M. Houston

Bio: M. Houston is an academic researcher from University of Pittsburgh. The author has contributed to research in topics: Onuf's nucleus & Gray commissure. The author has an hindex of 1, co-authored 1 publications receiving 209 citations.

Organization of afferent and efferent pathways in the pudendal nerve of the female cat.

Abstract: Application of horseradish peroxidase to the pudendal nerve in the female cat labelled lumbosacral afferent and efferent neurons and their processes. Afferent axons entered the spinal cord primarily at the S1 and S2 segments and traveled rostrocaudally in Lissauer's tract and the dorsal columns. A distinctive component of the dorsal column projection was located at the lamina I-dorsal column border as a densely labelled, compact bundle that distributed fibers to the dorsal horn at spinal levels near the segments of entry of the afferent axons. Afferent terminal labelling was located in the marginal zone, the intermediate gray matter, and the dorsal gray commissure in the lumbosacral and coccygeal spinal cord. A well-defined terminal field restricted to the S1 and rostral S2 segments was present in the medial third of the nucleus proprius and substantia gelatinosa. Labelled motoneurons in Onuf's nucleus (S1 and S2) exhibited longitudinal dendrites that extended rostrocaudally within the nucleus and three groups of transverse dendrites that emanated periodically from the nucleus and passed to the ventrolateral funiculus, the intermediate gray, and the dorsal gray commissure. Components of the pudendal nerve that innervate the anal and urethral sphincters were also labelled by injecting HRP into the respective sphincter muscles. Motoneurons innervating the anal and urethral sphincters were located in the dorsomedial and ventrolateral divisions, respectively, of Onuf's nucleus. Afferent projections from the two sphincters were similar; the most prominent terminations were present in the marginal zone, intermediate gray, and dorsal gray commissure. These results are discussed with respect to the physiological function of the pudendal nerve and its relationship with sacral autonomic pathways.

The neural control of micturition

Abstract: Micturition, or urination, occurs involuntarily in infants and young children until the age of 3 to 5 years, after which it is regulated voluntarily. The neural circuitry that controls this process is complex and highly distributed: it involves pathways at many levels of the brain, the spinal cord and the peripheral nervous system and is mediated by multiple neurotransmitters. Diseases or injuries of the nervous system in adults can cause the re-emergence of involuntary or reflex micturition, leading to urinary incontinence. This is a major health problem, especially in those with neurological impairment. Here we review the neural control of micturition and how disruption of this control leads to abnormal storage and release of urine.

Extracellular stimulation of central neurons: influence of stimulus waveform and frequency on neuronal output.

Abstract: The objective of this project was to examine the influence of stimulus waveform and frequency on extracellular stimulation of neurons with their cell bodies near the electrode (local cells) and fibers of passage in the CNS. Detailed computer-based models of CNS cells and axons were developed that accurately reproduced the dynamic firing properties of mammalian motoneurons including afterpotential shape, spike-frequency adaptation, and firing frequency as a function of stimulus amplitude. The neuron models were coupled to a three-dimensional finite element model of the spinal cord that solved for the potentials generated in the tissue medium by an extracellular electrode. Extracellular stimulation of the CNS with symmetrical charge balanced biphasic stimuli resulted in activation of fibers of passage, axon terminals, and local cells around the electrode at similar thresholds. While high stimulus frequencies enhanced activation of fibers of passage, a much more robust technique to achieve selective activation of targeted neuronal populations was via alterations in the stimulus waveform. Asymmetrical charge-balanced biphasic stimuli, consisting of a long-duration low-amplitude cathodic prepulse phase followed by a short-duration high-amplitude anodic stimulus phase, enabled selective activation of local cells. Conversely, an anodic prepulse phase followed by a cathodic stimulus phase enabled selective activation of fibers of passage. The threshold for activation of axon terminals in the vicinity of the electrode was lower than the threshold for direct activation of local cells, independent of the stimulus waveform. As a result, stimulation induced trans-synaptic influences (indirect depolarization/hyperpolarization) on local cells altered their neural output, and this indirect effect was dependent on stimulus frequency. If the indirect activation of local cells was inhibitory, there was little effect on the stimulation induced neural output of the local cells. However, if the indirect activation of the local cells was excitatory, attempts to activate selectively fibers of passage over local cells was limited. These outcomes provide a biophysical basis for understanding frequency-dependent outputs during CNS stimulation and provide useful tools for selective stimulation of the CNS.

Neural Control of the Lower Urinary Tract

Abstract: This article summarizes anatomical, neurophysiological, pharmacological, and brain imaging studies in humans and animals that have provided insights into the neural circuitry and neurotransmitter mechanisms controlling the lower urinary tract. The functions of the lower urinary tract to store and periodically eliminate urine are regulated by a complex neural control system in the brain, spinal cord, and peripheral autonomic ganglia that coordinates the activity of smooth and striated muscles of the bladder and urethral outlet. The neural control of micturition is organized as a hierarchical system in which spinal storage mechanisms are in turn regulated by circuitry in the rostral brain stem that initiates reflex voiding. Input from the forebrain triggers voluntary voiding by modulating the brain stem circuitry. Many neural circuits controlling the lower urinary tract exhibit switch-like patterns of activity that turn on and off in an all-or-none manner. The major component of the micturition switching circuit is a spinobulbospinal parasympathetic reflex pathway that has essential connections in the periaqueductal gray and pontine micturition center. A computer model of this circuit that mimics the switching functions of the bladder and urethra at the onset of micturition is described. Micturition occurs involuntarily in infants and young children until the age of 3 to 5 years, after which it is regulated voluntarily. Diseases or injuries of the nervous system in adults can cause the re-emergence of involuntary micturition, leading to urinary incontinence. Neuroplasticity underlying these developmental and pathological changes in voiding function is discussed.

Integrative control of the lower urinary tract: preclinical perspective

Abstract: Storage and periodic expulsion of urine is regulated by a neural control system in the brain and spinal cord that coordinates the reciprocal activity of two functional units in the lower urinary tract (LUT): (a) a reservoir (the urinary bladder) and (b) an outlet (bladder neck, urethra and striated muscles of the urethral sphincter). Control of the bladder and urethral outlet is dependent on three sets of peripheral nerves: parasympathetic, sympathetic and somatic nerves that contain afferent as well as efferent pathways. Afferent neurons innervating the bladder have A-δ or C-fibre axons. Urine storage reflexes are organized in the spinal cord, whereas voiding reflexes are mediated by a spinobulbospinal pathway passing through a coordination centre (the pontine micturition centre) located in the brainstem. Storage and voiding reflexes are activated by mechanosensitive A-δ afferents that respond to bladder distension. Many neurotransmitters including acetylcholine, norepinephrine, dopamine, serotonin, excitatory and inhibitory amino acids, adenosine triphosphate, nitric oxide and neuropeptides are involved in the neural control of the LUT. Injuries or diseases of the nervous system as well as disorders of the peripheral organs can produce LUT dysfunctions including: (1) urinary frequency, urgency and incontinence or (2) inefficient voiding and urinary retention. Neurogenic detrusor overactivity is triggered by C-fibre bladder afferent axons, many of which terminate in the close proximity to the urothelium. The urothelial cells exhibit ‘neuron-like' properties that allow them to respond to mechanical and chemical stimuli and to release transmitters that can modulate the activity of afferent nerves.