Detection and Central Processing of Touch

Question 1

what is the dorsal column medial lemniscus pathway

Answer 1

major afferent pathway for fine discriminatory touch pressure, vibration and conscious proprioception.
Each area of our body is subserved by a particular peripheral nerve that corresponds to a particular part of the brain so it knows what part of our body is involved

Question 2

What properties are encoded by touch?

Answer 2

Spatial dimensions - size, shape, weight
Surface compliance - hard or soft
Surface texture - smooth or rough
Motion - speed and direction
Object recognition
Mechanical stimulation - vibration, pressure, stroking or prodding
Amalgamated with other sensory information reach the brain e.g visual, auditory, temperature and proprioception

Question 3

what are the sensory receptors in the skin?

Answer 3

Pacinian corpuscle
Meissner’s corpuscle
Ruffini’s ending
Merkel’s disks

Question 4

why do we need different touch receptors?

Answer 4

Different receptive field areas encode fine and broad spatial information
Specialise for dynamic and static sensitivity
Motion sensors - rapidly adapt to stimuli
Pressure sensors - slowly adapt to stimuli
Different sensory thresholds extend range of intensity encoded
Parallel processing of different information from receptors allows the brain to process many features at once - assists speed of identification

Question 5

what is the significance of different sized receptive fields

Answer 5

RF- spatial location ver which it responds to an appropriate stimuli
Different neurons have different but overlapping RFs
Helps to give very fine identification within RFs
Large RF - allow detection of changes over a wider area, but leads to less precise perception
Small RF - allow detection over a small area but with precise perception
RFs field changes with body position
Body cannot discriminate between two points that lie in the same receptive field.
Most sensitive: fingers, face, feet.

Question 6

what are the properties of Merkel’s disks ?

Answer 6

Located in the epidermis, where they are aligned with the papillae that lie beneath the dermal ridges.
Very dense in the fingertips, lips and external genitalia
Stimulation of these receptors gievs a sensation of light pressure.
Extremely sensitive, very small receptive fields useful for fine touch.
Static slowly adapting pressure detector (frequency range 0.3-3 Hz)
Useful in small object discrimination features such as static shapes, edges and rough textures.
Merkel’s disks are stimulated by the angels, the points and the curves and provide the spatial characteristics of the braille symbols

Question 7

What are the properties of Meissener’s corpuscles?

Answer 7

Lie between the dermal papillae just beneathe the epidermis of fingers, palms and soles of feet.
Elongated receptor formed by connective tissue that comprises several lamellae of schwann cells
Centre of the capsule contains one afferent fibre that will contain the mechanical receptors.
Detection motion on surface, detect change in texture (vibrations around 3-40 Hz) and adapt rapidly.
Small receptive fields
They are sensitive to shape and rough textural changes in exploratory and discriminatory.
Meisner’s corpuscle supply the temporal or dynamic information of the fingertips moving over the dot in braille

Question 8

what is the effect of age on discriminatory touch?

Answer 8

As people age their ability to discriminate between two points get much worse - poor discrimination thresholds
Decrease in density and distribution of receptors with small RFs

Question 9

why do we need slow adapting receptors?

Answer 9

In slowly adapting receptors e.g Merkel’s discs and Ruffini’s endings, the firing rate reflects the absolute level of indentation.
Intensity of stimulus is encoded in frequency of action potentials - force.
The firing rate can also reflect size/ shape of the indenting object.

Question 10

why do we need fast adapting receptors

Answer 10

In rapidly adapting receptors e.g Meissner’s corpuscle and Pacinian corpuscles, the firing rate can reflect the speed of indentation.
Temporal pattern.
Stops firing action potentials when the stimulus is constant.
Allows a response to new changes in sensory input to be encoded.

Question 11

what are the properties of Ruffini’s Endings?

Answer 11

Enlarged dendritic endings with elongated capsules
Spindle shaped receptor sensitive to skin stretch and contributing to the sense of movement and control of the finger
Highest in density around the fingernails where they are useful in monitoring slippage of objects along the surface of the skin which allows you to modulate the grip on your object.
Respond to sustained pressure and show very little adaptation
Have a very large receptive field
High frequency range 14-400Hz
Receptors pick up slightly different frequency ranges
Combination of all receptors together in parallel pathways going up to the brain that give us all the information we need to identify an object.

Question 12

how does tactile object recognition work?

Answer 12

Working hypothesis of how we recognise objects
Eyes closed. The shape of your hand (proprioception) and stretch of the skin help to identify object shape.
Different hand shapes for pickling up different objects. Also set up into the brain in different parallel pathways.
haptics - active touching and exploration
Information sent to the brain about any object is in the form of action potentials from the different sensory receptors
The brain then puts this complex fragmented pattern of temporal and spatial information together to give the perception of an object.

Question 13

What are the properties of Pacinian corpuscles?

Answer 13

Located in subcutaneous tissues
Detection of motion, deep pressure and high frequency vibrations (10-500Hz)
Respond to pressure changes such as when you are grasping and releasing an object
Involved in the discrimination of fine surfaces (less than 1 micron) running against the skin as this creates high freq vibrations
Stimulation of the fibres produces a sensation of vibration or tickle
Rapidly adapting
Large receptive field

Question 14

How is information about touch processed in the CNS?

Answer 14

Dorsal column medial lemniscus pathway
The brain knows exactly where on the body the information has originated from
Peripheral nerves come into the spinal cord at a very specific place
Dermatome: area of the skin that is supplied by a single spinal nerve
Shingles: varicella-zoster virus reactivation along a dermatome - helps to visualise them
31 pairs of spinal nerves

Question 15

How does sensory information enter the spinal cord?

