Lecture 38 4/12/24 Flashcards
What are the two main steps of micturition?
-bladder progressively fills until the tension in its walls rises above a threshold level
-nervous reflex known as the micturition reflex causes a conscious desire to urinate
What type of nervous reflex is the micturition reflex?
autonomic spinal cord reflex
What are the two main parts of the urinary bladder?
-the body, where the urine collects
-the neck, which is funnel shaped and connects with the urethra
What is the posterior urethra?
the lower part of the bladder neck
What is the detrusor muscle?
-smooth muscle component of the bladder
-extends in all directions
-increases bladder pressure when contracted
How is the detrusor muscle able to contract the bladder all at once?
the smooth muscle cells are fused together to allow for low-resistance electrical pathways; action potentials easily spread from cell to cell
How is the trigone identified?
it has smooth mucosa, in contrast to the folded mucosa elsewhere in the bladder
What are the characteristics of the ureters entering the bladder?
-course obliquely through the detrusor muscle
-pass 1 to 2 centimeters beneath the bladder mucosa before emptying into bladder
What is the internal sphincter?
-involuntary smooth muscle composed of detrusor muscle interlaced with elastic tissue
-prevents bladder emptying until threshold is reached
What is the external sphincter?
-in urogenital diaphragm
-voluntary skeletal muscle
-consciously prevents urination
Which nerves provide the main parasympathetic supply to the bladder?
pelvic nerves
Which spinal cord segments do the pelvic nerves connect to?
segments S2 and S3 via sacral plexus
What is the function of the sensory fibers in the pelvic nerves?
detect the degree of stretch in the bladder wall
What is the function of the motor fibers in the pelvic nerves?
innervate the detrusor muscle
What is the function of the skeletal motor fibers of the pudendal nerve?
innervate and control the voluntary skeletal muscle of the external sphincter
Which nerves provide the main sympathetic supply to the bladder?
hypogastric nerves
Which segment of the spinal cord do the hypogastric nerves connect to?
L2
What are the functions of the hypogastric nerve fibers?
-stimulate the blood vessels
-provide sensations of fullness and possibly pain
What happens when a ureter becomes blocked?
-reflex constriction occurs, which is associated with severe pain
-pain leads to constriction of the renal arterioles, which reduces urine output from kidney (ureterorenal reflex)
What are the characteristics of the ureter walls?
-contain smooth muscle
-innervated by sympathetic and parasympathetic nerves
-intramural plexus of neurons and nerve fibers extends along entire length
What are the steps of urine transport to the bladder?
-urine flows from collecting ducts into the renal pelvis; no more compositional changes
-calyces stretch and stimulate peristaltic contractions; urine flows towards bladder
-detrusor muscle tone holds ureters shut during bladder contraction to prevent backflow
What are micturition waves?
periodic acute increases in pressure that last from a few seconds to more than a minute and are caused by the micturition reflex
What is the threshold amount of urine in the bladder before the pressure begins to rise rapidly?
300 to 400 mL
What does it mean for the micturition reflex to be self-regenerative?
initial contraction of the bladder activates stretch receptors to cause a greater increase in sensory impulses from the bladder and posterior urethra, which causes further increase in reflex contraction of the bladder
What occurs in a complete cycle of the micturition reflex?
-progressive and rapid increase in pressure
-period of sustained pressure
-return of pressure to basal tone
What happens if the micturition reflex becomes too powerful?
it can cause a reflex which inhibits the pudendal nerve control over the external sphincter, causing involuntary urination
Which centers in the brain are involved in the inhibition or facilitation of the micturition reflex?
-pons
-cerebral cortex
What happens when the sensory nerve fibers from the bladder to the spinal cord are destroyed?
-micturition reflex contraction cannot occur
-stretch signals from bladder are not transmitted
-individual loses bladder control; bladder fills to capacity and overflows a few drops at a time
What happens when the spinal cord is damaged above the sacral region?
-micturition reflexes can still occur, but are no longer controlled by the brain
-periodic but unannounced bladder empyting occurs
What is uninhibited neurogenic bladder?
-partial damage in spinal cord or brain stem interrupts most of the inhibitory signals
-results in frequent, relatively uncontrolled micturition
