In chapter 9 we introduced the cell's responses to external stimuli. This was the third apex of the internal-state/gene-expression-pattern/stimulus-response triangle by which we characterised "livingness". Now we shall introduce the brain's responses to external stimuli. This principle forms the third apex of the brain-state/neuronal-activity-pattern/stimulus-response triangle by which we might characterise "mind".
Just as a cell interacts with its environment via receptors on the surface membrane, so the brain interacts with its environment through specialised sensors. (The "environment" of the brain includes the rest of the body as well as the outside world.) So now we can extend our analogy between cell and brain:-
Sensors such as light-sensitive cells in the retina, vibration-sensitive cells in the inner ear and the pressure-sensitive cells in major arteries all activate or inhibit neurones, sending impulses to the brain. The mechanism by which an external stimulus is converted to a change in neurone firing rate can be complicated52 but the effect is simple: there is a change in the frequency of action potentials in neurones connecting the sensor to the brain.
52 For example, consider a rod cell in the retina. When the cell is dark-adapted, its membrane sodium channels are jammed open by a "molecular wedge" (a cyclic nucleotide). In this state, the cell is active. It releases inhibitory neurotransmitters and prevents signals travelling to the brain. When light strikes the cell, a membrane
Cross-talk among neuronal pathways can take place at all levels between the sensor and the cerebral cortex. A signal to which the brain is primed to respond, perhaps by memory (facilitated synapses), can attenuate other simultaneous signals. For instance, you can switch attention from one nearby conversation to another during a noisy party. A woman can sleep through a thunderstorm but wake up when her baby cries. These examples involve pathway cross-talk in the higher parts of the brain. Near the sensor itself, processes such as lateral inhibition sharpen the focus of a signal pathway. Lateral inhibition works roughly as follows. Suppose an external stimulus activates five sensor cells: A and E rather weakly, B and D moderately and C strongly. At the first synapse after the sensory surface, each cell stimulates its own postsynaptic neurone but inhibits those on either side. Thus, although five sensory cells are activated, only the neurone from C carries information to the brain.
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