Chapter 15- Special Senses Flashcards
What are special senses?
Any of the senses with special sensory receptors. These receptors are extremely specific and will only respond to one type of stimulus. Only found in the head and sit very close to the surface (considered exteroceptors).
Special senses (5)
- Vision
- Olfaction (smell)
- Gustation (taste)
- Hearing
- Equilibrium
Why is touch not considered a special sense?
Touch doesn’t count because receptors are scattered throughout the body and the receptors are mechanoreceptors
What is our most dominant sense?
Vision. 50% of our sensory receptors are photoreceptors and more than 50% of the cerebral cortex is responsible for integrating visual information. The eyes are anatomically and physiologically complex and very well protected
Function of eyebrows
Provide shade, prevent sweat from running into eyes. Sweat has a high salt concentration and can be irritating to the eyes. Eyebrows are usually darker in color, which provides the shade.
Conjunctiva
Transparent mucous membrane- there are 2 types. Produces lubricating mucus.
Palpebral conjunctiva
Portion of the conjunctiva that covers the inner eyelids
Bulbar conjunctiva
Portion that covers the anterior surface of the eye (except cornea)
Conjunctivitis
“Pink eye”- the color is caused by irritation as the conjunctival membrane secretes more mucus. This infection can be viral or bacterial.
Palpebrae function
Eyelids- open and close the eyes
Which muscles allow the eye to open and close (2)?
- Orbicularis oculi- encircles the eye
- Levator palpebrae superioris
Both of these muscles are skeletal
Lacrimal caruncle
Located on medial portion of the eyelid- this is the pink tissue you can see in the medial corner of the eye. Crusty secretions you sometimes see in the corner of your eyes is the secretion of the lacrimal caruncle. The sebaceous and sweat glands here produce an oily secretion.
What is the function of eyelashes?
Eyelashes project from the upper and lower lid to protect the eye from debris, which can be irritating or damaging.
Lacrimal apparatus function
Production and drainage of tears
Parts of the lacrimal apparatus (3)
- Lacrimal glands
- Lacrimal canaliculi
- Nasolacrimal duct
Lacrimal glands
Produces and releases dilute saline solution (tears)
Lacrimal canaliculi
Drains tears from the eye surface at the medial portion of the eye.
Nasolacrimal duct
Drains tears from lacrimal canaliculi into nasal cavity. We produce tears constantly, but not enough to constantly produce a runny nose
What is the function of tears?
Tears contain lysozyme, an enzyme that kills bacteria. They also lubricate the eye surface and wash away foreign bodies in the eye
Function of extrinsic eye muscles
Allows movement of the eye in the orbit- lets you follow the movement of objects without turning your head.
Where do extrinsic eye muscles attach?
To the sclera of the eye
Extrinsic eye muscles (6)
- Superior rectus
- Inferior rectus
- Lateral rectus
- Medial rectus
- Superior oblique
- Inferior oblique
Rectus muscles function
Rectus muscles (4) pull the eye in the direction indicated by the name of the muscle.
Oblique muscles function
Oblique muscles (2) either elevate or depress the eye and turn it laterally
Why do we need oblique muscles?
Lateral pull of oblique muscles resists medial pull of rectus muscles
Layers of the eye (3)
- Fibrous layer- superficial
- Vascular layer
- Retina- deep
Regions of the fibrous layer of the eye (2)
- Sclera
2. Cornea
Sclera
The whites of the eyes. Functions- gives the eyeball shape, provides sturdy anchors for extrinsic muscles.
Cornea
Transparent layer at the most anterior region of the eye. Supplied with many pain receptors- can be very painful if anything gets in the eye, causes tear production and reflexive blinking. High regenerative and repair capacity, but doesn’t have any blood vessels. If there were blood vessels, we would be able to see them and it would obscure your vision.
Cornea functions
Functions- allows light to enter the eye, bends light as it passes.
Why don’t cornea transplants have a risk of rejection?
The cornea doesn’t have blood supply, so there is a lack of immune system supply.
Regions of the vascular layer of the eye (3)
This is the middle layer.
- Choroid
- Ciliary body
- Iris
Choroid
Well vascularized layer, dark in color (to absorb light). Blood vessels here nourish surrounding layers of the eye
Ciliary body
The region of the eye that encircles the lens. Includes the ciliary muscle, ciliary processes, and suspensory ligaments
Ciliary muscle
Smooth muscle bundles that control lens shape
Ciliary processes
Secrete aqueous humor
Suspensory ligaments
Extend from ciliary processes to lens. Functions- holds lens in place, transmits tension from ciliary muscle to lens
Iris
The colored portion of the eye
What does the color of the iris depend on?
Color depends on amount of brown pigment in the eyes. People with blue eyes do have melanin, but in much smaller amounts. If you have no melanin at all, you would have red eyes (seen with albinism).
