How do we learn? How do our brains allow us to acquire new information? We can ask this question for many different types of learning and memory. In addition to learning from books, we learn from new experiences. Our response to certain stimuli, such as spicy foods, can change over time. But how do we adapt to things that are initially so aversive?
The answer lies in neuroplasticity. Plasticity is the brain’s capacity for change. To understand how this occurs, scientists often study simpler forms of learning in simpler systems. Take, for example, the sea snail Aplysia californica. In the ocean, the Aplysia must learn to adapt to dangerous surroundings. When her siphon is touched, the Aplysia’s natural response is to withdraw her delicate siphon and gill. This is a basic defensive reflex, and it’s controlled by a very simple neural circuit.
Touching the siphon generates an electrical signal called an action potential in the sensory neuron. The action potential travels along the sensory neuron toward the motor neuron. At the connection between these two neurons, the synapse, chemicals called neurotransmitters are released from one cell and activate receptors on the next cell. If the signal is strong enough, an action potential will also be generated in the motor neuron. This sends the message to the gills to retract.
Now what happens when the Aplysia siphon is touched again, and again, and again? Over time she learns that it’s not a sign of danger, so her defensive response becomes progressively weaker, a type of learning called habituation. In order for this to occur, something in the Aplysia’s brain has to change. But what?
The defense reflex is controlled by the strength of the connection between the sensory and motor neurons. During habituation, this synaptic connection is weakened. Now when the siphon is touched, the signal from the sensory neuron is no longer strong enough to generate an action potential in the motor neuron. As a result, the gill and siphon will not be withdrawn, no matter how hard we try. This form of neuroplasticity has allowed the Aplysia to learn and respond in a way that is more adaptive to her environment.
Like Aplysia, humans can also habituate and adapt over time. Our responses to stimuli that were initially aversive can change completely. By studying neuroplasticity in simple organisms like the Aplysia, we can begin to understand the neural basis for all forms of learning, from fine tuning the simplest reflexes, to acquiring the most complex skills and knowledge that make us who we are.