Sense Organs

Part 3: Integration & Control

Sensation

Sense Organs: Organs specifically designed to increase the ability to detect and transmit sensory signals to the CNS, which then sends the appropriate efferent response. Thousands of signals detected every second.

Sensation: The conscious or unconscious awareness of stimuli.

Perception: The conscious interpretation of sensations. Not all stimuli reach perception.

Sensory Receptors

Sensory Receptor: Any structure specialized to detect a stimulus. Transducers: Receptors that translate

stimulus energy into nerve energy via action potentials.

Can be simple nerve endings or a complete sense organ.

Adaptation: If a stimulus is prolonged, firing frequency & conscious sensations decline.

Sensory Receptors

Sensory receptors transmit 4 types of information: Modality: Refers to the type of stimulus or sensation

it produces. E.g. vision, hearing, taste.

Location: Determined by which afferent (sensory) nerve fiber are firing & gives the brain the ability to identify the site or location of the stimulus.

Intensity: Determined by the number & kind of nerve fibers which are firing & the time intervals between action potentials.

Duration: Refers to the way nerve fibers change their firing frequencies over time.

Classification of Sensory Receptors – General

Somesthetic (General) Senses: Includes receptors in the skin, muscle, tendons, joint capsules, and viscera. Somatic Senses: Tactile senses, thermal sensations,

pain sensations, proprioceptive sensations. Visceral Senses: Provides information about

conditions within the internal organs.

Special Senses: Limited to the head & innervated by the cranial nerves. Includes: Vision, hearing, equilibrium, taste & smell.

Classification of Sensory Receptors – Overlapping Systems

Chemoreceptors: Respond to chemicals, including odor & taste.

Thermoreceptors: Respond to temperature (heat & cold).

Nociceptors: Pain receptors. Mechanreceptors: Respond to the physical deformation

of the plasma membrane. E.g. caused by touch, pressure, stretch, tension, or vibration.

Photoreceptors: Respond to light. Osmoreceptors: Respond to osmotic pressure. Proprioreceptors: Sense the body position, muscle

tension, & movement in space.

Classification of Sensory Receptors – Microscopic Features

Unencapsulated: Nerve endings of sensory dendrites are not wrapped in connective tissue. Example: Free nerve endings, Merkel discs, hair receptors.

Encapsulated: Nerve endings are dendrites wrapped in glial cells or connective tissue & most are pressure, stretch, & touch receptors. Meissner Corpuscles: Tactile sensors located in the dermal papillae –

respond to light touch. Pacinian Corpuscles: Lamellated sensors located deep in the dermis –

respond to deep pressure, stretch, & vibration. Ruffini Corpuscles: Tonic receptors – sense heavy touch & pressure. Krause End Bulbs: Similar to Meissner corpuscles but only found in

mucous membranes. Separate Cells: A separate receptor cell synapses with the sensory

neuron. Typically found with special sense synapses such as… Photoreceptors in the eye Hair cells in the inner ear Taste buds on the tongue.

Special Sense

5 Special Sense: TouchSmellTasteHearingVision

An addition: Equillibruim!

Touch

Touch Receptors responsible for detecting stimuli against the skin were discussed in Chapter 5.

Pain Sensation

Nocioceptors: Pain sensory neurons found in every bodily tissue except the brain.

Analgesia: The relief of pain. Oligopeptides: Chemicals released by the

CNS that have analgesic qualities. AKA edogenous opiods.

E.g. Endorphins & dynorphins.

Types of Pain

Slow Pain: Dull, achey pain signals carried along an unmeylinated neuron.

Fast Pain: Sharp, stabbing, sudden pain signals carried along a myelinated neuron.

Superficial Somatic Pain: Pain felt by receptors in the skin.

Deep Somatic Pain: Pain felt in the muscles, joints, tendons, and fascia.

Visceral Pain: Pain sensation from the receptors in the visceral organs.

Referred Pain: Pain caused by an organ or specific area but felt in other areas served by the same spinal nerve.

