Mats Djupsjöbacka

Influences on the γ-muscle spindle system from muscle afferents stimulated by increased intramuscular concentrations of bradykinin and 5-HT

There is evidence that static muscular contractions induce a release of bradykinin (BK) in the working muscle, and that increased concentration of BK and 5-HT in a muscle increases the discharge rate of a subpopulation of group III and group IV muscular afferents. It is also known that activity in group III and IV muscle afferents may activate gamma-motoneurones to both homonymous and heteronymous muscles. The aim of the present study was to investigate whether increased concentration of BK and 5-HT in one muscle may influence the activity in primary and secondary muscle spindle afferents (MSAs) from the chemically affected muscle and from surrounding muscles, via fusimotor reflexes. The experiments were made on six cats anaesthetised with alpha-chloralose. The triceps surae (GS) and the posterior biceps and semitendinosus (PBSt) muscles were subjected to sinusoidal stretches. Simultaneous recordings of 2-11 MSAs from these muscles were made and the mean rate of firing and the modulation for each MSA were determined. Responses of 47 MSAs (26 PBSt and 21 GS) were recorded. The responsiveness of the MSAs to injections of BK (9-100 mg/ml, 0.5-1.0 ml) and 5-HT (25-150 mg/ml, 0.5-1.0 ml) was 89% and 83%, respectively, for injections into the arterial supply of the ipsilateral GS muscle, and 84% and 40% respectively for injections to the contralateral GS muscle. Of 10 secondarŁy MSAs, only one was unresponsive to BK injections, while several MSAs responded to both ipsilateral and contralateral BK injections.(ABSTRACT TRUNCATED AT 250 WORDS)

Influences on the γ-muscle-spindle system from muscle afferents stimulated by increased intramuscular concentrations of arachidonic acid

There is evidence that static muscular contractions induce a release of arachidonic acid (AA) in the working muscle and that increased concentration of AA in a muscle increases the discharge rate of a subpopulation of groups III and IV muscular afferents. It is also known that activity in groups III and IV muscle afferents may activate gamma-motoneurones to both homo- and heteronymous muscles. The aim of the present study was to investigate if increased concentration of AA in one muscle may influence the activity in primary and secondary muscle spindle afferents (MSAs) from the chemically affected muscle and from surrounding muscles, via fusimotor reflexes. The experiments were made on five cats anaesthetized with alpha-chloralose. The triceps surae (GS) and the posterior biceps and semitendinosus (PBSt) muscles were subjected to sinusoidal stretches. Simultaneous recordings of 2-12 MSAs from these muscles were made and the mean rate of firing and the modulation for each MSA were determined. Responses of 36 MSAs (17 PBSt and 19 GS) were recorded. The responsiveness of the MSAs to injections of AA (0.3-1.0 mg; 0.3-1 ml) was 86% (n = 36) for injections into the arterial supply of the ipsilateral GS muscle and 45% (n = 20) for injections to the contralateral GS muscle. Out of 14 secondary MSAs, only one was unresponsive to ipsilateral AA injections while two of eight were unresponsive to contralateral AA injection. The majority of responses were compatible with predominantly static or mixed dynamic and static fusimotor activation. None of the effects were compatible with inhibition of fusimotor activity. The duration of the effects were usually 2-4 min. However, on some occasions the elevations in MSA activity persisted for up to 1 h. Local anaesthesia of the nerve to the injected muscle always abolished the effects of the injections and control injections of the solution in which the AA was dissolved were ineffective in changing the MSA responses. I.v. injections occasionally induced effects on the MSAs, but such effects were significantly different from those caused by close arterial muscle injections. Thus, increased concentration of AA may excite primary and secondary MSAs from homo- as well as heteronymous muscles, including contralateral muscles, most probably via fusimotor reflexes evoked by activity in chemosensitive muscle afferents.

Influences on the γ-muscle-spindle system from contralateral muscle afferents stimulated by KCl and lactic acid

