PSY Learning 21/9 - Chapter 2

Limbic System and Emotional Processing: Hippocampus and Amygdala

• System Overview:

  • The limbic system is responsible for emotional memory, processing specific subsets of emotion rather than all emotional experiences universally.

  • The two foundational cornerstones of the limbic system are the hippocampus and the amygdala.

• Hippocampus Anatomy and Nomenclature:

  • Humans possess two hippocampi: a left hippocampus and a right hippocampus.

  • Anatomical Location: Positioned approximately $1.5\,\text{inches}$ inward from the top of the ears.

  • Structure: Bilateral, C-shaped structures resembling ram's ears.

  • Etymology: Named after the Latin/Greek term for "seahorse." The term originates classically from descriptions of Poseidon's steeds in The Odyssey.

• Amygdala Anatomy and Nomenclature:

  • An amygdala branches off from the end of each hippocampus (left and right).

  • Etymology: "Amygdala" is Latin for "almond."

  • Size and Shape: Conveniently identical in size and shape to an almond.

• Olfactory and Neural Connections to Emotion:

  • The olfactory system is directly hardwired into the limbic system, allowing olfactory stimuli to evoke vivid emotional memories (e.g., past romantic partners, a grandmother at Thanksgiving, or the smell of Christmas Day).

  • Specific regions of the mouth are also directly wired into particular neural nuclei within the limbic structures.

  • While approximately eight or nine distinct structures comprise the broader limbic system, the hippocampus and amygdala remain its primary operational core.

• Functional Role of the Amygdala:

  • Functions as the brain's central alarm bell, registering fear-inducing and threatening stimuli unconsciously.

  • Triggers the autonomic fight-or-flight response.

  • Processes threats through dual circuits:

    • Unconscious circuit: Allows immediate, impulsive reactions (e.g., direct aggression or flight) when time does not permit conscious thought.

    • Conscious circuit: Sends information to the frontal lobes for deliberate behavioral planning (e.g., evaluating how to handle a spider in the shower).

  • Behavioral Responses: Directs fight, flight, or freeze responses. Freezing is driven by amygdala-mediated motivation, as passive waiting usually carries severe survival risks.

  • Neurochemistry of Threat Processing:

    • Surges of epinephrine and norepinephrine motivate fight-or-flight behaviors.

    • Epinephrine functions primarily within peripheral body systems.

    • Norepinephrine operates primarily within the central nervous system (brain).

  • Role in Memory Construction:

    • The hippocampus processes and lays down objective factual information into long-term memory.

    • The amygdala explicitly encodes and attaches the emotional tone or affective component to the memory.

Non-Verbal Intent, Social Cues, and Intuitive Processing

• Social Intent Decoding via the Amygdala:

  • The amygdala decodes non-verbal social intent through physical proximity, body language, and tone of voice to evaluate potential interpersonal threats or romantic interest.

• Physical Indicators of Social and Romantic Interest:

  • Tactile Cues:

    • Touching the upper body or arm indicates crossing a personal space boundary due to romantic or interpersonal attraction.

    • Touching the leg indicates immediate romantic or sexual desire.

  • Directional Orientations:

    • Foot Orientation: Feet pointed directly toward a person signal interpersonal interest; feet turned away signal a desire to disengage or depart.

    • Belly Button Orientation: Orienting the torso and belly button directly toward someone indicates engagement, as it exposes vulnerable internal organs toward that individual.

• Visceral Intuition ("Vibes"):

  • Intuitive feelings or "vibes" represent subcortical processing via the amygdala circuit alerting the conscious mind to subtle threat signals before rational analysis registers them.

  • Case Study (Ted Bundy):

    • Serial killer Ted Bundy was widely perceived as attractive, wealthy, educated, smart, and charming.

    • A student's aunt was set up on a blind date with Bundy. Despite strong peer encouragement to go on the date based on his superficial merits, she experienced an intuitive negative visceral reaction ("bad vibes") and canceled the date, avoiding potential harm.

