Micturition- How The Human Urinary System Works? | ScienceMonk

Micturition or Urination is the process of periodic excretion of urine from the urinary bladder. In infants and young children with an age range of 2 – 3 years, micturition occurs involuntarily, whereas the process is under voluntary control in adults.

The entire process is regulated by a highly complex neural circuit involving the brain, spinal cord, and peripheral nervous system. In addition, smooth and striated muscles in the urinary bladder, bladder neck, urethra, and urethral sphincter are involved in the process of urination.

Micturition

Sustained or intermittent contraction of detrusor, which is the main contractile muscle of the urinary bladder, together with contractions of other lower urinary tract muscles plays a major role in controlling the urine excretion from the body.

The process of voiding (urine elimination) primarily depends on the amount of urine accumulated in the bladder. Usually, the feeling of the void starts with a urine volume of 100 ml – 150 ml.

The accumulation of 350 ml – 400 ml of urine increases the signal intensity of the pelvic nerves that regulate the sensation of bladder filling, bladder distention, and the ultimate need for urination. In a nutshell, the entire process of micturition involves storage of urine in the bladder and elimination of urine from the bladder.

The storage phase is mainly associated with regulation and maintenance of continence, whereas the voiding phase (elimination) is related to the relaxation of the urethral sphincter and simultaneous contraction of bladder muscles. All these processes are controlled by the central nervous system, including the autonomic and somatic nervous systems.

The Neural Circuit Involved in Micturition

Micturition is a spinobulbospinal reflex controlled by higher brain centres such as the pontine micturition centre (PMC). The autonomic and somatic pathways are involved in the voluntary regulation of the lower urinary tract.

The sympathetic innervation originates in the thoracolumbar outflow of the spinal cord, and the parasympathetic and somatic innervations originate in the sacral segments of the spinal cord. Both efferent and afferent signals propagate through these nerves.

Activation of the parasympathetic efferent pathway to the bladder and urethra along with the inhibition of somatic pathway to the urethral sphincter causes simultaneous contraction of the bladder and relaxation of the urethral sphincter.

These responses are entirely regulated by the PMC, which ultimately facilitate the process of micturition. Studies have shown that any interruption between PMC and lumbosacral spinal cord causes disruption of bladder function and retention of urine.

Neural Regulation of Micturition

Distention of the urinary bladder due to urine accumulation stimulates low-intensity sympathetic signals in the hypogastric nerves to the bladder base and urethra, in addition to stimulating pudendal signals to the urethral sphincter.

These responses are collectively known as guarding (storage) reflexes that ultimately control urinary continence (capacity of preventing accidental leakage of the urine from the bladder).

Contraction of the detrusor muscle and neurotransmission in the bladder ganglia is also modulated by these sympathetic signals. During voiding, spinobulbospinal reflex pathways, which pass through the PMC, are stimulated by the intense afferent firing in the pelvic nerves.

This subsequently induces the parasympathetic signalling to the bladder and urethral smooth muscle and inhibits the sympathetic and pudendal signaling to the urethral outlet.

Cholinergic neurotransmitters released by the parasympathetic postganglionic nerves act on the M3 muscarinic receptors and induce the contraction of the detrusor muscle. This is the main excitatory mechanism that initiates micturition.

As a feedback mechanism, activation of muscarinic receptors present in the parasympathetic nerve terminals and ganglia results in elevation or reduction in neurotransmitter release, depending on the intensity of neural signals.

Non-cholinergic neurotransmitters released by the parasympathetic nerves mediate the excitatory function via ATP that acts on P2X purinergic receptors present in the detrusor muscle. Nitric oxide released by the parasympathetic nerves mediates the inhibition of urethral smooth muscle.

Urination Disorders

Micturition disorders mainly include urine retention and/or incontinence. These disorders can be either neurogenic or non-neurogenic.

Neurogenic Disorders

They primarily occur due to two kinds of pathologies:

  • Dysfunction of the upper motor neuron due to suprasacral spinal cord lesion; and
  • Dysfunction of the lower motor neuron due to sacral spinal cord lesion, peripheral neuropathy, myopathy, neuromuscular junctionopathy, or dysautonomia.
  • These impairments are associated with malfunctioning of the bladder and urethra.

Non-Neurogenic Disorders

  • In contrast, non-neurogenic disorders are mainly associated with structural or functional abnormalities in the lower urinary tract.
  • Urine retention related disorders are generally caused by abnormal contractions of the detrusor muscle or blockages in the urethra.
  • Disorders related to urinary incontinence are caused by structural or functional abnormalities in the urethra or impaired urine storing capacity of the bladder. In addition, mechanical stress can induce incontinence.
  • Stress incontinence mainly occurs due to a sudden increase in abdominal pressure during sneezing or coughing. Such condition frequently occurs due to hormonal imbalance, particularly after prostatectomy in men and after childbirth and during menopause in women.
  • Medical conditions that are related to abnormal micturition include diabetes mellitus, diabetes insipidus, enlarged prostate, inflammation in the prostate, urinary tract infections, etc. In addition, overactive bladder due to involuntary contraction of bladder muscle often leads to abnormal urination. Such involuntary contraction may occur even when the bladder is not sufficiently full of urine.

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