Mesosegmental motor neurons

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Mesosegmental motor neuron activity causes movement in the anterior segment of the animal (Fig. 1). The mesosegment is the largest region of the Aplysia californica body, and correspondingly the mesosegmental motor neurons make up the largest population of unidentified motor neurons in the Aplysia californica pedal ganglia. The mesosegmental motor neurons can be divided in to two classes: transverse motor neurons, which produce transverse contractions, and longitudinal motor neurons, which produce longitudinal contractions in the mesosegment.

Figure 1. Schematic representation of the Aplysia body, divided to illustrate target regions of pedal ganglia nerves. Modified from Hening et al, 1979.
Figure 2. Schematic representation of the dorsal Aplysia pedal and pleural ganglia. Left pedal ganglion shows designated sectors, right pedal ganglion shows approximate locations of identified neurons. Modified from Hening et al 1979.

Anatomy and location

Transverse contractor motor neuron soma varied in size between 50-150um, may be pale or pigmented, and may be homogenous in color or show a ring of pigmentation. Most soma were located in Sector II along the posterior margin, in Sector IIIc along the anterior margin, or dispersed throughout Sector II and IIIa. A small number were also located in Sectors Ia and Ib.

  • Identified transverse contractor motor neurons: P2, P3

Longitudinal motor neuron soma have not been anatomically described in the literature. Most were found in the center and medial edge of Sector II, or were scattered in Sectors I, II, and III.

  • Identified longitudinal motor neurons: P4

Motor effects and electrophysiology

Transverse contractor motor neurons usually caused dorsoventral (transverse) contractoin in the ipsilateral tegument at the caudal neck when stimulated. Some motor neurons in this category were also observed to cause transverse contraction of the ventral foot. These cells exhibit differing responses depending on their target area. Motor neurons that targeted the anterior portion of the mesosegment were found to be excited by tail stimulation. Motor neurons that targeted posterior mesosegment areas exhibited excitation or inhibition upon tail or head stimulation.

Longitudinal motor neurons


1. Hening, W.A., Walters, E.T., Carew, T.J., Kandel, E.R. (1979) Motorneuronal control of locomotion in Aplysia. Brain Research, 179. 231-253.