Hippocampal Memory Dynamics and False Memories

• Hippocampal Memory Operations:

  • Essential for encoding and retrieving conscious long-term memories.

  • Historical Model vs. Modern Neuroscience Model:

    • Historical Model: Memory was compared to a book stored on a library shelf, where retrieval involved removing the book, reading it, and returning it in an unchanged state.

    • Modern Model: Memory retrieval functions like opening a computer text file. Every time a memory is accessed, it must be saved and re-encoded back into long-term storage using the hippocampus.

• Mechanisms of False Memory Creation:

  • Because retrieval requires re-encoding via the hippocampus, subtle alterations are introduced upon every recall event.

  • Minor edits—such as altering word order, swapping word choices (e.g., replacing "for" with "such as"), or changing punctuation (changing a period to a comma)—permanently rewrite the stored memory file.

  • Accumulated re-encoding events over time lead to significant deviations from the original event, producing false memories.

• Neuropathology of Alzheimer's Disease:

  • Over $60$ distinct diagnosable dementias exist, with Alzheimer's disease accounting for approximately $70\%$ of all dementia cases.

  • Initial clinical presentation almost universally manifests as memory impairment because the disease process selectively attacks the temporal lobes and hippocampi first.

  • Neuroimaging reveals severe tissue destruction in the left and right hippocampi, where lost neural tissue is replaced by dark spaces filled with cerebrospinal fluid ($\text{CSF}$).

Conscious Long-Term Memory Circuits: The Papez Circuit

• Circuit Architecture:

  • Long-term conscious memory relies on three primary circuits, chief among which is the Papez circuit.

  • Consists of six interconnected anatomical components divided into three input structures and three output structures.

• Input Sequence (Short-Term to Long-Term Processing):

  1. Cingulate Cortex: Acts as an environmental sensory "antenna" within the limbic system that detects anomalies or unexpected environmental changes (e.g., noticing an open window upon returning home).

  2. Parahippocampal Tissue: Cortical tissue located immediately adjacent to and running alongside the hippocampus.

  3. Entorhinal Cortex: Specialized tissue positioned between the hippocampus and the olfactory system ("ento" meaning within, "rhino" referencing the nose).

  4. Hippocampus: Formulates and converts short-term representations into raw long-term memory structures (does not store memories permanently).

• Output Sequence (Consolidation and Circuit Closure):

  1. Fornix: A primary fiber pathway containing approximately $1 \times 10^6$ ($1\,\text{million}$) neurons, equivalent in diameter to the optic nerve. Carries output signals away from the hippocampus.

  2. Mammillary Bodies of the Hypothalamus: Receives signals from the fornix and attaches chronological timeline context to the memory.

  3. Anterior Nucleus of the Thalamus: Receives signals from the mammillary bodies and initiates memory consolidation (the setting or solidifying process).

  4. Cingulate Cortex: Receives output from the anterior thalamic nucleus, closing the circuit loop and routing processed memory to designated cortical storage regions.

• Clinical Case Study (Patient NA):

  • Patient NA suffered a penetrating trauma from a fencing foil that traveled directly into the anterior nucleus of his thalamus.

  • Although his hippocampus, fornix, and mammillary bodies remained intact, the destruction of the anterior thalamic nucleus prevented memories from solidifying or consolidating, resulting in severe anterograde amnesia.

Neural Memory Consolidation and REM Sleep

• Role of Sleep in Memory Consolidation:

  • Memory consolidation requires Rapid Eye Movement (REM) sleep to finalize the structural setting of information processed through the anterior nucleus of the thalamus.

  • Sleep deprivation (e.g., pulling an "all-nighter") disrupts REM sleep, preventing memory consolidation and leaving stored information in an unorganized, fluid state ("hot soup").

Interhemispheric Communication: Corpus Callosum and Clinical Implications

• Anatomy of the Corpus Callosum:

  • A massive subcortical band of myelinated white matter fibers located approximately $1.5\,\text{inches}$ deep within the longitudinal fissure beneath the cerebral cortex.

