Mechanosensation Flashcards
Detection of mechanical stimuli is also sometimes called the (1) sensory system
Somatic
Membrane receptors can be of 2 kinds:
- Lock and key receptors
- Force-gated receptors (stretch-activated)
The somatic sensory system detects primarily 2 kinds of stimuli
- Mechanical stimuli
- Noxious stimuli
The main organ of somatosensation is…
The skin
Why is the somatic sensory system important? (3)
- Identify shapes and textures
- Monitor internal and external forces acting on the body
- Detect potentially harmful stimuli
(Exploration, pain, healing)
Non-hairy skin is called “(1) skin”
Glabrous skin
Describe receptors in the skin, generally
Can have no capsule (free nerve endings), or nerve endings can be associated with some kind of associated cell
Free nerve endings in the skin are typically associated with detection of (1) stimuli
Noxious
What are the 4 kinds of somatosensory receptors in the skin?
- Merkel’s discs
- Meissner’s corpuscle
- Pacinian corpuscle
- Ruffini’s endings
What is the primary role of a capsule around a cutaneous nerve ending?
Helps to regulate the response to stimuli: When stimulus is applied to skin, the skin is deformed which activates nerve endings. The type of capsule around the nerve ending affects the ionic permeability of the cell membrane
Capsules around nerve endings are especially common in (glabrous/non-glabrous) skin
Glabrous
Describe rapid adaptation
Changes in a stimulus are encoded
Describe slow adaptation
Persistence of a stimulus is encoded
Which kind of subcutaneous receptor is associated with glabrous and hairy skin?
Merkel’s discs
40% of sensory innervation to the hand comes from these subcutaneous receptors
Meissner’s corpuscles
Describe the histology of Meissner’s corpuscles
Capsules made of connective tissue: layers of Schwann cells. These may surround one or more nerve fibers
Meissner’s corpuscles are (rapidly/slowly) adapting
Rapidly
What kind of mechanosensory stimulus are Meissner’s corpuscles tuned to?
Vibration and movement across the hand
Meissner’s corpusles are associated with (1) skin
Glabrous
Other than in the skin, where else can Pacinian corpuscles be found? (2)
- Gut
- Skeletal system
Describe the histology of a Pacinian corpuscle
Onion-like arrangement of cells with fluid between cell layers
What kind of stimulus are Pacinian corpuscles tuned to?
Vibration
Ruffini’s corpuscles are found (cutaneously/deep) in the skin
Deep
Other than skin, where are Ruffini’s corpuscles found?
Ligaments and tendons
What kind of stimulus are Ruffini’s corpuscles tuned to?
Stretch
Describe the positioning of Ruffini’s corpuscles relative to direction of stretch
Parallel to the direction of skin stretching
Where are Merkel’s disks localized in the skin?
Fingertips
Which kind of subcutaneous receptor would be the most useful for reading braille?
Merkel’s disks
What kinds of stimuli are Merkel’s disks tuned to? (2)
Light pressure
Shapes/edges/texture
Describe the anatomy of Merkel’s disks
Surrounded by supporting cells (Merkel’s cells) which release peptides to modulate the activation of the nerve terminal
Compared to other subcutaneous receptors, Merkel’s disks are (more/less) encapsulated
Less
Describe lanceolate endings
Associated with hairy skin, specialized nerve endings which wrap around a hair follicle
Which is likely more responsible for mechanosensation, the nerve fiber or the surrounding capsule cell?
Both are likely equally responsible
True or false: free nerve endings act independently from other cells
False: receive input and have connections to multiple different cell types
Free nerve endings are associated primarily with these kinds of cells
Keratinocytes
What is the role of keratinocytes? (3)
Provide a water barrier (keep cells hydrated)
Secrete neuromodulatory factors (like ATP, which likely activates neurons)
Express TRP channels
ATP released from keratinocytes binds to other keratinocytes via this receptor
P2Y2
Activation of P2Y2 receptors induces… (2)
Release of calcium from internal calcium stores
Induces further release of ATP
Where are muscle spindles located?
Within organs - primarily in muscle tissue
What kind of sensory information do muscle spindles provide?
Stretch info within muscles, proprioception
Which muscles have a lot of muscle spindles, why?
Extraocular muscles, muscles in head and neck have a lot of muscle spindles – Would be really painful to strain these muscles, could impact survival more heavily than straining a leg, for example
Describe the very general pathway to the cortex from somatosensory receptors. What area of the cortex do these neurons terminate in?
