chapter 12 part 3
12.6 Functional Brain Systems
Functional brain systems consist of networks of neurons that collaborate across extensive brain regions.
Key Networks:
Limbic system
Reticular formation
Limbic System (1 of 4)
The limbic system comprises structures located on the medial aspects of the cerebral hemispheres and the diencephalon.
Fornix:
A fiber tract that connects various regions of the limbic system.
Components:
Involves portions of the diencephalon and several cerebral structures that form an encircling structure around the brain stem.
Limbic System (2 of 4)
The limbic system is a significant part of the emotional brain or affective brain.
Amygdaloid Body:
Plays a critical role in recognizing angry or fearful facial expressions, assessing potential danger, and eliciting the fear response.
Cingulate Gyrus:
Involved in expressing emotions through gestures and helping to resolve mental conflict.
Limbic System (3 of 4)
The limbic system also processes emotional responses to odors.
Example: The smell of skunks is perceived as unpleasant due to associated emotional responses.
Most outputs from the limbic system are relayed through the hypothalamus.
The hypothalamus is implicated in psychosomatic illnesses, where emotional states influence physiological conditions.
Limbic System (4 of 4)
Interaction with prefrontal lobes:
This interaction enables emotional reactions to stimuli that we cognitively understand, contributing to an awareness of the emotional richness of our lives.
Memory: The hippocampus and amygdaloid body significantly contribute to memory processing.
Reticular Formation (1 of 3)
The reticular formation is a region that extends through the central core of the brain stem.
Three Broad Columns:
Raphe nuclei
Medial (large cell) group of nuclei
Lateral (small cell) group of nuclei
Extensive axonal connections exist with other brain regions:
Includes connections with the hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cord.
These connections regulate brain arousal.
Reticular Formation (2 of 3)
Reticular Activating System (RAS):
Responsible for sending impulses to the cerebral cortex to maintain consciousness and alertness.
Functions as a filter for repetitive, familiar, or weak stimuli, managing approximately 99% of stimuli that do not reach consciousness.
The system is suppressed by sleep centers, alcohol, and drugs.
Severe injuries can induce permanent unconsciously termed coma.
Reticular Formation (3 of 3)
The motor function of the reticular formation aids in controlling coarse limb movements through reticulospinal tracts.
Furthermore, reticular autonomic centers oversee visceral motor functions, which include:
Vasomotor Centers
Cardiac Center
Respiratory Centers
Higher Mental Functions
The analysis of higher mental functions encompasses various areas:
Language
Memory
Brain waves and EEGs
Consciousness
Sleep and sleep-wake cycles
Language
The language implementation system primarily operates within the association cortex of the left hemisphere, specifically including:
Broca’s Area:
This area is essential for speech production.
Patients with lesions in Broca’s can comprehend speech but struggle to produce it.
Wernicke’s Area:
Involved in understanding both spoken and written language.
Patients with lesions in Wernicke’s can produce speech, but the output lacks coherence and sense.
The corresponding areas on the right side are responsible for nonverbal components of communication.
Memory (1 of 4)
Memory: Defined as the processes of storing and retrieving information.
Types of memory include:
Declarative Memory: Facts such as names, faces, and dates.
Procedural Memory: Skills such as playing an instrument.
Motor Memory: Skills related to motor functions like riding a bike.
Emotional Memory: Experiences tied to emotions, e.g., the physiological reaction one feels upon encountering a rattlesnake.
Memory (2 of 4)
Stages of Declarative Memory Storage:
Short-term Memory (STM) or Working Memory: Temporary holding capacity, limited to about seven or eight pieces of information.
Long-term Memory (LTM): Vast capacity that can store seemingly unlimited amounts of information.
Memory (3 of 4)
Factors that Influence the Transfer from STM to LTM:
Emotional State: Optimal transfer occurs when the individual is alert, motivated, surprised, or aroused.
Rehearsal: Repetitive practice enhances memory consolidation.
Association: Connecting new information to existing memories aids in retention.
Automatic Memory: Subconscious information is stored in long-term memory without intentional effort.
Memory (4 of 4)
Memory Consolidation: Process involving classifying new facts into existing categories within the cerebral cortex.
Key structures in consolidating memory include:
Hippocampus
Temporal cortical areas
Thalamus
Prefrontal cortex
Brain Wave Patterns and the EEG (1 of 5)
Brain waves reflect the electrical activity associated with higher mental functions.
