Saltatory conduction
As excitation line relaying excitation in the nerve cells or muscle cells is called, for example in the neuron, the transmission of an action potential along the axon, which can be wrapped in different ways than axon of glial cells. Depending on the formation of these Gliahülle different types of conduction are possible by a highly trained myelin sheath of the nerve fiber conduction velocity is considerably increased.
Over very short distances can alone elektrotonisch a thrill to be fast forwarded, but with decreasing voltage difference. Therefore, the slowing repeated formation of action potentials by ion currents is necessary for longer distances, which can continuously happen progressively. An adequate insulation by multiple myelin -containing wraps allowed a jump point process, in which a short isolated sections ( internodes ) alternates elektrotonisch Propagated depolarization with the building of action potentials at the intermediate exposed membrane area of the axon ( Ranvierscher constriction ).
The term " conduction " occasionally used here is inaccurate, since not irritable, but a is routed through that caused excitement.
- 3.1 Continuous Conduction
- 3.2 saltatory conduction
Basics
Simplified, an axon be considered as a long cylinder, which consists of a sequence of sections. The wall of said cylinder is formed by the lipid bilayer of the axon membrane, whose electrical properties can be described as a parallel circuit of an electric resistor and a capacitor with the capacity. The resistance of the membrane is so great that the lipid double layer functions as a dielectric, so that. By electrostatic forces effective across the membrane between the intra - and extracellular space, a capacity is formed in the unexcited state Whose magnitude is proportional to the surface of the membrane, and inversely proportional to its thickness.
Membrane time constant
The axon is not excited, it has a resting membrane potential of about - 70mV, that is to say between the two plates of the capacitor, there is just this potential difference. During a depolarization now changes the membrane potential; the capacitor has to be discharged - or even reloaded if the potential difference is positive. The time required for this process can be determined using the membrane time constant and is calculated as the product of the membrane resistance and the membrane capacitance:
The time constant is the time in seconds for the exponential process, after which the amplitude of the potential difference dropped to 1 / e, or about 36.8 % of the initial value and is reduced by a factor; This constant is a measure of the rate of potential change. As this process of really time-consuming is in the conduction of excitation - and for each membrane section that is depolarized, must be repeated - can accelerate the excitation line when the membrane time constant is reduced or the frequency reduced, with which the action potential are formed again needs. The latter is made possible by increasing the membrane longitudinal constant described below.
Membrane length constant
In addition to the series resistance of each axon also has a membrane resistance. Together with the longitudinal resistance is calculated from a longitudinal membrane constant. It shows the distance along an axon after which the amplitude of the potential is decreased to 36.8 %. This suggests that the distance after which a triggered action potential in a place by opening of voltage-gated cation channels is still able again to trigger an action potential is greater, the greater the membrane longitudinal constant. According to the above equation it can be to both increase by an increase in membrane resistance. In the human body this happens by means of isolation of the axon myelination by, thus preventing the occurrence of leakage current is reduced, and so the loss of the charge carriers, which are responsible for the formation of the potential difference is minimized. On the other hand, allows the membrane longitudinal constant increase by a lowering of the series resistance. It is inversely proportional to the cross-sectional area of the axon: doubling the Axondurchmessers leads to a decrease of the series resistance to a quarter. Since, however, increases while increasing the surface of the axon, at the same time the membrane capacitance and the membrane resistance is reduced, the effect on the conduction velocity in practice will be less.
Electrotonic conduction
The electrotonic conduction occurs only over short distances. Since the membrane is around the axon is a relatively poor insulator, the electric potential decreases with increasing distance. An example of an electrotonic excitation is found in the human retina. This is where the excitement is forwarded as undergraduates, lovely analog potential change elektrotonisch. This applies to the photoreceptors, such as the bipolar cells; only in the ganglion cell action potentials are formed. This form of conduction is sufficient because of the unfavorable conditions of the ion conduction inside and the insulation outward to only a few hundredths of a millimeter wide. When the potential is then raised by action potentials again, a further propagation of information is possible.
Conduction by action potentials
In axons of nerve cells causes a depolarization of the temporary opening of voltage activated sodium channels. The resulting depolarization runs as action potential of the nerve fiber. Depending on whether the axon is myelinated or not, there are two different ways:
Continuous Conduction
In unmyelinated nerve fibers, ie in the absence of myelination, the impulse through the axon of section is transferred to section by the previous section directs an action potential at the adjacent, still not excited section. The previous section is already in the repolarization phase, while the newly aroused section already changes its permeability to achieve an action potential itself. This form of transmission is relatively slow (usually only 1-3 m / s, a maximum of 30 m / s) and is found in nerves, which supply the internal organs, quite frequently. Also known are the rather low conduction velocities in nociceptors, which have diameters of less than one micrometer. The line speed can be increased by a thickening of the axon. Are particularly well known in this context, the well-studied so-called Riesenaxone with squid and sea snails of the genus Aplysia with diameters of up to one millimeter. However, the increase in diameter is not very effective, since the reduction of the lead resistance is partially offset by an increase in the membrane capacitance and a decrease in the membrane resistance again. A doubling of the diameter leads to a theoretically doubling of the line rate, is practically still below.
