Neurons as Circuits

Neurons As Biological Circuits

The neuron is frequently compared to an electrical circuit, as it has many of the same properties and can be mathematically defined using similar (in many cases the same) equations and relations. Though the neuron and an electrical circuit have similar characteristics and follow similar basic principals, they differ in the expression of them.

When the neuronal membrane is discussed, it is important to identify the membrane potential. The membrane potential is described in volts, similar to the voltage of an electrical circuit. Voltage may be defined as the separation of charges and the amount of work required to move a unit of charge from one point to another. In the cell this is called a potential difference and is achieved by the separation of charges across the membrane. In electrical circuits voltage is related to a battery, which has a pole with an excess of electrons.

Current is nothing more than the movement of charge past a point. The more charge that moves past a point in a unit of time the greater the current. This characteristic is measured in amperes (A). In the neuron current is achieved by the movement of charged ions in an aqueous solution. The electrical circuit expresses current as the movement of free electrons through a conducting wire.

Once the idea of particle movement is introduced it is important to discuss the resistance. Resistance represents the difficulty a particle experiences while moving in a medium. It is measured in ohms. The inverse of resistance is conductance. Conductance is the ease at which a particle can move through a medium. It is measured in siemens. Because they are inversely related, high conductances are correlated to low resistance, and vice versa. It is important to note that generally speaking resistance and conduction in the neuron are dealing with the ability of ions to cross the membrane. Thus it often referred to as membrane resistance or membrane conductance. As such, when the majority of ion channels are closed, few ions cross the membrane, and membrane resistance is said to be high. On the other hand, during depolarization events in which many ion channels are open and the cell experiences large influxes and effluxes of ions, membrane conductance is said to be high.

The cell membrane is also said to act as a capacitor, and has a property known as capacitance. A capacitor consists of two conducting regions separated by an insulator. A capacitor works by accumulating a charge on one of the conducting surfaces. As this charge builds, it creates an electric field that pushes like charges on the other side of the insulator away. This causes an induced current known as a capacitative current. It is important to realize that there is no current between the conducting surfaces of the capacitor. Capacitance then, may be defined two ways, 1) as an ability to store and separate charge, or 2) as the quantity of charge required to create a given potential difference between two conductors. Thus given a set number of charges on each side of the membrane, a higher capacitance results in a lower potential difference. In a cellular sense, increased capacitance requires a greater ion concentration difference across the membrane.

In the neuron the membrane is the insulator between the two conducting surfaces (represented by the aqueous intra and extracellular fluids). A neurons capacitance is proportional to it?s membrane surface area, so large neurons, have larger capacitances. Capacitance also decreases with the distance between the two conducting surfaces. Capacitance plays the most important role in the axon, and is involved in action potential generation and propagation. Myelin plays a very important role in this also. Myelination not only increases the membrane resistance of the axon (there is very little ion exchange across the membrane in myelinated regions of the axon), but increases the distance between the conducting surfaces, so decreases membrane capacitance. These factors lead to more rapid conduction of action potentials down the axon and is responsible for the phenomenon known as saltatory conduction.

To review, increased capacitance corresponds to a decreased impulse velocity (speed), and decreased capacitance means a faster impulse velocity. Action potential velocity is slower in unmyelinated axons due to membrane capacitance, short distances between action potential sites (the shorter the distance between action potentials, the more action potentials that are required to travel a given distance of axon), and the presence of passive ion leak channels (these cause a slight decrease in depolarization magnitude due to the loss of some ions). Myelinated axons have much faster action potentials because of three main properties. First, in the internodal regions of myelinated axons the myelin eliminates the loss of ions, and subsequent decrease in depolarization magnitude, by drastically increasing membrane resistance (there is basically no diffusion through passive leak channels under myelin). Second, myelin greatly increases the distances between the extra and intra cellular solutions (the conducting surfaces if the axon where a capacitor), thus decreasing membrane capacitance. Finally, due to myelination, there is a greater distance between the sites of action potentials in myelinated vs. unmyelinated cells. This corresponds to a smaller number of action potentials needed to travel a given axonal distance, and thus a shorter amount of time is required. These properties allow myelinated axons to conduct impulses up to 150 times faster than unmyelinated axons. Many axons however remain unmyelinated. This may seem to be detrimental, but these axons are generally transmitting signals significantly shorter distances, where the increase in speed is not necessary.

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