Neural organization of the viscero-cardiac reflexes

The reflex responses evoked in the postganglionic nerves to the heart were tested in chloralose-anaesthetized cats. Electrical stimulation of the A delta afferent fibres from the left inferior cardiac nerve evoked spinal and supraspinal reflex responses with the onset latencies of 36 ms and 77 ms respectively. The most effective stimulus was a train of 3-4 electrical pulses with the intratrain frequency of 200-300 Hz. Electrical stimulation of the high threshold afferent fibres (C-fibres) from the left inferior cardiac nerve evoked the reflex response with the onset latency of 200 ms. The C-reflex was present in intact animals and disappeared after spinalization. The most effective stimulus to evoke this reflex was a train of electrical pulses delivered at a frequency of 1-2 Hz with an intratrain frequency of 20-30 Hz. The most prominent property of the C-reflex was its marked increase after prolonged repeated electrical stimulation. We conclude that: (1) viscero-cardiac sympathetic reflexes may be organized at the spinal and supraspinal level; (2) viscero-cardiac sympathetic reflexes evoked by stimulation of the A delta and C afferent fibres from the left inferior cardiac nerve have different central organization.     

Analysis of reflex activity in cardiac sympathetic nerve induced by myelinated phrenic nerve afferents

Electrical stimulation of the phrenic nerve afferents evoked excitatory responses in the right inferior cardiac sympathetic nerve in chloralose-anaesthetized cats. The reflex was recorded in intact and spinal cats. The latency and threshold of the volley recorded from the phrenic nerve as well as of the cord dorsum potentials evoked by electrical stimulation of the phrenic nerve indicated that group III afferents were responsible for this reflex. The phrenicocardiac sympathetic reflex recorded in intact cats was followed by a silent period. The maximum amplitude of the reflex discharges was 800 microV, the latency was 83 ms and the central transmission time 53 ms. Duration of the silent period lasted up to 0.83 s. In spinal cats the reflex was recorded 5.5-8 h after spinalization. The maximum amplitude of the spinal reflex discharges ranged from 22 to 91 microV and the latency from 36 to 66 ms.     

Responses of cardiac vagus and sympathetic nerves to excitation of somatic and visceral nerves

Somato-vagal and somato-sympathetic reflex responses were studied by recording simultaneously the activity of cardiac vagal and sympathetic efferents following excitation of various somatic (and 1 visceral) nerves in chloralose-anesthetized dogs.Stimulation of pure cutaneous (infraorbital, superficial radial, sural nerves), muscle (gastrocnemius, hamstring nerves) and mixed nerves (sciatic, brachial, intercostal, spinal) with short trains of pulses inhibited the activity of cardiac vagus nerve and excited that of cardiac sympathetic nerve after a latency of approximately 40–60 ms, depending on the nerve stimulated. These responses were followed by the opposite response, i.e. excitation of vagus and long-lasting inhibition (`silent period') of sympathetic nerve activity. These biphasic reflex responses recorded from both autonomic nerves had similar latencies so that a clear reciprocal relationship was observed. In addition to the above reflex responses which were observed in most instances, two peaks of excitation of short duration were recorded from the vagus nerve, in some instances, and an ‘early (spinal) reflex’ in sympathetic nerve was also observed. Both excitatory and inhibitory responses described above in either nerve were readily evoked by excitation of Group II (Aβ), but not Group I (Aα), afferent fibers and increased in magnitude when Group III (Aδ) afferents were also excited. Group IV (C) afferent contributed insignificantly to the somato-vagal reflex. The vagus nerve discharge evoked by sinus nerve stimulation was inhibited during reflex inhibition produced by somatic nerve stimulation. The latency of such inhibition was less than 20 ms and lasted for 100 ms after sural nerve stimulation. We conclude that, as in case of the baroreceptor reflex and autonomic component of the ‘defense reaction’, the somato-vagal and somato-sympathetic reflex responses are reciprocal in nature.     