Kinesthesia

Kinesthesia: The perception of body movements.

Proprioceptors: Sensory receptors that can sense the location of body parts, their movements, & muscle tension without visual input. Mostly done through the hair cells in the inner

ear responsible for balance & equilibrium. Muscle Spindles: Involved in our stretch

reflexes.

Smell

Olfaction: The sense of smell. Located in the nose – around 20 million receptors!

Olfactory Receptors: First-order neurons whose dendrites contain the olfactory hairs (cilia) that respond to chemicals. Only sensory receptor in contact with the outside world. Receptors are replaced every few months. Supported by… Basal stem cells, Columnar supporting cells

and.. Olfactory glands (aka Bowman’s glands): Produce the mucus

that moistens the epithelium. Olfactory Epithelium: Contains most of the olfactory

receptors. Odorant: A chemical responded to by the olfactory

hairs.

Smell

Olfactory Epithelium: Contains most of the olfactory receptors.

Hyposomia: The gradual loss of smell as we get older.

Process of Smelling

Odorants (chemicals) bind to the Olfactory Receptors. Odorants must be water-soluble, volatile, & reach the

minimum threshold to be detected. Olfactory receptors send a signal to the

Olfactory Nerves (Cranial nerve 1). Signal continues to the Olfactory Bulbs (first-

order neuron) & synapses with the Olfactory Tract (second-order neuron).

Signal is then transmitted to the brain, through the cortex & on to the thalamus to be perceived.

Taste

Gustation: The chemically-based sense of taste. 5 Primary Tastes: Sweet: Least sensitivity. Sour Bitter: Most sensitive. Salty Umami: Meaty or Savory flavor.

Flavor: A combination of taste, smell, and texture.

MOST of taste perception is driven by scent.

Taste

Taste Buds: Small organs made up of several components that allow us to perceive taste. Over 10,000 total, mostly on the tongue. Can be located on the papillae.

Taste

Components of a taste bud: Gustatory Receptor Cells: Around 50 total in

each taste bud. Taste Pore: A small opening in the top of the

taste bud.Gustatory Hair: Projects through the taste

pore to help trap chemicals to analyze.

Taste

Papillae (elevations on the tongue that give it a rough texture). 4 types: Vallate Papillae: Twelve large circular

papillae forming an inverted V at the back of the tongue.

Each contains a few hundred taste buds.

Fungiform Papillae: Mushroom-shaped papillae.

Each contains around give taste buds. Found all over the tongue.

Taste

Foliate Papillae: Found in the trenches on the sides of the tongue.

These taste buds degenerate rapidly.

Filiform Papillae: Threadlike papillae found all over the tongue.

Contain no taste buds. Contain tactile receptors to detect texture. Increase friction between the tongue and food &

move food around in the oral cavity.

Taste

Tastant: Any chemical that stimulates our taste. Each tastant stimulates the receptors in a different

way. G-Proteins: A specific protein some tastants must be

linked to in order to reach threshold.

Facial Nerve (CN 7) and Glossopharyngeal Nerve (CN 11) serve the tongue.

Vagus Nerve (CN 10) serves the throat and epiglottis.

Taste

Pathway of Taste Perception: Taste bud to the appropriate cranial nerveCranial nerve to the gustatory nucleus in the

medulla oblongataMedulla oblongata to the thalamusThalamus to the gustatory cortex

Hearing

Ear: The main sensory organ responsible for translating the smallest of vibrations into electrical signals to be interpreted as sound & provide sensory information on equilibrium. Responds to sound almost 1,000 times faster than the

eyes due to light.

3 Regions of the Ear: External Ear Middle Ear Inner Ear

Hearing

External Ear: The portion of the ear we can see! Auricle (Pinna): The flap of cartilage that funnels

vibrations toward the auditory canal. Helix: The external rim. Lobule: The inferior portion we pierce for earrings.

External Auditory Canal: Tube located just inside the ear; curved & approximately 1 inch long. “The ear canal”.

