15.3 Central Control

The autonomic nervous system keeps your body running smoothly without you having to think about it. Your brain's control centers, like the hypothalamus, work behind the scenes to regulate things like heart rate, digestion, and body temperature.

Emotions can trigger automatic responses in your body too. When you're scared, your heart races and you breathe faster. This happens because brain areas like the amygdala activate your fight-or-flight response, preparing you to face danger or run away.

Central Control of Autonomic Functions

Regulation of autonomic functions

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• The cerebral cortex influences autonomic functions
  • Prefrontal cortex and insular cortex have efferent connections to autonomic centers in the brainstem (hypothalamus, medulla oblongata)
  • Limbic system structures, such as the amygdala and hippocampus, modulate autonomic responses (fear, anxiety, stress)
• The hypothalamus is a key integrator of autonomic functions
  • Receives input from higher brain centers (cerebral cortex, limbic system) and sensory systems (visual, olfactory, gustatory)
  • Controls autonomic output through connections with the brainstem (medulla oblongata, pons) and spinal cord (thoracolumbar, sacral regions)
• The brainstem contains autonomic control centers
  • Medulla oblongata: cardiovascular center regulates heart rate and blood pressure, respiratory center controls breathing
  • Pons: pneumotaxic center modulates respiratory rate, apneustic center promotes inhalation
  • Midbrain: periaqueductal gray matter involved in pain modulation and defensive behaviors (freezing, fight-or-flight)
• The thalamus acts as a relay station for sensory information to the cerebral cortex, influencing autonomic responses

Hypothalamus and homeostasis

• The hypothalamus is the primary link between the endocrine and nervous systems
  • Synthesizes and secretes releasing hormones (corticotropin-releasing hormone) and inhibiting hormones (somatostatin) to control the anterior pituitary gland
  • Produces hormones secreted by the posterior pituitary gland: antidiuretic hormone regulates water balance, oxytocin stimulates uterine contractions and milk letdown
• Regulates body temperature
  • Anterior hypothalamus contains heat-sensitive neurons that detect increased body temperature
  • Posterior hypothalamus contains cold-sensitive neurons that detect decreased body temperature
  • Integrates temperature information and initiates appropriate autonomic responses: sweating and vasodilation to cool the body, shivering and vasoconstriction to conserve heat
• Controls hunger and satiety
  • Lateral hypothalamus: hunger center stimulates appetite by responding to ghrelin and low blood glucose levels
  • Ventromedial hypothalamus: satiety center suppresses appetite by responding to leptin and high blood glucose levels
  • Responds to hormones (leptin from adipose tissue, ghrelin from stomach) and nutrient levels in the blood to maintain energy balance
• Regulates circadian rhythms and sleep-wake cycles
  • Suprachiasmatic nucleus receives light input from the retina via the retinohypothalamic tract
  • Coordinates the release of melatonin from the pineal gland to promote sleep during darkness
• Plays a crucial role in maintaining homeostasis by regulating various physiological processes

Brain regions for emotional autonomic responses

• Limbic system structures modulate autonomic functions
  • Amygdala: processes fear and anxiety, initiates fight-or-flight response by activating sympathetic nervous system (increased heart rate, blood pressure, and respiration)
  • Hippocampus: involved in memory formation and recall, influences stress responses by regulating hypothalamic-pituitary-adrenal axis (cortisol release)
  • Anterior cingulate cortex: regulates emotional responses and pain perception by modulating autonomic functions (heart rate variability, skin conductance)
• Prefrontal cortex: involved in decision-making, planning, and emotional regulation
  • Ventromedial prefrontal cortex: processes risk and fear, inhibits amygdala activity to reduce anxiety and stress responses
  • Dorsolateral prefrontal cortex: regulates emotions and attention, exerts top-down control over autonomic functions (heart rate, blood pressure)
• Insular cortex: integrates sensory information and autonomic responses
  • Anterior insula: processes emotional and cognitive aspects of pain, involved in interoceptive awareness (perception of internal body states)
  • Posterior insula: receives visceral sensory input (heart rate, blood pressure) and regulates autonomic functions to maintain homeostasis
• The basal ganglia contribute to motor control and emotional processing, indirectly influencing autonomic functions

Descending pathways of autonomic control

1. Hypothalamic-spinal pathway
   • Originates in the paraventricular nucleus of the hypothalamus
   • Descends through the brainstem (midbrain, pons, medulla) and lateral horn of the spinal cord
   • Synapses with preganglionic sympathetic neurons in the thoracolumbar spinal cord (T1-L2) to control cardiovascular, respiratory, and thermoregulatory functions
2. Medullary-spinal pathway
   • Originates in the rostral ventrolateral medulla (RVLM) and the rostral ventromedial medulla (RVMM)
   • Descends through the spinal cord via the lateral funiculus
   • Synapses with preganglionic sympathetic neurons in the thoracolumbar spinal cord (T1-L2) to regulate blood pressure and preganglionic parasympathetic neurons in the sacral spinal cord (S2-S4) to control bladder and bowel functions
3. Vagal pathway
   • Originates in the dorsal motor nucleus of the vagus nerve and the nucleus ambiguus in the medulla oblongata
   • Vagus nerve (cranial nerve X) carries preganglionic parasympathetic fibers to thoracic and abdominal viscera (heart, lungs, stomach, intestines)
   • Regulates heart rate, digestion, and inflammation through the release of acetylcholine and vasoactive intestinal peptide (VIP)

Autonomic Nervous System and Neurotransmission

• The autonomic nervous system consists of sympathetic and parasympathetic divisions
• Neurotransmitters play a crucial role in synaptic transmission within the autonomic nervous system
• The cerebellum coordinates motor functions and indirectly influences autonomic responses through its connections with other brain regions