Module 6: Brain Regions and Structures
Module 6: Brain Regions and Structures — Major Divisions of the Vertebrate Brain
Learning Objective (LOQ 6-1): What are the hindbrain, midbrain, and forebrain?
Three Primary Structural Divisions:
Hindbrain: Contains fundamental brainstem structures directing autonomous, life-sustaining functions such as respiration, sleep cycles, wakefulness, motor coordination, and physical balance.
Midbrain: Sits directly atop the brainstem, functioning as a structural and functional conduit connecting the hindbrain to the forebrain; coordinates selective motor actions and transmits visual and acoustic sensory streams.
Forebrain: The highest and largest division, managing advanced cognitive processing, associative functions, complex sensory integration, and conscious voluntary motor control.
Evolutionary Specialization and Environmental Pressures:
Natural selection shapes nervous systems according to distinct survival needs (Cesario et al., 2020).
Human Forebrain Specialization: Humans possess disproportionately large, well-developed forebrains that allow for high-level abstraction, analytical reasoning, planning, and intricate decision-making.
Predatory Shark Hindbrain Specialization: Sharks feature an exceptionally complex hindbrain tailored for dynamic equilibrium, rapid underwater maneuvering, and agile pursuit of evasive prey (Yopak et al., 2010).
FIGURE 6.1 — Brain Divisions: Forebrain, Midbrain, Hindbrain:
Overview: Illustrates the hierarchical organization where the brainstem (pons and medulla) extends from the spinal cord, the thalamus attaches to the top of the brainstem, and the reticular formation threads through both.
The Brainstem and Essential Survival Functions
Learning Objective (LOQ 6-2): What structures make up the brainstem, and what are the functions of the brainstem, thalamus, reticular formation, and cerebellum?
Core Architecture:
The brainstem is the brain's innermost and phylogenetically oldest core, beginning where the spinal cord swells as it enters the skull.
Manages vital homeostatic functions automatically, without conscious intervention.
Medulla (Medulla Oblongata):
Located at the base of the brainstem as a continuation of the spinal cord.
Directly controls vegetative survival reflexes: breathing pacing and cardiovascular heart rhythm.
Patients with extensive cortical damage can maintain cardiorespiratory survival provided the medulla remains undamaged.
Pons:
Sits immediately above the medulla.
Serves as an integrated relay station that coordinates movement across muscle groups and participates in sleep regulation and arousal states.
Contralateral Cross-Wiring (The Crossover Point):
The brainstem acts as the major crossover nexus where nerve fibers connect each side of the brain with the contralateral (opposite) side of the body.
Left hemisphere tracts innervate the right side of the body; right hemisphere tracts govern the left.
FIGURE 6.2 — The Body's Wiring:
Shows the crossover point in the brainstem where ascending and descending pathways switch sides.
Decerebration and Purposeful Action (Klemm, 1990):
Surgically isolating a cat's brainstem from superior structures (midbrain and forebrain) leaves vegetative functions intact (breathing, running, climbing, self-grooming).
However, the animal loses teleological agency and will not execute purposeful actions to locate food.
The Thalamus: Sensory Control Center
Location and Structure:
Paired, egg-shaped structures belonging to the forebrain that sit directly on top of the brainstem.
Derived from the Greek term for "inner chamber."
Sensory Hub Function:
Operates as the central routing switchboard for all incoming sensory inputs, except smell (olfaction).
Dispatches visual, auditory, gustatory (taste), and tactile signals to dedicated cerebral cortical zones.
Descending Feedback Loop:
Receives descending feedback from cortical regions and redirects these instructions down to the medulla and the hindbrain's cerebellum.
The Rail Network Analogy: Acts like Seoul in South Korea's train system: an essential transit hub through which traffic converges before dispersing to various destinations.
The Reticular Formation: Sensory Filtering and Arousal
Anatomy:
A netlike ("reticular") nerve network extending inside the brainstem from the upper spinal cord directly through the thalamus, positioned between the ears.
