g., aspirin).

  • Mechanism: Analogous to phantom limb pain, caused by neural reorganization in the auditory pathway.

Meniere’s Syndrome

  • Definition: A disorder of the inner ear affecting the vestibular apparatus and hearing.

  • Symptoms: Vertigo, nausea, vomiting, tinnitus, and hearing loss.

  • Treatment: Managed with antimotion drugs, low-salt diet, diuretics, or surgery in severe cases.

Vision

  • Development:

    • Eyes develop from optic vesicles by the 4th week of gestation.

    • Newborns have poor vision, but depth perception and color vision develop by age 3.

    • Age-related changes include presbyopia (loss of lens elasticity), cataracts, and reduced visual acuity.

  • Disorders:

    • Age-Related Macular Degeneration (ARMD): Deterioration of the macula lutea, leading to central vision loss.

    • Glaucoma: Increased intraocular pressure damaging the optic nerve.

    • Cataracts: Clouding of the lens.

Taste/Smell

  • Development:

    • Functional at birth but decline with age due to loss of receptors.

    • Women and nonsmokers generally have a sharper sense of smell.

  • Disorders:

    • Ageusia: Loss or impairment of the taste sense.

    • Anosmia: Loss of the sense of smell.

Hearing/Balance

  • Development:

    • Ears develop from otic placodes in the embryo.

    • Newborns can hear, but critical listening develops in early childhood.

    • Age-related hearing loss (presbycusis) affects high-pitched sounds and is increasingly common in younger people due to noise exposure.

  • Disorders:

    • Otitis Media: Inflammation of the middle ear, often causing conduction deafness.

    • Labyrinthitis: Inflammation of the inner ear, affecting balance and hearing.

Extra:

Cornea can be transplanted due to lack of blood supply

Transduction


Chemical senses, chemoreceptors, olfactory and gustatory


Chapter 16

Stimuli for hormone release

  • Three types of stimuli trigger endocrine glands to manufacture and release their hormones

  • Humoral Stimulus

    • Hormone release caused by altered levels of certain critical ions or nutrients

    • Stimulus: ion concentration of Ca in capillary blood response: parathyroid glands secrete parathyroid hormone (PTH) when increase blood Ca

  • Neural Stimulus

    • Hormone released by neural input

    • Stimulus: AP in preganglionic sympathetic fibers to adrenal medulla

    • Response: adrenal medulla cells secrete epinephrine and norepinephrine

  • Hormonal Stimulus

    • Hormones release caused by another hormone

    • Stimulus: hormones in hypothalamus

    • Response: anterior pituitary gland secretes hormones that stimulate other endocrine glands to secrete hormones

Cell response to hormone

  • Cell must have specific receptor proteins on its plasma membrane or interior to which the hormone can bind. 

  • Hormone receptor responds to hormone binding by prompting the cell to perform. Degree of target cell activation depends on:

    • Blood levels of hormone

    • Relative number of receptors

    • Affinity (strength) of binding

Upregulation

  • Persistent low levels of a hormone can cause its target cells to form additional receptors for that hormone

Down regulation

  • Prolonged exposure to high hormone concentrations can decrease the number of receptors for that hormone 

    • Desensitizes target cells

Half Life

  • Length of time for hormone blood level to decrease by ½ 

    • Depend on solubility

  • Water soluble hormones have shortest half life

  • Steroid hormones take days before effect seen

Permissiveness

  • Situation where 1 hormone requires another to be present in order to exert its full effects

  • Reproductive system hormones regulate development of the reproductive system, but thyroid hormone is necessary for normal development. 

Synergism

  • When 2 hormones have the same effect on the same target cell which amplifies the overall effect

  • Glucagon and epinephrine cause the liver to release glucose to blood.

