Module 6 - PNS Afferent Division Flashcards
Describe the sensation of pain
• Uses nociceptors
• Activated by intense thermal, mechanical, or chemical stimuli
o Accompanied by behavioural and emotional responses unlike other stimuli
• Located everywhere but the brain
• Free nerve endings that do not adapt to sustained or repeated stimuli
• Sensations may be out of proportion to damage or appear for no obvious reason
• Influenced by prostaglandins, histamine, potassium, serotonin, and substance P (peptides)
Describe fast pain
- Uses myelinated A-delta (δ) fibres
- Responds to cold, warmth, and mechanical stimuli
- Occurs immediately after stimulus applied
- Not felt in deeper tissues of the body
- Precisely located to stimulation
Describe slow pain. Why does this pain last even after the stimulus is removed?
• Also called chronic, burning, aching, or throbbing pain
• Uses unmyelinated C fibres
• Responds to pain, heat, cold, and mechanical stimulation
• Activated by bradykinin released to the ECF from damaged tissue
o Contributes to pain and inflammatory response
o These may be the reason the pain sensation lasts after removal of stimuli
• Polymodal nociceptors
o Respond to more than one type of noxious stimuli
• Begins a second or more after stimulus is applied
• Gradually increases in intensity over seconds or minutes
• Can be felt in skin and deeper tissues
• Felt in affected area but more diffuse
Describe afferent neurons
Receive stimuli from the environment and transmit the impulse to the CNS
Single long dendrite that is functionally similar to an axon
Short axon
Smooth rounded cell body
Describe the two types of neurons in the afferent division of the autonomic nervous system
Visceral Afferent Neurons
• Convey information to the CNS from receptors
• Located in the smooth muscle of the visceral organs (mainly thorax, abdomen, and pelvis)
• Usually happens no conscious awareness except pain signals coming from viscera
Somatosensory Neurons • Consists of receptors in the PNS and processing centers in the CNS • Person is consciously aware of this input • 4 primary types of receptors are o Thermoreceptors o Mechanoreceptors o Photoreceptors o Chemoreceptors • Input from the body surface o Somaesthetic sensation from the skin o Proprioception from the muscles, joints, skin, and inner ear • Includes input from special senses
What is perception?
Conscious interpretation of the external world
What is transduction?
o Energy conversion process that converts mechanical or chemical stimulation into an electrical signal
What is adequate stimulus?
o Property of a sensory receoptor that determines the type of energy to which a sensory receptor responds with the intention of sensory transduction
Ex. Receptors in the eye are most sensitive to light etc.
o Some receptors can respond weakly to stimuli other than their adequate stimuli
Receptors are activated to a lesser degree
Same sensation detected
Ex. The eye sees “stars” in response to mechanical pressure
What is modality?
o A particular way of sensing the stimulus
o Determined by the type of sensory neuron that’s activated and its point of termination in the brain
What stimulus do photoreceptors respond to?
Visible wavelengths of light
What stimulus do mechanoreceptors respond to? What are some examples of these receptors and their specific stimulus?
Mechanical energy
Skeletal muscle receptors - stretch
Hair cells in the ear - bend to sound waves
Baroreceptors - blood pressure
Osmoreceptors - change in osmotic pressure
What stimulus do thermoreceptors respond to?
Varying amounts of heat
What receptors are responsible for the sensation of wetness?
A combination of touch, pressure, and thermal input at the central integration area
What stimulus do chemoreceptors respond to?
Specific chemicals
May include receptors for smell and taste
Chemical content of digestive tract
Oxygen and CO2 levels in the blood
What may the structure of the sensory neurons be and what is the depolarization of each type called?
A specialized ending of the afferent neuron - generator potential
A separate receptor cell closely associated with the peripheral ending of the neuron - receptor potential
Where are action potentials created in afferent neurons?
• All receptors have graded potential with action potentials initiated at the peripheral end of the afferent neuron next to the receptor
What is stimulus intensity determined by?
o Frequency of action potentials generated
o Number of receptors activated in the area
What is adaptation?
- Decrease in sensation during prolonged stimulus
* Frequency of action potentials generated in afferent neuron decreases
Describe the 2 types of receptors based on their speed of adaptation?
Phasic Receptors
• Rapidly adapting receptors
• Activated when stimulus is first encountered
• Important to signal a change in stimulus intensity rather than relay status quo information
o Ex. Tactile (touch) receptors so you can wear clothes
Off Response
• Slight depolarization when stimulus is removed
Tonic Receptors
• Slow to adapt or do not adapt at all
• Will maintain sending signals even with a sustained stimulus to allow to tell duration of the sensation
• Located where it is valuable to maintain information about stimulus
o Ex. Proprioceptors
Describe labelled lines of afferent neurons.
• Chains of neurons that create the somatosensory pathways
• Sends specialised information to specific area in the somatosensory cortex
o Allows brain to interpret specific sensations
First Order (Primary) Neurons
• Sensory neurons in PNS carrying information to CNS
• Axons synapse with second order neurons
Second Order (Secondary) Neurons
• Usually located in brain or spinal cord
• Axons ascend to the thalamus and synapse with third order neurons
Third Order (Tertiary) Neurons • Synapse located in the thalamus • Project axons into primary somatosensory cortex
What is the receptive field?
The region of skin surface surrounding a somatosensory neuron in which it will respond to a stimulus
Size is inversely related to number of receptors in the area
What is discriminative ability?
The greater acuity created by a small receptive field the more a neuron is able to sense and discriminate between different sensations (compare sensation of the same object between fingertips and elbows)
What is lateral inhibition?
Occurs within the CNS, the most activated stimulus area inhibits the less exited pathways. It’s what allows us to feel a pin point sensations
Most lateral inhibition occurs with touch and vision, allowing the most accurate localization
Describe Pacinian corpuscles. What do they respond to? What is their appearance? How quickly do they adapt?
Lamellar Corpuscle
• Also called a Pacinian corpuscle
• Respond to
o High frequency vibration
o Touch
o Deep pressure
o Allows for sensation from texture
• Located in the dermis, subcutaneous tissue, and other tissues of the body
• Myelinated neuron ending surrounded by a multilayered connective tissue capsule that resembles a sliced onion
o Onion structure causes constant pressure to slip, resulting in adaptation
• Phasic receptor so is rapidly adapting
o Respond to transient touch rather than sustained pressure
Describe Meissner’s corpuscles. What do they respond to? What is their appearance? How quickly do they adapt?
Tactile Corpuscles
• Also called Meissner’s corpuscles
• Respond to
o Low-frequency vibrations
Sensation of texture and if an object is sliding past the skin
o Light touch
• Located in dermal papillae of hairless skin and concentrated in areas sensitive to light touch such as fingers, lips, and nipples
• Elongated oval structure with a core of modified Schwann cells and neuronal endings enclosed by a capsule of connective tissue
• Afferent neuron is myelinated
• Phasic receptor so is rapidly adapting
• Small receptive fields
Describe Merkel’s discs. What do they respond to? What is their appearance? How quickly do they adapt?
Merkel’s Discs • Also called a tactile disc • Respond to o Low frequency vibrations o Tissue displacement (very sensitive) Provides touch information • Located in superficial layers of skin, mammary glands, and mucosa, clustered beneath fingerprints • Myelinated afferent neurons with endings clustered in specialized epithelial structures called touch domes or hair disks • Slow to adapt
Describe Ruffini corpuscles . What do they respond to? What is their appearance? How quickly do they adapt?
Bulbous Corpuscles
• Also called Ruffini corpuscles
• Highly sensitive to stretching and pressure
• Located in the dermis, subcutaneous tissue, and joints
• Branched neuronal endings that are distributed between collagen bundles and Schwann cells enclosed in a fibrocellular capsule with a myelinated nerve
• Slowly adapting
• Large receptive fields
Describe free nerve endings. What do they respond to? What is their appearance? How quickly do they adapt?
Free Nerve Endings
• Bare dendrites
• Respond to
o Touch and pressure
o Temperature
o Nociception (pain)
o Tickling and itching
• Certain chemicals can result in an itch sensation (such as mosquito saliva)
• Located in skin, hair roots, around the eyes, and many others
• Bare dendrites and neuron may be myelinated or unmyelinated
Describe the basics of pain sensation
• Uses nociceptors
• Activated by intense thermal, mechanical, or chemical stimuli
o Accompanied by behavioural and emotional responses unlike other stimuli
• Located everywhere but the brain
• Free nerve endings that do not adapt to sustained or repeated stimuli
• Sensations may be out of proportion to damage or appear for no obvious reason
• Influenced by prostaglandins, histamine, potassium, serotonin, and substance P (peptides)
What are the 2 main neurotransmitters used along pain pathways?
Substance P and Glutamate
Describe substance P’s role along the pain pathways
• Activates ascending pain pathways • Signals sent to o Cortex Somatosensory area localizes the pain o Thalamus Can perceive pain o Reticular formation Increases level of alertness o Interconnections with hypothalamus and limbic system Elicit behavioural and emotional response
Describe glutames role along the pain pathways
• Major excitatory neurotransmitter
• AMPA receptors
o Results in permeability changes that initiate action potentials in dorsal horn
o These action potential head up to higher levels for processing
• NDMA receptors
o Calcium entry into dorsal horn cell
o Not involved in pain pathway but instead activates a second-messenger system that makes the dorsal horn neuron more excitable than normal
o Contributes to exaggerated sensitivity that resolves with healing
Describe how fast pain is experienced
- Uses myelinated A-delta (δ) fibres with a free nerve ending receptor
- Responds to cold, warmth, and mechanical stimuli
- Feels sharp, stabbing, or acute
- Occurs immediately after stimulus applied
- Not felt in deeper tissues of the body
- Precisely located to stimulation
Describe how slow pain is experienced
• Also called chronic, burning, aching, or throbbing pain
• Uses unmyelinated C fibres with a free nerve ending receptor
• Responds to pain, heat, cold, and mechanical stimulation
• Activated by bradykinin released to the ECF from damaged tissue
o Contributes to pain and inflammatory response
o These may be the reason the pain sensation lasts after removal of stimuli
• Polymodal nociceptors
o Respond to more than one type of noxious stimuli
• Begins a second or more after stimulus is applied
• Gradually increases in intensity over seconds or minutes
• Can be felt in skin and deeper tissues
• Felt in affected area but more diffuse
Describe the bodies internal analgesic system including what endogenous opiates are and how they function in the body
• Suppresses transmission in pain pathways at entrance to spinal cord
• Two regions provide profound analgesia
o Periaqueductal grey matter
o Reticular formation
o Stimulation here blocks release of substance P from afferent pain-fiber terminals
• Endogenous opiates
o Internal substances that bind to opioid receptors on afferent pain-fiber terminals and dull pain
o Include
Endorphins
Enkephalins
Dynorphins
o Released from descending analgesic pathway
o Binding blocks release of substance P
What are the protective mechanisms for the eye?
Bony socket
Eyelids
Tears
Eyelashes
What are the 3 layers of the eye and their primary structure(s)?
Outer - fibrous tunic - sclera and cornea
Middle - vascular tunic - choroid, ciliary body, and iris
Inner - retina
How does the eye prevent any reflection or scattering of light within the eye?
Highly pigmented choroid and outer layer of the retina
What is glaucoma, what is it caused by, and what are the results?
Excess pressure within the anterior chamber of the eye
Caused by aqueous humor not draining as quickly as it is created (eg blockage of drainage canal)
Can push lens back into vitreous humor, results in pressure against the inner neural layer of retina which can lead to retinal and optic nerve damage and ultimately blindness
Describe the sclera of the eye
- White of the eye
- Made of tough connective tissue
- Maintains shape and protects inner structures
- Provides attachment points for extraocular muscles
Describe the cornea of the eye
- Transparent
- Avascular connective tissue with regularly spaced collagen fibers
- Allows light to enter the eye
- Curved surface a refractory structure for incoming light
Describe the choroid of the eye
- Posterior portion of vascular tunic
- Lines most of the internal surface of the sclera
- Contains blood vessels that also supply the retina
- Contains melanocytes to produce pigment that minimize light scattering
Describe the ciliary body. Where is the aqueous humor secreted and how does it attach to the lens?
• Anterior portion of vascular tunic
• Extends from anterior margin of retina to just posterior of the corneoscleral junction
• Contains melanocytes producing melanin
• Ciliary processes
o Protrusions or folds on internal surface of the ciliary body
o Contain capillaries secreting aqueous humor
• Zonular fibers
o Also called suspensory ligaments
o Extend from the ciliary process
o Attach to the lens
o Composed of thin, hollow fibrils resembling elastic connective tissue fibers
• Accommodation
o Contraction or relaxation of ciliary muscles changes tension of zonular fibers
o This changes the shape of the lens adapting it for near or far vision
Describe the iris and the 2 muscles that make it work
• Muscular, coloured portion of the eye
o Colour determined by amount of melanin produced
• Contraction and relaxation change the size of the pupil regulating the amount of light that enters
• Suspended between cornea and lens as an anterior projection of the ciliary body
• Sphincter pupillae muscle
o Also called the circular (constrictor) muscle
o Flat, thin band of circularly oriented muscle fibers
o Contraction causes constriction of the pupil
o Stimulated by parasympathetic fibers of oculomotor (III) nerve
• Dilator pupillae muscle
o Also called the radial (dilator) muscle
o Attaches to the outer circumference of the sphincter pupillae
o Projects like spokes towards the base of the iris
o Stimulated by sympathetic neurons causing dilation
What is light?
