Thermoregulation, and the Hypothalamus 1. Neural Signal Pathway After Eating a Meal • Stimulus: Food stretches the stomach and intestines, activating mechanoreceptors (stretch receptors) and chemoreceptors. • Sensory Signals to the Brain: – Signals travel via the vagus nerve (cranial nerve X) and spinal afferent nerves to the brainstem, particularly the medulla oblongata. • Integration in the Brainstem: – The medulla oblongata activates the parasympathetic nervous system. – The hypothalamus helps coordinate autonomic and endocrine responses. • Parasympathetic Response (Efferent Signals): – Increased gastric secretions and intestinal motility. – Stimulation of pancreatic enzyme secretion. – Gallbladder contraction to release bile for fat digestion. • Local Enteric Nervous System Activity: – Fine-tunes digestion and directly controls peristalsis and secretion. • Hormonal Interaction: – Hormones like gastrin, secretin, and cholecystokinin (CCK) regulate digestion further. 2. Thermoregulation Feedback Response Negative Feedback Mechanism • Stimulus: A deviation from normal body temperature. • Receptors: – Peripheral thermoreceptors in the skin. – Central thermoreceptors in the hypothalamus. • Control Center: Hypothalamus. 1 • Effector Responses: – If Too Hot (Hyperthermia): ∗ Vasodilation: Blood vessels widen to release heat. ∗ Sweating: Evaporation cools the body. – If Too Cold (Hypothermia): ∗ Vasoconstriction: Blood vessels constrict to retain heat. ∗ Shivering: Muscle contractions generate heat. Positive Feedback Mechanism • Example: Fever temporarily raises the body’s set point to enhance immune response. 3. Role of the Hypothalamus • Integration of Sensory Signals: – Inputs include temperature, blood pressure, osmolarity, and glucose levels. – Signals come from the nervous system and bloodstream. • Control of Endocrine Glands: – Releases hormones to regulate the anterior pituitary (e.g., CRH, TRH, GnRH). – Inhibits hormones like somatostatin and dopamine. • Regulation of the Autonomic Nervous System: – Controls involuntary processes such as heart rate, digestion, and stress response. • Secretion of Neurohormones: – Oxytocin: Involved in uterine contractions and bonding. – ADH: Regulates water balance and blood pressure. 4. Integration of Signal Pathways • The hypothalamus acts as a central hub for neurohormonal pathways but not all signals go through it. • Examples: – Sensory signals are processed in the somatosensory cortex. – Motor signals are processed in the motor cortex and cerebellum. – Reflexes bypass the brain and are processed by the spinal cord. 2