Antagonism

  • Occurs when 1 hormone offsets the action of another 

  • Insulin which lowers blood glucose is antagonized by glucagon

Posterior Pituitary Gland

  • Largely neural tissue such as pituicytes and nerve fibers

  • Releases neurohormones received ready-made from hypothalamus

  • Hormone storage and not a true endocrine gland that manufactures hormones

  • Oxytocin

    • Stimulated by impulses from hypothalamus neurons in response to stretching of the uterine cervix or suckling of an infant of breast

    • Targets: Uterus and Breakfast

  • Antidiuretic Hormones

    • Stimulated by hypothalamic neurons in response to increased to increase blood solute concentration or decrease blood volume

    • Also stimulated by pain, some drugs, lower blood pressure inhibited by adequate hydration of the body and by alcohol

    • Target: Kidneys

    • Effects of hyposecretion: decrease diabetic 

    • Effects of Hypersecretion: 

Anterior Pituitary Gland

  • Composed of glandular tissue

  • Hypothalamic hormones released into special blood vessels (hypoglossal) control the release of anterior pituitary hormones

  1. When stimulated, hypothalamic neurons secrete releasing or inhibiting hormones into the primary capillary plexus

  2. Hypothalamic hormones travel through portal veins to the anterior pituitary where they stimulate or inhibit release of hormones made in the anterior pituitary

  3. In response to releasing hormones, the anterior pituitary secretes hormones into the secondary capillary plexus. This in turn empties into the general circulation

  • Secretes: growth hormone, thyroid stimulating hormone, Adrenocorticotropic hormones, follicle-stimulating hormones, luteinizing hormone, prolactin

Oxytocin

  • Strong stimulant of uterine contraction

  • Increased amounts during childbirth

  • Number of oxytocin receptors peak at end of pregnancy and uterine smooth muscle becomes more sensitive to the hormones stimulatory effects 

  • Stretching of cervix as birth approaches dispatches afferent impulses to hypothalamus

  • Hypothalamus then synthesizes oxytocin and triggering its release from posterior pituitary

  • Oxytocin acts as hormonal trigger for milk ejection in response to prolactin

  • Childbirth and milk ejection are positive feedback mechanisms

Antidiuretic 

  •  Peptide, mostly from neurons in supraoptic nucleus of the hypothalamus

  • Stimulated by impulses from hypothalamic neurons in response to increased blood solute concentration or decreased blood volume

  • Also stimulated by pain, some drugs, and low BP

  • Inhibited by hydration of body by alcohol

  • Target organ: Kidneys

    • Stimulate kidney tubule cells to reabsorb water from the forming urine back into blood

  • Hyposecretion causes diabetes insipidus

  • Hypersecretion causes syndrome of inappropriate ADH secretion

Growth

  • Protein, somatotropic cells

  • Stimulated by GHRH release, which is triggered by low blood sugars

  • Secondary triggers: Deep sleep, hypoglycemia

  • Increases blood levels of amino acids, low levels of fatty acids, exercise

  • Can be inhibited by GH and insulin growth factors 

    • Caused by increase in GHIH or decrease in GHRH

  • Low levels in pituitary dwarfism

  • High levels in gigantism in children and acromegaly in adults

Table 16.3

Diabetes

  • Antidiuretic Hormone (ADH) deficiency

  • Marked by intense thirst and huge urine output

  • Can be caused by a pituitary tumor or a blow to the head that damages hypothalamus or posterior pituitary

  • Can be life-threatening in unconscious or comatose patients

  • Diabetes Mellitus: Insulin deficiency caused by large amounts of blood glucose lost in urine

    • Type 1

    • Type 2

Thyroid Gland

  • Butterfly shaped gland, anterior neck on trachea, just inferior to larynx. 

    • Isthmus: median mass connection two lateral lobes

    • Follicles: Hollow sphere of epithelial follicular cells that produce glycoprotein thyroglobulin

    • Colloid: fluid of follicle lumen containing thyroglobulin and iodine and is precursor to thyroid hormone

    • Parafollicular cells: produce hormone calcitonin

Thyroid Hormone

  • Two iodine-containing amine hormones. Both constructed from 2 linked tyrosine amino acids.

    • Thyroxine (T4): major hormone secreted by the thyroid follicles. Has 4 bound iodine atoms. 