Pupil
Central opening of iris that lets light enter the eye
What structures allow for constriction or dilation of the pupil?
Smooth muscle layers of iris allow for constriction or dilation of the pupil. Sphincter pupillae and dilator pupillae
Sphincter pupillae
When contracted, the muscle becomes thicker and the pupil constricts
Dilator pupillae
When contracted, the pupil dilates
If an individual’s pupils are different sizes, what can that indicate?
Pupils that are different sizes are indicators of head trauma, “blown pupils”.
What factors influence pupil size?
With pupil size, we are concerned with levels of light, autonomic nervous system influence, and strong emotions- if you look at someone you love, your pupils will get larger (more light to see them better)
Retina
This layer contains all the photoreceptors of the eye. Two layers: the pigmented layer and the neural layer.
Pigmented layer of the retina
Lies against the choroid, most superficial. Pigment here absorbs light, phagocytes here help with photoreceptor renewal
Neural layer of the retina
Innermost layer of the retina, contains photoreceptor cells (rods and cones). Also contains bipolar cells and ganglion cells. Both are used to generate action potentials in response to light stimuli.
Rods
Used for dim light and peripheral vision. Only one visual pigment (photopigment) in rods- no color vision, and rod vision tends to be blurry. Most numerous, found mostly in retinal periphery. Several rods all synapse on a single ganglion.
Cones
Used for bright light and high resolution color vision- have low sensitivity to light. Found mostly in the fovea centralis and macula lutea. A single cone has 1 of 3 (red, green, or blue) visual pigments, and each cone synapses on its own ganglia.
Optic disc
Associated with the retina. This is the point at which the optic nerve exits the back of the eye. No photoreceptors are found here, causing a “blind spot”. The brain fills in the missing information, so we don’t notice the blind spot
Macula lutea
This is where retinal structures are displaced to the side. Result- light passes directly to photoreceptors- there is increased visual acuity here.
Fovea centralis
Associated with the retina- found at the center of the macula lutea. Contains only cones- provides extremely detailed color vision, but is only 1/1000th of the total visual field. This is why if you want to see something really clearly, you have to look directly at it
Anterior segment
The interior chamber located in front of the lens, contains aqueous humor
Aqueous humor
Watery fluid found in the anterior segment. It is continuously drained and produced. Functions- supplies nutrients and oxygen to structures in the anterior chamber and some retinal cells, removes waste.
Glaucoma
People with glaucoma either have too much aqueous humor or aren’t draining it enough. Eventually the fluid will push on the optic nerve and increase the pressure in the eye.
Posterior segment
The interior chamber located behind the lens, contains vitreous humor
Vitreous humor
Jelly-like fluid found in the posterior segment. Functions- transmits light, stabilizes the lens, and holds the retina place, contributes to intraocular pressure. Vitreous humor lasts a lifetime.
Lens
Convex, transparent, flexible structure in the eye. Function- used to bend light as it enters the eye
Lens epithelium
Covers the anterior portion of the lens. Functions- coordinates metabolic activity of the lens, provides more cells for lens fibers.
Lens fibers
Makes up the bulk of the lens. Fibers are laid down over a lifetime, old fibers are not broken down. Disadvantage- the lens becomes thicker and more dense with time- loses ability to focus light. Older individuals have to hold objects farther away to see them
Cataracts
Distortion of vision due to thickening of the lens. Fibers clump together unevenly and they turn white in color, changing how light can pass through the lens- like looking through fog. Can only be fixed by replacing the lens
Human eyes only respond to
Electromagnetic radiation in the visible light spectrum (400-700 nm).
The color of a particular object is caused by
Which wavelengths are absorbed and which are reflected. Ex- green grass reflects green wavelength and absorbs others
What causes the color white?
The object reflects all wavelengths of light
What causes the color black?
The object absorbs all wavelengths of light
Light travels at a constant speed-
Through a given medium
Refraction
Occurs when light travels at an oblique angle from one medium into a medium with a different density (ex- a straw in a glass of water).
In the human body, what structure is responsible for refraction of light?
The lens refracts light that enters the eye. Light rays bend so they converge at a single point- the focal point. However, the real image is upside down and reversed
Where is light bent as it enters the eye? (3)
Light is bent 3 times as it enters the eye:
1. Cornea
2. Anterior surface of lens
3. Posterior surface of lens
The cornea is mostly responsible for bending light, but it can’t change shape. The lens is used to fine tune refraction- forms a clear image
What structures are responsible for changing lens shape?
Ciliary muscles and suspensory ligaments around the lens
Relaxation of the ciliary muscles causes
Increased tension in suspensory ligaments. Effect- suspensory ligaments are pulled tight
Contraction of the ciliary muscles causes
Decreased tension in suspensory ligaments. Effect- suspensory ligaments go slack
How does a flat lens affect vision?
