The detrusor muscle detrusor urinae muscle, muscularis propria of the urinary bladder and muscularis propria, is smooth muscle found in the wall of the bladder. The detrusor muscle remains relaxed to allow the bladder to store urine, contracts during urination to release urine. Related are the urethral sphincter muscles which envelop the urethra to control the flow of urine when they contract. In older adults over 60 years in age, the detrusor muscle may cause issues in voiding the bladder, resulting in uncomfortable urinary retention; the fibers of the detrusor muscle arise from the posterior surface of the body of the pubis in both sexes, in the male from the adjacent part of the prostate. These fibers pass, in a more or less longitudinal manner, up the inferior surface of the bladder, over its apex, descend along its fundus to become attached to the prostate in the male, to the front of the vagina in the female. At the sides of the bladder the fibers intersect one another; the 3 layers of muscles are arranged longitudinal-circular-longitudinal from innermost to outermost.
During urination the muscle is contracted via parasympathetic branches from the pelvic splanchnic nerves to empty the bladder. At other times the muscle is kept relaxed via sympathetic branches from the inferior hypogastric plexus to allow the bladder to fill. External sphincter muscle of female urethra External sphincter muscle of male urethra Internal urethral sphincter Histology at KUMC urinary/renal18- Detrusor myectomy for detrusor overactivity: 1-year follow-up
A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, whose axon projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs muscles and glands. There are two types of motor neuron -- lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and directly onto lower motor neurons; the axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, gamma motor neurons. A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch; as a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through summation or a tetanic contraction. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch.
The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. A tetanic contraction is caused by constant high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, tension rises smoothly reaching a plateau. Motor neurons begin to develop early in embryonic development, motor function continues to develop well into childhood. In the neural tube cells are specified to either ventral-dorsal axis; the axons of motor neurons begin to appear in the fourth week of development from the ventral region of the ventral-dorsal axis. This homeodomain is known as the motor neural progenitor domain. Transcription factors here include Pax6, OLIG2, Nkx-6.1, Nkx-6.2, which are regulated by sonic hedgehog. The OLIG2 gene being the most important due to its role in promoting Ngn2 expression, a gene that causes cell cycle exiting as well as promoting further transcription factors associated with motor neuron development.
Further specification of motor neurons occurs when retinoic acid, fibroblast growth factor, TGFb, are integrated into the various Hox transcription factors. There are 13 Hox transcription factors and along with the signals, determine whether a motor neuron will be more rostral or caudal in character. In the spinal column, Hox 4-11 sort motor neurons to one of the five motor columns. Upper motor neurons originate in the motor cortex located in the precentral gyrus; the cells that make up the primary motor cortex are Betz cells. The axons of these cells descend from the cortex to form the corticospinal tract. Corticomotorneurons are neurons in the primary cortex which project directly to motor neurons in the ventral horn of the spinal cord. Axons of corticomotorneurons terminate on the spinal motor neurons of multiple muscles as well as on spinal interneurons, they are unique to primates and it has been suggested that their function is the adaptive control of the distal extremities including the independent control of individual fingers.
Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas. Nerve tracts are bundles of axons as white matter. In the spinal cord these descending tracts carry impulses from different regions; these tracts serve as the place of origin for lower motor neurons. There are seven major descending motor tracts to be found in the spinal cord: Lateral corticospinal tract Rubrospinal tract Lateral reticulospinal tract Vestibulospinal tract Medial reticulospinal tract Tectospinal tract Anterior corticospinal tract Lower motor neurons are those that originate in the spinal cord and directly or indirectly innervate effector targets; the target of these neurons varies, but in the somatic nervous system the target will be some sort of muscle fiber. There are three primary categories lower motor neurons, which can be further divided in sub-categories. According to their targets, motor neurons are classified into three broad categories: Somatic motor neurons Special visceral motor neurons General visceral motor neurons Somatic motor neurons originate in the central nervous system, project their axons to skeletal muscles, which are involved in locomotion.
