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Neurophysiology > Detection and central processing of touch > Flashcards

Detection and central processing of touch Flashcards

1
Q

What is the dorsal column-medial lemniscus pathway

A

The major afferent pathway for fine discriminatory touch, pressure, vibration and conscious proprioception

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2
Q

What properties of objects are coded for by sense of touch

A

Spatial dimensions, surface compliance (hard/soft), surface texture, motion- all combine for object recognition

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3
Q

What are different types of mechanical stimulation

A

Vibration, pressure, stroking or prodding

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4
Q

What is the proportions of different mechnoreceptors in the hand

A

40% Meissener’s, 25 % Merkel’s, 20% Ruffini’s, 1-15% Pacinian

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5
Q

Benefits of multiple touch receptors

A

Different receptive field sizes, can be specialised for dynamic and static sensitivity, broader range of intensities, parallel processing

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6
Q

What are the consequences of large receptive fields

A

Allow detection of changes over a wider area, leads to less precise perception

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7
Q

What are the consequences of small receptive fields

A

Allow detection over a small area, but with precise perception

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8
Q

What technique can be used to record sensory fields

A

Microneurography- single sensory axon recordings in the hand allow mapping of single receptive fields

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9
Q

How does the distribution of receptor types differ across the hand

A

eg high conc of Merkel’s and Meissner’s in fingertips allows fine discrimination

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10
Q

Why does 2 point discrimination ability differ across the body

A

Acuity of touch sensatino varies across the body as receptor field size changes- smaller fields allow greater acuity as points don’t fall in the same receptive field

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11
Q

Why does the spatial acuity of skin ni the fingertip deteriorate noticably with age

A

May be due to a decrease in the density and distribution of Meissner’s corpuscles and Merkel’s disks in the skin

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12
Q

What types of mechanoreceptors are specialised for dynamic or static sensitivity

A

Motion sensors that rapidly adapt to stimuli, pressure sensors that slowly adapt to stimuli

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13
Q

What is the benefit of lots of differenr mechanoreceptors with different sensory thresholds

A

extends the range of intensities encoded

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14
Q

What is the benefit of parallel processing from different mechanoreceptors

A

Parallel processing of different information from receptors allows brain to process many features at once, speeding up identification

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15
Q

Which mechanoreceptors are slowly adapting

A

Merkel’s disks and Ruffini’s endings

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16
Q

What does the firing rate in slowly adapting receptors reflect

A

The absolute level of indentation- intensity of stimulus is encoded in AP firing rate
AP firing rate can also reflect the size/shape of the indenting object

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17
Q

What mechanoreceptors are rapidly adapting

A

Meissner’s corpuscle and Pacianian corpuscle

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18
Q

What does the firing rate reflect in rapidly adapting receptors

A

Speed of indentation- stops firing APs when stimulus is constant, can encode responses to new changes in sensory input
Provides a temporal pattern

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19
Q

What qualities of the object can be knwon by combining responses of rapidly and slowly adapting receptors

A

Dynamic and static qualities about a stimulus

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20
Q

Where are Merkel’s disks located in the skin

A

Epidermis, aligned with the papillae that lie beneath the dermal ridges

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21
Q

Where are Merkel’s disks found in the body

A

Very dense in fingertips, lips and external genitalia

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22
Q

What sensation does stimulating Merkel’s disks give rise to

A

Light pressure

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23
Q

What are Merkel’s disks suited for

A

Extremely sensitive with very small receptive fields

Useful for fine touch and small object discrimination features eg static shapes/edges/rough textures

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24
Q

What are the adaptive properties of Merkel’s disks

A

Static, slowly adapting pressure detector (frequency range 0.3-3Hz, created by rubbing your features over an object)

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25
Q

Which mechanoreceptors are superficial and which are deep

A

Meissner’s corpuscle and Merkel cells- superficial

Pascinian corpuscle and Ruffini’s endings- deep

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26
Q

How do superficial vs deep mechanoreceptors have different recptive field sizes

A

Superficial have much smaller receptive fields, deep much larger

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27
Q

Where are Meissner’s corpuscles located in the skin

A

Lie between the dermal papillae just beneath the epidermis

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28
Q

Where are Meissner’s corpuscles located in the body

A

Fingers, palms and soles of feet

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29
Q

What are Meissner’s corpuscles suited for

A

Rapidly adapting, small receptive fields

Sensitive to shape and rough textural changes in discriminatory touch, detect motion on surfaces

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30
Q

What sensation does stimulation of Meissner’s corpuscles give rise to

A

Flutter (vibrations around 3-40Hz)

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31
Q

What mechanoreceptors does Braille reading use

A

Receptors with small receptive fields in high density in the fingertips- Merkel’s disks and Meissner’s corpuscles