Ceruminous Glands: Glands producing ear wax that line the external auditory canal.

Tympanic Membrane aka the Eardrum: Thin flap of connective tissue separating the outer & middle ear.

Hearing

Middle Ear: The epithelium-lined, air-filled cavity located within the temporal bone. Oval Window: Opening that leads to the inner ear. Contains

the. Auditory Ossicles: Smallest bones in the body!

Malleus (Hammer) Incus (Anvil) Stapes (Stirrup)

Round Window: Located beneath the oval window. Tensory Tympani Muscle & Stapedius Muscle: Smallest

muscles in the body – stabilize the stapes and reduce the possibility of damage from loud noises.

Eustachian Tube: Connects the middle ear to the nasopharynx – this is what opens when we swallow or yawn to equalize pressure in the ear.

Hearing

Inner Ear aka Labrynth: A series of maze-like passages & canals. Bony Labrynth: Outer passage.

Perilymph: Lines the bony labrynth & helps protect the membranous labrynth.

Membranous Labrynth: Inner passage. Endolymph: Potassium-rich fluid found in the

membranous labrynth.

Hearing

Inner ear structures: Vestibule: The first chamber of the inner ear,

important in maintaining equilibrium. Contains 2 sacs (Utricle & Saccule) & leads to the semicircular canals.

Semicircular Canals: 3 bony canals that end in ampulla (swollen ducts) which lead to the cochlea.

Hearing

Inner ear structures: Cochlea: The main organ of hearing that resembles a snail shell. Made

up of 3 fluid-filled chambers: Scala Vestibuli: Contains perilymph fluid.

Lies superior & begins near the oval window. Vestibular Membrane: Separates the Scala Vestibuli & Scala Media

Scala Media aka Cochlear Duct: Contains endolymph fluid. Lies in the middle. Basilar Membrane: Separates the Scala Media & Scala Tympani.

Scala Tympani: Contains perilymph. Lies inferior & ends at the round window.

Organ of Corti: Rests on the basilar membrane – contains thousands of hair cells, each with stereocilia bundles projecting into the endolymph.

Inner Hair Cells: Located within the Organ of Corti & transmit the actual sound impulses.

Outer Hair Cells: Change the cochlea’s response to different frequencies.

Hearing

Process of Hearing: Auricle directs sound waves to the external auditory

canal Sound waves strike the tympanic membrane

causing vibrations. Low-frequency sounds cause slow vibrations High-frequency cause fast vibrations

Vibrations cause the malleus to vibrate, which vibrates the incus, which vibrates the stapes.

Movement of the stapes pushes the membrane of oval window open & closed.

Hearing

Process of Hearing: Vibrations of the oval window cause the perilymph

of the cochlea to move in waves. Waves move to the scala vestibuli, the scala

tympani, then to the round window. The waves cause the round window to bulge into the

middle ear & the walls fo the scala vestibuli & scala tympani to change.

Wall changes cause pressure waves in the endolymph inside the cochlear duct.

Waves in the cochlear duct cause vibration of the basilar membrane.

Cochlear waves move the organ of Corti hairs against the tectorial membrane.

Hearing

Process of Hearing: Tectorial membrane vibrations cause

receptor potentials, then nerve impulses, via opening potassium channels.

Auditory Pathway travels from the first-order sensory neurons in the cochlear branch of the vestibulocochlear nerves (CN 8) to the medulla oblongata.

Signal travels from the medulla oblongata to the midbrain, to the thalamus, then the temporal lobe for conscious perception.

Hearing

Tuning: Basilar membrane is divided into pitches. High intensity sound waves create longer

vibrations & creates an interpretation of high pitch.

Low intensity sound waves create shorter vibrations & creates an interpretation of low pitch.

Equilibrium

Equilibrium: The sense of balance maintained by our inner ear. Static Equilibrium: Refers to our ability to

keep our position in response to gravity. Dynamic Equilibrium: Refers to our ability to

keep our body position in response to sudden movements.

Equilibrium

Vestibular Apparatus: The receptor organ responsible for maintaining equilibrium.

