Tactile and Proprioceptive Sensation - lecture 7 Flashcards
What are sensory receptors?
Highly specialised nerve cells that convert energy from stimuli into nervous impulses detected at the brain - these impulses identify the type of the stimulus, its strength/intensity and sometimes it’s location
What are the 2 sub systems of somatosensory receptors?
Mechanoreceptors - (all types of touch, proprioception) detect mechanical deformations
Free nerve endings (pain, temperature)
How is sensory information transmitted from stimulus to cell body?
Excitatory signals from receptors converge onto the cell body. The summation of these inputs determine whether an action potential is generated
What is the dorsal root ganglion ?
A cluster of nerve cell bodies located near the spinal cord, each level of the spinal cord has its own DRG
What is the somatosensory neural pathway? (See image for reference)
Stimulus is detected by the receptor, then travels to the DRG by a peripheral afferent nerve fibre. From the it travels via spinal cord (central afferent nerve fibre) to the thalamus and then the SSC.
What is a dermatome?
Each level of the spine is responsible for sensation for a specific area of the body - these regions are called dermatomes
What is the receptor neural pathway?
Tactile mechanoreceptors serve as the excitatory or inhibitory inputs to their neuron cell bodies located in the DRG’s
What is sensory homoculus?
Sensation for the entire human body is represented according to inputs into the SSC. The size of the SSC area representing a body region is reflective of the density of sensory receptors it contains.
What are the 3 types of touch?
Static pressure/force
Dynamic pressure/force
Vibration
In order of superficial to deep describe the regions of the cutaneous system?
Epidermis , dermis , hypodermis/subcutaneous layer
What are the two superficial touch mechanoreceptors?
Meissner corpuscle and merkel disc
What touch receptor responds to skin stretch?
Ruffin organ
Meissner and merkel disc both respond to touch, how do they differ?
Meissner corpuscle responds to touch / dynamic pressure
Merkel discs respond to touch / static pressure
Where can the ruffini organ be found?
Middle of the dermis
What does the pacinian corpuscle respond to?
Deep pressure/vibration
What are the 2 rapidly adapting touch receptors?
What are the 2 slowly adapting touch receptors?
RA1 - meissner
RA2 - pacinian
SA1 - merkel discs
SA2 - Ruffini organ
Which two receptors have small/specific receptive field size and which two receptors have large/broad receptive field size?
Small/specific - meissner and merkel
Large/broad - ruffini organ and pacinian
What does it mean if a touch receptor is rapidly adapting?
A short burst (neural spike train) on stimulus application but rapidly adapt to silent when the stimulus is sustained
What does it mean if a touch receptor is slowly adapting?
Takes far longer to reduce firing and become silent when the stimulus is sustained
What is unique about the meissner corpuscle?
Senses changes at low magnitude, low frequency skin motion/indentation
Contributes to detection of Braille patterns as fingertips run over dots
What is unique about merkel discs?
Detects the nature of curved surfaces according to sustained firing rate during prolonged stimulus application
A nerve bundle subserves these receptors
As object size increases the force/skin depression is shared between multiple receptors so the sustained firing rate of individual receptors decreases
What is unique about the ruffini organ?
Located in skin folds to detect skin stretch, increased sustained firing rate when stretch increases
Indicates the shape of large objects grasped with entire hand but also information about shape and finger movement with empty hand
What is unique about the pacinian corpuscle?
Highly sensitive to vibration
Allows sensation of conditions at the working surface of a tool in our hands
What is proprioception?
Perception of where the hands and limbs are in space in the absence of vision
What are the 2 subdivisions of proprioception?
Static - (joint/limb position sense)
Dynamic - (kinaesthetic/motion sense, rate at which limbs are moving around
What are the 3 main types of proprioceptive receptors?
Muscle spindles, GTO’s and joint receptors
What do muscle spindles respond to?
