Johan Thunberg

Influences on the fusimotor-muscle spindle system from chemosensitive nerve endings in cervical facet joints in the cat: possible implications for whiplash induced disorders

&NA; The aim of the present study was to establish if there exists reflex connections from ligamentous structures in cervical facet joints and the fusimotor system of dorsal neck muscles. In seven cats, anaesthetized with &agr;‐chloralose, bradykinin (BK) of concentrations between 12 and 50 &mgr;g was injected into the facet joint between C1 and C2. Recordings were made from single muscle spindle afferents (MSA) originating in contralateral trapezius and splenius muscles (TrSp). Fusimotor induced changes in the sensitivity of the muscle spindle afferents were assessed by recording the responses to sinusoidal stretches of the TrSp muscles. The mean rate of discharge and the depth of modulation of a fitted sine were taken as quantitative estimates of the response. A total of 25 MSAs were recorded, and 21 of these showed clear‐cut alterations in their responses to the sinusoidal stretches following Bk. injections into contralateral facet joint. The majority of the responding afferents (13/21) showed changes in their responses indicating an increased activity of static fusimotoneurones, although responses of dynamic and mixed static and dynamic nature were also seen. Local anaesthetics applied to the intraarticular receptors abolished the effects. Injection (i.v.) of a general anaesthetic (pentobarbital) abolished the effects. The results show that there exist reflex connections between receptors in cervical facet joints and fusimotoneurones of dorsal neck muscles, and this might be of importance in the pathophysiology behind whiplash associated disorders (WAD).

Elevated Intramuscular Concentration of Bradykinin in Jaw Muscle Increases the Fusimotor Drive to Neck Muscles in the Cat

Patients suffering from temporomandibular dysfunction exhibit clinical findings such as fatigue, painful muscles, and muscles that are tender to palpation, not only in the temporomandibular area, but also in the neck/shoulder region. The mechanisms behind this are not known, although previous studies of similar spreading phenomena have revealed a possible involvement of the fusimotor system. In the present study, we evaluated the activity of this system by recording the activity of muscle spindle afferents from dorsal neck muscles after intramuscular injections of 6 μg to 25 μg bradykinin in the ipsilateral masseter muscle. A total of 23 muscle spindle afferents from the trapezius and splenius muscles was recorded at the C3-C4 level in 7 adult cats anesthetized with a-chloralose. Of these 23 afferents, 17 (74%) showed significantly different responses to bradykinin injections compared with control injections (dissolving agent, Tyrode), and the majority of the effects were compatible with an increased static fusimotor drive to the muscle spindle system. Thus, the results demonstrate potent reflex connections from groups III and IV masseter muscle afferents to fusimotor neurones on the C3-C4 level. It is concluded that the fusimotor system might play a significant role in the mechanisms behind the spread of muscle pain and tension from the temporomandibular region to the neck.

Comparison of brain activation after sustained non-fatiguing and fatiguing muscle contraction: a positron emission tomography study

The concept of fatigue refers to a class of acute effects that can impair motor performance, and not to a single mechanism. A great deal is known about the peripheral mechanisms underlying the process of fatigue, but our knowledge of the roles of the central structures in that process is still very limited. During fatigue, it has been shown that peripheral apparatus is capable of generating adequate force while central structures become insufficient/sub-optimal in driving them. This is known as central fatigue, and it can vary between muscles and different tasks. Fatigue induced by submaximal isometric contraction may have a greater central component than fatigue induced by prolonged maximal efforts. We studied the changes in regional cerebral blood flow (rCBF) of brain structures after sustained isometric muscle contractions of different submaximal force levels and of different durations, and compared them with the conditions observed when the sustained muscle contraction becomes fatiguing. Changes in cortical activity, as indicated by changes in rCBF, were measured using positron emission tomography (PET). Twelve subjects were studied under four conditions: (1) rest condition; (2) contraction of the m. biceps brachii at 30% of MVC, sustained for 60 s; (3) contraction at 30% of MVC, sustained for 120 s, and; (4) contraction at 50% of MVC, sustained for 120 s. The level of rCBF in the activated cortical areas gradually increased with the level and duration of muscle contraction. The fatiguing condition was associated with predominantly contralateral activation of the primary motor (MI) and the primary and secondary somatosensory areas (SI and SII), the somatosensory association area (SAA), and the temporal areas AA and AI. The supplementary motor area (SMA) and the cingula were activated bilaterally. The results show increased cortical activation, confirming that increased effort aimed at maintaining force in muscle fatigue is associated with increased activation of cortical neurons. At the same time, the activation spread to several cortical areas and probably reflects changes in both excitatory and inhibitory cortical circuits. It is suggested that further studies aimed at controlling afferent input from the muscle during fatigue may allow a more precise examination of the roles of each particular region involved in the processing of muscle fatigue.

Increased intra-articular concentration of bradykinin in the temporomandibular joint changes the sensitivity of muscle spindles in dorsal neck muscles in the cat.

