Aplysia cerebral ganglia
Cerebral ganglia of Aplysia are part of the circumesophogeal nervous system that consists of the paired buccal ganglia (BG), pedal ganglia (Pe), and pleural ganglia (Pl) (Fig. 1). The cerebral ganglion has oval shape and consists of two bilaterally symmetric ganglia. In mature animals (>250 g weight), a cerebral ganglion without connective tissue sheath, is approximately 3 mm across its width and 2 mm caudo-rostrally.
Ganglionic Nerves and Connectives
Five major nerves extend from each Cerebral Ganglion to the periphery and there are three connectives to other ganglia (Fig. 2). The names of the nerves are listed in Table 1. In addition, there is also a sixth nerve, which is difficult to observe, the statocyst nerve (S) in the connective tissue sheath surrounding the C-P and C-PL connectives.
The neurons in the Cerebral ganglia are organized in clusters that are separated by glial boundaries.
The neurons within the cluster A are located in the caudal part of the ganglion, on top of the C-PL connectives. There are about 20-30 large neurons (100-120 µm) in each cluster and they have dark pigmentation. They are characterized by silent spontaneous firing pattern and predominantly inhibitory spontaneous synaptic activity. From all nerves in the head they receive EPSPs and IPSPs, but twice more EPSPs.
The neurons in this cluster are found in caudal part of the ganglion, medial to the A cluster, in the connective tissue sheath. There are approximately 25 large, pigmented neurons on each side 100 µm in diameter. They are characterized by irregular spontaneous firing pattern receive excitatory spontaneous synaptic input. They receive both EPSPs and IPSPs, but nine times more EPSPs than IPSPs.
Identified neurons within this cluster: CB1.
The C cluster is located in the central part of the ganglion, rostral and lateral from the B cluster. The neurons are less pigmented and smaller comparing to the neurons in the B cluster. There are large cells with about 75 µm in diameter, but many of the deeper and peripheral cells have cell bodies with 30 µm in diameter. In each C cluster there are about 80 to 100 neurons. When it comes to spontaneous firing pattern these neurons are very heterogeneous, some of them are silent, while others show irregular or pacemaker activity to primarily excitatory spontaneous synaptic input. By electrical stimulation they produce more EPSPs than IPSPs.
The D cluster is found between the C and E cluster with around 50 neurons in each hemiganglion. They are very small with somata diameter in the range between 20 and 50 µm and very lightly pigmented or even clear. As with the C cluster, some of the cells show silent, while other show irregular or pacemaker activity to primarily excitatory spontaneous synaptic input. The spontaneous activity was predominantly IPSPs in the former and EPSPs in the latter.
The neurons within this cluster are located at the lateral edges where the C-P connective, AT nerve, C-B connectives and LLAB nerves enter. In each cluster there is one large cell with about 150 µm in diameter, while other neurons are smaller. Majority of these neurons are pigmented and each cluster contains between 20 and 25 neurons. Most neurons show pacemaker discharges and some have bursting spontaneous firing pattern. Spontaneous synaptic input is mainly EPSPs. By stimulating C-B connectives, ULAB and AT nerves, cells within the E cluster showed synaptic activity, with two times more EPSPs than IPSPs.
The F cluster is found in the center of the ganglion. Each cluster has approximately 50 light cells with diameter range between 30 and 50 µm. Most neurons show silent or irregular spontaneous firing pattern. Spontaneous synaptic inputs were mainly IPSPs but evoked synaptic inputs were two times more EPSPs than IPSPs. As shown in Figure 3, the two symmetrical F clusters are located laterally in both cerebral hemiganglia on the dorsal side and contain ~70 neurons with cell body size >50 µm and around 100 neurons with <50 µm in diameter. Cell bodies are distributed in three distinct layered structures and because of that, they are labeled as top, middle and bottom layer F-cluster neurons (CFT, CFM and CFB).
- The CFT neurons are large cells, with wide range of somata diameters from 50 to 150 µm (92 ± 25 µm), white, non-uniformly shaped and generally unipolar. Upon electrical stimulation of the anterior tentacular (AT) nerve they show weak electrical coupling, the presence of synchronized spontaneous changes in membrane potential and a generalized inhibitory input.
- The CFM neurons are located in a granular neuron region characterized by unipolar, round and smaller cell bodies than ones in CFT, with somata diameters from 30 to 60 µm (46 ± 12 µm). The CFM neurons are mainly silent but do not show electrical coupling or synchronized changes in membrane potential and exhibit weak action potential broadening during constant current injection.
- The CFB neurons are tiny neurons with somata diameters from 10 to 30 µm located in the third layer just above the neuropil.
Using mass spectrometry, three different peptides are detected according to cell size and location, which supports the morphological and electrophysiological data for the F cluster.
The G cluster is found rostrally in each hemiganglion. Each cluster contains between 30 and 40 cells that vary in anatomy and physiology. Most cells have diameters less than 75 µm, but the biggest cells in cerebral ganglia are also in these clusters: the metacerebral cells MCC, which are 250 µm in diameter. (5) Pigmentation of cells in the G cluster goes from light to dark. Majority of the cells have either silent, or irregular and pacemaker spontaneous firing patterns, and a few neurons have bursting discharge pattern. Among silent cells majority have IPSPs, while other cells have mainly EPSPs.
This is the only asymmetrical cluster; it is found only in the rostral part of the right cerebral ganglion. Estimated number of cells within this cluster is between 40 and 60 and they all have dark pigmentation and diameter between 75 and 85 µm. Most cells show either irregular or pacemaker spontaneous firing pattern.
Clusters J and K
Mechanosensory neurons in the cluster J and K innervate the head and mouth of the animal and according to their pharmacological properties and synaptic connections these neurons are functionally heterogeneous, some of them have defensive roles and others have a role in feeding. J/K cerebral sensory neurons send axons to the periphery via ipsilateral cerebral nerves, with approximately 50 % of them that have axons in the ipsilateral cerebral–pleural or cerebral–pedal connectives. Cerebral J/K neurones make monosynaptic connections to a subpopulation of neurons within the B clusters of the cerebral ganglion