Nerves carry an electrical signal

Signal conduction in and between nerve cells

  • Information conduction along the axon
  • Importance of the myelin sheath
  • Synaptic transmission of the nerve signal
  • A nerve signal is an electrical signal and is generated by the flow of ions (in H.20 dissolved salts) caused by channels in the membrane. Such channels, also called channel proteins, are important for the transmission of information, and they are not equally permeable to all ions. If the membrane were equally permeable to everyone, it would be in chemical-electrical equilibrium. That is, the concentration of positive sodium and potassium ions and negative chlorine ions inside and outside the membrane would be the same.
    This is not so because potassium ions are let through more strongly than sodium ions. Chlorine ions can hardly cross the membrane at all. Therefore, when the membrane is at rest there is a difference in the concentration of the three most important types of ions: sodium, potassium and chlorine. The fluid outside the nerve cell has a twelve times higher sodium concentration than inside. Inside, the concentration of potassium ions is forty times higher than outside.

    Due to the greater permeability of the membrane for potassium ions, these can flow out in higher numbers than negative ions. This creates an imbalance of positive and negative charges between the inside and outside of the cell, which creates an electric field. When the charge difference has reached a certain size, this can compensate for the diffusion current of the ions, and a state of rest occurs. In the quiescent state there is a potential difference of approximately -70 mV from the inside to the outside. Despite the partially permeable membrane, the concentration gradient between inside and outside is maintained by special pump proteins (special molecules) that transport ions to the other side. One such pump is the sodium pump.

    See Fig.3.1: Membrane of the nerve fiber. The ion pump transports sodium ions outwards and potassium ions inwards. When open, the sodium channel allows sodium ions to flow inwards, the potassium channel allows potassium ions to flow outwards.



    See Figure 3.2: Propagation of the nerve signal along an unmyelinated axon.

    If an axon is at rest, there is a potential difference of -70 mV from the inside to the outside. If an electrical nerve signal then comes to the cell, its electrical field causes sodium channels to open and thus sodium ions can flow from the outside to the inside, but no potassium ions can flow to the outside. This reduces the negative membrane potential, which is the reason that even more sodium channels open and a relatively high current of sodium ions enters the cell, so that the membrane potential is further reduced, becomes positive and increases to a value of 30 mV .
    Now potassium channels are opened, potassium ions get out, so that the state of rest can be restored. The phase in which the cell potential is below the resting potential is called the refractory phase. At this time, the cell membrane reacts with difficulty to stimuli. The ion pumps are now fully utilized to bring the ions that have flown in and out via the channels back inwards or outwards.
    In order to understand the transmission of the nerve signal, it is important that the action potential has the same shape and amplitude everywhere. Therefore no single signal can carry any information. The message about the strength of the initial stimulus is encoded by the frequency of the action potential. This frequency coding allows the action potential to be transmitted unchanged over long distances.

    Many vertebrate nerve cells are surrounded by a certain whitish layer called the myelin sheath. This acts as a kind of "electrical insulator" of the nerve fiber to accelerate the transmission of stimuli from nerve cells and to effectively use the energy for stimulus transmission.

    See Fig.3.3: Nerve cell with myelin sheath.

    The speed of movement of a nerve signal in an unmyelinated axon depends on the diameter of the cross-section of the axon. In order to conduct signals twice as fast, the nerve fiber must be four times as thick. Myelinated nerve fibers require less volume for the same information transfer speed. Their thickness is proportional to the transmission speed. The human spinal cord would have to be several meters thick if it were unmyelinated.
    However, such a myelin layer does not completely surround the axons, but has constrictions, the Ranvier constrictions, at one meter intervals, from which the layer is missing. In the case of unmyelinated cells, the influx of sodium ions results in a polarization and then a depolarization of the respective section of the nerve fiber again due to the outflow of potassium ions.
    The electric field lines of a nerve signal are denser the closer the still unpolarized section of the axon is to the already depolarized area. Therefore, the electric field causes sodium channels to open in the vicinity of the depolarized section and thereby enables the action potential to be passed on. The field lines cannot penetrate the insulating myelin sheath, but jump from one depolarized section to the next and polarize the membrane there by opening sodium channels.
    In this way, the action potential then moves from one ring to the next much faster. Mammalian brains couldn't be so compact without myelin.

    When an electrical action potential reaches a synapse, special ion channels open and many ions flow into the synapse. This causes some synaptic vesicles from the synapse to bind to the presynaptic membrane, fuse with it and thereby bring the messenger substance, the neurotransmitter, into the synaptic gap between the starting nerve cell and the target cell.
    The messenger substance enters the new cell through this gap, where it fuses with special receptors of chemically controlled ion channels and thus becomes the cause of the opening of the channels through which ions now flow in again. This influx results in a reduction in the amount of the negative resting potential of the neuron. This difference is commonly called synaptic potential. The respective cell sums up all synaptic potentials.
    If the synaptic potential exceeds a certain value, for example due to special excitation by the starting cell or due to a lot of stimuli from many other neurons, action potentials are triggered again at the base of the nerve fiber of the neuron.

    See Fig.3.4: Synapse enlarged.

    These processes all take place through signal conduction via an exciting synapse. Another type of synapse are inhibitory synapses. They do the opposite. A lowering of the negative resting potential is triggered and thus the triggering of action potentials at the base of the neuron is prevented.
    The strength of the connection through a synapse can be changed. If information is passed on very often via a synapse, its effectiveness is more pronounced than that of a less active synapse, i.e. synapses are capable of learning. This is probably the most important process of learning for living beings with a neural system. In most artificial neural networks, these properties serve as the decisive basis.