# 29 - Ascending Reticular Activating System Neurotr

# Ascending Reticular Activating System - Neurotransmitters

© SPMM Course 
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Often they occur after sleep disturbances (e.g., brief sounds) and, like K complexes, may occur 
during brief semiarousals. 
 
D. Regulation 
Hypothalamic controls 
 The master clock of the brain is the 
suprachiasmatic nucleus (SCN) located in the 
anterior hypothalamus - this orchestrates 
circadian rhythms and is synchronized by signals 
from the retina. 
 SCN is reset each day by signals of light from the 
retina. Specialized melanopsin-containing retinal 
ganglion cells project via retinohypothalamic 
tract to the SCN. This provides light input 
independent of vision. 
 In the absence of solar guidance, the 24-hour 
sleep-wake cycle will gradually increase to 
approximately 26 hours –this is called freerunning. 
 Pineal melatonin secreted during darkness can 
also reset the SCN. Thus, melatonin promotes 
sleep in those with delayed sleep onset or jet lag. 
 The ventrolateral preoptic nucleus (VLPO) is called the sleep switch nucleus. It has projections to the 
main components of the ascending arousal system. The VLPO induces sleep by putting the brakes on 
the arousal nuclei. People with damage to their VLPO have chronic insomnia. 
 The VLPO must be inhibited so that people can wake up. This is brought about by a negative feedback 
from the monoaminergic system. The switching to arousal is then stabilised by orexin (also called 
hypocretin) neurons in the hypothalamus. Orexin neurons are mainly active during wakefulness and 
reinforce the arousal system. Patients with narcolepsy have reduced number of orexin neurons, 
leading to repeated somnolence during the day. 
Ascending Reticular Activating System - Neurotransmitters 
Neurotransmitter 
Cell Bodies 
Function 
Cholinergic 
Midbrain-pons nuclei 
REM on neurons. Activation brings on REM sleep 
Noradrenergic 
Locus coeruleus 
REM off neurons. Activation reduces REM sleep. 
Dopaminergic 
Periaqueductal gray matter 
D2 possibly enhances REM sleep 
Serotoninergic 
Raphe nuclei 
5HT2 stimulation possibly maintains arousal 
Histaminergic 
Tuberomammillary nucleus 
H1 stimulation possibly maintains arousal 
 
SLEEP & AGEING 
 
Newborns sleep about 16 hours a day. They spend >50% 
of sleep time in REM sleep. Sleep onset REM is also seen 
in neonates. 
 
By 3-4 months of age, the pattern shifts so that the total 
percentage of REM sleep drops to less than 40, and entry 
into sleep occurs with an initial period of NREM sleep. 
By late teens adult pattern of sleep is established. 
 
This distribution remains relatively constant until old 
age. Absolute reduction occurs in both slow-wave sleep 
and REM sleep in older persons. An increase in 
frequency of awakenings after sleep onset also occurs 
with age.