The three types of these neurons are the alpha efferent neurons, beta efferent neurons, gamma efferent neurons. They are called efferent to indicate the flow of information from the central nervous system to the periphery. Alpha motor neurons innervate extrafusal muscle fibers, which are the main force-generating component of a muscle, their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells. A single motor neuron may synapse with 150 muscle fibers on average; the motor neuron and all of the muscle fibers to which it connects is a motor unit. Motor units are split up into 3 categories: Main Article: Motor Unit Slow motor units stimulate small muscle fibers, which contract slowly and provide small amounts of energy but are resistant to fatigue, so they are used to sustain muscular contraction, such as keeping the body upright, they gain their energy via oxidative hence require oxygen. They are called red fibers. Fast fatiguing motor units stimulate larger muscle groups, which apply large amounts of force but fatigue quickly.
They are used for tasks that require large brief bursts on energy, such as jumping or
External sphincter muscle of male urethra
The external sphincter muscle of urethra sphincter urethrae membranaceae, sphincter urethrae externus, surrounds the whole length of the membranous urethra, is enclosed in the fascia of the urogenital diaphragm. Its external fibers arise from the junction of the inferior pubic ramus and ischium to the extent of 1.25 to 2 cm. and from the neighboring fascia. They arch across the front of the urethra and bulbourethral glands, pass around the urethra, behind it unite with the muscle of the opposite side, by means of a tendinous raphe, its innermost fibers form a continuous circular investment for the membranous urethra. The muscle helps maintain continence of urine along with the internal urethral sphincter, under control of the autonomic nervous system; the external sphincter muscle prevents urine leakage as the muscle is tonically contracted via somatic fibers that originate in Onuf's nucleus and pass through sacral spinal nerves S2-S4 the pudendal nerve to synapse on the muscle. Voiding urine begins with voluntary relaxation of the external urethral sphincter.
This is facilitated by inhibition of the somatic neurons in Onuf's nucleus via signals arising in the pontine micturition center and traveling through the descending reticulospinal tracts. During ejaculation, the muscle contracts to prevent mixing between urine and semen and backward flow of semen into the bladder. During ejaculation, the external sphincter opens and the internal sphincter closes. Levator ani External sphincter muscle of female urethra Internal urethral sphincter Prostatic urethra This article incorporates text in the public domain from page 429 of the 20th edition of Gray's Anatomy Anatomy figure: 41:06-04 at Human Anatomy Online, SUNY Downstate Medical Center - "Muscles of the female urogenital diaphragm and structures located inferior to it."
External sphincter muscle of female urethra
The external sphincter muscle of female urethra is a muscle which controls urination in females. The muscle fibers arise on either side from the margin of the inferior ramus of the pubis, they are directed across the pubic arch in front of the urethra, pass around it to blend with the muscular fibers of the opposite side, between the urethra and vagina. The term "urethrovaginal sphincter" is sometimes used to describe the component adjacent to the vagina. TA: A09.5.03.006F, FMA: 30439 The "compressor urethrae" is considered a distinct, adjacent muscle by some sources, TA: A09.5.03.005F, FMA: 30438 The muscle helps maintain continence of urine along with the internal urethral sphincter, under control of the autonomic nervous system. The external sphincter muscle prevents urine leakage as the muscle is tonically contracted via somatic fibers that originate in Onuf's nucleus and pass through sacral spinal nerves S2-S4 the pudendal nerve to synapse on the muscle. Voiding urine begins with voluntary relaxation of the external urethral sphincter.
This is facilitated by inhibition of the somatic neurons in Onuf's nucleus via signals arising in the pontine micturition center and traveling through the descending reticulospinal tracts. External sphincter muscle of male urethra Internal urethral sphincter Levator ani This article incorporates text in the public domain from page 431 of the 20th edition of Gray's Anatomy