A flat lens decreases refractory power because the light passes through the lens faster
Far point of vision
The point at which the lens no longer needs to change shape to focus light. In the normal eye, this is 20 feet. More parallel rays won’t need to be refracted as much and the lens doesn’t need to work as hard to bend light
When looking at a distant object, what is occurring with the light rays?
When looking at a distant object- light rays entering the eye are nearly parallel. Cornea and lens easily focus light on retina
When looking at a distant object, what is happening with the ciliary muscles?
Ciliary muscles are relaxed- the tension of the suspensory ligaments flattens the lens
Near point of vision
The closest point to face that still allows clear vision. In the normal eye, this is 4 inches. Anything closer than that would be blurry.
When looking at a close object, what is occurring with the light rays?
The closer an object is, the more divergent the light rays. Result- lens must work harder to refract and focus light
What 3 processes must occur for close vision?
- Accommodation of the lens
- Constriction of pupils
- Convergence of the eyes
Accommodation of the lens
Contraction of ciliary muscles. Effects- suspensory ligaments go slack- the lens bulges
Why do the pupils need to constrict for close vision?
It prevents divergent rays from entering the eye- would cause blurred vision
Why do the eyes need to converge for close vision?
Keeps object focused on foveae. The closer the object, the more the eyes must converge
Emmetropic eye
This is the “normal” eyeball shape- people whose eyes have this shape have 20/20 vision because the cornea and lens can focus light without help. The focal point is easily and efficiently focused on the retina
Myopia
Elongated eyeball shape (past the focal point of the retina). Effect- objects are focused in front of the retina. More pronounced in distant objects. This results in nearsightedness- difficulty seeing far way.
What is the treatment for myopia?
Concave shaped corrective lenses
Hyperopia
Shortened eyeball shape. Effect- objects are focused behind the retina. More pronounced in close objects. This results in farsightedness- difficulty seeing close objects
What is the treatment for hyperopia?
Convex shaped corrective lenses
Anatomy of rods and cones
The outer segment of the rod/cone is embedded in pigmented layer of retina. It contains photopigments (visual pigments) folded into discs. The inner segment is embedded in the neural layer of the retina
Photopigments
Part of the photoreceptor that responds to light- no other part will respond (looks like stacked coins)
Why is the outer segment of a rod longer than that of a cone?
Provides more photopigments to respond to light (for a higher sensitivity to light).
Rods and cones synapse with
Bipolar cells
Phototransduction
Process of converting light energy into a graded receptor potential that begins when a photoreceptor catches light
How are photoreceptors different from typical neurons?
Normal rules for neurons don’t technically apply here- photoreceptors are very highly specialized neurons. Photoreceptors never create action potentials.
Cells involved in light processing (3)
- Photoreceptor cells
- Bipolar cells
3, Ganglion cells
Photoreceptor cells
Create graded potential in response to incoming light stimuli
Bipolar cells
Create either IPSP or EPSP- has an effect on the ganglion cell. Bipolar cells and ganglion cells do not respond to light
Ganglion cells
Generate action potential that is propagated along the optic nerve toward the visual cortex. Bipolar cells and ganglion cells will not react to light
How does darkness affect photoreceptor ion channels?
In the dark, photoreceptor ion channels are open. Result- receptor is depolarized to -40 mV. Allows positively charged ions to enter the photoreceptor and depolarizes it (in the absence of light- different from what we have seen)
How does light affect photoreceptor ion channels?
In the light, photoreceptor ion channels close
Result- receptor is hyperpolarized to -70 mV
This process uses a G protein signaling system
What occurs in the bipolar cell in darkness?
Ca +2 channels open in the terminal of the photoreceptor- neurotransmitter released between the photoreceptor and bipolar cells. Next, the neurotransmitter causes IPSP at bipolar cells.
What occurs in the ganglion cell in darkness?
Hyperpolarization of bipolar cell prevents neurotransmitter release between bipolar cell and ganglion cell- no action potential generated. No AP generated because no neurotransmitter was released
What occurs in the bipolar cell in the light?
Ca +2 ion channels close, neurotransmitter not released. Bipolar cell depolarizes in absence of IPSP, cell depolarizes (generates EPSP). Ca +2 channels open, neurotransmitter released between bipolar cell and ganglion cell
What occurs in the ganglion cell in the light?
Action potential generated on the ganglion cell
Light adaptation
Occurs when moving from dark to light conditions- the retina is the part of the eye that must adapt. Adaptation to bright light takes about 60 seconds, highest visual acuity and color vision is reached in about 5 minutes
What type of vision dominates in the dark?
In the dark, rod vision dominates, retinal sensitivity is high
What happens to rods and photoreceptors when moving to a bright environment?