  • Etymology: "Corpus" translates to body; "callosum" translates to colossal or huge.

  • Structural Subdivisions:

    • Genu: The anterior portion, named after the Latin term for a "bent knee" (derived from the posture of genuflection).

    • Splenium: The posterior portion.

  • Primary Function: Facilitates direct interhemispheric communication between the left and right cerebral hemispheres.

• Attention-Deficit/Hyperactivity Disorder (ADHD) Neuroanatomy:

  • Corpus Callosum Morphometry: Structural MRI studies demonstrate that individuals with ADHD present with a smaller corpus callosum overall, with volumetric reductions localized to the genu, the splenium, or both regions.

  • Basal Ganglia Integrity: Children with ADHD display a volumetric reduction and decreased cerebral blood flow within the right basal ganglia.

  • Prefrontal Asymmetry:

    • Neurotypical Brain: The right prefrontal cortex is structurally larger than the left prefrontal cortex.

    • ADHD Brain: Structural symmetry is observed (meaning the right prefrontal area is pathologically smaller than normal), or the right prefrontal lobe is smaller than the left.

    • Because the right prefrontal cortex allocates attention and sustains mental focus, structural reductions in this area and its interhemispheric callosal connections disrupt attentional control.

Cortical Architecture and Functional Lobellar Specialization

• General Cortical Structure:

  • The cerebral cortex forms the outer mantle of the brain and is organized into four major lobes.

  • Sulci and gyri (folds and grooves) maximize surface area within the cranial cavity.

• Functional Specialization of the Four Lobes:

  • Frontal Lobes:

    • Comprises approximately $\frac{1}{3}$ ($33.3\%$) of total cortical tissue, exceeding a symmetric quarter-share ($25\%$).

    • Serves as the executive center ("CEO") of the central nervous system.

    • Houses the prefrontal cortex, which governs executive functions: working memory, short-term memory storage, complex problem-solving, goal-directed task execution, organization, sequencing, and selective attention allocation.

  • Parietal Lobes:

    • Processes somatosensory and perceptual inputs, including taste and touch.

    • Governs spatial navigation and orientation through physical environments.

    • Directs procedural learning and motor/muscle learning (e.g., executing motor programs required for driving an automobile).

  • Temporal Lobes:

    • Stores conscious, long-term declarative memories and factual information.

    • Houses schemas: mental organizational frameworks or categorical files used to classify concepts (e.g., categorizing music into pop, rock, country, or hip-hop; categorizing footwear into boots, sandals, sneakers, high heels, flip-flops, wedges, mules, clogs, or ankle boots).

    • Relates to node-and-map distributed representations of semantic knowledge (as described by David Rumelhart).

    • Secondary Alzheimer's Symptom: Loss of schema frameworks leads to anomia (inability to name objects). Patients misname objects using adjacent files in the same categorical framework (e.g., referring to a bottle as a "small cup, glass, or jar").

  • Occipital Lobe:

    • Dedicated to processing visual inputs.

    • Facilitates visual learning, observational learning, and reading comprehension.

Comparative Neuroanatomy: Human vs. Non-Human Brains

• Dolphin Brain Morphology:

  • Dolphins possess large, highly convoluted brains with two prominent cerebral hemispheres that are physically far more separated than human hemispheres.

  • Display unihemispheric slow-wave sleep: one cerebral hemisphere sleeps at a time while the opposite hemisphere maintains continuous swimming and navigation.

  • Possess a proportionately massive cerebellum compared to humans to support continuous locomotion in aqueous environments.

• Comparative Structures:

  • Most non-human vertebrates possess a dramatically larger cerebellum relative to cerebral cortex volume due to constant physical movement.

  • An elephant's brain is visually shaped like a loaf of French bread.

Neuronal Structure, Synaptic Transmission, and Microscopic Anatomy

• Foundational Units:

  • Neurons are specialized cell types responsible for generating all cognition, emotion, and physical behavior.

