Autonomic Nervous System & Higher-Order Function III
Overview of Higher-Order Functions
• Higher-order functions describe complex brain activities that go beyond basic reflexes or simple sensory-motor loops.
• They involve widespread networks that include—but are not limited to—the cerebral cortex, limbic structures (especially hippocampus & amygdala), thalamus, hypothalamus, basal nuclei, cerebellum, and the reticular activating system (RAS).
• Processing can be either conscious (explicit, declarative) or unconscious (implicit, procedural) and may shift from one to the other with practice (e.g., learning to drive).
• These functions are plastic: synaptic strength, connectivity, and even white-matter architecture can change with experience, injury, or therapy.
• Classic examples: memory & learning, regulation of sleep/arousal/alertness, decision making, language, reasoning, moral judgments, and creativity.
Memory
Major Classifications
• Short-Term (Working) Memory:
– Limited capacity (≈7±2 items) and duration (seconds–minutes).
– Maintained by sustained neuronal firing in prefrontal and parietal circuits.
– Vulnerable to interruption, fatigue, or interference.
• Long-Term Memory: subdivided into secondary and tertiary stores.
– Secondary (recent) memory: hours to weeks; susceptible to disruption.
– Tertiary (remote) memory: months to lifetime; comparatively stable.
• Memory Consolidation: the process that converts short-term traces into long-term representations, typically during sleep.
Anatomical Locations & Specializations
• Cerebral Cortex: distributes long-term declarative (fact) memory across association areas.
• Hippocampus (medial temporal lobe): necessary for consolidation; lesions cause anterograde amnesia (e.g., patient H.M.).
• Amygdala: attaches emotional salience; modulates strength of consolidation (e.g., fear learning).
• Premotor & Supplementary Motor Cortex, Cerebellum: house motor (skill) memories—riding a bike, typing.
• Occipital & Temporal Association Areas: specialize in faces (fusiform gyrus) and voices; damage yields prosopagnosia or phonagnosia.
• Modality-Specific Association Areas: tactile, gustatory, olfactory memories are stored near their respective sensory cortices—“the smell of cookies” triggers somatosensory, gustatory, and limbic circuits.
Physiological Mechanisms
• Synaptic Facilitation: repetitive stimulation keeps elevated, boosting vesicle release and EPSP amplitude.
• Long-Term Potentiation (LTP): NMDA-receptor–dependent insertion of receptors; hallmark of hippocampal plasticity.
• Structural Remodeling:
– Growth of new dendritic spines & synapses.
– Pruning of unused connections ("use it or lose it").
– Activity-dependent myelin thickening to speed conduction.
• Retrograde Firing: in rare circuits, post-synaptic depolarization can send messages “backwards,” tuning the presynaptic neuron.
• Neurotransmitter Modulation: sustained release of glutamate, acetylcholine, dopamine, or can mark a circuit as important, enhancing consolidation.
Factors Enhancing Consolidation
• Repetition/Practice: reactivates hippocampal-cortical loops.
• Emotional Arousal: amygdala releases epinephrine, cortisol; evolutionary advantage for remembering threats.
• Sleep: hippocampal sharp-wave ripples replay recent experiences to cortex (slow-wave stages 3–4).
• Contextual Diversity: retrieving information in multiple settings strengthens recall cues.
Sleep, Arousal, and Consciousness
Functional Spectrum
• Consciousness is not binary; ranges from deep coma → deep sleep → light sleep → quiet wakefulness → full attention.
• Brain maintains a “preconscious buffer” that tracks environment (e.g., hearing your name across a noisy room—Cocktail-party effect).
Sleep Architecture
• Non-REM (NREM) Sleep: stages 1–4; progressively lower cortical activity, ↓heart rate, ↓BP, ↓respiratory rate, ↓core temperature. Stage 3–4 (slow-wave) critical for memory consolidation & growth hormone release.
• REM (Paradoxical) Sleep: EEG resembles waking, vivid dreams, skeletal muscle atonia (glycinergic inhibition in spinal cord prevents acting out dreams). High limbic activity, frontal inhibition (explains bizarre dream logic).
• Typical 90-min cycles: NREM → REM, repeated 4–6× per night; REM periods lengthen toward morning.
Neurochemical Switches
• Melatonin (pineal gland): secreted when retinal photoreceptors report low luminance. Inhibits RAS, promotes sleep.
• Reticular Activating System (RAS): diffuse reticular formation projections through thalamus to cortex.
– Activated by norepinephrine and histamine (from hypothalamic tuberomammillary nucleus) → wakefulness.
– Inhibited by GABAergic ventrolateral preoptic nucleus (VLPO) → sleep.
• Adenosine accumulates with prolonged wakefulness; binds A1 receptors in basal forebrain, dampening RAS activity (caffeine blocks these receptors).
Practical Implications
• Sleep deprivation impairs frontal-lobe executive functions—planning, moral judgment, error detection.
• Shift-work disorder & jet lag result from circadian misalignment; associated with metabolic syndrome, depression, and cancer risk.
• REM behavior disorder may presage Parkinson’s disease (α-synuclein pathology spreads from brainstem upward).
Reticular Activating System (RAS)
• Anatomy: interlaced gray-matter columns in brainstem (midbrain → pons → medulla) surrounding the reticular formation.
• Inputs: visual (optic tract collaterals), auditory (cochlear nuclei), somatosensory (spinoreticular), vestibular, cerebellar, and limbic.
• Outputs: ascends via intralaminar nuclei of thalamus & basal forebrain to almost every cortical region; descends to spinal cord to influence posture & reflex tone.
• Neurotransmitters & Effects:
– (locus coeruleus) → vigilance, attention.
– Serotonin (raphe nuclei) → mood, onset of sleep.
– Acetylcholine (pontine tegmentum) → cortical desynchronization (wake & REM).
– Dopamine (ventral tegmental area) → motivation, reward-based alertness.
• Clinical note: lesions → coma; overactivation → insomnia, anxiety.
Connections to Earlier Material & Real-World Relevance
• Autonomic changes during sleep align with earlier discussion of sympathetic/parasympathetic balance (e.g., parasympathetic predominance in deep sleep).
• Memory deficits in Alzheimer’s correlate with hippocampal atrophy and cholinergic loss—emphasizing neurotransmitter roles described above.
• PTSD exemplifies amygdala-driven overconsolidation of fear memories; therapeutic strategies (e.g., propranolol) target -adrenergic signaling to weaken reconsolidation.
• Learning a motor skill (e.g., playing a sport) demonstrates cortical-cerebellar cooperation; sleep enhances procedural gains—athletes and musicians schedule practice to leverage this.
Ethical & Philosophical Considerations
• Manipulating memory (optogenetics, pharmacological blockers) raises questions of personal identity and consent.
• Enhancement drugs (modafinil, amphetamines) vs. fair competition and societal pressure.
• The definition of consciousness bears on end-of-life decisions (coma vs. vegetative vs. minimally conscious states).
Key Take-Away Equations & Data Snippets
• Hebbian Rule (conceptual, not formalized in slides): “Cells that fire together, wire together.”
• Capacity of Working Memory: items (Miller’s law).
• 90-Minute Sleep Cycle: empirical average; individual cycles range .
Study Hacks & Mnemonics
• HIPPO wears a CAP: HIPPOcampus for Consolidation of Associative & Place memories.
• RAS = Radio Antenna System: keeps the “station” (cortex) tuned in.
• STEPS to Memory: Sleep, Time (spacing), Emotion, Practice, Structure (organize information).
• Before exams, aim for at least full slow-wave cycles the night prior to lock in material.