This week UCSD is proud to feature the work of Peter Strick, Chair of Neurobiology at the University of Pittsburg. Dr. Strick’s lab investigates the circuitry of the brain, focusing on cortical motor areas as well as the basal ganglia and cerebellum.
Motor cortex has long been known to contain somatotopically organized motor “homunculi,” regions with point-for-point correspondence between the body and its representation in the brain. Recent work in Dr. Strick’s lab has discovered that primary motor cortex (M1) and rostromedial motor area (M2) also contain somatotopic map of visceromotor functions.
The autonomic nervous system is the functional subdivision of the central nervous system responsible for unconscious monitoring and control of visceral organs. It allows us to maintain homeostasis despite constant changes in temperature, food intake, and physical activity. This system is not exclusively reactionary, suggesting possible allostatic regulation in which higher brain centers generate anticipatory activity to prevent imbalances before they occur.
In this paper Strick injected rats’ kidneys with a modified rabies virus to identify the cortical origin of their autonomic innervations. Rabies virus is a useful tool for tracing circuits because it travels exclusively in the retrograde direction (from a postsynaptic neuron into the presynaptic neurons that contact it) and does so in a time-dependent fashion (infection spreads at a constant rate). Varying the time the virus is allowed to spread can tell us which neurons are involved in each stage of a circuit: first-order neurons (those directly innervating the kidneys) are infected first, followed by second-order, and so on and so forth.
Figure 1. Retrograde transneuronal transport of rabies virus through neural circuits that innervate the kidney.
Contrary to classical models in which cingulate and insular cortex provide the primary cortical control of visceral function, the first cortical neurons infected were located overwhelmingly in M1 or M2. Further, these neurons were concentrated heavily in areas representing the trunk, consistent with the known spinal location of renal sympathetic preganglionic neurons. These results suggest that sympathetic visceromotor control originates in motor cortex and is organized in a somatotopic manner similar to motor control.
Figure 2. The first neurons to reach cortex are located most densely in the trunk representations of M1 and M2.
The discovery of visceromotor control in motor cortex has important implications. M1 and M2 have different roles in the control of conscious movement; while M1 is involved in the execution of movement, M2 is critical to the planning and preparation of movements. If visceromotor control parallels this distinction, M2 may provide the anticipatory processing necessary for allostatic regulation. Further, interconnectivity between M1 and the basal ganglia (BG) may explain why some Parkinson’s patients (a neurodegenerative disease that affects the BG) experience autonomic dysfunction (symptoms often assuaged by deep brain stimulation of BG).
If you are interested in this and other work by the Strick lab, join us tomorrow at 4:00 at the UCSD Center for Neural Circuits and Behavior, where Dr. Strick will be speaking about unraveling the “brain-body” connection.
Levinthal DJ, Strick PL. The motor cortex communicates with the kidney. J Neurosci 32: 6726–6731, 2012.
Michael Metke is a first year graduate student in the Neuroscience program at UCSD.