Explore the complex mechanisms governing sleep and wakefulness. Learn how the brain's thalamus and cortex regulate our states of consciousness, from light sleep to REM sleep. Discover the role of neurotransmitters like histamine, acetylcholine, serotonin, and orexin in orchestrating the sleep-wake ...
The sleep-waking cycle
The cerebral cortex receives two types of input: information and regulatory inputs.
- All information reaches the cortex from the thalamus, that collects, elaborates and relays
information from the ascending sensory paths, the cerebellum and the basal nuclei. The thalamus
can regulate the relevance of the incoming information (from periphery) with respect to internal
cortical activity (attention level and wake/sleep state)
- A number of other ascending paths reach the cortex bypassing the thalamus; these mostly originate
from the brainstem; they do not carry information; they rather have a regulatory function and
may change the intensity and mode of information processing by the cortex. They are referred
to as the activating reticular ascending system (ARAS).
® Lesion of the ARAS or suffering or compression of the brainstem produce coma.
® A change in the function of the thalamus produces instead an interference in sensory data
flow (and possibly cerebellar and basal nuclei feedback) to the cortex, so that the cortex may be
driven to elaborate information that it already contains (if we shut off the information arriving
from the periphery); this tends to occur when the mind wanders in a fully relaxed person, or
during sleep.
The thalamus functioning mode defines the source of the data and the way the cortex elaborates
them and produces a state of wake or sleep.
The interference with ascending information is based on the two possible modes thalamic relay neurons
can work (obviously, intermediate and locally diverse situations are possible):
• Relay neurons in the thalamus express T-type calcium channels, that produce a relative
prolonged depolarizing current (several tens of ms) before inactivating; this current can elicit a
rapid series of action potentials in response to any activation of the cell (bursting mode).
• If the thalamic neuron is maintained in a state of slight depolarization, the T-type calcium
channels remain in the inactivated state, and the neuron responds in the normal way, integrating
the inputs and firing single spikes when they produce an over-threshold depolarization
(transmission mode).
198 Body At Work II
, Enrico Tiepolo
Thalamic neurons typically activate, simultaneously, principal pyramidal cells
in the cortex and parvalbumin positive (PV) interneurons that project on the
soma of the same pyramidal cell; the pyramidal cell EPSP7 is thus quickly
followed (about 1 ms later) by an IPSP that prevents other stimuli that do not
arrive in perfect synchrony to add up to produce a spike.
If the thalamic neuron fires repetitively (giving bursts of activity
instead of single spikes), the PV neuron desensitizes (responds less and less)
and the effectiveness of its synapse on the principal neuron declines as well. So,
no inhibition follows the activation of the pyramidal cell and the time window
for input summation on the latter widens. Elaboration becomes less
precise and less discriminative.
Also, neighboring neurons tend to become activated synchronously and repetitively, giving rise to large
and slow waves in EEG recordings, when the thalamus is in bursting mode.
Conversely, when the thalamus is in transmission mode, the activity of the neurons is
desynchronized (each neuron is performing a different type of elaboration), and the cortex elaborates in
a discriminative mode, giving rise to an EEG characterized by small, disordered, high frequency waves.
During wake, activity in the 8-13 Hz band (alpha) tends to predominate in most areas, while higher
frequencies (15-30 Hz, beta) tend to prevail during elaboration of sensory inputs and other cerebral
activities. When the thalamus shifts to bursting mode, slower activity (4-8 Hz, theta, or 2-4 Hz, delta)
appears and prevails, characterizing the phases of slow-wave sleep.
Sleep is characterized by different stages that can be more or less deep and are associated with different
waves recorded through EEG. There is stage 1, which is of light sleep (easy to awake) and phases 2, 3, 4
of increasingly deep sleep, characterized by increasedly slow (decreased frequency) and large (increased
amplitude) EEG waves (SWS). Phase 2 is sometimes followed by a phase in which the EEG becomes
more similar to the awake state, and rapid eye movements appear (REM phase of sleep).
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