CNS & Homeostasis

• The central nervous system (CNS) plays a role in maintaining homeostasis.

Homeostasis role of CNS

• Homeostasis is the maintenance of a relatively stable internal environment despite changing conditions.
• Homeostatic functions include maintaining fluid, electrolyte, acid-base balance, blood pressure, blood glucose, oxygen concentrations, biological rhythms, and body temperature.
• The nervous and endocrine systems maintain homeostasis, working together with their own mechanisms for vital homeostatic regulation.
• The endocrine system secretes hormones into the blood to regulate cell functions, with effects that are typically slow.
• The nervous system sends action potentials to excite or inhibit target cells, with generally immediate actions.
• Two CNS structures directly involved in maintaining homeostasis are the reticular formation, which controls internal organ functions and behavior aspects, and the hypothalamus, closely associated with the pituitary gland.
• The reticular formation and hypothalamus have interconnections, enabling them to coordinate many homeostatic functions.
• Vital functions such as heart pumping, blood pressure, and digestion are largely controlled by the autonomic nervous system (ANS), which regulates the body's viscera.
• The ANS is a component of the PNS that is controlled by components of the CNS, mainly the hypothalamus.
• The hypothalamus receives sensory input from viscera, the limbic system, and the cerebral cortex.
• The hypothalamus can respond to normal physiological and emotional changes and adjust ANS output to maintain homeostasis.
• The hypothalamus maintains homeostasis by relaying instructions to nuclei in the reticular formation of the medulla, including the vasopressor center, which increases heart rate and blood pressure.
• The vasodepressor center, inferior and medial to the vasopressor center, decreases heart rate and blood pressure.
• Additional centers in the reticular formation regulate digestion and urination.
• Respiration is one of the few vital functions not under ANS control.
• The rate and depth of breathing are regulated by neurons in the anterior medullary reticular formation, influenced by input from cerebral and limbic regions, and certain nuclei in the pons.

Body Temperature Homeostasis

• The hypothalamus regulates body temperature and acts as the body's thermostat, setting a normal body temperature around 37°C or 98.6°F.
• Temperature-sensitive neurons located in the skin, deeper body areas, and the hypothalamus itself provide input.
• When body temperature rises above the set point, a negative feedback loop cools the body.
• When body temperature falls below the set point, a different feedback loop conserves heat.
• Fever is a result of a temporary increase in the body temperature set point.

Fever

• Fever, marked by elevated body temperature, can occur in infectious and noninfectious conditions.
• Pyrogens, chemicals secreted by immune cells and certain bacteria, cross the blood-brain barrier and interact with hypothalamic nuclei to control temperature.
• Pyrogens increase the hypothalamic set point, leading to shivering, muscle aches, and constricted blood vessels.
• Treatment is not always required, and addressing the underlying cause is often more important.
• Antipyretics, like acetaminophen and aspirin, treat fever by blocking pyrogen formation, allowing the hypothalamus to return to its normal set point.

Regulation of Feeding

• The hypothalamus regulates feeding behaviors.
• Stimulation of certain hypothalamic nuclei induces hunger and feeding behaviors, preserving glucose homeostasis.
• Orexins, neurotransmitters, may be related to the secretion of these hunger signals.
• Orexins are secreted by hypothalamic neurons during fasting, inducing eating and regulating the sleep/wake cycle.
• Other hypothalamic nuclei inhibit feeding, with food intake controlled by complex mechanisms involving hormones and various brain regions.

Sleep and Wakefulness

• Sleep is a reversible, normal suspension of consciousness and a fundamental homeostatic process.
• How sleep is regulated and brain activity during sleep are known, but why animals sleep is unknown.
• Sleep appears to serve an energy restoration function and replenish's the brain's glycogen supply.
• Sleep is required for survival, and deprivation can cause temperature imbalances, weight loss, cognitive decline, hallucinations, and even death.
• Adults typically need 7-8 hours of sleep, while infants need about 17 hours per night.

Circadian Rhythms and Biological Clock

• Human sleep follows a circadian rhythm (24-hour cycle) with periods of wakefulness and sleep.
• Rhythm is controlled by the hypothalamus, causing changes in wakefulness in response to day and night cycles.
• Nerves from the eye signal the hypothalamus when light levels decrease, stimulating the pineal gland to secrete melatonin, decreasing reticularformation activity.
• Decreased activity in reticular formation "disconnects" the thalamus from the cerebral cortex, decreasing consciousness.
• Arousal from sleep is mediated by hypothalamic neurons secreting orexin.

