Types[ edit ] Most cases involve the cranial nerves , which innervate many small cranial muscles, such as the facial muscles and the extraocular muscles. This is in contrast to areas of body where miswiring of the larger muscles is less evident due the size of the muscles. Synkinesis can also involve the upper limbs, especially hands which is quite rare, at 1 case in 1 million. As the nerve attempts to recover, nerve miswiring results see Mechanism of Action below. In patients with severe facial nerve paralysis, facial synkinesis will inevitably develop.
|Published (Last):||18 August 2009|
|PDF File Size:||18.15 Mb|
|ePub File Size:||9.15 Mb|
|Price:||Free* [*Free Regsitration Required]|
Early experiments, like those by du Bois-Reymond,  demonstrated that the firing of a primary nerve may induce the firing of an adjacent secondary nerve termed "secondary excitation". This effect was not quantitatively explored, however, until experiments by Katz and Schmitt  in , when the two explored the electric interaction of two adjacent limb nerves of the crab Carcinus maenas. Their work demonstrated that the progression of the action potential in the active axon caused excitability changes in the inactive axon.
These changes were attributed to the local currents that form the action potential. For example, the currents that caused the depolarization excitation of the active nerve caused a corresponding hyperpolarization depression of the adjacent resting fiber. Similarly, the currents that caused repolarization of the active nerve caused slight depolarization in the resting fiber. Katz and Schmitt also observed that stimulation of both nerves could cause interference effects.
Simultaneous action potential firing caused interference and resulted in decreased conduction velocity , while slightly offset stimulation resulted in synchronization of the two impulses.
In Arvanitaki  explored the same topic and proposed the usage of the term "ephapse" from the Greek ephapsis and meaning "to touch" to describe this phenomenon and distinguish it from synaptic transmission. Over time the term ephaptic coupling has come to be used not only in cases of electric interaction between adjacent elements, but also more generally to describe the effects induced by any field changes along the cell membrane.
This was accomplished in one study in two experimental conditions: increased calcium concentrations, which lowered the threshold potential, or by submerging the axons in mineral oil, which increased resistance.
While these manipulations do not reflect normal conditions, they do highlight the mechanisms behind ephaptic excitation. Depending on the location and identity of the neurons, various mechanisms have been found to underlie ephaptic inhibition. In the simpler case of adjacent fibers that experience simultaneous stimulation the impulse is slowed because both fibers are limited to exchange ions solely with the interstitial fluid increasing the resistance of the nerve. More recent research, however, has focused on the more general case of electric fields that affect a variety of neurons.
It has been observed that local field potentials in cortical neurons can serve to synchronize neuronal activity.
This coupling may effectively synchronize neurons into periods of enhanced excitability or depression and allow for specific patterns of action potential timing often referred to as spike timing. This effect has been demonstrated and modeled in a variety of cases. Such phenomenon was proposed and predicted to be possible between two HR neurons, since in simulations and modeling work by Hrg. Hence the phenomenon is of not only fundamental interest but also applied one from treating epilepsy to novel learning systems.
Synchronization of neurons is in principle unwanted behavior, as brain would have zero information or be simply a bulb if all neurons would synchronize. Hence it is a hypothesis that neurobiology and evolution of brain coped with ways of preventing such synchronous behavior on large scale, using it rather in other special cases. Cardiac tissue[ edit ] The electrical conduction system of the heart has been robustly established. However, newer research has been challenging some of the previously accepted models.
The role of ephaptic coupling in cardiac cells is becoming more apparent. There are also a number of mathematical models that more recently incorporate ephaptic coupling into predictions about electrical conductance in the heart. Knowing the role that ephaptic coupling plays in maintaining synchrony in electrical signals, it makes sense to look for ephaptic mechanisms in this type of pathology.
One study suggested that cortical cells represent an ideal place to observe ephaptic coupling due to the tight packing of axons, which allows for interactions between their electrical fields. They tested the effects of changing extracellular space which affects local electrical fields and found that one can block epileptic synchronization independent of chemical synapse manipulation simply by increasing the space between cells.
A number of studies have shown how inhibition among neurons in the olfactory system work to fine tune integration of signals in response to odor. This inhibition has been shown to occur from changes in electrical potentials alone.
The inhibition due to ephaptic coupling would help account for the integration of signals that gives rise to more nuanced perception of smells. Cable theory is one of the most important mathematical equations in neuroscience.
However, many authors have worked to create more refined models in order to more accurately represent the environments of the nervous system. For example, many authors have proposed models for cardiac tissue that includes additional variables that account for the unique structure and geometry of cardiac cells  varying scales of size,  or three-dimensional electrodiffusion. It was shown that an action potential of one axon could be propagated to a neighboring axon.
The level of transmission varied, from subthreshold changes to initiation of an action potential in a neighboring cell, but in all cases, it was apparent that there are implications of ephaptic coupling that are of physiological importance. It was found that rhythmic electrical discharge associated with fetal neurons in the rat spinal cord and medulla was still sustained.
This suggests that connections between the neurons still exist and work to spread signals even without traditional synapses. These findings support a model in which ephaptic coupling works alongside canonical synapses to propagate signals across neuronal networks. The firing of these basket cells, which occurs more rapidly than in the Purkinje cells, draws current across the Purkinje cell and generates a passive hyperpolarizing potential which inhibits the activity of the Purkinje cell.
Although the exact functional role of this inhibition is still unclear, it may well have a synchronizing effect in the Purkinje cells as the ephaptic effect will limit the firing time. A similar ephaptic effect has been studied in the Mauthner cells of teleosts. Many people believed that the micro electrical fields produced by the neurons themselves were so small that they were negligible. Whether it is a true lack of evidence or simply obstinance in the face of change, many in the field are still not entirely convinced there is unambiguous evidence of ephaptic coupling.
Research continues and in , surprising results were announced .
Selected References These references are in PubMed. This may not be the complete list of references from this article. Bennett MV. Physiology of electrotonic junctions. Ann N Y Acad Sci. Continuous conduction in demyelinated mammalian nerve fibers. The internodal axon membrane: electrical excitability and continuous conduction in segmental demyelination.
Ephaptic transmission between single nerve fibres in the spinal nerve roots of dystrophic mice.
Curr Opin Neurobiol. Ephaptic interactions within a chemical synapse: hemichannel-mediated ephaptic inhibition in the retina. Kamermans M 1 , Fahrenfort I. A third, less well known, form of communication is ephaptic transmission, in which electric fields generated by a specific neuron alter the excitability of neighboring neurons as a result of their anatomical and electrical proximity.