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Jan. 15, 2010 Research Highlight Biology

Shaping and sharpening movements

A new recording method shows how microcircuitry in the motor cortex of the brain controls voluntary movements

Image of pyramidal neurons and FS interneurons Figure 1: Schematic diagram showing that pyramidal cells in layers 2/3, 5 and 6 of the motor cortex are involved in preparation, initiation, execution and termination of voluntary movements, whereas fast-spiking interneurons are involved only in the execution phase. © (2009) RIKEN

Using a novel technique to record the electrical activity of multiple nerve cells in moving animals, RIKEN researchers have clarified how the brain controls self-initiated voluntary movements. Their findings, published in Nature Neuroscience 1, reveal the dynamic properties of small networks of neurons, and show that movements are controlled by the co-ordinated activity of distinct groups of at least two different types of cell.

Voluntary movements are controlled by the primary motor cortex, which is subdivided into five or six layers, each containing distinct populations of pyramidal cells that are presumably activated at different times during the preparation, initiation and execution phases of a movement. The dynamics of these groups of cells is, however, poorly understood. Each layer also contains diverse populations of interneurons, the most abundant being fast-spiking (FS) interneurons, which are thought to regulate the output of the motor cortex by inhibiting the activity of pyramidal cells.

Yoshikazu Isomura of the RIKEN Brain Science Institute and his colleagues trained rats to spontaneously pull a lever with their forelimbs. While the animals performed the movement, the researchers were able to identify the type and layer of the active cells using their new recording method. They also used electrodes to simultaneously record the activity of multiple pyramidal cells and interneurons within the motor cortex to examine how they are connected to one another.

The researchers found that the pyramidal neurons in all cortical layers fire during every phase of voluntary movement. They also identified several distinct patterns of pyramidal cell activity, which occur in sequence and at different times, corresponding to each phase of movement. By contrast, FS interneurons were found to be activated during the execution of movements, but not during the preparation phase (Fig. 1).

FS interneurons therefore appear to modulate the output of pyramidal neurons, rather than inhibit their activity until onset of a movement as previously thought. They shape the motor commands sent from the motor cortex to the spinal cord and other brain regions involved in controlling movement, and make them sharper by limiting pyramidal neuron activity to shorter time windows, according to Isomura. The interneurons may also suppress irrelevant motor commands during the execution of movements.

“Our findings will eventually lead to more efficient rehabilitation for people with brain damage,” says Isomura. “They will also enable easier detection of motor control signals, so that brain-machine interfaces for such patients, which consist of implantable electrode arrays, will become less invasive.”

References

  • 1. Isomura, Y., Harukuni, R., Takekawa, T., Aizawa, H. & Fukai, T. Microcircuitry coordination of cortical information in self-initiation of voluntary movements. Nature Neuroscience 12, 1586–1593 (2009). doi: 10.1038/nn.2431

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