Circuits of neurones

Suppose a stimulus is detected by touch, vision or some other sense. Suppose the animal needs to respond rapidly by moving towards the stimulus source (if it means food) or away from it (if it means danger). This might be achieved through a reflex arc. The sensory organ stimulates neurone A. Action potentials in neurone A activate neurone B, which in turn activates neurone C. Neurone C brings about the required response.

41 How many different neurotransmitters are there in mammalian brains? There seem to be several hundred; more are discovered every year. Some are quite simple molecules, often derivatives of amino acids. Others are peptides (fragments of proteins), often quite large ones. Different neurotransmitters have different effects on postsynaptic cells; they might activate or inhibit; the effects might be transient or longer term. All neurones specialise in the neurotransmitters they make, but most neurones make more than one. Therefore, a single action potential can have different effects on different postsynaptic neurones.

Dedicated pre-programmed circuits of this sort are found in all animals with organised nervous systems. Reflexes are staple parts of all animal behaviour. Reflex behaviour is not "intelligent" because the stimulus is directly and inflexibly linked to the response. There is no variability, no choice, no requirement for learning or memory, and the response is caused by the stimulus, not internally in the brain. Nevertheless it is possible to intervene in these dedicated circuits. Suppose neurone B is inhibited by another neurone, X. When X is activated it blocks the A-B-C circuit and diminishes or even eliminates the reflex. Thus, reflexes can be overridden. Alternatively, if X weakly stimulates B, it will potentiate the reflex, and this can lead to an elementary form of learning. Suppose X is stimulated just before A is stimulated. If this sequence of stimuli is repeated often enough, it can change the circuit permanently. In time, the stimulus through X will elicit the response as effectively as the stimulus through A. This is "conditioned learning", a process made famous by Pavlov and his dogs42.

More subtly, if one neuronal circuit is activated immediately before another, and if a neurone in circuit I interacts with a neurone in circuit II, then learning by association can result. After the same activation sequence has been repeated many times, the input to circuit I will evoke the output of circuit II. Associative learning was first postulated by Donald Hebb in the middle of the 20th century. It might partly account for an intelligent animal's ability to predict events. There is evidence that Hebbian associative learning happens in a wide variety of animal species.

Intelligent behaviour, which originates in the brain rather than from external stimuli, must involve far more complicated processes than conditioned reflexes and associative learning; but it too must require interacting circuits of neurones. The "models in the brain" that we mentioned in the first section of this chapter must be particular sequences of circuits through which information is processed and behavioural outcomes are implemented. Information seems to "coded" in these circuits by -among other things - the frequencies of the action potentials and the actual location of each circuit in relation to others; but here we reach the present limits of neurobiological knowledge

42 Ivan Pavlov pioneered the strategy of reducing behaviour (and mental processes) to physiology. He discovered that if a bell was always rung immediately before his dogs were fed, then after a time the dogs began to salivate in response to the sound of the bell, irrespective of whether food was offered. Conditioned learning is critically dependent on the temporal order of the stimuli. Pavlov would not have obtained interesting results if he had rung the bell immediately after feeding the dogs.

Fig. 16-2: associative learning. A schematic outline of Hebb's hypothesis. Hebb suggested that memories might be formed by the mechanism outlined in this simplified diagram. Two incomi ng neurons (on the left of the picture) synapse on to a third. The cell body or dendrites of the third neuron are illustrated by the large ellipse, and the axon of the third neuron extends to the right of the picture. The input from the upper incoming neuron is strong enough on its own to make the third neuron fire, but the input from the lower incoming neuron is too weak to elicit a response. However, if both incoming neurons fire simultaneously a sufficient number of times, biochemical changes occur that strengthen the lower synapse. After these changes, an input from the lower incoming neuron suffices to make the third neuron fire, even when the upper incoming neuron is 'silent .

Embryonic neurones are called neurites. They grow towards specific targets along fibres of the extracellular matrix (the meshwork to which many of the body's cells are anchored). This growth lengthens them dramatically; that is how axons are formed. Both the target and the extracellular matrix are essential for neurite growth and therefore for nervous system development.

Chemical signals secreted by the target determine the direction of growth and also sustain the survival and maturation of the neurites. Each neurite has a set of receptors to ensure that it heads towards the appropriate target. It is possible to interfere experimentally with this process. Eliminating the target, introducing an artificial chemical gradient or altering the receptors will send the neurite in the wrong direction (or kill it). Such experiments have improved our understanding of embryonic brain development.

The extracellular matrix is necessary for an organised system of neurones to develop as the embryo matures. It is also necessary for establishing synapses, and for signals from the target to be correctly "interpreted" by a neurite. However, it is only required during the embryo stage. It is absent from mature brains. Like scaffolding, it is dismantled when the building is complete. Therefore, the growth of new axon branches and new postsynaptic dendrites in the mature brain - i.e. the formation of new synapses - takes place without any "support system".

Brain development is just one aspect of embryo development, a topic that we touched on in chapters 8 and 9. Embryo development involves the sequential expression and suppression of various groups of genes, a process roughly analogous to a chord progression in music. Each new pattern of gene expression is associated with a new internal state of the cell and responsiveness to a new set of stimuli. Also, the cell's ability to send messages to other cells is changed. Within each cell, the three-way reciprocity of internal state, responsiveness to signals from other cells and gene expression pattern causes a programmed progression of changes. This programmed progression is initiated by the expression of just one or two genes, known as immediate-early genes. During the execution of the developmental programme, hosts of other genes are expressed and suppressed in every cell.

The key genes in brain development are equally crucial for the development of other organs. In the favourite species of geneticists, the fruit-fly Drosophila, gene defects that alter behaviour, learning and memory also alter muscle activity, female fertility and other functions quite remote from the brain. It is therefore absurd to speak of genes "for" behaviour, memory or learning, and particularly absurd to speak of genes "for" a particular type of behaviour. Rather, we have genes that play key roles in embryo development generally. A defect in one of these genes will lead to abnormal development, including abnormal brain development. This will result in deviations from normal behaviour, along with other anatomical or physiological anomalies43.

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