Detection and central processing of touch Flashcards
What is the dorsal column-medial lemniscus pathway
The major afferent pathway for fine discriminatory touch, pressure, vibration and conscious proprioception
What properties of objects are coded for by sense of touch
Spatial dimensions, surface compliance (hard/soft), surface texture, motion- all combine for object recognition
What are different types of mechanical stimulation
Vibration, pressure, stroking or prodding
What is the proportions of different mechnoreceptors in the hand
40% Meissener’s, 25 % Merkel’s, 20% Ruffini’s, 1-15% Pacinian
Benefits of multiple touch receptors
Different receptive field sizes, can be specialised for dynamic and static sensitivity, broader range of intensities, parallel processing
What are the consequences of large receptive fields
Allow detection of changes over a wider area, leads to less precise perception
What are the consequences of small receptive fields
Allow detection over a small area, but with precise perception
What technique can be used to record sensory fields
Microneurography- single sensory axon recordings in the hand allow mapping of single receptive fields
How does the distribution of receptor types differ across the hand
eg high conc of Merkel’s and Meissner’s in fingertips allows fine discrimination
Why does 2 point discrimination ability differ across the body
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
Why does the spatial acuity of skin ni the fingertip deteriorate noticably with age
May be due to a decrease in the density and distribution of Meissner’s corpuscles and Merkel’s disks in the skin
What types of mechanoreceptors are specialised for dynamic or static sensitivity
Motion sensors that rapidly adapt to stimuli, pressure sensors that slowly adapt to stimuli
What is the benefit of lots of differenr mechanoreceptors with different sensory thresholds
extends the range of intensities encoded
What is the benefit of parallel processing from different mechanoreceptors
Parallel processing of different information from receptors allows brain to process many features at once, speeding up identification
Which mechanoreceptors are slowly adapting
Merkel’s disks and Ruffini’s endings
What does the firing rate in slowly adapting receptors reflect
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
What mechanoreceptors are rapidly adapting
Meissner’s corpuscle and Pacianian corpuscle
What does the firing rate reflect in rapidly adapting receptors
Speed of indentation- stops firing APs when stimulus is constant, can encode responses to new changes in sensory input
Provides a temporal pattern
What qualities of the object can be knwon by combining responses of rapidly and slowly adapting receptors
Dynamic and static qualities about a stimulus
Where are Merkel’s disks located in the skin
Epidermis, aligned with the papillae that lie beneath the dermal ridges
Where are Merkel’s disks found in the body
Very dense in fingertips, lips and external genitalia
What sensation does stimulating Merkel’s disks give rise to
Light pressure
What are Merkel’s disks suited for
Extremely sensitive with very small receptive fields
Useful for fine touch and small object discrimination features eg static shapes/edges/rough textures
What are the adaptive properties of Merkel’s disks
Static, slowly adapting pressure detector (frequency range 0.3-3Hz, created by rubbing your features over an object)
Which mechanoreceptors are superficial and which are deep
Meissner’s corpuscle and Merkel cells- superficial
Pascinian corpuscle and Ruffini’s endings- deep
How do superficial vs deep mechanoreceptors have different recptive field sizes
Superficial have much smaller receptive fields, deep much larger
Where are Meissner’s corpuscles located in the skin
Lie between the dermal papillae just beneath the epidermis
Where are Meissner’s corpuscles located in the body
Fingers, palms and soles of feet
What are Meissner’s corpuscles suited for
Rapidly adapting, small receptive fields
Sensitive to shape and rough textural changes in discriminatory touch, detect motion on surfaces
What sensation does stimulation of Meissner’s corpuscles give rise to
Flutter (vibrations around 3-40Hz)
What mechanoreceptors does Braille reading use
Receptors with small receptive fields in high density in the fingertips- Merkel’s disks and Meissner’s corpuscles
What corresponds exactly to the dimensions of Braille characters
The threshold of sensitivity on the skin
What do Merkel’s disks and Meissner’s corpuscles both provide info about when reading Braille
Merkel’s- stimulated by angles/points/curves so provide the spatial characteristics of the symbols
Meissner’s- temporal info of fingertip moving over dot
Where are Ruffini’s endings located in the skin
Oriented parallel to stretch lines in the skin
What are Ruffini’s endings suited for
Large receptive field, slow adapting
Kinesthetic sense of/control of finger position and movement, grip and limb movement