Answer 15

Sensory fibres entering the spinal cord via a given dorsal root convey information from a distinct strip of body surface.
Sensory receptor (pacinain corpuscle) → sensory nerve → spinal cord → dorsal columns → brain
Cell bodies congregate in dorsal root 
Dorsal column send information about joint sensation, vibration, pressure, discriminatory touch.

Question 16

What happens if the sensory pathway is damaged?

Answer 16

Advanced syphilis: the whole of the dorsal columns becomes hardened so aren’t able to transmit information, person becomes reliant on crude touch.
Crude touch has a very large receptive field.
Free nerve endings.
Pathway that is part of the anterolateral system.
Only detect that you’ve been touched but not where.

Question 17

What are the dorsal column nueclei?

Answer 17

cuneate nucleus - information from T6 and above
Gracilis nucleus which deals with T7 and below
All the information is kept separate and comes up in parallel pathways from the major receptors from the individual dermatomes in the skin so the brain knows where it comes from

Question 18

What is the touch pathway?

Answer 18

Dorsal column medial lemniscus system
3 neuron relay with two synapses
stimuli –> sensory neurons –> dorsal column nuclei (synapse in medulla in brainstem) decussates –> thalamus (synapse) –> primary somatosensory cortex

Question 19

what circuit phenomenon make information ‘sharper’?

Answer 19

The neuron that is stimulated the most sends out collaterals which inhibit the neurons either side of it
The information that goes up to the somatosensory cortex is clean because the noise from the side is inhibited
Contacts GABAergic interneurons which cause an IPSP
Voltage-gated Ca2+ channels don’t open so neurotransmitter is not released.
Lateral inhibition
Neurons either side of the edge are inhibited so that firing rates are driven below normal
Gives good contrast and increases resolution - can find point of pin although don’t see absolute signal as this has been dampened

Question 20

what does somatotopic mapping mean?

Answer 20

Disproportionate cortical representation of some parts of the body
Hand, mouth and face disproportionately represented.
Areas that perform well in the two point discrimination test, so have very small RFs and very densely packed small RF receptors need more cortical space to process the information
Cortical magnification of the areas that are important for fine discriminatory touch
Hand area is very close to the face - phantom limbs

Question 21

what is the vertical column hypothesis?

Answer 21

Information is organised into vertical cortical columns to begin the processing of information from the receptors
All slow adapting receptors from D1 come into one particular place from a very selected area of skin
All rapidly adapting receptors went into another area
These areas appear to be in stripes of alternate slow and fast repeatedly over the area of the cortex that subserved those digits
Single neurons within the columns receive inputs from 300-400 mechanoreceptors of a single type due to convergence
All neurons within a column receive inputs from the same local area of skin

Question 22

What is area 3B

Answer 22

the primary somatosensory cortex
All information from the peripheral and touch receptors come in here
If stimulated, humans report sensations in very particular parts of their body
If lesioned, using ablation studies, that person no longer has a conscious perception of any touch
area 3B sends information to area 1 about texture and sends size and shape information to area 2 - hierarchical processing

Question 23

What happens if area 1 is lesioned?

Answer 23

lose the ability to perceive texture

Question 24

what happens if area 2 is lesioned

Answer 24

lose the ability to perceive shape and size through touch

Question 25

what is binding theory?

Answer 25

How the brain binds information together to come up with a perception of an object
hierarchical processing
Sending information across the brain so the RFs become larger and the modality specificity diminishes

Question 26

how is the posterior parietal cortex involved in touch?

Answer 26

somatosensory processing
Higher order processing
Final place where separate aspects of the stimulus come together to form a meaningful object - where the binding occurs
RFs even larger, character changed again, become very complex
Incorporated information from other sensory systems
Stimulation of areas of 5 and 7 give rise to sensations to whole objects - real sensations rather than parts of the body

Question 27

what is stereogenosis?

Answer 27

The cerebral cortex uses signals from all the different receptors to build up a perception of size, shape, texture, mass and temperature of an object to identify - process of stereognosis
Damage to the posterior parietal cortex → astereognosis
Inability to recognise objects by feeling them with their eyes shut
Sense of touch is normal, they can describe attributes of the object but are unable to identify object
Pairs of spinal nerves - if only one side of nerve/brain is damaged only lose the sense of touch on the opposite side of your body
Damage is limited to the side contralateral to the damage
Object can be easily identified by sound and vision - processed by different parts of the brain which are in tact so work normally

Question 28

How are sensory maps plastic?

Answer 28

Sensory maps are dynamic and can change, subject to the amount of sensory input
territory invasion following disuse
If put back in action the space is reclaimed
The collateral sprouts will die back
overuse of areas results in larger cortical representations
more pronounced in children as brains are more plastic

Question 29

How can we learn about cortical maps from amputees?

Answer 29

Individual had lower arm amputated reported when he touched different parts of his face it felt like he was touching his arm
When sensory information was no longer coming in from the hand meant that areas associated with the face sent out collateral sprouts which now use the cortical area that used to process the information from the arm as its unused space.
fMRI - when face is stroked, the area that used to belong to the thumb starts to light up. Disappears overtime.
Maps in humans are very dynamic

Question 30

How is tactile acuity enhances in blind people?

Answer 30

People who don’t have vision can use touch to compensate
Practise theory - touching more makes you better at touching
those with the highest tactile experience had the highest tactile acuity

Question 31

summary

Answer 31

Receptors involved in detection of discriminatory touch and vibration
Receptive fields, adaptation rates, stimulus frequencies and location in glabrous skin.
Anatomical pathway to the cortex. Processing within the cortex, vertical columns, converging information and more complex receptive fields as information is deciphered. Role of lateral inhibition at both synapses.
Role of the posterior parietal cortex in identification of the object
Cortical plasticity and its importance in normal function.