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BIO 132
Neurophysiology
Lecture 36
Motivation (cont)
Commonalities of Hypothalamic Homeostatic
Feedback Loops
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The homeostatic feedback loops involve integration of
sensory information by the hypothalamus.
The hypothalamus effects change in monitored
variables through three effectors:
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Humeral response – hormones released from the pituitary
Visceral motor response – Autonomic NS
Somatic motor response – Motivation/behavior
Effectors controlled by different hypothalamic areas.
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Periventricular zone – Humeral and visceral motor responses
Lateral zone – Somatic motor response
Short-term Feeding Behavior
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Goal of short-term feeding behavior: Maintain
short-term energy stores (blood glucose levels).
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The body has a set-point for the concentration
of glucose in the blood.
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The hypothalamus has different ways of
monitoring blood glucose concentrations.
Monitoring of Blood Glucose
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Direct monitoring of blood glucose is done by
specialized neurons in the arcuate nucleus.
Sensory input of indicators of blood glucose enter the
hypothalamus from three sources.
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Blood insulin levels – insulin released by pancreas in proportion to
glucose levels is monitored by specialized neurons of the arcuate nucleus
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Cholocystokienin (CCK) levels – sensory neurons of the vagus nerve,
located in the stomach and intestines, relay to the hypothalamus
information about CCK levels. CCK is released by the GI tract in
proportion to the nutrient levels in the gut.
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Gastric distention – sensory neurons of the vagus nerve located in the
stomach and intestines relay information about the stretch of the
stomach and intestines.
Effects of Eating a Meal
nutrients
Blood glucose
Insulin
Lateral zone
Satiety
ventrical
inhibit
Periventricular
zone
Hunger
GI stretch
CCK
Feeding behavior
Pituitary
via vagus n.
Arcuate nucleus
TRH
CRH
TSH
ACTH
Caloric intake
SNS
thyroid hormone
Metabolic rate
cortisol
Diffuse Modulatory Systems and Food Intake
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VTA of the dopaminergic system releases dopamine
into the lateral zone of the may reward you for eating
foods your body needs.
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Thought to increase the craving for but not the palatability of
food
Parts of the serotonergic system are activated during a
meal which seem to increase mood and decrease the
motivation to eat.
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Pharmaceutical companies tried to capitalize on this
Serotonin as an Appetite Suppressant
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Once released, serotonin is taken up by platelets in the
blood or degraded by the enzyme monoamine oxidase
(MAO).
Fenfluramine blocks uptake of serotonin by platelets
and worked as an effective appetite supressant.
Phentermine is a MAO inhibitor and blocks the
breakdown of serotonin. This also worked as an
effective appetite supressant.
Fen-Phen, a combination of the two drugs was even
more effective than either one alone.
Dangerous Side-effects
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Doctors noted that patients taking fen-phen had
a much higher incidence of abnormal heart
valves.
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In 1997 the FDA announced the withdrawal of
both fen-phen and fenfluramine, citing the
dangerous side-effects of heart valve problems.
Drinking Behavior (Thirst)
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Goal: maintenance of fluid volume and
osmolarity (saltiness) of the body.
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The body has a set-point for both fluid volume
and osmolarity.
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The hypothalamus has a way of monitoring each
of the above variables.
Types of Thirst
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Volumetric thirst – triggered by too low of
blood volume.
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Volume of blood is monitored by specialized
neurons embedded in the walls of the aorta and
carotid arteries.
Increased volume leads to increased stretch of the
arteries, leading to increased firing of the neurons.
 The neurons send their information to the hypothalamus
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Types of Thirst
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Osmometric thirst – triggered by too high a
level of osmolarity (saltiness).
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Osmolarity is monitored by neurons in a specialized
region of the hypothalamus called the vascular organ
of the lamina terminlus (OVLT).
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Increased osmolarity leads to water leaving the neurons
causing them to shrink, which closes mechanoreceptive
channels and decreases the firing of the neurons.
Effects of Blood Volume and
Osmolarity Changes
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Input from the baroreceptors sensing a decrease in
blood volume and/or the OVLT sensing an increase in
osmolarity will cause the same humeral, visceral motor,
and somatic motor responses.
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Humeral response: increase release of ADH to increase water
re-absorption (decrease fluid loss)
Visceral motor response: increase activation of the SNS to
increase mean arterial pressure
Somatic motor response: feeling of thirst (seek out water to
drink)
Temperature Effects on Behavior
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Goal: maintenance of body temperature at the internal
set-point.
The hypothalamus has a set-point for body temperature
that is usually around 98.6 °F (37 °C), but can be
altered by immune states (fever).
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Thermo-sensitive neurons throughout the body feed into the
hypothalamus but the most sensitive and effective are found
in the hypothalamus itself
Changes in body temperature outside of the normal
range can be dangerous since it alters enzyme shape
and function.
Effects of Decreased Body Temperature
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Cold-sensitive neurons in the hypothalamus fire as the
temp falls, bringing about a humeral, visceral motor,
and somatic motor responses.
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Humeral response: increase release of thyroid releasing
hormone (TRH) from parvocellular neurons to signal the
release of thyroid stimulating hormone (TSH) from the
anterior pituitary. TSH cause the release of thyroid hormone
from the thyroid gland, which increase heat production by
making mitochondria of the body less efficient.
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Thyroid hormone requires cells to burning more glucose to make the
appropriate amount of ATP.
Effects of Decreased Body Temperature
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Visceral motor response:
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Blood is shunted away from the skin and towards the core so as to
minimize heat loss.
Piloerectation (goose-bumps) – constriction of tiny muscles
connected to hairs causes hair to stand up in an attempt to make
“fur” thicker (increasing the thermal barrier).
Somatic motor response:
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Involuntary shivering of skeletal muscle – causes increase energy use
and heat production
Desire to seek warmth
Effects of Increased Body Temperature
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Warm-sensitive neurons in the hypothalamus fire as the
temp increases, bringing about a humeral, visceral
motor, and somatic motor responses.
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Humeral response: decrease release of TRH from
parvocellular neurons leading to decreased release of TSH
and subsequent decrease release of thyroid hormone, which
decrease heat production by making mitochondria of the
body more efficient.
Visceral motor response: Blood is shunted toward the skin to
increase heat loss.
Somatic motor response: Sweating (or panting) and desire to
seek cooler conditions
Effects of Decreased Body Temperature
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Visceral motor response:
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Blood is shunted away from the skin and towards the core so as to
minimize heat loss.
Piloerectation (goose-bumps) – constriction of tiny muscles
connected to hairs causes hair to stand up in an attempt to make
“fur” thicker (increasing the thermal barrier).
Somatic motor response:
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Involuntary shivering of skeletal muscle – causes increase energy use
and heat production
Desire to seek warmth