What sensation does stimulation of Ruffini’s endings give rise to
Stretching (vibrations around 15-400Hz)
What features of the hand encode object shape
Hand posture and skin stretch
What is the working hypothesis of tactile object recognition
Each finger touching the object receives cutaneous input that provides info about local shape and texture
Where are Pacinian corpuscles located in the skin
Subcutaneous tissues (deep)
What are Pacinian corpuscles suited to
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
What sensation does stimulation of the Pacinian corpuscles give rise to
Vibration (10-500Hz)
What is tactile discrimination
Uses all 4 receptors simultaneously to investigate an object by touch alone
What is haptics
Active touching and exploration
What info does the brain combine to give a perception of an object by tactile discrimination
Brain combined complex fragmented pattern of temporal and spatial info together from all the different receptors in the form of APs
How many neurons are in the relay sending touch info to the brain
3 neuron relay with 2 synapses
When does lateral inhibition of sensory synapses occur
When several sensory neurons next to one another are costimulated by eg objects touching the skin
How does lateral inhibition occur in mechanoreceptors
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
What is the benefit of lateral inhibition in touch
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
What is Wilder Penfield’s ‘cerebral cortex of man’
Used electrodes to stimulate somatosensory cortex in awake epileptic patients, used where they felt sensations to map the body surface onto the somatosensory cortex
What is the concept of cortical magnification
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
What is the vertical column hypothesis
Mountcastle (1957)- microelectrode recording and mapping studies show info in the cortex is organsied into vertical columns (300-600 microns across)
Describe the input to neurons in the columns
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
Evidence of the vertical column hypothesis in owl monkeys
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)
What does horizontal connectivity between vertical cortical colmumns allow
Parallel processing of features of the stimulus
Which of Brodmann’s areas is the primary somatosensory cortex
Broadmann’s area 3b
Where do parallel streams of somatosensory info start to converge
Somatosensory cortex
What happens to sensory info as the brain binds it together
Receptive fields become larger, modality specificity diminishes, cortical neural responses become more complex
What areas of the cortex does 3b send info to
Texture info->area 1
Shape and size info -> area 2
Lesion ablation evidence of the sending of different stimulus info to different cortex areas
Lesion of 3b- loss of texture, shape and size
1- loss of texture
2- loss of shape and size
What happens in the posterior parietal cortex
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
What areas of the brain is the posterior parietal cortex located in
Brodmann’s area 5 and 7 (association areas, involved in higher order processing)
Evidence of the posterior parietal cortex being located in areas 5 and 7
Stimulation of areas 5 and 7 give rise to sensations of whole objects
What is the process of stereognosis
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
What is the result of damage to posterior parietal cortex
Asterognosia- inability to recognise objects by feeling them with eyes clsoed
What is the behaviour of patients of asterognosia
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)
Study showing example of territory invasion following disuse
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)
Study showing functional expansion of a cortical representation
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)
What do Jenkin’s (1990) studies of cortical remapping in owl monkeys suggest
Cortical maps are dynamic and adjust depending on the amount of sensory input
What happens in the somatosensory cortex following limb amputation
The areas in the cortex near to the one now receiving no input will ‘remap’ this cortical region
Study first showing somatosensory cortex remapping following amputation
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
Study showing cortical remapping in Braille readers
Supported tactile experience hypothesis- blind individuals outperformed sighted individuals on all fingers but not lips, proficient blind readers outperformed blind non-readers
What is the tactile experience hypothesis
The increased tactile experience in the fingers when frequently Braille reading increases tactile acuity in blind people
What is the visual deprivation hypothesis
Blind individuals gain increased tactile acuity generally, in fingers, lips etc
What does the Braille reader tactile acuity study suggest
Increased practice of reading Braille leads to dynamic changes in corticalmaps meaning we have more brain processing power for the info coming in
