BIO 132

1
BIO 132

• Neurophysiology

Lecture 36 Motivation (cont)
2
Commonalities of Hypothalamic Homeostatic
Feedback Loops

• The homeostatic feedback loops involve
  integration of sensory information by the
  hypothalamus.
• The hypothalamus effects change in monitored
  variables through three effectors
• Humeral response hormones released from the
  pituitary
• Visceral motor response Autonomic NS
• Somatic motor response Motivation/behavior
• Effectors controlled by different hypothalamic
  areas.
• Periventricular zone Humeral and visceral motor
  responses
• Lateral zone Somatic motor response

3
Short-term Feeding Behavior

• Goal of short-term feeding behavior Maintain
  short-term energy stores (blood glucose levels).
• The body has a set-point for the concentration of
  glucose in the blood.
• The hypothalamus has different ways of monitoring
  blood glucose concentrations.

4
Monitoring of Blood Glucose

• 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.
• Blood insulin levels insulin released by
  pancreas in proportion to glucose levels is
  monitored by specialized neurons of the arcuate
  nucleus
• 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.
• Gastric distention sensory neurons of the vagus
  nerve located in the stomach and intestines relay
  information about the stretch of the stomach and
  intestines.

5
Effects of Eating a Meal
nutrients
Periventricular zone
Lateral zone
inhibit
Blood glucose
Insulin
GI stretch
Feeding behavior
CCK
Pituitary
Caloric intake
via vagus n.
Arcuate nucleus
6
Diffuse Modulatory Systems and Food Intake

• VTA of the dopaminergic system releases dopamine
  into the lateral zone of the may reward you for
  eating foods your body needs.
• 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.
• Pharmaceutical companies tried to capitalize on
  this

7
Serotonin as an Appetite Suppressant

• 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.

8
Dangerous Side-effects

• Doctors noted that patients taking fen-phen had a
  much higher incidence of abnormal heart valves.
• In 1997 the FDA announced the withdrawal of both
  fen-phen and fenfluramine, citing the dangerous
  side-effects of heart valve problems.

9
Drinking Behavior (Thirst)

• Goal maintenance of fluid volume and osmolarity
  (saltiness) of the body.
• The body has a set-point for both fluid volume
  and osmolarity.
• The hypothalamus has a way of monitoring each of
  the above variables.

10
Types of Thirst

• Volumetric thirst triggered by too low of blood
  volume.
• 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

11
Types of Thirst

• Osmometric thirst triggered by too high a level
  of osmolarity (saltiness).
• Osmolarity is monitored by neurons in a
  specialized region of the hypothalamus called the
  vascular organ of the lamina terminlus (OVLT).
• Increased osmolarity leads to water leaving the
  neurons causing them to shrink, which closes
  mechanoreceptive channels and decreases the
  firing of the neurons.

12
Effects of Blood Volume and Osmolarity Changes

• 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.
• 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)

13
Temperature Effects on Behavior

• 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).
• 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.

14
Effects of Decreased Body Temperature

• Cold-sensitive neurons in the hypothalamus fire
  as the temp falls, bringing about a humeral,
  visceral motor, and somatic motor responses.
• 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.
• Thyroid hormone requires cells to burning more
  glucose to make the appropriate amount of ATP.

15
Effects of Decreased Body Temperature

• Visceral motor response
• 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
• Involuntary shivering of skeletal muscle causes
  increase energy use and heat production
• Desire to seek warmth

16
Effects of Increased Body Temperature

• Warm-sensitive neurons in the hypothalamus fire
  as the temp increases, bringing about a humeral,
  visceral motor, and somatic motor responses.
• 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

17
Effects of Decreased Body Temperature

• Visceral motor response
• 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
• Involuntary shivering of skeletal muscle causes
  increase energy use and heat production
• Desire to seek warmth