Receptors and transduction in sensory processes

Question 1

what is sensory transduction?

Answer 1

process of converting a signal such as light, taste, touch, pressure or smell from the environment into an electrical signal in the sensory neuron.

Dependent on: what the stimulus is, which type of receptor is involved, how specific the receptor is and how large an area the receptor covers.

Collected by the peripheral nerves, integrated with our senses and processed by the CNS.

Question 2

What are the key properties of receptors ?

Answer 2

To be consciously aware of environmental stimuli, you have to detect them, convert them into signals your brain can read and send them to your brain for processing.
Receptors are specific for an individual stimulus.
Receptors change the stimulus energies into electrochemical energy in the form of action potentials generating temporal and spatial coding which is translated by the brain.
Many of sensory receptors can adapt to stimuli

Question 3

Which senses adapt?

Answer 3

all but pain, which amplifies

Question 4

What are the different receptors for different modalities?

Answer 4

Visual: light detected by photoreceptors called rods and cones
Auditory: sound detected by mechanoreceptors called hair cells (cochlea)
Vestibular: gravity detected by mechanoreceptors called hair cells (vestibular labyrinth)
Gustatory: chemicals detected by chemoreceptors in the taste buds
Olfactory: chemicals detected by chemoreceptors called olfactory sensory neurons

Somatosensory (dorsal root ganglion neurons)

touch: pressure detected by cutaneous mechanoreceptors
proprioception: displacement detected by mechanoreceptors in muscles and joints
temperature: thermal energy detected by cold and warm thermoreceptors
pain: chemical/ thermal/ mechanical energy detected by chemo/thermo/mechano receptors called polymodal/ thermal/ mechanical nociceptors

Question 5

What is Mueller’s law?

Answer 5

Mueller recognised that neurons that are specialised to valuate a particular type of stimulus energy will produce an appropriate sensation regardless of how they’re activated. Univariance.
Specificity for each modality is ensured by the structure and the position of the sensory receptor.

Question 6

How do mechanoreceptors transduce stimulus energy?

Answer 6

Mechanoreceptors are ionotropic
Mecahnsical pressure pushes down on receptor causes a distortion in the actin cytoskeleton and this causes the channel to open up and ions to flow
Typically non-specific cation channels
Channel that is stimulated is the same one that lets ions through so called an ionotropic channel

Question 7

How do chemoreceptors transduce energy?

Answer 7

chemoreceptors are metabotropic
g-protein couple receptors trigger a second messenger cascade resulting in the opening of an ion channel
chemical detected
G protein coupled second messenger signalling cascade
increased expression of the second messenger cyclic AMP
cAMP binds to that channel
channel then opens and allows cations through.

Question 8

How do photoreceptors transduce energy?

Answer 8

metabotropic
G protein coupled second messenger signalling cascade
in the dark the photoreceptor is held open by cGMP.
When the light comes in, the photoreceptor responds to this by activating a second messenger system which downgrades the amount of cGMP present so the channel closes.

Question 9

How do we feel touch?

Answer 9

Detection of pressure and vibrations
When we touch an object the skin copies its surface shape and texture.
This deformation pattern shifts according to the object’s movement

Different receptors at different skin positions detect and signal different sensations of discriminatory touch
unique shapes, location and structure
Varying activation levels at anyone time combine to give a specific perception of touch

Meisner's corpuscle
Pacinian corpuscle
Merkel's disks 
Ruffini's Endings
Free nerve endings

Question 10

What happens when we touch a lightly textured surface?

Answer 10

high frequency vibrations and pressure indentation in the receptor
Stimulates the pacinian corpuscle
transferred to the free nerve ending underneath, opening the mechanically channels receptors present.
Allows Na+ to move in along the non-specific cation channel
Increase in positive charge inside the nerve ending - receptor potential
If this surpasses the threshold of an action potential an impulse will travel without detriment up the Aß peripheral nerve fiber, into the spinal cord - ready to be signalled to the brain.

