Lecture 8- Touch

Question 1

What can you tell from the size of a receptive field?

Answer 1

-tells you how the info starts to get analysed, tells you about the resoultion of that system

Question 2

What are the four types of receptors in the skin?

Answer 2

-Various encapsulated nerve endings -Merkel, Meissner, Pacinian, Ruffini

Question 3

What does the somatosensory system do?

Answer 3

-mediates sensations from the whole body surface, including skin and deeper tissues

Question 4

What is the overall structure of the skin?

Answer 4

-most of the body covered by hairy skin. -Palmar surface of the hands and the soles of the feet are covered by glabrous skin, with skin ridges a prominent feature

Question 5

What type of receptors are the receptors in the skin?

Answer 5

-mechanoreceptors= respond to mechanical stress/stimuli

Question 6

What 2 mechanoreceptors are close to the surface?

Answer 6

Meissner corpuscles and Merkel complex

Question 7

What 2 mechanoreceptors are deeper in the skin?

Answer 7

Ruffini organs and Pacinian corpuscles

Question 8

What are the receptors innervated by and where are their cell bodies?

Answer 8

-innervated by large myelinated axons with cell bodies in dorsal root ganglia

Question 9

What is hepatic manipulation?

Answer 9

-can work out what an object is just by touch

Question 10

What is special about the Merkel complex?

Answer 10

• the Merkel cells are not neural
• it is a complex and the nerve endings end around these Merkel cells and transmit information to the CNS

Question 11

What does the Meissner corpuscle look like and why?

Answer 11

• like a corn cob sort of
• the shape of the end= connective tissue, important as it determines how the nerve will be stimulated depending on its shape

Question 12

What does the Pacinian corpuscle look like?

Answer 12

• like a marine worm
• flat= because it reacts to vibrations (again the shape confers its properties)

Question 13

What does the Ruffini organ look like?

Answer 13

• like a Neptune’s necklace (sea geep)
• long= thus reacts to stretch of the skin

Question 14

In which layer are the Merkel and Meissner?

Answer 14

-epidermis (upper)

Question 15

In which layer is the Ruffini organ?

Answer 15

-dermis (below epidermis)

Question 16

In which layer is the Pacinian corpuscle?

Answer 16

-lower bit of dermis and subcutaneous layer (below dermis) hypodermis

Question 17

How does transduction work in mechanoreceptors (afferent fibre)?

Answer 17

• afferent fibre is encapsulated by Schwann cells
• then the receptor at the end
• in the membrane of the receptor there are mechanically gated channels (when you touch sth or move sth across= the membrane stretches) it opens a Sodium channel and that results in DEPOLARISATION of the cell
• if the stimuli is strong enough, reach the threshold and fire an action potential (all or nothing system)
• all the skin receptors do it but respond differently to ongoing stimuli

Question 18

What are the two types of receptors in terms of how they adapt to ongoing stimuli and which one is more common?

Answer 18

1. Slowly adapting 2.Rapidly adapting=by far the most common in the nervous system -most mechanoreceptors are rapidly adapting -burst in activity when the stimuli begins, we are interested in change

Question 19

Which are the slowly adapting mechanoreceptors and what do they respond to?

Answer 19

1. Merkel complexes=found at the bottom of the epidermal ridges where they respond to indentation 2.Ruffini endings=found in the upper dermis, have sustained response to skin movement

Question 20

Which are the rapidly adapting mechanoreceptors and what do they respond to?

Answer 20

1.Meissner receptors= found near the skin surface and have a transient response to skin movement 2. Pacinian receptors= located deep in the dermis and hypodermis and have a transient response to vibration

Question 21

How much of the total do the Merkel complexes make up and how big are their receptive fields?

Answer 21

25% of total -smallest receptive fields -capable of fine discrimination

Question 22

How much of the total do the Meissner receptors make up and how big are their receptive fields?

Answer 22

40% of the total -small receptive fields -more sensitive

Question 23

How much of the total do the Pacinian receptors make up and how big are their receptive fields?

Answer 23

15%

• huge receptive fields
• vibration sensitive

Question 24

Do muscles have mechanoreceptors?

