Sensory Flashcards
Is there a receptor for each type of stimuli
There is not a receptor for every specific stimulus meaning that some stimuli require multiple receptors. An example of this is the sensation of feeling wet using thermo, touch and pressure.
What is a sensory receptor
Specialised peripheral ending of afferent neurons
Each type responds to a specific stimulus
Receptor translate the energy from the stimulus into electrical signals through a process called signal transduction
What are the two receptor types
Specialised afferent ending - single cell receptor (free nerve endings)
Example is pain receptor
- Stimulus opens stimulus sensitive channels permitting Na entry causing local potential
- Local current flow between depolarised receptor and adjacent open voltage Na
Separate receptor cell (more common)
- Sensory receptors in sperate cell that once depolarised will open voltage gate Ca channel
- Ca will cause neurotransmitter release which will bind to chemically gated receptor channels allow Na entry
What are the four modes of information provided by sensory systems
Modality, location, intensity and timing (when and how often)
How does the brain recognise a specific modality if all receptors just produce AP’s
Modality is nerve specific not receptor specific meaning the brain knows what the stimulus is based on the afferent nerve.
What is a receptor field
Receptive field is the area of skin monitored by one sensory neuron.
Some receptors will share nerve fibers forming a receptive field between the two fibers.
Highly sensitive areas have very small receptive fields.
What is lateral inhibition
Lateral inhibition (local inhibition) increases accuracy
Receptor field direction under stimulus is stimulated more while the others are inhibited by interneurons.
Ability to localise touch is directly related to the density of tactile receptors _> higher density means higher ability to localise touch.
How can intensity of sensory system be interpreted if all nerves just produce AP’s of same amplitude
- More APs
- High frequency Aps
- Larger stimulus also leads to activity of more receptors
Tonic vs phasic receptors
Tonic receptors are very slow adapting and produce Aps as long as the stimulus is present (pain)
Phasic receptors are fast to adapt and stop firing AP just after stimulus is detected and dire again when it stops. (touch)
Nerve distribution in the homunculus
Highly sensitive areas have more nerves meaning they need more area
Types of cutaneous receptors (encapsulated and non encapsulated)
Encapsulated (cutaneous tissue and nerve ending encapsulated in thin sheath)
- Meissner corpuscles
- Pacinian corpuscles
- Ruffini corpuscles
Non encapsulated
- Hair follicle receptor
- Merkel’s disk
- Free nerve ending
Phasic and tonic with respect to light and deep touch
The light touch receptors are phasic meaning they will adapt over time but the deeper touch receptors are slow adapting.
Number of hot and cold receptors
There are separate receptors for hot and cold sensations and the receptors for cold are more abundant.
Thermal receptor adaptation
Note that thermoreceptors adapt quickly so hand in cold bath will adapt and not feel cold after some time.
Thermoreceptors adapt meaning that changes in temperature are sensed rather than the actual temperature itself. If you take the hand out of the cold bath and put it into room temp water it will feel hot.
Normally thermal stimulation is non noxious but extreme temperature can also trigger pain responses.
Three types of nociceptors
- Mechanical nociceptors
- Mechanical damage such as cutting or crushing
- Thermal nociceptors
- Temperature EXTREMES
- Polymodal nociceptors
- Respond equally to all kinds of damaging stimuli
- +/- Silent (sleep) nociceptors
- Mechanical stimulation during inflammation ONLY
Characteristics of fast pain
Fast pain (more tolerable):
- Occurs on stimulation of mechanical and thermal nociceptors
- Carried by small myelination A delta fibers
- Produces sharp, prickling sensation
- Easily localise
- Occurs first
Characteristics of slow pain
Slow pain:
- Occur on stimulation of polymodal nociceptors
- Carried by small myelination C fibers (slower conduction velocity)
- Produces dull, aching, burning sensation
- Poorly localised
- Occurs seconds and persists for longer time making it more unpleasant
How is the change from fast to slow pain mediated
The change between these two type of pain is mediated centrally. Nociceptors do not adapt meaning that the transition from fast to slow pain comes centrally.
How do local anaesthetics stop pain
Block Na channels on nociceptors
Ice will decrease speed of chemical reactions and therefore reduce pain
Pain neurotransmitters
- Substance P - Activates ascending pathways (sensory nerve fiber to synapse onto ascending) that transmit nociceptive signals to higher levels for further processing
- Glutamate - major excitatory neurotransmitter
In a state of hyper sensitivity glutamate will increase the excitability of dorsal horn neuron meaning even a little substance P will take it to threshold (faster)
What is the CNS analgesic system and how can it effect pain
- Suppresses transmission in pain pathways as they enter the spinal cored via opioid receptors
- Depends on presence of opiate receptors
- Endogenous opiates - endorphins, enkephalins, dynorphin
If pain persists for long time and individual can no longer avoid the pain (fight/flight) body will release endorphins that bind to opioid receptors that are inhibitory. May also bind to brain to help tolerate pain.