Question 11

how is the intensity of a stimulus encoded

Answer 11

in the frequency of action potentials, with greater spiking when the stimulus is at its highest level.

Question 12

What are the properties of the Pacinian Corpuscle?

Answer 12

rapidly adapting (phasic) to constant stimuli - due to encapsulated nerve endings (removing them eliminates phasic properties)
layers of connective tissue sensitise sensory nerve for sensing vibrations but make it unresponsive to steady pressure
reacts to new pressure (fires in response to applied pressure, then stops as this pressure is constant, lifting the pressure cause it to fire again)
the capsule layers of the pacinian corpuscle are slick with viscous fluid between them
If the stimulus pressure is maintained, the layer slip past one another and transfer the stimulus energy away so that the underlying axon terminal is no longer deformed and the receptor potential dissipates.
When the pressure is release the events reverse themselves and the terminal is depolarised.
means temporal coding for different stimuli are unique

Question 13

How is temporal coding for different stimuli unique?

Answer 13

Temporal coding for different stimuli are unique - only possible as a consequence of the rapid adaptability of the pacinian corpuscle
Steps stimulus: medium response at the start - then adaptation to continuous stimuli and spikes start to drop off
Vibration stimulus: periodic response showing spike increases only at the start of each stimulus.
Fast ramp: speed of ramp determines the spacing of action potentials. Large spike response only at start of stimulus due to the speed of the ramp spike adaptation to continued stimulus
Slow ramp: small spike response as stimulus slowly continues to change then

Question 14

How do we smell?

Answer 14

Odour is detected by sensory organs within nasal cavity
Odourants bind to particular receptors
Olfactory receptor cells are activated and send electrical signals up to the brain
The signals are relayed via converging axons
The signals are transmitted to the higher regions of the brain

Question 15

what are olfactory receptors?

Answer 15

Olfactory cilia at the top - large surface area covered in up to a thousand different chemoreceptors for the collection of the different odorants.
Weak shape Theory: an overall perception of an individual smell is built up from a large profile of receptors which are all activated at the same time.
Profile of receptors gives us the overall perception of a particular smell.

Question 16

What how do olfactory chemoreceptors work?

Answer 16

ligand/ odorant binds to g protein coupled receptors: G(olf)
G(olf) is activated and alpha subunit activates adenylate cyclase
Adenylate cyclase causes production of cAMP from ATP
cAMP gated cation channel is opened when cAMP binds - metabotropic receptors
Na+ and Ca2+ enter causing depolarisation of a receptor potential
Starts the cascade of events which result in action potentials being sent to the brain

Question 17

How do olfactory receptors adapt to smells?

Answer 17

After constant exposure to the same smell for a while you will no longer smell at the same level
If exposure stops and then starts again you will smell with the same intensity
Entry of Ca2+ through cAMP gated channel as part of odour detection
Ca2+ binds to calmodulin (CaM) to form a complex
Ca2+-CaM complex activates the enzyme phosphodiesterase (PDE) which converts cAMP to AMP
Some cAMP dependent channels close
Less depolarisation and receptor potential despite continued exposure to the odorant
Less action potential spikes sent to the brain
Less perception of an odour

Question 18

What is sound?

Answer 18

sound is a mechanical pressure wave that results from the back and forth vibration of particles of the medium e.g air/liquid through which the sound is moving
Areas of high pressure and low pressure throughout the travelling wave
Typically wide range in loudness of sound we can hear 10dM-130dB
Capacity to hear sounds from a range of pitches 20Htz-20kHtz (most sensitivity between 2000-5000 Htz) region of human speech

Question 19

What are the physiological mechanisms of hearing?

