Somatosensation and pain L1 Flashcards
what are the 4 principle mechanoreceptive systems innervating the glabrous (hairless) skin of the hands
Slowly adapting type 1 (SAI) afferents that end in Merkel cells;
rapidly adapting (RAI) afferents that end in Meissner’s corpuscles;
rapidly adapting Pacinian corpuscles (PC)
and slowly adapting type 2 (SAII) afferents that terminate in Ruffini endings
describe the pacinian corpuscle
concentric layers of cellular membranes alternating with fluid-filled spaces.
dsitribution of pacinian corpuscle?
It is distributed widely, including connective tissue in muscles, periosteum of bones and mesentery of the abdomen.
sensitivity of pacinian corpuscle?
They are extremely sensitive, responding to 10 nm of skin motion at 200 Hz, and are thought to play a role in the perception of events through an object held in the hand.
what ends in meissners corpuscle?
Meissner’s corpuscles are attached to RA I afferents.
Meissner’s corpuscles are attached to RA I afferents. They have receptive fields averaging ___ mm in diameter and they respond best to ….
Meissner’s corpuscles are attached to RA I afferents. They have receptive fields averaging 3-5 mm in diameter and they respond best to low frequency vibration.
Meissners corpuscle cells are analogous to what in the visual system?
hese endings are analogous to the receptors used in scotopic vision—they show enhanced sensitivity and poorer spatial resolution
merkel cells vs meissners corpuscles?
Meissners: high sensitivity - poor spacial resolution
merkel cells: higher spacial resolution, decreased sensitivity
what do rapidly adapting receptors respond to?
only to the onset of a stimulus
what do slowly adapting receptors respond to?
tonic response to a steady stimulus
describe phase locking in rapidly adapting stimuli?
A rapidly adapting receptor responds to low frequency sinusoidal mechanical stimuli with a single action potential for each phase of the stimulus—it effectively treats each period of the waveform as a new stimulus. This is known as phase-locking.
how is the intensity of a sinusoidal stimulus for rapidly adapting receptors conveyed?
Note that the intensity of the sinusoidal stimulus must be encoded by the number of sensory fibres that are active rather than the frequency of firing. The number of active fibres is linearly related to the amplitude of vibration.
which is slowly adapting
which is radpily adaptinh
The Pacinian corpuscle is _______ adapting (_A II).
The Pacinian corpuscle is rapidly adapting (RA II).
response of a pacinian corpuscle if we peel the lamina off?
de-sheathed = slowly adaptingh
what are accessor structures
Accessory structures:
Structural components of sense organs which may play an important role in protection, conduction, concentration, analysis, sensitization or inhibition; but they are not directly involved in the transduction process e.g. lamellae of the Pacinian corpuscle, the intrafusal fibres of the muscle spindle, eye structures, basilar membrane.
describe
Human vibration sensitivity can be altered differentially by changing the responsiveness of two different receptors— RA I and RA II.
The normal situation is shown in (A). The solid black line indicates the threshold of human vibration sensitivity. The dashed lines correspond to the sensitivity functions for the two receptors.
In (B) local anaesthetic has been applied to the superficial layers of the skin. This reduces the effectiveness of the Meissner’s corpuscles as they lie close to the surface.
In C) the effectiveness of the Pacinian corpuscle has been reduced by pre-adapting the receptor to stimulation around 250 Hz.
In contrast, pre- adapting the skin with a low frequency stimulus reduces the effectiveness of the Meissner’s corpuscles (D).
Merkel cells are attached to ___I afferents
Merkel cells are attached to SAI afferents
merkel cell receptive fields?
They have small, highly localised receptive fields.
do merkel cells innervate the skin densely?
yes
Merkel cells are attached to SAI afferents. They have small, highly localised receptive fields.
They innervate the skin densely and respond to indentation with a _____ response to _____.
Merkel cells are attached to SAI afferents. They have small, highly localised receptive fields. They innervate the skin densely and respond to indentation with a linear response to 1500μm.
Merkel cells are ten times _____ sensitive to dynamic stimuli and their spike discharge is largely invariant (i.e. very good at discrimination).
are ten times more sensitive to dynamic stimuli and their spike discharge is largely invariant (i.e. very good at discrimination).
how do merkel cells (SA1) encode the magnitude of stimuli?
The firing rate (A) of a SAI afferent can encode perceived stimulus magnitude (B)
describe the threshold for indentation of SA1 afferents
Note that the threshold indentation for this afferent was around 200 μm and so the dynamic range was less than an order of magnitude. Compare this with the large dynamic range for stimulus intensity in the visual and auditory systems.
do merkel receptors lie below the crest of the finger print ridge?
yes
are merkel cells attached to slowly adapting afferents?
yes
how many merkel cells attach to onw afferent fibre?
about 20 / afferent
what do merkel cells respond best to?
