BIOL 2052 - Sensory systems: vestibular, auditory and touch

sensory systems

• have one for each of the 5 senses

• can hep us to detect changes in internal/external environments which can help us to survive

• sensory organs send info to the CNS and the sensory info sent to the cortical structures via the subcortical strictures (thalamus)

transduction —> encoding —> processing

TRANSDUCTION

• sensory cues are transduced into electrical signals

• a sensory receptor is a protein in the csm

• activation of this protein leads to a current across the membrane

• current must surpass threshold —> allows us to filter out weak stimuli

• stimulus strength is encoded by

  • amplitude of generator potential

  • frequancy of action potential

• driven by mechanogated ion channels

• vibrational cues can lead to change in shape of the membrane in the flattened cells

ENCODING

• sensory neuron carries signal to the brain

PROCESSING

• signal sent to the cortical areas which will give us overall perception of sensory cue

• when we do this we have to gain info about:

  • modality

  • intensity

  • duration

  • location

mechanoreceptors

• 4 different types

• split into superficial and deep which further divides into rapidly adapting and slowly adapting

• also specialised exteroceptors (mechanoreceptors in the cochlear)

• mechanoreceptors that fall into the category of proprioceptors: in the golgi tendon organ, the otolith organs, hair cells and semi lunar canals

methods of signal transduction

SPECIALISED SENSORY NEURON

• receptor protein embedded in membrane and responds to stimuli

• E.g: pacinian cell

• 

SPECIALISED EPITHELIAL RECEPTOR

• has receptor protein that responds to the stimulus

• depolarisation, influx of calcium, vesicle release

• neurotransmitter release binds to receptor on sensory neuron membrane

• E.g: a hair cell

• 

tonic receptors vs phasic receptors (fast vs slow adapting)

TONIC RECEPTORS

• slow adapting

• stimulus is continuous during the duration of the stimulus

• very consistent frequency

• e.g: nociceptor

PHASIC RECEPTOR

• when stimulus comes on initial burst of activity even though stimulus still there

• when stimulus turns off theres another burst of activity

• good at signalling continuous stimuli

• E.g: some touch receptors

somatosensory system - fine touch

• somatosensory afferents convey fine touch into central circuits

• cell bodies in dorsal root ganglia

• mechanosensory afferents are pseudo unipolar neurons

• the cell bodies for the trigeminal neurons located in the trigeminal ganglia

• somatic neurons organised into segments according to the region of the periphery they innervate

• each segment recieves afferents from different regions called dermatomes (a region of skin innervated by the spinal nerve of a single dorsal root ganglion)

sensory afferents

PROPERTIES

• A beta fibres

• midrange sensory afferents

• myelinated

• involved in touch

• 4 types of receptor: merner, meissner, pacinian and ruffini cells

receptive field

• an area of skin over which stimulation will cause a change in the rate of action potentials

• size can vary depending on where they are in the body

• greater the density of receptors = small receptive field

2 point discrimination

• the abaility to distinguish between 2 points of touch

• if 2 points within the same receptove field then they are percieved as one

• when one point of touch is percieved then it is known as having a low spatial accuity

• will vary across the body

CORTICAL MAPS

• somatosensory cortex in post central gyrus

• certain regions of the body have mcuh larger region in the brain dedicated to recieving their signals

transmission of signals - touch

• transmission of signal for touch goes through dorsal column of medial leminiscal system

• separated into 1st, 2nd and 3rd order neurons

• gracile nucleus recieves input from the pathways from the lower body

• cuneate nucleus recieves input from pathways in the upper body

TRIGEMINOTHALAMIC SYSTEM

• signals from the face go through the trigeminothalamic pathway

• its different but does go through the thalamus

• synapse between 1 and 2 is the principal nucleus of the trigeminal complex

touch receptors

MERKEL DISKS

• tonic

• sensory function is shape and texture

• stimulated by edges and points and curves

• 

MEISSNERS CORPUCULE

• phasic

• motion detection, grip control

• stimulated by skin motion

• 

PACINIAN

• terminal end of the afferent encapsulated

• encapsulated alongside flattened cells

• terminal processes of the afferent wind around collagen molecules

• large receptive fields

• phasic

• 

RUFFINI

• large receptive field

• tonic

• good at responding to stretch as theres a collagen bundle that the receptor is stretched around

