The Primordial Emotions: The Dawning of Consciousness

Denton 2006

Christina Weng

October 17, 2008

The Physiology of the Primordial Emotion of Thirst

Questions to Keep in Mind

1) How is thirst generated?

2) Where is the change in physical state that generates thirst detected?

3) Where does the thirst system fit on the evolutionary development timeline?

4) How is immediate gratification of thirst possible?

The Mechanisms Producing ThirstCellular Dehydration

Change in osmotic pressure Rise in extracellular [Na]

Sensors in the hypothalamus

Other areas of the brain

Consciousness of thirst

caused by

detected by

impulses to

Aquatic Animals Conserve Water

Reptiles/birds Mammals

eggs urine limit to water intake freshwater seawater

consciousness

Evolution of thirst

The Dry Mouth Theory

Claude Bernard: continuous wetting of the mouth does not satisfy thirst

In animals: some salivation continues even when water is withheld—wetting of mouth is not sufficient to satisfy thirst

Claude Bernard’s Experiment

In humans: less salivation dry mouth sensation exacerbates signals from hypothalamic sensors.

Rapid Gratification of Thirst

Loss of thirst occurs long before the water is absorbed from the gut and the chemical imbalance (origin of thirst sensation) is corrected.

In experiments, animals drink just enough to repair the water and/or sodium deficit.

HOW?

Taste of water Esophageal metering Filling of stomach

Satisfaction

The Neuroimaging of Thirst by Positron Emission Tomography

PET/Radioactive Water Technology

Detects local change in cerebral blood flow

Regional cerebral blood flow Neural activity

Activation

Deactivation

Anatomy

The Experiment

9 males: 24-36 years

1) Baseline scans2) Rapid venal infusion of 3% NaCl (vs. 0.9% in normal blood)3) Sensation of thirst appears (+25 min) Image4) Thirst sensation maximum (+40 min) Image5) Mouth rinse Image6) Drink Image

Effect of Salt Solution on Blood Plasma and Thirst

Primary Sensation of Thirst (+25/+30 Minutes)

Evolutionary Ancient Areas

Parahippocampus (limbic system)

Cingulate gyri (limbic emotional system)

Insula (visceral sensory area)

Cerebellum

Activation: yellow to red

Deactivation: green to blue

Maximum Thirst(+ 40 min)

Activation of Papez Circuit: linked to emotional behavior

Maximum activation in left parahippocampus, major deactivation in parahippocampus; bypass Papez circuit

Activation of parietal region: “dry mouth” sensation

Activation of thalamus: sensory inflow from “dry mouth,” transmission of sensation

Wetting the Mouth

Knee of corpus callosum: remains highly activated

Strong activation of frontal lobe: taste

Drinking Water to Satiation

ACTIVATION IN KNEE OF CORPUS CALLOSUM DISAPPEARS!

Other ObservationsCerebellum: more significant role than expected

Posterior cingulate area: reflected correlation with thirst score

Cingulate regions: cortical response to rise in salt concentration, concious sensation of thirst

Receives most impact from more primitive regions of brain

Robinson and Mishkin Experiment

Electrical stimulation of the anterior cingulate area in monkeys

Water drinking behavior

The Cerebellum

Contains 70% of neurons in the brain

Traditionally associated with motor control and learned tasks

Possible connection with the hypothalamus and thalamus

(http://www.mult-sclerosis.org/cerebellum.html)

fMRI Imaging of Thirst

Agrees with PET scan results

Strong activation in third ventricle:

Site of receptors that detect changes in salt concentration.

Activation of lamina terminalis vs. AC

Future Directions

1) Activity of the anterior cingulate with respect to rapid increase of sodium concentration in the plasma.

2) Involvement of the cerebellum in response to rising sodium concentration

3) Role of the cerebellum in the emergence of consciousness of thirst