    • Triiodothyronine (T3): Has 3 bound iodine atoms

  • Enters target cells and binds to intracellular receptors within nucleus

  • Increases metabolic rate and heat production

  • Regulates tissue growth and development

  • Maintains blood pressure

Thyroid Synthesis

  1. Thyroglobulin is synthesized and discharged into the follicle lumen

  2. Iodide is trapped (actively transported in)

  3. Iodide is oxidized to iodine

  4. Iodine is attached to tyrosine in colloid, forming DIT and MIT

  5. Iodinated tyrosines are linked together to form T3 and T4

  6. Thyroglobulin colloid is endocytosed and combined with a lysosome

  7. Lysosomal enzymes cleave T4 and T3 from thyroglobulin and hormones diffuse into bloodstream

Thyroid Transport and Regulation

  • Most T4 and T3 released into blood, immediately binds to TBGs

    • TBG: thyroxine binding globulins

  • T34 binds more tightly and is more active (10x)

  • Falling TH triggers release of thyroid stimulating hormone (TSH), increasing levels of TH

  • Infants = exposure to cold stimulate hypothalamus to secrete thyrotropin releasing hormone (TRH)

    • Thyrotropin releasing hormone which increases thyroid-stimulating hormone (TSH) release and enhances body metabolism/heat production

  • Inhibit TSN

    • GHIH

    • Dopamine

    • Increase glucocorticoids

    • Increased blood iodine concentration

Calcitonin

  • Polypeptide hormone released by parafollicular cells (C-cells) of thyroid

  • Secreted in response to rise in Ca levels in blood

  • No known physiological role in humans

  • Does not need to be replaced if thyroid is removed

  • At high doses it is given to treat Paget’s disease and osteoporosis

  • Inhibits osteoclast activity and inhibits bone resorption/Ca release from bone

  • Stimulates Ca uptake and incorporation into bone matrix

Parathyroid gland

  • Posterior aspect of thyroid gland

  • Parathyroids glandular cells are arranged in thick, branching cords containing oxyphil cells and parathyroid cells, which secrete parathyroid hormone

  • Parathyroid hormone: controls Ca balance in the blood

  • Failing Ca levels trigger parathyroid hormone release and increased blood Ca levels inhibit its release 

  • Parathyroid hormone increases Ca levels in blood by stimulating 3 target organs

    • Skeleton

    • Kidneys

    • Intestines

  • Stimulates osteoclasts to digest Ca rich bony matrix and release Ca and phosphates into blood

  • Enhances kidney reabsorption of Ca

  • Promotes Activation of vitamin D, which increases Ca absorptivity

Feedback loop

Adrenal cortex

  • Encapsulate medulla, bulk of gland, glandular tissue derived from embryonic mesoderm

  • Synthesizes corticosteroids = helps with stress

  • Release rate depends on 3 zones

  1. Zona glomerulosa: cell clusters produce mineralocorticoid hormones (aldosterone) that helps control mineral and H2O balance in blood

  2. Zona Fasciculata: linear cords produce metabolic glucocorticoids (cortisol)

  3. Zona Reticularis: net like cells produce minimal gonadocorticoids (adrenal sex hormone)

Feedback loop aldosterone

ANP

  • Decreases increased blood pressure/blood volume

  • Decreased heart rate

Adrenal medulla

  • Medullary chromaffin cells which crowd around porous blood-filled capillaries, are modified postganglionic sympathetic neurons

  • They synthesize the catecholamines epinephrine and norepinephrine via a molecular sequence from tyrosine to dopamine to NE to epinephrine

  • When a short-term stressor activates fight-or-flight, the sympathetic NS is mobilized

  • Blood vessels constrict, heart beats faster, and blood is diverted to skeletal muscles

  • Blood glucose levels rise and preganglionic sympathetic nerve endings wearing through the adrenal medulla signal for release of catecholamines, which reinforce and prolong fight-or-flight

  • Unequal amounts of the 2 hormones are released and stored

  • 80% epinephrine, 20% NE

  • With a few exceptions, they exert the same effects

  • Epinephrine is the more potent stimulator of metabolic activities and dilator of small airways

  • NE has greater influence on peripheral vasoconstriction and blood pressure

Pineal gland

  • Hangs from the roof of the 3rd ventricle in the diencephalon

  • Secretory cells called pinealocytes are arranged in cords and clusters

  • Secretes melatonin

  • Pineal gland indirectly receives input from the visual pathways

  • (Retina → Suprachiasmatic nucleus of hypothalamus → superior cervical ganglion → pineal gland) concerning the intensity and duration of daylight