The aim of the present study was to investigate to what extent activation of bradykinin-sensitive nerve endings of the temporomandibular joint (TMJ) might induce changes in the muscle spindles output from neck muscles through reflex effects on cervical fusimotor neurones. To this end, 26 muscle spindle afferents (MSAs) emanating from the trapezius and splenius muscles of the anaesthetised cat (alpha-chloralose, initial dosage 60 mg/kg) were recorded during injection of Bradykinin (BK) (12.5-50 microg/ml) in the ipsilateral TMJ. Fifteen (58%) MSAs exhibited statistically significant fusimotor effects to injection of BK into the TMJ. Of the 15, ten MSAs showed a response related to activation of static fusimotor neurones, three MSAs showed a response related to an activation of both static and dynamic fusimotor neurones and two MSAs showed a inhibition of both static and dynamic fusimotor neurones. The control experiments suggests that the effects observed were due to activation of BK sensitive nerve endings in the TMJ. It seems possible that the reflex connections between TMJ nociceptors and the fusimotor-muscle spindle system of dorsal neck muscles might be involved in the pathophysiological mechanisms responsible for the sensory-motor disturbances in the neck region often found on patients with temporomandibular disorders.

Brain processing of tonic muscle pain induced by infusion of hypertonic saline

Most of the previous studies on the effects of pain on Regional Cerebral Blood Flow (rCBF) had been done with brief cutaneous or intramuscular painful stimuli. The aim of the present study was to investigate the effect on rCBF of long lasting tonic experimental muscle pain. To this end we performed PET investigations of rCBF following tonic experimental low back pain induced by continuous intramuscular infusion of hypertonic (5%) saline (HS) with computer controlled infusion pump into the right erector spinae on L3 level in 19 healthy volunteers. Changes in rCBF were measured with the use of 15O labelled water during four conditions: Baseline (before start of infusion), Early Pain (4 min after start of infusion), Late Pain (20 min after start of infusion) and Post‐Pain (>15 min after stop of infusion) conditions.

Low Apgar score, neonatal encephalopathy and epidural analgesia during labour : a Swedish registry-based study

Maternal intrapartum fever (MF) is associated with neonatal sequelae, and women in labour who receive epidural analgesia (EA) are more likely to develop hyperthermia. The aims of this study were to investigate if EA and/or a diagnosis of MF were associated to adverse neonatal outcomes at a population level.

Spontaneous bursting neuronal discharges recorded from peripheral nerve in human: injury discharges or not?

This paper deals with a spontaneous, bursting neuronal activity which can not be altered by any stimulation in the periphery or voluntary actions or by cognitive tasks. An initial description of such units led to the conclusion that this activity was generated ectopically at the site of a previous or present impalement of a nerve fibre. The aim of the current study was to record a larger number of these units by using microneurography, in order to characterise their firing properties and particularly, see if any subtypes of units could be identified. In conclusion, this paper suggests that some of these discharges could be related to an injury of the nerve fibre, however most of them could not. Some hypothesis regarding the nature of these bursting activities are suggested.

Information-theoretic analysis of de-efferented single muscle spindles.

Abstract. The information transmission properties of single, de-efferented primary muscle-spindle afferents from the hind limb of the cat were investigated. The gastrocnemius medialis muscle was stretched randomly while recording spike trains from several muscle-spindle afferents in the dorsal root. Two classes of input stimuli were used: (i) Gaussian noise with band-limited flat spectrum, and (ii) Gaussian noise with a more “naturalistic” 1/fn spectrum. The “reconstruction” method was used to calculate a lower bound to the information rate (in bits per second) between the muscle spindles and the spinal cord. Results show that in response to the flat-spectrum input, primary muscle-spindle afferents transfer information mainly about high frequencies, carrying 2.12 bits/spike. In response to naturalistic-spectrum inputs, primary muscle-spindle afferents transfer information about both low and high frequencies, with “spiking efficiency” increasing to 2.67 bits/spike. A simple muscle-spindle simulation model was analyzed with the same method, emphasizing the important part played by the intrafusal fiber mechanical properties in information transmission.

Adrenal response after trauma is affected by time after trauma and sedative/analgesic drugs

BACKGROUND The adrenal response in critically ill patients, including trauma victims, has been debated over the last decade. The aim of this study was to assess the early adrenal response after trauma. METHODS Prospective, observational study of 50 trauma patients admitted to a level-1-trauma centre. Serum and saliva cortisol were followed from the accident site up to five days after trauma. Corticosteroid binding globulin (CBG), dehydroepiandrosterone (DHEA) and sulphated dehydroepiandrosterone (DHEAS) were obtained twice during the first five days after trauma. The effect of time and associations between cortisol levels and; severity of trauma, infusion of sedative/analgesic drugs, cardiovascular dysfunction and other adrenocorticotropic hormone (ACTH) dependent hormones (DHEA/DHEAS) were studied. RESULTS There was a significant decrease over time in serum cortisol both during the initial 24 h, and from the 2nd to the 5th morning after trauma. A significant decrease over time was also observed in calculated free cortisol, DHEA, and DHEAS. No significant association was found between an injury severity score ≥ 16 (severe injury) and a low (< 200 nmol/L) serum cortisol at any time during the study period. The odds for a serum cortisol < 200 nmol/L was eight times higher in patients with continuous infusion of sedative/analgesic drugs compared to patients with no continuous infusion of sedative/analgesic drugs. CONCLUSION Total serum cortisol, calculated free cortisol, DHEA and DHEAS decreased significantly over time after trauma. Continuous infusion of sedative/analgesic drugs was independently associated with serum cortisol < 200 nmol/L.