Download Neuroscience 14a – Introduction to Consciousness

Neuroscience 14a - Introduction to Consciousness
Anil Chopra
1. Define consciousness
2. Outline briefly the structure of the reticular formation
3. Explain how the reticular activating system modulates the activity of the cerebral
cortex
4. Define the main EEG rhythms and state their functional significance
5. Define the main altered states of consciousness and the 3 observations upon which
the Glasgow coma scale is based
6. Give examples of metabolic and non-metabolic causes of coma
7. Distinguish between brain death and persistent vegetative state
Consciousness is defined as the level or arousal or state of awareness.
There are different level’s of arousal:
- Full awakefullness and responsiveness –normal arousal
- Obtundation – drowsy and not fully responsive.
- Stupor – appears to be asleep, little or no spontaneous activity however
rousable when stimulated.
- Coma – completely unresponsive and unrousable.
Reticular formation
This regulates many vital functions including the sleep/awake cycle. It is a
polysynaptic network located in the pons, midbrain and upper medulla and is poorly
differentiated. It consists of 3 parts:
 Lateral Reticular Formation
 Has small neurones
 Receives information from ascending tracts for touch and pain.
 Receives vestibular information from median vestibular nerve.
 Receives auditory information from superior olivary nucleus.
 Visual information from superior colliculus.
 Olfactory information via medial forebrain bundle.
 Paramedian Reticular Formation
 Has large cells.
 Receives signals from lateral reticular formation.
 Projects onto cerebral hemispheres.
 Nucleus coeruleus contains noradrenergic neurones
and projects onto the cerebral cortex.
 Ventral tegmental nucleus contains
dopaminergic neurones that project directly
onto the cortex.
 Cholinergic neurones project onto the thalamus.
 Raphe nuclei (Median RF)
 In the midline of the reticular formation
 Contain serotonergic projections to the brain and spinal
cord.
Thalamus
The thalamus is contained in the mid-part of the diencephalon and is split up into a
number of different nuclei which perform 3 main tasks:
o Cholinergic projections excite the individual thalamic relay nuclei which lead to
activation of the cerebral cortex.
o Cholinergic projections to the intralaminar nuclei, which in turn project to all
areas of the cortex .
o Cholinergic projections to reticular nuclei regulate flow of information through
other thalamic nuclei to the cortex.
Tuberomammillary nucleus in the hypothalamus projects to the cortex and is involved
in maintaining the awake state.
This collectively is known as the reticular activating system, which is triggered by
sensory input – cholinergic projections to the thalamus which then stimulates the
cerebral cortex.
Nucleus
Group
Type
Anterior
Anterior
Dorsal medial
(DM)
Centromedian
(CM)
Medial
nuclear
Diffuse
projection
Diffuse
projection
Intralaminar
-
Other IL nuclei
Intralaminar
Diffuse
projection
Ventral anterior
(VA)
Lateral
nuclear
Relay
Ventral lateral
(VL)
Lateral
nuclear
Relay
Ventral
posterolateral
(VPL)
Lateral
nuclear
Relay
Ventral
posteromedial
(VPM)
Lateral
nuclear
Relay
Lateral dorsal
Lateral
(LD) and Lateral
nuclear
posterior (LP)
Input
mammillary bodies,
hippocampus
olfactory cortex,
amygdala
globus pallidus
(inhibitory)
pontine and
mesencephalic
reticular formation
Output
Function
cingulate gyrus
memory formation
hypothalamus, cingulate
and orbitofrontal cortex
emotional behavior
modulation of basal
ganglia
diffuse projections to frontal thalamic portion of
cortex and other thalamic ascending reticular
nuclei
activation system
Primary, Pre and
Basal Ganglia (GPi,
motor relay. activation
Supplementary motor
SNr)
facilitates movement
cortex (areas 4 & 6)
Primary, Pre and
Basal Ganglia (GPi,
motor relay. activation
Supplementary motor
SNr) & Cerebellum
facilitates movement
cortex (areas 4 & 6)
cuneate, gracile nuclei,
somatosensory relay for
somatosensory cortex
marginal zone and
body, relays info from ALS
(areas 1,2,3)
substantia gelatinosa
and DCML tracts
somatosensory relay for
spinal and principal
somatosensory cortex
face, relays sensory info
nuclei of V, nucleus
(areas 1,2,3), taste cortex
(from trigeminothalamic
solitarius
(area 43)
tract) and taste info
caudate and putamen
Diffuse
projection
sensory cortex, other
thalamic nuclei
frontal, parietal and
cingulate cortex
sensory and emotional info
integration
visual cortex (area 17),
cuneate and lingual gyri via visual relay
optic radiations
LGN
Lateral
nuclear
Relay
retina (60% feedback
from cortex)
MGN
Lateral
nuclear
Relay
inferior colliculus (via
auditory cortex (area 41),
brachium of the inferior
via auditory radiations
colliculus)
auditory relay
Pulvinar
Lateral
nuclear
Diffuse
projection
reciprocal input from all
output areas, superior parietal and temporal
colliculus, primary
association areas
visual cortex
integration of sensory
information, modulation of
spatial attention (?)