Efferent activity in the phrenic nerve during the startle reflex in chloralose-anesthetized cats

Reflex activity in the phrenic nerve was studied in chloralose anesthetized cats during development of somatic startle reflexes in limb and lower intercostal nerves. It was shown that the main component of this activity during low-threshold reflexes evoked by acoustic, tactile and low-threshold somatic afferent stimulation was depression of phrenic inspiratory activity. The following reflex discharges were prevalent components of phrenic responses to high-threshold afferent stimulation: early, propriospinal (intercostal-to-phrenic reflex) and late, suprasegmental ones. The latter were of two types: inspiratory (observed mainly during inspiration in about 75% of experiments) and expiratory (observed during expiration in 25% of experiments) which could be classified as "phrenic startle reflexes". Modulation of all responses during the respiratory cycle was described. Structural characteristics of reflex responses evoked in the phrenic nerve by stimulation of various respiratory and nonrespiratory bulbar sites as well as their respiratory modulation have been analyzed. Organization of possible neurophysiological mechanisms of phrenic responses during startle reflexes is discussed.     

Supraspinal regulation of spinal reflex discharge into cardiac sympathetic nerves

(1) In chloralose anaesthetized cats, reflex responses were recorded in inferior cardiac nerves following stimulation of intercostal nerves and hind limb afferent nerves. (2) In 80% of cats, a long latency reflex response alone was recorded, whereas, in the others, a short and long latency response was present to intercostal nerve stimulation. (3) In cats displaying only a long latency somatocardiac reflex response, damage to the ventral quadrant of the ipsilateral cervical spinal cord, through which runs a bulbospinal inhibitory pathway, resulted in the appearance of shorter latency reflexes to intercostal nerve stimulation. Lesions elsewhere in the cervical cord did not do this. (4) The characteristics of the early responses indicated that they were somatosympathetic reflexes and not dorsal root reflexes. (5) The early reflexes remained and the late reflex disappeared on subsequent complete transection of the spinal cord. The early reflexes were therefore spinal reflexes, and suppressed in the animal with cord intact. (6) Lesions at C4, which included a contralateral hemisection and a section of dorsal columns extending into the dorsal part of the lateral funiculus, abolished the inhibition of a sympathetic reflex that followed stimulation of some somatic afferent nerve fibres. These sections did not release the spinal reflex. Therefore, this reflex inhibition was not responsible for the suppression of the spinal somatosympathetic reflex. (7) The descending inhibitory influence on the segmental reflex pathway was not antagonized by strychnine, bicuculline or picrotoxin. (8) The possibility is discussed that the spinal reflex pathway into cardiac sympathetic nerves is tonically inhibited by a bulbospinal pathway originating from the classical depressor region of the ventromedial reticular formation.     

Effects of morphine on somatocardiac sympathetic reflexes in spinalized cats

The effects of morphine on sympathetic reflexes, recorded in the inferior cardiac nerve, to myelinated A and unmyelinated C afferent stimulation were tested in 17 acutely spinalized cats. Stable sympathetic A and C reflexes of short latency (approximately 30 ms and 140 ms in the case of the ulnar nerve, respectively) could be recorded in the inferior cardiac sympathetic nerve to stimulation of somatic A and C afferents in the ulnar and upper thoracic intercostal nerves, ipsilaterally. Spinal sympathetic A reflexes, which were primarily evoked from stimulation of A delta afferent fibers, could be elicited from more segmental levels than could sympathetic C reflexes. Additionally, smaller reflexes, only from A afferent fiber activation, were identified from stimulations on the contralateral side of the body. Small doses of morphine (0.02 mg kg-1, i.v.) proved to be ineffective at altering sympathetic A and C reflexes, while somewhat larger doses (0.2 mg kg-1, i.v.) produced a clear 62% decrease in C reflexes and a 33% decrease in A reflexes, Dosages of 1 and 2 mg kg-1 severely depressed both A and C reflexes. All of the above effects of morphine administration were completely and immediately reversible by naloxone (i.v.). The results are discussed with regard to the effects of morphine on sympathetic A and C reflexes in CNS intact, anesthetized cats.     

