Ultrastructure of synapses in the first-evolved nervous systems
Abstract
The phylum Cnidaria represents the first group of animals to evolve a recognizable nervous system. A comparison of the ultrastructural features of synaptic loci in animals representing all four classes of the cnidaria has provided an overview of the first-evolved synapses that can be compared morphologically to synapses in higher forms. Synapses in these watery jellylike animals with unmyelinated axons are sparse and difficult to fix well. However, we now have sufficient evidence to define an early synapse as one with paired electron dense plasma membranes separated by a 13-25 nm gap containing intracleft filaments and with vesicles on one or both sides of the synaptic cleft. The vesicles are of three types: dense-cored, clear, and opaque. Neuromuscular synapses resemble neuronal synapses and lack the postsynaptic specializations of higher animals. However, some coelenterates, such as the jellyfish Chrysaora, have a postsynaptic cisterna in the muscle. Neuromuscular and neuronematocyte synapses can have either clear or dense-cored vesicles. Opaque vesicles at two-way interneuronal synapses and at neuromuscular synapses in the oral sphincter muscle of sea anemones can be labelled with antisera to the neuropeptides Antho-RFamide (Antho-Arg-Phe-NH2) and Antho-RWamides (Antho-Arg-Trp-NH2) I and II, respectively. That suggests that neuropeptides evolved as neurotransmitters early in the animal kingdom. The basic differences between first evolved synapses and synapses of higher animals are the lack of postjunctional folds at neuromuscular synapses and the presence of fewer and somewhat larger synaptic vesicles, generally containing granular cores, in the more primitive animals.
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Nerve net-like arrangements of neurons cover almost the entire ectoderm in cnidarians and ctenophores.
“Nerve net-like arrangements of neurons cover the almost entire ectoderm in cnidarians and ctenophores [61,129,130–136].”
From Gastric pouches and the mucociliary sole: setting the stage for nervous system evolution (2015) · cites this paper as [131]
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Roots and early routes of neuroendocrinology
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- Authors
- Westfall, J. A.
- Affiliations
- Kansas State University (US)
- Corresponding author
- Westfall, Jane A.
- Venue
- Journal of Neurocytology
- Published
- 1996-01
- Volume
- 25
- Issue
- 1
- Pages
- 735-746
- ISSN
- 0300-4864 (ISSN-L)
- 1573-7381
- OpenAlex
- W2107357933
- Cited by
- 78
- References
- 59
- Citations by year
- 2026: 3 · 2025: 5 · 2024: 2 · 2022: 3 · 2021: 1 · 2020: 2 · 2019: 4 · 2018: 3 · 2017: 4 · 2016: 5 · 2015: 6 · 2014: 2 · 2013: 2 · 2012: 1
- FWCI
- 0.54
- Type
- journal article
- Language
- en
- Open access
- closed
- Licence
- www.springer.com/tdm (tdm)
- Locations
- Journal of Neurocytology (publishedVersion)
- PubMed (publishedVersion)
- Funders
- National Institutes of Health (2r01ns010264-15a1)
- National Institute of Neurological Disorders and Stroke (NS-10264)
- Topics
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Marine Invertebrate Physiology and Ecology
- Life Sciences › Neuroscience › Cellular and Molecular Neuroscience › Neuroscience and Neuropharmacology Research
- Life Sciences › Neuroscience › Cellular and Molecular Neuroscience › Neurobiology and Insect Physiology Research
- Keywords
- synaptic ultrastructure, Cnidaria, synaptic cleft, synaptic vesicles, neuromuscular synapse, coelenterates
- MeSH terms
- Animals, Invertebrates, Microscopy, Electron, Neuromuscular Junction, Neurons, Neuropeptides, Sea Anemones, Synapses, Neurotransmitter Agents, Scyphozoa
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