Higher Mental Functions

• Cognition is the collective term for tasks performed by cerebral cortex association areas.
• Cognitive functions include processing complex stimuli, recognizing related stimuli, processing internal stimuli, and planning responses.
• Cognitive processes are responsible for social and moral behavior, intelligence, thoughts, problem-solving, language, and personality.
• Cognitive function localization involves parietal association cortex for spatial awareness and attention.
• Temporal association cortex is responsible for recognizing stimuli, especially complex stimuli like faces.
• The prefrontal cortex, the largest and most complex association cortex, handles cognitive functions related to personality, gathers information, integrates it to create self-awareness, and allows for planning and executing appropriate behaviors.
• Cerebral lateralization is the unequal representation of cognitive functions in the right and left hemispheres, representing a division of labor, and some functions appear to be lateralized although not an absolute.
• Emotional functions are located in the left frontal cortex being responsible for "positive" emotions, while the right cortex handles "negative" emotions.
• Attention is lateralized to the right parietal cortex; facial recognition is lateralized in the right temporal cortex.
• Language-related recognition and the ability to identify an object with its proper name are lateralized in the left temporal cortex.

Dementia

• Patients with dementia exhibit progressive loss of recent memory, degeneration of cognitive functions, and changes in personality.
• Treatments have not been proven to prevent or cure dementia, but some drugs may slow the progression of Alzheimer's disease in certain patients.

Forms of Dementia include

• The most common form of dementia is Alzheimer's disease (AD), with it's key signs including neurofibrillary tangles(aggregates of proteins in neurons), senile plaques (extracellular deposits of protein around neurons), and degeneration of cortical neurons synaptic connections, mainly in cortical association areas and the hippocampus.
• Early signs of Alzheimer's disease are recent memory loss and forgetfulness, progressing to impaired attention, language skills, critical thinking, visual-spatial abilities, personality changes, motor skills, sensory perception, and long-term memory.

Cognition and Language

• Language is a crucial cognitive function for the brain, involving: comprehending, producing, assigning, and recognizing the correct symbolic meaning of words through speaking, writing/signing.
• Multiple brain regions are required for communication, including: two multimodal association areas that are critical.
• Broca's area, in the frontal lobe, is involved in language production; including planning and ordering words with proper grammar and syntax.
• Wernicke's area, in the temporal lobe, is essential for understanding language and linking words with their correct symbolic meaning.
• Aphasia is a language deficit that occurs when either of these critical areas is damaged.
• Broca's aphasia involves the inability to correctly plan and order grammar and syntax, resulting in disordered language comprehension.
• Wernicke's aphasia impairs the ability to understand language; but still speaking fluently (adequate grammar and syntax, but incorrect word linkage), but language comprehension isn't intact.

Learning and Memory

• Two basic types of memory: declarative (fact) memory, things readily available to consciousness; and nondeclarative (procedural or skills) memory, involving largely unconscious skills and associations.
• Memory is classified by the length of time it is stored: immediate memory (few seconds), short-term (working) memory (several minutes), and long-term memory (days, weeks, or lifetime).
• Process of converting immediate or working memory into long-term memory, called consolidation, and declarative memory formation and storage appears to require the hippocampus (component of the limbic system).
• Long-term potentiation (LTP) is the mechanism by which hippocampal neurons encode long-term declarative memories, increasing synaptic activity between associated neurons.
• The hippocampus is required to form new declarative memories; long-term memories are stored in the cerebral cortex that correlates with their functions and retrieval of memories are mediated by pathways involving the hippocampus and prefrontal cortex.
• Nondeclarative memory formation and storage doesn't rely on the hippocampus; memories tend to be stored in cerebral cortex, cerebellum, and basal nuclei.
• Emotion, the complex combination, involves visceral motor responses mediated by the hypothalamus, somatic motor responses mediated by the hypothalamus and limbic cortex through the reticular formation, and feelings integrated with sensory and/or cognitive stimuli.
• Amygdala analyzes the stimuli's significance, creates associations; projects to prefrontal cortex, receives input from the brainstem, thalamus, cerebral cortex, and basal nuclei.