Answer 19

Sounds waves gathered by the pinna (outer ear)
Travel along the auditory canal towards the tympanic membrane/ eardrum which vibrates according to the frequency and loudness of the sound
Malleus, incus and the stapes (ossicles) are the circular bones which amplify and transmit the vibrations from the eardrum to the oval window - the entry point to the fluid filler cochlea

The cochlea is a set of three fluid filled tubes spiral 2.5x around the modulus
Vibration of fluids in cochlea bend the sensory hair cells which are found on the sensory organ, the organ of corti which runs along the entire length of the cochlea
Different types of hair cells: inner and outer
The travelling sound waves sets up a travelling wave within the fluid which goes along the entire length of the cochlea
Different frequencies of sound will generate different patterns in the wave

Question 20

What is the structure of the organ of corti?

Answer 20

Basal membrane on the bottom
Tectorial membrane on the top
There are different types of hair cells - inner and outer with different functions
Inner hair cells: responsible for the detection and signalling of sound to the brain for it to be perceived
Outer hair cells: are involved in controlling the sensitivity of the system to quiet sounds 3:1 ratio to inner cells. Embedded in the tectorial membrane
On Top of the hair cells are hair like processes called stereocilia - when there is a pressure wave (high pressure and areas of low pressure) set up a travelling wave of fluid which moves flexible membranes and stimulates hair cells

Question 21

How do hair cells work?

Answer 21

Individual hair cells have height graded protrusions of stereocilia that move as a bundle. Mechanically linked to adjacent partners by tip links which are physically linked to mechanically gated channels which will all open when the hair moves in one direction and close in the other.
Mechanodetectors that can detect both intensity and frequency
Bending of the hair cells one way opens a mechanically gated channel allowing depolarisation and neurotransmitter release onto the auditory nerve
bending of hair cells the other way closes the same channels anc causes a hyperpolarisation
Neurotransmitter release causes action potentials to be transmitted to the brain in the auditory nerve.
When the channels are open K+ enters causing depolarisation opening voltage gated Ca2+ stimulating the release of neurotransmitter (Glu) onto sensory nerve
Action potentials in the sensory nerves associated with the inner hair cells send information to the brain for the perception of sound

Question 22

how can we adapt to quiet sounds?

Answer 22

Outer hair cells adapt the auditory system to be more sensitive to quiet sounds - the cochlear amplifier. Provides mechanical amplification of the signal.
If damaged then deafness occurs.
Due to protein preston which is selectively expressed in the outer membrane of the outer hair cells.
It is considered a motor protein activated by depolarisation increases the power of the sound wave by increasing/decreasing length of outer hair cells.
Have a long and short form.
Prestin is the motor protein on the surface of the outer hair cell. This combines with chloride ions.
When the outer hair cell is hyperpolarised, chloride is bound to prestin and its elongated.
When depolarised, the positive charge attracts the chloride away from the prestin motors which are subsequently shortened so the hair cell is shorted.
The change in length increases the power of the high and low pressure elements of the sound wave.
The inner hair cell is stimulated to a greater level when the power is greater. So the outer hair cell amplifies the sound that is generated generated by the inner hair cell.

Question 23

how can we adapt to loud sounds

Answer 23

Circular chain - three small bones of the inner ear that help to transmit the soundwaves from the tympanic membrane into the oval window of the cochlea.
The muscles controlling these bones - the stapedius muscle and the tensor tympani muscles are part of an involuntary protective reflex loud sounds
The tensor tympani muscle is attached to the malleus and when this contracts it stiffens the ossicular chain and the stapedius muscle attached to the stapes, pulling it away from the oval window.
Less sound is presented to the tympanic membrane so less sound is transmitted to the cochlea via the oval window.
Reflex: delay of 50-100 ms so won’t protect against sudden sounds but will help with continuing loud noises.
Reflex activates when eating so that you don’t hear the noise of chewing

Question 24

What are the receptors involved in vision?