different section of ear anatomy
- External ear
- Pinna (funnels sound waves), external auditory meatus and tympanum
- Transmits airborne sound waves to fluid filled inner ear
- Amplifies sound energy
- Middle ear
- Transmits airborne sound waves to fluid filled inner ear
- Amplifies sound energy
- Inner ear
- Houses the cochlea and vestibular apparatus
Sound transmission from tympanic membrane to mechanical deformation of hair cells
- Tympanic membrane vibrates when struck by sound waves
- Middle ear transfers vibrations through ossicles ( malleus, incus and stapes) to oval window which is the entrance to the fluid filled cochlea
- Waves in cochlea fluid set basilar membrane in motion
- Receptive hair cells are bent as basilar membrane is moved up and done from waves
- Mechanical deformation of specific hair cells is transduced into neural signals that are transmitted to auditory cortex in temporal lobe of brain for sound perception
How is frequency interpreted
Frequency we interpret based on which hair cells are stimulated not anything to do with the specifics of the sound waves.
Perilymph is displaced which will activate hair cells
What are stereocillia
The stereocilia from the hair cells of the basilar membrane contract the above tectorial membrane. These hairs are bent when the basilar membrane s deflected as the tectorial membrane is stationary.
Inner hair cells
The inner hair cells are stuck in the tectorial membrane - organ moved in relation to membrane
- Main sensory cell
- Single line
- Receptor potential following bend of stereocilia
Outer hair cells
The outer hair cells amplify sound we want to hear - APs from brain to contractile component of the hair cell at same frequency of the inner hair cell.
- Receive efferent input
- Move to amplify the wave in basilar membrane - prestin is a transmembrane protein that mechanically contracts and elongates
- Tunes and enhances response by inner hair cells
What are tip links
Tip links between hair cells that are connected to mechanically gated ion channels
When tension these will open the channels
Conductive vs sensorineural hearing loss
Conductive - transmission of sound waves
- Glue ear, foreign material ect
Sensorineural - Within the cochlear
- Presbycusis - age related loss
- Noise damage
- Iatrogenic - ototoxic drugs
Rinne test - mastoid bone
Rinne test - mastoid bone
Normal if air conduction is greater than bone conduction
Conductive hearing loss = BC>AC
Sensorineural hearing loss = AC>BC but both are equally depressed (cochlear is affected or auditory nerve)Rinne test - mastoid bone
Normal if air conduction is greater than bone conduction
Conductive hearing loss = BC>AC
Sensorineural hearing loss = AC>BC but both are equally depressed (cochlear is affected or auditory nerve)
Weber test
Someone with unilateral conductive hearing loss would hear the tunning fork loudest in affected ear.
This is because the conduction problem masks the ambient noise of the room, whilst the well-functioning inner ear picks the sound up via the bones of the skull causing it to be perceived as a quieter sound in the unaffected ear.
Tuning fork is position on top of head
How can hearing aids be problematic
Amplify every sounds, cant direct hearing
What is the blind spot and why is it an oval
No light cells present where optic nerve leaves retina, this is known as the blind spot and is oval in shape due to exciting blood vessels at the poles.
Para and symp stimulation in reference to pupillary contraction
In bright light OR/OR BOTH parasympathetic stimulation the iris will contract therefore decreasing size of pupil. - Circular constrictor muscle
In dim light OR/OR BOTH sympathetic stimulation the iris will relax therefore increasing size of pupil. (radial dilator muscle)
What is the fovea
- Pinhead sized depresion in excat center of retin
- Point of most distinct vision and point of fusion of vision
- Has only cones
- Bipolar and ganglion cells are pulled aside so light can strike cones direction (doesn’t have to go through network of nerve fibers)
Only images that hit this are fused
What is accomodation
The change in strength and size of the lens is known as accommodation
Age related reduction in accommodation is called presbyopia and is due to the cell death from longer diffusion distances.
Para and symp innervation to keep image on retina
Sympathetic relaxes muscle which tightens ligament making lens weaker (long distance)
For long distance the focal point is much closer as the light rays have already become parallel.
At shorter distance the focal point would be behind the retina therefore needed parasymp innervation to strengthen the lens and move the focal point forward onto the retina
What is a near sighted eye (longer eye)
The near source is already focused on the retina without any accommodation, a far source is focused in front of the retina.
Corrected with concave lens meaning far source focused without accommodation and near source needs accommodation.
What is a far sighted eye (shorter eye)
The far source is already focused on the retina without any accommodation, a near source is focused behind the retina.
Corrected with convex lens meaning far source focused without accommodation and near source needs accommodation.
Main difference between rods and cones
Cones are sensitive to colour and are mainly located on the fovea.
