Not every conducting tissue, is nervous tissue. Indeed, the presence of an electrochemical potential between the interior of the cell and the exterior of the cell is one of the fundamental aspects of cellular organization and energetics. The ability of cells to propagate an electrical current across a population predates the emergence of animals and multi-cellularity. What distinguishes neurons from other cells is:
They are only found in animals, but are not the only neuroid cell types in animals.
They have localized their points of electrical and chemical contact to specialized and restricted points of contact on the cell membrane, i.e. the presence of the synapse and the synaptic cleft.
They have an evolved a highly plastic physiology that is exquisitely responsive to epigenetic events and the prevailing physiological conditions that enables sustained recognition, memory and learning on the multicellular level.
The distal ends of multi-neuron networks innervate either sensory or effector cell types.
In 1970, the British-Canadien zoologist George Owen Mackie (October 20, 1929 - ) published Neuroid Conduction and the Evolution of Conducting Tissues in The Quarterly Review of Biology,[1] where he explored the functional cellular precursors to nerves in protozoa, porifera, cnidaria, and other invertebrates. His studies in the behavioral physiology of these simple organisms revealed the relationship between electrical conduction in the cell and the initiation of a behavioral response.
Mackie defined Neuroid conduction as "the propagation of electrical events in the membranes of non-nervous, nonmuscular cells."[2] He identifies examples of neuroid conduction in algae, simple and higher plants, and in the non-neural tissue of sponges, jellyfish, and ctenophores. This demonstration of non-neural conduction shows that depolarization is not a nervous system specialization, but rather the intrinsic potential of cells more broadly - it is, their primitive evolutionary state.
In cnidaria, we start to see the first signs of genuine nerve cells but they appear to be late-comers to an already established neuroid conduction pathway between different cell populations in these rudimentary radial organisms.
Neuroid Conduction Facilitates Rapid Response
"In hydromedusae and siphonophores neuroid conduction occurs in the exumbrellar ectoderm and subumbrellar endoderm, the two layers being linked as a transmission pathway for excitation going to ectodermal smooth muscle systems. The "crumpling" behavior of medusae is transmitted by this system, but nervous components may be involved in the generation of the full response. Reverse locomotion in physonectid siphonophores (e.g., Nanomia) involves activation of neuroid pathways. In the siphonophore Hippopodius, neuroid conduction in the exumbrella is coupled to luminescent and blanching reactions. Neuroid conduction in all these forms provides a rapid and efficient method of information transfer. It is typically associated with the spread of protective and locomotory responses and is general rather than local in effect. The more complex and local responses are believed to be organized by the nervous system."[2] - G.O. Mackie (1970)
The capacity of neuroid conducting tissues to both rapidly sense and transmit environmental influences across entire populations of cells within the organism, positions them ideally for the evolutionary transition to the cellular architecture of nervous system tissues. By the time we get to advanced cnidarians, they exhibit full-fledged bidirectionally propagating radial nerve nets of considerable cognitive capacity. The transition to Bilateria will condense the nerve-net into a centralized unidirectionally propagating nervous system with afferent and efferent pathways. In addition to conducting influences and responses across the body, the neuroid tissues of early animals mediate the first indications of a generalized cognition in our pre-neural ancestors.
Cellular communication systems evolved early in the evolution of cells, if its foundations weren't already present at the point of membrane-cellwall encapsulation at the origin of cells. For bacteria, genetic exchange between partners via transduction, transformation and/or conjugation requires at least a minimal form of cell to cell communication. The evolution of social bacteria shows us that quorum sensing is a common phenomena in bacteria - Not only are they able to communicate between others of their own species, but they can communicate with other species as well. Eukaryotes exhibit an even greater range of social communication between individual cells in response to epigenetic events. What all these cell to cell communication systems hold in common are membrane-embedded macro-molecular systems that link environmental events occurring external to the cell to metabolic and genetic response elements within the cell.
Many of the biogenic amines and catecholamines identified as "neurotransmitters", were identified as such because they were first discovered in the nervous systems of animals.[a] Like the phenomena of neuroid conduction in pre-neural cells, some of these substances have an evolutionary history that is much deeper than the origin of animals. Many so called "Neurotransmitters" predate animals, therefore neurons as well.
