Vast amounts of detailed, often trivial information sits between our ears, waiting on the off-chance of being recalled: important dates, past events, and how to walk, talk and eat. Learned information and procedures must be stored in order to be useful, but how we do this, often without awareness, remains ambiguous.
The idea that our brains grew new neurons to accommodate new memories was exposed as highly impractical by thinkers in the 19th Century. Thinking turned to ways in which neurons might change to allow memories to be stored, including Hebb’s law that ‘neurons that fire together wire together.’ Modern thinking on memory had begun, but conclusive evidence was still far away.
However, deep in the Gulf of California, munching on red algae in a blissfully unaware state, lies an ideal model for studying these ideas: the sea slug, Aplysia Californica.
Due to its primitive anatomy, neural processes can be observed much more clearly in Aplysia than in more complex animals, like mammals. Aplysia have much fewer neurons (around 20,000), which, unlike those of mammals, are visible to the naked eye. This means that individual neurons can be easily identified, with few differences in anatomy between animals. In Aplysia, sensory neurons, processing touch, and motor neurons, inducing movement, are connected by single synapses, rather than the complex networks seen in mammals, so the pathways of neural activity can be much more easily traced.
They also possess a simple but useful behaviour, the Gill/Siphon Withdrawal Reflex (GSWR), whereby a light touch to the animal’s siphon or mantle causes the gill and siphon to retreat inwards, protecting the Aplysia from potential harm. Manipulations of this reflex have provided a useful model for studying several types of learning at the level of the synapse.
Several kinds of simple learning are useful to Aplysia. They live in kelp forests, so would really struggle to breathe if they tucked their gill away every time a kelp plant wafted their way, but would be in danger if they didn’t bother to do so on making contact with a predator. Ergo, using habituation to zone out to the kelp’s gentle nudges, whilst remaining responsive to different kinds of touch, is vital.
These kinds of learning seems to be achieved by changes in chemical transmission at the synapse. During habituation, repeated gentle electrical stimulation of the siphon leads to a gradual decrease in the magnitude of the GSWR, as the synapse from the sensory to motor neurons becomes less sensitive, and the signal becomes too weak to pass to the motor neuron. This can then be reversed by dishabituation, in which an electrical shock reinstates the GSWR. The GSWR can also be amplified by presenting a novel, unpleasant stimulus, such as a tail shock. This results in sensitisation, in which chemical messaging from the presynaptic sensory neuron, headed up by serotonin, leads to a temporary increase in the strength of the synapse, meaning that the signal is more easily passed to the motor neuron.
These responses are graded, such that the longer the period of stimulation, the longer the learning is retained, with some accounts of these behaviours lasting days and even weeks. Not bad for an invertebrate!
However, Aplysia have also shown the ability to learn an association between events through classical conditioning – at least at the neural level. Whilst a gentle siphon touch elicits a small GSWR, a tail shock induces a much bigger withdrawal response. However, after repeated, simultaneous pairings of a gentle siphon touch and a tail shock, a siphon touch alone will elicit a larger GSWR than any stimulation before training. This is because the two kinds of stimulation are now associated.
The synaptic changes that allowed this associative learning to happen were up for debate. If classical conditioning in Aplysia was underpinned by the same neural processes as those in mammals, Aplysia would be invaluable for studying the neural basis of learning and memory, and applying these findings to more complex animals.
In mammals, a mechanism of synaptic plasticity had already been proposed: that of long-term potentiation, or LTP. This refers to an enduring increase in synaptic strength due a particular pattern of activity, which allows a neural signal to be transmitted more easily when that pattern is activated. LTP has several requirements in order to occur, including the need for both the presynaptic and postsynaptic neurons to fire a signal. It was first observed by Terje Lømø, whilst delivering electrical stimulation to the hippocampi of anaesthetised rabbits; the hippocampus being the part of the brain most often implicated in memory.
Originally, it was thought that conditioning in invertebrates, like Aplysia, was not underpinned by LTP, and that this explained the species’ more primitive learning abilities. Instead, processes induced solely by the presynaptic neuron were alleged to account for conditioning, similar to the sensitisation process.
However, several researchers then found evidence for LTP during conditioning in Aplysia. Most convincing was a paper showing that blocking either the presynaptic or postsynaptic neuron from firing resulted in an absence of synaptic strengthening after behavioural associative learning. In other words, both presynaptic and postsynaptic chemical messaging was necessary for conditioning to occur. Such messaging also used the same neurotransmitter as Lømø’s bunnies.
It began to seem that LTP was responsible for the neural changes allowing classical conditioning in both vertebrates and invertebrates – suggesting that LTP may represent associative learning at its simplest level in all organisms possessing the ability for this kind of learning.
Most current research in this area is now directed towards understanding how LTP might cause structural changes to neural networks, and investigating the link between LTP and behaviour. As the nervous system of Aplysia is much simpler than that of a mammal, such a link is easier to demonstrate in these little marine animals. But let’s be clear: it is the Aplysia’s nervous system that shows signs of learning, not the animal per se. These little slugs remain as blissfully unaware of their hidden talents as ever before…