How do we hear, taste and smell? Flashcards
What is audition and what is it important for?
Hearing - the sense of detecting sounds
Important for signalling and communication
What is sound?
The movement or vibration of objects - movement alternately squeezes air molecules together and pulls them apart - this pressure wave spreads outward from the source of sound to the listener
What is frequency and pitch?
rate of vibration in cycles per second i.e. Hertz (Hz)
What is amplitude and loudness?
intensity of sound i.e. decibels (dB)
What does your ear have to do to hear sound?
- direct the sound waves into the hearing part of the ear
- Sense the fluctuations in air pressure
- Translate fluctuations into an electrical signal that brain can understand
What is complexity and timbre
Most tones have a mixture of frequencies
Complexity determines timbre (eg. difference between violin and flute)
What does the external ear do?
Captures, focuses and filters sound
What are the 2 parts of the external ear?
Pinnae and ear canal
What is the pinnae?
funnel-like structure that channels sound into the ear
What is the function of the ear canal?
Narrowing amplifies sound waves towards the eardrum (tympanic membrane)
What does the middle ear do?
Concentrates/amplifies sound energies
What are the ossicles?
bones of the middle ear
Malleus (hammer), Incus (anvil) and stapes (stirrup)
What does the inner ear do?
Converts vibrational energy into waves of fluid
Auditory part of the vestibulocochlear nerve
What is the cochlea in the inner ear?
The auditory part of the vestibular nerve
Coiled structure (bony exterior with hollow compartments filled with fluid)
What happens in the cochlea?
Vibrations from ossicles to oval window membrane sends waves through fluid filled cochlea
- Vibration transmitted to organ of corti via basilar membrane (structures in cochlea_
- movement of sensory hair cells transduce sound waves into neural activity which is sent along auditory nerve to brain
What does the tonotopic basilar membrane do?
Vibrates when stapes moves fluids of inner ear.
Different sound wave frequencies cause peak bending of membrane at different points along its path
What is the organ of corti and what does it do?
includes three rows of outer hair cells and one row of inner hair cells. Vibrations caused by sound waves bend the stereocilia on these hair cells via an electromechanical force.
What is the auditory pathway to the brain?
The auditory nerve transmits auditory information up a series of nuclei to the cortex where perception occurs. These nuclei include 1) cochlear nucleus, 2) superior olivary nuclei, 3) lateral lemniscus, 4) inferior colliculus, and 5) medial geniculate nuclei.
Where is the primary auditory cortex?
In temporal lobe within Heschl’s gyrus in the sylvian fissure
Tonotopic
Where is the secondary auditory cortex?
Surrounds primary auditory cortex
Planum temporale - behind Heschl’s gyrus and superior temporal gyrus
How do we hear pitch?
Enabled by tonotopic organisation
Each hair cell responds to a small range of frequencies based on location on basilar membrane.
Bipolar cell axons (form cochlear nerve) connect to only one hair cell each (order of these reproduces tonotopic representation)
- systematic representation maintained all the way to primary auditory cortex
(temporal coding - the rate of sound waves - also helps)
How do we hear loudness?
Larger pressure changes produce more intense basilar vibration –> greater shearing of hair cells –> cochlear bipolar neurons fire more frequently
From this, auditory system determines changes in loudness
How do we detect location?
Difference in timing at each ear and relative loudness are detected by nuclei in the brain stem that receive input from both left and right ventral cochlear nuclei.
Superior olive (medial) neurons respond to differences ARRIVAL TIME between each ear.
Superior olive (lateral) and trapezoid body neurons respond to differences in LOUDNESS
This info is conveyed to inferior colliculus in midbrain where an auditory spatial map aids localization of sound
What is hearing loss?
decreased sensitivity to sound
What is deafness?
loss of hearing - speech not perceivable with hearing aids
When may hearing loss/deafness arise?
early or later in life - developmental or acquired (eg. damage from loud sounds/infections/side effect of drugs)
How does conduction deafness occur?
Pathology of outer/middle ear: prevent vibrations reaching cochlea
How does sensorineural deafness/hearing loss occur?
