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Win7 from XP This 29 page paper raised a number of issues I'd like to come back to. Below is a brief, initial survey: CotA The most recent reviews on amphioxus are by Ruppert (1997), for the general anatomy of the animal as a whole, and Nieuwenhuys (1998), for the nervous system. corpuscles of de Quatrefages (Fig. 3), putative mechanosensory organs consisting of primary sensory cells enclosed in a capsule (Baatrup 1982) The epithelia that line the buccal and atrial cavities, as well as the organs embedded within them, are also supplied with an extensive set of neural plexuses collectively known as the atrial nervous system. Though the system is more or less continuous, it is usually subdivided on the basis of the organs it innervates; i.e., there are buccal, velar, gonadal, parietal, pterygeal, pharyngeal, and endostylar subdivisions, and so on (Bone 1961). The connection to the nerve cord via nerves 1–7 (1–8 according to Dogiel 1903; Kutchin 1913) is highly asymmetrical. Nerves 3 and 4 on the left side are exceptional in having contralateral branches that connect with the inner buccal plexus on the right side. In addition, a subsidiary branch from the contralateral branch of the left nerve 4 connects to the right side of the velar plexus, while its left side connects to nerve 5 by means of a caudal branch from that nerve. This is all a consequence of the fact that the larval mouth develops initially on the left side and is innervated entirely by nerves emerging from the left side of the nerve cord (Lacalli et al. 1999). The initial connections are then retained during subsequent development, so the nerves are dragged along as the mouth is repositioned. Solitary receptors are widely distributed over the entire epidermis but are most common in the region of the rostrum, buccal cirri, and tail (Dogiel 1903; Franz 1923; Bone 1960b; Stokes and Holland 1995a; Holland and Yu 2002). They form small clusters in some instances (Sinnesknospen, Franz 1923; Schulte and Riehl 1977), especially along the buccal cirri. The most common receptor cell types are referred to by convention as types I and II (Schulte and Riehl 1977; Bone and Best 1978). Type I cells are primary sensory neurons with an apical circlet of microvilli, a single cilium, and a basal neurite. There are several subtypes, but all are probably mechanosensors (Baatrup 1981; Lacalli and Hou 1999). Their axons project to the CNS via the dorsal nerves; once there, they travel along the cord in two fiber tracts, dorsal and subdorsal in the terminology of Holland and Yu (2002), which may correspond to the somatosensory and viscerosensory tracts of Bone (1960a; see Fig. 4). The central axons of type I cells reach considerable lengths, so an axon entering the CNS via the first nerve can typically project caudally to mid-spinal levels, at least in larvae (Holland and Yu 2002). Little is known about the neurotransmitters released by peripheral neurons, but there is evidence that at least some type I cells are GABAergic (Anadón et al. 1998). Anadón, R., Adrio, F., and Rodríguez-Moldes, I. 1998. Distribution of GABA immunoreactivity in the central and peripheral nervous system of amphioxus (Branchiostoma lanceolatum Pallas). J. Comp. Neurol. 410: 293–307. Copied to: Amphioxus Neurotransmitters . Ruiz, M.S., and Anadón, R. 1991b. The fine structure of lamellate cells in the brain of amphioxus (Branchiostoma lanceolatum, Cephalochordata). Cell Tissue Res. 263: 597–600. Copied to: Lamellar Body = Pineal Eye . Ruiz, M.S., and Anadón, R. 1991c. Some considerations on the fine structure of rhabdomeric photoreceptors in the amphioxus, Branchiostoma lanceolatum (Cephalochordata). J. Hirnforsch. 32: 159–164. Link to Frontal Eye . Type II receptors (Fig. 4) are secondary sensory cells with synaptic terminals borne on short basal processes, usually three per cell (Stokes and Holland 1995a; Lacalli and Hou 1999). Apically, they have a modified nonmotile cilium surrounded by a collar of branched microvilli. This extensive elaboration of the apical surface suggests a chemoreceptive function, but essentially nothing is known for certain about chemoreception in amphioxus, either in terms of structures or physiology (Lacalli 2004). The synaptic zones in each segment consist of two distinct domains, the ventral and dorsal synaptic compartments (Figs. 3, 4). Both utilize acetylcholine as a transmitter (Flood 1974). Flood, P.R. 1974. Histochemistry of cholinesterase in amphioxus (Branchiostoma lanceolatum, Pallas). J. Comp. Neurol. 