When the brain is asleep, cortical neurones show synchronous rhythmic activity. Arousal occurs when this synchronous activity is disrupted, as a result of non-specific sensory activity - pain, bright lights, loud sounds, imposed movements, etc. The ascending reticular activating system (ARAS) of the brainstem initiates arousal.
The activity of cortical neurones can be monitored in humans using the electroencephalogram (EEG). During sleep, cortical neurones show synchronised activity, but arousal is associated with desynchronisation, i.e., independent activity of cortical columns. Synchronised activity during sleep is generated by reciprocal connections between the thalamus and cortex, and desynchronisation occurs as a result of strong non-specific sensory inputs. The latter is integrated by the ascending reticular activating system, and distributed to the hypothalamus, thalamus and cortex by the projections of cholinergic neurones of the basal forebrain (the Nucleus Basalis and the nucleus of Meynert), the noradrenergic locus coeruleus, and the serotoninergic raphe nuclei.
The Sleep-Waking Cycle is controlled by the hypothalamus, with two centres alternating in activity; the anterior hypothalamus actively promotes sleeping, whereas the lateral hypothalamus promotes arousal. The'Flip-Flop' hypothesis is outlined below.
Image source: napwell.com
The diagram shows the changes in the EEG during different stages of sleep. The EEG is a measure of the voltage difference between different points on the scalp, and shows rhythmic activity due to the activity of the underlying cortical neurones. In deep sleep the rhythm is slow (delta rhythm: less than 4Hz) and the voltage changes are large due to synchronous activity in cortical neurones. In the awake individual, low voltage waves of high frequency (beta rhythm; greater than 13 Hz) are observed, and the cortical activity is said to be desynchonised - i.e. each cortical column is working independently. The brain of a relaxed awake person has an alpha rhythm (8-13 Hz).
When dreaming, the cortex becomes active (with low voltage high frequency activity) accompanied by rapid eye movements - and known as REM sleep.
EEG and Sleep: Brain Waves
A single channel in the EEG measures the voltage between two points on the scalp, and these potentials can be described in terms of their frequency (number of cycles per second) and amplitude (microvolts). The normal waves are:
Alpha waves: 8-13 Hz
Beta waves: >13 Hz
Delta waves: <4 Hz
Alpha waves are present in awake relaxed people; thinking and attention cause an increase in frequency (beta waves).
Sleep is associated with a slowing of the rhythm and in deep sleep the frequency drops below 4Hz.
Arousal causes desynchronisation of the EEG: the frequencey of the brain waves becomes faster and the amplitude of the waves is decreased.
The arousal system is activated by strong inputs in any sensory system and causes waking - loud noises, bright lights, pain, imposed movement etc.
The arousal system consists of
Cholinergic neurones at the pontine/midbrain border (including the dorsolateral tegmental nucleus and the pedunculopontine nucleus). Cholinergic activity is highest when awake and remains high during REM sleep, and is reduced or absent in non-REM sleep.
Noradrenergic neurones of the locus coeruleus in the upper pons. Brainstem monoaminergic activity is highest while awake, reduced during non-NREM sleep, and absent in REM sleep.
Serotoninergic neurones of the raphe nuclei in the midline of the medulla and pons.
Histaminergic neurons of the tuberomamillary nucleus. These become active as soon as a person is awake but are completely silent during REM sleep.
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Classical experiments established that a large area of the brainstem is concerned with arousal and desynchronisation of the EEG.
The Ascending Reticular Activating System (ARAS)
Early studies of the mechanisms of sleep in experimental animals identified areas of the brainstem that caused desynchronisation of the EEG
We now know that these can be refined into regions of the brainstem containing neurones that release acetylcholine, or noradrenaline or serotonin. The pathways of these specific neurotransmitter systems are shown in images  and .
Note that the terminals of these axons are found throughout the cortex.
Typically these terminals exist on the superifical apical dendrites of pyramidal cells.
When ARAS becomes active these terminals produce mild depolarisations in the pyramidal cells, which disrupt any synchronised rhythms.
The relation between ARAS, the Reticular Nucleus of the Thalamus, and Cortico-thalamic reverberating circuits is covered in the tutorial on Sleep and Waking in Chapter 3