A form of electromagnetic radiation composed of particle-like individual packets of energy called photons that travel in a wavelike fashion
What is a wavelength and what are the wavelengths of visible light? How do these wavelengths correspond to the colours we see?
The distance between two wave peaks
400-700 nm
Shorter wavelengths are violet and blue
Longer wavelengths are orange and red
What changes to the waveform of light in a bright light vs a dim light?
Light energy changes in intensity, so the amplitude or height of the wave is changed but the length between two wave peaks remain the same
What is a light ray?
The forward movement of a light wave in a particular direction
How does the body take divergent (radiating) lights and change it to an accurate image of the light source?
By bending the light rays inward and focusing them back to a point on the retina called the focal point
What two structures contribute to the eyes refractive ability? Which ones contributes the most?
Cornea and lens; the cornea contributes the most due to the curve and the difference in densities between the air and the cornea
What is astigmatism?
Uneven curvature of the cornea resulting in unequal refraction
Describe the accommodation used for far vision
Ciliary muscle is relaxed and suspensory ligaments (zonular fibers) are taut. this pulls lens into flattened, weakly refractive shape
Describe the accommodation for near vision
Ciliary muscle is contracted and suspensory ligaments (zonular fibers) are slack. This lens becomes more convex and stronger allowing for near vision
How does the autonomic nervous system adjust for far/near sightedness?
Sympathetic system causes relaxation of ciliary muscle, resulting in far vision
Parasympathetic stimulation causes contraction of ciliary muscle for near vision
What is presbyopia?
Loss of elasticity in the lens so it does not accommodate for near vision well, resulting in people needing reading glasses. Age related, and affects most people by middle age (45-50)
What is a cataract?
A now opaque elastic fiber in the lens that prevents light rays from passing through; can be surgically replaced with an artificial lens to correct vision
What is emmetropia?
Regular vision where incoming light can be focused on the retina without assistance
What is myopia?
Nearsightedness
Far light lands in front of the retina and is blurry. Close objects can be brought to retina without accommodation (even though this is normally needed)
What is hyperopia?
Farsightedness
Far objects focused on the retina with accommodation (which isn’t usually needed for far objects). Close objects focus behind the retina and remains blurry
What are the 3 layers of excitable cells (called the neural layer) in the retina?
- Photoreceptor cell layer (rods and cones)
- Bipolar cell layer (see activating action potentials below)
- Ganglion cell layer (see activating action potentials below)
Are the rods and cones a peripheral organ?
No, they are a part of the CNS; during embryonic development the retinal cells back out of the nervous system so the retinal layers are facing backwards
In the eye, what do the axons of ganglion cells join to form?
Optic nerve
Describe the point in retina where optic nerve and blood vessels pass through
Optic Disk
• Also called the blind spot
• Optic (II) nerve creates blind spot as no photoreceptors here
• Your brain patches in the information with what’s in the surroundings
Describe the fovea centralis
- Small depression in the center of the macula
- Vision is sharpest
- Full of cones, no rods
- Bipolar and ganglion cells displaced to periphery as they scatter some light
Describe the fovea centralis
- Small depression in the center of the macula
- Vision is sharpest
- Full of cones, no rods
- Bipolar and ganglion cells displaced to periphery as they scatter some light
Describe the macula lutea?
- Also called the yellow spot
- Exact center in posterior portion of the retina
- Large concentration of cones
- High acuity that is slightly less than the fovea because the ganglion and bipolar cells are overlying the rods and cones here
Describe macular degeneration
• Also called age-related macular disease (AMD)
o Occurring in those 50 years or older
o Most common cause of blindness in those over 75
• Leading cause of blindness in western hemisphere
• Degenerative disorder of retina and pigmented layer
• Abnormalities of the macula occur
• Symptoms
o Blurring and distorting vision at center of visual field
o Gradual vision loss as pigmented layer atrophies and degenerates (dry AMD)
o Loss of ability to see straight ahead
o Maintain peripheral vision
• wet AMD causes new blood vessels to form in choroid and leak plasma or blood under the retina
o Vision loss can be slowed by laser surgery destroying leaking vessels
• Smoking increases risk
• No effective treatment
Describe the 3 main parts of rods and cones
• Outer segment o Detects light stimulus o Face away from incoming light o Either rod or cone shaped o Contains stacked, flattened, membranous discs containing photopigment • Inner segment o Forms mid-region of the cell o Contains metabolic machinery • Synaptic terminal Closest to eyes interior
Describe photopigments
• Undergoes chemical alterations when activated by light initiating the action potential
• Opsin
o Protein integral to disc membrane of the outer segment of the rod or cone
• Retinene
o Bound in interior of opsin molecule
o Light-absorbing part of the photopigment
o Derivative of vitamin A
• Rhodopsin used in rods
• Photopsin used in cones
Describe the location, functions, and photopigments of rods
• Highest concentration in the periphery of the retina
• Low light
• Rhodopsin is the photopigment
o Absorbs all wavelengths of light but brain cannot distinguish them
o Interpreted only in shades of grey
o Detects different intensities rather than different colours
• More sensitive but less acuity than cones
• Over 100 rods may converge via bipolar cells to a single ganglion cell
Describe the location, functions, and photopigments of cones
• • Highest concentration in the fovea and macula
• For bright light
• 3 slightly different photopigments, giving blue, green, red cones
• Photopigment is photopsin
• Brain interprets the various receptors to determine colour
• Generally, 1 cone to 1 ganglion cell
Found in the fovea
What is phototransduction?
• Converting light stimuli into electrical signals
What is phototransduction and how to light and dark effect it?
- Converting light stimuli into electrical signals
- Photoreceptors inhibited by adequate stimulus (hyperpolarized by light)
- Photoreceptors excited in absence of stimulation (depolarized by darkness)
How do photoreceptors work in the dark?
• Internal second messenger cyclic GMP (cGMP)
o Cyclic guanosine monophosphate
o Binds to sodium channels to keep them open
• Slow influx of sodium keeps receptor cell slightly depolarized
• Depolarization at synaptic end causes calcium channels to open releasing inhibitory neurotransmitter
• This prevents you from seeing things in the dark
How do photoreceptors work in the light?