Normal brain functions are continuous and challenging to quantify.
Brain Wave Patterns and the EEG (2 of 5)
Electroencephalogram (EEG): Records electrical activity linked to brain function.
Applications of EEG include:
Diagnosing epilepsy and sleep disorders.
Localizing brain lesions, tumors, infarcts, and infections.
Research applications, including determining brain death.
Electrodes are positioned on the scalp to measure electrical potentials across various cortical zones.
Brain Wave Patterns and the EEG (3 of 5)
EEG measures neuronal electrical activity patterns produced by synaptic events in the cerebral cortex.
Each individual's brain activity is distinctive.
Wave patterns can change based on age, sensory input, brain diseases, and body chemistry.
Brain waves are measured in Hertz (Hz), with frequency indicating the number of peaks per second (1 Hz = 1 peak per second).
The waves are classified into four categories based on their frequency:
Alpha waves
Beta waves
Theta waves
Delta waves
Brain Wave Patterns and the EEG (4 of 5)
Wave Classifications:
Alpha Waves (8–13 Hz): Regular, rhythmic, low-amplitude waves indicating a calm or “idling” brain state.
Beta Waves (14–30 Hz): Rhythmic yet less regular waves occurring during moments of mental alertness.
Theta Waves (4–7 Hz): Irregular and more common in children, uncommon in fully awake adults.
Delta Waves (4 Hz or less): High-amplitude waves associated with deep sleep; their presence during consciousness indicates potential brain damage.
Brain Wave Patterns and the EEG (5 of 5)
Clinical Notes:
Epileptic Seizures: Characterized by intense electrical discharges from groups of neurons disrupting normal messages.
Symptoms may include:
Loss of consciousness
Stiffness and uncontrollable jerking
While epilepsy does not inherently lead to intellectual impairments, it affects 1% of the population.
Contributing factors involve genetics, brain injuries, strokes, infections, and tumors.
Consciousness (1 of 2)
Consciousness: Encompasses:
Perception of sensations
Voluntary control and initiation of movements
Capabilities related to higher mental processing (memory, logic, judgment).
Clinically, consciousness is assessed on a continuum that includes:
Alertness
Drowsiness (lethargy)
Stupor
Coma
Consciousness (2 of 2)
Current Theories:
Suggest that consciousness depends on the simultaneous activity of extensive cortical regions.
It is interwoven with other neural activities and perceived as holistic and utterly interconnected.
Clinical – Homeostatic Imbalance 12.7 (1 of 2)
Consciousness Loss: Outside of sleep, the loss of consciousness typically indicates impaired brain function.
Fainting (Syncope): A brief consciousness loss often due to inadequate cerebral blood flow caused by low blood pressure, ischemia, hemorrhage, or severe emotional stress.
Coma: An extended unconscious state where oxygen consumption is reduced.
Clinical – Homeostatic Imbalance 12.7 (2 of 2)
Brain Death: Defined as an irreversible coma, raising ethical and legal concerns regarding the determination of life and death status.
Sleep and Sleep-Wake Cycles (1 of 6)
Sleep: Described as a state of partial unconsciousness that can be reversed with stimulation.
During sleep, cortical activity diminishes, though brain stem activity remains unchanged.
Types of Sleep:
Two principal categories defined by EEG patterns:
Non-Rapid Eye Movement (NREM) sleep
Rapid Eye Movement (REM) sleep
Sleep and Sleep-Wake Cycles (2 of 6)
During sleep's initial 30-45 minutes, NREM progresses through the first two stages, transitioning into stages 3 and 4, collectively referred to as slow-wave sleep.
In this phase, wave frequency reduces while the amplitude increases.
Several physiological metrics change, including EEG, heart rate, respiratory rate, blood pressure, and gastrointestinal motility.
Sleep and Sleep-Wake Cycles (3 of 6)
After about 90 minutes, the fourth stage concludes, leading to the onset of REM sleep.
REM sleep is marked by temporary paralysis (except for quick eye movements), increases in oxygen consumption, heart rate, and breathing rates—higher than those during wakefulness.
Most dreaming occurs during REM sleep.
Sleep and Sleep-Wake Cycles (4 of 6)
Regulation of Sleep:
The alternating sleep-wake cycles are reflective of the natural circadian rhythm, approximately 24 hours long.
RAS activity is suppressed during sleep, and RAS helps mediate sleep stages.
