M. Michaelis

SILENT AFFERENTS : A SEPARATE CLASS OF PRIMARY AFFERENTS ?

1. In recent years, fine sensory nerve fibres have been detected that are not excited by physiological stimuli, even at potentially tissue damaging intensities. These silent afferents are known to supply knee joint, skin and viscera; in the last case, silent afferents seem to be particularly numerous.

Classification of preganglionic neurones projecting into the cat cervical sympathetic trunk.

1. The spontaneous and reflex activity patterns of 167 single preganglionic axons dissected from the cervical sympathetic trunk were examined in chloralose‐anaesthetized cats. Each neurone was classified into one of four major groups, on the basis of three principal criteria: the presence or absence of significant cardiac rhythmicity of the activity, the response to noxious stimulation of the skin, and the coupling of its activity to central inspiratory drive (phrenic nerve activity). Most neurones were also subjected to additional tests, which included carotid chemoreceptor stimulation, nasopharyngeal probing, systemic hypercapnia (ventilation with 8% CO2), hyperventilation, adrenaline‐induced blood pressure rises and retinal illumination. 2. Group I neurones (n = 69; 41%) showed significant cardiac rhythmicity, indicating strong baroreceptor control. Most (54/69) were excited by noxious stimuli, the rest being unaffected. Their activity showed variable degrees of excitatory coupling to the central inspiratory drive, and was enhanced by hypercapnia (35/39). Their responses to stimulation of arterial chemoreceptors (12/15) and nasopharyngeal receptors (24/35) were excitatory. 3. Group II neurones (n = 39; 23%) were inhibited by noxious stimulation of skin. With nine exceptions, they showed no significant cardiac rhythmicity, although they were weakly inhibited by an adrenaline‐induced blood pressure rise. Their coupling to central inspiratory drive was weak or absent, and their responses to hypercapnia and hyperventilation were variable. By contrast to other groups, they were inhibited by both chemoreceptor stimulation (9/10) and nasopharyngeal stimulation (17/18). 4. Group III neurones (n = 33; 20%) showed no significant cardiac rhythmicity, but their activity was closely coupled to central inspiratory drive. They were inhibited by hyperventilation (9/9) and excited by hypercapnia (20/21), but only fired during the central inspiratory phase and sometimes during late expiration. Their responses to noxious stimulation (28/33), chemoreceptor stimulation (8/11) and nasopharyngeal probing (24/24) were excitatory, but the induced activity was ‘gated’ by the respiratory cycle, occurring primarily during inspiration and avoiding the postinspiratory phase. 5. Group IV neurones (n = 26; 16%) showed no significant cardiac or respiratory related activity and were either excited (n = 22) or unaffected (n = 4) by noxious stimuli. One of the latter and three group II neurones were inhibited by retinal illumination; thirty‐one other neurones of all classes were unaffected. 6. Approximately 45% of thoracic sympathetic neurones were silent under the experimental conditions. About 25% of these could be recruited by systemic hypercapnia leaving 34% without spontaneous and reflex activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Respiratory modulation of the activity in sympathetic neurones supplying muscle, skin and pelvic organs in the cat.

1. The respiratory‐related modulation of activity in neurones of the lumbar sympathetic outflow to skeletal muscle, skin and pelvic organs was investigated in anaesthetized, paralysed and artificially ventilated cats, using single‐ and multi‐unit recordings. The activity of the neurones was analysed with respect to the phrenic nerve discharge under various experimental conditions. 2. Neurones tentatively classified as muscle vasoconstrictor and visceral vasoconstrictor neurones exhibited two activity peaks, one caused by baroreceptor unloading during the declining phase of the second order blood pressure waves and a respiratory drive‐dependent peak in parallel with inspiration. The two peaks were separated by depressions of activity in early inspiration and post‐inspiration. After cutting vagus and buffer nerves the activity peak during inspiration remained and was followed and sometimes preceded by a depression of activity. 3. The majority of the neurones tentatively classified as cutaneous vasoconstrictor neurones exhibited no respiratory modulation in their activity. Others exhibited an activity peak in expiration, an activity peak in inspiration, or a respiratory profile similar to that in muscle vasoconstrictor neurones. During increased respiratory drive (induced by hypercapnia) some neurones with unmodulated activity changed to an inspiratory or an expiratory pattern. Neurones discharging predominantly in inspiration projected preferentially to hairless skin. 4. Neurones which were tentatively classified as sudomotor neurones discharged predominantly in early expiration. 5. Some preganglionic neurones which were tentatively classified as motility‐regulating neurones discharged during expiration. The majority of these neurones disclosed no respiratory modulation of their activity. 6. The study shows that different types of neurone of the lumbar sympathetic system exhibit distinct patterns of respiratory modulation in their activity. We conclude that the type and degree of central coupling between respiratory system and sympathetic nervous system may vary according to the destination of the sympathetic neurones.

