Anodal Block Research Articles

Pulpal anodal blockade of trigeminal field potentials elicited by tooth stimulation in the cat

Trigeminal field potentials elicited by monopolar and bipolar stimulation of the canine tooth pulp in the barbiturate-anesthetized cat were recorded to characterize the effects of direct current anodal blocks on pulpal excitability. Anodal blocking currents ranging as high as 50 μa result in a dose-dependent attenuation of pulp excitability. An induction time, inversely related to the blocking current and lasting up to several minutes, is required for the establishment of a new steady-state level of excitability. Blocks of these low current intensities or of short duration are rapidly (seconds) reversible upon block cessation. In general, blocking currents greater than 50 μa further attenuate pulpal excitability in a dose-dependent manner and if terminated immediately were also rapidly reversible. However, following any prolonged administration of these more intense blocking currents, pulpal excitability did not return to control values but was seen to fall within tens of seconds to some intermmediate value which was then maintained for the duration of the monitoring interval (10 min). For all intensities, application of the blocking current to the occlusal end of the tooth was required for the effects to be widespread throughout the pulp chamber.

Anodal blocking of Aσ tooth pulp afferents

Anodal blocking of Aσ tooth pulp afferents

Mechanism of action of dental electro-anaesthesia.

NEWMAN1 has proposed that the analgesia produced by passing a small electric current from the dental drill (positive) to a hand electrode (negative) is due to anodal blocking of nerve fibres within the tooth. Fields et al.2 feel that the likely mechanism is electrocoagulation of odontoblasts in the dentinal tubules under the site of drilling. The difference is clinically important, for anodal blocking is a harmless reversible process whereas the destruction of odontoblasts leaves permanent dead tracts in the dentin under the site of application of the current. These two mechanisms can be separated by applying stimulation and electro-analgesic current to different sites on the same tooth, since the local mechanism (electrocoagulation) would have no effect in blocking a stimulus applied elsewhere.

Intraspinal latency, cutaneous fiber composition, and afferent control of the dorsal root reflex in cat

The intraspinal delay, fiber composition, and excitability of the cutaneous dorsal root reflex (DRR) was studied in pentobarbital-anesthetized cats. A maximal cutaneous DRR is elicited by a volley of cutaneous fibers with conduction velocities above the A-delta range. The addition of A-delta and C fibers does not increase DRR amplitude or duration; nor does an isolated A-delta or C fiber volley, delivered during anodal polarization block of the larger fibers, elicit a DRR. Collision experiments reveal that the initial phase of the cutaneous DRR recorded from nerves is composed of activity in large myelinated fibers; the later phase is due to active A-delta fibers. Thus, the observations reveal that A-delta fibers carry, but do not elicit, the DRR. A collision technique shows that the minimum delay for the inter-fiber interaction producing the DRR may be as short as 1.5 msec, a value compatible with DRR mediation by one interneuron. Prolonged DRR depression follows mechanical stimulation of the skin and single shock or repetitive electrical stimulation of the A-alpha cutaneous afferents capable of DRR excitation; neither large (A-alpha) fiber volleys below DRR thresh-old nor isolated A-delta volleys depress the DRR. In the case of cutaneous nerves, this depression affects unconditioned nerves, but the effects of dorsal rootlet tetany are restricted to DRRs elicited by or recorded from the conditioned rootlet. The results are summarized in a model in which A-alpha cutaneous afferent fibers activate interneuronal systems mediating DRR excitation and depression.

Self-stimulation in rats: Tip alignment influences the effectiveness of bipolar electrodes

Bipolar electrodes implanted in the posterior hypothalamus and ventral tegmentum of midbrain stimulate the reinforcement system more effectively when their tips are aligned in the medio-lateral direction then when these are aligned anteroposteriorly. This could in part explain the response variability in intracranial self-stimulation experiments. The current probably stimulates the tegmental cells and the lateral hypothalamic path neurons of the medial forebrain bundle with dendritic fields in the coronal plane, and anodal block prevents the sagittal propagation of rewarding impulses in the axons of these cells.

Reflex cardiovascular and respiratory responses originating in exercising muscle.