Typically maintained by the saccule & utricle within the vestibule of the inner ear. Macula: A small patch of hair cells on both the

saccule & utricle. Contains around 50 stereocilia & a single motile kinocilium.

Otolithic Membrane: Thick, gelatin-like layer of glycoproteins weighted down by otoliths (calcium crystals).

Shifting the head causes changes in these structures that generate nerve signals.

Equilibrium

Semicircular Ducts: The three ducts, each containing ampulla, help to maintain balance. Crista Ampularis: Mounds of hair on each

ampulla. Cupula: Gelatinous material covering the crista

ampularis.

Changes in these structures help to maitain dynamic equilibrium.

Vision

Light: Elecromagnetic radiation measured in wavelengths, from short to long.Short wavelengths appear violet to blueLong wavelengths appear red.

Eyes: Contain over half the sensory receptors in the body & are responsible for relaying visual sensations.

Occipital Lobe: The portion of the brain dedicated to visual perception.

Vision

Accessory structures of the eyes: Eyebrows & Eyelashes: Protect the eyes by keeping

harmful materials & perspiration out. Sebaceous Ciliary Glands: Found at the base of the

eyelashes – blockage or infection causes a sty. Palpebrae aka Eyelids: Shade the eyes during

sleep, protect against foreign material, & keep eyes moist.

Tarsal Plate: The thick fold of connective tissue that supports the eyelid.

Conjunctiva: Inner lining of the eyelid that serves to keep the eye moist.

VERY sensitive to pain & highly vascular. Dilation and constriction of blood vessels in this layer causes

“bloodshot eyes”.

Vision

Accessory structures of the eyes: Lacrimal Apparatus: Structure responsible

for producing lacrimal fluid (aka tears). Lacrimal glands produce the lacrimal fluid which

is transported by the lacrimal ducts & the lacrimal canals to the lacrimal sacs (which store the tears).

Tears serve to keep the eye clean & moisturized.

Vision

6 Extrinsic Muscles that move the eye: Superior rectus Inferior rectus Medial rectus Lateral rectus Superior oblique Inferior oblique

Vision

Orbit: The opening in the skill and protective tissue surrounding the eyeball.

Eyeball: The actual “eye” – we only see 1/6th of the eye exposed! 3 layers:Tunica FibrosaUvea (Vascular Tunic)Retina

Vision

Tunica Fibrosa (Fibrous Tunic): The outer layer of the eyeball consisting of the..Cornea: Cures to help focus light on the

retinaSclera: The white of the eye that holds the

eye’s shape & protects the internal portion.

Vision

Vascular Tunic (Uvea): Middle layer of the eye – highly vascular. Choroid: Supplies the eye with nutrients. Ciliary Body: Secretes aqueous humor (the

jelly-like substance in the interior of the eye).Ciliary Muscle: Adjust the shape of the lens

to focus vision.

Vision

Iris: The colored portion of the eye that controls the diameter of the pupil.

Chromatophores: Contribute to the color of the eye.

Pupil: The opening in the center of the iris that regulates the amount of light in the eye.

Pupillary Reflex: The process of letting in less or more light depending on the brightness of the surrounding environment.

Pupillary Constrictor: Muscle that constricts the pupil.

Pupillary Dilatory: Muscle that widens the pubil

Vision

Retina: The inner layer of the eye that starts the visual pathway. Lines the posterior ¾ of the eye & is lined with blood vessels.

Optic Disc: The place where the optic nerve leaves the eye.

AKA Blind Spot due to a lack of receptors in this area. Pigmented Layer: The layer that contains melanin

to absorb stray light & prevent light scattering. Neural Layer: Outgrowth of brain that processes

visual input – contains 3 layers: Photoreceptor layer Bipolar cell layer Ganglion layer

Vision

Macula Lutea: Exact center of the back of the retina. Central Fovea: The depression in the center of the macula

lutea that contains only cones – this is the point of highest visual acuity.

Lens: Responsible for focusing light rays onto the retina.