Properties of afferent nerve fibres supplying the saphenous vein in the cat.

1. We examined the responses of primary afferent neurones supplying a vascularly isolated segment of the saphenous vein to mechanical and chemical stimuli in anaesthetized cats. Activity was recorded from centrally cut axons of afferent nerve fibres which were isolated from the saphenous nerve near its junction with the femoral nerve. 2. A total of thirty units responded to one of these stimuli and twenty‐three of them were activated by local mechanical stimulation of the venous wall. Most receptive fields were circular spots. The response of the isolated venous segment to distension was tested in fifteen out of thirty units and eight out of fifteen were activated. Intravasal threshold pressures inducing discharges were in the range of 35‐250 mmHg with a mean of 120 mmHg. 3. Twenty‐seven out of the thirty units were tested for both mechano‐ and chemosensitivity. Thirteen were classified as A fibres and fourteen as C fibres with conduction velocities of 5‐30 m s‐1 and less than 2.5 m s‐1 respectively. Twenty fibres (12/13 A, 8/14 C) were mechanosensitive. Two‐thirds of the mechanosensitive A (8/12) and all of the mechanosensitive C fibres (8/8) responded to at least one of the chemical stimuli used: hypertonic saline, bradykinin (BK) or capsaicin. 4. The remaining seven units (6 C, 1 A) were activated by injection of BK into the isolated venous segment but failed to respond to mechanical stimuli. Six were found during five experiments in which BK was used as a search stimulus. Injection of bradykinin into the isolated venous segment repeatedly induced an increase in systemic blood pressure. 5. The proportion of unmyelinated fibres responding to mechanical stimulation of the venous segment was systematically examined in three experiments and amounted to about 1% of the unmyelinated afferents in the saphenous nerve. 6. In conclusion, a small proportion of afferent nerve fibres in the saphenous nerve responds to presumably noxious mechanical and/or chemical stimuli applied to the saphenous vein. These fibres, together with some chemospecific venous afferents, may be capable of encoding nociceptive information from the vein especially under pathological conditions.

Respiratory‐related activity patterns in preganglionic neurones projecting into the cat cervical sympathetic trunk.

1. Activity in 233 single sympathetic preganglionic neurones that project to the superior cervical ganglion was analysed with respect to central components of respiration (phrenic nerve discharge) and to the afferent feedback generated by mechanical events occurring with ventilation in anaesthetized and artificially ventilated cats. 2. The activity in ninety‐one neurones was modulated during the respiratory cycle in two ways: directly by the central inspiratory drive, and indirectly by ventilation‐related blood pressure changes, acting via the systemic baroreceptors. The direct influence was prominent in vagotomized animals or those with a raised respiratory drive, and consisted of an inspiratory increase in activity and decreases of activity in early inspiration and postinspiration. The indirect influence (excitation due to baroreceptor unloading) usually dominated in normocapnic cats with intact vagus nerves. This population of neurones showed both similar reflex responses and a similar respiratory modulation of activity as postganglionic neurones supplying hindlimb skeletal muscle. 3. Sixty‐one neurones discharged exclusively, or almost exclusively, during central inspiration. This discharge pattern neither depended on the integrity of vagal nor baroreceptor afferents. The activity of these neurones was abolished during hyperventilation and enhanced during hypercapnia. In the latter state, a small activation was often seen in stage II expiration. 4. In normocapnia the remainder of neurones (n = 81) exhibited no, or no pronounced, respiratory modulation of activity, except three neurones which showed a prominent expiratory pattern being of central and not of reflex origin. They were not a homogeneous population and included neurones exhibiting reflex responses similar to those of postganglionic neurones supplying hindlimb skin (n = 36), neurones responding to light (n = 4), and others (n = 41). 5. It is concluded that distinct types of thoracic preganglionic neurone differ with respect to respiratory modulation of their activity stemming from both central and reflex sources. Thus, the temporal profile of activity in these neurones in relation to respiration is another functional characteristic which can be used to distinguish between populations of sympathetic neurones.