Receptors > neurons > brainstem > thalamus > primary somatosensory cortex in the postcental gyrus of the parietal lobe
All touch information goes to this area of the brain
Primary somatosensory cortex
Which kind of neurons are associated with non-painful touch?
AB (A-beta)
Which kinds of neurons are associated with pain? Which is the faster conducting fiber?
A-delta and C
A-delta conducts faster
Describe the shapes of A-beta, A-delta and C fibers (size, myelination)
A-beta: Largest diameter, myelinated
A-delta: Medium dimeter, lightly myelinated
C: Small diameter, unmyelinated
Why is 2-point discrimination the best on a human’s hands and face?
Because these are the areas of the body with the largest concentration of somatosensory receptors
Where on the face are the largest amounts of somatosensory receptors?
Mouth
The star appendages of the star-nosed mole are used exclusively for this mode of sensation
Touch (not used for smell)
Describe the role of appendage #11 of the star-nosed mole
Used for fine discrimination, called the “tactile fovea”
Each appendage of the star of a star-nosed mole is composed of 1000+ (1)
Eimer’s organs
Describe Eimer’s organ
Merkel-like complexes which are highly sensitive to touch: shape/edges/texture
Mechanosensory cells of the skin project to (1), which terminate in the (2) of the spinal cord
- A dorsal root ganglion
- Lateral ventral horns of the spinal cord
Why are mechanotransduction channels thought to not use a second messenger cascade?
They respond so quickly that a 2nd messenger cascade is probably unlikely
What are the 3 theories as to how mechanosensory channels open?
- Stretch of the membrane directly pulls a channel apart
- Receptors are tethered to the cytoskeleton or the ECM which when deformed could open the channel
- A movement-sensitive protein is associated with a channel, this could be tethered or directly sensitive to a channel inducing opening
Which channel types are involved in mechanoreception? (3)
Epithelial sodium channels (ENaCs and ASICs)
TRP channels
Piezo channels
ENaC channels are important for mechanosensation in this organism
Light touch in C. elegans
Describe the changes in mechanoreception which occur in mice lacking ASIC receptors
Experience small changes in mechanosensitivity
Which of the mechanosensitive channels is likely the most important for mechanosensation in mammals?
Piezo channels
How could you investigate the mechanism for mechanosensation in drosophila without knowing which channels are involved?
Apply a mutagen to flies (creates random genomic mutations) and screen for flies which lack touch sensitivity
What are the drosophila mechanosensory organs?
Johnston’s organ
Chordotonal organs
Bristles (act as external sense organs)
Describe the proposed mechanism for bristle mechanosensation in flies
A bristle is connected to a neuron, which has a little neuron cap. When the bristle move, it makes contact with the cap, moves, and induces an action potential
Chordotonal organs in flies detect this kind of motion
Stretch
Flies which lacked mechanosensation usually had mutations in these genes
nompA-D
One of the proteins which associates with the nompD genes in flies is (1). What does this mean?
- TRP channels
It means that TRP channels must be involved in mechanosensation in drosophila
The external sense bristles on drosophila are connected to their underlying neurons (the dendritic caps of neurons) through…
Ankyrin repeats
Describe how pulling on the ankyrin repeat of a bristle sense cell causes it to open
Ankyrin is probably tied to cation channels, when it moves it pulls the channel apart so it opens
Describe the anatomy of a TRP channel
6TM, N=4
What ions pass through TRP channels?
They are nonspecific cation channels
What evidence suggests that TRP channels involved in drosophilia mechanotransduction are likely not using a 2nd messenger cascade?
They are activated too quickly: low latency of onset (200 us)
If TRP channels don’t use a 2nd messenger, how do they initiate changes in the cell to open?
There is likely a direct coupling of mechanoreceptors to the TRP channel (e.g. like ankyrin repeats connecting to the cytoskeleton)
Of the 2 kinds of Piezo channel (Piezo1 and Piezo2), which is the dominant transducer of mechanical forces for touch sensation in mice?
Piezo2
What 2 pieces of evidence led us to believe that Piezo2 channels are the dominant mechanosensory channels in mammals?
- They are localized around subcutaneous nerve endings
- Piezo2 null mice have signficant touch defects
Describe the movement to open Piezo2 channels
Trimer: rotate around the central pore (twisting back and forth motion like a camera shutter)
What causes Piezo2 channels to open?
Movement of the cell membrane
What are 3 locations in hairy skin where Piezo2 channels are found?
Merkel cell-neurite complexes
Lanceolate endings
Circumferential fibers
What are 2 locations in glabrous skin where Piezo2 channels are located?
Merkel cell-neurite complexes
Meissner corpuscles
