The Electric Brain: The Nature of Neural Coding

"[the brain is] the most complicated material object in the known universe"
Gerald Edelman, 1992
The chemical composition of mind is not unusual, what is unique is its organization. Estimates of the number of neurons in the brain vary from 10 to 100 billion. Neurons connect to each other at specialized sites called synapses and there are about one million billion connections in the cortex. Greenfield gives us some of the analogies typically drawn to try to envision such large numbers: if you counted the number of connections between neurons at a rate of one a second it would take 32 million years; there are a billion connections in a piece of brain the size of a match head. Between 10,000 and 100,000 neurons make contact with any particular neuron. When you consider all the possible patterns of response that could be coded by such a huge and vastly interconnected network the number becomes bafflingly huge. The structure of neurons was discovered by Camillo Golgi in 1872. Using what has since become known as the Golgi method, he stained cortex with silver nitrate and was able to visualize individual neurons. An example Golgi stained brain image is available here. (Just hit the Back button to return to this text.)
This cartoon version of two connected neurons represents the basic structures involved in neural transmission. Neural signalling is a combination of electrical and chemical transmission. The electrical signalling is dependent on the flow of sodium, potassium, calcium or chloride ions (charged particles) across the cell membrane. When the cell is at rest the balance of ions is such that the inside of the membrane is more negative than the outside (~70mV). When electrical impulses accumulate on the postsynaptic terminal and add up to a significant voltage channels in the cell membrane open to allow the passage of sodium ions into the cell. The influx of positively charged sodium ions depolarizes the membrane potential to about 50mV. The resting potential is then restored by an outflow of potassium ions and a closing of the sodium channels. This characteristic depolarization followed by hyperpolarization is known as the action potential. It is brief (less than1/1000th of a second) and uniform.The action potential is regenerated all the way down the axon, a process known as saltatory conduction, which can be as fast as 220 miles per hour.

A schematic zooming of the synapse shown above.


When the action potential reaches the axon terminal of the presynaptic cell, the electrical changes cause the vesicles containing neurotransmitter to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft, the gap between neurons. The neurotransmitter diffuses across the synaptic cleft and binds to specialized receptors in the postsynaptic membrane, in a lock and key fashion. The binding of neurotransmitter to receptors causes channels to open and thus initiates the influx of ions that causes IPSPs (Inhibitory Postsynaptic Potentials) and EPSPs (Excitatory Postsynaptic Potentials) that may sum to trigger an action potential.

Neurotransmitters were first discovered by Loewi in his famous experiment on "vagus stuff" (acetylcholine) that Greenfield describes.
As Greenfield points out, chemical transmission is not as fast as electrical transmission, but it does endow the brain with enormous versatility. The already vast power and flexibility of neural signalling is increased by the variety of excitatory and inhibitory neurotransmitters available.

The electrician fallacy

Although the preceding description makes the brain appear to be a vast electrical network, Edelman cautions us against pushing the analogy too far. The brain is self-organizing: the network of the brain is created by cellular movement during development and subsequent selection and strengthening of synaptic connections. Precise point-to-point wiring cannot occur because there is considerable variation in the shapes of individual neurons and in their connection patterns. Any satisfactory developmental theory of higher brain function must remove the need for homunculi and electricians at any level. In addition, the theory must account for object categorization of a world whose events and objects do not come prelabeled.

Edelman's theory emphasizes the extensive interconnections between cortical areas. As he puts it: "the matter of the mind interacts with itself at all times". This is the same feature of brain organization that Greenfield highlights when she talks of the "incessant dialogue" between brain areas.

And on to memory...

I'd like to end with another insight from Edelman. A vital feature of brain organization is the emergence of the system property of memory: previous changes alter successive changes in specified and special ways. This point of Edelman's evokes a major contribution of Donald Hebb, a Canadian pioneer in neuroscience. Hebb proposed that a successful synapse is strengthened: "When an axon of cell A is near enough to excite cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased." This feature of neural signalling was described by Sigmund Freud in his unpublished "Project for a Scientific Psychology", and has since become known as a Hebbian synapse.

Sources

  • Edelman, G. (1992) Bright Air, Brilliant Fire: On the Matter of the Mind, Harper Collins Publishers: New York.
  • Churchland, P.S. (1986) Neurophilosophy, MIT Press:Cambridge, MA.
  • Greenfield, Susan A. (1997) The Human Brain: A Guided Tour, HarperCollins: New York.