157: 407–438. Copied to: Amphioxus Neurotransmitters . The ventral synaptic compartments are where the deep, anaerobic, fast muscle cells receive their innervation. The presynaptic motoneurons involved belong to a class of cells that Bone (1960a) called somatomotor (SM) cells; they may therefore also be called ventral compartment motoneurons. They are found in the ventral parts of the grey matter and have a tendency to cluster opposite the synaptic contact zones, and each has a broad apical process connecting it to the ventricular cavity. Some have internal vacuoles, and this character, together with size and positional differences, has been used to define several subtypes (Bone 1960a; only one such type, the SM1 cell, is shown in Fig. 4). The axons of the SM cells project laterally into the bundle of somatomotor fibers adjacent to the synaptic zone of the ventral compartment. The dorsal compartment is where the superficial, aerobic, slow muscle cells of the myomeres receive their innervation. The DC motoneurons are known from larvae (Lacalli and Kelly 1999; Lacalli 2002a) but have not yet been identified with certainty in adults. From the larval data, however, it seems that the whole of the DC innervation along the nerve cord may derive from motoneurons located in the anterior cord at the level of somites 2–6 (see below). This is approximately equivalent to the zone fated to become the IR of the anterior cord, which extends from myotomes 2 to 4. Thus, there are serially (but not segmentally) repeated neurochordal synaptic contacts at the base of the nerve cord. These are thought to be cholinergic (Flood 1970), but their source within the nerve cord has not been identified. GABA, neuropeptide Y, and several other neuropeptides have been detected in various cells loosely classified as interneurons (Uemura et al. 1994; Anadón et al. 1998; Castro et al. 2003) Castro, A., Manso, M.J., and Anadón, R. 2003. Distribution of neuropeptide Y immunoreactivity in the central and peripheral nervous systems of amphioxus (Branchiostoma lanceolatum Pallas). J. Comp. Neurol. 461: 350–361. Copied to: Amphioxus Neurotransmitters . Second, a novel class of interneurons, Anadón’s cells, has been identified in the vicinity of the ventral expansion of the central canal (Anadón et al. 1998; see Fig. 5). These are very small GABAergic cells interspersed between the cell bodies of SM and VM neurons (cf. Figs. 4, 5). Anadón et al. (1998) have suggested that they might be comparable to the inhibitory Renshaw cells of vertebrates. Whether by light microscopy (e.g., Edinger 1906; Franz 1923; Ekhart et al. 2003) or EM (Meves 1973), it is difficult to discern much about the neuronal and glial cells of the anterior vesicle, since most cells are small and rather densely stained and have few visible distinguishing features. Franz (1923, 1927) therefore concluded that the entire anterior vesicle consisted only of glial cells. However, GABAergic (Anadón et al. 1998) and serotoninergic neurons (Moret et al. 2004) have since been identified in this region in adult specimens Moret, F., Guilland, J.-C., Coudouel, S., Rochette, L., and Vernier, P. 2004. Distribution of tyrosine hydroxylase, dopamine and serotonin in the central nervous system of amphioxus: implications for the evolution of catecholamine systems in vertebrates. J. Comp. Neurol. 468: 135–150. Copied to: Amphioxus Neurotransmitters . Then, just ventral to the Joseph cells and surrounding the dorsal expansion of the central canal, there are bilateral, longitudinal bands of neurons immunoreactive for urotensin and FMRFamide (Uemura et al. 1994), GABA (Anadón et al. 1998), neuropeptide Y (Castro et al. 2003), and catecholamines (the catecholaminergic population I of Moret et al. (2004)). Firstly, the more posterior alm cells (slightly rostral to the junction of myomeres 1 and 2; black squares in Fig. 6) seem to correspond to the anterolateral serotoninergic cells of Holland and Holland (1993) that were also observed by Moret et al. (2004). Slightly more anterior (black circles in Fig. 6) is another group of immunocytochemically identifiable cells within the alm group. This is the catecholaminergic population II of Moret et al. (2004). There is some uncertainty about the exact positions of these two cell groups, however. Moret et al. (2004) place