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

REFLEX ACTIVITIES OF AUTONOMIC NERVOUS SYSTEM

Autonomic reflexes

• These reflexes arc, similar to somatic reflex arc have
  • Sensory part
  • Motor part.
• Visceral sensory fibers enter the dorsal horn of spinal cord, branching into spinal sensory column.
• They employ interneurons.
• Interneurons transmit impulses to the preganglionic neurons in the spinal cord or to the brain stem.
• Efferent part represented by preganglionic axon to carry the visceral motor stimuli to ganglionic cell.
• Post ganglionic axons innervate visceral effectors.
• Examples of Autonomic reflexes are
  • Peristaltic reflex and
  • Micturition reflex.

Peristaltic reflex

• Peristalsis is the motor activity of the intestinal musculature to cause onward propulsion of the contents.
  • Sensory information by way of stretching of the wall.
  • This intum stimulates intra mural sensory neurons.
  • Sensory neuron either directly or through interneuron synapse with Post ganglionic parasympathetic neuron.
  • This reflex is unique, that it can be seen in isolated preparatious also and is known as a type of Extra spinal reflex.
  • In normal animals, this reflex has an additional influence from brain stem.
  • Control is mediated through preganglionic vagal, pelvic nerves.

Micturition Reflex

• Act of voiding urine is known as micturition.
• It is a reflex act, under the control of higher centres.
• But the uniqueness is that this reflex persist even after the transection of the spinal cord.
• This reflex involves stretch receptors of the bladder wall, primary axons of cell bodies in the sacral spinal segment and dorsal grey column of sacral segments.
• The urinary bladder is the storage component and allows urine to be accumulated.
• Contractions of the smooth muscles in the walls of renal pelvis and ureter by way of action potentials move down towards urinary bladder. The contractions are initiated by stretch and distension as a result of continued production of urine.
• This peristaltic activity is autonomous and myogenic.
• Junction of uretar and urinary bladder is not provided with any anatomical sphincter.
• There is a functional sphincter developed by the bladder muscle viz Detrusor muscle.

• The functional sphincter act as a valve to prevent reflux of urine into ureter.
• The smooth muscle fibers of Detrusor muscle interlaces and converge at the urethral ori face.
• It allows the bladder as a leak proof compartment while it is filling, that is as the bladder expands , the neck of the bladder at the level of urethra constricts due to the convergence of smooth muscle fibers and vice versa as it relaxes.
• Detrusor muscle is controlled primarily by parasympathetic and sympathetic innervation.
• Activity of Detrusor muscle in expulsion of urine is a reflex mechanism.
• This micturition reflex centre is located within the sacral spinal card and this centre is intum controlled by the brain stem and cerebral cortex.
• Filling of urinary bladder activate stretch receptors.
• Sensory neuron via pelvic nerve which has its cell bodies in the dorsal root ganglia of sacral spinal nerves.
• They employ inter neurons.
• Fibers take two routes from here, one is for local reflex and other passes to fasciculus gracilius to the nucleus gracilius of medulla.
• Some fibers through another set of interneurons synapse with spinothalamic tract.
• Thus, information about the stretch reaches medulla / brainstem and also to cerebral cortex for conscious perception and voluntary emptying of the bladder.
• Motor pathway is mediated via pelvic nerves, whose cell bodies are in intermediate mucleus of the sacral spinal segment.
• When motor activity is initiated to contract the bladder Detrusor muscle, the neck of the bladder relaxes simultaneously with contracting of bladder wall allowing the urine to flow down.
• Pudental nerve serve as motor nerve to external urethral sphincter which is a skeletal muscle.
• As micturition reflex is in progress, the external urethral sphincter relaxes allowing voiding of urine.