BETHESDA, MD (February 25, 2004) When we want to go, why can we "wait"? In other words, when we sense that a bowel movement will be necessary, the body has the ability to defer that action until an appropriate time. A new research study examines this issue and the findings could have beneficial implications for those patients with fecal incontinence resulting from a cerebrovascular accident and injuries to the frontal lobe.
Voluntary control of the external anal sphincter (EAS) plays an essential role in maintaining fecal continence, or the ability to retain the body’s waste material until the time for proper discharge. This sphincter, known by a more common — and sometimes less complimentary — name, is comprised of deep, superficial, and subcutaneous striated muscle groups located around the boundary of the gastrointestinal tract. Innervation to this sphincter is provided by the somatic fibers of the second, third, and fourth sacral routes through nerves associated with genitalia.
Despite the considerable information existing about the peripheral and reflex control of the EAS, there are sparse data about the central control of voluntary contraction of this sphincter in humans. What is known mostly addresses its contraction response to experimental electrical or magnetic stimulation of the motor cortex, the region of the cerebral cortex most nearly immediately influencing movements of the face, neck and trunk, and arm and leg. Past studies have shown that the contracting of the EAS or pelvic floor to direct stimulation of the motor cortex, the cerebral cortical activity map related to voluntary contraction of this sphincter, could be quite different from the cortical topography of its control determined by direct stimulation of the cerebral cortex.
A New Study
To obtain a better understanding of the cortical control of the continence mechanism a new study attempted to determine the areas of the human cerebral cortex involved in voluntary contraction of the EAS and the relationship between cerebral cortical activity and two effort levels of willful contraction of this sphincter. The authors of "Cerebral Cortical Representation of External Anal Sphincter Contraction: Effect of Effort" are Mark K. Kern, Ronald C. Arndorfer, James S. Hyde, and Reza Shaker, all from the Medical College of Wisconsin, Milwaukee, WI. Their findings appear in the February 2004 issue of the American Journal of Physiology Gastrointestinal and Liver Physiology. The journal is one of 14 peer-reviewed journals published each month by the American Physiological Society (APS).
The researchers acknowledged the critical role of the external anal sphincter (EAS) in maintaining fecal continence. They also maintained that cerebral cortical control of voluntary EAS contraction is not completely understood. Accordingly, this research effort was designed to establish the cortical areas associated with voluntary EAS contraction and to determine the effect of two levels of sphincter contraction effort on brain activity.
Seventeen adults (ages 21-48, nine males), all without symptoms, were studied using functional magnetic resonance imaging (fMRI) to detect brain activity. Studies were done in two stages. In stage 1 (10 subjects, 5 male), anal sphincter pressure was monitored from a catheter-affixed bag. Subjects performed maximal and submaximal EAS contractions during two fMRI scanning sessions consisting of alternating 10-second intervals of sustained contraction and rest. In stage 2 studies, seven subjects (4 males) performed only maximum-effort sphincter contractions without a catheter. EAS contraction was associated with multifocal fMRI activity in sensory/motor, anterior cingulate, prefrontal, parietal, occipital, and insular regions.
All 17 subjects were able to complete their studies. Key findings in both stages of the research study were:
Stage 1: Comparison of objectively monitored maximal and submaximal EAS contractions found that during maximal anal sphincter contraction, cerebral cortical fMRI signal changes were detected in the sensory motor cortex in 10 of 10 subjects, cingulate gyrus and prefrontal cortex in seven of 10, parietooccipital activity in nine of 10, and insular cortex in six of 10 subjects.
Stage 2: Self-reported maximum EAS contraction without the presence of a measurement device resulted in changes in cerebral cortical blood oxygenation level in four distinctive areas of the brain, similar to studies conducted with the presence of the manometric device. The cortical activity was detected in the sensory motor cortex in seven of seven subjects, the cingulate gyrus and prefrontal regions in four of seven, the parietooccipital area in five of seven, as well as the insular cortex in three of seven subjects. Because the studied groups were different, no comparison could be done on magnitude of volume of cortical activation or its maximum percent fMRI signal change in activity between monitored maximum contraction and self-reported maximum contraction.
Comparison of the incidence of cortical activity in various regions associated with maximal and submaximal anal sphincter contraction between male and female subjects (5 males, 5 females) using a Fischer exact test revealed that insular activation was significantly more prevalent among female subjects compared with male subjects. Insular activity was observed in 100 vs. 20% for maximal and 80 vs. 20% for submaximal anal sphincter contraction among female and male subjects, respectively. There were no gender differences for activation incidence in other studied brain regions.
The research findings determined the cerebral cortical regions activated during voluntary contraction of the EAS. Results indicated that voluntary motor control of the EAS involves multiple cortical regions of the brain. These regions include the sensory/motor, cingulate gyrus, prefrontal, insular, and parietooccipital regions of the cerebral cortex. Furthermore, we have shown that the volume of cerebral cortical activity was directly related to the level of effort applied in generating two levels of contraction, in that generation of higher levels of contraction is accompanied by recruitment of larger volumes of the cerebral cortex exhibiting higher levels of fMRI activity.
Findings of the present study documenting a significantly larger area of brain activity with significantly higher signal intensity during maximum compared with submaximum contraction of the EAS confirm previous findings and indicate that the cerebral cortical activity measured by fMRI signals is directly related to EAS contractile output.
In summary, voluntary contraction of the EAS is associated with correlated, multifocal cerebral cortical activity. These regions of activity include the primary and secondary sensory/motor cortices, the insula, as well as the association areas of the brain such as the cingulate gyrus, prefrontal cortex, and the parietooccipital region. The volume and intensity of activation of recruited cortical regions during EAS contraction are commensurate with the level of contractile effort.
Source: American Physiological Society, February 26, 2004.