Special sense - ear Flashcards
Anatomy of the ear
3 parts
External ear: pinna, ear canal, tympanic membrane
Middle ear: 3 ossicles, oval and round window, eustachian tube
Internal ear: cochlear, semi-circular canals
Middle ear 3 ossicles
Maleus, Incus and Stapes
Hearing and balance
-The outer and middle ear are involved with hearing
-The inner ear functions in both hearing and equilibrium
-Receptors for hearing and balance
Respond to separate stimuli
Are activated independently
-Afferent fibres from:
Hearing receptors in cochlear vestibular division
Transmit information from the inner ear in from of action potentials via vestibulocochlear nerve to the brain
The inner ear
2 things
Bony Labryinth:
Tortuous channels worming their way through the temporal bone, filled with perilymph
Contains the vestibule, the cochlea and the semicircular canals
Membranous labyrinth:
Series of membranous sacs within the bony labyrinth, filed with potassium-rich fluid
How we perceive sound
-The outer ear and external auditory canal act passively to capture the acoustic energy
-Sound waves strike the tympanic membrane causing it to vibrate
-Formation of a perilymph (fluid) wave within the cochlea
-Wave travels the length of the cochlea = displacement of the basilar membrane
-Stereocilia are bent due to a shearing force causing change in resting membrane potential of hair cell
3 structures in the inner ear amplify the sounds
Sound transduction by hair cells
- Oscillations of cochlea membranes cause hair cell sterocilia to bend
- Sterocilia are different lengths
- Bend either towards or away from tallest stereocilium (transmit the sounds to the brain by creating the action potential)
- Signals from hair cells transmitted to brain via cochlear nerve
- In ends of stereocilia of hair cells are mechanically-gated K+ channels
- Bending of stereocilia towards tallest stereocilium (kinocilium) opnes mechanically0gates K+ channels in ahir cells
- Allows K+ to enter cell depolarisation Ca2+ enters cell release of neurotransmitter = action potential
Auditory pathway to the brain
- Vestibulocochlear nerve
- Spiral ganglion transmit to the brainstem via cranial nerve VIII
- Synapses on the 2 cochlear nuclei
- Main pathway involves axons from the ventral cochlear nucleus to the inferior colliculus
- Travels to and synapse at the medial geniculate body of the thalamus = MGN
- Finish at the auditory cortex
- Vestibular nerve (balance) joins the cochlear nerve (hearing) entering the internal acoustic meatus from this point on they are collectively called vestibulocochlear nerve
Deafness = hearing aids
3 basic parts
Microphone
Amplifier
Speaker
Vestibular apparatus –> balance
Comprised of
Comprides: 3 semicircular canals, 1 utricle, 1 saccule
- Semicircular canals contain endolymph
- At base of each semicircular canal is ampulla (jug)
- Within each ampulla is cupula (cap)
- Utricle and saccule = linear acceleration
- Cupula and ampulla = rotational acceleration
Structure of ampulla
- Ampulla sits as base of each semicircular canal
- Each ampulla has ridge (crista) that extends with cupula = cap into the lumen of fluid filled inside the ampulla
- Cupula bridges width of ampulla
- Forms mobile barrier through which endolymph (fluid) cannot circulate
- Mechanoreceptor hair cells extend out of crista into gelatinous cupula
- If head it tilted endolymph pushes against hair bundles in cupula
- Mechanoreceptor hair cells transmit information to vestibulocochlear nerve
Transduction of rotation
- The movement of stereocilia towards or away from Kino cilium causes K+ channels to open or close
- Causes depolarisation or hyperpolarisation of hair cells, increasing or decreasing Ca2+ concentration within cells
- End result is increase or decrease in number of action potentials
- Information is interpreted in the brain
Anatomy of utricle and saccule
- Utricle and saccule detect linear acceleration in same way that semicircular canals that detect rotational acceleration
- Receptor cells are hair cells with stereocilia that extend into gelatinous layer containing small calcium carbonate crystals
Chemical sense - smell and taste
Chemoreceptors
-Respond to chemicals in aqueous solution
-Membrane of sensory neurons becomes depolarised action potentials
-Respond to different classes of chemicals
-Complement each other
-Smell and taste are related
in severe rhinitis loss of specific taste of food
Smell
- Chemoreceptor animals have much better olfactory ability to humans
- This superior ability is reflected in:
Greater surface are of nasal cavity lining
The size of the olfactory region of the brain
Olfactory nerves
- Sensory nerves of smell
- Run from nasal mucosa to olfactory bulbs
- Pass though cribriform plate of ethmoid bone
- Fibres synapse in olfactory bulb
- Pathway terminates in primary olfactory cortex
- Purely sensory (olfactory) function
Olfactory anatomy
-Smell is difficult to research
-At least 1000 smell genes active in the nose
-Extremely sensitive
-Olfactory epithelium: spans 3cm2 = cilia located on dendrites
Ciliated pseudostratified epithelium
- Nasal cavity also contains pain and temperature receptors (respond to e.g. ammonia, chilli
- Chemical signal gets translated into electrical signal receptor stimulation –chemical changes (G protein cascade) opening/closing of membrane channels
Olfactory physiology
-Resting membrane potential of olfactory receptors is -55mV
-Continuous impulses arise from these receptors at a rate 1:3/sec
-Stimulation of receptors by odorant substance decreases potential from -55 to -30mV = depolarisation
-Detection of odours at 1 in 10 million molecules
Stronger the odour = increased odour molecules
-Odorant molecules must cross the mucosal layer
-Odorant binding proteins mostly wok via G coupled activation
Triggers depolarisation – leads to action potential
-Smell depends on the pattern of activation of olfactory receptor neurons
-Mitral cells from the olfactory tract, eventually terminate in the olfactory cortex and limbic system
Taste - gustation
- Within papillae are taste buds that contain specialised gustatory receptor cells
- Receptors selectively bind to various chemicals in food
- Binding of chemical to receptor causes opening/closing of ion channels in cell membrane
- Causes change in electrical potential of receptor cell membrane
- Increase in intracellular Ca2+ causes release of transmitter, which signals afferent nerve
- Most taste buds respond to 2-4 taste qualities
- Bitter buds are most sensitive (protective)
- Sugar and salt most pleasurable
- Taste receptors are fast adaptors
- Salty taste is simply the perception of sodium ions in the saliva
Neural code for tasting
In humans, localisation of receptors means that some regions are more sensitive to certain tastes
Several receptors communicate with single afferent system, making perception of test complex
Gustatory pathway
- Release of neurotransmitter - AP
- Facial nerve (anterior 2/3 of tongue) and glossopharyngeal nerve (posterior 1/3 of tongue)
- Solitary nucleus of medulla (initiate PsNS reflexes to trigger saliva and gastric secretion
- Thalamus to gustatory cortex of parietal lobes and limbic system
