Audition and chemical senses

Functions of hearing

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* Sounds carry important information about others and our surroundings 
* Through hearing, we can detect different sound attributes: complexity, intensity, frequency 

What is sound?

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* Sounds are produced by vibrating objects 
* Vibrations displace the surrounding medium (liquid or air) creating pressure changes
* Complex sounds are most common and pure tones (single frequency) are rare in the environment 

Frequency

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* different animals are sensitive to / can detect sounds within different frequency ranges (e.g.: humans have a range from 20Hz to 20000Hz)
* cycles per time unit
* perceived as pitch

Amplitude

changes in magnitude of sound, same frequency, measured in decibels

perceived as loudness

Complexity

frequency composition, varies from pure tones to mixtures of frequencies

perceived as sound quality

How do we perceive sound?

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* The auditory system can detect changes in air pressure across time in a frequency specific manner 
* Human ears can perceive each individual frequency + its amplitude variation
* The brain receives the information of sound detection and assigns meaning to it

**Outer ear: sound capture and amplification**

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* captures and amplifies sound waves 
* Tympanic membrane / eardrum : separates the outer ear from the inner ear, when sounds reach the membrane they cause it to vibrate and that vibration is sent to the bones in the middle ear

**Middle ear : sound transmission**

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*  amplifies and transmits vibrations 
* Air filled cavity occupied by ossicles : malleus, incus and stapes 
* The ossicles vibrate in response to tympanic vibration and they amplify + transmit sounds to the inner ear (oval window) 

**Inner ear: structure and function**

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*  translates vibrations into neural activity 
* composed by the oval window, cochlea, helicotrema, vestibular organs

Oval window function

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* leads to movement of fluid within the cochlea and activation of receptors for hearing 

Cochlea function

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* sound waves are transduced into electrical impulses that the brain can interpret as individual frequencies of sound 

Helicotrema function

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* connects the scala tympani and the scala vestibule which allows fluid to move  between them , slightly impedes the travel of sound + the hair cells in this area best detect low frequency sounds 

Vestibular organs functions

provides your brain with information about balance, motion, and the location of your head and body in relation to your surroundings.

**Organ of corti**

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* transduction of auditory signals , sound waves enter the ear via the auditory canal and cause vibrations of the tympanic membrane.
* It is also capable of modulating the auditory signal → outer hair cells can amplify the signal  

Basilar membrane

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* acts as a spectral analyzer that translates vibration frequencies within the cochlear fluid pressure waves into positions of maximal displacement along its length.

Basilar membrane tonotopy

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* Tonotopy = tones spatial arrangement 
* When very high-frequency sound waves reach the ear, only the region nearest the cochlear base vibrates.
*  As the frequency of the sound is lowered, the place of maximal amplitude of vibration shifts toward the cochlear apex. 
* Because of this resonance gradient, the basilar membrane is said to be “tonotopically” organized.

Pressure transmission along the canals

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* Vibrations of the stapes that push and pull the flexible oval window in and out of the vestibular canal at the base of the cochlea 
* Pressure waves deflect the basilar membrane in a frequency specific manner 
* All pressure ends up moving the round window dissipates 

Tectorial membrane

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* attached on one end, projects into the middle canal.
* Floats above inner hair cells and touching outer hair cells 
* Vibrations of the basilar and tecorial membrane, makes stereocilia bend 

Stereocilia

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* hair-like extensions on the tips of hair cells. Molecular filaments connect the tip of each cilia to neighbouring potassium channels 
* In resting state there is a basal K+ influx and neurotransmitter release 
* vibration induce bending of stereocilia which increase K+ influx, increasing neurotransmitter release at the cell base 

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Place code in the cochlea

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* frequency information is coded by the place along the cochlea with the greatest mechanical displacement 

Amplitude code in the cochlea

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* louder sounds produce larger vibrations of the basilar membrane, making the inner hair cells release more neurotransmitter 

Auditory pathways

* auditory nerve enters the medulla -- making synapsis in a tonotopic manner
* axons from the cochlear nuclei ascend to the superior olivary complex in the pons
* inputs from each ear are processes by both olivary nuclei
* tonotopic representation is preserved up to A1

Transitory hearing loss

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* obstruction of the ear canal,damage to the tympanic membrane , problems in the ossicles (conductive hearing loss)

Otosclerosis (permanent hearing loss)

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excessive growth of ossicles,requires surgery

Sensorial hearing loss (permanent)

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(most common defect): due to defects in cochlea or auditory nerve. Damage to hair cells caused by toxicity or excessive exposure to noise 

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Hearing aids – cochlea implants

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1. Miniature flexible electrode array surgically implantes in the cochlea through the oval window 
2. A receiver defects and process sound into radio signals, which are sent to the stimulator 
3. Miniature electrodes positioned in frequency specific regions of the cochlea emit electrical signals, activating neighbouring bipolar cells and the auditory nerve

Bheaviours supported by olfactory cues

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1. Food or mate seeking
2. Feeding
3. Co-specific identification 
4. Marking territories 
5. Reproduction 
6. Aggression 

Functions of the human nose

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* Primary : humidify and warm air going into the lungs 
* Secondary : olfaction 
* Odorants interact with the olfactory epithelium → mucus in the epithelium captures odorants 

Olfactory epithelium

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* Supporting cells: metabolic and physical support 
* Basal cells: olfactory cell progenitors 
* Olfactory sensory neurons: detect odors and produce mucus

Olfactory receptors

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* Specific receptors in the cilia of OSNs recognise odorants
* Olfactory receptors are G-coupled proteins whose activation opens Na+/Ca2+
* OSN is depolarized by Na+/Ca2+ influx, firing action potentials

Olfactory pathways

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* Axons from OSNs pass through the tiny holes in the cribriform plate to enter the brain
* Each type of OSN projects its axon to a single glomerulus within the olfactory bulb
* OSN axons make synapsis with mitral and tufted cells that project to the primary olfactory cortex and other brain regions 

Modulation of olfaction

\-- defection threshold can be affected by:

* gender-- women have lower thresholds than men , especially during ovulation
* training
* age : by 85, 50% of the population has effectively lost their sense of smell

Taste

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* Short-range information (inside the mouth)
* Taste recognition guide appetite and trigger physiological processes for absorbing nutrients and adjusting metabolism 
* Important for identifying nutrients and avoiding chemical threats 
* Taste is greatelly influenced by culture 
* Liking / disliking a certain flavour is already present in newborns 

Structure and function of taste sensors

taste receptors are arranged in taste buds

* taste buds are arranhed in three kind of papillae
* receptors for different taste group togther in the same bud
* receptor activation sends neural signal through taste nerves

Taste neural pathway

\-- three cranial nerves collect taste information

* chroda tympani
* glosso-pharyngeal
* vagus

\-- synapse at nucelus of the medulla -→ hypothalamus → insula or gustatory primary cortex → orbitofrontal cortex