L7. Peripheral Sensory Mechanisms

Question 1

What are cutaneous mechanoreceptors?

Answer 1

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.

Question 2

What is the difference in distribution between hairy and glabrous skin?

Answer 2

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

Question 3

What are the four main types of mechanoreceptors in GLABROUS SKIN? Classify them in terms of location

Answer 3

SUPERFICIAL (at the dermal/epidermal border) Meissner corpuscles Merkel complexes DEEP (Dermis) Ruffini organs Pacinian corpuscles

Question 4

Describe the morphology of the axons innervating the mechanoreceptors to the DRG and why this is so

Answer 4

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

Question 5

There are free nerve endings superficially in the epidermis of the skin. What are these responsible for?

Answer 5

Nociception, pain and temperature recognition (also some mechanoreceptor information)

Question 6

Where are the Meissner corpuscle mechanoreceptors located?

Answer 6

They are located superficially in the dermis (immediately under the epidermis).

Question 7

Where are the Merkel cell neurite complexes located?

Answer 7

They are located superficially in the troughs of the epidermal regions

Question 8

Where are the ruffini corpuscles located and what do they look like?

Answer 8

The are located deeper in the dermis and are cigar shaped cells

Question 9

Where are the pacinian corpuscles located?

Answer 9

Deep in the dermis (just above the subcutaneous layer)

Question 10

All of the mechanoreceptors are encapsulated nerve endings, what is this capsule made up of and what does it appear like?

Answer 10

They have a corpuscular structure with sheaths made up of CONNECTIVE TISSUE forming a specialised blob at the end - onion like structure

Question 11

What is the force range that the mechanoreceptors of the skin generally receptive to?

Answer 11

10nm to larger sub-damaging distortion of skin (then the pain receptors take over)

Question 12

What is the sensitivity and dynamic range of the mechanoreceptors? (in terms of the frequency of forces)

Answer 12

0-1000Hz

Question 13

What is the range of receptive field sizes for mechanorecptors?

Answer 13

Ovaloid fields ranging from 10 mm2 to the entire hand

Question 14

How does mechanical distortion cause an action potential to occur through the axons of the sensory fibres?

Answer 14

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.

Question 15

Describe the all-or-none concept in terms of action potential firing in response to stimuli

Answer 15

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.

Question 16

Describe the difference between a slowly adapting mechanoreceptor and a rapidly adapting mechanoreceptor

Are most sensory receptors slowly or rapidly adapting?

Answer 16

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

Question 17

Describe the difference between a tactile (mechanoreceptive) afferent compared to a nociceptive one

Answer 17

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.

Question 18

Which 2 of the 4 mechanoreceptors are the slowly adapting ones?

Answer 18

Merkel (superficial) which respond to indentation

Ruffini (deep) have sustained responses to skin movement

Question 19

Which 2 of the 4 mechanoreceptors are rapidly adapting?

Answer 19

Meissner (superficial) have transient responses to skin movement

Pacinian (deep) have a transient response to vibration

Question 20

Why do we have four different types of mechanoreceptors?

Answer 20

Because they have different functionalities in terms of:

Location, density, stimuli (response to different things), rapid/slow adaptation

Question 21

Describe the receptive fields of the different types of mechanoreceptors

Answer 21

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

Question 22

Why do the deeper mechanoreceptors generally have larger receptive fields than the superficial ones?

Answer 22

Because they are harder to stimulate and require more energy at the top of the skin to transmit it deep enough to activate them

Question 23

Draw the firing rates of action potentials for each of the types of mechanoreceptors for the following two stimuli scenarios:

Answer 23

Question 24

Do mechanoreceptors occur at low or high threshold energies?

Answer 24

Low energies

Question 25

There are other mechanoreceptors in the body (apart from those present in the skin). Give some examples

Answer 25

* Muscles * Tendons * Those involved in hearing * Gut tube (peristalsis control)

Question 26

What is the major determining factor for the ability to distinguish spatially, between two different stimuli

Answer 26

The size of the receptive fields and where in the receptive field the objects lie

Question 27

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?

Answer 27

Green: One stimulus Red: unable to distinguish: likely one stimulus Blue: able to tell it is two

Question 28

What is the spacial two-point discrimination like in the fingers compared to somewhere more distally like the wrist?

Answer 28

The spatial resolution of the finger tips is much more senstivity and finer than distally

Question 29

Define dermatome

Answer 29

A region of skin that is supplied (sensory) by a spinal nerve

Question 30

Why isn't there a region of the body for the C1 spinal nerve?

Answer 30

C1 spinal nerve is purely motor (all others are mixed motor and sensory)

Question 31

What nerve supplies the sensory aspect of the face?

Answer 31

The trigeminal nerve (a cranial nerve and not a spinal nerve - but behaves similarly going to a trigeminal root ganglia)

Question 32

Is the dermatome map a definite and clear cut description for every individual?

Answer 32

No the dermatome map is diffferent for different people and the overlap between regions is quite large

Question 33

Generally describe the dermatome map (in terms of cranial supply, thoracic, lumbar and sacral)

Answer 33

Question 34

What is the difference between a dematome and a peripheral nerve (cutaneous innervation)?

Answer 34

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**

Question 35

What is the afferent axon type (fibre type) of the mechanoreceptors? What is the conduction velocity normally ranging between?

Answer 35

Ab (beta) fibres 6-12 um diameter of axons Conduction velocity of 35-75m/s (quite fast)

Question 36

How do mechanosensory afferent fibres relay the information from the periphery to the CNS?

Answer 36

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)**