Answer 24

-yes -all muscles have muscle spindles= mechanoreceptors -golgi tendon organ= also mechanoreceptors - used to tell stretch of muscle etc.

Question 25

How sensitive a field is depends on?

Answer 25

-the size of the receptive field --green = looks like one point of contact - red= also can't tell if two points - blue= can tell it is two - this is the idea of resolution

Question 26

Are there big differences in the sensory discrimination in various parts of the body?

Answer 26

- depends on the entire system, peripheral and central mechanisms - differs a lot - on your back can't tell if two point even 4cm apart

Question 27

What fibres are the ones linking the mechanoreceptors of touch with the CNS?

Answer 27

-A beta fibres -second fastest transmission(after muscles) (35-75m/s) - axon diameter= 6-12micrometers

Question 28

What fibres are the ones linking the mechanoreceptors of pain with the CNS?

Answer 28

-thinner, completely different fibres -the slowest fibres (together with the temp ones) -pain= sustained effect, can still feel it after you remove your hand from the fire or so, so you can learn!

Question 29

What is a dermatome?

Answer 29

- region of skin that is supplied by nerves of one spinal segment - (31 spinal segments)

Question 30

Why are the dermatomes a funny shape?

Answer 30

-because of the shape the embryo has

Question 31

How is the AP transported from the touch mechanoreceptor to the CNS?

Answer 31

- starts at the receptor ending, then goes down the mechanosensory afferent fiber and then to the dorsal ganglion cells (that is where the cell body is) - then the axon goes further into the spinal cord and has some local connections there but goes up into the brain into the medulla (dorsal column nucleus)and that is where it synapses with the second neuron

Question 32

How does the pain circuit work?

Answer 32

- very differently to the mechanosensory circuit, - goes from the receptor to the dorsal root ganglion cells and then to spinal cord, there it crosses over and goes to the ventral side and up to the brain

Question 33

What is the main mechanosensory pathway?

Answer 33

1. Mechanosensory receptors from the body to the spinal cord (lumbar= if lower body, cervical if upper) 2. goes up to the Gracile nucleus(lower body)/Cuneate nucleus(upper) 3. There the first neuron synapses with the second and crosses over 4. Second neuron now goes through the brain to the thalamus (the ventral posterior lateral nucleus) 5. Third neuron is from thalamus to the primary somatic sensory cortex

Question 34

Where is the primary somatic sensory portion of the thalamus and the cortical target?

Answer 34

-target= postcentral gyrus (cenetral gyrus= functional barrier of the cortex)

Question 35

How many areas is the primary somatosensory cortex divided into?

Answer 35

-4 Area 1, 2, 3a,3b

Question 36

What does somatotopic mean?

Answer 36

-the map of what is for what -areas of the skin that are next to eah other are innervated ny neurons next to each other= in the somatosensory cortex -almost a continous map

Question 37

What is the somatotopic order in the human somatic sensory cortex?

Answer 37

- how much is devoted to one depends on how sensitive - so hands a lot!

Question 38

What is this?

Answer 38

- homonculus - scaling of importance - how much of the cortex devoted to each part

Question 39

What are the relationships between the different areas of the primary somatic sensory cortex?

Answer 39

each of those regions has to have its own map - functionally separated - 3a= size and shape - most input to 3b - some info interchange

Question 40

What happens with the information from individual receptors in the primary somatic sensory cortex?

Answer 40

-still separated even here the ruffini will be elswhere than pacini

Question 41

What happens to the cortical map of a digit if it is amputated?

Answer 41

-the space devoted to it will be devoted to the others

Question 42

What is the story with the lactating rats?

Answer 42

rats before and after they have babies -how much space devoted to the feeling the nipples - when lactating then much more space devoted to it -plastic, the mapping

Question 43

Can using one digit a lot, influence the mapping for it?

Answer 43

-if you stimulate a finger more you can expand the space devoted to it

Question 44

How much of the total do the Ruffini receptors make up and how big are their receptive fields?

Answer 44

20% -stretch sensitive -larger