This diagram recalls the reciprocal dependence between cell structure and metabolism (i.e. cell function) discussed in chapter 4.
The brain is self-regulating. To take a simple example: when you bend your elbow the biceps muscle contracts and the triceps relaxes. When you straighten it again the opposite happens: the biceps relaxes and the triceps contracts. The part of the brain responsible for these movements contains reciprocal control systems. When the nerves to the biceps fire, the nerves to the triceps are inhibited, and vice-versa. This control depends crucially on the construction of the nerve pathways. For example, one set of axon terminal branches activates an excitatory nerve, one an inhibitory nerve51. It also depends on pathway function: which neurones become active, which transmitters are released at which synapses. In turn, the control mechanism ensures that some neurones and synapses are active and others are not. Since brain function affects brain structure, the control processes also indirectly affect brain structure.
Some of the consequences of this are familiar from everyday experience. Skills such as riding a bicycle or typing have to be learned. During such learning, the parts of the brain that control the relevant muscles are modified. Their structural organisation is changed and therefore so is their function. Structure, function and control in neural circuits all depend on one another.
51 The motor nerves (the ones that actually cause muscle contraction) respond to two different neurotransmitters, acetylcholine (which activates) and gamma-aminobutyrate (which inhibits). An instruction to contract the biceps releases acetylcholine on to the biceps nerve and gamma-aminobutyrate on to the triceps nerve. Some poisons such as strychnine cause convulsions by interfering with the gamma-aminobutyrate inhibition, causing both opposing sets of muscles to contract at one. The effect is to tear muscles and tendons, break bones and cause exhaustion. Homeostasis within the brain is essential for survival.
There are more intricate examples, but they all depend on the principle that the probability of an action potential in a particular neurone at a particular time depends on activities in other parts of the brain and on the connections of these parts to the neurone. We define the "brain state" at any instant by the following diagram:-
Fig. 18-2: "brain state" is roughly analogous to the "internal state" of a cell as defined in chapter 6.
"Brain state" bears some comparison to the internal state of a cell (chapter 6), although cellular transport has no obvious counterpart in the brain. Like a cell's internal state, brain state changes from moment to moment. The structures and functions of the numerous circuits, and the control process operating in them, are never constant. Since "brains cause minds", it follows that the workings of the mind are underpinned by an ever-shifting pattern of activities and an ever-changing set of connections among the 1015 or so synapses in the human brain.
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