Neuroanatomy and neurophysiology of pelvic floor muscles
Pelvic floor muscles (PFM) support pelvic organs, they are actively involved in their function, and probably the main culprits in some dysfunctions. A good example is stress urinary incontinence (SUI), which may develop due to weakness and/or activation and coordination disturbances of PFM. All activity of PFM is mediated (controlled) by the nervous system.
INNERVATION OF PELVIC FLOOR MUSCLES
Somatic motor pathways
The motor neurons that innervate the striated muscle of the external urethral and anal sphincters originate from a localized column of cells in the sacral spinal cord called Onuf’s nucleus (Mannen et al 1982), expanding in humans from the second to third sacral segment (S2–S3) and occasionally into S1 (Schroder 1985). Within Onuf’s nucleus there is some spatial separation between motor neurons concerned with the control of the urethral and anal sphincters. Spinal motor neurons for the levator ani group of muscles seem to originate from S3–S5 segments and show some overlap.
Sphincter motor neurons are uniform in size and smaller than the other alpha motor neurons. They also differ with respect to their high con centrations of amino acid, neuropeptide, nor adrenaline (norepinephrine), serotonin and dopamine-containing terminals, which represent the substrate for the distinctive europharmacological responses of these neurons, and differ from those of limb muscles, the bladder and the PFM.
The somatic motor fi bres leave the spinal cord in the anterior roots and fuse with the posterior roots to constitute the spinal nerve. After passing through the intravertebral foramen the spinal nerve divides into a posterior and an anterior ramus.
Somatic fibres from the anterior rami (also called the sacral plexus) form the pudendal nerve. Traditionally the pudendal nerve is described as being derived from the S2–S4 anterior rami, but there may be some contribution from S1, and possibly little or no contribution from S4. The pudendal nerve continues through the greater sciatic foramen and enters in a lateral direction through the lesser sciatic foramen into the ischiorectal fossa (Alcock’s canal). In the posterior part of Alcock’s canal the pudendal nerve gives off the inferior rectal nerve; then it branches into the perineal nerve, and the dorsal nerve of the penis/clitoris. Although still controversial, it is generally accepted that the pudendal nerve supplies not only the anal but also the urinary sphincter. On the other hand it is mostly agreed that the main innervation for the PFM is through direct branches from the sacral plexus (‘from above’) rather than predominantly by branches of the pudendal nerve (Fig. 4.1).
Significant variability of normal human neuroanatomy is probably the source of remaining controversies originating from anatomical studies of peripheral innervation of pelvis, which have so far been performed in only a small number of cases. Higher nervous system regions control spinal cord motor nuclei by descending pathways; these inputs to PFM motor neurons are manifold, and mostly ‘indirect’ (through several interneurons). More direct connections to Onuf’s nucleus are from some nuclei in the brainstem(raphe, ambiguous) and from paraventricular hypothalamus.
Functional brain imaging is a powerful new tool to demonstrate functional anatomy of the human brain, and has already increased our knowledge in the realm of neural control of the lower urinary tract (LUT). Functional brain imaging techniques are based in particular on registering – directly or indirectly – the blood flow in the living human brain. Those brain areas, which during a particular manoeuvre (e.g. pelvic fl oor contraction) are controlling that particular activity, are more metabolically active than other ‘nonactive’ brain areas.
The increase in metabolism is accompanied by an increase in blood fl ow through the particular area, and this can be recorded. The established way of recording the ‘amount’ of blood fl ow in parenchymatous organs is by nuclear medicine techniques, by making the blood fl ow ‘visible’ by a radioisotope injected into the blood. Positron emission tomography (PET) relies on this principle and is able to render enough anatomical detail to be useful also for functional anatomical studies.
Using a different recording principle (but based on similar physiological principles), functional magnetic resonance tomography (fMR) is even better for providing detailed functional anatomical data. (These techniques can also demonstrate brain areas with ‘less activity’ as in the ‘resting state’, thus indicating inhibition of certain brain areas during execution of some manoeuvres.) PET studies have revealed activation of the (right) ventral pontine tegmentum (in the brainstem) during holding of urine in human subjects.
This finding is consistent with the location of the ‘L region’ in cats, proposed to control PFM nuclei. The connections serve the coordinated inclusion of PFM into ‘sacral’ (LUT; anorectal, and sexual) functions. Individual PFM and sphincters need not only be neurally coordinated ‘within’ a particular function (e.g. with bladder activity), but the single functions need to be neurally coordinated with each other (e.g. voiding and defecation, voiding and erection).
Sacral function control system is proposed to be a part of the ‘emotional motor system’ derived from brain or rainstem structures belonging to the limbic system. It consists of the medial and a lateral component. The medial component represents diffuse pathways originating in the caudal brainstem and terminating on almost all) spinal grey matter, using serotonin in particular as its neurotransmitter. This system is proposed to ‘set the threshold’ for overall changes in muscle activity, such as for instance in muscle tone under different physiological conditions (e.g. sleeping).
The lateral component of the emotional motor system consists of discrete areas in the hemispheres and the brainstem responsible for specifi c motor activities such as micturition and mating. The pathways belonging to the lateral system use spinal premotor interneurons to infl uence motor neurons in somatic and autonomic spinal nuclei, thus allowing for confl uent interactions of various inputs to modify the motor neuron activity.
PFM nuclei also receive descending corticospinal input from the cerebral cortex. PET studies have revealed activation of the superomedial precentral gyrus during voluntary PFM contraction, and of the right anterior cingulate gyrus during sustained PFM straining. Not surprisingly, PFM contraction can be obtained by electrical or magnetic transcranial stimulation of the motor cortex in man.