Respond to, or increasing firing rate when they are stretched. They signal changes in muscle length (both absolute and rate of change).
Explain the role of nuclear bag fibres (in muscle spindles)
They are sensitive to rate of change and also static/absolute muscle length
Dynamic bag (type 1) and static bag (type 2)
Explain the role of nuclear chain fibres
They are sensitive to absolute muscle length only. Not concerned with rate of change.
How do group 1a fibres differ from type II fibres when referring to afferent spindle innervation?
Group 1a fibres signal rate of change and absolute length as they are attached to both bag and chain fibres
Group II fibres signal absolute length alone as they are attached to the ends of chain and static bag fibres
What do dynamic nuclear bag fibres do?
They are connected to group 1a only and act as the main receptor in signalling rate of change
Group II fibres elicit a static response, what does this mean?
The group II fibres are slower conducting and change their rate of firing proportionally to muscle length
The group 1a fibres demonstrate a dynamic response,what does this mean?
They are faster conducting and signal the same static type response but also increase firing proportionally to the rate of stretch
Discuss the evidence for spindles in terms of vibration?
Vibration stimulates firing of type 1a afferents
E.g. Bicep tendon
Or holding nose gives Pinocchio illusion
What are GTO’s and what do they do?
Small encapsulated regions at the junction between the muscle fibres and tendon that signal changes in muscle tension or force.
How do GTO’s signal changes?
As a muscle contracts, collagen fibres straighten and compress mechanoreceptors regions of 1b afferent axons -> this increases firing rate and signals increased tension
What are joint receptors?
Contained within the joint are free nerve endings and GTO type endings (in ligament instead of tendon).
These receptors have a high threshold and typically signal extreme mechanical stress pain
Other ruffini and pacini type endings signal capsule / tissue stretch and deformation / vibration respectively.
What is the key evidence against joint receptors being crucial for proprioception
Primarily seen to be for extreme limit detection in joint motion,key evidence being that the normal sensation of joint rotation is maintained after hip replacement surgery
What does the gamma motor neuron do?
Since spindles only signal muscle stretch/extension, a mechanism is required to keep them sensitive when the muscle is stretched/contracted - this is the gamma motor neuron
What is alpha-gamma co activation?
When a muscle is contracted/shortened, muscle spindles become slack and fall silent. Gamma drive contracts intrafusal fibres to make them taught again. The alpha (extrafusal) and gamma (intrafusal) motor neurons are activated to maintain spindle sensitivity - alpha -gamma co activation
Describe a simple monosynaptic stretch reflex (tendon hammer tap)??
1) tap occurs which produces a brief stretch of quad
2) increases the firing rate of the muscle spindles along primary 1a afferent fibre
3) travels to the DRG and then to the spinal cord where it synapses with the alpha motor neuron
4) result impulse innervates extrafusal quad muscle and evokes a contraction
Why is the stretch reflex needed?
The stretch reflex is needed for maintaining muscle tone, which makes movements more efficient/effective when initiated
Can also be employed to define a new desired limb position but this requires the updating of both systems (alpha and gamma)
How is a new limb position activated?
The sensitivity of spindles can be changed via the gamma motor neuron drive and exploitation of the monosynaptic reflex arc. In this instance, the alpha motor neuron works as a slave to correct errors between actual spindle firing rate and normal spindle firing rate. This is all mediated in SC so the regulation of only 1 system is required.
What did sainburg et al (1995) find with deafferented patients?
That they did not have the control to draw line at specific angles.
Describe 2 studies looking at proprioceptive / motor deficits with ageing ?
One study looking p’s pressing a button when they could detect movement at the wrist. Results showed that old sedentary were slowest then old active then young.
Helsen et al (2016) found no measurable correlation between proprioceptive activity and motor performance.
Describe the results of Herter et al (2014) looking at stroke patients?
Despite some correlation with clinical measures, no direct relationship was found between proprioceptive loss and impaired reaching performance