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32
Q

What corresponds exactly to the dimensions of Braille characters

A

The threshold of sensitivity on the skin

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33
Q

What do Merkel’s disks and Meissner’s corpuscles both provide info about when reading Braille

A

Merkel’s- stimulated by angles/points/curves so provide the spatial characteristics of the symbols
Meissner’s- temporal info of fingertip moving over dot

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34
Q

Where are Ruffini’s endings located in the skin

A

Oriented parallel to stretch lines in the skin

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35
Q

What are Ruffini’s endings suited for

A

Large receptive field, slow adapting

Kinesthetic sense of/control of finger position and movement, grip and limb movement

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36
Q

What sensation does stimulation of Ruffini’s endings give rise to

A

Stretching (vibrations around 15-400Hz)

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37
Q

What features of the hand encode object shape

A

Hand posture and skin stretch

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38
Q

What is the working hypothesis of tactile object recognition

A

Each finger touching the object receives cutaneous input that provides info about local shape and texture

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39
Q

Where are Pacinian corpuscles located in the skin

A

Subcutaneous tissues (deep)

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40
Q

What are Pacinian corpuscles suited to

A

Large receptive field, rapidly adapting
Respond to pressure changes and motion detection, high frequency vibration created by discrimination of fine surfaces running against the skin

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41
Q

What sensation does stimulation of the Pacinian corpuscles give rise to

A

Vibration (10-500Hz)

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42
Q

What is tactile discrimination

A

Uses all 4 receptors simultaneously to investigate an object by touch alone

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43
Q

What is haptics

A

Active touching and exploration

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44
Q

What info does the brain combine to give a perception of an object by tactile discrimination

A

Brain combined complex fragmented pattern of temporal and spatial info together from all the different receptors in the form of APs

45
Q

How many neurons are in the relay sending touch info to the brain

A

3 neuron relay with 2 synapses

46
Q

When does lateral inhibition of sensory synapses occur

A

When several sensory neurons next to one another are costimulated by eg objects touching the skin

47
Q

How does lateral inhibition occur in mechanoreceptors

A

There is mutual inhibition between receptors using GABA-expressing interneurons- they cause an IPSP that prevents voltage-gated Ca2+ channels from opening and prevents release of neurotransmitter from secondary neurons

48
Q

What is the benefit of lateral inhibition in touch

A

Info send to somatosensory cortex is clean, as noise from the neurons peripheral to the stimulus is inhibited
Improves contrast and resolution, allows Braille reading

49
Q

What is Wilder Penfield’s ‘cerebral cortex of man’

A

Used electrodes to stimulate somatosensory cortex in awake epileptic patients, used where they felt sensations to map the body surface onto the somatosensory cortex

50
Q

What is the concept of cortical magnification

A

The size of cortical area devoted to receiving info from a spot on the skin reflects the density of receptors there- doesn’t represent the correct proportions of the skin area

51
Q

What is the vertical column hypothesis

A

Mountcastle (1957)- microelectrode recording and mapping studies show info in the cortex is organsied into vertical columns (300-600 microns across)

52
Q

Describe the input to neurons in the columns

A

Single neurons receive inputs from 300-400 mechanoreceptors of a single type through convergence
Each column only receive input from one local area of skin

53
Q

Evidence of the vertical column hypothesis in owl monkeys

A

Each adjacent digit is represented by an adjacent area of cortex
Within the area of each cortical finger representation are alterating columns of rapidly/slowly adapting sensory responses (Kaas et al, 1981)

54
Q

What does horizontal connectivity between vertical cortical colmumns allow

A

Parallel processing of features of the stimulus

55
Q

Which of Brodmann’s areas is the primary somatosensory cortex

A

Broadmann’s area 3b

56
Q

Where do parallel streams of somatosensory info start to converge

A

Somatosensory cortex

57
Q

What happens to sensory info as the brain binds it together

A

Receptive fields become larger, modality specificity diminishes, cortical neural responses become more complex

58
Q

What areas of the cortex does 3b send info to

A

Texture info->area 1

Shape and size info -> area 2

59
Q

Lesion ablation evidence of the sending of different stimulus info to different cortex areas

A

Lesion of 3b- loss of texture, shape and size
1- loss of texture
2- loss of shape and size

60
Q

What happens in the posterior parietal cortex

A

Binding theory- separate aspects of a stimulus (including info from other sensory systems) come together to form a meaningful object
Receptive fields get larger and incredibly complex

61
Q

What areas of the brain is the posterior parietal cortex located in

A

Brodmann’s area 5 and 7 (association areas, involved in higher order processing)

