Hermissenda Ib interneuron
NeuronBank Accession ID: Her0002676
The type Ib interneuron is a neuron found in the anterior-lateral region of the cerebral ganglion in H. crassicornis. The interneuron has a contralateral homologue and projects across the cerebral commissure to the contralateral pedal ganglion. Type Ib interneurons receive excitatory input from identified rostral and lateral hair cells in the statocyst. The type Ib interneurons excite ventral contraction motor neurons (VCMNs), identified tail contractile neurons (TC1 and TC2), and an identified lateral foot contractile (LFC) neuron. This excitation causes the animal to contract its foot along the rostro-caudal axis. Type Ib interneurons also excite VPI cilia-activiating motor neurons in the contralateral pedal ganglia. Thus, when particular hair cells are activated type Ib interneurons are excited which causes H. crassicornis to turn and crawl in the desired direction.
Neuronal Type: Local interneuron.
See Figure 1 (Crow and Tian, 2004).
- Type Ib interneurons are located in the anterior-lateral region of the cerebral ganglion.
- The axon projects across the cerebral commissure to the contralateral pedal ganglion.
- There are 2 type Ib interneurons (one in each cerebral ganglion).
- The molecular profile of type Ib interneurons is uninvestigated.
Contralateral type Ib interneurons are electrically coupled (Figure 2; Crow and Tian, 2004).
Type Ib interneurons are excited by input from ipsilaeral rostral and lateral, but not caudal hair cells in the statocyst. This connection appears to be polysynaptic (Crow and Tian, 2004). Light also excites type Ib interneurons (Crow and Tian, 2009). Type Ib interneurons display a significant increase in EPSP activity and spike frequency (when type Ib interneurons are depolarized to reach spike threshold) in light-adapted as compared to dark-adapted animals. This activity is dependent on light intensity.
Type Ib interneurons monosynaptically excite VCMNs in the pedal ganglion (Figure 3; Crow and Tian, 2004). Type Ib interneurons also excite identified tail contractile neurons (TC1 and TC2) and an identified lateral foot contractile (LFC) neuron (Crow and Tian, 2009). These projections to contractile efferents result in contraction of the ipsilateral tail and lateral foot along the rostro-caudal axis of the foot. Type Ib interneurons also monosynaptically excite VP1 cilia-activating motor neurons which elicits cilia movement in semi-intact preparations (Figure 3; Crow and Tian, 2004).
Type Ib interneurons are relatively quiescent at rest. As mentioned above, light-adapted animals receive significantly more EPSPs than dark-adapted animals, but type Ib interneurons rarely spike in either setting (Crow and Tian, 2009).
Based on work in semi-intact preparations, type Ib interneurons appear to be involved in directionally motivated crawling in H. crassicornis based on input from the graviceptive system. Type Ib interneurons excite VP1 cilia-activating motor neurons. Cilia activation is a component of crawling in H. crassicornis. Type Ib interneurons also excite VCMNs, TC neurons, and LFC neurons. Excitation of these motor neurons causes foot contraction along the rostro-caudal axis of the foot in the slug. Finally, specific populations of hair cells in the statocyst excite the type Ib interneurons. Light also excites the type Ib interneurons, but this excitation is modest and doesn't appear to be a major player in light-induced modulation of locomotion. Thus, type Ib interneurons are excited when the animal is moving in a direction that activates particular hair cells of the graviceptive system. This excitation would presumably lead to excitation of VCMNs, TCs, LFCs, and VP1s which would cause the animal to turn and crawl until the hair cells were no longer activated and the slug was now moving in or facing the desired direction.
The Ib interneurons appear to be homologues of the Tritonia diomedea C2 neurons (Josh Lillvis, unpublished results).
- Crow T and Tian L-M (2004) Statocyst Hair Cell Activation of Identified Interneurons and Foot Contraction Motor Neurons in Hermissenda. J Neurophysiol, 92: 2874-2883. PMID: 14985407
- Crow T and Tian L-M (2009) Polysensory Interneuronal Projections to Foot Contractile Pedal Neurons in Hermissenda. J Neurophysiol, 101(2): 824-33. PMID: 19073803
- Tian L-M and Crow T (2006) Serotonin-Immunoreactive CPT interneurons in Hermissenda: Identification of Sensory Input and Motor Projections. J Neurophysiol, 96: 327-335, 2006. PMID: 16801502