The aim of the present study was to investigate whether increased concentrations of lactic acid and potassium chloride (KCl) in contralateral muscles can influence the sensitivity of primary and secondary muscle spindle afferents (MSAs) from ipsilateral extensor and flexor muscles. The experiments were performed on 7 cats anaesthetised with alpha-chloralose. Recordings were made simultaneously from 2-12 single MSAs from the triceps surae (GS) and/or the posterior biceps and semitendinosus muscles (PBSt). The mean rate of firing and the amplitude of a fitted sine curve of MSA responses to sinusoidal stretching of the receptor bearing muscles were determined. Responses of 42 primary MSAs (17 from PBSt and 25 from GS) were recorded. On 33 of these, clear-cut alterations in sinusoidal response were evoked by injection of 1 ml KCl (200-400 mM) or 1 ml lactic acid (20-50 mM) into the arterial supply of the contralateral GS or PBSt muscles. Six out of 8 secondary MSAs showed sizeable effects to increased intramuscular concentrations of KCl and/or lactic acid (3 from PBSt and 3 from GS). On both primary and secondary MSAs, from GS as well as from PBSt muscles, the large majority of effects were excitatory. All effects on secondary MSAs were compatible with reflex actions on static fusimotor neurones, whereas on primary MSAs different types of reflex responses were observed (i.e. pure static, pure dynamic and mixtures of static and dynamic fusimotor actions). Stimuli related alterations in MSA responses were completely abolished when the contralateral GS or PBSt nerves were anaesthetised.(ABSTRACT TRUNCATED AT 250 WORDS)

A neural network approach to real-time spike discrimination during simultaneous recording from several multi-unit nerve filaments

A multi-channel, real-time, unsupervised spike discriminator was developed in order to reconstruct single spike trains from several simultaneously recorded multi-unit nerve filaments. The program uses a Self Organising Map (SOM) algorithm for the classification of the spikes. In contrast to previous similar techniques, the described method is made for use on a PC, and the method may thus be implemented at relatively low cost. In order to test the accuracy of the program, a robustness test was performed, where noise with different RMS levels was superimposed on the spikes. Furthermore, the maximal classification rate was determined. The program is easy to use, since the only manual inputs needed are the voltage threshold for spike detection, and the number of units present in each recorded nerve filament.

A multichannel hook electrode for simultaneous recording of up to 12 nerve filaments

This paper describes a simple multichannel hook electrode specially adapted for simultaneous recording from several dorsal root filaments in the cat. The electrode consists of a 3-mm-thick PVC plate and accommodates 12 silver wire hooks placed in notches in the plate. It has a semicircular shape, and the notches are milled radially in the plate. With this electrode we have made successful recordings of single-unit activity from 12 muscle afferents for periods of up to 8 h. In contrast to more rigid and complicated devices, e.g., developed to assess activity of populations of individual neurones from single-channel recordings of multi-unit activity, it also permits selection of the types of afferents to be recorded. The use of this simple device and an appropriate number of amplifiers and window discriminators can greatly enhance the quality of studies on populations of afferents in animals and may also decrease the number of laboratory animals needed.

A method for analysis of encoding of stimulus separation in ensembles of afferents

This paper describes a new method for the analysis of ensemble coding in populations of receptor afferents. The method is based on principal component analysis (PCA) combined with algorithms for calculation of stimulus separation (discrimination). It allows a quantification of the ability of ensembles of afferents to discriminate between stimuli of different intensity, and it can also be used to calculate to which extent each afferent in an ensemble contributes to the discrimination. Since the emphasis is put on discrimination of stimuli rather than on similarities between the representations of the stimulus in the ensemble response and the stimulus itself, it involves no implicit assumption about the unknown decoding mechanisms in the CNS. Examples of results obtained with analysis of and comparison of simultaneously recorded and pooled sequentially recorded populations of primary muscle spindle afferents are given. These results support some of the general arguments in favour of the ensemble coding theory, and indicate that the results of studies made on populations of simultaneously recorded receptor afferents may differ considerably from results obtained in studies on pooled sequentially recorded afferents. In contrast to when ensemble responses are based on parallel recordings, population responses constructed from sequential recordings will be distorted as a result of unavoidable temporal variations in physiological conditions.

Analysis of Encoding of Stimulus Separation in Ensembles of Muscle Afferents

In sensory physiology there have been two main theories relating to the processing of afferent information. These are the “labelled line” theory and the “ensemble coding” theory (cf. Ray & Doetsch, 1990a). The labelled line theory concentrates on the response properties of single receptors, while the ensemble coding theory places the emphasis on encoding in a neuronal population rather than by individual receptors or afferents (Erickson, 1968; Ray & Doetsch, 1990b).

Regulation of the γ-Muscle Spindle System by Chemosensitive Muscle Afferents

Appelberg, Hulliger, Johansson & Sojka (1983a) has shown that electrical stimulation of group III muscle afferents evokes excitatory effects on both homo- and heteronymous γ-motoneurones. It is also well known that increased intramuscular concentrations of substances liberated during static muscle contractions and/or inflammation increase the activity of group III and IV muscle afferents. Furthermore, it has been shown that the activity in the primary muscle spindle afferents (MSAs) is most important for the reflex mediated muscle stiffness.