Drop what you're doing: Cataplexy and corticomotor structure and function

The motor cortex is located on the precentral gyrus, caudal to the prefrontal cortex, and rostral to the somatosensory cortex and central sulcus, extending from the midline, laterally (Carlson, 2010, p. 273). Whereas the control of sensory systems is largely in the posterior cortex, movement is largely anterior and takes several forms: reflex actions, automatic repetitive actions (like walking), semivoluntary actions (yawning), and voluntary actions (picking up an object) (Zillmer, Spiers, & Culbertson, 2008, p. 189).

The motor cortex is organized (according to one theory) in a top-down, hierarchical fashion (another theory suggests a parallel process within the motor cortex, several circuits working simultaneously, see also Haines as cited in Zillmer, et al., 2008, p. 189). The motor cortex follows this pattern thus: first, highly integrated sensory information from the sensory association areas (parietal lobes, subcortical basal ganglia, and the cerebellum) and gets funneled through the primary motor cortex (responsible for fine details for movement, mapped as a motor homunculus) down through three other areas, the secondary motor cortex, the supplemental motor area (or the supplementary motor area or medial premotor area – functions in organization and sequential timing of movement) (pp. 189-91) (SMA - Carlson, 2010, p. 274); the premotor area (PMA or premotor cortex – plays a role in planning and movement readiness), and the cingulate motor area (CMA or cingulate motor area – likely involved in emotional and motivational impetus for movement, though less is known about this, but its proximity to the anterior cingulate suggests its emotional component) with the areas in the parietal lobes (posterior, Brodmann’s areas 5 & 7 – spatial mapping and motor programming coordination) and dorsolateral prefrontal cortex (dlPFC - involved in premotor planning and deployment Zillmer, et al., 2008, pp. 189-92).

In order to control movement, there are two groups of cortical efferent pathways, descending from the structures listed above (funneled down through the primary motor strip down through the midbrain, pons, medulla, and terminating in the spinal cord): the lateral (containing the corticospinal, corticobulbar, and rubrospinal tracts – controlling independent limbs for grasping objects and face & tongue movement) and the ventromedial group (the vestibulospinal – involved in posture, the tectospinal tract – eye, trunk, head coordination, the lateral and medial reticulospinal tract – involved in walking, and the ventral corticospinal tract – involved with both locomotion and posture) (Carlson, 2010, pp. 274-7).

These brain structures and descending pathways have myriad disorders that are beyond the scope of this post; however, one very interesting phenomenon is that of narcoleptic-cataplexy (NC). This is a condition by which the victim has a sudden and pathologic onset of REM sleep, where “the associated motor inhibition relates to a variety of nervous system dysfunctions, including massive nonreciprocal excitation of spinal inhibitory interneurons and active inhibition of motoneurons” (Zillmer, et al., 2008, pp. 462-3). This unusual and extreme form of narcolepsy is a complex sleep-wake disorder and is a sudden and brief loss of muscle tone (atonia) triggered by strong (mostly positive) emotions such hearing a funny joke (Schwartz, Ponz, Poryazova, Werth, Boesiger, Khatami, & Bassetti, 2008, p. 514). This cataplexic response is when suprapontine (that which lies supra, or above, the pons) structures fail to control the activation of the ponto-medullary mechanisms that normally underlie muscle atonia in REM-sleep (p. 514). Interestingly enough, Schwartz and colleagues found that when people see humorous pictures activation in the hypothalamus (reduced) and amygdala (increased) show up on MRI concluding that in cataplexy, people have a dysfunction in the hypothalamic-amygdala due to positive emotions and that hypocretin (a hypothalamic peptide that regulates sleep-wake, motor and feeding) modulates emotional inputs within the amygdala (pp. 514-5).

These results beg the question can this information be used in other areas where amygdalic overactivation is a problem (as in PTSD)? Time will tell.



References

Carlson, N. (2010). Physiology of behavior, (10th ed.). Boston: Allyn & Bacon.

Schwartz, S., Ponz, A., Poryazova, R., Werth, E., Boesiger, P., Khatami, R., & Bassetti, C. L. (2008). Abnormal activity in hypothalamus and amygdala during humour processing in human narcolepsy with cataplexy. Brain: A Journal of Neurology, 131(2), 514-522. doi:10.1093/brain/awm292

Zillmer, E., Spiers, M., & Culbertson, W. (2008). Principles of neuropsychology, (2nd ed.). Belmont, CA: Thomson Wadsworth.