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Abstract
Octopuses have the extraordinary ability to control eight prehensile arms with hundreds of suckers. With these highly flexible limbs, they engage in a wide variety of tasks including hunting, grooming, and exploring their environment. While the neural circuitry generating these movements engages every division of the octopus nervous system, much of the control circuitry is found within the arms themselves. The axial nerve cords (ANC), which lie in the center of every arm, are the largest neuronal structures in the octopus, containing four times as many neurons as are found in the central brain. There are also five peripheral nerve centers in the arm: a sucker ganglion in the stalk of every sucker and four intramuscular nerve cords. In this thesis, I explored the structure of the extensive nervous system in the arm of the octopus, focusing on the cellular and molecular composition of the ANCs and sucker ganglia. I first studied the neuronal organization of the adult axial nerve cord (ANC) of Octopus bimaculoides. In transverse cross section, the cell body layer (CBL) of the ANC wraps around its neuropil (NP) with little apparent segregation of sensory and motor neurons or nerve exits. Strikingly, when studied in longitudinal sections, the ANC is segmented. ANC neuronal cell bodies form columns separated by septa, with 15 segments overlying each pair of suckers. The segments underlie a modular organization to the ANC neuropil: neuronal cell bodies within each segment send the bulk of their processes directly into the adjoining neuropil, with some reaching the contralateral side. Cellular analysis establishes that adjoining septa issue nerves with distinct fiber trajectories, which across two segments (or three septa) fully innervate the arm musculature. Sucker nerves also use the septa, setting up a nerve fiber “suckerotopy” in the sucker-side of the ANC. The ANC also contains many longitudinal fiber tracts, well suited for intermediate and long-distance communication. The neural modules described here provide a new template for understanding the motor control of octopus soft tissues. The segments in the CBL supply the structure for local sensorimotor control, and the longitudinal tracts provide the pathways for coordinating motor acts between suckers and along the arm. Comparative analysis suggests that arm nervous system structure varies based on appendage morphology, environmental context and behavioral needs. Examination of the axial nerve cord in the arms of the pelagic squid Doryteuthis pealeii reveals a reduced complexity, with fewer number of segments and an expansion of longitudinal tracts, compared to O. bimaculoides, a benthic animal. Additionally, the ANC in the sucker-poor tentacle stalk in D. pealeii is unsegmented, suggesting a strong link between segmentation and suckers. I next examined the cellular organization and molecular composition of the sucker ganglion in O. bimaculoides. The sucker ganglion has an ellipsoid shape and features an unusual organization: the neuropil of the ganglion is distributed as a cap aborally (away from the sucker) and a small pocket orally (towards the sucker), with neuronal cell bodies concentrated in the space between. Using in situ hybridization, we detected positive expression of sensory (PIEZO) and motor (LHX3 and MNX) neuron markers in the sucker ganglion cell bodies. Nerve fibers spread out from the sucker ganglion, targeting the surrounding sucker musculature and the oral roots extending to the ANC. These results indicate that the sucker ganglion is composed of both sensory and motor elements and suggest that this ganglion is not a simple relay for the ANC but facilitates local reflexes for each sucker.