The human nervous system is a beautiful and complex network of cells working together to supervise bodily functions, a wide repertoire of behaviours and enable higher reasoning. It is composed of 1) the central nervous system with its brain and spinal cord and 2) the peripheral nervous system with its nerves and ganglia (groups of neuron cell bodies in the periphery). Over the centuries, researchers have tried to shed light upon the nervous system’s machinery and hidden mechanisms in hope for a better understanding of what makes us who we are. But to answer the question “How does this work?”, it is sometimes useful to take a few steps back and focus on understanding “Where does this come from?” and “How has it evolved into this?”.
The study of the evolution of the nervous system over the years and across species officially started in the 19th century and has been associated up until now to many debates and controversies. The two main sources of tension oppose the view that:
Even though the study of evolution can be heated, the core understanding of the innate differences and similarities between different nervous systems has brought science forward considerably. The study of the human nervous system is inherently limited by the ethical aspect of research on humans. Advances in genomics, biochemistry and even biophysics have contributed to expand our knowledge on the origins of the nervous system and in this way, has allowed for a more refined and translational use of animal models to better understand the complexity of the human brain.
During this short time together, we will look at an array of invertebrate species, from the simple corals, leeches, flatworms to the smart octopuses. These species have all led to incredible discoveries in the neuroscience field and have also unravelled a part of the full evolutionary path (or rocky road) leading to the human brain.
Even though hard to believe due to their lack of bones inside their bodies, invertebrates display some very original nervous systems. In 1822, Saint-Hilaire, a French naturalist, proposed the theory that the nervous system of arthropods (insects, spiders, scorpions…), which is located in the front of the animals (ventral), is similar to the nervous system of vertebrates, which is located in the back (dorsal). This theory was bitterly disputed, but also led to a new wave of questioning, reassessing the potential relationships between the nervous systems of vertebrates and invertebrates.
Let’s net with the flow
To start this journey across species, let’s talk about what is considered the most primitive and simple kind of nervous system: the diffuse nerve nets of cnidarians.
The cnidarian phylum is a group of species including corals, sea anemones, hydras and jellyfishes... In these organisms, the nerve cells, instead of being highly concentrated in specific places, are distributed throughout the organism. Although simple, these organisms are able to perceive some stimuli, integrate information and change their behaviours accordingly. This is, for example, the case for the sea anemones that can move in reaction to their environment.
Some cnidarians, such as the hydras, have diffused systems, called ‘nerve nets’, composed of nerve cells and fibres. Some species of hydras own two nerve nets that form multiple connections at specific locations. In these organisms, neurons are making contact in a fashion resembling the vertebrates’ chemical synapses. Interestingly, some small proteins present in the hydra’s nervous system also exist in mammals and can modify brain function.
Guiness world nerve cord
Surprisingly, the organisation of the flatworms’ nervous systems displays some structural similarities to those of mammals. They, indeed, present long nerve cords running from their head to the end of their body, reminiscent of spinal cords, and a collection of nerves divided in two main centres in their head, reminiscent of a brain. These organisms still present nerve nets, however, the presence of a centralized network of neurons brings them closer to more evolved species.
Nowadays, these organisms have been very well-characterized. In the Planaria and Nematodes species, structural, chemical, and behavioural nervous characteristics have been identified and used, for instance, in neuropharmacology for the study of neurotransmitters. Indeed, the nematodes produce some neurotransmitters also found in humans such as acetylcholine, GABA, glutamate, serotonin, adrenaline, noradrenaline and histamine. In these animals, they have sometimes been associated with specific behavioural roles. Serotonin has, in this way, been linked to the initiation of egg laying, the inhibition of locomotion and mating. On the other hand, dopamine has been shown to inhibit these behaviours.
Brain waving goodbye
In the same way, in annelids, nervous functions became more centralised in the head, which started the formation of brain-like structures. In some more predatory species of annelids, the organisation is more complex. Indeed, the brain is further subdivided into a forebrain, midbrain, and hindbrain. Furthermore, they present giant axons that are travelling from the brain, along the nerve cord. These giant neurons are allowing for a rapid conduction of the information to the muscles to contract the worm’s body in response to threats.
Synapse your fingers to the beat
In the bivalves, most of the sense organs are found at the edge of the mantle and are innervated by groups of nerve cells (ganglia) supervising specifically their function. Contractions of the muscles in response to a stimulation sent by the ganglia closes the shell of the mollusc.
One of my favourite molluscs (no, it’s not weird to have a favourite mollusc) is the aplysia. This very simple organism has, indeed, allowed us to study learning and memory at a cellular level and as a result, has allowed us to unravel two processes relating to synaptic plasticity: habituation and sensitisation. In a nutshell, in response to a mild tactile stimulus, the aplysia usually retracts its gill (organ that underwater species use to breathe). When the stimulus is repeated many times, the action potentials created by the motor neurons to retract the gills become weaker and weaker, until it is too low to induce a motor response. This process is habituation. As for sensitisation, through association, the aplysia can become more sensitive to certain stimuli than it was previously. By associating a neutral stimulus to a painful one, the aplysia becomes more warry and retract the gills even in absence of danger. This phenomenon seems very organic, but this model allowed for a conclusion on this phenomenon at a molecular level.
Mind your own tentacles
Cephalopods have the characteristic of having multiple integration centres argued to be related to the evolution of spatial-, social- and self-motivated learning. They are considered to have attained the highest degree of nervous system development amongst the invertebrates. Although the organisation of their nervous system is similar to the one of annelids, they present some novelty, with, for instance, a higher degree of similarity with the human brain as well as a concentration of the nervous functions in the brain of the animals. The periphery displays less nerve cells, mostly concentrated within nerve fibres and ganglia (groups of nerve cells). Nerves extend from the brain to the ganglia (mostly located at the base of the tentacles) and from the ganglia along the length of arms. Because of their large sizes, these fibres conduct the information rapidly to the tentacles and allow for quick movements. This more complex organisation of the nervous system is inherently linked to a wider behavioural repertoire. Primarily, cephalopods are predators: they move very quickly, use their eyes to spot their food, use their tentacles to obtain and discriminate tactile information about their prey, environment, and learn and memorise past events and best ways to catch their prey… These more-developed functions are centralized and linked to very specific centres such as associative areas linked to object recognition, learning and memory.
Studies led on Octopus vulgaris are currently trying to determine whether octopuses can recognise a reflection of themselves. If indeed octopuses are capable of self-awareness, they would be the first invertebrates shown to possess such higher brain functions.
There are fundamental differences between the nervous system of vertebrates and invertebrates, however, some similarities can also be observed across the species. It is hard to tell exactly how the nervous system evolved and which steps were involved in the process. It is, however, possible to infer a general outline of evolution throughout the species. Little by little, it would seem that the nervous system became more and more complex and similar to what we are used to seeing in vertebrates. As for the human brain, there was still a long way to go and a lot of different nervous system versions until it finally reached its current level of complexity and organisation. Understanding simpler organisms has helped and will continue to help us better grasp some more difficult concepts like mentioned previously with habituation and sensitisation in the aplysia reflecting the concept of synaptic memory.
Sometimes, all it takes to see things clearer is a small flashlight.
Stay tuned to learn more about the nervous system across species of vertebrates 🙂
Author: Jennifer Morael
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