them adjacent to the rostral half of the second myomere. In an independent immunocytochemical study, H. Wicht (unpublished data) localized them more anteriorly, adjacent to myomere 1 and thus within the confines of the alm group (see Fig. 6). Wicht’s study did confirm, however, that both the catecholaminergic population II and the anterolateral sertoninergic neurons have long descending projections to the spinal cord. In retrograde tracing experiments, Fritzsch (1996) found pairs of labelled cells in late larvae that may correspond to the anterolateral serotoninergic cells, even though he did not specify their exact position, but Ekhart et al. (2003), in a similar study in adults, did not find such cells. Assuming the latter result is a false negative, the cells and projections appear to be real; it is only their exact axial position that is a matter of some uncertainty. Serotonin-containing neurons are absent in this region (Moret et al. 2004), but a relatively large number of GABAergic and peptidergic cells (Uemura et al. 1994; Anadón et al. 1998; Castro et al. 2003) do occur. In addition, there are four relatively large catecholaminergic cells (population III of Moret et al. 2004) with translumenal processes in the vicinity of the roots of the fifth dorsal nerves. Uemura, H., Tezuka., Y., Hasegawa, C., and Kobayashi, H. 1994. Immunohistochemical investigation of neuropeptides in the central nervous system of the amphioxus, Branchiostoma belcheri. Cell Tissue Res. 277: 279–287. Despite the usefulness of the infundibular cells as anatomical markers, there is no obvious transition in terms of neuronal cell type at this point. Instead, cells of essentially anterior character are found from the preinfundibular region to the beginning of the PMC. "Anterior" here refers to cells with irregular basal neurites that form repeated varicosities containing mixed vesicle types and few, if any, synapses. These are features that are generally associated with slow transmission, often involving neuropeptides (Burns and Augustine 1995). Beginning in the PMC, most of the neurons have well-defined axons and separate dendritic structures, either arbors or spines (both occur), and synaptic junctions, often with clear vesicles, predominate. This implies fast transmission and aminergic or amino acid transmitters, which is perhaps logical for neurons directly involved in the locomotory control circuits. The LPN3s are thus the best candidates for neurons exerting a direct controlling influence over both fast and slow swimming, which appear to have a similar neuromuscular basis in amphioxus and vertebrates (Bone 1989). Fast or escape swimming occurs in response to sensory inputs, which are a massive and redundant input to the VC system. The VC system also receives synaptic input from fibers in the postinfundibular neuropile and may be subject to additional paracrine input as well, via fibers passing through the neuropile, all of which provides an opportunity to modulate the response to sensory stimuli. In contrast, the slow system, which drives vertical migration, is almost devoid of synaptic input. Besides its link via junctions to the LPN3s, this pathway seems to be mainly under the control of the PPN2s mentioned above, a class of preinfundibular projection neurons that make repeated junctional contacts with the axons of the DC motoneurons. There are a number of specialized cell groupings at the anterior end of the adult nerve cord that are not present in early larvae, including various types of migrated cells described above from the adult IR. Judging from the time that the anterolateral serotoninergic cells first appear (Holland and Holland 1993), these cell groupings probably develop in the late larval phase or during metamorphosis. Despite the proliferative activity this entails, the anteriormost region fails to thicken as much as the rest of the cord, so the CV progressively disappears as an externally recognizable zone. Of the late-developing cell groups, the dorsal (population I) dopamine-containing cells reported by Moret et al. (2004) are especially noteworthy. These are as dorsal and anterior as one can get in the nerve cord, which is precisely where a telencephalic homolog would be predicted to form if amphioxus had one. For this and other reasons, Lacalli (2004) suggested that the population I cells may represent a primitive version of the olfactory bulb. CotA AmphAuth NrvSysStrDevEv |