Homeostasis & Autonomic Nervous System

Homeostasis and the Autonomic Nervous System (ANS)

• Positive feedback loops amplify the signal that creates an effect, causing it to grow continuously.
• Negative feedback loops cause the effector to stop what caused the signal.
• Ghrelin, the hunger hormone, employs a positive feedback loop.
• Body temperature regulation uses a negative feedback loop to maintain a specific range.
• The autonomic nervous system (ANS) has two divisions: parasympathetic (rest/digest) and sympathetic (fight/flight).
• Both divisions maintain homeostasis via hormones and neurotransmitters, which unconsciously regulate bodily functions.
• One ANS system regulates excitement and fear responses, while the other regulates relaxation and rest.

Maintaining Blood Pressure and Regulating Breathing

• Baroreceptors in the aortic arch and carotid sinuses detect blood pressure changes.
• Baroreceptors then send signals to the medulla oblongata in the brainstem.
• The medulla oblongata activates the sympathetic or parasympathetic nervous system based on BP.
• The parasympathetic system decreases heart rate, activates vasodilation, and reduces cardiac output.
• The sympathetic system increases heart rate, activates vasoconstriction, and releases epinephrine and norepinephrine to increase fight/flight effects.
• Central Pattern Generators (CPGs) are neuron networks in the spinal cord and brainstem that automatically control rhythmic patterns, like walking or chewing.
• The respiratory CPG in the medulla oblongata regulates breathing based on oxygen and activity levels.
• The chemoreflex regulates blood properties, including CO2, O2, and pH.
• Central chemoreceptors in the medulla oblongata detect pH changes in the cerebrospinal fluid.
• Peripheral chemoreceptors in arteries detect changes in O2.
• Glomus cells detect low oxygen and high CO2 levels, increasing breathing to balance O2/CO2.

Hypothalamus and Homeostasis

• The hypothalamus regulates homeostasis indirectly through the anterior pituitary by releasing tropic hormones, which trigger the release of other hormones. -Examples of tropic hormones include TRH, CRH, GnRH, GHRH, SST, DA.
• The hypothalamus directly controls the posterior pituitary using neural connections, releasing ADH and oxytocin.
• Thirst is regulated by urination and hydration; dehydration causes cells in the paraventricular and supraoptic nucleus to shrink, opening stretch-inactivated receptors.
• Opening of stretch-inactivated receptors leads to the release of vasopressin, which binds to kidney receptors, causing urine retention.
• Hunger is regulated by the hypothalamus, using leptin (hunger suppressing) and ghrelin (hunger stimulating).
• Low energy levels are detected by the lateral hypothalamus resulting in the release of signals, causing hunger.
• Ghrelin released from the stomach increases hunger, while the ventromedial hypothalamus suppresses hunger with leptin.
• Both thirst and hunger are hypothalamus-controlled, hormone-driven responses that cause behaviors to seek water or energy.
• Main difference between these two is the use of different key hormones and effective organs.

Biological Rhythms

• The suprachiasmatic nucleus (SCN) regulates the sleep-wake cycle based on perceived light from the retina.
• The SCN also regulates melatonin, body temperature, and metabolism, and adjusts for jet lag.
• Internal factors in maintaining circadian rhythms include sleep-wake cycle genetics, melatonin and hormones, body temperature, and cellular clocks.
• External factors include light, diet, schedules, drugs, and exercise.

Types of Memory

• Explicit memory is active recall, such as remembering a family member's birthdate.
• Implicit memory occurs naturally, like riding a bike, and happens automatically.
• The two forms of implicit memory are short-term (habituation/sensitization) and long-term (habits and motor skills).
• Habituation involves decreased reaction to a harmless stimulus over time, while sensitization is increased reaction to a strong, likely harmful stimulus.
• Neuroplasticity is the brain's ability to form new connections by strengthening pathways, pruning unused pathways, and adapting based on experiences.
• There's evidence that neurons can rewire after injury and remain flexible.
• "Neurons that fire together, wire together" simplifies the Hebbian theory, meaning that neurons can form connections and fire together if they perform the same task simultaneously over time.
• Long-term potentiation (LTP) strengthens neural connections in the hippocampus for memory and learning.
• Glutamate activates AMPA receptors, leading to depolarization and removal of the magnesium block from NMDA receptors to allow calcium entry, triggering structural changes that reinforce connections.

Explicit Memory & the Hippocampus

• The prefrontal cortex is involved in working memory tasks.
• The hippocampus is necessary for remembering and forming new memories, but motor functions can still be learned without it.
• The hippocampus is meant for making and compartmentalizing new memories
• Encoding requires attending to information and connecting it to prior knowledge within the hippocampus.
• Storage happens in the cerebral cortex.
• Consolidation turns memory from short to long term.
• Retrieval is the recall of stored information.

The Frontostriatal System

• The frontostriatal system includes neural circuits connecting the prefrontal cortex with striatal brain regions (caudate nucleus and putamen).
• Both loops of the system involve the striatum, which receives input from the prefrontal cortex and regulates motor and cognitive functions.
• The system influences motivation, decision-making, and motor control, playing a part in disorders like Parkinson's, ADHD, and OCD.
• The direct pathway promotes activity, while the indirect pathway suppresses it, ensuring balanced control over movement and cognition.
• The frontostriatal system regulates action selection and goal pursuit, integrating cognitive control, motivation, and reward processing.
• The prefrontal cortex guides decision-making and impulse control, while the striatum evaluates rewards and habits.
• Dopamine reinforces behaviors, ensuring beneficial actions are prioritized.
• The direct pathway facilitates movement and goal-directed behavior.
• The indirect pathway suppresses competing actions to prevent distractions, allowing individuals to plan, adjust, and stay focused.