Introduction to Sensory Receptors and Sensory Neurones Flashcards
what does the somatosensory system mediate the sensory modalities of?
fine discriminatory touch and stretch (mechanosensation)
joint and muscle position sense (proprioception)
temperature (thermosensation)
pain (nociception)
itch (pruriception)
what is the role of the exteroceptive division of the somatosensory system?
cutaneous senses - registers information from the surface of the body by numerous receptor types
what is the role of the proprioceptive division of the somatosensory system?
sensors in muscle, tendon and joints - monitors posture and movement
what is the role of the enteroceptive division of somatosensory system?
reports upon the internal state of the body and is closely related to autonomic function
which order neurone is located in PNS?
1st order neurone (primary sensory afferent)
where is cell body of 1st order neurone found?
dorsal root ganglia (innervation of limbs, trunk, posterior head)
or
cranial ganglia (innervation of anterior head)
which order neurones are located in CNS?
2nd order neurone (projection neurone)
3rd order neurone (projection neurone)
as well as the order neurones, what else is located in CNS?
somatosensory cortex
where is cell body of 2nd order neurone located?
dorsal horn of spinal cord
or
brainstem nuclei
where is cell body of 3rd order neurone located?
thalamic nuclei
where is cell body of somatosensory cortex located?
primary somatosensory cortex (S-I)
stimulis (mechanical, thermal or chemical) opens cation-selective ion channels in peripheral termination of primary sensory afferent - what type of response does this elicit?
depolarising receptor potential
the amplitude of receptor potential is graded and is proportional to?
stimulus intensity
what type of action potential does supra-threshold receptor potential trigger?
all or nothing
conducted by axon, at a frequency proportional to its amplitude
action potentials arriving at the central terminal cause what kind of release of neurotransmitter on to second order neurones?
graded
when is there a greatest sensitivity to change when plotted against amplitude of receptor change?
at low stimulus strength
what is the definition of modality?
what type of stimulus excites the sensory receptor
what is the definition of threshold?
what intensity of the stimulus is required for excitation of sensory receptor
what is the definition of adaption rate?
does the sensory unit discharge action potentials continuously during stimulus, or does it respond preferentially or to a changing stimulus
eg when going from brightly lit area to dim room, eyes take a while to adapt due to stimulus change
what is the definition of conduction velocity?
how rapid does the sensory unit conduct action potentials along its axon
for the modality of touch, pressure and vibration, what is the:
a) stimulus?
b) sensory unit?
a) mechanical forces acting on skin
b) skin mechanoreceptors
for the modality of proprioception, what is the
a) stimulus?
b) sensory unit?
a) mechanical forces acting on joints and muscles
b) joint and muscle mechanoreceptors
for the modality of temperature, what is the
a) stimulus?
b) sensory unit?
a) heat (thermal energy)
b) cold and warm thermoreceptors
for the modality of pain, what is the
a) stimulus?
b) sensory unit?
a) strong mechanical force on skin, viscera. Heat on skin, mucous membranes and viscera
b) mechanical, thermal and polymodal nociceptors
for the modality of itch, what is the
a) stimulus?
b) sensory unit?
a) irritant (eg chemical) on skin or mucous membranes
b) itch receptors
what do low threshold mechanoreceptors (LTMs) mediate?
fine discriminatory touch
what do low threshold thermoreceptors mediate?
cold through to hot
what do high threshold units (nociceptors) respond to?
high (noxious, potentially damaging) intensity stimuli
what do high threshold mechanoreceptors (mechanical nociceptors) respond to?
high intensity mechanical stimuli
what do thermal nociceptors respond to?
extreme degrees of heat (>45oc) or cold (<10-15oc)
what do chemical nociceptors respond to?
substances in tissue (eg inflammation - prostaglandins, bradykinin, serotonin, histamine, K+, H+ and ATP)
what do polymodal nociceptors respond to?
at least two of above
what is the definition of adaption?