• Cyclic GMP decreased triggered by photopigment activation
• Retinene changes shape when absorbing light activating the photopigment
• Transducin activated
o G protein
o Activates phosphodiesterase enzyme
• Enzyme degrades cGMP, permitting sodium channels to close
• Resulting hyperpolarization is receptors potential, spreading to synaptic terminal decreasing neurotransmitter release
• Brighter lights cause hyperpolarization resulting in even more decreases in neurotransmitter release
Describe ganglion cells and how they play a role to information received from both rods and cones
• First order neurons in site pathway
• Conduct action potentials to the brain
• Cones
o 1 to 1 ratio of cone to ganglion
o Only bright lights capable of reaching threshold
o Very small receptive field on retinal surface
• Rods
o Significant convergence of rods to a ganglion
o Summation allows response to much lower light condition
o No ability to discern exact location of exact rod that was activated resulting in low acuity
o Poor ability to distinguish between 2 objects
o Objects often fuzzy
Describe dark adaptation
- Gradual process of being able to see in darkened surroundings after bright conditions
- Photopigment broken down during light exposure gradually regenerated
- Newly rejuvenated rods are the only ones sensitive enough to be turned on by the dim light
Describe light adaptation
- Initial exposure to bright lights causes vision to appear bleached
- Photopigments begin to break down leading to decreased sensitivity
- Once broken down, they can no longer respond to light
Describe night blindness
- Result of dietary deficiencies of vitamin A needed for retinene
- Enough cone photopigment to respond to bright light
- Decrease in rods reduces sensitivity so they are no longer functional
What is colour vision and what does it depend on?
Perception of many colours
Depends on the 3 cone types various ratios of simulation in response to different wave lengths
Describe colourblindness and its possible causes
- Lack of a type of cone, so they only see combinations of 2 colours
- Cannot distinguish as many varieties of colours
- Often genetic but may be due to eye, nerve, or brain damage
Describe types of retinal processing
• Strong stimulation to a cone suppresses activity in surrounding cones to increase contrast
Ganglion Cell Processing
• On-center ganglion cells
o Increases firing rate when light is most intense at the center of its receptive field
• Off-center ganglion cells
o Increases firing rate when light is most intense at the periphery of its receptive field
• Information in absolute brightness sacrificed for relative brightness which enhances contours of an image
Describe what part of the visual field is sent to each side of the visual cortex
Left half of cortex receives information from right half of visual field from both eyes
Right half of cortex receives information from left half of visual field from both eyes
What is the term for double vision and what are some possible causes?
Diplopia
- Defects to external eye muscles that make the eyes not focused on the same object simultaneously
- Binocular information improperly integrated during visual processing
Describe the pathway to the brain that the optic nerve takes once it leaves the eye
• Retinal ganglion axons form optic (II) nerve
• Optic chiasm
o Crossing point of optic nerves
o Information from medial fibers are crossed over
Left side of visual field from both eyes sent to right side of visual cortex
Right side of visual field from both eyes sent to left side of visual cortex
• Optic tract
o Fibers leaving the optic chiasm
• Optic radiations project to the primary visual cortex via the lateral geniculate nucleus of the thalamus
o Some information sent from visual cortex to higher-level visual areas for further processing
• Other optic tract fibers terminate in the
o Superior colliculi which controls extraocular muscles
o Pretectal nuclei which controls pupillary and accommodation reflexes
Describe the visual field
- Field of view that can be seen without moving the head
* Image at retina is upside down and backwards, which is interpreted correctly in the brain
Describe depth perception
- Overlapping visual fields of the eyes produce binocular vision (same area seen at the same time by both eyes) which allows for depth perception
- Some depth perception is possible with a single eye using experience and comparison with other cues
Describe the visual cortex processing including the cell types used for interpretation of visual input
- Hierarchy of cells responding to increasingly complex stimuli
- Respond to patterns built by converging connections originating from closely aligned photoreceptor cells in the retina
- Each cell responds only to a specific pattern for which its programmed
- Each level of neuronal cells have increased capacity for abstraction
- Higher visual regions integrate information into a single picture we see
Simple Cells
• Stacked on top of one another within cortical columns of primary visual cortex
• Evaluating for specific simple patterns such as lines
Complex Cells
• Stacked on top of one another within cortical columns of primary visual cortex
Hypercomplex Cells
• Located in higher visual-processing areas
• Responds only to particular edges, corners, and curves giving dimension
What is pitch or tone of a sound?
Eg if it is a C or G note
Determined by frequency of vibrations
Greater the frequency the higher the pitch
Humans can hear 20 - 20 000 cycles per second
Most sensitive to 1000-4000 cycles per second
What is intensity of a sound?
Also called loudness
Depends on amplitude of soundwaves, greater the amplitude, the louder the sound
Measured in dB
What is the timber of a sound?
Also called quality
Depends on overtones, which are additional frequencies superimposed on the fundamental pitch or tone
What allows us to determine which instrument is playing a G even though they play the same note
Allows us to distinguish characteristic differences in voices
How do we know where a sound is coming from?
• Minor changes allow us to determine where a noise is coming from
• From behind
o Shape of pinna changes timber of sounds approaching from behind
• Left and right
o Sound from the left will
Reach the left ear first with a slight delay to the right
Be more intense to the left ear than to the right
How does the body equalize the air pressure between the middle and outer ear so that the tympanic membrane can vibrate freely?
The eustachian (auditory) tube connects the middle ear to the pharynx Can be opened with stretching, which permits middle ear to reach the same air pressure as the outside ear
Describe the tympanic membrane
• Thin, semitransparent membrane
• Sound waves that reach the membrane cause it to vibrate
• Very sensitive to pain
• Fibrous cartilage ring holds it to the temporal bone
• Made of three layers, from outside to inside are
o Covering of stratified squamous epithelium
Continues with the epidermis
o Dense connective tissue
Contains collagen, elastin fibers, and fibroblasts
o Simple cuboidal epithelium
• Convex towards middle ear
• Umbo
o Apex of the eardrum
What are the 3 main portions of the outer ear?
