VPB 121: Veterinary Physiology - II (2+1)

MICTURITION REFLEX

• Micturition is the complex phenomenon involves initiation from stretch receptors in the bladder wall.
• Axons have their cell bodies located in the sacral spinal ganglia and dorsal grey column of the spinal sacral segments.
• Stretching of the bladder stimulate the sensory fibres which in turn causes the generation of action potential that passes via spinal nerves to the spinal cord.
• They may employ sacral inter neurons prior to terminate in the higher centre or travel straight to end in the higher centre.
• If sacral inter neurons are involved, spinal cord completes the reflex arc by synapsing with pre ganglionic parasympathetic neurons.
• These parasympathetic system causes contraction of smooth muscles of urinary bladder.
• There is involvement of sympathetic system and is evident in the early stages of filling of the bladder by activating beta adrenergic preganglionic sympathetic neurons by the synapsing of interneurons. These fibres inhibit smooth muscle contraction allowing still more stretching to hold the urine.
• Spino thalamic tract from spinal segments at the level of bladder can also serve as a afferent fibers to end in fasciculus gracilis of medulla where they synapse in the nucleus gracilis. These fibers pass via thalamus and reach somesthetic cortex to cause conscious perception of bladder sensation.
• Stimulation of cell bodies in pelvic plexus especially in bladder wall via axons of post ganglionic para sympathetic system causes contraction of detrusor muscle and relaxes the striated sphincter of the bladder.
• This in turn causes evacuation of urine from bladder and allows the flow via urethra.
• Sympathetic activation via alpha receptor system potentiate contraction and smooth muscle tone of the bladder allows urine output.
• Involvement of brain stem via tecto spinal and reticulo spinal tract results in voluntary micturition.