BBB - Brief Notes on the Sense of Touch

References are to Figures and pages in chapter 9 of Book 2.

Tactile information reaches the cortex in the brain via pathways from sensory cells in the skin. In very simple terms, this can be summarised as follows:-

1. Each touch-sensitive neuron has nerve endings in the skin (Fig. 9.19). Receptive field of this cell is the region of skin which, if touched, results in a change in the firing rate of the cell. If the nerve ending is highly branched over a large area, the cell has a large receptive field - and vice versa (Figs. 9.20, 9.21 and last paragraph on p.252).
2. The cell's sensory axon runs to its cell body in a dorsal root ganglion of the spinal cord (Fig. 9.17). The axon runs on into the spinal cord itself.
3. Among other things (see p.250), the axon synapses, within the dorsal column of the spinal cord, with axons which ascend towards the brain (Fig. 9.18). These synapse in the medulla with axons running to the thalamus, where in the ventrobasal nucleus these synapse with neurons connecting to the sensory cortex.
4. If we consider a single interneuron in the ventrobasal nucleus (VBN) of the thalamus, there is a convergence of incoming sensory axons on to it. The pattern of this convergence results in the concentric receptive field of this VBN cell.
5. Consider recordings taken from a single VBN cell - as shown in Fig. 9.24 (cell A). Explanation of receptive field (RF):-
1. The RF is an area of skin which, if touched, results in a change in the firing rate of the cell.
2. The RF is circular with two concentric regions (Fig. 9.23). Tactile stimulation in the central circular area causes an increase in firing rate; stimulation in the outer ring area causes a decrease in firing rate.
3. Thus the two areas are antagonistic; stimulation across the whole RF means that the two areas cancel out and the firing rate stays at the background level.
6. This arrangement of antagonistic areas is termed lateral inhibition. It also occurs in most other sensory systems -- see especially vision, where retinal ganglion cells have similarly organized concentric antagonistic regions in their RF.
7. Why? What's the point of such an arrangement? The important aspect of sensory information is to signal when there is a change. Edges are important - they show the animal where things are. If an edge covers part of the concentrically organised RF, the imbalance between excitatory and inhibitory areas will result in a change of firing rate. Thus, a signal is registered indicating the "edge" = a change in stimulation level which can correspond to the boundary of an object. So, in the tactile system, each of these VBN cells can "tell" its destination cell in the cortex whether or not there's an edge.
8. Knowing where the edges are is much more efficient than having to record every single point in the sensory environment.

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