Photoreceptors bombarded with stimuli- “white light” occurs. Rods “turned off” and cones “turned on”- retinal sensitivity decreases. Rods stop responding when light increases
Dark adaptation
Occurs when we move from light to dark conditions. Adaptation to dark takes up to 30 minutes
What type of vision dominates in the light?
In the light, cone vision dominates, retinal sensitivity is low
What happens to rods and cones when moving to a light environment?
Sudden lack of light causes a temporary loss of vision. Cones “turn off”, rods “turn on”- retinal sensitivity increases
What is the pathway to the visual cortex from the back of the eye?
The optic nerve exits the back of the eye. Medial fibers from each optic nerve cross over at the optic chiasm, and the optic tracts continue to the visual cortex
Optic tract function
- Carries fibers from the lateral portion of the eye on the same side
- Carries fibers from the medial portion of the eye of the opposite side
- Contains all information from the same half of the visual field
How does each eye receive visual input from the other side of the body?
The lens flips/reverses the image, so the medial portion of the eye receives input from the temporal part of the visual field and the lateral portion of the eye receives input from the medial part of the visual field
Lateral geniculate nucleus
This is an area in the thalamus where most fibers in the optic tract synapse with neurons. These fibers then project to the primary visual cortex
Other than the lateral geniculate nucleus, where else can fibers from the optic tract travel to?
- Superior colliculi- visual reflex center that controls extrinsic eye muscles
- Pretectal nuclei- mediates pupillary response to light
- Suprachiasmatic nucleus- sets biorhythms
How does depth perception occur?
The complete visual fields of left and right eye overlap. The visual cortex combines two images to create depth perception. Importance- depth perception allows us to locate objects in space
When would depth perception be lost?
If a person loses an eye or loses vision in one eye.
Chemoreceptors only respond to
Stimuli dissolved in solution
How is olfaction adaptive?
Serves as a warning system for our surroundings- can cause us to move away from bad smells
Where are olfactory receptors found?
The olfactory epithelium in the roof of the nasal cavity
3 cell types in the olfactory receptors
- Olfactory sensory neurons
- Supporting cells
- Olfactory stem cells
Olfactory cilia
Hair like projections found in epithelium, part of the structure of olfactory sensory neurons.
Function- increase receptive surface area of neuron. Mucus surrounding cilia dissolves airborne odorants
Structure of olfactory sensory neurons
Contain olfactory cilia. Axons of multiple sensory neurons form small fascicles- filaments of the olfactory nerve. They travel through the cribriform foramen of the cribriform plate and synapse with olfactory mitral cells in olfactory bulb
How many smells can the human nose identify?
The human nose can identify 10,000-1 trillion odorants. A single smell is due to combination of hundreds of different odorants
How do receptors correspond to odorants?
Olfactory epithelia has about 350 different odorant receptors. Each receptor responds to 1 or more odorants, and each odorant can bind 1 or more receptors
How long do olfactory sensory neurons last?
Olfactory sensory neurons have a superficial location, so they are prone to destruction. The life span of olfactory sensory neurons is 30-60 days, but olfactory stem cells replace damaged/destroyed neurons
What two things must take place for the sensation of smell to occur?
- Activation of sensory neurons
2. Transduction of smell
How are sensory neurons activated?
The odorant in a gaseous state dissolves in epithelium. Odorant binds receptor proteins in olfactory cilium membrane
How does transduction of smell occur?
Transduction involves G protein. Na+ influx depolarizes olfactory sensory neuron- creates receptor potential. A Ca 2+ influx causes adaptation- decreased response to sustained odorant stimulus
Glomeruli
Site of synapse of olfactory sensory neurons with mitral cells
Pathway to the olfactory cortex?
In the olfactory bulb, olfactory sensory neurons synapse with mitral cells, and impulses flow from the olfactory bulb via the olfactory tract. From the olfactory tract, information can be sent to the olfactory cortex or to the limbic system.
What happens when information about smell is sent to the olfactory cortex?
Smell is consciously interpreted and identified
What happens when information about smell is sent to the limbic system?
Smell elicits an emotional response. This can result in activation of the sympathetic or parasympathetic system or in activation of protective reflexes- coughing, sneezing
Where are taste buds located?
Most taste buds are located on the papillae of the tongue, but some are located on the soft palate of the mouth, inside of the cheeks, and the pharynx.
What are papillae (of the tongue)?
Where taste buds are located, papillae are small peg like projections you can see on the tongue.
Types of papillae (3)
- Fungiform papillae
- Vallate papillae
- Foliate papillae
Fungiform papillae
Found all over tongue, contain 1-5 taste buds each. Named for their shape- shaped like a mushroom
Vallate papillae
Found at back of tongue (form an inverted V), have many taste buds each
Foliate papillae
Found on side of tongue, taste bud number varies with age. Young children have a lot of these taste buds, but we lose them as we get older
What are the 2 types of epithelial cells found in taste buds?