  • Neurotransmitters act as the chemical language of the brain, mediating signal transmission between neurons and body tissues.

• Anatomical Components of a Neuron:

  1. Soma (Cell Body):

     • Contains standard cellular machinery: the cell nucleus, DNA, and mitochondria.

  2. Dendrites:

     • Etymology: Derived from the Greek "dendro," meaning tree (as seen in rhododendron).

     • Branching arborizations lined with specialized receptor proteins designed to capture chemical neurotransmitter messages released into the synaptic cleft.

     • Form complex pathways containing $10,000$ to $50,000$ ($10\text{k} - 50\text{k}$) interconnected neurons (e.g., dopamine or serotonin pathways).

     • Potential Brain Connectivity: The human brain contains more potential dendritic connections than there are stars in the observable universe. The potential connection value is $1 \times 10^{15}$ ($1\,\text{quadrillion}$).

     • Scale Visualization of Time Intervals:

       • $1 \times 10^6$ ($1\,\text{million}$) seconds = $11\,\text{days}$..

       • $1 \times 10^9$ ($1\,\text{billion}$) seconds = $31\,\text{years}$..

       • $1 \times 10^{12}$ ($1\,\text{trillion}$) seconds = $31,000\,\text{years}$..

       • $1 \times 10^{15}$ ($1\,\text{quadrillion}$) seconds = $31,000,000\,\text{years}$..

  3. Axon:

     • A single elongated fiber extending from the soma that propagates electrical impulses (action potentials) away from the cell body.

     • Axon lengths range from less than $1\,\text{mm}$ inside subcortical brain structures to $3\,\text{feet}$, $5\,\text{feet}$, or $6\,\text{feet}$ in peripheral sensory and motor nerves.

  4. Axon Terminals (Synaptic Terminals / Terminal Buttons):

     • Distal terminal expansions containing membrane-bound vesicles filled with neurotransmitter molecules.

     • Arrival of an electrical impulse triggers membrane fusion and vesicular release of neurotransmitters across the synapse to adjacent dendritic receptors.

Axonal Transport, Myelination, and Developmental Neurobiology

• Myelin Sheath Architecture:

  • Myelin is a thick, fatty, white fibrous membrane composed of specialized glial cells wrapped concentrically around axons.

  • Neuronal cell bodies lack myelin and constitute gray matter; myelinated axons constitute white matter.

• Saltatory Conduction and Speed Optimization:

  • Myelin insulation prevents electrical charge leakage.

  • Nodes of Ranvier: Periodic uninsulated gaps along the myelinated axon identified by Louis-Antoine Ranvier.

  • Action potentials hop rapidly from node to node via saltatory conduction.

  • Increases neural conduction velocity from an unmyelinated baseline of $2\,\text{mph}$ up to $200\,\text{mph}$.

  • Enables near-instantaneous sensorimotor processing (e.g., stepping on a sharp shell barefoot on a beach triggers sensory transmission up a $5\,\text{foot}$ nerve and motor return within milliseconds).

• Developmental Myelination Sequence:

  • Myelination occurs predominantly postnatally in a strict posterior-to-anterior (back-to-front) spatiotemporal pattern.

  • Dietary Requirement: Infants and young children require whole fat milk until age $6$ to supply essential lipids for glial myelin synthesis.

  • Functional Milestones Maturation:

    • Visual Cortex: Myelinated during early infancy, transitioning vision from blurry uncoordinated input to focused clarity within weeks, followed by depth perception within months.

    • Motor Cortex: Myelinated around $1\,\text{year}$ of age, enabling independent bipedal walking.

    • Broca's Area: Myelinated subsequently, enabling articulate speech production.

    • Prefrontal Cortex: Begins myelination around age $3$, but requires an additional $20\,\text{years}$ to complete, fully maturing between $25\text{ to } 30\,\text{years}$ of age.

• Administrative Announcement:

  • The upcoming course examination is formally scheduled for Wednesday, September 30.