• sensory function: hand shape, finger position

• stimulated by skin stretch

• 

auditory system

• detection of movement of air molecules

• sound waves caused by repeating patterns of compressed air and less compressed air

human ear

INNER EAR

• houses hair cells that respond to mechanical cues

• contains the cochlear, made up of three structures

  • scala vestibuli

  • scala tympani

  • scala media

MIDDLE EAR

• mallius, incus, stapes

• transfers mechanical movement across space

• important fr amplifying signal

• goes from air environment to aqueous which has a high resistance to movement of molecules

• large amount of signal is lost as its transferred to the oval window of cochlear

• we can still generate movement in inner ear due to amplification

OUTER EAR

• made up of concha, pinna and meatus

• hervest sound waves and channel them to the eardrum (tympanic memb)

• eardrum sets out mechanical movement

• helps amplify 2-5kHz frequencies and localise the source of the soundwaves

how does the inner ear amplify the signal

• focuses force of large tympanic membrane down onto the much smaller oval window

• level action of the malleus incus

inner ear anatomy

• organ of corti within the cochlear

• made up of inner and outer hair cells

• also contains supporting cells and tectorial membrane which sits over the hair cells

• supporting cells and the hair cells sit on the basilar membrane

BASILAR MEMBRANE

• tapered structure - narrow at one end and gets progressively wider

• different regions of basilar membrane will respond to different frequencies

• waveforms will move along the basilar membrane until it causes undulation in the corresponding region

trandsuction of signal - auditory system

• two pivot points for tectorial and basilar membrane sets up shearing force on hair cells when membrane starts to move

• movement of basilar membrane converts to movement of stereocilia in 2 directions

• when stereocilia move shortest to longest theres an opening of mechanogated ion channels which causes depolarisation

hair cells - auditory system

• length of stereocilia increase in a stepwise fashion

• stereocilia contain actin filaments and so are rigid

• also anchored at pivot points

• kinocilium is the longest cilia

• have tight junctions which maintains the ionic gradient

• they have synaptic ribbons - cytoskeletal adaptations which prime the vesicles for fast release

• 

• is a specialised type of epithelial cell which speaks to afferent neurons that speak to the CNS

• the outer hair cells recieve signals from the CNS whilst the inner hair cells send the signals to the CNS

• hair cells are attached to bipolar neurons - one projection is sent to a hair cell whilst the other is sent to the CNS

• the cell bodies lie in the spiral ganglia

• 

tip links - auditory system

• link together tips of stereocilia

• links together mechanogated ion channels in the membrane of the stereocilia

• this means that when one stereocilia moves so do the others

the ionic gradient of the auditory system (endonuclear potential)

• depolarisation in the hair cell is caused by K+

• K+ gradient caused by high concenytration in the endolymph and a low concentration in the perilymph

• when the hair cell is at rest there’s a tonic release at basal level

• sound waves will cause the release of neurotransmitter

  • hair cells moving to the right causes the release of neurotransmitter whilst hair cells moving to the left will cause hyperpolarisation

labelled line coding - auditory system

• each auditory afferent is tuned to a specific frequency

• a single neuron will respond maximally to a very specific stimulus

outer hair cells of the auditory system

• primary role is to amplify cochlear signal

• can change shape in response to depolarisation/hyperpolarisation

  • depolarised = contraction

  • hyperpolarised = relaxed

• amplifies motion of bascillar membrane to amplify inner hair cells —> CNS

major pathways - auditory system

• bipolar nerve cells have cell bodies in spiral ganglion

• integration between two sides of the head occurs in the superior olive

• projections from superior olive —> interior colliculus —> thalamus —> auditory cortex where theres a map of where inputs go based off frequency

how is localisation encoded in the auditory system

• to get from one side of the hed to the other theres a time delay

• instead we have delay lines

• the signal must travel further to get to the coincidence detector so signals from left and right occurs at the same time

