The human body contains trillions of cells. For the most part, they work together to accomplish the everyday tasks which make up our lives. They accomplish this cooperative structure by communicating.
Cellular communication boils down to two major methods. The first is electrical signals carried by nerves which stimulate cells to engage in particular activities. The second is through chemicals, such as hormones, which similarly stimulate cells. These two systems often work in partnership to control all the system functions in the body.
Electrical signals are the basis of the nervous system and the basic cell type of the nervous system is the neuron. There are approximately 75 billion in a typical human brain with an equal number of glial cells composed of astrocytes, oligodendrocytes, and other bodies. While neurons do a lot of the heavy lifting, the glia cells do a lot of the housekeeping, providing support and nutrients to the neurons while also cleaning up the synaptic clefts. The glial cells also communicate with one another via chemical means.
It is the neurons which really perform the functions we associate with our minds. Part of what makes these cells unique is their structure.
Typically, cells are small and round, such as red blood cells or they are squished together into brick-like arrangements in organs such as the liver or pancreas.
Neurons, in contrast, are highly asymmetric better resembling a tree than a typical round blob. They have projections covering each end forming a myriad of connections - sort of roots and branches - with a long trunk in-between called the axon.
They are also very large cells and in the human body, the axon can stretch to a metre in length. In a blue whale, single neurons have been found over 10 yards long.
Perhaps more importantly, from the perspective of signaling, neurons make connections with other neurons. On average, each neuron talks to 10,000 others and listens to what 10,000 others have to say. While neurons don't literally talk, they communicate with other cells through synapses so that the whole of our brain is made up of approximately 100 trillion connections.
The "ears" of a neuron are called dendrites and extend from the cell body. They accept input from other neurons in a unidirectional fashion. That is, they can only listen to what one other dendrite from a sending neuron is saying.
At the other end of the axon, the dendrites can only speak and only to one receiving neuron. Thus, information flows from one end of a neuron to the other and never the other way around.
This is important in the overall structure of our nervous system. It would be very strange and counter-productive if moving our finger, for example, caused the thought to appear in our brain.
The conduction of signals is via an electrochemical influx. Specifically, the membrane of a neurons has an electrical potential generated by pumping potassium into the cell and sodium out. The balance between the concentrations of these ions results in a resting potential across the cell's surface of -89 mV. It doesn't sound like a lot, as a typical battery is 1,500 mV but, because the cell membrane is so thin, it amounts to a field voltage of around 200,000 V/cm.
So what happens when a signal is received? The dendrite is excited and the membrane depolarizes, allowing a flood of sodium into the cell followed by the slower release of potassium out. The resulting potential change triggers the next portion of the membrane to fire which cause the next portion to fire and so on. It is a bit like watching dominoes fall.
But a single neuron firing through a single dendrite isn't really sufficient to activate a neuron to pass on the message. Multiple dendrites must be stimulated simultaneous. The signals are ganged together with each signal adding to others.
At the base of the axon, adjacent to the cell body, is a specialized region called the axon hillock. If enough signals are summarized simultaneously they can surmount the hillock producing a ripple down the rest of the axon. Once past the hillock, a different set of channels opens faithfully carrying the full action potential to the end of the axon where each dendrite passes it on to the next neuron in the chain.
Which channels in the receiving neuron or neurons get activated determines the fate of the impulse. Generally speaking, the more receiving dendrites from a single receiving neuron, the better. It is as if you have a whole crowd shouting a single phrase at you from 100 metres away. You are more likely to hear what is being said than if a single individual tries to do the same.
All of this is the basic electrical architecture of the nervous system. More on how it is organized next week.
