Nerve conduction is the ability of nerve fibers to transmit bioelectrical impulses at a specific rate to both directions of conduction. Conduction occurs via action potentials in salvatory excitatory conduction. In diseases such as polyneuropathy, nerve conduction is impaired.
What is nerve conduction?
Nerve conductance is the ability of nerve fibers to transmit bioelectrical impulses at a specific rate to both directions of conduction. Nerve fibers are capable of conducting bioelectrical impulses throughout the body. Physically, each nerve fiber is composed of an insulating myelin sheath and a conductive mass inside this sheath. Signal transmissions in the nervous system occur through the transmission of action potentials, which are passed on as bioelectrical voltages. However, because voltage decay occurs rapidly along nerve fibers, impulses in the nervous system are transported only over short distances as actual bioelectric voltages. In addition, voltage-dependent ion channels are therefore located in the membranes of the nerve fibers. These channels of the nerve fibers complementarily serve to carry voltage potentials along individual nerves. Without the ion channels, nerve conduction would be much less abrupt. The speed of the nerve conduction channels can be measured today. In this context, we talk about the nerve conduction velocity, which in mammals is equivalent to between one and 100 m/s. This nerve conduction velocity depends on temperature because molecular structures are involved in nerve conduction.
Function and task
When certain nerves are irritated, this irritation can be propagated thanks to nerve conduction. For example, when nerves in the extremities are irritated, this impulse spreads in both directions of the nerve fiber, changing the body’s voltage field. The impulse is transmitted to the brain, where it passes into consciousness. Motor impulses sent to the muscles from the central nervous system also reach their destination only because of nerve conduction. Within this framework, the nerve conduction velocity determines how long the impulse takes to propagate and ultimately reach its target. The myelin layer of the axons serves as electrical insulation and achieves an extreme amplification of the conducted signal. Only at the exposed parts of the nerve fiber the impulse has to be amplified. Therefore, ion channels are interposed at these sites to make the signal strong enough to depolarize the membrane of the next nerve fiber and trigger an action potential there as well. This system is also known as salvatory excitation conduction. At a nerve fiber, there is initially resting membrane potential. There is thus a potential difference between the extracellular and intracellular spaces, but there is no potential difference along the axon. When the nerve fiber at resting potential is reached by a pulse that depolarizes it across the threshold potential, this voltage opens the voltage-dependent Na+ channels of the fiber. Na+ ions thus flow from the extracellular space into the intracellular space of the nerve fiber. The plasma membrane depolarizes and an excess of positive charges is present compared to the surrounding environment. In this way, an electric field is created. As a result, a potential difference occurs along the axon. Thus, charge shifts occur, which positively affect the membrane potential of the next nerve fiber. In addition to the transmission of action potentials in the peripheral nervous system, the transmission of impulses in the central nervous system also occurs via the processes described.
Diseases and disorders
If the peripheral nerve costume and thus the nerve conduction in the individual nerve tracts is damaged, then numbness and even motor impairments can occur. Damage to the nerve tracts manifests as slowed nerve conduction velocities. One of the best-known diseases in this context is polyneuropathy. Information into the brain and out of the brain into the body is transmitted only slowly, no longer at all or at least incompletely in the context of polyneuropathies. The reason for this is damaged nerve pathways that inhibit the flow of information. There are various causes for this phenomenon. Basically, medicine distinguishes between acquired and congenital polyneuropathies.Acquired forms of the disease may be due, for example, to toxins or inflammation and harmful metabolic products. Congenital variants, on the other hand, are genetically determined. High alcohol consumption and poor nutrition are the most common triggers of acquired polyneuropathy. Both blood sugar and metabolic products from the breakdown of alcohol attack the nervous system and can damage it. However, infections such as leprosy can also be associated with polyneuropathies. In some infections with a polyneuropathy, the pathogen even remains undetected. This is the case, for example, with Guillain-Barré syndrome. In this disease, there are sudden inflammatory changes in the peripheral nervous system, usually originating from the nerve roots on the spinal cord. Even more common than polyneuropathy is carpal tunnel syndrome, which usually results from pressure damage to the median nerve of the carpus. To be distinguished from the above-mentioned diseases are demyelinating diseases of the central nervous system, which impair nerve conduction by the breakdown of insulating myelin in switching centers such as the brain. One of the best known of these diseases is the degenerative disease multiple sclerosis. Neuropathies such as acute motor axonal neuropathy also fall into this category.
