Biomedical subjects

K A Scheepstra

Publications and source records attributed to K A Scheepstra.

4 recordsLinked to original sources

PubMed1998

Correlated histological and physiological observations on a case of common sensory output and motor input of the bag1 fibre and a chain fibre in a cat tenuissimus spindle.

In muscle spindles of the cat, independent control of dynamic and static components of the response of the primary sensory ending to stretch is provided by separate motor inputs to the various kinds of intrafusal muscle fibre: dynamic axons (gamma or beta) to the bag1 fibres and static axons to the bag2 (typically gamma only) and chain (gamma or beta) fibres. Nonlinear summation of separately evoked effects during combined stimulation of dynamic and static motor axons appears to be due to mutual resetting by antidromic invasion of separate encoding sites, leading to partial occlusion of the momentarily lesser response by the greater. The encoding sites are thought to be located within the primary ending's preterminal branches which from first-order level are normally segregated to the bag1 fibre and to the bag2 and chain fibres. Here we describe the analysis of a special case that arose in a histophysiological study which had shown that the degree of occlusion was related to the minimum number of nodes between the putative encoding sites. Three-dimensional reconstruction of the primary ending revealed that the terminals of one chain fibre were derived entirely from the first-order branch that supplied the bag1 fibre, including one terminal that was shared directly with the bag1 (sensory cross-terminal). The other first-order branch supplied the bag2 and remaining chain fibres as normal. The degree of occlusion seen during simultaneous stimulation of a dynamic beta axon and a static gamma axon indicated that the encoding sites were separated by both first-order branches. Schematic reconstruction of the motor innervation revealed that the static gamma axon was most unlikely to have supplied the chain fibre which shared sensory terminals with the bag1, but that these fibres also shared a motor input with histological characteristics of beta type. Ramp-frequency stimulation of the dynamic beta axon at constant length evoked a driving effect which persisted after fatiguing the extrafusal component and was therefore explicable on the basis of the observed pattern of motor innervation, though the identity of the axon could not be conclusively proved. Individually, instances of shared sensory terminals and motor input of bag1 and chain fibres are rare in the cat; their combination in a single spindle with correlated physiology is described here for the first time. The observation is considered in relation to the importance of dynamic and static segregation in motor control, since it may imply that there is a lower limit to the degree of segregation that the developmental programme can provide.

Animals↗

PubMed1997

Pacemaker activity in a sensory ending with multiple encoding sites: the cat muscle spindle primary ending.

1. A combined physiological, histological and computer modelling study was carried out on muscle spindles of the cat tenuissimus muscle to examine whether there was any correlation between the functional interaction of putative encoding sites, operated separately by static and dynamic fusimotor neurones, and the topological structure of the preterminal branches of the primary sensory ending. 2. Spindles, whose I a responses to stretch and separate and combined static and dynamic fusimotor stimulation were recorded in physiological experiments, were located in situ. Subsequently the ramifications of the sensory ending were reconstructed histologically, and the topology of the branch tree was used in computer simulations of I a responses to examine the effect of the electronic separation of encoding sites on the static-dynamic interaction pattern. 3. Interactions between separate static and dynamic inputs, manifest in responses to combineed stimulation, were quantified by a coefficient of interaction (Ci) which, by definition, was 1 for strictly linear summation of separate inputs and zero for maximum occlusion between inputs. 4. For the majority of spindles static-dynamic interactions were characterized by pronounced occlusion (C1 < 0.35). In these spindles putative encoding sites (the peripheral heminodes of the branches supplying the intrafusal fibres activated by individual fusimotor efferents) were separated by a minimum conduction path of between three and ten myelinated segments (2-9 nodes of Ranvier). In contrast, significant summation (C1, approximately 0.7) was found in only one spindle. In this case putative encoding sites were separated by a single node. 5. Occlusion was not due to encoder saturation and it could not be accounted for by any other known physiological mechanisms (intrafusal fatigue or unloading). It is therefore attributed to competitive pacemaker interaction between encoding sites which are largely selectively operated by static and dynamic fusimotor efferents. 6. Model simulations of real preterminal-branch tree structures confirmed that short conduction paths between encoding sites were associated with manifest summation, whereas longer minimum conduction paths favoured pronounced occlusion. 7. In the extreme, occlusion could be so pronounced as to give rise to negative values of C1 during critical segments of response cycles. This was associated with lower discharge rates during combined static and dynamic stimulation than the higher of the individual stimulation effects. This phenomenon is referred to as hyperocclusion. Computer simulations demonstrated that hyperocclusion could be accounted for by a slow ionic adaptation process. e.g. by a very slowly activating K+ conductance.

Animals↗

PubMed1995

A model study on the influence of a slowly activating potassium conductance on repetitive firing patterns of muscle spindle primary endings.

The general mathematical model of Frankenhaeuser and Huxley, which describes the generation of action potentials in myelinated nerve fibres, has been used as a kernel for a model of a sensory nerve ending. Two types of modifications were implemented. First, the four original permeability constants (those of potassium, sodium, non-specific and leak) were changed simultaneously (using an automated tuning algorithm), in order to introduce low-frequency repetitive firing capability (down to 15 Hz), keeping the deviations from the original values as small as possible. Second, a slow potassium conductance was added, in order to model slow processes (like accommodation) with time constants longer than those required to simulate short-lasting action potentials. Sensory stimuli were simulated as changes in passive conductance. The model displayed the following properties, which are typical of many sensory endings in general and of muscle spindle primary endings in particular: (i) The range of sustained repetitive firing was extended into the domain of low discharge rates, so as to span the entire physiological range (from 2 to 700 sec-1). (ii) The relation between firing rate and receptor potential was roughly linear over the full range of firing. (iii) Following a step increase of stimulus, the firing rate showed adaptation with a time constant of about 70 msec. (iv) Sudden reduction of the stimulus was followed by post-release silence. (v) Following a step increase of stimulus, the receptor potential showed a short dynamic peak. (vi) Following a step decrease of stimulus, the receptor potential displayed a post-release undershoot and recovery with a time constant of approximately 100 msec. (vii) With sinusoidal stimuli the receptor potential showed band-pass filter properties with phase advance below and phase lag above 12 Hz and a peak in gain at about 20 Hz. Thus the present equations adequately describe a range of known properties of muscle spindle primary endings. Based on minimal modification and extension of the Frankenhaeuser-Huxley model of action potential generation in myelinated fibres, they constitute a theory of sensory encoding. This theory is further corroborated by the experimental evidence of the presence of calcium-activated potassium channels in numerous sensory--including primary spindle--endings.

Action Potentials↗

PubMed1993

Computer-simulated neural networks: an appropriate model for motor development?

The idea of an artificial neural network is introduced in a historical context, and the essential aspect of it, viz., the modifiable synapse, is compared to the aspect of plasticity in the natural nervous system. Based on such an artificial neural network, a model is presented for the way in which (the path along which) the connectivity in the spinal cord is modified during the period that a newborn 'learns' to control the movement of his forearm. In this way an automatic calibration of the receptors and the antagonists' recruitment of motor units is represented. The learning process is described in non-mathematical terminology. The model is then shown to be able after learning to reach target angles outside the training set of angles, and to be able to relearn when an important receptor has been made inoperative. In this way it is shown that the model is able to generalize, and that it is robust against at least some damage.

Computer Simulation↗