Neuroscience 14a Introduction to Consciousness



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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:

  • Cholinergic projections excite the individual thalamic relay nuclei which lead to activation of the cerebral cortex.

  • Cholinergic projections to the intralaminar nuclei, which in turn project to all areas of the cortex .

  • 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






Input






Output






Function






Anterior  

Anterior  

Diffuse projection  

mammillary bodies, hippocampus  

cingulate gyrus  

memory formation  

Dorsal medial (DM)  

Medial nuclear  

Diffuse projection  

olfactory cortex, amygdala  

hypothalamus, cingulate and orbitofrontal cortex  

emotional behavior  

Centromedian (CM)  

Intralaminar  

-  

globus pallidus (inhibitory)  

caudate and putamen  

modulation of basal ganglia  

Other IL nuclei  

Intralaminar  

Diffuse projection  

pontine and mesencephalic reticular formation  

diffuse projections to frontal cortex and other thalamic nuclei  

thalamic portion of ascending reticular activation system  

Ventral anterior (VA)  

Lateral nuclear  

Relay  

Basal Ganglia (GPi, SNr)  

Primary, Pre and Supplementary motor cortex (areas 4 & 6)  

motor relay. activation facilitates movement  

Ventral lateral (VL)  

Lateral nuclear  

Relay  

Basal Ganglia (GPi, SNr) & Cerebellum  

Primary, Pre and Supplementary motor cortex (areas 4 & 6)  

motor relay. activation facilitates movement  

Ventral posterolateral (VPL)  

Lateral nuclear  

Relay  

cuneate, gracile nuclei, marginal zone and substantia gelatinosa  

somatosensory cortex (areas 1,2,3)  

somatosensory relay for body, relays info from ALS and DCML tracts  

Ventral posteromedial (VPM)  

Lateral nuclear  

Relay  

spinal and principal nuclei of V, nucleus solitarius  

somatosensory cortex (areas 1,2,3), taste cortex (area 43)  

somatosensory relay for face, relays sensory info (from trigeminothalamic tract) and taste info  

Lateral dorsal (LD) and Lateral posterior (LP)  

Lateral nuclear  

Diffuse projection  

sensory cortex, other thalamic nuclei  

frontal, parietal and cingulate cortex  

sensory and emotional info integration  

LGN  

Lateral nuclear  

Relay  

retina (60% feedback from cortex)  

visual cortex (area 17), cuneate and lingual gyri via optic radiations  

visual relay  

MGN  

Lateral nuclear  

Relay  

inferior colliculus (via brachium of the inferior colliculus)  

auditory cortex (area 41), via auditory radiations  

auditory relay  

Pulvinar  

Lateral nuclear  

Diffuse projection  

reciprocal input from all output areas, superior colliculus, primary visual cortex  

parietal and temporal association areas  

integration of sensory information, modulation of spatial attention (?)  

Reticular  

Reticular  

Diffuse projection  

thalamic nuclei (excitatory input), collateral projections from cortical feedback to thalamus)  

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 1


in response to pain 2

in response to speech 3

spontaneous 4
Verbal responses

none 1


incomprehensible sounds 2

inappropriate words 3

disoriented speech 4

oriented speech 5


Motor responses

none 1


extensor response to pain (decerebrate rigidity) 2

flexor response to pain (decorticate rigidity) 3

withdrawal to pain 4

localisation of pain 5



obeys commands 6
There are different altered states of consciousness:


  • Concussion or contusion - temporary loss of consciousness lasting for a few minutes.

  • Confusion – least, sustained disturbance of consciousness – mental processes are slowed. May be inattentive, disoriented, have difficulty carrying out simple commands or speaking.

  • Stupor – more profound, can only be roused by strong sensory stimuli

  • 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.
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