Auricle (pinna)
External auditory meatus
Tympanic membrane
Describe the auditory ossicles and their function
• Made of 3 smallest bones in the body
o Malleus, incus, and stapes
• The malleus attaches to the tympanic membrane in the superior inner surface
• Stapes fits into the vestibular (oval) window
• Ossicles connected by synovial joints
• Amplifies pressure of air sound waves to the fluid of the inner ear
o Size difference between tympanic membrane and oval window increasing force
o Lever action of ossicles provide mechanical advantage
Describe the function of the vestibular (oval) window
- Connects to the inner ear
* Movements of the oval window cause movement of the fluid in the inner ear
Describe the muscles found in the middle ear and their function
• Dampens large vibrations of stapes due to loud noises (over 70dB)
o Contraction reduces movement of ossicles
o Only works for prolonged noises
Tensor Tympani Muscle
• Skeletal muscle innervated by mandibular branch of trigeminal (V) nerve
Stapedius Muscle • Innervated by facial (VII) nerve • Is the smallest skeletal muscle • Protects vestibular window • Decreases hearing sensitivity
Describe the structure of the cochlea
• Fluid filled
• Contains sensory receptors responsible for hearing in the organ of Corti
• Modiolus
o Central bony core
• Bony part separated into
o Membranous cochlear duct
Also called scala media
Continuation of membranous labyrinth
Filled with endolymph
Roof is the vestibular membrane
Floor is the basilar membrane and is the site of the organ of corti
o Scala vestibuli
The bony channel located above the cochlear duct
Begins at the oval window
Ends at the vestibular window
Filled with perilymph
Vestibular membrane separates cochlear duct from scala vestibuli
o Scala tympani
The bony channel located below the cochlear duct
Ends at the cochlear window
Filled with perilymph
Basilar membrane separates cochlear duct from scala tympani
• Helicotrema
o Opening at the apex of the cochlea
o Only place the scala vestibuli and scala tympani are not completely separated by cochlear duct
Describe the structure of the organ of Corti
• Site of mechanoreceptors
• Spiral organ resting on the basilar membrane of the cochlea
• The 16 000 hair cells within the organ of Corti organized as
o Inner hair cells arranged in a single row
Changes mechanical sound to electrical impulses
o Outer hair cells arranged in three rows
Enhances response of inner hair cells
• Hair bundle
o Apical tip of each hair cell extends into endolymph of cochlear duct and consists of
Stereocilia
• Not cilia but rather long, hairlike microvilli stiffened with actin
• Arranged in rows of graded height
• About 100 per hair cell
Kinocilium
• Also called cilium
o Basal end synapse with
First order sensory neurons (mostly inner hairs)
• Cell bodies are located in the spiral ganglion
Efferent neurons from cochlear branch of vestibulocochlear (VIII) nerve (mostly outer hairs)
• Tectorial membrane
o Flexible gelatinous membrane
o Overlies hair cells
o Apical ends of stereocilia embedded here
Describe the mechanism of hearing from an incoming sound to arrival at the cochlear duct (scala media)
- Sound waves directed into external acoustic meatus by auricle
- Strikes tympanic membrane which vibrates back and forth
- Vibration transmitted from malleus, to incus, to stapes
• Due to the difference in surface areas, the sound waves intensify through the middle ear - Stapes moves back and forth against the vestibular (oval) window
- Movement at vestibular window pushes on perilymph of scala vestibuli
- Pressure waves transmitted from scala vestibuli to scala tympani and eventually to cochlear (round) window, which bulges outward (at number 9)
• The bulging of the cochlear window dissipates sound energy - Waves travel through perilymph of scala vestibuli, the vestibular membrane and into the endolymph inside the cochlear duct
Describe the mechanism of hearing inside the cochlear duct
- Pressure waves in endolymph cause basilar membrane to vibrate
• Stereocilia of the inner hair cells bend against the tectorial membrane as they oscillate on the basilar membrane
• Mechanically gated ion channels in the hair cells are opened and closed with the movement
• Depolarizing graded potentials occur as basement membrane moves upward and neurotransmitter release increases
• Hyperpolarizing graded potentials occur as basement membrane moves downwards and neurotransmitter release decreases - The cochlear branch of the vestibulocochlear (CN VIII) nerve receives neurotransmitters across the synapse
Describe the role of the outer hair cells in the organ of Corti
• Do not communicate with cochlear branch of vestibulocochlear (VIII) nerve
• Amplify the movement of the basilar membrane
• Electromotility
o Behaviour of rapidly changing length in response to changes in membrane potential
o Shorten on depolarization
o Lengthen on hyperpolarization
Describe how pitch is determined
• Various frequencies cause certain regions to vibrate more than others
o Basement membrane near the vestibular (oval) window is narrow and stiff
Vibrates best with high frequency sounds
o Basement membrane near the helicotrema is wide and flexible
Vibrates best with low frequency sounds
• Sound wave dies out at its region of maximum displacement as the oscillation dissipates the wave
• Brain interprets pattern of hair cell stimulation to determine the frequency
Describe how intensity (loudness) is determined
- Loud sounds cause a greater deflection of tympanic membrane
- Greater amplitude of basilar membrane movement in the region of peak responsiveness which the brain interprets as louder
Where is the primary auditory cortex and how is it organized
Temporal lobe
Tonotopically (different regions linked to specific areas of basil membrane)
Where do the cochlear nerves cross over, what does this mean for brain injury to one side of the brain
In the medulla, so it doesn’t matter which side of the brain is damaged, hearing from both ears will still occur in the undamaged side
What is the auditory pathway from the hair cells to the arrival at the primary auditory cortex
First Order Neurons
• Stimulated by neurotransmitters released from the bending hair cells
Cochlear Nerve
• Branch of the vestibulocochlear (VIII) nerve
• Formed by axons of first order neurons
Cochlear Nuclei
• Located in the medulla
• Site of synapse for cochlear nerve
• Cross over occurs just superior to here
Superior Olivary Nucleus
• Located in the pons
• Difference in timing between left and right side allows us to locate source of the sound
• Action potentials ascend midbrain to end in the inferior colliculus
Inferior Colliculus
• Located in the midbrain
• Some axons terminate here
• Brought by lateral lemniscus
o Ascending tract of axons that have decussated (crossed over) in the medulla
Medial Geniculate Nucleus
• Located in the thalamus
• Axons travel here from the colliculi
What is deafness?
• Significant or total hearing loss which may be temporary or permanent, partial or complete
Described as conductive deafness or sensorineural deafness
• Most do not notice hearing loss until destruction is extensive and they have difficulty understanding speech
• Testing is Weber’s test
o Can distinguish between types of deafness
o Stem of vibration fork held to the forehead
Normal hearing will hear sound equally in both ears
Conduction issues will hear sound best in affected ear
If sound is heard best in normal ear, likely sensorineural
Describe sensorineural deafness
• Caused by impairment of hair cells in cochlea or damage to cochlear branch of vestibulocochlear (VII) nerve
• Caused rarely by damage to ascending auditory pathways or auditory cortex
• Causes include
o Atherosclerosis reducing blood supply
o Certain drugs such as aspirin and streptomycin
o Repeated exposure to loud noises
Damages hair cells of cochlea, the louder the noise, the more rapid the hearing loss
• Neural presbycusis
o Partial hearing loss due to age-related wearing out of the hair cells over time
Describe conduction deafness
• Caused by impairment of external and middle ear mechanisms for transmitting sounds to the cochlea
• Causes include
o Otosclerosis which is deposition of new bone around oval window
o Impacted cerumen
o Injury to ear drum
o Aging resulting in thickening of eardrum and stiffening of joints in auditory ossicles
When are hearing aids helpful?
