8.2 The brainstem, arousal, sleep and consciousness Flashcards
what is arousal?
the emotional state associated with some kind of goal or avoidance of something noxious
what is consciousness?
difficult to define but something to do with ‘awareness’ of both external world and internal states
what is exteroception?
the perception of stimuli originating outside or at a distance from the body.
what is interoception?
sensitivity to stimuli originating inside of the body
what are the 2 main neural components of consciousness?
cerebral cortex
reticular formation
how does the cerebral cortex influence consciousness?
As it is the site where thoughts arise
how does the reticular formation influence consciousness?
the reticular activating system in the brainstem is the circuitry that keeps the cortex ‘awake’. It receives many inputs from the cortex and the sensory system
how are the cortex and the reticular system connected?
connected by reciprocal excitatory projections, forming a positive feedback loop
when do we see positive feedback loops?
Positive feedback loops are seen when there is a binary outcome (e.g. sleep/awake, ovulating/not ovulating etc)
outputs from the reticular system to the cortex occur via three major relay nuclei. what are they?
Reticular formation sends cholinergic (excitatory) projections to these relays
- Basal forebrain nuclei send excitatory cholinergic fibres to cortex (think sedative side effects of anticholinergics)
- The hypothalamus sends excitatory histaminergic fibres to the cortex (think sedative side-effects of sedating antihistamines)
- The thalamus sends excitatory glutamatergic fibres to the cortex
where are the basal forebrain nuclei?
The basal forebrain structures are located in the forebrain to the front of and below the striatum
the reticular formation sends many projections to the cortex. what other structure does the reticular system send projections to?
The reticular formation also sends projections down the cord, responsible for
maintaining muscle tone
what tool do we use to clinically assess consciousness?
GCS - Glasgow coma scale
what are the 3 components of the GCS?
eye opening
motor response
verbal response
how is eye opening scored in the GCS?
Spontaneous eye opening (4) suggests normal cortical and brainstem function Response to speech (3) suggests slightly diminished cortical function but still functioning brainstem Response to pain (2) suggests impaired cortical function but brainstem preserved so that reflex opening can occur No response (1) suggests severe damage to brainstem +/- cortex
how is motor response scored in the GCS?
o Obeys commands (6) suggests normal function with
working connections from auditory system to brainstem /cord
o Localises to stimuli (5) suggests diminished higher
cortical function but still connections working from sensory to motor cortex
o Withdraws to pain (4) suggests that there is still a ‘physiological’ reflex response to stimuli
o Flexor response to pain (3) suggests a lesion above the
level of the red nuclei. This response is still ‘semi- physiological’
o Extensor response to pain (2) suggests a lesion below the red nuclei. This response is not physiological at all
o No response to pain (1) suggests severe damage to
brainstem +/- cortex
how is verbal response scored in the GCS?
Oriented in time/place (5) suggests normal cortical function Confused conversation (4) suggests diminished higher cortical function but language centres are still functioning adequately Inappropriate words (3) suggest language centres have been damaged Incomprehensible sounds (2) suggests cortical damage with brainstem mediated groans No response (1) suggests severe damage to brainstem +/- cortex/.
what is the function of the electroencephalogram?
Measures the combined activity of thousands of neurones in a particular region of cortex
Good for detecting neuronal synchrony and evidence of normal cerebral function
what is neuronal synchrony?
Deprived of sensory input, neurones in the brain tend to fire synchronously. Occurs naturally in sleep as we progress through the stages. Shown on EEG by decreasing frequency and increasing amplitude of the waves.
an electroencephalogram has high temporal resolution and low spatial awareness. what does this mean?
high temporal resolution - can acquire and revisit data quickly
low spatial awareness - cannot be used to precisely localise lesions
what are the functions of sleep?
Generally unknown
Energy conservation / repair?
Memory consolidation?Clearance of extracellular debris?
‘Resetting’ of the CNS?
what are the major stages of sleep? how are they characterised?
Awake with eyes open - beta waves, irregular 50Hz
Awake with eyes closed - alpha waves - regular 10Hz
Stage 1 sleep - background of alpha and interspersed theta waves ( theta at around 5Hz, regular)
Stage 2/3 sleep - background of theta and interspersed sleep spindles and k-complexes. Sleep spindles are high frequency bursts arising from the thalamus. K-complexes represent the emergence of the ‘intrinsic rate’ of the cortex
Stage 4 sleep - Delta waves – regular, 1Hz. Related to k-complexes seen in stages 2/3 REM sleep - EEG similar to beta waves. Dreaming occurs in this stage, so similar to the EEG in a conscious patient
how many cycles of sleep do we typically pass through a night?
Typically pass through around 6 cycles of sleep per night
what is the neural mechanism of non-REM sleep/
complex mechanism involving the deactivation of the reticular activating system (and hence cortex) + inhibition of the thalamus. This deactivation is facilitated by removal of sensory inputs (fewer positive influences on positive feedback loop)