Saltatory conduction
In vertebrates ( vertebrates ), most axons have a myelin sheath wrapped ( myelinated nerve fiber ), which is formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system and that is interrupted at a distance of 0.2 mm to 1.5 mm. We call such an interruption nodus, knot or Ranvierscher constriction. The myelinated, ie isolated section, called internodes. This isolation increased the membrane longitudinal constant (see above) of the axon of a few hundredths of a millimeter to a few millimeters. Since the isolation also leads to a reduction of the electrical capacity of about 300 nF / m to about 0.8 nF / m, is also reduced, the membrane time constant. Only by this effect real propagation speeds in excess of 100 m / s with an unchanged cross-section of the axon are possible. In addition, voltage-dependent Na channels and Na / K - ATPases are located in a 100- fold higher density at the constriction rings. All these components allow an action potential was generated on one of up to 1.5 mm away constriction, the membrane is depolarized the next constriction sufficiently to there initiate another action potential. The exact electrophysiological processes that take place here are described in the following example.
In the unexcited nerve fiber, there is at each point along the axon, the resting membrane potential which is shown in Figure 5 at -90 mV. This means that there is a potential difference between the intracellular and extracellular space; along the axon, eg between N1 and N2, this is not the case. Comes now the first constriction N1 to an excitation in the form of an action potential, which depolarizes the membrane over the threshold potential, which is shown in Figure 5 at -60 mV, ,, in the opening of voltage-gated Na channels. Following their electrochemical gradient now flow Na ions from the extra - into the intracellular space of the axon. This leads to depolarization of the plasma membrane in the region of Schnürrings N1, ie the capacitor formed by the membrane (see Basics ) is transhipped in Figure 5 on 30 mV. A time of about 0.1 ms is required for this procedure, which is a function of the declared already in the section on the basics membrane time constant. Due to the influx of positively charged sodium ions, an excess is intracellularly formed of positive charge carriers in comparison with the surroundings at N1. This has immediately the formation of an electric field and thus a potential difference along the axon result: the resulting electric field exerts a force directly also to more distant charged particles: an N2 learn negatively charged particles (eg, Cl - ions) an attractive force towards the positive charge excess of N1. At the same positive charge carriers, which are located between N1 and N2 are moved by the electric field in the direction of N2. Through these charge transfers occur almost instantaneously to a positive view of the membrane potential at N2, without the need for ion have traveled all the way from N1 to N2. This process is comparable with powering a light bulb by operating a remote light switch: The light bulb will begin without delay to light because the electrons are everywhere at once puts the cable in motion and therefore also in the light bulb has a current flows, even though each individual electron has only moved a few centimeters.
As shown in Figure 5 below, there is the electrotonic spread of depolarization through the internode thus almost without loss of time, during a relatively long time must be applied for the regeneration of the action potential at the constriction rings. Since the excitation thus appears to jump from node to node, it is called a saltatory conduction.
The membrane potential along the axon now runs as well as by the blue curve shown in Figure 5 and would vary with increasing distance from N1 continues the resting membrane potential approach (dashed curve ), if it is not by suprathreshold depolarization of the membrane of N2 to the opening of the local voltage-dependent Na channels would come. This leads to a regeneration of the action potential, and a gradient of the membrane potential in accordance with the purple curve to be repeated again to N3 from the events.
At a line speed of 120 m / s, a nerve impulse duration of 1 ms has a length of 120 mm. That is, during the passage of a pulse around 80 to several hundred Schnürringe are simultaneously excited. At the front of the propagating electrical pulse there is a constant exchange between the electrotonic conduction in the internodes and the regeneration of the amplitude of the action potential in the constriction rings.
At birth, the myelin sheaths are missing when people in some places. For example, the pyramidal tracts are not fully myelinated, which means that in infants reflexes can be triggered, which are considered pathological ( diseased ) in adults (see Babinski reflex). After two years, however, should be observed no pathological reflexes more. In demyelinating diseases such as multiple sclerosis in the central nervous system, the myelin sheaths are degraded, which leads to various neurological deficits.
Excitation transfer
Achieved an action potential or a graded depolarization of the presynaptic, this triggers a sequence of reactions, which leads to small vesicles, the so-called synaptic vesicles with the presynaptic membrane merge ( exocytosis ) and thereby distribute neurotransmitter into the synaptic cleft. Open this transmitter either directly ligand- controlled ( ionotropic ) or indirectly mediated ( metabotropic ) ion channels in the postsynaptic membrane. The specificity of these ion channels determines whether the post-synaptic cell (nerve, muscle, or glandular cell) depolarized (energized) or hyperpolarized ( inhibited ) is. Depending on the nature of the transmitter caused by the cellular response is generated in the slave cell locally either exciting or an inhibitory postsynaptic potential, which is passed over the membrane elektrotonisch.
At the neuromuscular junction of the skeletal muscle, the motor end plate as a point of attachment to a nerve cell with a muscle fiber, the acetylcholine is secreted from the vesicles, which passes through the synaptic cleft. The neurotransmitter molecules bind to receptor molecules on the membrane of the muscle cell ( sarcolemma ). Subsequently split ( in this case) the acetylcholinesterase in the transmitter acetylcholine and choline acetate. The choline is taken up by a Cholinkanal in the presynaptic membrane again, combined with acetic acid and stored again as acetylcholine in a vesicle.
Excitation propagation in the heart
The excitation propagation in the heart is through the combination of conduction system and excitation transfer from cell to cell, a uniqueness in the body dar.