62
Q

Evidence of the posterior parietal cortex being located in areas 5 and 7

A

Stimulation of areas 5 and 7 give rise to sensations of whole objects

63
Q

What is the process of stereognosis

A

The cerebral cortex uses signals from different receptor types and adjacent receptive fields to build up a perception of the size, shape, texture and mass of an object to identify it

64
Q

What is the result of damage to posterior parietal cortex

A

Asterognosia- inability to recognise objects by feeling them with eyes clsoed

65
Q

What is the behaviour of patients of asterognosia

A

Sense of touch is normal- can describe attributes of the object but can’t combine them to recognise the object
Object can be easily identified by sound or vision (processed by different brain areas)

66
Q

Study showing example of territory invasion following disuse

A

When digit 3 of owl monkey is amputated so sensory input from digit 3 is silent, the area representing digit 3 in the somatosensory cortex disappears due to extensive remapping of remaining digits (Jenkins, 1990)

67
Q

Study showing functional expansion of a cortical representation

A

Owl monkey trained to use 3 digits to rotate a disk to get food
Cortical area representnig these digits expanded at the expense of the other digits, and average receptor field size was smaller thus more precise (Jenkins, 1990)

68
Q

What do Jenkin’s (1990) studies of cortical remapping in owl monkeys suggest

A

Cortical maps are dynamic and adjust depending on the amount of sensory input

69
Q

What happens in the somatosensory cortex following limb amputation

A

The areas in the cortex near to the one now receiving no input will ‘remap’ this cortical region

70
Q

Study first showing somatosensory cortex remapping following amputation

A

Ramachandran et al (1992)- stroking different face areas led to the perception of being touched on different parts of the missing limb, fMRI showed the regions previosly for the missing limb were now stimulated by stroking the face

71
Q

Study showing cortical remapping in Braille readers

A

Supported tactile experience hypothesis- blind individuals outperformed sighted individuals on all fingers but not lips, proficient blind readers outperformed blind non-readers

72
Q

What is the tactile experience hypothesis

A

The increased tactile experience in the fingers when frequently Braille reading increases tactile acuity in blind people

73
Q

What is the visual deprivation hypothesis

A

Blind individuals gain increased tactile acuity generally, in fingers, lips etc

74
Q

What does the Braille reader tactile acuity study suggest

A

Increased practice of reading Braille leads to dynamic changes in corticalmaps meaning we have more brain processing power for the info coming in

75
Q

What are mechanoreceptors sensitive to

A

Physical distortions eg bending, stretching, vibration

76
Q

What sort of bodily things to mechanoreceptors monitor

A

Skin contact, pressure in heart and blood vessels, stretching of the digestive organs and urinary bladder

77
Q

Evidence for the different size receptive fields of different mechanoreceptord

A

Valbo et al (1984)- the stimulus probe was moved around the ski surface, allowing them to map the receptive field of a single mechanoreceptor, M/M were small, P and R were big

78
Q

How can hairs act as a receptor system

A

Follices embedded in the skin are richly innervated by free nerve endings, bending the hair deforms the follicle and surrounding tissue and nearby nerve endings, affecting their AP firing rate

79
Q

Study showing the different frequency sensitivity of different mechanoreceptors

A

Schmidt (1987)- the skin was indented w a pressure probe at various frequencies while recording from the nerves, amplitude increased until it generated APs

80
Q

What is the special nerve ending surrounding the Pacianian corpuscle

A

Football-shaped capsule with 20–70 concentric layers of connective tissue surrounding an axon terminal

81
Q

How does the Pacinian corpuscle capsule work to generate APs

A

When the capsule is compressed, energy is transferred to the nerve terminal which deforms its membrane, opening mechanoreceptive channels

82
Q

How does the Pacinian corpuscle capsule allow rapid adaption

A

The capsule layers are slick with viscuous fluid between them- if stimulus pressure is maintained, the layers slip past one another and transfer the stimulus energy so the axon terminal is no longer deformed- when pressure is released, the events reverse themselves

83
Q

Study showing what happens if the Pacinian corpuscle is removed

A

Loewenstein (1960s)- naked nerve terminal is less sensitive to vibrating stimuli and more to steady pressure, showing the capsule determines what the corpuscle is sensitive to

84
Q

Recent study of how Merkel’s disks/their associated nerve terminal work in mechanosensation

A

A mechanosensitive channel called Piezo2 in the Merkel cell opens to depolarise the cell, triggering synaptic release of an unknown transmitter from the cell that excites the nearby nerve ending
The nerve ending is also mechnically sensitive because of a second ion channel in its own membrane