Respond to edges and bars with high spatial resolution
describe the dynamic range of merkel cells?
linear dynamic range.
narrow
what is Piezo2
in mice:
when Piezo2 is abolished, no inward current when touched = failure to demonstrate a sustained response
Piezo2 is the transduction cahnnel apparently
T or F
The SA1 nerve terminals are responsible for the rapid and dynamic response to mechanical stimuli while the Merkel cells are responsible for the sustained response. The expression of Piezo2 has been observed in the dorsal root ganglion, Aβ, Aδ and C-fibre low threshold mechanoreceptors.
T
describe ruffini endings/
- 1 receptor / fibre
- Respond to skin stretch but less sensitive to skin indentation comparent to SAI afferents
- Deep location shields receptor from confounding indentation
- Perception of hand shape and finger position
- They contact SAII afferents.
brail - which is which?
which afferents are most accurate for reading braille?
These spatial event plots show that the response of SAI fibres looks rather similar to the original dot pattern.
In contrast, the response of other fibre types is less convincing!
what determines tactile acuity?
Receptive field size and density determines tactile acuity
is tactile acuity good everywhere?
Tactile acuity is better in some parts of the body e.g. hands and fingers, than other parts e.g. back.
how do you measure tactile acuity?
In the nineteenth century the ‘compass’ test was devised by Weber to determine the smallest discriminable distance between two points of contact, the so called two-point limen. In general, the two-point limen improves up to twenty-fold from the shoulder (40mm) to the fingers (2mm).
what does thi sshow?
different tactile acuity across the body
more mobile the body part = better tactile acuity?
yes - not so much for the lower body though
Areas with high tactile acuity have ____ receptive fields.
Areas with high tactile acuity have small receptive fields.
If two points contacting the skin stimulate just one receptive field, then we have no information that two points on the skin were stimulated.
In line with this prediction whicih fibres have small receptive fields and the highest density on the fingertips.
In line with this prediction RAI and SAI fibres have small receptive fields and the highest density on the fingertips.
what are warm and cold spots?
spots of skin which either detect warmth or coldness
concetnration of cold spots exceeds the warm sports?
yes -
30x more cold spots
do many receptors encode temperature?
yes.
describe the temperature receptor Trpv1
Trpv1 channels respond to the active ingredient of chilli peppers, capsaicin, and also to painful increases in temperature above 43°C
describe the temperatuer receptor Trpm8
Trpm8 channels respond to menthol and are activated by non-painful decreases in temperature below 28°C.
whats the coldest temperature receptor/
TRPA1
whats meant by paradoxical cold?
When a heat stimulus of > 45o is applied to a cold spot on the skin a sensory illusion occurs called paradoxical cold. Such a high temperature is usually perceived as painful when applied to a diffuse area of the skin, however, when applied to a single cold spot it is perceived as cold. Therefore, activity in the cold fibre is experienced as cold irrespective of the physical nature of the stimulus—this is an example of labelled line coding.
Our sensory experience is determined by … what?
Our sensory experience is determined by the neuron’s central connections, not by the stimulus that evokes the action potentials
Pain is mediated by ______
Pain is mediated by nociceptors
The axon terminals of nociceptors (noci – to
injure/hurt) do not possess any specialised
endings and for this reason they are often
referred to as…
The axon terminals of nociceptors (noci – to
injure/hurt) do not possess any specialised
endings and for this reason they are often
referred to as bare or free nerve endings
how can pain be separated ?
Pain is often separated into an early (first), sharp pain and a second, dull, burning pain.
why are free neve endings good characteristics for noiciceptors?
This
property of nociceptors makes them particularly
sensitive to chemicals produced or released at a
site of injury.
how can we abolish the 2 components of pain?
Blocking A fibres abolishes the first component while blocking ‘C’ fibres abolishes the second pain.
The majority of the non-myelinated ‘C’ fibres are polymodal
what does this meam?
The majority of the non-myelinated ‘C’ fibres are polymodal in that they respond to thermal (< 15°C and > 43°C), mechanical (strong not mild) and chemical stimuli (e.g. chilli peppers, acid).
describe how pleasant touch is encoded:
set of fibres that respond to light touch, low- velocity stroking
stimulation = pleasant
characterised by low conduction velocity (~ 1m/s) and are only found in hairy skin
CT afferents
As stroke velocity increases there is a ………… relationship with action potential firing
As stroke velocity increases there is a non-linear relationship with action potential firing
fat
mamba