- Gustatory epithelial cells
2. Basal epithelial cells
Gustatory epithelial cells
Receptor cells for taste- sensory dendrites wrap around gustatory cells and form the first part of the pathway to the brain. Contain gustatory hairs.
Gustatory hairs
Microvilli projecting from tips of gustatory epithelial cells. Function- receptor membrane of gustatory epithelial cells. Hairs respond to taste stimuli- this is the taste sensitive portion
Basal epithelial cells
Stem cells- replace lost/damaged gustatory epithelial cells. Without these cells, we would lose our sense of taste within days.
How often are taste buds replaced?
Taste buds are replaced every 7-10 days. We lose taste buds from eating, and hot or spicy foods can burn or destroy the taste buds
What are the 5 basic modalities of taste?
- Sweet
- Sour
- Salty
- Bitter
- Umami
Sweet taste is produced by
Produced by most sugars, alcohols, some amino acids, lead salts
Sour taste is produced by
Produced by acids- anything with a high concentration of hydrogen ions, like lemons
Salty taste is produced by
Produced by metal ions (inorganic salts)
Bitter taste is produced by
Produced mostly by alkaloids, some nonalkaloids. Caffeine is a consumable alkaloid
Umami taste is produced by
Produced by amino acids glutamate and aspartate- causes the beef taste of steak and the tang of ageing cheese
Simulation of the cells for a taste modality will result in
The conscious perception of taste. Most substances produce a combination of the taste modalities, but a single taste cell responds to only one modality
What could potentially be the 6th taste modality?
Long chain fatty acids. These are found in lipids- might explain why people like eating fatty foods
What two processes must occur for taste to take place?
- Activation of taste receptors
2. Transduction of taste- different mechanisms affect how we taste
What areas of the tongue detect each taste modality?
All areas of the tongue can detect all taste modalities. It isn’t true that different parts of the tongue are responsible for different tastes
What occurs during activation of taste receptors?
Chemical tastant must dissolve in saliva and diffuse into taste pore associated with the taste bud. Tastant binds gustatory epithelial cell- graded potential occurs and will depolarize gustatory epithelial cell- releases neurotransmitter. Neurotransmitter release to sensory dendrite elicits action potential
What mechanism results in salty taste?
Na+ influx through Na+ channels directly depolarizes gustatory epithelial cell
What mechanism results in sour taste?
H+ acts intracellularly to open ion channels
What mechanism results in bitter, sweet, or umami taste?
G protein gustducin activation leads to opening of cation channels to depolarize the membrane
Which nerve carries information from anterior ⅔ of tongue?
Facial nerve
Which nerve carries information from posterior ⅓ of tongue?
Glossopharyngeal nerve
Pathway to the gustatory cortex
Information travels from the facial or glossopharyngeal nerve. Most fibers synapse at the solitary nucleus in the medulla, and travel to the primary gustatory cortex, others travel to limbic system and hypothalamus, others travel to the limbic system and hypothalamus
What is the purpose of sending taste information to the hypothalamus and limbic system?
Gives an appreciation/emotional response to food you like or dislike
Why are taste likes/dislikes adaptive?
Taste likes and dislikes have homeostatic value. Cravings usually mean that we are short on a macronutrient, ion, etc. (craving salt supplies the body with minerals, craving sweets supplies the body with carbohydrates). Some tastes indicate spoiled food (extreme sour) or poison (extreme bitter). Function- protects us from consuming something that can harm us
3 major regions of the ear
- External ear
- Middle ear
- Inner ear
What 3 structures are associated with the external ear?
- Pinna
- External acoustic meatus
- Tympanic membrane
Pinna
Outermost cartilaginous (elastic cartilage and skin) part of the ear. Function- funnels sound into inner part of the ear
External acoustic meatus
Tube extending from auricle to tympanum. Has sebaceous glands, some hairs, and ceruminous glands
Tympanic membrane
Eardrum- translucent membrane that divides outer ear from middle ear. Function- vibrates in response to sound waves- transmits vibrations to the bones in the middle ear
What structures are associated with the middle ear?
The middle ear is an air filled cavity. Contains:
- Auditory ossicles and associated muscles
- Oval window and round window
- Pharyngotympanic tube
Auditory ossicles (3)
Malleus, incus, and stapes. 3 smallest bones in the body. The malleus is most lateral and connected directly to the tympanic membrane. The incus is in the middle and the stapes is the most medial and connects to the oval window in the inner ear.
What is the function of the auditory ossicles?
Ossicles vibrate in response to incoming sound waves- transmit sound to inner ear. The ossicles are all joined by small ligaments- they can move in relation to each other. When the malleus moves, the incus and the stapes will move as well. When the stapes vibrates, it sends vibrations to the inner ear.
What 2 muscles are associated with the auditory ossicles?
