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June 2002 Vol 3 No 6 HIGHLIGHTS
Nature Reviews Neuroscience 3, 418 (2002)
SENSORY SYSTEMS
Fast adaptors
Rachel Jones
To deal with the huge range of inputs to which they are exposed, most
sensory systems show adaptation -- if a stimulus is repeated or sustained,
the sensory response will decrease or even disappear. Adaptation seems to
occur at various levels in a sensory pathway, and new data from Chung et al.
suggest that synaptic depression at thalamocortical synapses is an important
component of this process, at least in the somatosensory system.
When a rat's whisker is deflected, neurons
in the somatosensory thalamus and cortex
fire strongly. But if the whisker is
deflected repeatedly, the responses
quickly wane. Recordings from the
thalamus and cortex show that cortical
neurons adapt to this kind of repetitive
stimulation more quickly and more
strongly than do thalamic neurons, and
that they recover more slowly. This
supports the idea that cortical adaptation reflects further processes, over and
above those that lead to thalamic adaptation.
There are several possible mechanisms of cortical adaptation. For example,
slower adaptation in the cat visual cortex results from membrane
hyperpolarization, probably due to activation of a potassium current. Another
proposed mechanism is enhancement of inhibitory transmission. But a
leading candidate is short-term synaptic depression, either at thalamocortical
synapses or at recurrent excitatory cortico-cortical synapses (which normally
amplify the signal).
Chung et al. recorded the responses of cortical neurons to direct stimulation
of the ventroposteriomedial nucleus (VPM) of the thalamus. After adaptation,
the responses of cortical neurons to thalamic stimulation were smaller,
indicating that the thalamocortical synapses were depressed. By contrast,
cortical responses to stimulation of other cortical neurons did not decrease
after adaptation, indicating that, under these conditions, recurrent cortico-
cortical synapses do not become depressed. The authors suggest that this
lack of depression might be observed because the cortical neurons respond
to only the first few stimuli in a train (owing to thalamocortical depression),
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so their synapses have time to recover during the rest of the train.
Further recordings ruled out other potential mechanisms of adaptation. The
cortical cells showed no changes in membrane potential, input resistance or
membrane excitability following adaptation, indicating that such postsynaptic
effects are unlikely to account for rapid adaptation in this system. This
contrasts with the previous finding that slower adaptation in the visual
system results from membrane hyperpolarization in the cortex, and may
represent a general difference between fast and slow adaptation.
References and links
ORIGINAL RESEARCH PAPER
Chung, S. et al. Short-term depression at thalamocortical synapses
contributes to rapid adaptation of cortical sensory responses in vivo. Neuron
34, 437-446 (2002) | PubMed |
FURTHER READING
Carandini, M. & Ferster, D. A tonic hyperpolarizaton underlying contrast
adaptation in cat visual cortex. Science 276, 949-952
(1997) | Article | PubMed |
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Nature  Macmillan Publishers Ltd 2002 Registered No. 785998 England
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