Cellular Mechanisms of Learning and the Biological Basis of Individuality”. Principles of Neuroscience

THROUGHOUT THIS BOOK we have emphasized that all behavior is a function of the brain and that malfunctions of the brain give rise to characteristic disturbances of behavior. Behavior, in turn, is shaped by learning. How does learning act on the brain to change behavior? How is new information acquired and, once acquired, how is it retained? In the preceding chapter we saw that memory—the outcome of learning—is not a single process but has at least two forms. Implicit (declarative) memory is unconscious memory for perceptual and motor skills, whereas explicit (nondeclarative) memory is a memory for people, places, and objects that requires conscious recall. In this chapter we examine the cellular and molecular mechanisms that contribute to these two forms of memory by exploring the mechanisms that underlie simple implicit forms of memory storage in invertebrates and the more complex explicit forms in vertebrates. We shall see that the molecular mechanisms of memory storage are highly conserved throughout evolution, and that the more complex forms of learning and memory depend on many of the same molecular mechanisms used in the simplest forms. Finally, we shall consider the idea that these mechanisms contribute to individuality by changing the connectivity of neurons in our brains. Figure 63-1 The cellular mechanisms of habituation have been investigated in the gill-withdrawal reflex of the marine snailAplysia. A. A dorsal view of Aplysia illustrates the respiratory organ (gill), which is normally covered by the mantle shelf. The mantle shelf ends in the siphon, a fleshy spout used to expel seawater and waste. A tactile stimulus to the siphon elicits the gill-withdrawal reflex. Repeated stimuli lead to habituation. B. This simplified circuit shows key elements involved in the gill-withdrawal reflex as well as sites involved in habituation. In this circuit about 24 mechanoreceptors in the abdominal ganglion innervate the siphon skin. These glutaminergic sensory cells form synapses with a cluster of six motor neurons that innervate the gill and with several groups of excitatory and inhibitory interneurons that synapse on the motor neurons. (For simplicity, only one of each type of neuron is illustrated here.