10 - Reticular Formation and Consciousness

Question 1

What is the definition of consciousness and arousal?

Answer 1

Question 2

What does consciousness require?

Answer 2

Cortex and Reticular Formation connected by reciprocal excitatory projections, forming a positive feedback loop

Consciousness = cortex + reticular formation

Question 3

What is the reticular formation and how does it maintain consciousness?

Answer 3

• Population of specialised interneurones in the brainstem.
• Reticular activating system devoted to arousal

- Inputs: sensory system (e.g hot room) and cortex (e.g own thoughts)

- Outputs: thalamus (glutamate), hypothalamus (histamine), basal forebrain nuclei (Ach), spinal cord (regulates muscle tone)

Question 4

Why do antihistamines and anticholinergics have drowsiness as a side effect?

Answer 4

They block the excitatory neurotransmitters being released from the hypothalamus and the basal forebrain nuclei so less stimulation of consciousness/awakefulness

Question 5

What is the basis of the Glasgow Coma Scale that is used to assess the level of consciousness?

Answer 5

• Best is 15, worst is 3
• Eye opening spontaneously would means normal cortical and brainstem function
• Eye opening in response to speech would mean lower level of cortical function but brainstem still good
• Eye opening to pain would mean cortex damaged as pain is a reflex in brain stem
• No response suggest severe damage to brainstem

Question 6

What is an electroencephalogram used for?

Answer 6

• Measures the combined activity of thousands of neurones in a given part of the cortex to a high temporal resolution but low spatial resolution
• MRI gives better spatial resolution but low temporal resolution

Question 7

What is neural synchrony?

Answer 7

• Synchronous oscillations of membrane potentials in a network of neurons connected with electrical synapses (gap junctions).
• Considered to be the neural correlate of consciousness
• Happens in sleep (deprivation of sensory input) and epilepsy

Question 8

What are the different stages of sleep?

Answer 8

• Usually cycle through sleep 6 times in a night
• Dream in REM
• As you go down the stages the EEG shows decreased frequency and increased amplitude as the neurones become synchronous

Question 9

What do each of the scorings for the motor and verbal response of the GCS mean?

Answer 9

Question 10

What would you see on the EEG in each stage of sleep?

Answer 10

Question 11

What is the mechanism of non-REM sleep?

Answer 11

- Deactivation of reticular activating system (and hence cortex) + inhibition of the thalamus

• This deactivation is facilitated by removal of sensory inputs e.g tv off (fewer positive influences on positive feedback loop)

Question 12

What is the mechanism of paradoxical REM sleep?

Answer 12

• Initiated by neurones in the pons

- Similar EEG to when awake with eyes open (beta waves), but difficult to rouse due to strong thalamic inhibition

- Decreased muscle tone due to glycinergic inhibition of lower motor neurones

- Eye movements and some other cranial nerve functions can be preserved (e.g. teeth grinding)

- Autonomic effects seen (e.g. penile erection, loss of thermoregulation)

• Essential for life – long term deprivation leads to death

Question 13

What are the functions of sleep?

Answer 13

• Generally unknown
• Energy conservation / repair?
• Memory consolidation?
• Clearance of extracellular debris?
• ‘Resetting’ of the CNS?

Question 14

What are some common sleep disorders?

Answer 14

Question 15

What is the pathology behind the following disorders of consciousness:

• Brain death
• Coma
• PVS
• Locked in Syndrome

Answer 15

PVS is when a patient is in a vegetative state for more than 4 weeks, they are in a state of partial arousal

Question 16

A 26-year-old female presents with double vision when looking to her left-hand side. She is normally fit and well except for an episode of paraesthesia in her left arm 6 months ago and right foot 2 years ago. Both of which resolved spontaneously within 24 hours and no medical attention was sought at the time.

Suggest a suitable diagnosis and explain why she has diplopia on left lateral gaze.

Answer 16

- MS so plaques often form in medial longitudinal fasiculus

• Left lateral rectus/abducens and right medial rectus/oculomotor need to contract to move left
• Allows for conjugate movement
• Also, inhibition of contralateral extraocular muscle movement
• Without the medial longitudinal fasiculus she cannot do this

Question 17

As you perform a lumbar puncture the patient complains of paraesthesia over the lateral aspect of the right foot, what have you hit?

Answer 17

Right S1 dorsal root

Question 18

If someone has a chronic raised ICP what can you do?

Answer 18

• Drain some fluid off using lumbar puncture
• Lose weight
• Diuretics

Question 19

A 45 year old woman is admitted following a stab wound to her back. She has sustained a complete hemisection of the left side of her spinal cord at the level of L3.

What is the pattern of sensory loss that may be expected?

Answer 19

• Complete paraesthesia at the level of L3
• Ipsilateral loss of dorsal column modalities (e.g vibration) from L3 down
• Contralateral loss of spinothalmic modalities from L4 down

Question 20

Identify the following structures.

Answer 20

* = caudate nucleus

A = Putamen

B = Global Pallidus

C = Thalamus

Question 21

What nuclei make up the striatum and the lentiform nucleus?

Answer 21

Striatum: caudate nucleus and putamen

Lentiform: putamen and global pallidus

Question 22

A 54 year old male has a known syringomyelia at T6 and gradually develops loss of pain and temperature bilaterally from T6 down. What is the precise structure in the spinal cord that has been affected?

Answer 22

Ventral white commisure

Question 23

A 45 year old patient is undergoing surgery for resection of a brain tumour. After the operation they have loss of sensation in all modalities in their left foot. From which site was the tumour removed?

Answer 23

Right, medial, post-central gyrus

Question 24

When you have nystagmus what does the fast phase show you?

Answer 24

Fast phase goes towards the lesion