The antiquity of the amino acid and purinergic transmitters is obvious because of their metabolic functions, but the biogenic amines are the mainstay of the global neurotransmitter fountains embedded in the integrating reticulum of the reticular activating system - and, the deeper history of the ancient role of these biogenic amines is little discussed, explored, or known about.
Many "neurotransmitters"[b] have been found to be utilized by single celled organisms, usually for the coordination of growth and metabolism in colonial contexts.[5] These molecules were connected to biological mechanisms of communication between cells long before animals and nervous systems existed - and, a clear understanding of their roles in this context sheds light on their modern role in animal nervous systems.
Receptors, Signal Transduction & Second-messenger Systems
Most cell-membrane bound receptors facing the exterior of the cell are coupled to second-messenger systems that communicate with the enzyme complexes, organelles, metabolism, and genome in the interior of the cell.
The main signal transduction pathways in cells are:
At the 1984 Conference of the Cognitive Neuroscience Institute, American neuroscientist Ira B. Black (March 18, 1941 – January 10, 2006) summarized the consensus the conference had come to regarding some of the key features and dynamics of memory at the cellular level in neurons.
Mnemonic Plasticity To Memory
"Memory must involve the alteration of neuronal function and therefore requires plasticity, a change in state with experience. Moreover, mnemonic plasticity is characterized by (a) codification within the neuron, (b) short onset, (c) long-lasting effects, (d) specificity, (e) a high degree of precision, (f) enhanced effects with repetition, and (g) alteration of neuronal function. Further mechanisms must allow for decay, or the phenomena of forgetting... In fact, neurotransmitter functions, the agents of synaptic communication, undergo relatively long-term changes in response to brief experimental stimuli, and most definitely alter behavior. Transmitters and associated regulatory molecules encode, store, and express environmental information in a highly precise manner, thereby exhibiting mnemonic characteristics. Transmitter metabolism and even phenotypic expression are altered by discrete environmental stimuli. Relatively brief environmental events evoke long-lasting alterations in transmitter function, providing the temporal amplification that is central to mnemonic phenomena (Black 1984). Transmitter metabolism and physiologic effects are precisely governed by specific regulatory molecules, many of which respond to environmental stimuli in a pattern characteristic of memory."[6] - Ira B. Black
Black goes on to examine the Catecholamine transmitters and the many components within the metabolic, genetic, and macro-molecular architecture of each specific transmitter system that are subject to regulatory action. "Metabolism of individual transmitters is organized into relatively discrete, self-contained functional units."[7] Black lists the following components as being typical of a catecholamine neurotransmitter system:[7]
biosynthetic enzymes,
storage vesicles,
receptor apparatus with its coupled second-messenger system components,
mechanism for high-affinity neuronal reuptake activation/deactivation,
catebolic enzymes.
He points out that this is a minimal system and has multiple potential points of regulation that are properly responsive to environmental changes, thereby allowing the system to be fine-tuned over physiological time. These are the component of a single neurotransmitter system, but neurons are capable of harboring multiple neuron transmitter systems within the cell - making it possible to employ a transmitter logic that is enormously sophisticated and selective.
In Neural Darwinism, Gerald Edelman picks up on the idea that a diversification of neurotranmitter substances in vertebrates added additional potential to regulate a highly variant neural architecture.
Transmitter Logic
"...instead of dividing input into only two classes - excitatory and inhibitory - and thinking of neuronal operations in Boolean terms, we might rather consider a kind of 'transmitter logic' in which each transmitter (in association with its post-synaptic partners) can lead to characteristic modifications of synapses receiving only certain other transmitters and located only on certain other parts of the dendritic tree."[8] - Gerald M. Edelman (1987)
Neurotransmitter and receptor combinations provide an additional level of degeneracy at each synaptic cleft within each neuron, at the pre- and post-synaptic points of contact on the neuronal cell body, axon, dendritic tree of itself and its contacts. Additionally, degenerate receptor types for a specific transmitter, can allow the transmitter to serve a variety of functions depending on the coupling.
