BIO 132

1. BIO 132 Lecture 36 Motivation (cont) Neurophysiology

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. Satiety ventrical Hunger ACTH TRH CRH SNS TSH thyroid hormone cortisol Metabolic rate 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