What are mechanoreceptors sensitive to
Physical distortions eg bending, stretching, vibration
What sort of bodily things to mechanoreceptors monitor
Skin contact, pressure in heart and blood vessels, stretching of the digestive organs and urinary bladder
Evidence for the different size receptive fields of different mechanoreceptord
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
How can hairs act as a receptor system
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
Study showing the different frequency sensitivity of different mechanoreceptors
Schmidt (1987)- the skin was indented w a pressure probe at various frequencies while recording from the nerves, amplitude increased until it generated APs
What is the special nerve ending surrounding the Pacianian corpuscle
Football-shaped capsule with 20–70 concentric layers of connective tissue surrounding an axon terminal
How does the Pacinian corpuscle capsule work to generate APs
When the capsule is compressed, energy is transferred to the nerve terminal which deforms its membrane, opening mechanoreceptive channels
How does the Pacinian corpuscle capsule allow rapid adaption
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
Study showing what happens if the Pacinian corpuscle is removed
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
Recent study of how Merkel’s disks/their associated nerve terminal work in mechanosensation
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
Which fibres carry information about touch sensation
Abeta
What 2 branches do the Abeta axons split into when they enter the dorsal horn
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
In which columns do the Abeta axons first ascend to the brain
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
What comprises the dorsal columns
Primary sensory axons and second-order axons from neurons in the spinal grey matter
Where do the axons of the dorsal column terminate
Dorsal column nuclei at the junction of the spinal cord and the medulla
What happens from the dorsal column nuclei
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
How do axons travel upwards through the medial lemniscus
Medial lemniscus rises through the medulla, pons and midbrain, its axons synapse on neurons of the ventral posterior nucleus of the thalamus
What happens once the thalamus has been reached
Thalamic neurons of the VP nucleus project to specific regions of the primary somatosensory cortex S1
How is somatosensory info altered as it passes through synapses in the brain
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
How can mutually inibitory conections between cortical columns have an effect on sensory discrimination
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
How can sideways inhibition within cortical columns have a effect
Can give rise to directional responses often seen in S1 to cutaneous stimuli moving in specific directions
How do different areas 1 2 and 3 correspond to different dept of receptor
2 and 3a- deep receptor predominance
1 and 3b- superficial receptor predominance
How are the cortical maps of the somatosensory cortex changeable within very short time frames
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
How is the somatosensory cortex layered
The thalamic inputs terminate mainly in area IV, and the neurons of layer IV project to cells in the other layers
What is somatotopy
The mapping of the body’s surface sensations onto a structure on the brain
What is a homunculus
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
What are the relative sizes of different body parts of the homunculus
Mouth tongue and fingers are incongruously large, while the trunk, arms and legs are tiny
How is the size of relative areas on the somatotopic map related to importance
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
Study showing how cortical maps differ across species
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
Evidence showing somatotopic mapping alters according to life experiences
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
What is another disorder similar to astereognosia that can result from damage to posterior parietal areas
Agnosia- the inability to recognise objects even though simple sensory skills are normal
Example of neglect syndrome from damage to parietal cortical area
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
What is the posterioir parietal cortex necessary for
Perception and interpretation of spatial relationships, movement planning, accurate body image
Evidence for contralateral processing in the somatosensory system
Fritsch and Hitzig (1870)- electrical stimulation of different specific parts of the cortex in dogs produces contractions of different contralateral muscles
How does the primary motor cortex also have somatic mapping
Parts of the body used in tasks requiring precision and fine control eg face and hands have proportionately larger representation in the motor map