• if the sound is coming from in front of us then the coincidence detector that has the maximal response is in the middle

• each neuron has a specific unique location

• the action potentials will converge on an MSO that responds most strongly if their arrival is coincident

• intraural time difference is the difference in time between reaching left and right ear

  

vestibular system

• located in the ear - same region as the inner ear

• coordinated reflexive movements like balance and posture

• integrates info from the proprioceptive system

• controls head position, self motion and spatial orientation

structure and function of the vestibular system

• series of interconnected chambers

• semi-circular canals give you info about rotational movement

• vestibular part of cranial nerve 8 sends info back to the brain

• scarpas ganglion holds the cell bodies of the neurons that send signal back to the brain

• they are fluid filled chambers - the same fluids as in the auditory system

  • endolymph - inside the membranous labyrinth

  • perilymph - between the membranous labyrinth and the bony labyrinth

vestibular navigation

• there are 3 axes, 2 types of movement

  • translational movement —> up, down, left, right

  • rotational movement —> eg roly poly

• translational movement detected by otolith organs, rotational movements detected by semi-circular canals

vestibular hair cells

• hair cells in ottolith organs and semi-circular canals

• the movement of fluid in the semi circular canals results in teh movement of hair cells

• there are no inner and outer hair cells, there is only one type

• cytoskeletal ribbon synapses result in the fast release of glutamate at the basal membrane

• hair cells communicate with afferent neurons with no efferent inputs

• there are also tight junctions to maintain the ion gradients

• tonic release of neurotransmitter which results in basal level of activity

• tip links attached to mechano-gated ion channels —> activation causes influx of K+ which causes depolarisation

otolith organs

• the utricle and the saccule

• sensory epithelium called the maculla

• the otolith membrane on top of the stereocilia

• crystals of calcium carbonate sit in the gel otolith membrane and act to increase the mass so that it responds to gravitational force

• when walking the hairs tilt backwards as the gel on top of the hair cell has a large mass and moves slower than the fluid

vestibular system - striola

• is an axis of symmetry -- all the hair cells are orientated towards the striola

• stereocilia are mirror imaged around the striola which creates a differential of electrical activity either side

• when one side is depolarised the other is hyperpolarised

• the utricle detects motion in the horizontal plane whilst the saccule detects movements in the vertical plane

orientation of the saccular and utricular maccula

• saccular maccula orientated vertically and the utricular maccula orientated horizontally

• stereocilia must move away from the saccule for depolarisation and nust move towards the utricle for depolarisation

• the utricle responds to left, right sideways head tilts whilst the saccule responds to translational movements in the vertical plane

• both of them will detect front back translational movements and front back head tilts

semi-circular canals

• superior, posterior and horizontal

• stereocilia embedded in the cupula

• the cupula is a barrier that spans across the base of the semi circular canals at the ampluae

• provides information about acceleration and deceleration

• it is a gelatonous structure which will return back to the baseline position at rest

• this results in electrical activity which will plateau rather than in the otolith organs where the signal is constant

• when the endolymph moves it causes movement in the cupula

• all stereocilia are arranged in the same plane —> theres no axis of symmetry

• however there are pairs of stereocilia either side of the head

equilibrium pathways in vestibular system

• vestibular nuclei recieves input from the visual, cutaneous, vestibular, proprioceptive systems

• sends output to spinal

vestibulo occular reflex

• reflex that controls the movement of eye sockets when the head moves

• the reflexive eye movements move the eye in the opposite direction to the head

• this stabilises the gaze when the eye is moving

• is a reflex —> no conscious control

• the eye muscles one side of the eye contract whilst the eye muscles on the other side relax which is controlled by movement of fluid in the semi-circular canal

vestibular spinal reflex

• goes through lateral and medial pathway of vestibular system

• signal is generated by the ottolith organs

• few synapses

• for maintenance of posture

• ascending pathway projects ventral posterior nucleus of the thalamus

• can also receive input from the muscle and touch

• thalamic nuclei then projects to the vestibular cortex

• allows you to integrate stimuli to respond to inputs and change body orientation