With conductive deafness but the receptor cell-neural pathway must still be intact
Less helpful for sensorineural deafness
Describe cochlear implants
Electronic devices implanted that transduce sounds into electrical signals and directly stimulate the cochlear nerve
Helps to bypass defective cochlear system
Success ranges from ability to “hear” sounds and being able to carry on a conversation
What is the organ that plays a role in the coordination of head movements with eye and postural movements?
Vestibular apparatus
What are the 2 structures of the vestibular apparatus?
• Otolith organs
o Utricle and saccule of vestibule
• Semicircular canals
What fluids are contained within the vestibular apparatus?
• All parts contain endolymph and are surrounded by perilymph
Describe the hair cells used in the vestibular apparatus
• Contain sensory receptors that depolarize or hyperpolarize based on which direction the endolymph pulls them
• Stereocilia
o A tuft of 20-50 microvilli arranged in rows of increasing height
o Tip links
Molecular bridges between adjacent stereocilia
• Kinocilium
o Conventional cilium that extends beyond the longest stereocilium
• Hair bundle
Collective name for stereocilia and kinocilium
Describe the otolithic organs
• Collectively called the vestibule
• Sac like structure located between the semicircular canal and cochlea
• Otolithic organs
o Utricle
Oriented vertically when individual is in an upright position
o Saccule
Oriented horizontally when individual is in an upright position
• Macula
o Thickened region attached to the inner wall of each organ
o Contains hair cells
• Supporting cells
o Columnar cells
o Secrete otolithic membrane
Thick gelatinous, glycoprotein layer
This rests on the hair cells
• Otoliths
o Dense calcium carbonate crystals
o Suspended within otolithic membrane making it heavier and giving it more inertia than the surrounding fluid
Describe how the otolithic organs detect movement
• Utricle responds to
o Moving the head away from vertical
I.e., Tilting head any way other than straight up and down
o Horizontal linear motion
I.e., walking backward, forward, etc.
• Saccule responds to
o Movement away from a horizontal position
i.e., getting up from bed
o Vertically directed linear acceleration and deceleration
i.e., jumping up and down
• Hairs are bent due to either movement or gravity because the otolithic membrane is top heavy due to the otoliths on the top
• Depending on the direction, depolarizing or hyperpolarizing occurs resulting in increased or decreased neurotransmitter release respectively
Describe the semicircular canals
• Detect rotational or angular acceleration or deceleration of the head
• 3 different rings in 3 separate planes
o Anterior and posterior are vertical
o Lateral is horizontal
• Contains both hair cells and supporting cells
• Ampulla
o Dilated portion of each duct
• Crista
o Small elevation (ridge) of each ampulla
o Site of hair and supporting cells
• Cupula
o Mass of gelatinous material that covers the crista
o Protrudes into the endolymph and sways in the direction the fluid is moving
Describe how the semicircular canals detect movement
• Cupula movement
o Leans in the opposite direction of the head movement
o If movement continues, fluid will stabilize
o If movement stops, the endolymph will flow in the reverse from the original movement
• Stereocilia within the cupula move pulling on the tip links
• The tip links pull on the mechanically gated ion channels
• Depolarization occurs if the stereocilia are bent towards the kinocilium
o Increases neurotransmitter release, increasing firing rates of first order neurons
• Hyperpolarization occurs if the stereocilia are bent away from the kinocilium
o Decreases neurotransmitter release, decreasing firing rates of first order neurons
• No stimulation is head is not moving or if it is rotating at a constant speed
Describe the equilibrium pathway after a hair cell has been bent?
First Order Neurons
• Stimulated by neurotransmitters released from the bending hair cells
• Axons for these cells is located in the vestibular ganglia
Vestibular Nerve
• Branch of the vestibulocochlear (VIII) nerve
• Formed by axons of first order neurons
Inferior Cerebellar Peduncles
• Located in the cerebellum
• Some axons synapse here
• There are bidirectional pathways between here and the vestibular nuclei
Vestibular Nuclei • Located in the medulla and pons • Most axons synapse here • Site of major integrating centers for equilibrium • Additional input received from o Eyes o Proprioceptors (especially those in the neck and limbs) • Send commands out to many areas
What are the locations that the vestibular nuclei sends information to regarding equilibrium and why are they stimulated?
Nuclei of oculomotor (III), trochlear (IV), and abducens (VI) nerves
• Move eyes with head to maintain focus on visual field
Nuclei of the accessory nerves (XI)
• Control head and neck movements to maintain equilibrium
Vestibulospinal tract
• Maintains muscle tone in skeletal muscles to maintain equilibrium
Ventral posterior nucleus
• Part of the thalamus
Vestibular Area • Located in the parietal lobe • Impulses arrive here via the thalamus • Part of the somatosensory cortex • Provides conscious awareness of position and movements of head and hands
What are tastants?
- Chemical that stimulates gustatory receptor cells
- Once dissolved in saliva, contacts gustatory microvilli of gustatory cells
- Action potential passed to first order neuron at base of gustatory cells
How does taste discrimination work if there are only 5 primary tastes?
• Taste itself is very limited
• Flavor is a combination of taste and olfactory information
• Sense of taste decreases with sinus infections, smoking etc.
• 5 primary tastes
• Each receptor cell responds in varying degrees to all 5 tastes but preferentially to 1
• Taste discrimination based on subtle differences in stimulation patterns
o Much like vision with only 3 colours
What are the 5 main tastes?
Salty Sour Sweet Bitter Umami
How is salty taste detected?
- Stimulated by chemical salts (especially NaCl)
* Na+ ions enter through specialised Na+ channels resulting in depolarization
How is sour taste detected?
- Stimulated by acids containing free H+ ions
* H+ blocks K+ channels decreasing passive movement of K+ out of the cell resulting in depolarization
How is sweet taste detected?
• Triggered by glucose but can be stimulated by similarly structured no calorie sweeteners
• G protein activation results in a cAMP second-messenger pathway
o Results in phosphorylation and blockage of K+ channels
o Leads to depolarization
How is bitter taste detected?
- Stimulated by more chemicals including alkaloids (coffee, nicotine, toxic plants etc.)
- May be a protective mechanism to prevent us eating dangerous compounds
- Each gustatory cell has 50-100 different bitter receptors, so a wide variety of chemicals create the same taste
- Gustducin (G protein for taste) sets of a second-messenger pathway in the cell
How is umami taste detected?
- Triggered by amino acids, especially glutamate
* A G protein and second-messenger system are currently unknown
What are taste buds?