Answer 24

Photoreceptors
Rod and cone cells
Rod: contains disks with a very large number of photopigments called rhodopsin. Capable of detecting and responding to dead light.
Cones: much smaller number of a different set of photopigments called photopsins. Can detect and respond to bright light and wavelengths responsible for colour.
Rhodopsin consists of an opsin which is a G protein and a retinal which is an aldehyde of vitamin A
Light causes the photoisomerisation of the retinal from a cis form to a trans form which in turn activates the G protein.

Question 25

How do we see?

Answer 25

When rods are resting: membrane potential -35mV fairly depolarised compared to a neuron: due to a steady flow of Na+ and Ca2+ into the rod through cation channels by high constitutive expression of cGMP. Lots of neurotransmitter release.
When photoreceptors activated by light: sets in process a cascade which results in the destruction of cGMP and the closure of the cation channels.
Causes hyperpolarisation of photoreceptors resulting in a reduction of Glu release.
This reduction is graded depending on the amount of light that is absorbed.
Evolutionary advantage of being able to detect shadows or darkness in visual scene which could depict danger.

Question 26

what is the sensory transduction pathway in vision?

Answer 26

Light induces a conformational change in opsin
Activates transducin, a G-protein
Triggers the enzyme phosphodiesterase (PDE) to break down cGMP
Reduces the cGMP levels
cGMP-gated channels close leading to hyperpolarisation
When the membrane potential hyperpolarises from -40 to -65mV, it is no longer able to respond to light - bleached.
Less glutamate being released when light has been accepted by the rod photoreceptor

Question 27

what is the pupillary light reflex?

Answer 27

controls the diameter of the pupil in response to the intensity of light that falls on the retina of the pupil in the eye.
Greater intensity of light, causes the pupil to constrict allowing less light in.
Low light intensity causes the pupil to dilate, allowing more light in.
Regulates the intensity of light entering the eye.

Question 28

how does the use of rods and cones shift?

Answer 28

Scotopic vision: Rods alone are used in starlight/ full moon conditions
Photopic vision: Cones alone are used in sunlight
Mesopic vision: both rods and cones
Photopigments used by the rods and cones show differential sensitivity to light capture. Rods very sensitive to light and bleach very quickly. Cones relatively insensitive to light and only bleach in very bright light.

Question 29

what is dark adaptation?

Answer 29

Visual system switches to pure rod vision. It takes time for the bleach rhodopsin photopigment to be regenerated.
After 25 mins in the dark the rods are at their most sensitive to light
Coloured spots of light can be used to prove that cones are responsible for the early adaptive response.
Time taken to regenerate pigment is short for the cones and long for the rods.

Question 30

what happens when we go from dim light to bright light?

Answer 30

Temporarily blinded
Need to adapt to high ambient light levels
All cGMP channels are closed by light as happens when we can no longer see any contrast, there is a drop in intracellular Ca2+ which triggers changes that decrease sensitivity to absolute light level and allow us to see contrast and detail again.
Activate guanylate cyclase:
Enzyme responsible for producing cGMP.
Increases cGMP levels which reopens channels so they can respond to light again
Inhibits phosphodiesterase
The enzyme responsible for the breakdown of cGMP
Increasing cGMP levels
Reopens channels
Increases channel affinity for cGMP
More likely to open
Changes in the signal transduction pathway to decrease the sensitivity to the overall light level to allow us to see contrast and detail. The purpose of vision.
Enables us to navigate environment

Question 31

what do Meisner’s corpuscles do?

Answer 31

found in the epidermis of the skin
Detects dynamic light touch
Single nerve ending encapsulated with connecting tissue

Question 32

what do Ruffini’s corpuscles do?

Answer 32

Found along stretch lines in the skin

Large dendritic endings with elongated capsules

Question 33

what do Merkel’s disks do?

Answer 33

Detect static touch - pick up edges shapes and rough textures
Dendritic ends covered in Merkel cells

Question 34

what do free nerve endings do?

Answer 34

No encapsulation at the end

Pick up pain and temperature