Excitation transmission
As neurotransmission in which is Physiology the transfer of the excitation of a cell refers to a different cell. Only nerve cells and muscle cells are electrophysiologically excitable ; they can form an action potential and conduct this excitation . The transmission takes place through synapses , whereby two forms are principally distinguished: chemical and electrical.
In most cases, excitation is transmitted between nerve cells via chemical synapses ( transmission ). One cell is not in direct contact with another, but uses a chemical messenger substance , called neurotransmitter , at presynaptic endings when excited , which is recognized by the other cell and transmits a signal . This allows, among other things, a modification of the transmitted signal and thus inhibiting inhibitory synapses in addition to the excitation of nerve cell-promoting excitatory . On the other hand, electrical synapses are not uncommon between muscle cells , through which cells are directly connected to one another ( gap junctions ), so that at these contact points the transmission of an excitation to another cell or to several muscle cells - such as in the heart muscle - is possible with little change .
Basics
Every cell has a membrane and every living cell has a so-called membrane potential . The cell membrane is the borderline at which a cell determines its relationship to its environment, closes, connects, forms inclusions and excludes. In this way, a different milieu is created inside the membrane-enclosed space than outside. This applies in particular to the content of salts or their dissolved charged particles, the ions migrating in the electric field . The ion concentrations in the interior of the cell can be adjusted to the appropriate content , primarily for small cations ( Na + , K + , Ca 2+ ) , via controlled passages and ducts in the membrane - passive ion channels or active ion pumps . In this way, single-cell life forms retain their (full) shape under changed osmotic conditions in the surrounding medium without bursting or shrinking.
The different distribution of charged particles in the outer and inner milieu creates a potential difference that is measured across the membrane as an electrical voltage and leads to an equalizing current when the two spaces on either side of the membrane are e.g. B. be short-circuited via electrodes. The potential difference has a characteristic value for most cells at rest under fixed external conditions; this resting membrane potential changes when the properties of the membrane change and its permeability changes , so that ion currents flow through membrane openings.
However, only with nerve cells and muscle cells is the phenomenon that changes in the membrane potential are not immediately compensated for, but can even be increased, built up, expanded and developed with a typical shape through voltage-controlled ion channels from a certain threshold , their action potential . These cells are excitable ,
- characteristic changes in their membrane potential can be induced in them as a response to stimuli or signals (conversion into signals of electrical form ( transduction ) or stimulation formation),
- They can also pass on these potential changes via their membrane, with longer distances as a series of signals of the same shape (conversion of analog signals into series of uniform signals ( transformation ) or excitation line ),
- and they can finally transmit these signals to other cells (transmission of signals to recipients, possibly by means of messenger substances ( transmission ) or excitation transmission ).
With nerve cells, the possible transmission of excitation becomes the basis of their function.
In each case shown as excitation of nerve cells, the recorded stimuli of different physical energies now all have the same energetic form in which they can be related to one another and compared, differentiated and integrated through the relationships between nerve cells and other nerve cells.
From nerve cell to nerve cell
Her excitement can across the membrane of the cell extension of a nerve cell as a series of action potentials even over long distances propagated are having quite different conduction velocity of nerve fibers , depending on how the neurite as axon of glial cells is encased. Usually an axon gives off side branches (axon collaterals) or branches out (telodendron) and has thickenings (end knobs , boutons terminaux ) at its often numerous ends , sometimes also those on the way ( boutons en passant ), where the excitation from one nerve cell to another can be transferred and so often to several different ones (divergence).
At the endings , the excitation of one nerve cell is transferred to another nerve cell mostly through chemical synapses. The terminal axon is designed as a presynaptic membrane region, which faces the postsynaptic membrane region of another cell, separated by a narrow synaptic gap that is bridged with the help of a transmitter as a messenger substance. In contrast to the transmission of excitation through electrical synapses, which is rare between nerve cells and which allows a signal to be passed on with hardly any changes, the transmission of excitation by means of chemical transmission from synaptic vesicles is a process that comprises several sub-steps and thus offers the possibility of signal transmission at different points change. Both restrictive changes (modification) and expanding adaptations ( modulation ) are possible here.
Different messenger substances are used as neurotransmitters - such as synapses as cholinergic , adrenergic , dopaminergic , serotoninergic , glycinergic , glutamatergic , GABA-ergic , peptidergic and others - and can also be combined with other messenger substances that convey additional effects as co-transmitters .
From nerve cell to muscle cell
To transfer the excitation of nerve cells to muscle fibers of the (striated) skeletal muscles
- see main article: Motorized End Plate
To transfer the excitation of nerve cells to muscle cells of the smooth muscles of hollow organs
- see article: Smooth muscles
From muscle cell to muscle cell
To transfer the excitation of muscle cells of the heart to other muscle cells of the (striated) heart muscles
- see main article: Excitation Conduction System
To transfer the excitation from muscle cells of other hollow organs to muscle cells of smooth muscles of the same organ
- see main article: Smooth muscles
See also
literature
- Robert F. Schmidt, Hans-Georg Schaible: Neuro- and sensory physiology. 5th edition. Springer, 2006, ISBN 3-540-25700-4 .
Individual evidence
- ↑ Stefan Silbernagl , Agamemnon Despopoulos : Pocket Atlas Physiology. 8th edition. Thieme Verlag, 2012, ISBN 978-3-13-567708-8 , pp. 54ff.