- Young adults have about 10 000 but this decreases with age
- Mostly located on the tongue but also located on the soft palate, pharynx, and epiglottis
- Taste pore is the opening of the taste bud to the surface
- Oval body consisting of 3 types of epithelial cells (gustatory, supporting, and basal)
Describe the gustatory epithelial cells of the taste buds
• Receptor cells for gustation
• About 50 per taste bud
• Microvilli project up to the taste pore and contain receptors for specific tastants
o Can only bind to liquids or molecules in saliva
• Only last about 10 days
Describe the supporting epithelial cells of the taste buds
• Contain microvilli and surround the gustatory epithelial cells
Describe the basal epithelial cells of the taste buds
- Located at periphery of taste bud near connective tissue layer
- Stem cells that develop into gustatory cells
What are lingual papillae and what are the 4 types found in the mouth?
• Elevations on the tongue where taste buds are located
• Increases surface area
Vallate papillae, fungiform papillae, foliate papillae, and filiform papillae
Describe the vallate papillae of the tongue
- Large and circular
- Form inverted V-shaped row at the back of the tongue
- About 12 that house 100 – 300 taste buds each
Describe the fungiform papillae of the tongue
- Mushroom-shaped elevations
- Scattered over entire tongue
- Each contain about 5 taste buds
Describe the foliate papillae of the tongue
- Located in small trenches on lateral sides
* Most taste buds degenerate in early childhood
Describe the filiform papillae of the tongue
- Don’t contain any taste buds
- Contain tactile receptors and increase friction allowing movement of food
- Pointed, threadlike structures
What are the nerves used for taste (including first order neurons)?
First Order Neurons
• Synapse at the base of gustatory cells
• Many dendrites receive inputs from many gustatory cells in several taste buds
Facial (VII) Nerve
• Anterior 2/3 of tongue
Glossopharyngeal (XI) Nerve
• Posterior 1/3 of tongue
Vagus (X) Nerve
• Serves throat and epiglottis
Where do the nerves for taste converge in the CNS and what are their final destinations?
Gustatory Nucleus
• Located in medulla
• Caudal part of the nucleus of the solitary tract
Final Destinations • Limbic system • Hypothalamus • Thalamus • Gustatory cortex o Taste signals arrive here via thalamus o Located in the insula o Here and operculum of frontal lobe give conscious perception of taste and discrimination of taste sensations
Describe the basics of olfaction and what smell is
• Scent molecule binds to olfactory chemoreceptor
• Humans recognize more than 10 000 different odours with about 10 million receptors
• Strength of smell is dependent on the number of odorant molecules
• Each receptor is specific to one chemical
o Distinct odors, or scents, are based on which neurons are depolarized
o Some people are more sensitive to some smells as they have more of the specific receptors
• Sensory neurons only live about 4 weeks and are constantly replaced
• Smell triggers limbic system and other parts of the cortex eliciting memories
Describe the olfactory epithelium
- Also called olfactory mucosa
- Location of receptors
- Occupies superior part of nasal cavity covering inferior surface of cribriform plate extending to superior nasal concha
Describe the olfactory sensory neurons
- First order neurons of olfactory pathway
- Bipolar neurons with knob-shaped dendrite and axon
- Olfactory cilia extend from dendrites
- Olfactory receptors located in plasma membrane of cilia
- Odorants are chemicals with odour and interact with receptors starting an action potential
- An increase of mucus production blocks the odor molecules from attaching to the neurons, decreasing the sense of smell
- Axons group into bundles that pass through cribriform plate and form olfactory (1) nerve
Describe the supporting epithelial cells of olfaction
• Columnar epithelial cells lining nose that make mucus
• Provide support, nourishment, and electrical insulation for sensory neurons
• Produce odour binding proteins that transport odorants to sensory neurons
• Detoxify chemicals that come into contact with olfactory epithelium
• Innervated by parasympathetic neurons of the facial (VII) nerve
o Impulses can also cause tears and a running nose after certain chemicals
Describe the basal epithelial cells of olfaction
• Stem cells located between bases of supporting cells
• Continuously divide to produce new sensory neurons
• Innervated by parasympathetic neurons of the facial (VII) nerve
o Impulses can also cause tears and a running nose after certain chemicals
Describe how olfactory sensory neurons are depolarized
- Each neuron looking for individual odorant molecule
- Connection activates a G protein triggering cAMP second-messenger system triggering Na+ channels to open causing depolarization
- Sufficient depolarization leads to an action potential
- Frequency of action potentials depend on the concentration of the stimulating chemical
Describe the olfactory pathway once a first order neuron has been depolarized
Olfactory Bulb
• Left and right olfactory nerves pass through cribriform plate and into the bulbs (one on each side)
Glomerulus
• Ball like arrangements located in the bulbs
• Each receives input from only one type of odour receptor helping with processing scents
• Site of synapse between receptors and mitral cells
Mitral Cells
• Second order neurons
• Convey information only about a select group of odorants depending on which glomerulus they associate with
Olfactory Tract
• Made of axons of mitral cells
Limbic System
• Some axons pass into here
• Produces emotional response to odours
Olfactory Cortex
• Some axons pass into here in temporal lobe
• Causes conscious awareness of smell
Orbitofrontal Cortex
• A pathway extends from olfactory cortex to here via thalamus
• Located in frontal lobe
• Allows for discrimination and odour identification
• More active on the right side than left side
Describe olfactory adaptation
- Sensory neurons are slow to adapt
* CNS is rapidly adapting to many scents but does not adapt for some scents
Describe the VNO
Vomeronasal Organ (VNO)
• Also called the sexual nose
• Used to detect pheromones and communicates with the limbic system
• Common in other mammals but doubts as to whether it works in humans
• Unconscious and odourless may explain chemistry and “vibes”
Which one of the following types of mechanoreceptors are located in the skin and respond to touch and deep pressure?
a) Meissner’s corpuscles
b) Merkel’s disks
c) Pacinian corpuscles
d) Ruffini corpuscles
C
The outermost (front) of the retina is comprised of which type of cells?
a) amacrine cells
b) bipolar cells
c) cones
d) ganglion cells
D
When sound waves reach the ear, the first step of hearing is vibration of the
a) basilar membrane
b) middle ear bones
c) oval window
d) tympanic membrane
D
List the 4 categories of sensory receptors
Photoreceptors, mechanoreceptors, thermoreceptors, and chemoreceptors
What is the role of the inner hair cells in the organ of Corti?
