Cardiopulmonary vagal afferents in the monkey: a survey of receptor activity

Firing properties of single axons with cardiac rhythmicity in the human cervical vagus nerve

Microneurographic recordings of the human cervical vagus nerve have revealed the presence of multi-unit neural activity with measurable cardiac rhythmicity. This suggests that the physiology of vagal neurones with cardiovascular regulatory function can be studied using this method. Here, the activity of cardiac rhythmic single units was discriminated from human cervical vagus nerve recordings using template-based waveform matching. The activity of 44 cardiac rhythmic neurones (22 with myelinated axons and 22 with unmyelinated axons) was isolated. By consideration of each unit's firing pattern with respect to the cardiac and respiratory cycles, the functional identification of each unit was attempted. Of note is the observation of seven cardiac rhythmic neurones with myelinated axons whose activity was recruited or enhanced by slow, deep breathing, was maximal during the nadir of respiratory sinus arrhythmia, and showed an expiratory peak. This is characteristic of cardioinhibitory efferent neurones, which are responsible for respiratory sinus arrhythmia. The remaining 15 cardiac rhythmic neurones with myelinated axons were categorised as cardiopulmonary receptors or arterial baroreceptors based on the position of their peak in firing with respect to the R-wave of the cardiac cycle. This latter method is not viable for neurones with unmyelinated axons due to their slow and unknown conduction velocities. With the exception of three neurones whose expiratory modulation implicates them as cardiac-projecting efferent neurones, this population is likely dominated by arterial baroreceptors. In conclusion, the activity of single units with cardiovascular function has been discriminated within the human cervical vagus, enabling their systematic study. KEY POINTS: Recordings of the electrical activity of the vagus nerve have recently been made at the level of the neck in humans. Examination of the gross activity of this nerve reveals subpopulations of neurones whose activity fluctuates in time with the heart's beat, suggesting that the neurones that monitor or modify cardiac function can be studied using this method. Here, the activity of individual cardiac rhythmic neurones was isolated from human vagus nerve recordings using template-based spike sorting. The relationship between this activity and the cardiac and respiratory cycles was used as a means of classifying each neurone. Neuronal firing patterns that are consistent with that of neurones that modify cardiac function, including heart-slowing 'cardioinhibitory' neurones, as well as neurones that inform the brain of cardiovascular status were observed. This approach enables, for the first time, the systematic study of the function of these neurones in humans in both health and disease.

Airway mechanosensors

A mechanosensory unit is a functional unit that contains multiple receptors (or encoders) with different characteristics, including rapidly adapting receptors, slowly adapting receptors, and deflation-activated-receptors. Each is capable of sensing different aspects of lung mechanics. The sensory unit is both a transducer and a processor. Significant information integration occurs at the intra-encoder and inter-encoder levels. Within an encoder, the information is encoded as analog signals and integrated by amplitude modulation. Information from each single stretch-activated channel is processed through several levels of temporal and spatial summation, producing a generator potential that encodes averaged overall information within the encoder. This analog signal is transformed into a digital signal in the form of action potentials that are encoded as frequency (frequency modulation). These all-or-none propagated action potentials from different encoders interact through a competitive selection mechanism. Such inter-encoder interaction may occur at several levels, because of the fractal nature of the sensory unit. Inter-encoder interaction retains representative information but eliminates redundant information, resulting in the final output to the central nervous system, where multiple decoders specific for different variables decipher the encoded information for further processing.

Sensory origin of lobeline-induced sensations: a correlative study in man and cat

1. Intravenous injections of lobeline HCl into twenty-six normal young male human volunteers produced sensations of choking, pressure or fumes in the throat and upper chest at a mean threshold dose of 12 micrograms kg-1. 2. Reflex changes in breathing pattern usually appeared just before the sensations. Increasing the dose of lobeline increased the intensity of the sensations gradually until a dry cough appeared at a mean threshold dose of 24.3 micrograms kg-1. At these doses there was a mean difference of 0.3s in the latencies for sensation and respiratory reflex; in four subjects there was no difference at all. 3. In cats anaesthetized with 35 mg kg-1 sodium pentobarbitone, injecting 25-67 micrograms kg-1 lobeline into the right atrium sensitized thirteen out of seventeen rapidly adapting receptors (RARs). In three out of four cats lobeline had no excitatory effect on the RARs in the absence of normal activity (i.e. when it was injected while artificial respiration was suspended), but on restarting the respiration the activity increased greatly. After injecting lobeline, the activity increased during inflation or deflation or in both phases of the respiratory cycle. It also increased greatly during deflation produced by suction of air from the lungs after lobeline. Such presumed increased activity in the RARs of man produced by forced expiration to residual volume at the time lobeline-induced sensations were expected did not enhance the sensations in any subject. 4. In all the subjects tested, forced expiration alone, which should stimulate RARs, never produced a dry cough or sensations similar to those produced by lobeline.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of rapidly adapting receptors of the airways in monkeys (Macaca mulatta)

The properties of rapidly adapting receptors (RARs) of the airways were examined in anaesthetised, artificially ventilated, paralysed and thoracotomised monkeys. The RARs were identified (i) by their rapid adaptation to a maintained inflation and forced deflation of the lungs and (ii) by their conduction velocity measurements. Right atrial (n = 17) and left atrial (n = 13) injections of histamine (10 micrograms/kg) stimulated the RARs. The stimulation was associated with an increase in peak intratracheal pressure. Right atrial injections of phenyl diguanide (n = 6, 10 micrograms/kg) and 5-hydroxytryptamine (n = 6, 10 micrograms/kg) did not produce a significant stimulation of the RARs. Administration of irritant vapours such as ammonia, (n = 12), cigarette smoke (n = 8), alcohol (n = 10), acetone (n = 10) and ether (n = 7), caused a significant stimulation of the RARs. This stimulation occurred in spite of a significant decrease or no change in peak intratracheal pressure. During mild degrees of pulmonary venous congestion produced by graded increments in mean left atrial pressure (+5 and +10 mmHg), there was a graded increase in RAR (n = 6) activity. The present study shows the existence of the RARs in the airways of the rhesus monkey. These receptors are stimulated (i) by administration of agents which cause bronchoconstriction (ii) by vapours which cause airway irritation and (iii) in conditions which cause an expansion of the extravascular space in airways.