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2.4 Biochemical and Electrical Activities in Neurons

The following is a description of the traditional view(s) on synaptic transmission: A very thin cell membrane separates the intracellular and extracellular regions of a biological cell shown in Figure 2.3. A high sodium and high chloride ion concentration but a low potassium concentration are found in the extracellular region, while high potassium but low sodium and low chloride concentrations are found in the intracellular region. The cellular membrane maintains this imbalance in composition through active ion transport. That is, a membrane protein, called the sodium pump, continuously passes sodium out of the cell and potassium into the cell. A neuron may have millions of such pumps, moving hundreds of millions of ions in and out of the cell each second. In addition, there are a large number of permanently open potassium channels (proteins that pass potassium ions readily into the cell, but inhibit passage of sodium). The combination of these two mechanisms is responsible for creating and maintaining the dynamic chemical equilibrium that constitutes the resting state of the neuron.

Under these resting conditions (steady state), one can ignore the sodium since the permeability of the biological membrane is relatively high for potassium and chloride, and low for sodium. In this case, positively-charged potassium ions (K+) tend to leak outside the cell (due to the membrane’s permeability to potassium) and the diffusion is balanced by an inward electric field that arises from the movement of these positive charges. The result is an intracellular resting potential of about -100 mV relative to the outside. When the cell is stimulated (due to synaptic inputs), the membrane permeability changes so that the sodium permeability greatly exceeds that of potassium and chloride. The sodium then becomes the dominant factor in establishing the steady state which arises when the inward diffusion of sodium (Na+) elicits a counterbalancing outward electric field (and the intracellular potential becomes positive by 40 mV).


Figure 2.3  Extracellular and intracellular spaces of a biological cell

Examining the process in greater detail, as conceived by Hodgkin and Huxley [28], the cell fires (or produces an action potential) when neurotransmitter molecules from the synapse reduce the potential to approximately -50 mv. At -50 mv, voltage-controlled sodium channels are opened; and sodium flows into the cell, reducing the potential even more. As a result, further increase in sodium flow occurs into the cell and, this process propagates to adjacent regions, turning the local cell potential to positive as it travels. This polarity reversal spreading rapidly through the cell causes the nerve impulse to propagate down the length of the axon to its presynaptic connections. (The cell which has provided the knob where the axonal branches end at the synapse is referred to as the presynaptic cell.) When the impulse arrives at the terminal of an axon, voltage-controlled calcium channels are opened. This causes neurotransmitter molecules to enter the synaptic cleft and the process continues on to other neurons.

The sodium channels close shortly after opening and the potassium channels open. As a result, potassium flows out of the cell and the internal potential is restored to -100 mv. This rapid voltage reversal establishes the action potential which propagates rapidly along the full length of the axon. An electrical circuit analogy to a cell membrane can be depicted as shown in Figure 2.4.

Action potentials refer to electrical signals that encode information by the frequency and the duration of their transmission. They are examples of ion movement. As the action potential travels down the axon, a large number of ions cross the axon’s membrane, affecting neighboring neurons. When many neurons exhibit action potentials at the same time, it can give rise to relatively large currents that can produce detectable signals. Thus, neuronal transmission physically refers to a biochemical activated flow of electric signals as a collective process across the neuronal assembly.

At the end of the axon (presynapse), the electrical signal is converted into a chemical signal. The chemical signal, or neurotransmitter, is released from the neuron into a narrow (synaptic) cleft, where it diffuses to contact specialized receptor molecules embedded within the membrane of the target, or the postsynaptic neuron. If these receptors in the postsynaptic neuron are activated, channels that admit ions are opened, changing the electrical potential of the cell’s membrane; and the chemical signal is then changed into an electrical signal. The postsynaptic neuron may be excited and send action potentials along its axon, or it may be inhibited. That is, the neurons are either excitatory or inhibitory (Dale’s law). A typical cell action potential internally recorded with a microelectrode is presented in Figure 2.5.

Considering a long cylindrical axon, the neuronal propagation is nearly at a constant velocity; and the action potential can be interpreted either as a function of time at a given site or a function of position at a given time. That is, the transmembrane potential can be regarded as satisfying a wave equation. The stimulus intensity must reach or exceed a threshold for the neuron to fire, but the form of the action potential is not related to the exact value of stimulus intensity in the occurrence or nonoccurrence of firing activity (normally specified as the all-or-none response) of the cell.


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