Responses of lumbar vasoconstrictor neurons supplying different vascular beds to graded baroreceptor stimuli in the cat

Lumbar sympathetic vasoconstrictor neurons supplying skeletal muscle, hairy skin and pelvic organs were tested for their responses to carotid baroreceptor stimulation in chloralose-anaesthetized cats. Using single- and few-fibre recordings, the responses of the different types of vasoconstrictor neuron to graded steps of non-pulsatile pressure ranging from 110 to 260 mmHg in a vascularly isolated carotid sinus were analyzed quantitatively during the first 10 s of stimulation. The activity in all postganglionic muscle vasoconstrictor (MVC) neurons, preganglionic visceral vasoconstrictor (VVC) neurons and one third of the postganglionic cutaneous vasoconstrictor (CVC1) neurons was strongly depressed by maximal baroreceptor stimulation. Moreover, quantitative analysis revealed no significant differences of the baroreceptor sensitivity of MVC and CVC1 neurons as compared with VVC neurons at all levels of carotid sinus pressure. In contrast, two-thirds of the postganglionic cutaneous vasoconstrictor (CVC2) neurons exhibited a significantly weaker barosensitivity. The functional implications are discussed.

Receptive properties of afferent nerve fibres associated with the rat saphenous vein

We tested afferent fibres isolated from the saphenous nerve for their responses to mechanical, osmotic and chemical stimuli applied to a vascularly isolated segment of the saphenous vein in the rat. Fifteen units were excited by local mechanical stimulation and many of them were also activated by distension of the venous segment, however exclusively by presumably noxious pressures. Most mechanosensitive fibres were excited by one of the three pain-producing substances, hyperosmotic saline (HS), potassium chloride (KCl) and bradykinin. Three fibres which were insensitive to mechanical stimulation, responded to injection of HS or KCl into the venous segment. We conclude that the venous afferents most likely have nociceptive function. Most are considered to be polymodal.

Viscero-sympathetic reflex responses to mechanical stimulation of pelvic viscera in the cat

Viscero-sympathetic reflex responses to mechanical stimulation of urinary bladder and colon were studied in cutaneous vasoconstrictor (CVC) neurones supplying hairy skin, in muscle vasoconstrictor (MVC) neurones supplying skeletal muscle and in sudomotor (SM) neurones supplying the sweat glands of the central paw pad of the cat hindlimb. The cats were anaesthetized, paralysed and artificially ventilated. The vasoconstrictor activity was recorded from the axons of the postganglionic fibres that were isolated in filaments from the respective peripheral hindlimb nerves. The activity in the sudomotor neurones was monitored by recording the fast skin potential changes occurring on the surface of the central paw pad. Afferents from the urinary bladder and from the colon were stimulated by isotonic distension and isovolumetric contraction of the organs. Most CVC neurones with ongoing activity were inhibited by these stimuli; only a few CVC neurones were excited. The MVC and SM neurones were generally excited by the visceral stimuli, yet the size of the evoked skin potential changes was variable. The reflex responses elicited in the sympathetic outflow to the cat hindlimb by stimulation of visceral afferents from the pelvic organs are uniform with respect to the different types of afferent input system but differentiated with respect to the efferent output systems. Graded stimulation of the visceral afferents from the urinary bladder by isotonic pressure steps elicited graded reflex responses in CVC (threshold less than 30 mmHg) and MVC neurones (threshold less than 20 mmHg) and a graded increase of the arterial blood pressure (threshold less than 20 mmHg). These graded reflex responses are closely related to the quantitative activation of sacral afferent neurones with thin myelinated axons innervating the urinary bladder that are also responsible for eliciting the micturition reflex, but not to the quantitative activation of sacral afferent neurones with unmyelinated axons. The latter have thresholds of 40-50 mmHg intravesical pressure at which the size of the vesico-sympathetic reflexes in the vasoconstrictor neurones was about 50% of maximal size. This does not exclude the fact that activation of unmyelinated vesical afferents contributes to the vesico-sympathetic reflexes.

