5.1 Synaptic Changes in Learning
Learning Objectives
- Describe the process of habituation in cellular learning
- Describe the process of sensitization in cellular learning
- Compare and contrast the processes of habituation and sensitization in cellular learning.
In Chapter 2, you learned about the importance of neuroplasticity in the brain as a foundation for learning and memory. This adaptability not only exists in humans, but in the nervous systems of all animals. In fact, much of what we know about what happens at the cellular level in neuroplasticity comes from the study of animals. In this chapter we explore some of the details on how neuroplasticity occurs at the level of the synapse. Eric Kandel was awarded a Nobel Prize in 2000 for his work investigating how this occurs. Much of his work was conducted with marine slugs, called Aplysia. Aplysia have a much simpler nervous system than humans, having only about 20,000 neurons. Yet, the structure of their neurons and synapses are remarkably similar to humans. This makes them a valuable model for studying the cellular basis of learning (Kolb et al., 2016).
Aplysia live on the sea floor, where they eat seaweed. They use their siphon for expelling excess water and seaweed and their gill for breathing. If the siphon is touched the gill and siphon withdraw into the mantle in a reflexive manner. The reflex involves a simple circuit of sensory and motor neurons. Eric Kandel and others have studied changes in this reflex. In the sections below, we will explore two ways that this reflex can change: habituation and sensitization. These changes represent learning that does not require conscious association.
Link to Learning
An interview with Eric Kandel on aplysia.
You can hear Eric Kandel’s nobel prize speech here.
Habituation
A behavior that shows habituation decreases in strength with repeated stimulation. This is a form of nonassociative learning. You have likely experienced this phenomenon. Our sensory systems are designed to habituate to repeated exposure of stimuli. Consider a loud hum from a fan in your dorm, as you first walk in the room it is loud and annoying. As time passes, the sound is in the background and you don’t notice it.
When studying habituation in the Aplysia in the lab, the researcher repeated touches or sprays a water jet on the siphon of the aplysia. As the sensory neurons are repeatedly stimulated, the levels of Ca2+taken in at the end feet in response to the action potentials decreases. This results in less neurotransmitter released at the synapse. This results in weaker excitatory postsynaptic potentials (EPSPs) at the motor neuron. This results in a decrease in the gill withdrawal response that can last as long as 30 minutes.
Sensitization
Sensitization is an increased response to a repeated stimulus. It can be considered as the opposite of habituation, an enhanced intensity of response as opposed to a reduced intensity of response. Both types of responses are influenced by the context and time frame in which they occur. Post-traumatic stress disorder (PTSD) has the characteristic of the enhanced response to a stimulus that we see with sensitization (Kolb, et al., 2016).
In studies with Aplysia in the lab, the researcher applies a small shock to the tail of the Aplysia. The shock to the tail, like an attack of a predator in the wild, results in an enhanced gill withdrawal response. This enhanced response can last from minutes to hours. The neural circuit in sensitization is different from that in habituation in that it also involves an interneuron. This interneuron receives a signal from the sensory neuron in response to the shock. The interneuron releases neurotransmitter that is taken up by the sensory neuron at the synapse. This results in an increase in the Ca2+ influx in the endfoot of the sensory neuron. This produces an increase in the amount of the neurotransmitter serotonin released by the sensory neuron. This causes more depolarization at the postsynaptic membrane, leading to an enhanced gill withdrawal response.
Link to Learning
Creating Long-term Changes at the Synapse
The changes at the synapse described above for habituation and sensitization occur rapidly, but do not last for extended periods of time. According to our definition in of learning, a change in behavior must be relatively permanent. To achieve such long-term changes, it is necessary for repeated stimulation. Repetitions over a short period of time, only produce short-term changes in behavior. Bailey et al. (2015) studied the effects of long-term training in Aplysia. In studies of Aplysia undergoing extensive habituation training, they found decreases in the number and sizes of the synapses, whereas the Aplysia in the long-term sensitization training showed increases in the number and sizes of the synapses. These synaptic changes are considered to be relatively permanent and may underlie long-term memory.
Test your knowledge:
- Explain the advantages and/or disadvantages of permanent habituation and sensitization.
a behavioral response that is weakened with repeated presentations of a stimulus.
an increased response to a repeated stimulus