The neuromuscular transform in a single segment of a segmented heart tube.

Leech hearts are hybrids; they are myogenic but need entrainment by a heartbeat central pattern generator (CPG) to execute functional constriction patterns. Leech hearts are modular: two lateral segmented heart tubes running the length of the animal. Moving blood through the segmented heart tubes of leeches requires sequential constrictions, timed by a heartbeat CPG and relayed to each heart segment by likewise segmental motor neurons.

Output variability across animals and levels in a motor system.

Rhythmic behaviors vary across individuals. We investigated the sources of this output variability across a motor system, from the central pattern generator (CPG) to the motor plant. In the bilaterally symmetric leech heartbeat system, the CPG orchestrates two coordinations in the bilateral hearts with different intersegmental phase relations (Δ) and periodic side-to-side switches.

The neural control of heartbeat in invertebrates.

The neurogenic heartbeat of certain invertebrates has long been studied both as a way of understanding how automatic functions are regulated and for how neuronal networks generate the inherent rhythmic activity that controls and coordinates this vital function. This review focuses on the heartbeat of decapod crustaceans and hirudinid leeches, which remain important experimental systems for the exploration of central pattern generator networks, their properties, network and cellular mechanisms, modulation, and how animal-to-animal variation in neuronal and network properties are managed to produce functional output.

Variation in motor output and motor performance in a centrally generated motor pattern.

Central pattern generators (CPGs) produce motor patterns that ultimately drive motor outputs. We studied how functional motor performance is achieved, specifically, whether the variation seen in motor patterns is reflected in motor performance and whether fictive motor patterns differ from those in vivo. We used the leech heartbeat system in which a bilaterally symmetrical CPG coordinates segmental heart motor neurons and two segmented heart tubes into two mutually exclusive coordination modes: rear-to-front peristaltic on one side and nearly synchronous on the other, with regular side-to-side switches.

Bringing up the rear: new premotor interneurons add regional complexity to a segmentally distributed motor pattern.

Central pattern generators (CPGs) pace and pattern many rhythmic activities. We have uncovered a new module in the heartbeat CPG of leeches that creates a regional difference in this segmentally distributed motor pattern. The core CPG consists of seven identified pairs and one unidentified pair of heart interneurons of which 5 pairs are premotor and inhibit 16 pairs of heart motor neurons.

Coping with variability in small neuronal networks.

Experimental and corresponding modeling studies indicate that there is a 2- to 5-fold variation of intrinsic and synaptic parameters across animals while functional output is maintained. Here, we review experiments, using the heartbeat central pattern generator (CPG) in medicinal leeches, which explore the consequences of animal-to-animal variation in synaptic strength for coordinated motor output. We focus on a set of segmental heart motor neurons that all receive inhibitory synaptic input from the same four premotor interneurons.

Constancy and variability in the output of a central pattern generator.

Experimental and corresponding modeling studies have demonstrated a twofold to fivefold variation of intrinsic and synaptic parameters across animals, whereas functional output is maintained. These studies have led to the hypothesis that correlated, compensatory changes in particular parameters can at least partially explain the biological variability in parameters. Using the leech heartbeat central pattern generator (CPG), we selected three different segmental motor neurons that fire in a functional phase progression but receive input from the same four premotor interneurons.

A central pattern generator producing alternative outputs: pattern, strength, and dynamics of premotor synaptic input to leech heart motor neurons.

The central pattern generator (CPG) for heartbeat in medicinal leeches consists of seven identified pairs of segmental heart interneurons and one unidentified pair. Four of the identified pairs and the unidentified pair of interneurons make inhibitory synaptic connections with segmental heart motor neurons. The CPG produces a side-to-side asymmetric pattern of intersegmental coordination among ipsilateral premotor interneurons corresponding to a similarly asymmetric fictive motor pattern in heart motor neurons, and asymmetric constriction pattern of the two tubular hearts, synchronous and peristaltic.

A central pattern generator producing alternative outputs: phase relations of leech heart motor neurons with respect to premotor synaptic input.

The central pattern generator (CPG) for heartbeat in leeches consists of seven identified pairs of segmental heart interneurons and one unidentified pair. Four of the identified pairs and the unidentified pair of interneurons make inhibitory synaptic connections with segmental heart motor neurons. The CPG produces a side-to-side asymmetric pattern of intersegmental coordination among ipsilateral premotor interneurons corresponding to a similarly asymmetric fictive motor pattern in heart motor neurons, and asymmetric constriction pattern of the two tubular hearts: synchronous and peristaltic.

Hemodynamics in the leech: blood flow in two hearts switching between two constriction patterns.

Two tubular, segmented hearts propel blood through the closed circulatory system of the medicinal leech and switch every 20-40 beats between two constriction patterns. We showed recently that within one heartbeat cycle, heart segments on one side constrict peristaltically rear-to-front (;peristaltic heart'), followed by nearly synchronous front-to-rear constrictions in the contralateral heart segments (;synchronous heart'). Using optical recordings from intact leeches, we now characterize the hemodynamic properties of the cardiac cycle of individual heart segments in different regions to ask whether the reversal of constrictions affects flow into, out of, and along the hearts.

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.

Heartbeat control in leeches. I. Constriction pattern and neural modulation of blood pressure in intact animals.

Two tubular hearts propel blood through the closed circulatory system of the medicinal leech. The hearts are myogenic but are driven by a centrally generated motor pattern that controls heart rate and intersegmental coordination. In two consecutive papers, we address the question of how the motor pattern is translated into the pattern of diastole and systole of leech hearts.

Heartbeat control in leeches. II. Fictive motor pattern.

The rhythmic beating of the tube-like hearts in the medicinal leech is driven and coordinated by rhythmic activity in segmental heart motor neurons. The motor neurons are controlled by rhythmic inhibitory input from a network of heart interneurons that compose the heartbeat central pattern generator. In the preceding paper, we described the constriction pattern of the hearts in quiescent intact animals and showed that one heart constricts in a rear-to-front wave (peristaltic coordination mode), while the other heart constricts in near unison over its length (synchronous coordination mode) and that they regularly switch coordination modes.

Organogenesis in the leech: development of nephridia, bladders and their innervation.

The formation of the definitive excretory system (nephridium and bladder complex) in Hirudo medicinalis during the last two thirds of embryonic development was observed with light- and electron microscopy, immunocytochemistry, and nuclear labeling. In jawed leeches, two excretory systems develop and function successively. The nephridia of the cryptolarva are associated with the larval sac and persist until the definitive nephridia are sufficiently developed to be functional.

Structural and ultrastructural features of the inlet and outlet regions of the urinary bladders of the leech, Hirudo medicinalis L.

Each of the 34 nephridia in the leech, Hirudo medicinalis, has its own separate bladder. Urine flows from the last portion of the nephridium, the final canal, into the bladder through a special inlet which prevents backflow of urine. This inlet consists of a vestibule formed by two serially arranged septa, each with a small pore.