Discharge characteristics of sympathetic efferents to the knee joint of the cat

Sympathetic postganglionic neurons to the knee joint of the cat were studied to characterize the nerve supply and response to somatic stimulation. In halothane anesthetized cats, the sympathetic postganglionic units from a branch of medial articular nerve (MAN) were dissected. The other branch of MAN was left intact. Most of the central filaments of MAN showed spontaneous discharge. The frequency of the spontaneous discharge of single units ranged from 0.2 to 2.9 impulses per second. Cardiovascular rhythmic modulation was observed in most of the filaments tested. Phenylephrine-induced baroreceptor stimulation caused inhibition of the discharges. Repetitive stimulation of the lumbar sympathetic trunk or the peripheral cut end of MAN led to a decrease in the local temperature inside the joint. The frequency threshold for decreasing the temperature of the joint was approximately 1 Hz. Maximum effects were obtained with 5 Hz stimulation. Histograms of MAN sympathetic efferent fibre activity following electrical stimulation of afferent nerve fibres in the MAN exhibited two response periods. The first, of about 280 ms latency, was elicited by myelinated fibre excitation (the A-reflex) and the second, with a latency of approximately 700 ms, was evoked by unmyelinated fibre excitation (the C-reflex). Electrical stimulation of radial afferent nerve produced similar A- and C-reflex discharges in sympathetic fibres of MAN. Passive movement of the knee joint within its normal working range (flexion, extension or outward rotation) had very little effect on sympathetic efferent nerve activity in MAN, whereas noxious outward rotation of the joint produced a reflex increase in activity to about 140% of the prestimulus control level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Somato-vesical reflexes in chronic spinal cats

The effects of afferent volleys in hindlimb cutaneous and muscle nerves on vesical tone and contractility and on the discharges in pelvic nerves to the bladder were measured in anesthetized CNS-intact and 2-19 months chronic spinal cats. In chronic spinal cats volleys in group III and IV fibers increased the tone of the quiet, empty bladder (excitatory somato-vesical reflex). The same volleys inhibited the slow, large, rhythmic micturition contractions of the expanded bladder (inhibitory somato-vesical reflex). In CNS intact cats single or short tetanic volleys induced a reflex discharge in pelvic vesical nerve branches with 3 distinct components. These reflexes could be observed during micturition contractions, not markedly between the contractions or when the bladder was empty and quiet. The latencies of the 3 components were 90, 320 and 770 ms, respectively. The two early components (AI- and A2-reflex) were evoked by volleys in group II and III hindlimb afferents. The late component (C-reflex) was induced by group IV volleys. In chronic spinal cats a group II and III-induced A-reflex (latency 90 ms) and a group IV-induced C-reflex (latency 340 ms) were observed. The central pathways and the physiological significance of the various somato-vesical reflexes are discussed.     

Nerve specific modulation of somatosensory inflow to cerebral cortex during submaximal sustained contraction in first dorsal interosseous muscle

Modulation of the early component (latency approximately 20-30 ms) of somatosensory evoked potentials (SEPs) and that of the middle and long latency cutaneous reflexes was examined in 13 healthy volunteers during fatiguing submaximal voluntary contraction (20% maximum) of the first dorsal interosseous muscle (FDI). The SEP was evoked by stimulating the ulnar nerve (U-SEP), a mixed nerve innervating the FDI muscle, the purely cutaneous nerves of the 2nd digit (D2-SEP) and the 5th digit (D5-SEP). The cutaneous reflex was recorded concurrently with D2-SEP. The size of D2- and D5-SEP significantly decreased during fatiguing contraction as compared to rest, and the decrease in both SEPs persisted throughout fatiguing contraction. In contrast, the significant decrease in the gating of U-SEP disappeared during the latter phase of fatiguing contraction. The ratio (reflex response/background EMG) of excitatory E2 (latency approximately 60-90 ms) and E3 (approximately 120-180 ms) responses following D2 stimulation significantly increased during the middle or latter phase of fatiguing contraction. In contrast, no significant changes in inhibitory I1 and I2 were seen. The release of the attenuation of U-SEP and a constant gating of the D2- and D5-SEP suggests that the brain selectively permits the muscular afferent inflow into the cortex during fatiguing contraction. An increase in the E2 and E3 reflex ratio of cutaneous reflexes during the later phase of fatiguing contraction most likely results from an increase in the excitability of the motor cortex.     