Originates from cochlear or auditory nerve lesions
Cochlea losses most common
What is presbycusis and how many/who does it affect?
Age-related hearing loss
~70% of over 65s
What are some potential causes of presbycusis?
-Changes in inner ear - slow decay and/or damage to sensory hair cells
-Long-term noise exposure
-Effects of high blood pressure/diabetes
What happens when there is damage to left primary auditory cortex?
Deficit in differentiating speech sounds
What happens when there is damage to right primary auditory cortex?
Deficit in discriminating musical sounds
What happens when there is damage to bilateral auditory cortex?
Agnosia for sound - everything sounds the same/sound is confused
What is the oldest sensory system?
Chemoreception - 500million yrs old
What is chemoreception and what does it do?
Taste and olfaction work together to analyse stimuli - detects presence of specific chemicals - creates flavour (why we are less sensitive to flavour when we have a cold)
What is olfaction?
Smell
How many olfactory receptors are there?
Large, diverse set olfactory receptors coded in 350 genes, allowing differentiation of 10,000+ smells (Axel & Buck, 2000)
How often do olfactory receptors need to be replaced due to hard work?
every 60 days
What differs between substances in olfaction?
Sensitivity of olfactory receptors
What 2 segments are in the nasal cavity?
respiratory segment and olfactory segment
Where is the olfactory segment and what is it lined with?
Along the roof of the nasal cavity - lined with olfactory epithelium (olfactory mucus membrane) which contains receptors for smell
how big is the olfactory epithelium?
2-4cm2
What is the structure of olfactory receptor cells?
Short, thick dendrite
end in 10-20 cilia which project into a mucous layer (olfactory mucosa)
Bipolar
Acuteness in different species
How do chemicals in the air come to interact with cilia?
Dissolve in mucosa
What is the olfactory pathway to the brain from the olfactory receptors?
Receptors project to olfactory bulb
The olfactory nerves are non-myelinated and consist of bundles of slenderfibresheldtogether by thin strips of connectivetissue; form olfactory cranial nerve
Receptors form synapses with dendrites of mitral cells at glomeruli
- Mitral cells send axons from olfactory bulb to a broad range of forebrain areas- Amygdala & pyriform cortex have no connection through thalamus
- Orbitofrontal cortex (role in emo & social behaviours) has thalamic connection
What is anosmia?
Absence of sense of smell
Can be anosmic for certain odours
Temporary/permanent
What is hyposmia?
Diminished olfactory sensitivity
- may be an early sign of Parkinsons
What is dyosmia?
Distorted sense of smell
Misinterpreted smells/hallucination of smells
What percent of over 80s have impaired ability to identify smells?
75%
What is gustation?
Taste
How do we taste?
Stimuli interact with microvilli (receptor tips) - open ion channels lead to change in membrane potential
What are the 5 tastes?
Bitter
Salty
Sweet
Umami
Sour
What forms the gustatory nerve?
Nerve fibres travel from taste buds to the brain stem via cranial nerves
anterior two thirds of the tongue via the facial nerve (VII).
back of the tongue via glossopharyngeal nerve (IX).
areas other than the tongue (epiglottis, pharynx) via vagus nerve (X).
What 2 pathways does the gustatory nerve split into on entering the brain?
- Through posterior medulla of ventroposterior medial nucleus of thalamus
- Projects through pons to hypothalamus and amygdala
What are the 2 pathways from the posterior medulla to ventroposterior medial nucleus of the thalamus?
- To primary somatosensory cortex (for tactile info) + To gustatory cortex of insula (for flavour)
What is the pathway that projects through pons to hypothalamus and amgydala hypothesised to do in gustation?
Play a role in feeding behaviour
What is ageusia?
Absence of sense of taste
- may be caused due to cranial nerve damage or problems with endocrine system (eg. salivary glands)
What is hypogeusia?
Diminished taste sensitivity
- often due to medication
What is dysgeusia?
Disturbed sense of taste
- diagnosis usually complicated since sense of taste is tied together with other sensory systems - various potential causes and treatments
