Criterios de Diagnóstico:

The Biological Basis of Thought and Perception. The Fundamentals

The brain is made of a vast number of richly interconnected neurons. There are two main parts to a neuron. It has a cell body, which is much like the body of any other type of cell. This takes care of the housekeeping, manages the genetic material and manufactures the proteins and other chemicals that are needed for its special role in the body. What is unusual about neurons is that they have projecting nerve fibres.

There are two kinds of nerve fibres projecting from the neuron cell body – input fibres and output fibres. The input fibres are called dendrites (from the Greek for tree), and receive electrochemical messages (both positive and negative) from many other neurons. The electrical charges make their way along the input fibres to the main body of the neuron. If a sufficiently strong positive charge builds up, the neuron fires and sends an electrical signal out along the output fibres (called axons). At the ends of the axons, the electrical signal causes the release of chemicals, which drift across small gaps to the next neuron’s input fibres, where they initiate either positive or negative electrical charges, which in turn pass to the cell body where they accumulate until they too are fired. In this way, very subtle and intricate ‘domino waves’ of nerve impulses are set in motion.

Neurons usually have more than one dendrite (input fibre). Along the length of the dendrites are numerous short spines, and on the ends of the spines are special sites (called receptors) which are capable of receiving the chemical messages from the axons of other neurons.

Each neuron usually has only one axon (output fibre) but this typically has many branches. At the end of each branch is a site we call the axon terminal, which, shortly after the cell fires, releases neurotransmitter chemicals, which drift across the small gap between the axon terminals of the fired neuron and the dendrite spine receptors on many connected neurons. This gap is called the synapse (from the Greek for point of contact or joint).

Figure 2.1 Simplified diagram of a neuron – inputs - switch - outputs.

Excitation and Inhibition

There are two main neurotransmitter chemicals – glutamate, which causes the receptors in the post- synaptic neuron’s dendrite spines to open, allowing positively charged ions to flow through this open channel into the neuron dendrite where a positive charge builds up and passes along the dendrite fibre to the cell body. The other neurotransmitter is called GABA and it has the reverse effect. It drifts across the synapse and opens different receptors, which allow negatively charged ions to flow into the neuron. The neuron will only fire when the balance of the many positive and negative charges in the cell body reaches a particular trigger level of positive charge. If it does fire, then the charge passes along the axon to the axon terminals, where it causes special chemical storage sites, called vesicles, to release the next cloud of neurotransmitter, which drifts across those synapses and opens channels

in the receptors of the next neurons in the network, and so on.

Figure 2.2 GABA and glutamate cross the synaptic gap. The cells that excite their neighbours by releasing glutamate tend to have long axons. The cells that inhibit cell firing in their neighbours through the release of GABA tend to have short axons and they group together into local inhibitory networks. This design principle of balancing long range excitation with local inhibition has interesting emergent properties. It damps the whole system down, so that an initial stimulus cannot escalate the network into a long drawn-out state of excitement. If that did happen, it would rapidly exhaust its electrochemical reserves and, more importantly, the system would no longer be able to discriminate between a really important signal and all the residual excitement and noise. With the GABA damping in place, important excitatory signals stand out clearly against the background. This is very clever. It produces a system which can make reasonably stable perceptions and decisions, and resist being thrown into chaos by every little change in circumstance, but which can also respond quickly to important changes. This

micro-level property of neural networks is clearly reflected in our macro-level behaviour.

There are a number of further refinements to this damping system. The receptors that receive the inhibitory signals can be on or near the cell body, whereas the receptors that receive the excitatory signals are typically located out on the extremities of the dendrites. The networks of inhibitory neurons tend to fire in a continuous background hum, whereas the excitatory projection neurons usually only speak when they have something important to say, and then they talk in loud bursts. As a result, the positive excitatory charges that flow inwards from the extremities have to pass along the dendrite and overcome any barriers of neutralising negative charge that have built up there as a result of the background inhibitory humming.

Figure 2.3 Neuron - inhibition and excitation.

It is only when they reach the cell body that the positively charged excitatory signals can cast their electrical vote in the ongoing decision as to whether or not the cell should fire. So it takes a good strong burst of almost simultaneous excitation from a substantial number of excitatory receptor sites to overcome this background inhibition and fire the cell.

In order to further protect the precious excitatory cells from exhaustion there is a process we call elicited inhibition. This happens when excitation neurons in an area are wired in such a way that they cause excitation in the local inhibitory cells as well. As a result, a pulse of

excitation is quickly followed by a wave of increased inhibition. Local variations in sensitivities cause local variations in the time delay between the excitation and inhibition cycles. This gives rise to a rich set of time- based emergent properties, which we might think of in musical terms such as pitch, amplitude, rhythm, wave shape, attack envelope, etc. These are extremely important, enabling for example, the very sensitive phase shift comparisons that we use to detect the direction of a sound source or the relative movement of two objects.

Figure 2.4 Two types of inhibition.