Spontaneous activity, conduction velocity and segmental origin of different classes of thoracic preganglionic neurons projecting into the cat cervical sympathetic trunk

Previously, in the anesthetized cat, thoracic preganglionic neurons projecting to the superior cervical ganglion were divided into four classes (groups I-IV) by way of their reflex pattern. Neurons of each class are probably involved in a distinct function, such as regulation of peripheral vascular resistance, regulation of blood flow through skin, regulation of pupil diameter, etc. Here it was tested whether the functionally different classes of thoracic preganglionic neurons also differ in the distribution of their segmental origin, their spontaneous activity and the conduction velocity of their axons. The segmental distribution of preganglionic neurons was almost identical to that determined previously with tracer methods. Distinct classes of neurons had different, although overlapping segmental distributions. Most group III neurons were located in segments T1 and T2, whereas group I, II and IV neurons showed a broader distribution. The subpopulations of preganglionic neurons did not differ in their rate of spontaneous activity. No significant difference was found in segmental distribution between neurons with spontaneous activity and silent neurons. No correlation was found between conduction velocity and spontaneous activity. The proportion of unmyelinated units was greater among group I (16.3%), group II (24.1%) and group IV (22.2%) neurons than among group III neurons (8%). The distributions of conduction velocity were significantly different between group I and group III and between group II and group III neurons. Axons of preganglionic neurons located in segments T1 and T2 conducted faster than axons of neurons located more caudally. The present study shows that distinct subpopulations of preganglionic neurons, as defined by their reflex patterns, differ in their segmental location within the spinal cord and with respect to the conduction velocity of their axons.

Responses of distinct types of sympathetic neuron to stimulation of the superior laryngeal nerve in the cat

Stimulation of afferents in the superior laryngeal nerve (SLN) leads to apnea and evokes reflexes in sympathetic neurons. It is not clear whether these reflexes are secondary to changes in the brainstem respiratory network or due directly to the afferent input on neurons belonging to central sympathetic pathways. To clarify this question, single thoracic preganglionic sympathetic neurons projecting into the cervical sympathetic trunk (CST) were classified as described previously and then tested for their responses to electrical stimulation of the superior laryngeal nerve (SLN) in chloralose-anesthetized, paralysed and artificially ventilated cats. SLN stimulation was performed with intensities sufficient to suppress central inspiratory activity detected by phrenic and recurrent laryngeal nerve recordings. Sympathetic neurons were tested under different levels of respiratory drive. Thirteen group I (putative muscle vasoconstrictor) neurons were mostly activated by SLN stimulation when respiratory drive was low, but depressed when it was high; this was due to the change in inspiration-related activity. Ten of eleven group II (putative cutaneous vasoconstrictor) neurons were depressed during SLN stimulation. This inhibition was independent of central respiratory drive. Inhibition also occurred in those neurons which predominantly discharged during postinspiration. Eight group III neurons which showed a discharge confined to inspiration were inhibited but mostly not silenced by SLN stimulation. Group IV (functionally unclassified) neurons either showed no response (n = 5) or were slightly inhibited (n = 2). The responses of group I neurons, but not the responses of group II and group III neurons, showed a significant positive correlation with those of systemic blood pressure. The observed responses corroborate the classification made previously. The results also demonstrate that the responses of sympathetic neurons to SLN stimulation are not merely due to the respiratory modulation of their activity, but rather consist of two components, one occurring independently of and the other secondary to, the changes in respiration.