1.5 Machines and Computers on the Microscale and Nanoscale

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in muscle contraction. Other neurotransmitters are epinephrine and norepinephrine

(also called adrenalin and noradrenalin, respectively), which are both part of the fight-

or-flight response, dopamine and serotonin, which are part of mood regulation, as well

as histamine, γ-aminobutyric acid (GABA), glycine, glutamate, aspartate, and nitric

oxide (NO).

When an action potential arrives at the end of the axon and the calcium channel is

opened, the calcium does not only change the membrane potential but the ions them-

selves also bind to the proteins that hold the neurotransmitter vesicles to the cytoskele-

ton, thus releasing the vesicles. The vesicles now act like any small particle phase in a

matrix (here the solution). The vesicle membranes are hydrophobic, so with the release

of the vesicles there is suddenly a high amount of hydrophobic surface in the hydrophilic

solution of the nerve ending. To release that high-energy state, the vesicles move to and

merge into the synapse membrane. The vesicle and synapse membranes have very sim-

ilar compositions and can simply merge; this automatically releases their contents, the

neurotransmitter, into the synaptic cleft. The neurotransmitter then diffuses to the other

side, initiating the action potential in the dendrite, as discussed above.

This is the barebones description of what happens at a synapse. In reality, there

are many regulatory mechanisms going on at the same time using other ion channels

or transmembrane channels, also signal transduction pathways that modify the activity

of the synapse itself, directly or allosterically, all in the name of homeostasis as well as

analysis of the incoming data. Here, “analysis” also stands for higher-order thinking,

memory, and consciousness. Only a fraction of these details and processes are currently

fully understood.

1.5 Machines and Computers on the Microscale and Nanoscale

Machines on the microscale and nanoscale usually end up being based on computer

chips. Why is that and how do they work? Let us start with what they made from: semi-

conductors. In conductors, usually metals, valence electrons have the energy needed to

conduct, i. e., are in the conduction band (Figure 1.40).

When the conduction band is energetically somewhat removed from the energy

of the valence electrons, you have a semiconductor (Figure 1.40). You can get electrons

moving in semiconductors by heating them, to get them energetically into the conduc-

tion band. More commonly, voltage is used as energy. To increase the number of charges

that are conducted, semiconductors are usually doped.

The most common semiconductor material is silicon, which has a valence of 4. If

silicon is doped with atoms with a valence of 3 (e. g., boron) you create “electron holes”

and with that a p-type semiconductor. Doping with atoms of a valence of 5 (e. g., phos-

phorous) will result in an n-type semiconductor that is conducting electrons.

Many parts of electric circuits can be built from semiconductors. An important one

is the transistor. A transistor can be both a switch and an amplifier. An example of a