Podcast with Sten Grillner on lamprey and central pattern generator

How does a 560-million-year-old fish illuminate the control architecture behind all vertebrate movement? Sten Grillner traces the neural circuits of locomotion from lamprey spinal cord to human basal ganglia.

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Sten Grillner presents decades of work on the lamprey, a jawless fish that emerged during the Cambrian explosion, as a model for understanding the conserved control systems underlying vertebrate motor behavior. He explains how the lamprey's spinal cord contains approximately 100 central pattern generators (CPGs) that produce rhythmic swimming through the interplay of excitatory premotor interneurons, inhibitory coordination neurons, and critical membrane properties including NMDA receptors, voltage-dependent calcium channels, and calcium-activated potassium channels. Even without sensory feedback, the isolated spinal cord generates well-coordinated locomotor patterns, though stretch receptors provide essential compensation for environmental perturbations.

The conversation reveals how detailed computational models of the lamprey spinal cord, incorporating biological variability in cellular properties across neuron populations, demonstrated that this variability is not noise but a design feature essential for stable motor output. A striking finding from large-scale simulations with 10,000 neurons showed that modifying just 5-10 percent of the network could entirely transform the pattern of activity, enabling transitions between forward and backward swimming.

Grillner then ascends the neural hierarchy to describe how basal ganglia circuits control behavior through a layered architecture. The substantia nigra reticulata and globus pallidus project directly to brainstem locomotor command centers and the tectum, providing powerful inhibitory gating of motor programs. The striatum receives cortical input and interfaces with the thalamus in recurrent loops. He presents this as a four-layered control structure: CPGs at the base, brainstem motor nuclei, the nigra-pallidus output layer, and the cortex-striatum input layer, with the thalamus providing modulatory feedback across the upper layers.

The discussion explores how this basic architecture has been elaborated through vertebrate evolution, from the emergence of paired fins in elasmobranchs to the development of limbs in tetrapods, while the fundamental circuit principles remain remarkably conserved. Grillner argues that new motor capabilities arise not from qualitative changes in spinal circuitry but from the parceling out of interneuron populations to independently control new appendages.