The inner hair cells of the organ of Corti transform the mechanical forces of sound into the electrical impulses of hearing
List the 5 different types of tastes
Sweet Salty Bitter Sour Umami
With regards to pain, capsaicin would activate which of the following?
a) chemical nociceptors
b) mechanical nociceptors
c) thermal nociceptors
A
Which of the following descriptions of eye sight is BEST associated with myopia?
a) The age-related reduction in accommodative ability
b) The elastic fibers of the lens becoming opaque
c) The lens is too strong; a light source is brought to focus on the retina without accommodation
d) The eyeball is too short; far objects are focused on the retina only with accomodation
C
Based on your knowledge of pain, which one of the following is involved in the analgesic pathway?
a) increased acetylcholine release
b) increased glutamate release
c) increased substance P release
d) increased endogenous opiate release
D
Compare and contrast tonic receptors and phasic receptors
Phasic receptors are rapidly adapting whereas tonic receptors either adapt slowly or do not adapt at all
Differentiate between receptor potentials and action potentials
Receptor potentials are local, graded to stimulus intensity, and diminish in amplitude as they spread
Action potentials are all-or-none events that are used for long-distance communication
With regards to hearing, distinguish between pitch and loudness
Pitch is determined by the frequency of the vibrations
Loudness depends on the amplitude of the vibrations
What would be the consequence of a lesion of the left optic tract?
a) a loss of the left visual field in both eyes
b) a loss of the right visual field in both eyes
c) a loss of vision in the left eye
d) a loss of vision in the right eye
B
Sensory receptors have four properties, which one of the following refers to modality?
a) receptors responding to different types of stimuli such as touch or heat
b) coded by the frequency of action potential firing
c) coded by action potentials with increased amplitudes
d) receptors for this property carried by efferent fibers
A
Several regions of the brain are involved in the processing of signals from pain afferent fibers. Which of the following is the role of the cortex in pain processing?
a) allows for emotional responses to pain stimuli
b) somatosensory processing localizing the pain to a discrete body region
c) allows for perception of pain
d) increases level of alertness following a pain stimulus
B
Explain how hearing aids work
In persons with intact receptor-cell neural pathways, hearing aids increase the intensity of airborne sounds and can modify the sound spectrum
Why is it that when you have a cold your sense of smell may be reduced?
Your sense of smell is reduced when you have a cold because the odorants do not reach the receptor cells as readily when the mucous membranes lining the nasal passageways are swollen and excess mucous is present
Suzanne complained to her physician of bouts of dizziness. The physician asked her whether by dizziness, she meant a feeling of lightheadedness, as if she were going to faint, or a feeling that she was surrounding objects in the room were spinning around. Why is the distinction important in the differential diagnosis of her condition? What are some possible causes of each of these symptoms?
Syncope most frequently occurs as a result of inadequate delivery of blood carrying sufficient oxygen and glucose supplies to the brain. Possible causes include
- circulatory disorders, such as impaired pumping of the heart or low blood pressure
- respiratory disorders, resulting in poorly oxygenated blood
- anemia, in which oxygen-carrying capacity of the blood is reduced
- low blood glucose, due to improper endocrine management of BGL
Vertigo typically results from viral infection or trauma, or abnormal neural processing of vestibular information, such as with a brain tumor
Define stimulus, receptor potential, labelled line, and perception
Stimulus - change detectable by the body
Receptor potential - graded potential change in a receptor in response to a stimulus
Labelled line - committed, incoming neural pathway that carries information regarding a particular sensory modality detected by a specialized receptor type at a specific site in the periphery, and delivers it to a defined area in the somatosensory cortex
Perception - is the conscious interpretation of the external world as created by the brain from the sensory input it receives
Compare the receptive field size for a sensory neuron on your tongue and a sensory neuron on your back
Size for your tongue will be smaller than the back so your tongue has greater discriminatory ability than your back does
Compare and contrast the classes of pain receptors
A-delta fibers
- small (1-5 micrometers) and myelinated
- free nerve ending receptor
- travels at 6-30 m/sec (fast)
- fast pain, cold, warmth, mechanical
- described as sharp, stabbing, or acute
C fibers
- small (0..5-2 micrometers) and unmyelinated
- free nerve ending receptor
- travels at <0.5-2 m/sec (slow)
- slow pain, cold, heat, mechanical
- described as burning, aching, throbbing
Compare the type of pain signals transmitted via A-delta fibers and C fibers
A-delta fibers constitute fast pain pathway that carries signals arising from mechanical and thermal nociceptors
C fibers constitute a slow pain pathway that carries impulses from polymodal nociceptors
Describe the role of endogenous opiates in the body’s natural analgesic system
Bind with opiate receptors at they synaptic knob of afferent pain fibers where they inhibit release of the pain neurotransmitter, substance P, blocking further transmission of the pain signal
What is emmetropia? Where does the focal point land with and without accommodation and what does this allow you to see?
Normal eye
No accommodation - focal point for far source on the retina
Accommodation - focal point for near source on the retina
What is myopia? Where does the focal point land with and without accommodation and what does this allow you to see?
nearsightedness
No accommodation - focal point for far source in front of retina so its blurry
No accommodation - focal point for near source on retina
Accommodation - not helpful
What is hyperopia? Where does the focal point land with and without accommodation and what does this allow you to see?
Farsightedness
No accommodation - not helpful
Accommodation - focal point for far source on the retina
Accommodation - focal point for near source is behind the eye so its blurry
Explain how light absorption by a photopigment leads to a hyperpolarizing receptor potential
When a photopigment absorbs light, retinal changes to the all-trans form, activating the photopigment. The activated photopigment activates the G protein transducin, which then activates the intracellular enzyme phosphodiesterase.
In the dark, the second messenger cGMP had been keeping chemically gated sodium channels open, resulting in a passive, inward, depolarizing sodium leak (dark current)
Activated phosphodiesterase degrades cGMP, permitting these chemically gated sodium channels to close, thereby stopping teh depolarizing sodium leak and causing hyperpolarization of the photoreceptor
Compare rod and cone vision
Rods
- 100 million per retina
- vision in shades of grey
- high sensitivity
- low acuity
- night vision
- much convergence in retinal pathways
- more numerous in periphery
Cones
- 3 million per retina
- colour vision
- low sensitivity
- high acuity
- day vision
- little convergence in retinal pathways
- concentrated in fovea and macula lutea
Describe the function of the middle ear
Amplifies the tympanic membrane vibrations and converts them into a wavelike movement in the inner ear fluid at the same frequency as the original sound waves
Compare the mechanisms for discrimination of pitch, loudness, and timbre
Pitch discrimination - Depends on which region of basilar membrane vibrates maximally with given sound frequency
Loudness discrimination - depends on amplitude of vibrations
Timbre discriminate - depends on overtones of varying frequencies, which cause many points along the basilar membrane to vibrate simultaneously, but less intensely, than the fundamental tone
When the head turns to the right, what direction does the fluid (and subsequent cupula and hairs) move in the left and right horizontal semicircular canals?
(head turn would be clockwise when viewed from above)
They would both flow counter clockwise when viewed from above
List the 5 established primary tastes and the stimuli that evoke each of these taste sensations
sweet - glucose
sour - H+
salty - Na+
Bitter - alkaloids and poisonous substances
Umami - amino acids, especially glutamate