The Suprachiasmatic nuclei and preoptic nuclei of the hypothalamus are responsible for timing the sleep cycle, releasing orexins crucial for waking the cortex.
Typical sleep patterns alternate between REM and NREM sleep phases.
Sleep and Sleep-Wake Cycles (5 of 6)
Importance of Sleep:
Slow-wave sleep (NREM stages 3 and 4) is presumed to be essential for restoration.
Lack of REM sleep can result in mood changes and depression.
REM sleep may serve two key functions:
Analyzing and processing the day's events and emotional experiences.
Eliminating unnecessary synaptic connections (dreams may serve a purpose in forgetting).
Daily sleep needs decrease with age, particularly with a noted decline in stage 4 sleep beginning after age 60.
Clinical – Homeostatic Imbalance 12.8
Narcolepsy: A sleep disorder characterized by sudden lapses into sleep from an awake state, which can be accompanied by cataplexy (loss of muscle tone while conscious).
Loss of orexins, the hypothalamic “wake-up” chemicals, may occur due to immune system destruction, with replacement therapies being a potential treatment.
Insomnia: Considered a chronic inability to achieve adequate sleep quality or quantity; causes may include depression, anxiety, caffeine overconsumption, or excessive screen time before bed.
Treatment strategies may involve blocking orexin signaling.
Protection of Brain Meninges
The meninges serve several protective functions for the CNS:
Covering and safeguarding the CNS.
Protecting blood vessels and enclosing venous sinuses.
Providing a medium for cerebrospinal fluid (CSF).
Forming partitions within the skull.
Three Layers of Meninges (from outermost to innermost):
Dura Mater:
The strongest layer, consisting of two fibrous connective tissue layers.
Periosteal layer: Adheres to the inner surface of the skull (found only in the brain).
Meningeal layer: The true external covering of the brain that extends into the vertebral canal as spinal dura mater.
In certain areas, layers separate to form dural venous sinuses, where venous blood collects before draining to the jugular veins of the neck.
Meninges (2 of 4)
The dura mater extends inward in specific areas to form flat partitions that divide the cranial cavity, referred to as dural septa, which serve to limit excessive brain movement.
Three Key Dural Septa:
Falx Cerebri: Situated within the longitudinal fissure, anchored to the crista galli.
Falx Cerebelli: Located along the vermis of the cerebellum.
Tentorium Cerebelli: A horizontal dural fold that covers the cerebellum at the transverse fissure.
Meninges (3 of 4)
Arachnoid Mater:
The middle layer characterized by spiderweb-like extensions.
Separated from the dura mater by the subdural space.
The subarachnoid space contains CSF and the largest blood vessels in the brain.
Arachnoid Granulations: Protrusions that extend through the dura mater into the superior sagittal sinus, facilitating the reabsorption of CSF into venous blood.
Meninges (4 of 4)
Pia Mater:
A delicate layer of connective tissue that closely adheres to the brain's surface, following the contours of each convolution.
Contains numerous tiny blood vessels supplying the brain.
Clinical – Homeostatic Imbalance 12.9
Meningitis: An inflammation of the meninges which may propagate to the CNS, causing encephalitis (inflammation of the brain).
Diagnosis of meningitis involves observing microbes in a CSF sample obtained via lumbar puncture.
Cerebrospinal Fluid (CSF) (1 of 2)
Cerebrospinal Fluid (CSF): Acts as a liquid cushion that maintains a constant volume around the brain.
Functions of CSF:
Provides buoyancy to CNS structures, reducing brain weight by approximately 97%, thereby preventing crushing under its own weight.
Protects the CNS from physical shocks and trauma.
Nourishes the brain and transports chemical signals.
The composition of CSF is a watery solution derived from blood plasma, with lower protein content and differing ion concentrations.
Cerebrospinal Fluid (CSF) (2 of 2)
Choroid Plexus: A cluster of capillaries hanging from each ventricle's roof, surrounded by pia mater and enveloped by ependymal cells.
Responsible for filtering CSF at a steady rate.
Ependymal cells utilize ion pumps to manage CSF composition and eliminate waste; cilia facilitate CSF movement.
The standard adult CSF volume is approximately 150 ml, replaced every eight hours.
Formation, Location, and Circulation of CSF (1 of 5)
Figure 12.24a: Illustration of the formation, location, and circulation of CSF.
Clinical – Homeostatic Imbalance 12.10
Hydrocephalus: Occurs when an obstruction hinders CSF circulation or drainage, leading to increased intracranial pressure.
