Paper 1: Neurobiology of a simple Memory (Wilson, 2008)
The olfactory system is a relatively simple system, highly conserved across vertebrate evolution and has long been implicated as having unique ties to memorial processes.
Although habituation is classically considered a simple form of memory, it may serve as an important building block in more complex forms of cognition and attention. Furthermore, disruptions in habituation and sensory gating are linked to disorders, as well as aging and substance abuse. Thus understanding the mechanisms of this form of memory could have far-reaching implications.
Odors activate olfactory sensory neurons (OSN) within the nose, which form glutamatergic synapses (excitatory) on second-order neuron called mitral cells. Miral cells project to the olfactory cortex, a major subdivision of which is called piriform cortex. Mitral cells are glutamatergic and target pyramidal cells within piriform cortex. Piriform cortical pyramidal neurons project to the basolateral amygdala, which in turn projects to the central amygdala.
As is the case in other sensory systems, central olfactory neurons show greater response decrement than peripheral receptors, with piriform cortical neurons showing rapid, nearly complete response adaptation within seconds or minutes of stimulation in both rodents and humans.
changes in the periphery could not account for the magnitude of decrease in perceptural intensity estimates.
cortical neurons (piriform cortical pyramidal cells) adapt to repeated or prolonged odor stimulation more rapidly and completely than their afferent mitral cells
this cortical adaptation is associated with depression of the glutamatergic mitral-pyramidal cell synapse and that this synaptic depression recovers with the same time course as the short-term adaptation of odor-evoked postsynaptic potentials.
depression of mitral cell afferents to the piriform cortex, via a presynaptic metabotraopic glutamate receptor-mediated mechanism, leads to both short-term cortical odor adaptation and short-term behavioral odor habituation.
- difference between short-term and long-term habituation
- short-term habituation happens after rapid, short presentations of odors and lasts only a few minutes; and long-term habituation can last > 1h following spaced, long presentations.
- short-term habituation is highly odor-specific, with very little cross-habituation even between molecularly similar odorants, whereas long-term habituation shows more stimulus generalization
- NMDA receptor antagonists blocking long-term habituation but having no impact on short-term habituation and metabotropic glutamate receptors blocking short-term habituation but having no effect on long-term habituation
- long-term habituation originates from olfactory bulb and short-term habituation is mediated by the olfactory cortex
- olfactory bulb mitral cells show strong cross-habituation whereas piriform cortical neurons show little cross-habituation
- cortical neurons adapt to repeated odors more rapidly and completely than their afferent mitral cells, cortical adaptation associates with depression of mitral-pyramidal cell synapse
- habituation in the mammalian brain is more complicated than generally described