Saturday, November 22, 2008
cell design 101.4, nerve cell
Every nerve cell is as unique as one’s fingerprints. But all nerve cells have something in common. Each one is excitable. Each one can change from a serene, hu hum cell to a fiery, pulsating one. How is this possible?
Well, we are of course familiar with the usual picture of a nerve cell, right? The picture is usually that of a star-like cell, with some cell processes sticking out. These cell processes even differ into two kinds; one is short and highly branching (dendrites), while the other is long and usually non-branching (axon). However, the latter usually terminates in small knob-like endings called axon terminals. So above is a diagram of a nerve cell.
As mentioned earlier, every nerve cell is unique, so this is not really how every nerve cell looks. However, it fairly represents one group of nerve cells called motor neurons.
So back to the question, how can a nerve cell change from a serene to a fiery cell?
Well, nerve cells have a unique way of distributing their channel proteins. Along their cell body membranes, the channel proteins (both leaky and gated) are distributed about almost evenly. So small batches of ions constantly move in or out through leaky channels, while gated channels open or close depending on the presence of some disturbance or stimulus. Then of course, ion pumps like the Na-K-ATPase pumps constantly pull back potassium ions (K+) that sneak out and bale out sodium ions (Na+) that sneak in. But, it’s a different story where the axon of the neuron starts. In this area, the neuron’s membrane is fully studded with Na and K gated channels. So, when the disturbance or stimulus is strong enough, it can set a whole bunch of gated channels open or close. The action is much like a floodgate opening or closing, it is sudden and it creates instant action. Na ions rush inwards while K ions rush outwards. Thus, the nerve cell is usually described as “firing” when it is in this state.
All this action takes place in milliseconds and the nerve cell once more goes back to its resting state, wherein only a few Na and K ions move in and out of the cell until the next disturbance or stimulus comes.
So the way things are arranged in space and time can spell the difference between a humdrum life and an exciting one. However, just like the neuron, we need both kinds of arrangement – one kind makes us grounded, the other kind makes us reach greater heights. Exciting, isn't it?