Diffuse
projection
thalamic nuclei
(excitatory input),
collateral projections
from cortical feedback
to thalamus)
Reticular
Reticular
inhibitory output to thalamic
nuclei from which input was
received (only thalamic
nucleus w/o projection to
cortex & w/ inhibitory
output)
regulate flow of info from
thalamus to cortex, part of
ascending reticular
activating system,
modulation of arousal &
sleep, generation of
oscillations (?)
Electroencephalogram – EEG
This is a technique used to record the electrical activity of neurones in the brain.
Electrodes are placed at a number of points on the heads of patients pick up both
action potentials and graded potentials generated in the brain (particularly the
superficial cortex).
The patterns produced by the EEG consists of waves, each with different patterns that
are normally recognisable.
- Amplitude: indicates degree of electrical activity. Synchronous firing also
results in an increased amplitude.
- Frequency: how often they change from maximum to minimum amplitude.
Lower frequencies are indicative of less active/responsive states. There are 4
distinctive frequency ranges:
Alpha: relaxed awake with
eyes open. (8-13Hz)
Beta: awake and
concentrating on
something. (13-30Hz)
Theta: early sleep /
drowsiness. (4-8Hz)
Delta: late stage sleep
(0.5-4Hz)
Altered States of Consciousness
Alertness is measured on the Glasgow Coma Scale.
Eyes open
none
in response to pain
in response to speech
spontaneous
1
2
3
4
Verbal responses
none
incomprehensible sounds
inappropriate words
disoriented speech
oriented speech
1
2
3
4
5
Motor responses
none
extensor response to pain (decerebrate rigidity)
flexor response to pain (decorticate rigidity)
withdrawal to pain
localisation of pain
obeys commands
1
2
3
4
5
6
There are different altered states of consciousness:
o Concussion or contusion - temporary loss of consciousness lasting for a few minutes.
o Confusion – least, sustained disturbance of consciousness – mental processes are
slowed. May be inattentive, disoriented, have difficulty carrying out simple
commands or speaking.
o Stupor – more profound, can only be roused by strong sensory stimuli
o Coma – cannot be roused by even strong sensory stimuli. Different from sleep as metabolic
activity of brain is depressed & there is total amnesia for period in coma.
Causes of Coma
-
Metabolic alteration: hypoglycaemia, hypoxia, intoxication, overdosing on
certain drugs such as sedatives, narcotics.
Lesions in Cerebral Hemispheres: only cause coma if they are large and
bilateral. Leads to a flat EEG.
Lesion in thalamus or brainstem: can be due to a number of reasons e.g. raised
intra-cranial pressure. EEG is a slow wave sleep.
Persistent Vegetative State
Patients who go into an irreversible coma can often enter persistent vegetative stage in
which sleep-wake cycles are present even though the patient is unaware of their
surroundings. Their brainstem is still able to function so reflexes and postural
movements are still present.
Individuals in a persistent vegetative state may smile, cry or react to elements of their
environment but there is no evidence that they can comprehend their behaviours.
Brain Death
Brain death is the point at which the entire brain does not function and there is no
possibility of it functioning again. The body may be kept alive artificially.
This may be caused by disconnection of the cortex from the brainstem or widespread
disease in the cerebral hemispheres. The EEG is not normally diagnostic although will
be flat. Spinal reflexes may be present.