1. In anaesthetized and decerebrate cats isometric exercise of the hind limb muscles was elicited by stimulating the spinal ventral roots L7-S1. This caused a rise in arterial blood pressure, with small increases in heart rate and pulmonary ventilation. These changes were abolished by cutting the dorsal roots receiving afferents from the exercising muscle.2. When the triceps surae muscle was made to exercise by ventral root stimulation, occlusion of the femoral artery and vein through and beyond the period of exercise caused the blood pressure to remain raised until the occlusion was removed. The ventilatory and heart rate responses were not markedly altered or prolonged by such circulatory occlusion.3. Injection of small volumes of 5% NaCl or isotonic KCl into the arterial blood supplying hind limb muscles gave cardiovascular and respiratory responses similar to those evoked by exercise. Like the responses to exercise, these responses were abolished by dorsal root section.4. Direct current anodal block of the dorsal roots receiving afferents from the exercising muscle was used to block preferentially large myelinated fibres: this form of block did not abolish the evoked cardiovascular and respiratory responses. Local anaesthetic block of the dorsal roots was used to block preferentially unmyelinated and small myelinated fibres: this form of block abolished the cardiovascular and respiratory responses. It is concluded that the reflex responses are mediated by fibres within groups III and IV (small myelinated fibres and unmyelinated fibres).

Contribution of unmyelinated afferent excitation to sympathetic reflexes

In this study of autonomic response to peripheral nerve afferent fiber excitation it was found that: (1) Stimulation of sural or saphenous A- and C-fibers together produced a reflex discharge followed by a silent period and a secondary late C-fiber reflex discharge in lumbar segment white rami and in the renal nerve. (2) By use of stimuli subthreshold for C-fibers and use of anodal block of A-fibers it was possible to stimulate A- and C-fibers independently. It was found that the C-fiber stimulation was responsible for the secondary reflex and increased overall efferent fiber activity. A rise in blood pressure was thus induced. A-fiber stimulation produced an earlier response and subsequent silent period. Following A-fiber stimulation the silent periods so reduced overall sympathetic discharge that depressor reactions occurred. (3) Deafferentation of baroreceptors abolished the normally occurring pulsatile surges in efferent discharge which tended to obscure the silent period following mixed fiber and A-fiber stimulation. The silent period and depressor reaction became more evident on A-fiber stimulation. Excitation of C-fibers was clearly seen to produce no silent period and only the C-fiber reflex of relative long latency. (4) Because of the latencies of response and presence of a silent period the responses recorded in this work were considered to be mediated through the medulla and not segmental spinal reactions. It was concluded that the A-fibers activate both a central excitatory and inhibitory system but the C-fibers only the excitatory complex.

The connections between the lateral giant fibers of earthworms

1. 1. Intracellular stimulation and recording techniques show elecrical connections between the lateral giant fiber of earthworms. In the species studied the connections attenuate electrical signals about three times. 2. 2. Anodal block of one lateral giant fiber does not delay conduction past the block. The bridges between the two fibers appear to conduct rapidly and the imblocked fiber determines conduction velocity for both fibers. 3. 3. Alcohol in the saline bath depresses conduction between the giant fibers and produces variable delay in crossover. Delayed crossings may be limited to particular regions of the nerve cord. Moving, labile sites of spike crossover, resembling quasi-artificial synapses, probably depends upon the demonstrated connections.

Anodal block of conduction in the optic tract.

Anodal polarization of optic tract fibers just behind the chiasm has been found to produce reversible block of conduction in first and second tract fibers within a sufficiently restricted distance from the tip of the polarizing electrode to suggest that this procedure may be applicable to studies of the function of pathways in the central nervous system. Rushton's expression for the steady current flow across the sheath of an insulated conductor indicates that threshold should vary inversely as the square root of fiber diameter. This expression gives a ratio of 1.4:1 for the threshold current for block in second tract fibers relative to the threshold current for first tract fibers, and our observed ratio of 1:1 was not believed to be significantly different from the theoretical value. The predicted increase in threshold of fibers less than 2.5 µ in diameter was confirmed by our finding that the first and second tract spikes could be depressed by anodal current that did not alter the later third and fourth tract spikes.