Special senses

somatic pathway

general: touch, proprioception

special: vision, hearing, balance

visceral pathway

• general: nociception, physiological receptors

• special: olfaction, gustation

how do we sense things?

• sensory receptors are specialised peripheral endings of afferent neurons

• each type of receptor responds to a specific type of stimulus

• signal transduction

what can we sense and their receptors

• visual (sight): photoreceptors

• auditory (hearing): mechanoreceptors

• somatosensory (touch): mechanoreceptors

• gustatory (taste): chemoreceptors

• olfactory (smell): chemoreceptors

• pain/nociception (complex): mechanoreceptors, thermoreceptors, osmoreceptors, chemoreceptors

what are the factors which show how intensity of the sensation is graded

• input intensity

• input duration

• no. fields activated

adaptation

decreased signalling despite same level of stimulus

localisation/central processing: each sense has a cortex

• visual cortex

• olfactory cortex

• gustatory cortex

• vestibular cortex

• somatosensory cortex

• auditory cortex

hearing

• neural perception of sound energy

  • identification of the sounds (“what”)

  • localisation of the sounds (“where”)

frequency and amplitude effect on hearing

• frequency → pitch

• amplitude → volume

sound

travelling vibrations of air

• consist of alternate regions of compressions and rarefaction of air molecules

external ear

• pinna, external auditory meatus, tympanium

• amplifies sound energy - contributes to locating sound

• transmits airborne sound waves

middle ear (auditory ossicles)

• transmits airborne sound waves to fluid filled inner ear

• amplifies sound energy

• ossicles are the little bones inside

• the last ossicle sits on the oval window of the cochlea

inner ear

• cochlea: conversion of sound waves into nerve impulses which allow hearing

• vestibular apparatus (sense of equilibrium): linear acceleration, rotation, balance

hearing loss parameter

>75dB continuous exposure, >100 dB acute

sound wave transmission

• tympanic membrane vibrates when struck by sound waves

• sound is heard as there is a deformation of the basilar membrane

detecting different sound frequencies

• high frequencies: vibrate and make movement at the start

• low frequencies: vibrate and make movement further down the cochlear membrane

• the mechanical receptors open

• sound air energy converts into mechanical chemical action potentials

• comes into contact with tectorial membrane

outer hair cells vs inner hair cells

outer: fine tune and amplify

• move to amplify the wave in the basilar membrane via the protein prestine

inner hair cells: detect the sound

conductive hearing loss and potential causes

failure of sound wave conduction

• ruptured ear drum

• glue ear

• ear canal physical blockage

• infections

sensorineural hearing loss and potential causes

failure of neural processing

• neural presbycusis - age related hearing loss → hair cell death

• noise damage = permanent partial hearing loss

• iatrogenic - ototoxic drugs damaging hair cells

number of taste buds present in the oral cavity and throat

10 000

taste pore

contained in each taste bud where fluids in the mouth come into contact with taste receptors

types of taste receptor cells

• sugars (sweet)

• amino acids (umami)

• sodium chloride (salty)

• alkaloids (bitter)

• acids (sour)

how often do taste buds regenerate

every 10 days

mechanism of taste transduction

• while each taste receptor uses a different mechanism, all increase in the same intracellular messenger i.e. an increase in calcium to release the neurotransmitter

• release of neurotransmitters (ATP and serotonin) from taste cells stimulates primary afferent gustatory nerve fibres

the olfactory mucosa

a 3cm squared patch of specialised epithelium in the ceiling of the nasal cavity

function of the olfactory system

• humidifies air before going into the lungs

• any bacteria/viruses are filtered to become fluid or deposits which are ejected from the nose or swallowed and digested where it is killed

• direct nerve transmission to the brain

• can detect 400 000 different substances

how often do smell cells regenerate

every couple of months

mechanism of smell transduction

• for a substance to be smelled it must be volatile in air and water soluble - so that it can be dissolved in mucus coating the olfactory mucosa

• like taste receptions, odours must be dissolved to be detected