Pancreas

  • Located behind stomach

  • Mixed gland: endocrine and exocrine cells

  • Acinar cells: form bulk of gland, produce enzyme rich juice for digestion

  • ~ 1 million pancreatic islets produce pancreatic hormones

  • Alpha-cells: release glucagon

  • Beta-cells: release insulin

  • Some islet cells also synthesize peptides

Glucagon

  • A 29-amino acid long polypeptide

  • Hyperglycemic agent

  • 1 molecule can cause the release of 100 mil glucose molecules into blood

  • Targets the liver to:

    • Break glycogen into glucose

    • Synthesize glucose from lactic acid and non-carb molecules

    • Release glucose into blood

  • Lowers amino acid levels in blood

  • Falling glucose levels will trigger alpha cells to secrete glucagon

Insulin

  • Secreted when blood glucose levels increase

  • Synthesized as proinsulin, then modified

  • Lowers blood-glucose by:

    • Enhancing membrane transport of glucose into fat and muscle cells

    • Inhibits breakdown of glycogen to glucose

    • Inhibits conversion of amino acids of fats to glucose

  • Plays a role in neural development, learning, and memory

  • Factors the influence insulin release

    • Elevated blood-glucose levels

    • Rising blood levels of amino and fatty acids

    • Release of acetylcholine by parasympathetic fibers

    • Glucagon, epinephrine, growth hormone, thyroxine, and glucocorticoids

    • Somatostatin and sympathetic nervous system inhibit insulin release

  • Glucogenesis: producing glucose

Diabetes Cardinal signs

  • Polyuria: A huge urine output that decreases blood volume and causes dehydration

  • Polydipsia: Dehydration stimulants hypothalamic thrust centers, causing excessive thirst

  • Polyphagia: excessive hunger and food consumption

    • Body breaks down protein and fat to supply energy rather than glucose

Ingestion

  • Eating

  • Taking food into the digestive tract

Propulsion

  • Moves food through the alimentary canal

  • Voluntary process, matched with peristalsis which is involuntary

Mechanic breakdown

  • Increases the surface area of ingested foods

  • Mechanical processes include chewing, mixing food with saliva, churning food

Digestion

  • Series of steps in which enzymes secrete into the lumen at the alimentary canal

  • Breaks down food into chemical building blocks

Absorption

  • Passage of digested end products by the active or passive transport into the blood or lymph

Defecation

  • Eliminates undigested substances from the blood from anus

GI Tract layers

  • Mucosa

    • Epithelium

    • Secrete

    • Absorb

    • Protect

  • Submucosa extends to mucosa

    • Areolar 

    • Blood and lymphatic muscularis externa

    • Muscularis

    • Segmentation and peristalsis

    • Circular and longitudinal layer (smooth muscle)

    • Forms sphincters

  • Serosa

    • Visceral peritoneum

    • Areolar covered with mesothelium

    • Single layer of squamous cells

    • Adventitia: outer layer of fibrous connective tissue surrounding organ

Enteric nervous system

  • Staffed by enteric neurons that communicate with one another to regulate digestive system activity

Short reflexes

  • Mediated entirely by ENS in response to stimuli within the GI tract.

Long reflexes

  • Involves CNS integration centers and extrinsic autonomic nerves.

  • ENS sends information to the CNS via visceral sensory fibers

  • Receives sympathetic and parasympathetic motor fibers from ANS

  • ENS acts as a way station for the ANS, allowing extrinsic controls to influence digestive activity

Saliva Function

  • Cleanses the mouth

  • Dissolves food chemicals for taste

  • Moistens food and helps compact into a bolus

  • Amylase = enzyme that starts digestion of starchy food

  • Moistens mouth to prevent infection and aide with chew/swallow

Composition of saliva

  • Hypoosmotic

    • Largely water

  • Slightly acidic

    • pH 6.75-7

  • Solutes include:

    • Electrolytes

    • Digestive enzymes, salivary amylase and lingual lipase

    • Proteins: mucin, lysozyme, IgA (antibodies)

    • Metabolic wastes: urea, uric acid