Biomedical subjects

Adam L Weaver

Publications and source records attributed to Adam L Weaver.

5 recordsLinked to original sources

PubMed2006

A central pattern generator producing alternative outputs: temporal pattern of premotor activity.

The central pattern generator for heartbeat in medicinal leeches constitutes seven identified pairs of segmental heart interneurons. Four identified pairs of heart interneurons make a staggered pattern of inhibitory synaptic connections with segmental heart motor neurons. Using extracellular recording from multiple interneurons in the network in 56 isolated nerve cords, we show that this pattern generator produces a side-to-side asymmetric pattern of intersegmental coordination among ipsilateral premotor interneurons. This pattern corresponds to a similarly asymmetric fictive motor pattern in heart motor neurons and asymmetric constriction pattern of the two tubular hearts, synchronous and peristaltic. We provide a quantitative description of the firing pattern of all the premotor interneurons, including phase, duty cycle, and intraburst frequency of this premotor activity pattern. This analysis identifies two stereotypical coordination modes corresponding to synchronous and peristaltic, which show phase constancy over a broad range of periods as do the fictive motor pattern and the heart constriction pattern. Coordination mode is controlled through one segmental pair of heart interneurons (switch interneurons). Side-to-side switches in coordination mode are a regular feature of this pattern generator and occur with changes in activity state of these switch interneurons. Associated with synchronous coordination of premotor interneurons, the ipsilateral switch interneuron is in an active state, during which it produces rhythmic bursts, whereas associated with peristaltic coordination, the ipsilateral switch interneuron is largely silent. We argue that timing and pattern elaboration are separate functions produced by overlapping subnetworks in the heartbeat central pattern generator.

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PubMed2003

Lobster (Panulirus interruptus) pyloric muscles express the motor patterns of three neural networks, only one of which innervates the muscles.

In several systems, including some well studied invertebrate "model" preparations, rapid, rhythmic inputs drive slow muscles. In this situation muscle contractions can summate temporally between motor neuron bursts, tonically contract, and low-pass filter broad-band input. We have investigated how the muscles innervated by each motor neuron type of the rapid, rhythmic (cycle period, approximately 1 sec) lobster pyloric network respond when driven by previously recorded in vitro pyloric network activity from intact stomatogastric nervous systems. Under these conditions the much slower gastric mill and cardiac sac networks of the stomatogastric nervous system are also active and modify pyloric activity. All of the muscles show pyloric timed phasic contractions that ride on a sustained tonic contraction; muscle activity can range from being almost completely phasic to almost completely tonic. The modifications of pyloric neuron activity induced by gastric mill (cycle period, approximately 10 sec) activity result in some pyloric muscles showing prominent, gastric mill-timed, changes in either phasic or tonic contraction amplitude. The strong modification of pyloric neuron activity induced by cardiac sac (cycle period, approximately 60 sec) activity alters the contractions of all pyloric muscles. These changes are sufficient that for some muscles, in some preparations, the primary muscle output is cardiac sac-timed. This is the first work to examine the motor responses of all pyloric muscle classes to spontaneous stomatogastric activity and shows that the pyloric motor pattern is a complex combination of the activities of three neural networks, although only one (the pyloric) innervates the muscles.

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PubMed2003

Relating network synaptic connectivity and network activity in the lobster (Panulirus interruptus) pyloric network.

The lobster pyloric network has a densely interconnected synaptic connectivity pattern, and the role individual synapses play in generating network activity is consequently difficult to discern. We examined this issue by quantifying the effect on pyloric network phasing and spiking activity of removing the Lateral Pyloric (LP) and Ventricular Dilator (VD) neurons, which synapse onto almost all pyloric neurons. A confounding factor in this work is that LP and VD neuron removal alters pyloric cycle period. To determine the effects of LP and VD neuron removal on pyloric activity independent of these period alterations, we altered network period by current injection into a pyloric pacemaker neuron, hyperpolarized the LP or VD neuron to functionally remove each from the network, and plotted various measures of pyloric neuron activity against period with and without the LP or VD neuron. In normal physiological saline, in many (or most) cases removing either neuron had surprisingly little effect on the activity of its postsynaptic partners, which suggests that under these conditions these neurons play a relatively small role in determining pyloric activity. In the cases in which removal did alter postsynaptic activity, the effects were inconsistent across preparations, which suggests that either despite producing very similar neural outputs, pyloric networks from different animals have different cellular and synaptic properties, or some synapses contribute to network activity only under certain modulatory conditions, and the "baseline" level of modulatory influence the network receives from higher centers varies from animal to animal.

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PubMed2002

Follower neurons in lobster (Panulirus interruptus) pyloric network regulate pacemaker period in complementary ways.

Distributed neural networks (ones characterized by high levels of interconnectivity among network neurons) are not well understood. Increased insight into these systems can be obtained by perturbing network activity so as to study the functions of specific neurons not only in the network's "baseline" activity but across a range of network activities. We applied this technique to study cycle period control in the rhythmic pyloric network of the lobster, Panulirus interruptus. Pyloric rhythmicity is driven by an endogenous oscillator, the Anterior Burster (AB) neuron. Two network neurons feed back onto the pacemaker, the Lateral Pyloric (LP) neuron by inhibition and the Ventricular Dilator (VD) neuron by electrical coupling. LP and VD neuron effects on pyloric cycle period can be studied across a range of periods by altering period by injecting current into the AB neuron and functionally removing (by hyperpolarization) the LP and VD neurons from the network at each period. Within a range of pacemaker periods, the LP and VD neurons regulate period in complementary ways. LP neuron removal speeds the network and VD neuron removal slows it. Outside this range, network activity is disrupted because the LP neuron cannot follow slow periods, and the VD neuron cannot follow fast periods. These neurons thus also limit, in complementary ways, normal pyloric activity to a certain period range. These data show that follower neurons in pacemaker networks can play central roles in controlling pacemaker period and suggest that in some cases specific functions can be assigned to individual network neurons.

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PubMed2002

Combinatorial and cross-fiber averaging transform muscle electrical responses with a large stochastic component into deterministic contractions.

Pyloric muscles of the stomatogastric neuromuscular system of the lobster Panulirus interruptus produce highly deterministic (range, less than +/- 6% of mean amplitude) contractions in response to motor nerve stimulation with unchanging spike bursts containing physiological (5-10) spike numbers. Intracellular recordings of extrajunctional potentials (EJPs) evoked in these muscles by motor nerve stimulation revealed a large, apparently stochastic amplitude variation (range, +/- 36% of mean amplitude). These observations raised the question of how do electrical responses with a large amplitude variation give rise to deterministic muscle output? We show here that this question is likely resolved by (1) combinatorial averaging within individual muscle fibers of the multiple EJPs that occur in motor neuron bursts, and (2) averaging across muscle fibers whose electrical responses are uncorrelated. Synapses with high inherent variability are also present in vertebrate CNSs. Combinatorial averaging in multispike inputs would also reduce variation in postsynaptic response at these synapses. The data reported here provide further support that bursting presynaptic activity could make such synapses functionally deterministic as well.

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