Tensor tympani and stapedius. The tensor tympani connects to the malleus. The stapedius connects to the stapes and is the smallest muscle in the human body
Function of the middle ear muscles
The tensor tympani and stapedius contract in response to extreme sound vibrations. The muscles contracting in response to loud sounds hold the bones in place so extreme vibration will be prevented and damage is less likely to occur.
How can very loud sounds damage the ossicles?
When we listen to very loud sounds, the tympanic membrane will vibrate very hard, which will also vibrate the ossicles really hard. This extreme vibration could damage the ossicles, which could prevent hearing from occurring at all
Oval window and round window
Small openings in middle ear, stapes attaches to oval window
Pharyngotympanic tube
Tube that runs from the middle ear to the nasopharynx. Function- opening of tube balances air pressure in the middle ear cavity
Why do your ears pop on an airplane?
“Popping” ears on an airplane is the pharyngotympanic tube opening to balance air pressure in the inner ear. The tympanic membrane will not vibrate if pressure is not equal on both sides of the ear. This means that the ossicles also will not vibrate and hearing won’t occur.
Otitis media
Otitis media (ear infection) is an infection that starts in the pharyngotympanic tube. The tube is usually collapsed and is a warm area, so it’s a bacteria breeding ground. Can cause inflammation once it gets to the middle ear cavity.
What are the 2 subdivisions of the inner ear?
- Bony labyrinth
2. Membranous labyrinth
Bony labyrinth of the inner ear
A system of channels that weave through the temporal bone- the bony labyrinth is the cavity inside the bone, not the bone itself. Channels are filled with perilymph- fluid similar to CSF. Perilymph surrounds the membranous labyrinth
Membranous labyrinth
Membranous sacs and ducts found in the bony labyrinth (run through the bony labyrinth). Filled with endolymph- fluid similar to ICF
Cochlea
The spiral bony chamber of the inner ear involved with sound perception. The cochlear duct runs through center and is membranous (membranous labyrinth of the cochlea). Contains the spiral organ- actual receptor organ for hearing . The cochlea ends blindly at the helicotrema- a dead end
3 regions of the bony labyrinth of the cochlea
- Scala vestibuli
- Scala tympani
- Scala media
Scala vestibuli
Part of bony labyrinth that begins at the oval window of the middle ear. Filled with perilymph.
Scala tympani
Part of bony labyrinth that ends at round window of the middle ear. Vestibuli and tympani are continuous- meet at the helicotrema. Both are filled with perilymph as they are part of the bony labyrinth
Scala media
Cochlear duct- part of membranous labyrinth. The spiral organ is found in the scala media. The stria vascularis secretes endolymph and forms a lateral wall in the scala media
Vestibular membrane
Divides scala media from scala vestibuli- forms a “roof”
Basilar membrane
Forms floor of scala media
The spiral organ
The receptor region for hearing, contains 2 cell types: cochlear hair cells and supporting cells
Cochlear hair cells
Have one row of inner hair cells and three rows of outer hair cells. Cochlear nerve fibers wrap around hair cells (hair cells are innervated by the cochlear nerve).
Supporting cells
Support hair cells and prevent damage to receptor cells
Sound waves are produced by
Compressions and rarefaction. Compression is when air molecules are pushed together and rarefaction is when air molecules spread apart. Air molecules bump against one another and travel outward from source-sound waves travel in one direction
Sound waves decrease in strength with
Distance
Speed at which sound travels is constant within
A given medium. Sound travels the slowest in less dense mediums (air) and travels the fastest in dense mediums (solid objects)
Frequency
Number of sound waves that pass a point in a given time- “pitch of a sound”. The human hearing frequency range is 20-20,000 Hz, but older individuals don’t hear higher pitched sounds as well as children
Wavelength
Distance between crests of a given sound wave. Shorter wavelength= higher frequency and higher pitch
Tone
Sound consisting of a single frequency. Most sounds we hear are mixtures of different tones
Amplitude
Height of crests for a given sound wave, denotes loudness of sound. Human hearing is from 0 dB-120 dB. Above 120 dB sound is painful, but hearing loss occurs from prolonged exposure to 90+ dB
Transmission of sound to the inner ear
The outer ear funnels the sound waves into the ear, and the sound waves vibrate the tympanic membrane. The malleus vibrates in response to tympanum, and the incus and stapes also vibrate. The stapes transmits vibrations to the oval window in the middle ear, where the scala vestibuli begins. The movement of the oval window causes the perilymph of scala vestibuli to move. Perilymph moves in pressure waves toward helicotrema. The round window acts as a pressure valve to allow perilymph movement
How does the round window act as a pressure valve?
The round window is flexible and bulges toward the middle ear with each wave of perilymph so the waves aren’t compressed- pressure must go somewhere.
What two paths can sound take once the perilymph is in motion?
