Neurons can be reliably sorted into classes based on their morphology, synaptic connectivity, physiological profile, and molecular contents. In invertebrate nervous systems, neuron classification can be so fine-grained as to reach the level of individually identifiable neurons–single neurons can be reliably found in every animal and distinguished from all other neurons in the nervous system (Bullock, 2000; Comer and Robertson, 2001; Leonard, 2000).
Classifying and identifying neurons allows neuroscientists to trace neural circuits, providing fundamental insight into information flow and processing in a nervous system. Over the past 40 years, mapped circuits of identified neurons have proven an invaluable framework for studying all aspects of nerual function. Examples of important invertebrate model circuits include the central pattern generator (CPG) for swimming in the mollusc Tritonia (Getting, 1989) , the gill and siphon withdrawal circuit and feeding circuitry in the mollusc Aplysia (Hawkins et al., 1993), circuits for heartbeat control, local bending, and swimming in the leech (Brodfuehrer et al., 1995; Brodfuehrer and Thorogood, 2001; Calabrese et al., 1995; Kristan, Jr. et al., 1995), visual circuits in insects (Borst and Haag, 2002), circuits for foregut movements in lobsters and crabs (Nusbaum and Beenhakker, 2002), and escape responses in crayfish (Edwards et al., 1999).
Identified Neurons in the Abdominal Ganglion of Aplysia californica
There are an estimated 1600 neurons in the adult Aplysia abdominal ganglion (Coggeshal, 1967). Approximately 78 classes of neurons have been identified in this ganglion, encompassing 247 individual neurons. Thus, nearly 16% of the ganglion has been elucidated. A complete list of cell types with references is here. Note that these totals do not include the bag cells.
The rate of discovery can be seen in the following table, which shows the total number of cells identified by year in 5-year blocks. The number of new identifications has dwindled, and no new cells have been identified since 1995 (Hickie and Walters, 1995)
| Year |
1965
|
1970
|
1975
|
1980
|
1985
|
1990
|
1995
|
2000
|
| Total | 59 | 61 | 23 | 45 | 27 | 26 | 6 |
0
|
|
Table1: Cell identifications / 5 years in Aplysia adbominal ganglion |
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Identified Cells in Tritonia diomedea Ring Ganglia
There are an estimated 8000 neurons in the adult Tritonia ring ganglia (Willows, et al., 1973). Approximately 45 neuron classes have been identified, encompassing 324 individual neurons (5% of total). A complete list of cell types with references is here.
As with Aplysia, the rate of discovery has slowed:
| Year |
1965
|
1970
|
1975
|
1980
|
1985
|
1990
|
1995
|
2000
|
| Total | 0 | 188 | 108 | 18 | 0 | 0 | 2 |
8
|
|
Table2: Cell identifications / 5 years in Tritonia ring ganglia |
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