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At least 199 records · Page 11Linked to original sources
PubMed1978

Burst generation by electrically coupled network in the snail helisoma: analysis using computer simulation.

The effectiveness of electrical coupling between neurons as a mechanism for mediating single and repetitive bursts is investigated here using computer simulation. The cyberchron network in the snail Helisoma generates repetitive bursts controlling the animal's feeding behavior and served as the basic model for the simulation studies described in this paper. The action potential properties of individual neurons were modeled by the Rall equations describing generalized action potentials. Several properties of electrical coupling and its role in burst generation were demonstrated, including: (1) A neuron in an electrically coupled network can generate action potentials at a higher frequency than an isolated neuron with similar membrane properties due to the loading through the electrical junctions. However, the ability of electrically coupled neurons to generate high-frequency bursts of action potentials requires a concomitantly greater amount of driving current to overcome the junctional loading. (2) Temporal and spatial summation of synaptic input onto a neuron is maintained at its most effective level because the postsynaptic current is integrated across the long postsynaptic membrane time constant. (3) Initial simulations concentrated on a pair of electrically coupled neurons which were below threshold. Stimulation of one of the two neurons with a short pulse resulted in a reverberation or regenerative excitation between the two neurons. The reverberation terminated after a number of action potentials dependent on the specific model parameters. Similar results were obtained with a network containing a greater number (20) of model neurons if approximately one-half of the neurons were stimulated simultaneously. However, none of the cases studied produced more than a single discrete burst. (4) Simulations were also conducted on 20-neuron networks containing two subpopulations of model neurons differing in their values of coupling resistance and excitability. Some networks of this type required stimulation of only one cell to make the two subpopulations of model neurons reverberate with one another. Such simulations suggest the possibility that 'preferred' input pathways involving a small number of neurons would be capable of 'turning on' the activity of the entire network.

Animals↗
PubMed1979

A proposed natural geometry of recovery from akinesia in the lateral hypothalamic rat.

The Eshkol-Wachmann movement notation is used to analyze and describe neurological recovery from the akinesia caused by severe bilateral lateral hypothalamic (LH) damage in rats. Exploratory movement recovers along several relatively independent dimensions which appear successively. First, lateral head scanning movements recover. At about the same time or later, longitudinal (backward-forward) head scans appear. After movements along these two dimensions increase in amplitude and involve the whole body, vertical (dorsal-ventral) head scans with snout contact (along vertical surfaces) typically appear, and increase gradually in amplitude. Later, vertical rearing without snout contact emerges. Recovery proceeds cephalocaudally, as more caudal limb and body segments are recruited along each of the above dimensions separately. LH rats show delayed recruitment of caudal limb and body segments ('strait-jacket phenomenon'). Support of the body and management of limb and body segments' contact with the ground also recover relatively independently, in a proximodistal fashion. In recovery, arrests between bouts of activity become shorter. Movement first becomes organized in relation to the animals' own body, and only much later, in relation to the environment. In each sequence of movement after pronounced immobility, the rat recapitulates the process of recovery; and, any time it starts to move, it repeats the movements at a particular amplitude several times until there is an increase to the next larger size movement ('warm-up' phenomenon). These regularities explain the apparently bizarre stereotyped behavior in partial enclosures ('behavioral traps') seen in LH rats recovering from akinesia. They also explain some aspects of exploration in rats and normal social behavior of wild animals, particularly in situations involving fear and conflict.

Animals↗
PubMed1979

A comparative study of ventrolateral and recurrent excitatory postsynaptic potentials in large pyramidal tract cells in the cat.

In acute cats deeply anesthetized with Nembutal, monosynaptic excitatory postsynaptic potentials (EPSPs) triggered by stimulation of the ventrolateral (VL) thalamic nucleus and the pes pedunculus were recorded in large pyramidal tract cells (PT cells). Deep anesthesia, low intensities of stimulation and an averaging technique were used in order to get VL and recurrent EPSPs free of polysynaptic potentials. Comparison of the time course of both EPSPs revealed a much faster rise time and shorter half-width for VL EPSPs than for recurrent EPSPs. This would suggest a more proximal location for VL synaptic contacts than for recurrent ones with respect to the soma of PT cells. The separation of the sites of origin of both EPSPs is further suggested by their almost perfect linear summation. It is suggested that VL EPSPs are produced on the apical dendritic tree, while recurrent EPSPs could originate on the basilar dendritic branches.

Animals↗
PubMed1980

Vagal and splanchnic effects at the level of the ventromedian nucleus of the hypothalamus (VMH) in the cat.

A systematic study of vagal and splanchnic projections to the VMH area was undertaken in anaesthetized cats by means of macroelectrodes and microelectrodes. Responses elicited by vagal and splanchnic nerve stimulation were recorded from the VMH nucleus and the region situated above it. The fact that they are identical, whatever the nerve stimulated, indicates that the afferences follow a common central pathway. From the response latency and the stimulation parameters, it is assumed that the small vagal and splanchnic fibres (A delta, B and C) are involved. Evoked potentials are generally constituted of two parts: the earlier implies certainly the VPL nucleus (since it disappears definitively after VPL coagulation), whereas the later concerns a different relay probably located in the neighbouring and associative structure (since it persists after VPL coagulation). Similar late responses were simultaneously recorded in VPL nucleus, suggesting that this structure was the same for both nuclei. Its exact location was discussed. On the other hand, effects of gastric distension on the evoked potentials produced by vagal and splanchnic nerve stimulation were studied. The results obtained (decrease or increase in the responses, occurrence of an additional potential) indicated clearly that the gastric afferences projected to the VMH region. This simple method could be used to determine the importance of the sensory innervation of the different parts of the digestive tract.

Afferent Pathways↗
PubMed1981

Comparison of the recruitment and discharge properties of motor units in human brachial biceps and adductor pollicis during isometric contractions.

Experiments were designed to assess the relative contribution of rate coding and motor unit recruitment to force production in two muscles of different fiber composition and function. Single motor unit action potentials were recorded during steady isometric contraction in biceps brachii, a large proximal limb muscle of mixed fiber composition, and adductor pollicis, a small hand muscle comprised mainly of type I muscle fibers. Action potential spike trains were obtained over the entire force range in each muscle. The results suggest that these two muscles are controlled in different ways. In biceps brachii, recruitment was observed from 0 to 88% maximum voluntary contraction (MVC). In adductor pollicis, no motor unit was observed to be recruited at forces greater than 50% MVC, with the majority of recruitment occurring below 30% MVC. On the average, motor units in adductor pollicis discharged at higher rates, with less regularity, and with a greater frequency of occurrence of short interspike intervals (intervals less than or equal to 20 msec) than those in biceps brachii. Such findings suggest that rate coding plays a more prominent role in force modulation in adductor pollicis, while recruitment plays a more important role throughout the contractile force range in biceps brachii.

Adult↗