a feature of sensory units that determines whether they change their firing rate only in response to a stimulus of changing intensity, or fire continuously throughout a constant stimulus
what is the role of slowly adapting (SA) or tonic / static response?
give example?
continuous info to CNS while terminal deformed
provides into about position, degree of stretch or force
eg stretch receptors
what is the role of fast adapting (FA) or phasic / dynamic response?
give example?
detects change in stimulus strength (eg rate of movement)
number of impulses proportional to rate of change of stimulus
eg some muscle spindle afferents, hair follicle afferents
what is the role of very fast adapting (very FA) or very phasic / dynamic response?
give example?
responds only to very fast movement such as rapid vibration
eg pacinian corpuscle
what is conduction velocity?
how rapidly does the sensory unit conduct action potentials along its axon
as the myelination gets thicker, does the velocity get higher or lower?
lower
what is meant by the site and extent of peripheral termination?
where is the peripheral terminal of sensory unit localised?
do its sensory receptors have small or large distribution?
the peripheral terminal of cutaneous afferent fibres branch into many fine processes. The tips of these processes can be which 2 things?
free nerve endings (partially naked)
associated with specialised structures
what is the definition of receptive field?
target territory from which sensory unit can be excited
does sensory acuity (fineness of discrimination) correlate directly or inversely with receptive field size?
inversely
high density of innervation = small RF = high acuity
what two things classify mechanoreceptors of skin?
rate of adaption (eg FA or SA)
size of receptive field (small = 1, wide = 2)
therefore, collectively they can be FA1, FA2, SA1, SA2
what do free nerve endings sense and what is their parent fibre type?
pain, heat, cold
Aδ or C
what do meissner’s corpuscles sense and what is their parent fibre type?
touch / vibration (where two-point discrimination highest at junction of dermis and epidermis but not hairy skin)
Aβ (subclass FA1)
what do merkel discs sense and what is their parent fibre type?
touch (same as meissners but also hairy skin)
Aβ (subclass SA1)
merkel discs are arranged in groups called what?
Iggo domes (sensory axon + merkel cell)
what do krause end bulbs sense?
touch in border or dry skin and mucous membranes
what do ruffini endings sense and what is their parent fibre type?
pressure within dermis and joint capsules
Aβ (subclass SA2)
what do pacinian corpuscles sense and what is their parent fibre type?
pressure within dermis and fascia
Aβ (subclass FA2)
what do follicular nerve endings sense and what is their parent fibre type?
touch
Aβ, or Aδ
what test is an important measure of somatosensory function?
two-point discrimination (spacial acuity)
how is the two point discrimination test carried out?
applying simultaneously two sharp point stimuli, separated by variable distance, at different sites on body surface
subject reports on whether one point, or two, are sensed and a threshold distance between two is established
most peripheral nerves communicate with CNS through what?
spinal cord
how many pairs of spinal nerves (arrangement of paired dorsal and ventral roots) are there?
31
what is the name given to area of skin innervated by the left and right dorsal roots of a single segment?
dermatome
which type of axon have the highest and lowest conduction velocities?
a-alpha = highest (80-120ms-1) due to high axon diameter and myelin sheath
c = lowest (0.5-2.0ms-1) due to no myelination and small axon diameter
describe the difference in receptive field for pacinian corpuscles and meissners corspuscles?
pacinian = large RF, there is area of max sensitivity but corpuscle responds when stimulus is strong enough anywhere within RF
meissners = small RF
what frequencies are pacinian and meissners corpuscles most likely to pick up?
pacinian = 200Hz meissners = 50Hz
meissners need vibration to be indented further into skin than pacinian corpuscles before becoming activated - true or false?
true
if a single dorsal root is cut, why does the corresponding dermatome not lose all sensation?
degree of overlap with the region of adjacent dorsal roots
therefore if T4 dorsal root was damaged, there would still be sensation in T4
to lose all sensation T4,5,6 would need to be damaged
what virus lies latent until reactivation in a dermatomal distribution?
shinges
lies in dorsal root ganglia until reactivated
