This question was posed by Dr. Mike Kaplan, our guide to cellular neuroscience at boot camp. Here’s the answer: while liver cells are fairly simple in their operations, neurons are amazingly complex. A single cell in the cerebellum can receive signals from up to 500,000 other cells in dozens of different ways. (That’s right, 500,000.) These signals can have an excitatory effect, making it more likely that the receiving neuron will fire, but they can also have an inhibitory effect, making it less likely to fire. In any single case, a neuron may be receiving some excitatory inputs, and some inhibitory inputs. Excitatory and inhibitory effects are accomplished via many different means including: dumping different sorts of particles into the cell body, which affects the “charge” of the neuron; by effecting levels of neurotransmitters between cells; and even by another neuron turning genes within the target neuron on and off, which might result in it generating more or less of something vital to firing. Once a neuron has been pushed past its threshold to fire (and by the way, threshold levels vary as well) it may fire quickly or slowly, and in certain rhythmic patterns. In addition, a neuron may fire big (releasing tons of neurotransmitter) or little (releasing less neurotransmitter).
Neurotransmitters are the particles released when a neuron fires, which can then be taken up by a neuron close by and trigger it to fire. There are dozens of neurotransmitters which effect the firings of neurons, even though most people have only heard of a couple of them, usually serotonin and dopamine. Drugs, illegal and legal, can effect neurotransmitter levels and thus have effects on neuronal firing.
In sum, there is a lot of variability in the firing of a single neuron. A LOT. When people are thinking about the brain in computer terms, they often think of neurons as being the basic binary bits within the computer, because they think a neuron is either on (firing) or off. But if you add together all of the variables that contribute to whether or not a neuron fires, and then add it the different ways in which the neuron may fire, the neuron itself may be described as a little computer. Computationally, they are very complex.
Dr. Kaplan’s conclusion from all of this is that once you learn about the individual properties of neurons, you realize that given the number of the neurons (maybe 80 billion), it no longer seems hard to believe that the brain produces the full-range of human experiences and behavior. Including the ability to study liver cells.