Question

Mr. Phettuccine is a 65 year old man who has been blind from birth and relies heavily on reading via Braille (touch-reading using raised dots). He has recently been diagnosed with a neurodegenerative disease and is worried it will affect his ability to read using Braille.

Using your knowledge of touch sensory physiology (M1L6), please answer the following questions with one or two (1-2) sentences each in the text box below.

Part A. Describe how lateral inhibition allows for localization of a touch stimulus. (1.5 marks)

Part B. If the neurodegeneration has reduced his touch lateral inhibition, what do predict would happen to his Braille reading? (1 mark)

Part C. What type of touch receptors do you predict would most be affected? Explain your reasoning. (1 mark 1).

Answer 1

Lateral inhibition in tactile discrimination.

Lateral inhibition is a phenomenon by which stronger inputs are enhanced and the weaker inputs of adjacent sensory units are simultaneously inhibited. Stimulus localization can be made more precise by lateral inhibition as explained:-

• When two stimuli are applied to the skin, in the absence of lateral inhibition they can be recognised as separate only e if they are applied in receptive fields that are separated from each other by a non stimulated receptive field, otherwise the stimuli will produce equal discharge in all these neurons.
• With lateral inhibition, the neuron with the receptive field in the centre is presynaptically inhibited by collaterals from the neurons with receptive field located laterally. As a result, the receptive field in the centre does not fire and two stimuli are perceived.

Braille reading is based only on the tactile localization phenomenon, which occurs with the help of lateral inhibition. If lateral inhibition gets damaged, the stimili will be perceived as one and so effective tactile localization will not happen. This will lead to inability to read using Braille language.

Merkel cells are the receptors present in the superficial layer of the skin. They are very sensitive to edges and corners of the stimuli. These are the receptors which are responsible for Braille reading. These will be most affected if lateral inhibition gets defected.