Electrophysiological analysis of the ascending and descending components of the micturition reflex pathway in the rat

Electrophysiological techniques were used to examine the organization of the spinobulbospinal micturition reflex pathway in the rat. Electrical stimulation of afferent axons in the pelvic nerve evoked a long latency (136 +/- 41 ms) response on bladder postganglionic nerves, whereas stimulation in the dorsal pontine tegmentum elicited shorter latency firing (72 +/- 25 ms) on these nerves. Transection of the pelvic nerve eliminated these responses. Firing on the bladder postganglionic nerves was evoked by stimulation in a relatively limited area of the pons within and close to the laterodorsal tegmental nucleus (LDT) and adjacent ventral periaqueductal gray. Stimulation at sites ventral to this excitatory area inhibited at latencies of 107 +/- 11 ms the asynchronous firing on the bladder postganglionic nerves elicited by bladder distension. Electrical stimulation of afferents in the pelvic nerve evoked short latency (13 +/- 3 ms) negative field potentials in the dorsal part of the periaqueductal gray as well as long latency (42 +/- 7 ms) field potentials in and adjacent to the LDT. The responses were not altered by neuromuscular blockade. Similar responses were elicited by stimulation of afferent axons in the bladder nerves. The sum of the latencies of the ascending and descending pathways between the LDT and the pelvic nerve (i.e. 72 ms plus 42 ms = 114 ms) is comparable although somewhat shorter (22 ms) than the latency of the entire micturition reflex. These results provide further evidence that the micturition reflex in the rat is mediated by a spinobulbospinal pathway which passes through the dorsal pontine tegmentum, and that neurons in the periaqueductal gray as well as the LDT may play as important role in the regulation of the micturition.     

Split medulla preparation in the cat: arterial chemoreceptor reflex and respiratory modulation of the renal sympathetic nerve activity

The study was undertaken in order to assess the changes in sympathetic output in a split medulla preparation of the cat which, as shown earlier, has impaired respiratory rhythm generation. The effects of medullary midsagittal sections on renal sympathetic nerve firing were investigated in chloralose anesthetized, paralyzed and artificially ventilated cats. Recordings of phrenic and recurrent laryngeal nerve activity served as indices of central respiratory rhythm generation. Sections, 5 mm deep from the dorsal medullary surface and extending 6 mm rostrally and 3 mm caudally to the obex, did not produce any significant changes in heart rate, blood pressure or tonic renal sympathetic nerve firing levels. They decreased or abolished, however, the respiratory rhythmicity in renal sympathetic nerve which paralleled the reduction of inspiratory discharges in phrenic and recurrent laryngeal nerves, and abolished the carotid body chemoreceptor-sympathetic reflex. The inspiratory activity remaining after the sections could still be enhanced by chemoreceptor stimulation. The inhibitory baroreceptor and pulmonary stretch receptor sympathetic reflexes, and the central excitatory effect of CO2 on renal sympathetic nerve firing were not altered. The effects of electrical stimulation within the midsagittal plane of the medulla have shown that descending pathways from the medullary inspiratory neurons (or their medullary collaterals) do not participate in the facilitation of spinal preganglionic neurons during inspiration and in relaying the pulmonary stretch receptor inhibitory sympathetic reflex. A region located close to the obex was identified from which excitatory responses in renal sympathetic nerves, compatible with the response obtained by carotid sinus nerve stimulation, could be evoked. It is concluded that a lesion in the midline of the lower medulla at the level of the obex selectively destroys cells or pathways which relay the carotid body chemoreceptor-sympathetic reflex.     

