Neuroanatomy and neurophysiology of pelvic floor
- 1. NEUROANATOMY AND NEUROPHYSIOLOGY OF PELVIC FLOOR KARISHMA MOHAMMAD MPT (OBGYN)
- 2. 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). – Spinal motor neurons for the levator ani group of muscles seem to originate from S3 to S5 segments and show some overlap (Barber et al., 2002).
- 3. – 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 (Marani et al., 1993). – 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.
- 4. ■ 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 (‘from below’) .
- 7. Functional brain imaging ■ PET studies have revealed activation of the (right) ventral pontine tegmentum (in the brainstem) during holding of urine in human subjects (Blok et al., 1997).
- 8. ■ The sacral function control system is proposed to be a part of the ‘emotional motor system’ derived from brain or brainstem structures belonging to the limbic system. ■ It consists of the medial and a lateral component (Holstege, 1998). – The medial component represents diffuse pathways 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).
- 9. – The lateral component of the emotional motor system, responsible for specific motor activities such as micturition and mating.
- 10. Afferent pathways ■ Send a long process to the periphery and a central process into the spinal cord where it terminates segmentally or – after branching for reflex connections – ascends in some cases as far as the brainstem (Bannister, 1995). ■ The afferent pathways from the anogenital region and pelvic region are divided into somatic and visceral. – Somatic afferents derive from touch, pain and thermal receptors in skin and mucosa and from proprioceptors in muscles and tendons. – The somatic afferents accompany the pudendal nerves, the levator ani nerve and direct somatic branches of the sacral plexus. – The visceral afferents accompany both parasympathetic and sympathetic efferent fibres
- 11. NEUROPHYSIOLOGY OF PELVIC FLOOR MUSCLES – Muscle activity is thoroughly dependent on neural control. ‘Denervated’ muscle atrophies and turns into fibrotic tissue. – A motor unit consists of one alpha (or ‘lower’) motor neuron (from the motor nuclei in the spinal cord), and all the muscle cells this motor neuron innervates. – By concomitant activity the PFM acts as the ‘closure unit’ of the excretory tracts, the ‘support unit’ for pelvic viscera and an ‘effector unit’ in the sexual response. – It is clear, however, that the coordination between individual PFM can definitively be impaired by disease or trauma.
- 12. ■ Tonic and phasic pelvic floor muscle activity – The normal striated sphincter muscles demonstrate some continuous motor unit activity at rest as revealed by kinesiological EMG. – This physiological spontaneous activity may be called tonic, and depends on prolonged activation of certain tonic motor units. – With any stronger activation maneuver (e.g. contraction, coughing), and only for a limited length of time, new motor units are recruited.These may be called ‘phasic’ motor units.
- 13. – On voiding, inhibition of the tonic activity of the external urethral sphincter – and also the PFM – leads to relaxation. – Similarly, the striated anal sphincter relaxes with defecation and also micturition.
- 14. Reflex activity of pelvic floor muscles ■ The reflex activity of PFM is clinically and electro physiologically evaluated by eliciting the bulbocavernosus and anal reflex. ■ The bulbocavernosus reflex is evoked on non painful stimulation of the glans (or – electrically – the dorsal penile/clitoral nerve). ■ The constant tonic activity of sphincter muscles is thought to result from the characteristics of their ‘low threshold’ motor neurons and the constant ‘inputs’ (either of reflex segmental or suprasegmental origin). ■ It is supported by cutaneous stimuli, by pelvic organ distension and by intra- abdominal pressure changes.
- 15. ■ To correspond to their functional (effector) role as pelvic organ supporters (e.g. during coughing, sneezing), sphincters for the LUT and anorectum, and as an effector in the sexual arousal response, orgasm and ejaculation, PFM have also to be involved in very complex involuntary activity, which coordinates the behavior of pelvic organs (smooth muscle) and several different groups of striated muscles. ■ This activity is to be understood as originating from so-called ‘pattern generators’ within the central nervous system, particularly the brainstem.These pattern generators (‘reflex centers') are genetically inbuilt.
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