Senses and perception 1 Flashcards
What is tactile input?
stimulation of skin receptors from touch, pressure, pain and temperature.
What features of the tactile input does the body process?
quality (type of touch), intensity/magnitude, duration, location
What structure converts a stimulus into an action potential (electrical signal)?
modality-specific sensory receptors
How is the information from the stimulus transmitted to the brain?
via modality-specific sensory pathways (differs for each quality e.g. pain, touch…)
What type of morphology does a sensory neuron have?
pseudounipolar morphology
Why is a sensory neuron described as pseudounipolar?
it has an axon that projects from the cell body for a short distance before splitting into a central and peripheral branch
Function of a sensory neuron (skin receptor)
relays information from the periphery to the CNS
What part of the sensory neuron is activated by the stimulus?
receptor ending
What determines the type of receptor ending of a neuron?
the modality of the sensory neuron
What type of receptor detects mechanical stimuli?
Mechanoreceptor
What type of receptor detects thermal stimuli?
thermoreceptor
What type of receptor detects noxious stimuli?
nociceptor
What type of sensation does a mechanical stimuli produce?
touch
What type of sensation does a thermal stimuli produce?
temperature
What type of sensation does a noxious stimuli produce?
pain
What are the special senses?
taste, smell, vision, pain
What type of stimuli is responsible for taste and smell?
chemical
What type of receptor detects chemical stimuli?
chemoreceptor
What type of stimulus is responsible for vision?
light
What type of receptor detects light stimuli?
photoreceptor
What modality of receptors are free nerve endings?
nociceptors (perceive pain)
Examples of different types of mechanoreceptors
Meissner’s corpuscle, Merkel disk, hair follicle receptor, Pacinian corpuscle, Ruffini ending
Why are there multiple, histologically distinct mechanoreceptors?
each mechanoreceptor is associated with a particular quality of mechanical tactile stimulation e.g. hair deflection (hair follicle receptor), vibration, slippage
What term defines the distribution of a sensory neuron’s receptor endings?
receptive field
Where are receptor endings (aka sensory receptors) located?
at the end of terminal/peripheral branches of sensory neurones
Why do receptive fields of different sensory neurons overlap?
to ensure complete coverage and to allow responses to a range of stimuli
What is the general trend in size variation of receptive fields in different body regions?
receptive fields are small in the periphery and increase in size towards the trunk (proximally)
What technique is used to estimate receptive field size (spatial resolution)?
two point discrimination
What does two point discrimination test?
ability to discern two separate mechanical stimuli
When will 2 distinct stimuli be perceived as one point?
if the stimuli are applied close enough together to be within the same receptive field
Why can 2 stimuli be felt as one point if they are applied closely together?
the 2 stimuli are applied within a single receptive field so only one sensory neurone is activated
How are receptive field sizes measured?
the maximum distance between two points that are perceived as one stimulus
What is meant by the transduction of a stimulus?
the conversion of a stimulus into an electrical signal (AP)
What is the outline of the sequence of stimulus transduction?
stimulus causes a change in receptor membrane permeability allowing an influx of cations. This leads to depolarisation/receptor potential which results in an action potential if the threshold is reached.
What is depolarisation also known as?
receptor potential
What is the resting membrane potential is mV approximately?
-70mV
What is the membrane potential in mV approximately at the peak of the action potential?
+35mV
What are the 2 subtypes of receptors (/receptor endings)?
ionotropic and G-protein coupled receptors
Describe the kinetics of ionotropic receptors
fast kinetics
Describe the kinetics of G-protein coupled receptors?
slow kinetics
What receptor modalities are ionotropic receptors?
mechanoreceptors and chemoreceptors
What receptor modalities are G-protein coupled receptors?
chemoreceptors
Describe an ionotropic mechanoreceptor in its inactive state
the ion channel is closed and tethered to the cell membrane by fibrils on the intracellular/cytoplasmic side of the phospholipid bilayer. Ions cannot enter the receptor ending
How does stimulus transduction via an ionotropic mechanoreceptor occur?
a force deforms the membrane which pulls on the underlying fibrils causing the ion channel to open. Na+ can enter causing depolarisation
Describe ionotropic chemoreceptors in their inactive state
the ligand binding sites on the extracellular surface of the ion channel are empty so the ion channel is closed. Ion passage cannot occur
How does stimulus transduction via an ionotropic chemoreceptor take place?
A specific ligand binds to the binding site causing a conformation change in the ion channel structure. The ion channel opens which allows Na+ influx and depolarisation.
Describe the structure of a G-protein coupled receptor
An extracellular ligand binding site is linked to an ion channel via a series of proteins
Which type of receptors have a slower rate of action?
G-protein coupled receptors are slower (chain of events required for activation) than ionotropic receptors
How are chemoreceptors activated?
by a ligand
Describe the stimulus transduction via G-protein coupled chemoreceptors
The ligand binding site becomes occupied when a specific ligand binds. This triggers a chain of events relayed along a series of proteins which results in the opening of the ion channel.
How is the quality (e.g. pain, temperature, touch) of the stimulus coded for?
via the type of receptor
What are the 2 ways the intensity of a stimulus can be coded for?
via the AP frequency or the number of neurones activated
Why does the number of activated neurones correspond to stimulus intensity?
different receptors have different thresholds therefore the greater the intensity, the more thresholds will be met and the more neurones depolarised
What happens to the frequency of AP discharge as the stimulus intensity increases?
AP frequency increases
Why is there a limit to the maximum AP frequency?
due to the refractory period (time taken for Na+ to become responsive and the resting membrane potential to be re-established)
How is stimulus duration coded for?
by the duration of AP firing
What is meant by adaptation of sensory receptors?
the decline in receptor potentials when sensory receptors are constantly exposed to a stimulus
What is the pattern of AP firing in slowly adapting receptors?
there is a high frequency of AP firing at the onset which decreases towards the end of the stimulus. But a level of discharge is maintained as long as the stimulus persists.
What is the pattern of AP firing in rapidly adapting neurones?
very high AP frequency at onset and then quickly adapts to become silent for remainder of stimulus
How is the location of the stimulus coded for?
by the receptive field which is activated
What is the name of the map depicting the cortical representation of sensory inputs from different body parts?
sensory homunculus
What is the name of a fold of the cerebral cortex?
gyrus
Where does the sensory homunculus run along?
the length of the gyrus of the primary sensory cortex
Why is the body map distorted in the sensory homunculus?
Receptive field size and density varies. Regions with smaller receptive fields and so higher RF density have greater cortical representation, whereas areas with larger receptive fields have a lower RF density resulting in limited cortical representation.
Example of an area of the body with small receptive fields and therefore a high density?
face, fingers
Example of area of the body with large receptive fields and therefore a low density?
trunk, proximal areas of limbs