Reflex control of sympathetic outflow and depressed baroreflex sensitivity following myocardial infarction

Reflex control of heart rate is frequently impaired following myocardial infarction. This is referred to as depressed baroreflex sensitivity. The aim of these experiments was to assess the function of other autonomic reflexes in dogs with depressed baroreflex sensitivity. Comparisons were made to dogs in whom baroreflex sensitivity was preserved or unchanged after myocardial infarction. Under chloralose-barbiturate anesthesia, reflex control of sympathetic outflow by the sinoaortic baroreceptors was determined by measurement of changes in systolic arterial pressure and efferent renal sympathetic nerve activity during infusion of phenylephrine. Following sinoaortic denervation, reflex control of sympathetic outflow by cardiac receptors with vagal afferent fibers was determined by measurement of changes in pulmonary capillary wedge pressure and renal nerve activity during blood volume expansion. Reflex decreases in renal nerve activity in response to increases in arterial pressure were similar in the two groups of dogs. In contrast, elevation of pulmonary capillary wedge pressure elicited significantly greater reflex decreases in renal nerve activity in dogs with depressed baroreflex sensitivity following myocardial infarction compared to dogs with preserved baroreflex sensitivity. Hemodynamic parameters and infarct sizes were similar in each group. In conclusion, activation of cardiac receptors with vagal afferent fibers elicited greater reflex inhibition of sympathetic outflow in dogs with depressed baroreflex sensitivity following myocardial infarction. These data suggest that these receptors are "sensitized". These results provide additional support for the hypothesis that depressed reflex control of heart rate following myocardial infarction is related to augmented afferent input from the left ventricle.     

Contribution of unmyelinated afferents from the sural nerve to spinal reflex activity

Ipsilateral ventral root reflexes evoked from electrical stimulation of the sural nerve were studied in an attempt to determine if the C or unmyelinated afferent fibers give rise to motor reflex responses because uncertainties in this area have persisted. Appropriate blocking techniques were utilized to isolate C fiber afferent barrages. Evidence was produced that a component of ipsilateral ventral root reflexes (including flexor reflexes) can be evoked solely with a C fiber afferent volley.     

Corneal reflex in normal and pathological subjects

The orbicularis oculi response can be evoked both by mechanical stimulation of the cornea (corneal reflex) and by electrical stimulation of the skin overlying the supraorbital nerve (blink reflex). Mechanical stimuli to the cornea activate A delta and C free nerve endings of the corneal mucosa. Electrical stimuli to the supraorbital nerve activate A beta, A delta and C fibers of the nerve trunk. Both reflexes present a bilateral late response, but the blink reflex shows in addition an early ipsilateral component (R1), which has never been observed with the corneal stimulation in man. We have developed a simple technique of electrical stimulation of the cornea which provides stable responses and allows precise measurements of threshold and latency of the reflex. In normal subjects, the threshold ranged from 50 to 350 microA, and the maximal stimulus that the subject could bear (tolerance level) ranged from 1000 to 2500 microA. The minimal latency to tolerance level stimuli was 39 +/- 3 msec. The latency difference between the direct responses evoked from the two opposite corneas never exceeded 8 msec and the difference between the direct and consensual responses elicited from the same cornea never exceeded 5 msec. An early ipsilateral component similar to the R1 response of the blink reflex was not observed, even with supramaximal stimulation. The electrically evoked corneal reflex was normal in 10 cases of essential trigeminal neuralgia, while the responses showed significant abnormalities in 18 subjects submitted to thermocoagulation of the Gasserian ganglion as a treatment of neuralgic pain, as well as in 2 cases of symptomatic neuralgia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparative study of sympathetic and parasympathetic discharges in the autonomic nerves during stimulation of different structures of the cat hypothalamus

