Using the restricted neuronal networks of an invertebrate to understand how coordinated rhythmic activity in produced by the nervous system

Understanding how rhythmic motor patterns are generated and coordinated by
central pattern generating neuronal networks (CPGs) has been one of the
major advances of the past 20 years in neuroscience. Owing to the relative
simplicity of the nervous systems of invertebrates, their analysis, both
physiological and computational, has contributed significantly to this
understanding. In my lab we have used the leech heartbeat CPG to
participate in this analysis.

The two longitudinal heart tubes of medicinal leeches are innervated in
each midbody segment (3–18) by the ipsilateral member of a single
bilateral pair of heart motor. The heart motor neurons fire rhythmically
in a coordinated pattern because they are inhibited ipsilaterally by a set
of four bilateral pairs of identified of premotor heart interneurons that
are members of the rhythmically active heartbeat CPG. The fictive motor
pattern for heartbeat is bilaterally asymmetric: motor neurons on one side
fire in a rear-to-front progression (peristaltic), whereas those on the
other fire in near synchrony (synchronous) and there is side-to-side
coordination of the two patterns. The asymmetry is not permanent, but
rather the motor neurons of the two sides change roles (patterns) every
20–40 heartbeat cycles. The temporal pattern of the activity of the
premotor interneurons has been described quantitatively and, like that of
the motor neurons, is bilaterally asymmetric with strict side-to-side
coordination and regular side-to-side switches of peristaltic and
synchronous patterns. In this talk, we will explore how the basic rhythm
is generated by network motifs of mutually inhibitory interneurons (so-
called half-center oscillators) in the CPG and how the two alternating
coordination modes are generated and switched.

1: Lamb DG, Calabrese RL. Neural circuits controlling behavior and
autonomic functions in medicinal leeches. Neural Syst Circuits. 2011 Sep
28;1(1):13. doi: 10.1186/2042-1001-1-13. PubMed PMID: 22329853; PubMed
Central PMCID: PMC3278399.  REVIEW

2: Kristan WB Jr, Calabrese RL, Friesen WO. Neuronal control of leech
behavior. Prog Neurobiol. 2005 Aug;76(5):279-327. Epub 2005 Nov 2. Review.
PubMed PMID: 16260077.  REVIEW