Connected to the ciliary body by a suspensory ligament & divides the interior of the eye into two cavities:

Anterior Cavity: Contains the aqueus humor, maintains intraocular pressure, & is located anterior to the lense.

Vitrus Chamber: Lies between the retina & lense. Contains vitreous body (jelly-like substance) that holds

the retina against the choroid & aid in visual acuity. This fluid is never replaced!

Vision

Types of photoreceptors: Rods: Low light threshold – allow for vision in dim

light. DO NOT detect color. Outnumber the cones – over 100 million per retina. Rhodopsin: The photopigment found in rods.

Cones: High light threshold – stimulated by bright lights and allow for color vision.

Around 6 million per eye. 3 types of cones in the average person, but up to 5 in

some, caused by photopigments called photopsins. Blue cones peak at 420 nm Green cones peak at 531 nm Red cones peak at 558 nm

Photopigments: Made up of opsin & retinal, both derived from Vitamin A.

Vision

The Visual Process: The process of image formation specific to the human eye. Similar to the function of a camera. Light rays enter the eye and are refracted (bent) by

the cornea & again by the lens. Image is then focused on the retina.

Images on the retina are inverted & reversed. E.g. upside-down and mirror-imaged!

Convergance: When the eyes move medially to allow light rays to strike the fovea centralis of each eye.

Pupils constrict or relax to adjust the amount of light. The lens undergoes accomodation or emmetropia to dovus

the image. The signal is transmitted up the optic nerve to the

brain.

Vision

Emmetropia: The process of an eye focusing on an object more than six meters away.

Accomodation: The process of an eye focusing on an object less than six meters away.

Near Point of Vision: The closest an object can be brought to the eye and still focus. Typically 10cm away for the average adult.

Vision

Photosensory Transduction: The process of translating light into action potentials. Occurs in the retina. Photopigments on the retina absorb light. Cis-retinal (a photochemical) creates a reaction and uses

isomerization to convert the light into trans-retinal. Sodium channels are closed by the breaking down of cyclic GMP

(cGMP) which causes hyperpolarization of the photoreceptors. and uses bleaching to separate out the opsin components. This sets of chemical reactions that leads to receptor potential

and an action potential by the bipolar cells. Regeneration combines opsin and trans-retinal back into cis-

retinal to prepare for the next reaction.

Vision

Visual Pathway: Retina processes the visual signal. Retinal Ganglion synapse with the optic nerve (CN

2). Optic nerve crosses the optic chiasm where it

crosses the nerve from the other eye. Optic nerves become the optic tract. Optic tract enters the brain, passes through the

midbrain, and moves to the hypothalamus (perceives light & dark), then the thalamus.

Optic tract then becomes the optic radiations and branches into the occipital lobe of the cortex to allow for conscious perception.

Vision

Light/Dark Adaptation: The pupils will quickly change when moving between areas of different light. From dark to light: Pupils constrict, rods

bleach, cones take over. From light to dark: Rhodopsin regeneration

overtakes bleaching, pupils dilate, rods take over main visual functioning.

Vision

Problems with Sight: Myopia: Nearsightedness that occurs when

the eyeball is too long, causing light rays to focus in front of the retina.

Hypermetropia: Farsightedness that occurs when the eyeball is too short, causing light rays to focus behind the retina.

Astigmatism: Irregular curvature of the cornea &/or lens which causes blurred or distorted vision. AKA Football Eye

Vision

Visual Disorders: Cataracts: Loss of transparency in the lens

due to changes in lens proteins. Can be caused by aging, injury, excessive UV

exposure, medication, & disease complications.

Can be corrected with surgery & artificial lens transplants.

Vision

Visual Disorders: Glaucoma: Increased ocular pressure causes

the lens to push on the neurons of the retina. Results in damage to the optic nerve. Can develop after injury or eye surgery, as a result

of tumors, or as a disease complication. Can be treated with medications, laser treatments,

or surgery to reduce pressure. Most common form of blindness in the US.