What are cutaneous mechanoreceptors?
Receptors present in the skin (and deeper tissues) that interpret mechanical stimuli (mainly distortion) and convert this physical energy into nerve impulses in a process called transduction.
What is the difference in distribution between hairy and glabrous skin?
Most of the body is covered by hairy skin The palmar surface of the hands and soles of the feet are covered by glabrous skin with ridges as a prominent feature
What are the four main types of mechanoreceptors in GLABROUS SKIN? Classify them in terms of location
SUPERFICIAL (at the dermal/epidermal border) Meissner corpuscles Merkel complexes DEEP (Dermis) Ruffini organs Pacinian corpuscles
Describe the morphology of the axons innervating the mechanoreceptors to the DRG and why this is so
The receptors are innervated by LARGE, MYELINATED axons with their bodies in the DRG This is in order to effectively and rapidly convey information back to the CNS
There are free nerve endings superficially in the epidermis of the skin. What are these responsible for?
Nociception, pain and temperature recognition (also some mechanoreceptor information)
Where are the Meissner corpuscle mechanoreceptors located?
They are located superficially in the dermis (immediately under the epidermis).
Where are the Merkel cell neurite complexes located?
They are located superficially in the troughs of the epidermal regions
Where are the ruffini corpuscles located and what do they look like?
The are located deeper in the dermis and are cigar shaped cells
Where are the pacinian corpuscles located?
Deep in the dermis (just above the subcutaneous layer)
All of the mechanoreceptors are encapsulated nerve endings, what is this capsule made up of and what does it appear like?
They have a corpuscular structure with sheaths made up of CONNECTIVE TISSUE forming a specialised blob at the end - onion like structure
What is the force range that the mechanoreceptors of the skin generally receptive to?
10nm to larger sub-damaging distortion of skin (then the pain receptors take over)
What is the sensitivity and dynamic range of the mechanoreceptors? (in terms of the frequency of forces)
What is the range of receptive field sizes for mechanorecptors?
Ovaloid fields ranging from 10 mm2 to the entire hand
How does mechanical distortion cause an action potential to occur through the axons of the sensory fibres?
By Mechanically gated sodium channels.
The mechanical distortion causes the opening of sodium channels and allowing an influx of sodium to depolarise the cell to threshold (action potential).
A complex tethering system of proteins and ion channels means that the opening of one channel induces opening of nearby channels.
Describe the all-or-none concept in terms of action potential firing in response to stimuli
The stronger the stimuli the more depolarisation of the cell will occur and the more likely the cell will reach threshold to cause an action potential to eventually percieve this stimuli.
Describe the difference between a slowly adapting mechanoreceptor and a rapidly adapting mechanoreceptor
Are most sensory receptors slowly or rapidly adapting?
A slowly adapting mechanoreceptor will respond gradually to the presence of an ongoing stimulus and will continue firing as long as the stimulus is still present.
A rapidly adapting mechanoreceptor is only interested in changes in the environment and thus will only fire when the stimulus is applied (and removed).
Most sensory receptors are rapidly adapting
Describe the difference between a tactile (mechanoreceptive) afferent compared to a nociceptive one
A mechanoreceptor is generally a slowly adapting mechanoreceptor that has the same level of firing for the duration of the stimulus.
A nociceptive is a special kind of slowly adapting receptor in which the firing increases the longer the stimulus is applied.
Which 2 of the 4 mechanoreceptors are the slowly adapting ones?
Merkel (superficial) which respond to indentation
Ruffini (deep) have sustained responses to skin movement
Which 2 of the 4 mechanoreceptors are rapidly adapting?
Meissner (superficial) have transient responses to skin movement
Pacinian (deep) have a transient response to vibration
Why do we have four different types of mechanoreceptors?
Because they have different functionalities in terms of:
Location, density, stimuli (response to different things), rapid/slow adaptation
Describe the receptive fields of the different types of mechanoreceptors
Merkel: smallest RF (ovaloid patches)
Meissner: small RFs (ovaloid patches)
Ruffini: larger patches of skin
Pancinian: largest patches of skin (up to whole digits) - respond to vibrations
Why do the deeper mechanoreceptors generally have larger receptive fields than the superficial ones?
Because they are harder to stimulate and require more energy at the top of the skin to transmit it deep enough to activate them
Draw the firing rates of action potentials for each of the types of mechanoreceptors for the following two stimuli scenarios:
Do mechanoreceptors occur at low or high threshold energies?
There are other mechanoreceptors in the body (apart from those present in the skin). Give some examples
- Those involved in hearing
- Gut tube (peristalsis control)
What is the major determining factor for the ability to distinguish spatially, between two different stimuli
The size of the receptive fields and where in the receptive field the objects lie
Predict what the CNS would interpret from each neuron (a,b,c) from this experiment (each for blue, red and green).
Would the individual recognise the stimuli as two objects or one? Why?
Green: One stimulus
Red: unable to distinguish: likely one stimulus
Blue: able to tell it is two
What is the spacial two-point discrimination like in the fingers compared to somewhere more distally like the wrist?
The spatial resolution of the finger tips is much more senstivity and finer than distally
A region of skin that is supplied (sensory) by a spinal nerve
Why isn't there a region of the body for the C1 spinal nerve?
C1 spinal nerve is purely motor (all others are mixed motor and sensory)
What nerve supplies the sensory aspect of the face?
The trigeminal nerve (a cranial nerve and not a spinal nerve - but behaves similarly going to a trigeminal root ganglia)
Is the dermatome map a definite and clear cut description for every individual?
No the dermatome map is diffferent for different people and the overlap between regions is quite large
Generally describe the dermatome map (in terms of cranial supply, thoracic, lumbar and sacral)
What is the difference between a dematome and a peripheral nerve (cutaneous innervation)?
Dermatomes are a reflection of the spinal roots and their sensory territory of the skin.
Peripheral innervation (cutaneous) refer to areas innervated by specific peripheral nerves (e.g. the radial or ulnar nerve), which can contain fibers from multiple spinal nerve roots because of how the spinal nerves form plexuses
What is the afferent axon type (fibre type) of the mechanoreceptors?
What is the conduction velocity normally ranging between?
Ab (beta) fibres
6-12 um diameter of axons
Conduction velocity of 35-75m/s (quite fast)
How do mechanosensory afferent fibres relay the information from the periphery to the CNS?
The axons travel from the skin to the soma in the dorsal root ganglion. They continue to the dorsal horn of the spinal cord (where they make multiple local synapses) and continue without any intermediate synapses/neurons through the white matter of the spinal cord up to the brain stem
ie. information travels through a single neuron from the periphery to the CNS (brainstem)