Simultaneous recordings of hypothalamo-parasympathetic and hypothalamo-sympathetic evoked discharges in anesthetized cats demonstrated that both parasympathetic discharges in the pelvic nerve and sympathetic discharges in the splanchnic nerve of the lowest threshold and shortest latency could be obtained from stimulation of the posterior hypothalamus. The focus of the maximal neuronal activated elicited by stimulation of afferent fibres of visceral nerves and the focus evoking maximal efferent reactions of parasympathetic and sympathetic nerves were located in the same region of the postero-lateral hypothalamus. It is supposed (as a working hypothesis) that convergent polysensory neurons of the hypothalamus are also polyeffector divergent elements of the hypothalamo-visceral reflex system.     

Responses in sympathetic nerves of the dog evoked by stimulation of somatic nerves

Responses in thoracic and renal sympathetic nerves evoked by electrical stimulation of cutaneous and muscle nerves in anaesthetized mongrel dogs were observed. Supramaximal stimulation of cutaneous nerves evoked two responses in both thoracic and renal nerves with latencies in the ranges 58--184 msec and 349--733 msec which are referred to as the early and late responses. It was shown that the early and late responses were evoked by group III and group IV afferent fibres respectively. Stimulation of muscle nerves of the forelimb and the hypoglossal nerve evoked smaller early responses which were considered to be due to activation of group III fibres and which had latencies in the range 92--157 msec. Supramaximal stimulation of muscle nerves in the hind limb failed to evoke any responses in approximately two-thirds of preparations and in the remainder only low level inconsistent early responses were observed. No matter how intense the stimuli applied to muscle nerves there were never any responses which could be related to the activation of group IV fibres.     

Reflex activation of the lower oesophageal sphincter in the cat induced by stimulation of the splanchnic afferent fibres

Reflex responses of the lower oesophageal sphincter (l.o.s.) to electrical stimulation of the splanchnic afferent fibres were recorded by electromyographic and manometric techniques. Repetitive stimulation of the central end of a splanchnic nerve induced a long latency excitation of the l.o.s., i.e. bursts of spike potentials concomitant with repetitive phasic contractions. Experiments involving nerve sections showed that the efferent pathways of this reflex were served either by stellate sympathetic and/or splanchnic fibres, or by vagal fibres. These responses were abolished following the administration of atropine. These results show that the splanchnic afferent fibres are involved in l.o.s. reflex motor responses through the activation of the sympathetic and parasympathetic efferent supply to the sphincter.     

Electrophysiological studies of a case of startle disease

A 53-year-old female with startle disease (major form) was reported. An abnormal startle response was the most prominent clinical feature. Physical examination revealed left lateral gaze palsy and left extensor plantar response. The caloric test evoked no responses bilaterally. Blood examinations including lysozomal enzymes and radiological examinations including MRI of the brain were all normal. A pathological startle reflex was elicited by the tap on the upper lip, causing the marked extension of the head with the elbow, hip, and knee joints slightly flexing. The earliest reflex activity in a surface-EMG study was recorded in the masseter muscle and the reflex then spread down the brain stem and the spinal cord. The duration of the discharge varied from 16 to 30 ms. The onset latencies of these responses from the tap were 11.2 ms, 12.7 ms, 14.5 ms, 25.7 ms, 38.5 ms, and 47.5 ms in the masseter, sternocleidomastoid, posterior-neck, biceps brachii, quadriceps femoris, and tibialis anterior muscle, respectively. An averaged electroencephalogram triggered by the taps showed no abnormal EEG activity preceding the pathological startle response, although a negative peak, which was thought as a normal early component of the trigeminal somatosensory evoked potentials, was followed by the reflex. High amplitude SEPs and long loop reflexes were observed following stimulation of the posterior tibial nerve but not of the median nerve. Blink reflexes and auditory evoked potentials were normal.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electro-acupuncture stimulation to a hindpaw and a hind leg produces different reflex responses in sympathoadrenal medullary function in anesthetized rats

