Latency: Function, Tasks, Role & Diseases

Neurological latency is the time between a stimulus and the stimulus response. It is thus equal in duration to the nerve conduction velocity. In addition, latency in medicine can mean the time between contact with a noxious agent and the first symptoms. Neurological latency is prolonged in demyelination.

What is the latency period?

Neurological latency is the time between a stimulus and the stimulus response. It is thus equal in duration to the nerve conduction velocity. The time interval between the perception of a stimulus and the stimulus response is called latency. The latency thus depends on the one hand on the neurological structures involved in stimulus perception and on the other hand on the respective type of stimulus. In neurology, latency is thus the basic duration of a conduction velocity in the nervous system. In clinical practice, however, the term latency is particularly associated with the exposure of an organism to noxious substances. These so-called noxious substances are absorbed by the body. Contact with the harmful substance is followed by a clinically asymptomatic interval. In this context, the latency period is the time between exposure to noxious substances, such as radiation, mechanical stress or poison, and the first manifestations of symptoms. If the acting noxious agent is microbiological in nature and thus corresponds, for example, to bacteria, fungi, parasites or viruses, instead of the latency period we speak of an incubation period. The neurological definition corresponds to the narrower definition. The damage-associated definition corresponds to an actual latency period only in the broadest sense.

Function and task

Any type of latency is ultimately a delay or reaction time. For noxious agents, for example, latency consists of the amount of time it takes an organism to respond to them. In the same sense, neurological latency corresponds to the reaction time it takes a nerve conduit to transmit a stimulus. The neurological latency depends not only on the type of stimulus, but also on the type of conduction and transmission speed of all neuronal structures involved in the stimulus transmission to the target organ. In most cases, the target organs are muscles. The nervous system contains different types of conduction whose transit times and structures are ideally matched to the particular stimulus responses desired. Each nerve fiber consists of an insulating myelin sheath and the conducting content. A voltage is conducted in the conduction according to electrodynamic laws. The nerve membrane is incomplete as an insulator. The electrolyte of the nerve pathway has a high resistance compared to, for example, copper veins. For this reason, there is a rapid voltage drop along the nerve fiber and nerve impulses can thus only be transmitted over short distances. Therefore, an additional change in ion permeability is initiated by the voltage-dependent ion channels of the membranes. The travel of stimuli along the nerve pathways to the response organ, such as a muscle, is the transit time or latency. The latency is subject to a temperature dependence. Thus, nerve conduction velocity increases by up to 2 m/s per degree Celsius. In addition, the thickness of the conduction has an influence on the latency. Thick axons, for example, transmit stimuli with higher nerve conduction velocity than thin axons. Other factors play a role in the latency associated with noxious agents. In addition to the type of noxious agent acting, for example, the immunological constitution of the individual may determine the latency time.

Diseases and complaints

Neurological latency is measured as a standard part of certain neurophysiological examinations. The measurement is not made on a single nerve fiber, but refers to the sum of all responses of fibers of a given nerve. A special case of the measurement is that of motor conduction time. At the skin surface, measurable nerve voltages are extremely small and prone to error. Therefore, motor nerves are stimulated to determine latency and the physician infers running ability from the muscle response and the span between stimulation and muscle movement. Strictly speaking, the time between the stimulus and the muscle response includes not only the latency and with it the nerve conduction time, but also the transmission time to the respective muscle group via motor end plates. This time is around 0.8 ms.In the type of measurement described, the transmission times to the muscles must be subtracted from the determined motor transmission time in order to obtain the latency time. If the latency is pathological and thus slowed down, the cause is usually a demyelination of the transmitting nerves. Such demyelination is associated with either neurological disease, mechanical nerve injury, or poisoning. Demyelination is always said to occur when the insulating myelin around individual nerve fibers has been degraded or shows degenerative manifestations. In the central nervous system, the cause of demyelination of the nerves can be, for example, the autoimmune disease multiple sclerosis. In this disease, the body’s immune system mistakenly sees the nerve tissue of the central nervous system as a danger and attacks central nerve tissue sections with autoantibodies that cause demyelinating inflammation. Unlike in the central nervous system, remyelination of demyelinated nerve fibers may well occur in the peripheral nervous system. Demyelination in peripheral nerves is subsumed under the term neuropathy. In most cases, such neuropathies are related to other diseases and thus are only the secondary manifestation of a certain primary disease. Sometimes most frequently, neuropathies and the associated demyelination of peripheral nerves are observed in the context of diabetes or after exposure to neurotoxic substances. The latter association explains, for example, why neuropathies are frequently observed in chronically alcohol-dependent individuals.