NB12-3 - Consciousness and DLA

Question 1

What is consciousness?

Answer 1

The ability to be aware of oneself and one’s place in the environment and respond appropriately to environmental stimuli.

Question 2

List the different aspects of consciousness and the regions of the brain responsible for them.

Answer 2

1. Arousal - simply being able to respond to stimuli; controlled by the reticular activating system
2. Conscious Awareness - being aware of general background stimuli; controlled by higher-order thalamic nuclei and the synchronous firing of broad heteromodal cortical networks
3. Content of conscious - the focus of your perception; controlled by specific thalamic relay nuclei and content specific sensory regions.

Question 3

Describe the concept of bottom-up and top-down activation, as it relates to consciousness. What neural circuits are responsible for these activations?

Answer 3

Bottom-Up - refers to information about external stimuli being received by the brain. Mostly just pain/touch pathways.

Top-Down - refers to the brains processing of the external stimuli information and applying context to it based on past experiences. Facilitated by long range re-entrant/recursive association fibers connecting prefrontal and parietal regions to sensory regions

Both of these are required for consciousness to exist

Question 4

Discuss the thalamocortical loops relevant to consciousness.

Answer 4

Conscious perception is also dependant upon re-entrant/recursive two bidirectional thalamocortical loops:

• One between the reticular nuclei and the higher levels of the cortex
• One between the LGN and the lower levels of the cortex

Thalamocortical oscillatory activity in the 30-70 Hz range is associated with conscious perception (absent during sleep)

Question 5

List and describe the levels of unconsciousness.

Answer 5

• Lethargic - patient is still able to be fully aroused
• Obtunded - patient cannot by fully aroused
• Stuporous - sleep like status
• Comatose - an extended loss of consciousness in which the patient is incapable of being aroused by external stimuli or inner need.

Question 6

Answer 6

B

Question 7

List the major systems that the reticular activating system (RAS) receives input from and what for?

Answer 7

1. Association cortices - internal plan to arouse yourself
2. Limb System - emotional arousal
3. Sensory Pathways - arousal in response to external stimuli
4. Thalamic reticular nucleus - inhibitory GABAergic pathways
5. Brain stem and hypothalamic circuits - controlling circadian rhythms

Question 8

On which thalamic nuclei to the cholinergic pathways from the RAS synapse? What is the effect of this?

Answer 8

The intralaminar nuclei

This abolishes slow wave cortical activity and promotes an alert state.

Question 9

Answer 9

E

Question 10

What are the common causes of a coma?

Answer 10

• There are two main pathways of the arousal system: RAS to thalamus, and Thalamus to cortex. Lesion of either could cause coma. Common examples include
  
  • Small lesions in mesencephalon (paramedian reticular formation)
  • Lesions of posterior lateral hypothalamus
  • Lesions of thalamus
  • Bilateral impairment to both hemispheres

Question 11

What is and what typically causes a vegitative state? What is another name for vegitative state?

Answer 11

A vegitative state typically develops after a coma and is usually caused by atrophy of the cortex. A person in a vegitative state has no awareness of their environment but the following functions typically remain intact:

• Breathing and circulation
• Autonomic functions
• Non-cognitive functions (eyes may track movement, blinking, teeth grinding)
• Normal sleep patterns
• Spontaneous emotional expression

Also known as an awake coma because some functions of being awake appear to return when a coma progresses to a vegetative state

Question 12

What causes locked-in syndrome? What voluntary functions is a locked-in patient sometimes still able to do and why?

Answer 12

Locked in syndrome is typically caused by a lesion to the anterior aspect of the pons (usually blockage of basilar artery) so that the corticospinal and corticobulbar fibers can’t function but the sensory pathways still function just fine. Sometimes these patients can still move their eyes vertically and open their eyelids because these functions are controlled at the level of the midbrain. It may appear that they can close their eyelids but this is usually just gravity closing them

Question 13

What is tetraplegia?

Answer 13

Paralysis of all voluntary muscles with exception of vertical eye movement. Classic symptom of locked in syndrome.

Question 14

How can you tell the difference between a vegetative state and locked-in syndrome?

Answer 14

Tetraplegia is characteristic of locked-in syndrome.

A person with locked-in syndrome will have a normal EEG and cortical metabolic activity

Question 15

Answer 15

A

Question 16

What are the criteria for diagnosing brain death? What are the typical causes of brain death

Answer 16

• Irreversible loss of all brain function characterized by:
  
  • Not responsive to any stimuli
  • No spontaneous respiration
  • Dilated, non-reactive, pupils
  • No vestibulo-ocular reflex
  • No corneal reflex
  • Isoelectric EEG
• Typically caused by anoxia, ischemia, intracranial hemorrhage, trauma, brain tumors, increase ICP, and uncal herniation

Question 17

Answer 17

2

Question 18

Describe the types of innervations that dilate/constrict the pupils and the routes those nerves take.

Answer 18

Dilation (mydriasis) - SNS cell bodies originate in the intermediolateral cell column of the upper thoracic spinal cord. The neurons will exit via the ventral root and enter and synpase within the superior cervical ganglion. The postganglionic fiber will follow the internal carotid plexus up to the dilator pupillae muscles.

Constriction (miosis) - PSNS cell bodies originate in the Edinger-Westphal nucleus in the midbrain and the axons follow CN-III to the ciliary ganglion where they will synapse. The postganglionics will then travel to the constrictor pupillae muscles.

Question 19

Describe the pupillary light responses seen with the various types of brain lesions we need to know.

Answer 19

Diencephalic lesion - super small but reactive pupils

CNIII (uncal) lesion - unequal pupils with one fixed dilated

Midbrain lesion - midposition not reactive

Pretectal (where PSNS originates)(pineal tumor) - large not reactive

Pontine lesion - pinpoint with slight reactivity

Diffuse effects of drugs, metabolic encephalopathy, etc typically causes small pupils

Question 20

Describe how the oculomotor responses would be for a patient with a right medial pontine lesion.

Answer 20

• Dolls head maneuver to the right - eyes move left - normal because left PPRF is intact
• Dolls head maneuver to the left - eyes stay in midposition because right PPRF is damaged
• Cool water in right ear - eyes don’t move
• Cool water in left ear - eyes turn left
• Cool water in both ears - eyes turn left

Question 21

Describe the oculomotor responses seen with a midbrain lesion.

Answer 21

• Doll head maneuver to the left - left eye doesn’t move but right eye moves right (PPRF is fine by CNIII is damaged)
• Doll head maneuver to the right - right eye doesn’t move but left eye moves left (PPRF is fine by CNIII is damaged)
• Cool water in right ear - left eye doesn’t move but right eye moves right
• Cool water in left ear - right eye stays in the middle but left eye moves left