The effects of electro-acupuncture stimulation (EAS) of two different areas of a hindlimb with different stimulus intensities on sympathoadrenal medullary functions were examined in anesthetized artificially ventilated rats. Two needles of 160 microm diameter and about 5 mm apart were inserted about 5 mm deep into a hindpaw (Chungyang, S42) or a hind leg (Tsusanli, S36) and current of various intensities passed to excite various afferent nerve fiber groups at a repetition rate of 20 Hz and pulse duration of 0.5 ms for 30-60 s. Fiber groups of afferent nerves stimulated in a hindlimb were monitored by recording evoked action potentials from the afferents innervating the areas stimulated. The sympathoadrenal medullary functions were monitored by recording adrenal sympathetic efferent nerve activity and secretion rates of catecholamines from the adrenal medulla. EAS of a hindpaw at a stimulus strength sufficient to excite the group III and IV somatic afferent fibers produced reflex increases in both adrenal sympathetic efferent nerve activity and the secretion rate of catecholamines. EAS of a hind leg at a stimulus strength sufficient to excite the group III and IV afferent fibers produced reflex responses of either increases or decreases in sympathoadrenal medullary functions. All responses of adrenal sympathetic efferent nerve activity were lost after cutting the afferent nerves ipsilateral to the stimulated areas, indicating that the responses are the reflexes whose afferents nerve pathway is composed of hindlimb somatic nerves. It is concluded that electro-acupuncture stimulation of a hindpaw causes an excitatory reflex, while that of a hind leg causes either excitatory or inhibitory reflex of sympathoadrenal medullary functions, even if both group III and IV somatic afferent fibers are stimulated.     

Altered control of the trapezius muscle in subjects with non-traumatic shoulder instability

OBJECTIVE: Reflexes of shoulder girdle muscles such as trapezius are evoked from muscle afferents supplying the forearm and hand in healthy subjects. These reflexes are thought to aid the stability of the shoulder during use of the arm and hand [Alexander CM, Harrison PJ. Reflex connections from forearm and hand afferents to shoulder girdle muscles in humans. Exp Brain Res 2003;148: 277-282.]. With this in mind, the objective of this investigation was to examine this trapezius reflex in subjects with non-traumatic shoulder instability (NTSI). METHODS: The occurrence and alteration to this supraspinal reflex pathway were investigated using electrical stimulation of the ulnar and the spinal accessory nerves as well as magnetic stimulation of the trapezius motor cortex. These results were compared to a healthy group. RESULTS: The reflexes to lower trapezius were not usually observed in subjects with NTSI. When evoked, the frequency of occurrence was 27% compared to 87% in a healthy population (p<0.002). When present, the latency of this lower trapezius reflex was later than that evoked in the healthy group (62.8 ms+/-28.1 ms and 38.4 ms+/-3.6 ms, respectively; p<0.009). Although both the conduction velocity of the effective afferents and the latency of the M response to lower trapezius did not differ to the healthy group (p<0.24 and 0.54, respectively), the latency and threshold of the corticospinal evoked potential of lower trapezius did differ (16.7 ms+/-4.7 ms vs 11.2 ms+/-1.8 ms; p=0.006; 57%+/-15.8% vs 36%+/-7.6%, p=0.003). Overall, these results contrast to these measures of control of upper trapezius, which did not differ to a healthy group. CONCLUSIONS: The delay or absence of these reflexes and the delay and change in threshold of the corticospinal response in lower trapezius in subjects with NTSI indicate that the feedback mechanisms that aid shoulder girdle stability, and the voluntary control of lower trapezius are not as proficient in these subjects. Significance: The control of lower trapezius should be considered when treating people with NTSI.