The Origin Of The Pre & Post Synaptic Clefts - Neuroid Conduction & The Nematocyst
Illustration of neuronal architecture showing elements of the internal cytostructure.
While the synapse appears to be one of the defining features of a neuron, it actually requires two cell to form a synapse. This is the point at which release and reception of "neurotransmitter" between contacting cells takes place in a localized fashion on the pre- and post-cell membrane on either side of the intervening gap.
Cells have a long history of chemical and mechanical communication. What distinguishes neurotransmission from other types of chemical communication between cells is the discretely localized transmission across the synaptic gap which occurs between two contacting cells.
It is important to remember that the evolutionary emergence of the synapse is a co-evolutionary process involving two cells and the pre- and post- synaptic side of the synapse.
The evolutionary and developmental, question is, "how did the receptors for these ancient substances came to be localized on the cell membrane of neuroid tissue cells to eventually emerge as the synaptic cleft. This transition appears to have occurred at the origin of the Cnidarian phyla when the first neurons and nervous system emerged in the form of a radial nerve net.
Cavalier-Smith suggests[9] that there is a pathway from the flask cells of sponges to cnidarian predatory feeding behaviors that will eventually lead to the emergence of the synapse:
From Nematocysts to Post Synaptic Cleft
"The larger larvae of true sponges provided a novel, hitherto unexploited, food for predators. One stem sponge lineage, I suggest, evolved nematocysts to catch and digest them, thereby becoming the ancestor of coelenterates (Cnidaria, Ctenophora), a clade on the best multigene trees. Nematocyst discharge of ECM anchors the aboral pole of settling cnidarian planula larvae just as do secretory flask cells at the aboscular pole (similarly anterior when swimming) of sponge larvae. Flask cells are the only larval sponge cell type to coexpress the majority of post-synaptic protein homologues, so I suggest, evolved directly into nematocytes by evolving capsular/tube minicollagens and cnidoin elastomer that facilitates their nanosecond discharge."[10] - Thomas Cavalier-Smith (2017)
In this scenario, the pre-synaptic cleft evolved as an effector organ in neuroid tissues for predatory feeding in pre-neural cnidarian ancestors employing the neuroid tissues to effect the global discharge of the nematocysts in a concerted fashion - the nematocysts being recently differentiated from the peripheral neuroid cells within the neuroid tissue population. The nematocysts occupies the post-synaptic side of the synaptic cleft - and, drives the evolutionary emergence of the pre-synaptic cleft and the emergence of the neurons and glia as the ancestral neuroid tissue evolves into nervous tissue.
The Intrinsic Characteristics Of The Unicellular Ancestor - Choanoflaggelates, Spermatozoa & Neurons
Venn diagram demonstrating the overlap between the human neuron and sperm proteomes[11]Choanoflagellate and human spermatozoon
Their are some interesting, and unexpected similarities in the capabilities and behaviors of Choanoflagellate and spermatozoan germ cells vis a vis neurons of the soma. But perhaps this is to be expected since both have been critical participants in the evolution of animals. Like neurons selectively releasing neurotransmitters at synaptic clefts between neighboring neurons, the spermatozoa cells must selectively localize the release of signalling molecules for entry iinto the egg, to a point of contact with the egg.
Matos et al.[12] list the following correspondences between neurons and spermatozoa.
In addition to sharing:
the presence of putative ‘neuronal’ receptors
excitable or neuroid cells
the presence of calcium ion channels
the presence of Ca2+ signalling pathways involved in the regulation of key functions
additional common signalling pathways
an abundant concentration of polyunsaturated fatty acids (PUFAs)
a high metabolic demand
Both are able to activate other cells:
Neurons - other neurons or somatic effectors
Spermatozoa: oocyte
Both engage exocytosis for critical functionality:
Neurons - release of neurotransmitters in the synaptic space via synaptic vesicles.
Spermatozoa - release of acrosomal-binding factors at the oocyte surface via acrosomal vesicles.
Such similarities suggest that perhaps neurons don't differentiate and specialize to the same extent or in the same manner as most other somatic cells, perhaps preserving their ancestral plasticity and epigenetic behavioral repertoires, reminiscent of their unicellular ancestors, for other purposes.