The thalamus is a meeting point of a great number of neurons. It functions as a relay station between the neocortex and the subcortical structures. It has a great number of relay cells. The functioning of these cells show the remarkable property that the forbidder is forbidden to forbid!

Figure 1. The relay cell of the Thalamus. Dots are excitatory vesicles and flatted disks inhibiting vesicles. (After Steriade, 1987)

Explanation of the figure. The action potential in fiber A cause the release of a transmitter from RL vesicles that excites three cells at the same time. It excites cell Rd and the two inhibitory neurons I1d and I2d. This results in the following cascade of events. The inhibitory neuron inhibits the cell Rd, but this inhibition is quickly removed because the inhibitory neuron I2d inhibits on his turn interneuron I1d, thus disinhibiting Rd. The forbidder is forbidden to forbid! This process allows the relay cell to tranfer fast (200 m/sec and even faster) messages with high fidelity.

Another remarkable property of the thalamus is to relay a visual stimulus in the first instance to subcortical structures (amygdala). This quick transmission allows the brain to respond to the possible dangerous situation. But in the meanwhile the thalamus sends the visual information to the visual cortex. This information will provide a more accurate representation of the detail of the stimulus, but it takes longer. In a situation of danger it is very useful to have an immediate crude response.

When a hiker walks in the wood is observing an object that might involve a dangerous situation. It could be anything also a snake or a non-dangerous branch of a tree. The thalamus takes no risk and sends the message directly to de emotional centers in the lower part of the brains and from there to the hart ( to increase the blood pressure) and the muscles to run away from the danger. This is survival mechanism built in our brain.