In neurobiology, a tetanic stimulation consists of a high-frequency sequence of individual stimulations of a neuron. It is associated with potentiation.
High-frequency stimulation causes an increase in release called post-tetanic potentiation (Kandel 2003). This presynaptic event is caused by calcium influx. Calcium-protein interactions then produce a change in vesicle exocytosis. The result of these changes is to make the postsynaptic cell more likely to fire an action potential.
Tetanic stimulation is used in medicine to detect a non-depolarizing block or a depolarizing block on the neuromuscular junction. Lower elicitations of tetanic stimulation in aged muscles were shown to be caused by lower levels of anaerobic energy provision in skeletal muscles.
Physiological basis
Tetanic stimulation is based upon temporal summation, a physiological process in which successive action potentials occur prior to complete muscle relaxation. These repeated impulses lead to a summation of muscle tension which maintains contraction. When a motor neuron delivers a rapid series of electrical stimuli to a muscle at the neuromuscular junction, tetanic stimulation is mediated by increased calcium influx, which encourages neurotransmitter release and eventually post-tetanic potentiation.
At the neuromuscular junction, each electrical impulse triggers the release of acetylcholine, a neurotransmitter involved in voluntary muscle control, memory, and regulatory functions, from presynaptic vesicles into the synaptic cleft. During the high-frequency stimulation involved with tetanic stimulation, calcium ions accumulate due to the increased rate of calcium influx compared to the lowered rate of clearance by physiological buffering systems. The continuous presence of these ions promote additional vesicle fusion and acetylcholine release, which ultimately amplifies the postsynaptic response.
In experimental/research settings, tetanic stimulation is often used to probe the dynamics of neurotransmitter release and calcium regulation. Frequencies between 50 Hz to 100 Hz are commonly required to produce complete tetanus, however this range varies among muscle types.
Clinical applications of tetanic stimulation
Neuromuscular monitoring in anesthesiology uses tetanic stimulation to evaluate depths of neuromuscular blockades through post-tetanic count. When patients are administered neuromuscular blocking agents (NMBAs) to induce muscle relaxation for procedures, clinicians have to assess the intensity and recovery of induced paralysis for the safety of the patient. Because of this, tetanic stimulation can be used to guide anesthesiologists in determining dosages and recovery steps. A common clinical application of this would be the post-tetanic count method in which single electrical impulses are delivered at 1 Hz immediately after a tetanic train. This allows for observable responses which can be indirectly used to measure recovery from muscle paralysis. As the neuromuscular blockade fades, the post-tetanic count increases gradually, which helps anesthesiologists in determining patient recovery. However, because tetanic stimulation can be painful in patients that are not fully anesthetized, it should only be applied under sedation for intervals no longer than 2–3 minutes.
In an even more specialized case, pudendal nerve tetanic stimulation was shown to enhance the amplitude of motor evoked potentials, making identification rates increase in pediatric craniotomies compared to general stimulation of other nerves. This method improved monitoring reliability as well as reduced false negatives during the procedure.
These findings suggest that tetanic stimulation can be used for diverse scenarios in clinical settings such as improving signal qualities and guiding decision making during operations and procedures.
Limitations and considerations
Although tetanic stimulation is incredibly informative and mostly beneficial during use, there are a few limitations and precautions that should be taken into account prior to use.
The high frequency stimulation involved in tetanic stimulation can produce intense and sustained muscle contractions that are painful to patients that are not properly anesthetized. This causes some limitations in research, as trials cannot be performed to determine applications of tetanic stimulation on human patients that are not under anesthesia.