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

SLEEP

• It is a state of reversible unconsciousness and relative immobility of an animal.
• It is due to temporary inhibition of the reticular activation system of the ascending reticular formation, which in turn reduces the normal excitability of the cortex.
• Sleep centres are located in the pons and medulla known as raphe nuclei. The nerve fibres of this area secrete serotonin. These nuclei send fibres to reticular formation, upward to thalamus, neocortex, hypothalamus and many areas of the limbic cortex. Fibres extend downward to spinal cord where they inhibit the incoming pain signals. The raphe nuclei provide inhibitory signals to the ascending reticular activation system (ARAS) and thereby induce sleep.
• Diffused Thalamic Projection System‚ (DTPS), the sleep-inducing centre is present in the thalamic reticular area, projects its ascending fibres to cortex and descending fibres to reticular formation. DTPS regulates cortical activity and is antagonistic to ARAS.
  Based upon the period of rest and activity, the animals may be classified as
  • Monophasic animals:  Have prolonged period of rest during night and are active during daytime (Man, birds).
  • Polyphasic animals:  Show several alternate periods of rest and activity (wild animals)
  • Intermediate type:  Has alternate periods of rest and activity during daytime followed by brief period of rest at night (domestic animals).

Changes during sleep

• Decrease in sensitivity and responsiveness of the individual to environmental stimuli.
• Reduction in metabolic processes.
• Fall in body temperature and BMR.
• Decrease in heart rate, B.P and respiratory rate.
• Decrease in alimentary secretions and motility.
• Pupillary constriction
• Decrease in muscle tone.

Endocrine secretions

• Decrease in corticoids
• Episodic increase in GH, melatonin and testosterone.

Theories of sleep

• Sleep is due to depression of the cortical centre by acid metabolites accumulated during  active periods.
• Increased physical efforts during active hours demand increased skeletal blood flow and relative cerebral ischemia which inhibits brain function.
• Sleep producing substance in the brain is stimulated follwing ischemia.
• Cortical excitation and cortical inhibition rhythmically alternate with each other, conditioned to an environmental cue ( day, light/ darkness) internal signals. This rhythm brings about the sleep wakefulness cycle.
• Hence sleep is defined as rhythmic phenomenon which involves the entire cortical excitability.
• Sleep centre- anterior hypothalamus - trop hotrophic area
• Waking centre-  posterior hypothalamus -  ergotrophic area