During the 1930s Wilder Penfield mapped the surface of the brain and showed that some areas were primarily involved in sensation (sensory cortex) while other areas were primarily involved in movement (motor cortex). The distinction wasn’t absolute and it’s been clear for at least 50 years that microstimulation, the electrical stimulation of cerebral cortex by very small currents, of sensory cortex can evoke motor responses. However, this knowledge mostly languished due to the lack of sophisticated techniques to tease out details of the neural circuits and the difficulty in placing the knowledge within a larger framework of how the brain worked.
A new paper “Motor Control by Sensory Cortex” published November 26, 2010 in Science reports on a study that used current techniques such as voltage sensitive dyes, optogenetic techniques, and neuronal inactivation through pharmacological techniques to good effect.
Microstimulation of primary somatosensory cortex resulted in whisker (vibrissae) retraction (movement of the whisker towards the mouse’s tail). The same thing happened at a longer latency when they microstimulated the whisker area in primary motor cortex. However, microstimulating another area in motor cortex resulted in whisker protraction (movement of the whisker towards the mouse’s nose). Interestingly, whisker movement occurs fastest on stimulation of the primary somatosensory cortex, next the primary motor cortex, and then finally the whisker protraction area in the motor cortex.
The spread of current to adjacent cortical areas is a potential problem with microstimulation. This research team used optogenetic techniques to provide better stimulus control including the possible spread of current. They modified cells in the primary somatosensory cortex or primary motor cortex by injecting a virus that caused those neurons to express light sensitive ion channels (channelrhodopsin). They used channels that stimulated the neurons when they shined blue light on the brain.
They showed that robust whisker retraction could be evoked by shining blue light on barrel cortex. They then began investigating the brain circuitry that may underlie the observed whisker movements. Using pharmacological manipulations the research team blocked excitatory synaptic transmission in the primary somatosensory cortex while they stimulated primary motor cortex. Whisker retraction evoked by optogenetic stimulation of primary motor cortex was blocked. However, whisker protraction emerged instead.
Through these and other manipulations the authors conclude that there are two parallel motor signaling pathways. One emerges from the primary somatosensory cortex which sends axons to the brainstem spinal trigeminal nuclei. From there, axons are sent to neurons in the facial nucleus that drive whisker retraction. The other pathway emerges from the primary motor cortex and sends axons to the brainstem reticular formation. From there, axons are sent to neurons in the facial motor nucleus that drive whisker protraction. When microstimulation is applied to the primary motor cortex whisker retraction is usually evoked through glutamatergic synaptic transmission in primary somatosensory cortex.
This is excellent work that will no doubt lead to further delineation of these circuits. A question that immediately comes to my mind is whether the “whisker protraction area” of motor cortex may be driving whisker protraction through the regular whisker related primary motor cortex?
Other whisker related blog posts:
Wiggling Whiskers: Neurons in the Barrel Cortex and Object Localization
Adult Brain is Continually Modified by Experience: Demonstration in the Whisker System
The Connectome: Video Journey Through Brain Microcircuitry
Adaptation Influences Synchronous Activity in the Thalamus and Information Content in the Cortex
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