Activation of two forms of locomotion by a previously identified trigger interneuron for swimming in the medicinal leech

Abstract

Higher-order projection interneurons that function in more than one behavior have been identified in a number of preparations. In this study, we document that stimulation of cell Tr1, a previously identified trigger interneuron for swimming in the medicinal leech, can also elicit the motor program for crawling in isolated nerve cords. We also show that motor choice is independent of the firing frequency of Tr1 and amount of spiking activity recorded extracellularly at three locations along the ventral nerve cord prior to Tr1 stimulation. On the other hand, during Tr1 stimulation there is a significant difference in the amount of activity elicited in the ventral nerve cord that correlates with the motor program activated. On average, Tr1 stimulation trials that lead to crawling elicit greater amounts of activity than in trials that lead to swimming.

Serotonin as an integrator of leech behavior and muscle mechanical performance

“…These can act antagonistically, or partially antagonistically in the case of the oblique layer, to the longitudinal layer, increasing segment length through reduction in segment circumference. Circular muscle activity has been reported during swimming (Baader, 1997) but is less intense than during the extension phase of crawling, likely reflecting partial recruitment of the muscle, and is in phase with that of longitudinal muscle motor neurons (Brodfuehrer et al, 2008). This suggests that antagonistic muscle activity is not a major source of work for longitudinal muscle elongation during swimming.…”

“…Reduced passive tension in the longitudinal muscles would require less work to be done by the circular muscle layer during the elongation part of the crawl cycle (Table 2). Retzius cell activity is highest during the elongation phase (Brodfuehrer et al, 2008), suggesting that reduction of passive resistance to antagonist work is an important aspect of 5-HT peripheral function during this behavior.…”

“…5-HT influences leech behavior both through modulation of motor pattern generation by the central nervous system and peripheral effects on tissue function (Mason and Kristan, 1982;Nusbaum and Kristan, 1986;Lent et al, 1988;Tian et al, 2007;Brodfuehrer et al, 2008;Gaudry et al, 2010). Elevated 5-HT levels are associated with pre-feeding search behavior, increased swimming activity and increased biting frequency (Willard, 1981;Lent and Dickinson, 1984;Lent et al, 1988;Wilson et al, 1996a).…”

Necessary, Sufficient and Permissive: A Single Locomotor Command Neuron Important for Intersegmental Coordination

“…In the medicinal leech, specific swim command-like neurons appear more dedicated or multifunctional (Brodfuehrer and Burns, 1995; Brodfuehrer et al, 1995; Brodfuehrer et al, 2008; Mullins et al, 2011). The swim trigger neuron Tr-2, for example, can initiate or at other times terminate swimming (Brodfuehrer and Friesen, 1986; O’Gara and Friesen, 1995).…”

Discontinuous locomotion and prey sensing in the leech

“…During prey localization, leeches readily switch between swimming, crawling and non-locomotion (for descriptions of these behaviors, see Fig.1) (Gray et al, 1938;Sawyer, 1986;SternTomlinson et al, 1986;Brodfuehrer et al, 1995a;Brodfuehrer et al, 2008), suggesting that leeches may serve as another example of stop-and-go predators. But if this is the case, then why would leeches employ two separate locomotor behaviors?…”