The helicotrema path or the basilar membrane path
Helicotrema path
If sound is low frequency (less than 20 hZ), the wave passes completely around helicotrema to the round window. Sound does not stimulate the spiral organ and sound perception will not occur
Basilar membrane
The sound is 20-20,000 hz. The sound waves are transmitted through the cochlear duct. Pressure waves vibrate the basilar membrane- stimulates hair cells in the spiral organ. The spiral organ is on top of the basilar membrane. Stimulation of hair cells generates action potential
Why is the basilar membrane “tuned” to specific frequencies in specific areas?
The fibers in the basilar membrane differ in length and elasticity from the oval window to the helicotrema, so they will respond to specific frequencies.
What is the length and elasticity of fibers near the oval window?
Near oval window, fibers are short and stiff, respond to high frequency waves
What is the length and elasticity of the fibers near the helicotrema?
Near the helicotrema, fibers are long and loose, respond to low frequency waves. Don’t need a lot of stimulation- basilar membrane responds better to low frequency.
How are inner hair cells stimulated?
Movement of basilar membrane stimulates inner hair cells. Inner hair cells have stereocilia embedded in the tectorial membrane. The stereocilia form the hair like structures of the hair cells. They get shorter as you move from one side of the hair cell to the other.
Stereocilia are joined together by
Tip links connect to mechanically gated ion channels- pulling tip links open ion channels and ions can move into/out of the hair cell. They are like trap doors on top of the stereocilia
What happens to the tip links when the basilar membrane is at rest?
Some tip links open, there is a small amount of ion flow. Inner hair cell is slightly depolarized- a small amount of neurotransmitter is released to the cochlear nerve
When happens when the sterocilia pivot toward the tallest hair (of the hair cell)?
There is increased tension between the tip links. When tip links open, all ion channels open. K+ and Ca 2+ flood into inner hair cell, and the inner hair cell depolarizes- creates receptor potential. Neurotransmitter is released to cochlear nerve- action potential
What happens when the stereocilia bend toward the shortest hair (of the hair cell)?
Tip link tension decreases, so the tip links close. K+ and Ca 2+ no longer enter inner hair cell, so the inner hair cell hyperpolarizes. Neurotransmitter no longer released- prevents generation of an action potential
What determines the role of outer hair cells?
Fibers that wrap around outer hair cells are efferent. Therefore, the function of the outer hair cells is determined by the brain.
Roles of the outer hair cells (2)
- Increases responsiveness of inner hair cells- this amplifies the motion of basilar membrane and lets the basilar membrane distinguish between different sounds
- Protection- outer hair cells stiffen in response to loud sound. Effect- basilar membrane becomes stiffer. This serves a protective function
Where must information go for conscious perception of sound to occur?
The auditory cortex
Pathway from the cochlea to the primary auditory cortex
Action potentials generated in cochlea pass through spiral ganglion to cochlear nuclei of the medulla. From there, fibers project to superior olivary nuclei. As they travel from the cochlear nuclei to the superior olivary nucleus, the fibers cross to opposite sides of the brain. The fibers pass through thalamus, project to primary auditory cortex.
What ensures that each auditory cortex receives information from both ears?
After passing through the thalamus, some fibers are contralateral, others are ipsilateral (crossed over again) before they get to the auditory cortex.
Pitch
Impulses from specific hair cells at specific regions of the basilar membrane are interpreted as specific pitch. Multiple frequencies stimulate multiple parts of the basilar membrane
Loudness
Louder sounds= more movement of fluid in cochlea. Larger waves of fluid/larger amplitude= more movement of basilar membrane, more deflection of inner hair cells. More neurotransmitter is released by more deflection of hair cells, which results in stronger graded potentials and more frequent action potentials at the cochlear nerve.
Localization of sound
Intensity and timing localize sound source
If intensity and timing of a sound are identical, where do you localize the sound?
If intensity and timing are identical, the source of sound is above, below, in front, or behind. These locations are more central and the tympanum on both sides vibrate at the same frequency.
If intensity and timing of a sound are different, where do you localize the sound?
If intensity and timing are different, the sound is localized on your left or right side. For a sound on your right side, the tympanum on your right side will vibrate harder and at a different frequency than the tympanum in the left ear. Action potentials arrive at different rates.
Equilibrium
Sense of our own location in space, involves structures found in the inner ear
What 2 structures are responsible for equilibrium?
- Vestibule
2. Semicircular canals
Vestibule
Most central portion of bony labyrinth, posterior to the cochlea. Contains 2 membranous sacs with endolymph: the
saccule- continuous with the cochlea, smaller of the two, and the utricle- continuous with semicircular canals
Function of the vestibule
Saccule and utricle contain maculae receptors- respond to linear acceleration and head position. Therefore, the saccule and utricle do not respond to a sensation of spinning or turning.
