General Neuronal Background information
Electrical impulses are sent along neurons (see Diagram 1). The neuron is a cell composed of dendrites, a cell body and an axon with its pre-synaptic structure (see Diagram 2). The synapse is a specialized space between neurons, in which information is transferred from one neuron to the next. At the synapse the electrical signal is converted into a chemical signal, in the form of a neurotransmitter. The post-synaptic structure contains receptors that bind the neurotransmitter, and convert the chemical signal back into an electrical signal (either by activating ion channels or metabolic processes) in the post-synaptic dendrite. The resting state, or "equilibrium" condition, of the neuron is a negative voltage potential (the inside is negative compared to the outside), which is created and maintained by ion channels and pumps that create a greater concentration of positive ions outside of the cell. The primary ions of concern in the creation of the differential are sodium (Na +) and potassium (K +). Throughout the body, sodium has a much higher concentration outside of cells and potassium has a much greater concentration inside cells. The cell membrane is made up of two layers of phospolipids, which have hydrophilic ends facing outward and hydrophobic ends facing inward. This creates a membrane barrier that is impermeable to most substances. To allow selected substances to pass, the membrane is imbedded with specific membrane channels that allow these substances to cross the membrane barrier.
The neuronal impulse is only sent in one direction, from the dendrites to the cell body to the axon. The dendrites are projections from the cell body that receive electrical stimulation from other cells. The stimulation of the dendrites results in the generation of changes in chemical concentrations along the membrane; these changes are produced by the activity of ion channels that result in graduated electrical signals that are either excitatory or inhibitory. The transmission of these signals through dendrites attenuates because the low resistance interiors are surrounded by very "leaky" high resistive membranes. The increase in diameter of the neuron increases the speed of transmission of the signal. Although as many as a thousand dendrites come into the cell body only one process, the axon, sends a signal out.
I will explain the electrochemical properties of the cell membrane that enable the interior of the neurons to be maintained at a voltage of approximately -60 mV. This is the result of the ion pumps and channels, which pump out Na + and pump in K +. The pumping process, which occurs continuously, results in three Na + being pumped out for every two K + that are brought into the cell. This results in the creation of ion gradients (K + high in, Na + high out). The continuous leakage of these ions down the gradients that are created leads to the negative potential.
There are also ion channels that allow for the selective transfer of ions through diffusion when the channels are open. Many of these ion channels are voltage dependant. This means that when an electrical signal is generated in the neuron, these ion channels can be activated to open or close thereby "transmitting" the signal along the neuron. This is the primary method of transmission of electrical signals. The cell membrane has a high resistivity, on the order of 10,000 ohms, whereas the cells interior has a resistivity of only a few hundred ohms. The problem is that there are many "holes" in the cell membrane, which are either ion channels or other protein-based pores that allow the electrical charge to be dissipated. Consequently, signals that are generated in the dendrites are not efficiently transmitted along the neuron. The distance that a signal can be transmitted is proportional to the radius of the neuron. The dendrites are the location where signals are generated. These appendages of the neuron cell body can be hundreds of "processes". Each dendrite will carry the signal that it receives from other neurons towards the cell body.
These electrical-chemical signals are graded, which means that they vary in strength depending on the strength of the input, unlike an action potential which is a fixed amplitude signal that is only sent when a certain threshold is reached. These graded potential signals from the dendrites can also be either excitatory or inhibitory, which means that they can be positive or negative and either contribute to the creation or suppression of an action potential in the neuron. All of the dendrites send their signals to the neuron cell body which then continue to the axon which is a single appendage. 1
The Action Potential and Transmission of a Signal
The initial segment of the axon, known as the "axon hillock", has an extremely high density of voltage-gated Na + channels. When the dendrites collectively generate a signal of sufficient strength, which exceeds the threshold for that neuron, this axon hillock generates an action potential. An action potential is an all or nothing event. As a result of the generation of an action potential at the hillock, an electrical signal is sent down the length of the axon. An action potential provides a sustainable signal that can travel as far as a meter or two.
Speed of Transmission
In addition to an increase in diameter of the neuron, the signal speed is also improved if the neuron is myelinated. Myelination is the coating of axons with "highly resistive" Schwann cells in as many as one hundred layers that acts like electrical tape to reduce the "leakiness" of the neuron. It results in improvement of speed of signal transmission by fifty times. There is also a complex advantage, known as saltatory signal transmission, through myelinated axons that causes the electrical signal to jump between the "Nodes of Ranvier," which are the areas between the myelinated segments of a neuron (see Diagram 1). Saltatory transmission is an extremely fast and efficient method. The Nodes of Ranvier are the intermittent areas of the neuron that are unmyelinated because myelination occurs in segments. The Nodes of Ranvier contain ion channels and the saltatory signal transmission allows the electrical signal to propagate from node to node without having to transverse the myelinated areas thus significantly increasing the speed of the signal (see Diagram 1). The myelination is white and consequently the areas of the brain referred to as white matter contain high densities of myelinated fibers, whereas the areas referred to as grey matter are enriched in unmyelinated cell bodies.
The Synapse
When the signal reaches the end of the axon it triggers an influx of Ca +2 ions. This flux activates the release of vesicles filled with neurotransmitters through the presynaptic membrane. These neurotransmitters carry the signal across the gap of the synapse and allow for the recreation of the signal in the new neuron. In fact, the neurotransmitters can assist in the sustainability or even increase in the signal in the new neuron by triggering the opening of ion channels in the postsynaptic cell. This is the incredible system of signal transmission, which allows for cells to transmit signals efficiently over distances.
Transmission of Information
On the most basic level it is the capacity to transmit information over distances that enables cells to communicate, respond to their environment, and exist in complex arrangements. However, the brain is able to do far more with these signals. The capacity to send signals enables the encoding of information within those signals as different frequencies or spikes of action potentials. The capacity to encode information in the action potential spikes leads to the ability for more complex signal generation and eventually the deciphering of that information. Much is unknown about how this cognitive process occurs, but it is clear that it does. Mathematical models have been proposed to explain these phenomena, but they simplify the neural networks and do not adequately explain how the signals are in fact a computation or a transfer of information. 2
Comments: