The Human Brain : From Neurone to Nervous System

VISCERAL SENSATION and REFLEXES


  1. Information concerning events in the viscera reach the brain via two pathways; the vagus (the tenth cranial nerve), and the spinal nerves. The former deals mainly with signals used in physiological regulation, whereas the latter is concerned mainly with sensation. However afferents in the sacral parasympathetic nerves are also involved in sensory and reflex events related to the physiology of pelvic viscera.

  2. Visceral signals that enter consciousness do so by utilising pathways used by somatic afferents entering the same spinal segments. Because of this arrangement, damaging events in the viscera give rise to sensations felt in the somatic domain, i.e. in skin or muscle. This is the basis of referred pain.

  3. Many functions of internal organs are monitored by afferent neurones in the vagus and in spinal nerves.

  4. The activity of some nerves that monitor events inside the body such as the levels of blood gases and pressures within the heart and blood vessels do not impinge on consciousness. Others, arising from the upper gastrointestinal tract, play a part in conscious behaviour, such as feeding and satiety. These types of afferent concerned with physiologcal regulation are found mainly in the vagal and glossopharyngeal nerves.

  5. Another group of afferent neurones travel to the spinal cord within sympathetic and sacral parasympathetic nerves, and mediate pain and sensations relating to normal events in the bladder and other internal organs in the pelvis.

  6. Key Words: Visceral afferents, vagal and spinal pathways; reflex and sensory functions






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