In newborns, non-fused skull bones allow head enlargement with increased pressure.
In adults, the rigidity of the skull can prevent expansion, potentially causing brain damage by compressing blood vessels and soft tissue.
Treatment often involves draining CSF with a ventricular shunt directed into the abdominal cavity.
Blood Brain Barrier (1 of 3)
Blood Brain Barrier (BBB): Maintains a stable environment for the brain, as chemical fluctuations could induce uncontrolled neuron firing.
Substances from blood must navigate through the continuous endothelium of capillary walls to enter neurons.
Tight junctions help ensure that substances pass through endothelial cells rather than around them.
Astrocyte foot processes and pericytes envelop endothelial cells, aiding the formation of tight junctions.
Blood Brain Barrier (2 of 3)
Substances traverse endothelial cells via:
Simple Diffusion: Allows lipid-soluble substances and blood gases to pass freely.
Specific Transport Mechanisms: Such as facilitated diffusion for vital substances like glucose, amino acids, and specific ions.
Transcytosis: Facilitates the transport of larger substances in and out of the brain.
Blood Brain Barrier (3 of 3)
The basement membrane, which constitutes the outermost part of the BBB, houses enzymes that deactivate certain chemicals that could otherwise activate brain neurons.
The BBB is absent in specific regions, including the vomiting center and hypothalamus, to enable monitoring of blood chemistry and body temperature.
Brain Injuries and Disorders
Traumatic Brain Injuries:
Include various types of injuries such as:
Concussion: A temporary alteration in brain function.
Contusion: Permanent damage to the brain tissue.
Subdural or Subarachnoid Hemorrhage: Blood accumulation that may exert pressure on the brain stem, potentially leading to fatal outcomes.
Cerebral Edema: A swelling of the brain that often accompanies trauma.
Cerebrovascular Accidents (CVAs) (1 of 2)
Often referred to as strokes.
Ischemia: Results from blood supply deprivation, leading to brain tissue necrosis.
Can result from blockage of cerebral arteries by blood clots.
Glutamate acts as an excitotoxin during ischemic conditions, exacerbating cellular damage.
Symptoms may include hemiplegia (paralysis on one side) or deficits in sensory perception and speech.
Cerebrovascular Accidents (CVAs) (2 of 2)
Transient Ischemic Attacks (TIAs): Temporary episodes of reversible cerebral ischemia.
Tissue Plasminogen Activator (TPA): The only approved treatment for stroke, aimed at re-establishing blood flow.
Degenerative Brain Disorders (1 of 3)
Alzheimer’s Disease (AD): A progressive degenerative brain illness resulting in dementia.
Key proteins within the brain misfold and malfunction.
Symptoms involve memory loss, shortened attention span, disorientation, language deficits, irritability, mood fluctuations, confusion, and hallucinations.
The presence of beta-amyloid plaques and neurofibrillary tangles disrupt neuronal function, leading to cell death and brain atrophy.
Degenerative Brain Disorders (2 of 3)
Parkinson’s Disease: Results from the degeneration of dopamine-releasing neurons in the substantia nigra.
The lack of dopamine leads to hyperactivity in the basal nuclei and manifests as tremors at rest.
While the cause remains uncertain, theories propose mitochondrial dysfunction or protein degradation pathways as potential factors.
Treatment options include L-dopa (a dopamine precursor), deep brain stimulation, and investigational gene therapy.
Degenerative Brain Disorders (3 of 3)
Huntington’s Disease: A hereditary and fatal disorder characterized by protein accumulation in brain cells.
Leads to degeneration of basal nuclei and the cerebral cortex.
Initial symptoms: wild, jerky movements (“flapping”).
Later symptoms: cognitive decline; typically fatal within 15 years of onset.
Treatment includes dopaminergic antagonists and ongoing research into stem cell transplant therapies.
Diagnostic Procedures for Assessing CNS Dysfunction
Basic tests may include knee-jerk reflex assessment using a hammer on the quadriceps tendon; abnormal responses can signal issues like intracranial hemorrhaging, multiple sclerosis, or hydrocephalus.
Advanced imaging techniques such as CT, MRI, and PET enable rapid identification of tumors, lesions, plaques, or areas of infarction.
Radioactive tracers are used to visualize specific regions in the brain.
Cerebral Angiography: Employing X-rays with dye to locate obstructions leading to clots responsible for strokes.
Ultrasound: Can track blood flow through arteries supplying the brain.