Tactile and Proprioceptive Sensation - lecture 7

Question 1

What are sensory receptors?

Answer 1

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

Question 2

What are the 2 sub systems of somatosensory receptors?

Answer 2

Mechanoreceptors - (all types of touch, proprioception) detect mechanical deformations

Free nerve endings (pain, temperature)

Question 3

How is sensory information transmitted from stimulus to cell body?

Answer 3

Excitatory signals from receptors converge onto the cell body. The summation of these inputs determine whether an action potential is generated

Question 4

What is the dorsal root ganglion ?

Answer 4

A cluster of nerve cell bodies located near the spinal cord, each level of the spinal cord has its own DRG

Question 5

What is the somatosensory neural pathway? (See image for reference)

Answer 5

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.

Question 6

What is a dermatome?

Answer 6

Each level of the spine is responsible for sensation for a specific area of the body - these regions are called dermatomes

Question 7

What is the receptor neural pathway?

Answer 7

Tactile mechanoreceptors serve as the excitatory or inhibitory inputs to their neuron cell bodies located in the DRG’s

Question 8

What is sensory homoculus?

Answer 8

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.

Question 9

What are the 3 types of touch?

Answer 9

Static pressure/force

Dynamic pressure/force

Vibration

Question 10

In order of superficial to deep describe the regions of the cutaneous system?

Answer 10

Epidermis , dermis , hypodermis/subcutaneous layer

Question 11

What are the two superficial touch mechanoreceptors?

Answer 11

Meissner corpuscle and merkel disc

Question 12

What touch receptor responds to skin stretch?

Answer 12

Ruffin organ

Question 13

Meissner and merkel disc both respond to touch, how do they differ?

Answer 13

Meissner corpuscle responds to touch / dynamic pressure

Merkel discs respond to touch / static pressure

Question 14

Where can the ruffini organ be found?

Answer 14

Middle of the dermis

Question 15

What does the pacinian corpuscle respond to?

Answer 15

Deep pressure/vibration

Question 16

What are the 2 rapidly adapting touch receptors?

What are the 2 slowly adapting touch receptors?

Answer 16

RA1 - meissner
RA2 - pacinian
SA1 - merkel discs
SA2 - Ruffini organ

Question 17

Which two receptors have small/specific receptive field size and which two receptors have large/broad receptive field size?

Answer 17

Small/specific - meissner and merkel

Large/broad - ruffini organ and pacinian

Question 18

What does it mean if a touch receptor is rapidly adapting?

Answer 18

A short burst (neural spike train) on stimulus application but rapidly adapt to silent when the stimulus is sustained

Question 19

What does it mean if a touch receptor is slowly adapting?

Answer 19

Takes far longer to reduce firing and become silent when the stimulus is sustained

Question 20

What is unique about the meissner corpuscle?

Answer 20

Senses changes at low magnitude, low frequency skin motion/indentation
Contributes to detection of Braille patterns as fingertips run over dots

Question 21

What is unique about merkel discs?

Answer 21

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

Question 22

What is unique about the ruffini organ?

Answer 22

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

Question 23

What is unique about the pacinian corpuscle?

Answer 23

Highly sensitive to vibration

Allows sensation of conditions at the working surface of a tool in our hands

Question 24

What is proprioception?

Answer 24

Perception of where the hands and limbs are in space in the absence of vision

Question 25

What are the 2 subdivisions of proprioception?

Answer 25

Static - (joint/limb position sense)

Dynamic - (kinaesthetic/motion sense, rate at which limbs are moving around

Question 26

What are the 3 main types of proprioceptive receptors?

Answer 26

Muscle spindles, GTO’s and joint receptors

Question 27

What do muscle spindles respond to?

Answer 27

Respond to, or increasing firing rate when they are stretched. They signal changes in muscle length (both absolute and rate of change).

Question 28

Explain the role of nuclear bag fibres (in muscle spindles)

Answer 28

They are sensitive to rate of change and also static/absolute muscle length

Dynamic bag (type 1) and static bag (type 2)

Question 29

Explain the role of nuclear chain fibres

Answer 29

They are sensitive to absolute muscle length only. Not concerned with rate of change.

Question 30

How do group 1a fibres differ from type II fibres when referring to afferent spindle innervation?

Answer 30

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

Question 31

What do dynamic nuclear bag fibres do?

Answer 31

They are connected to group 1a only and act as the main receptor in signalling rate of change

Question 32

Group II fibres elicit a static response, what does this mean?

Answer 32

The group II fibres are slower conducting and change their rate of firing proportionally to muscle length

Question 33

The group 1a fibres demonstrate a dynamic response,what does this mean?

Answer 33

They are faster conducting and signal the same static type response but also increase firing proportionally to the rate of stretch

Question 34

Discuss the evidence for spindles in terms of vibration?

Answer 34

Vibration stimulates firing of type 1a afferents

E.g. Bicep tendon
Or holding nose gives Pinocchio illusion

Question 35

What are GTO’s and what do they do?

Answer 35

Small encapsulated regions at the junction between the muscle fibres and tendon that signal changes in muscle tension or force.

Question 36

How do GTO’s signal changes?

Answer 36

As a muscle contracts, collagen fibres straighten and compress mechanoreceptors regions of 1b afferent axons -> this increases firing rate and signals increased tension

Question 37

What are joint receptors?

Answer 37

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.

Question 38

What is the key evidence against joint receptors being crucial for proprioception

Answer 38

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

Question 39

What does the gamma motor neuron do?

Answer 39

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

Question 40

What is alpha-gamma co activation?

Answer 40

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

Question 41

Describe a simple monosynaptic stretch reflex (tendon hammer tap)??

Answer 41

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

Question 42

Why is the stretch reflex needed?

Answer 42

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)

Question 43

How is a new limb position activated?

Answer 43

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.

Question 44

What did sainburg et al (1995) find with deafferented patients?

Answer 44

That they did not have the control to draw line at specific angles.

Question 45

Describe 2 studies looking at proprioceptive / motor deficits with ageing ?

Answer 45

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.

Question 46

Describe the results of Herter et al (2014) looking at stroke patients?

Answer 46

Despite some correlation with clinical measures, no direct relationship was found between proprioceptive loss and impaired reaching performance