Neural Conduction
From MyMCAT
Contents |
Introduction
In the previous section we looked at the structure of a neuron and how neurons are connected. But how does an individual neuron actually transmit a signal and propagate it along the length of its own axon? Here, this MCAT concept, along with the general concepts of nerve conduction and signal integration, are examined.
The Neural Membrane
The structure of a neuron's membrane is critical to the ability of a neuron to transmit and integrate a signal. The cell membrane maintains intracellular conditions that differ from those of the extracellular environment and as a consequence, changes to this difference can be used as a cellular signal. In general, there is an excess of negative ions (Cl-) inside the cell membrane and an excess of positive ions (Na+) outside. This electrochemical gradient (across the membrane) is the means of nerve impulse transmission. K+ is also unequally distributed, but in this case, K+ is found inside the cell. While this results in some positive charge inside the cell, it is not nearly as much as the Cl- inside the cell or the Na+ outside the cell, and so overall a negative charge still predominates the inside of the cell.
Nerve cells use both passive diffusion and active transport to maintain these differentials across their cell membranes. The unequal distribution of Na+ and K+ is established by an energy-dependent Na+/K+ pump, which moves 3 Na+ out of the cell and 2 K+ into the cell at the cost of one ATP. The continuous action of this pump acts to establish the unequal charge between the the sides of the membrane as more positive ions (Na+) are pumped out than pumped in (K+).
During a nerve impulse, specialized proteins embedded in the nerve cell membrane function as voltage-dependent channels allowing Na+ and K+ to passively enter or exit the cell. It is these changes in voltage and ion concentration which act as a signal for propagation. In general, if the outside of the cell is considered to have a electrical potential of 0mV, a resting nerve cell will have a potential of -70mV inside.
The Stimulated Nerve Cell
In general, weak or minor stimuluses to a nerve cell will be resisted or ignored by the cell membrane, but if a physical or chemical stimulus is strong enough to cause depolarization from the resting potential of -70 mV (to around -50 mV), the voltage-dependent Na+ transmembrane channels open. These activated Na+ channels cause a slew of Na+ to enter the cell at the location of the stimulus as it is favoured by both the concentration gradient and the electric gradient. This influx of Na+ causes a local reversal in the polarity of the membrane changing the electric potential to about +40 mV.
Signal Propagation
When an incoming signal stimulates the membrane to open some Na+ channels, either the stimulus is not strong enough to open enough channels and the membrane eventually returns to its resting state or the threshold is met and a maximum strength signal propagates. Action potentials, therefore, are said to be all-or-none, since they either occur fully or they do not occur at all.
If a stimulus is strong enough to cause a complete depolarization at a site on the cell membrane, the Na+ concentration gradient (outside > inside) becomes balanced by the electric gradient (due to the membrane potential now having become positive on the inside), and depolarization is complete at the site of the original stimulus. The K+ channels respond to the changes in polarity, sending K+ flowing out of the cell. The movement of K+ ions and slower action of the always active Na+/K+ pump soon restore concentration differentials and electric gradient to the resting state. The transient change in the electric potential across the membrane is the action potential. After depolarization, for a brief period (milliseconds), the Na+ channels cannot be stimulated because the ion gradient has not been restored. This period is called the refractory period.
If one considers how each of the components acts along a cell's membrane it becomes clear how the signal propagates. The local depolarization at the site of the original stimulus causes a change in polarity in adjacent sites to the stimulus. These regions thus react in the same way, causing Na+ channels to open passively allowing Na+ to enter followed by K+ channels opening causing K+ to exit. This effect travels down the length of an axon and because the previous region is incapable of "firing" again until the Na+ and K+ gradient is restored, the nerve impulse can only be propagated in one direction, away from the site of stimulus (dendrites) towards the terminal branches of the axons where neurotransmitters can be released.
Termination
Action potentials that reach the synaptic knobs generally cause a neurotransmitter to be released into the synaptic cleft. Neurotransmitters are small molecules that may open ion channels in the postsynaptic cell; most axons have the same neurotransmitter at all of their termini. The arrival of the action potential opens voltage-sensitive calcium channels in the presynaptic membrane; the influx of calcium causes vesicles filled with neurotransmitter to migrate to the cell's surface and release their contents into the synaptic cleft.
Postsynaptic cells can either respond to incoming neurotrasmitters as excitery or inhibitory. If enough neurotransmitter is released or multiple presynaptic neurons are exciting a postsynaptic cell, then a new action potential may be generated and the signal can continue down that cell. Inhibitory signals on the other hand reduce a cells ability to reach action potential, making it harder for other incoming excitory signals to activate the signaling cascade.