85
Q

Which fibres carry information about touch sensation

A

Abeta

86
Q

What 2 branches do the Abeta axons split into when they enter the dorsal horn

A

One branch synapses in the deep part of the dorsal horn on second-order sensory neurons for rapid/unconscious reflexes
One branch ascends straight to the brain

87
Q

In which columns do the Abeta axons first ascend to the brain

A

Enter the ipsilateral dorsal column, the white matter tract medial to the dorsal horn, that carries info about tactile sensation and limb position to the brain

88
Q

What comprises the dorsal columns

A

Primary sensory axons and second-order axons from neurons in the spinal grey matter

89
Q

Where do the axons of the dorsal column terminate

A

Dorsal column nuclei at the junction of the spinal cord and the medulla

90
Q

What happens from the dorsal column nuclei

A

Axons from cells in the dorsal column nuclei arch toward the ventral and medial medulla and decussate- from then on, the somatosensory system is contralateral
Ascend within a white matter tract called the medial lemniscus

91
Q

How do axons travel upwards through the medial lemniscus

A

Medial lemniscus rises through the medulla, pons and midbrain, its axons synapse on neurons of the ventral posterior nucleus of the thalamus

92
Q

What happens once the thalamus has been reached

A

Thalamic neurons of the VP nucleus project to specific regions of the primary somatosensory cortex S1

93
Q

How is somatosensory info altered as it passes through synapses in the brain

A

Inhibitory interactions between adjacent sets of inputs in the DC-ML enhance the responses to tactile stimuli
Thalamus and DCN can also enhance responses to tacile stimuli as the synapses in their nuclei can change their strenght depending on recent activity

94
Q

How can mutually inibitory conections between cortical columns have an effect on sensory discrimination

A

Each column is surrounded by neighbours of different modality but similar location, so inhibitory connections between columns ca accentuate differences in their activity via a kind of lateral inhibition to help refine sensory discrimination

95
Q

How can sideways inhibition within cortical columns have a effect

A

Can give rise to directional responses often seen in S1 to cutaneous stimuli moving in specific directions

96
Q

How do different areas 1 2 and 3 correspond to different dept of receptor

A

2 and 3a- deep receptor predominance

1 and 3b- superficial receptor predominance

97
Q

How are the cortical maps of the somatosensory cortex changeable within very short time frames

A

Bandaging a monkey’s hand for only a few hours is sufficient to cause a reduction in the hand’s representation in the cortical map

98
Q

How is the somatosensory cortex layered

A

The thalamic inputs terminate mainly in area IV, and the neurons of layer IV project to cells in the other layers

99
Q

What is somatotopy

A

The mapping of the body’s surface sensations onto a structure on the brain

100
Q

What is a homunculus

A

A name sometimes used for a somatic map, maps roughly resembles a body with legs and feet at the top of the postcentral gyrus and head at the opposite lower end of the gyrus

101
Q

What are the relative sizes of different body parts of the homunculus

A

Mouth tongue and fingers are incongruously large, while the trunk, arms and legs are tiny

102
Q

How is the size of relative areas on the somatotopic map related to importance

A

Larger and more used areas have a larger area on the somatotopic map eg mouth area is large as tactile sensations are important in producing speech, and your tongue is the last line of defence when deciding if something is nutritrious or poisonous/dangerous

103
Q

Study showing how cortical maps differ across species

A

Woolsey and Durham (1985)- whiskers of rodents have a large share of S1 while digits of the paws take up little
Sensory signals from each vibrissae go to one barrel in S1, 5 rows of barrels match 5 rows of facial vibrissae

104
Q

Evidence showing somatotopic mapping alters according to life experiences

A

Violinists continually finger the strings with their left hand- fMRI of S1 shows amount of their cortex devoted to their Lh is greatly enlarged, an exaggerated version of the continual remapping process in everyones brain every day

105
Q

What is another disorder similar to astereognosia that can result from damage to posterior parietal areas

A

Agnosia- the inability to recognise objects even though simple sensory skills are normal

106
Q

Example of neglect syndrome from damage to parietal cortical area

A

Springer and Deutsch (1989)- a patient who had a stroke in the right posterioir parietal cortex couldn’t reproduce many of the features on the let side of a model drawing when asked to copy it- entire left visual field is ignored

107
Q

What is the posterioir parietal cortex necessary for

A

Perception and interpretation of spatial relationships, movement planning, accurate body image

108
Q

Evidence for contralateral processing in the somatosensory system

A

Fritsch and Hitzig (1870)- electrical stimulation of different specific parts of the cortex in dogs produces contractions of different contralateral muscles

109
Q

How does the primary motor cortex also have somatic mapping

A

Parts of the body used in tasks requiring precision and fine control eg face and hands have proportionately larger representation in the motor map

Neurophysiology (23 decks)

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