Main difference is that robs have much more light sensitive discs
How do the receptive fields differ within the eye
At the centres of the fovea the receptive fields are quite small but in periphery they’re large
Hair receptor
- Afferent neuron ending spiral around base of hair follicle
- Stimuli = mechanical displacement of hair
- Fast adapting (phasic receptor)
- Discriminative touch - response best to moving objects and single the direction and speed of the moving object
Increases touch sensitivity as it allows sensation of light touch over a greater areas as the hair follicle has a large number of sensory nerve endings
Merkle cell
Comprised of Merkel cell and Merkel disc
- Enlarged afferent neuron ending form Merkel Disc
- Respond to fine tactile stimuli (pressure) and slow vibration
- Force applied leads to distortion of cell receptor potential causing neurotransmitter release in cell which leads to AP in disk.
- Slow adapting
Meissner corpuscle
- Most sensitive to flutter (low frequency vibration) and movement
- Rapidly adapting response
- Small receptive fields
- Respond best to rubbing against skin and movement across surface
Ruffini corpuscle
- Sensitive to skin stretch and sustained pressure
- Deep in skin
- Afferent fiber branches within capsule endings are intertwined with collagenous fibres
- Slowly adapting receptor
- Monitor slippage of object (modulation of grip)
Pacinian corpuscle
- Onion shaped with layers of epithelial cells
- Sensitive to vibratory pressure and deep touch
- Epithelial cells contain fluid that is displaced when force is applied, movement of fluid dissipates force
- Mechanical deformation = opening of pressure
- Sensitive channel = receptor potential
- Fast adapting
- Function - grasping releasing objects, surface texture discrimination
What is presbyopia
Age related vision loss
Loss of accommodation ability
Due to cell death from longer diffusion distances
How does endolymph movement activate hair cells in vestibular system
Hair cell consists of 1 kinocilium and 20-50 stereocilia which are connected by tip links (mechanically gated ion channels)
Note that the activation of hair cells comes from the movement of the endolymph. This movement is opposite to the direction of the head movement due to inertia.
The endolymph in same plane as movement pushes on cupula and bends hair cells int he opposite direction. If the movement continues then the endolymph moves at the same speed as the movement meaning no force on cupula. When the movement stops, the opposite occurs.
Semicircular canals work in pairs, depolarisation on one side means hyperpolarisation on the other side
Depolarisation in the hair cells causes increased neurotransmitter release and therefore more APs.
What is vestib occular reflex
- Reflex eye movement
- When vestibular system detects head rotation the eyes move in the opposite direction to maintain the image on fovea.
- This is a good test of function for vestibular system
- Inhibits extraocular muscles on one side while exciting the other
Signal transduction at vestib and ow dose this differ from cochlea
- Mechanical deformation of hair cells occurs with movement. The bending causes depolarising or hyperpolarising potentials depending on movement direction.
- Differs from cochlear as most of the info form vestib wont reach the conscious level but sound will.
3 causes of sensorineural hearing loss
- Presbycusis - age related loss
- Noise damage
- Iatrogenic - ototoxic drugs
Shortcut for remembering sensory receptors modalities
M’s = light touch
Merkel and Meissner
Ini’s = deep pressure
Pacinian and Ruffini
Shortcut to remember sensory receptor reaction
Merkle = T
Meissner = P
Pacinian = P
Ruffini = T
Signal transduction at hair cells in cochlea
Follow the leader
Stereocilia bend towards tallest member K enters causing depolarisation. This open voltage gated Ca channels which allows it to enter causing the release of neurotransmitter and therefore AP.
If stereocilia bend away from kinocilia the tip links slacken and close channels when stereocilia bend away from tallest member meaning no K enters and the hair cells hyperpolarises. Ca channels remain closed.
Vision transduction in the dark
Increased cyclic GMP will open voltage gated Na channels
This depolarises the membrane opening Ca channels
Increased release of inhibitory neurotransmitter -> inhibiting bipolar cells and no AP
Vision transduction in the light
Photopigment is activated which activates transducin
This will cause s cascade which will decrease cyclic GMP
Therefore Na channels -> membrane hyperpolarisation -> close Ca channels leading to decreased neurotransmitter release
Leads to bipolar disinhibition leading to AP generation
Test for hearing loss
Rinne test - mastoid bone
Normal if air conduction is greater than bone conduction (mastoid bone)
Conductive hearing loss = BC>AC
Sensorineural hearing loss = AC>BC but both are equally depressed (cochlear is affected or auditory nerve)
Weber test
Someone with unilateral conductive hearing loss would hear the tunning fork loudest in affected ear.
This is because the conduction problem masks the ambient noise of the room, whilst the well-functioning inner ear picks the sound up via the bones of the skull causing it to be perceived as a quieter sound in the unaffected ear.
Tuning fork is position on top of head