Linear acceleration
Moving straight forward, straight back, or to one side or the other.
Semicircular canals
System of 3 fluid filled canals- each canal lies in different plane of space (anterior, posterior, lateral). The semicircular duct passes through each canal.
Ampullae
Swelling at the end of each semicircular duct with receptor crista ampullaris
Semicircular canals function
Respond to rotational movement
Maculae anatomy
Flat patch with supporting cells and hair cells (stereocilia, kinocilia). Base of hair cells are supplied by the vestibular nerve- branch of the vestibulocochlear nerve. Tips of stereocilia and kinocilia embedded in the otolith membrane
Otolith membrane
Jelly like base with small otolith stones embedded in membrane. Otoliths are dense and heavy, they cause membrane to move in response to linear head movement
How does the sensation of movement occur in the maculae?
Movement of otolith membrane and jelly like base bends hair cells.
How does bending toward the kinocilium in the maculae influence sensation of movement?
Bending toward kinocilium (tallest hair cell)- hair cells depolarize- more neurotransmitters are released and more action potentials are generated.
How does bending away from the kinocilium in the maculae influence sensation of movement?
Bending away from kinocilium (bending head backwards)- hair cells hyperpolarize. Action potentials are still generated, just not as many. Brain interprets many more action potentials than usual as head movement.
Maculae in the utricle
Maculae are horizontal, hair cells are vertical, responds to forward and backward movement.
Macule in the saccule
Maculae are vertical, hair cells are horizontal. Responds to upward/downward movement
What do maculae respond to?
Maculae only respond to CHANGES in head position- it’s not important if your head is in the same position for a longer amount of time
Cristae ampullares anatomy
Contains hair cells and supporting cells that project into the ampullary cupula. No otoliths in the semicircular canals. Vestibular nerve fibers supply hair cells
Ampullary cupula
Gel that surrounds hair cells in the cristae ampullares
How does sensation of rotational movement occur?
Endolymph flows through canals in opposite direction as rotational movement- if spinning to the right, endolymph flows to the left. When hairs are deflected- depolarization occurs, increased neurotransmitter released- vestibular fibers generate action potentials
What happens in the cristae ampullares with consistent speed of rotation?
Consistent speed of rotation- endolymph travels at same speed as rotation. Hair cells are not stimulated
What happens in the cristae ampullares when you stop rotating?
Endolymph flows in opposite direction. Hair cells hyperpolarize- less neurotransmitter released, less action potentials generated so the brain interprets this as a change in rotational movement
Why is information about balance sent directly to the reflex centers of the brain?
This is adaptive. We shouldn’t have to think about balance because if you lose your balance, you want to react quickly. We are consciously aware if we lose balance, but will fix our balance before we are consciously aware
Where can impulses from the balance center travel to (2)?
- Vestibular nuclei
2. Cerebellum
Vestibular nuclei
Major integrative area for balance, also receives visual and somatic receptors. Vestibular nuclei sends impulses to brain stem- information used to correct body position
Cerebellum
Function- coordinates skeletal muscle activity and muscle tone to maintain head position, posture, and balance
Why can dysfunction in the inner ear result in multiple sensory deficiencies?
The inner ear plays multiple roles in the special senses
Deafness
Complete or partial loss of hearing. There are 2 types: conduction and sensorineural.
Conduction deafness
Anything that inhibits sound conduction through the ear (from outer ear to cochlea). Ex- ruptured tympanum, excessive cerumen, function of ossicles, otitis media. A ruptured tympanum can’t vibrate, so nothing can be transmitted to the cochlea, excessive cerumen physically blocks sound waves from getting through the external acoustic meatus
Sensorineural deafness
Hearing loss due to damage to neural structures. The typical cause is loss of hair cells over time. This can be due to explosive sounds, degeneration of cochlear nerve, stroke, or tumor in auditory cortex.
Explosive sounds can vibrate the basilar membrane so hard that the hair cells can be ripped apart
What is the treatment for sensorineural deafness?
Cochlear implant. A cochlear implant converts sound waves to electrical energy (basically what the inner ear does). One part of the implant is implanted in the temporal bone, the other part is an electrode that is attached to the cochlea. Different electrodes respond to different frequencies of sounds that can be interpreted by the brain
Meniere’s Syndrome
Affects all parts of the inner ear (cochlea, vestibule, semicircular canals). Sensory receptors in the inner ear are very specific to the amount of endolymph in the inner ear and its consistency. If the amount of the endolymph changes, you affect the function/responsiveness of the sensory receptors. The syndrome is caused by excessive endolymph production in the inner ear.
Meniere’s Syndrome symptoms
Extreme vertigo, nausea, vomiting, tinnitus (ringing in ears if the cochlea is affected, occurs in the absence of sound), deafness
Meniere’s Syndrome treatment
Motion sickness medication, diuretics to decrease endolymph production