Sensory Filtering and Multitasking:
Continuously monitors incoming sensory streams from the spinal cord, filtering out background noise and routing critical inputs to relevant brain areas (Wimmer et al., 2015).
Control of Consciousness and Arousal (Moruzzi & Magoun, 1949):
Stimulation: Direct electrical stimulation of a sleeping cat's reticular formation produced instant alertness and waking state.
Transection: Severing the reticular formation while sparing parallel sensory tracts plunged the cat into a permanent, irreversible coma.
The Cerebellum: Agility, Motor Memory, and Nonconscious Processing
Structure and Cellular Density:
Positioned at the rear of the brainstem; nicknamed the "little brain" because of its two miniature wrinkled hemispheres.
Contains more than half () of the brain's total neurons despite occupying a compact cranial footprint.
Voluntary Movement and Balance:
Coordinates smooth, dynamic motor agility in conjunction with the pons (e.g., Naomi Osaka returning a tennis serve, soccer ball footwork).
Maintains posture, somatic equilibrium, and kinetic balance.
Nonverbal Learning and Conditioning:
Cooperates with the basal ganglia (deep forebrain structures involved in movement) to enable nonverbal procedural conditioning and motor skill memory.
Vulnerability to Alcohol and Lesions:
Sensitive to alcohol intoxication, causing uncoordinated, jerky, and exaggerated movements.
Lesions prevent delicate tasks such as texting, typing, balanced walking, or playing the guitar.
Implicit Cognitive and Temporal Tracking:
Operates nonconsciously to estimate elapsed time (e.g., judging reading duration), register subtle tactile sensations (e.g., tight vs. loose clothing), and modulate emotional tone.
FIGURE 6.3 — The Brain's Organ of Agility:
Illustrates the cerebellum coordinating motor dynamics.
The Limbic System: Emotion, Drives, and Memory
Learning Objective (LOQ 6-3): What are the limbic system's structures and functions?
System Architecture:
Situated primarily within the forebrain, bordering the older brainstem and newer cerebral hemispheres.
Drives primal survival urges, emotional responsiveness (such as appreciating humor: "A skeleton walks into a café… 'I'll take a latte and a mop'"), and explicit memory consolidation.
FIGURE 6.4 & FIGURE 6.5 — The Limbic System and Brain Overview:
Depicts the interconnected loop of the amygdala, hypothalamus, and hippocampus, and their spatial relation to the pituitary gland and cortex.
The Amygdala: Aggression, Fear, and Social Threat
Structure: Two almond-shaped neural clusters resting deep inside the temporal lobes.
Ablation Studies:
Klüver & Bucy (1939): Surgical excision of the amygdala in an aggressive rhesus monkey transformed it into a gentle, mellow subject.
Confirmed in wild lynxes, wolverines, and wild rats.
Human Pathology: A patient with congenital destruction of both amygdalae displayed complete fearlessness when exposed to venomous snakes, public speaking, or direct gunpoint threats (Feinstein et al., 2013).
Volumetric Correlation: Smaller amygdala volumes correlate with reduced physiological arousal to perceived threats ("little amygdala, little fear") (Foell et al., 2019).
Electrode Stimulation:
Stimulating one specific locus in a domestic cat provokes an acute attack posture (arched back, dilated pupils, hissing, piloerection).
Moving the electrode slightly induces stark terror, causing the cat to cower before a mouse.
Social Threat and Criminality:
Amygdala damage reduces defensive wariness toward unfamiliar people (Harrison et al., 2015).
Abnormal structure and function correlate with psychopathy and criminal violence (Dotterer et al., 2017; Ermer et al., 2012).
Activated by angry faces rather than happy faces (Mende-Siedlecki et al., 2013); negative emotional events heighten memory consolidation (Admon et al., 2018).
Car Battery Metaphor: Removing a car's battery stops the vehicle, but the battery alone does not drive the car; fear and rage depend on extensive interconnected neural circuits.
Figure Reference (Amygdala and Hypothalamic Imaging):
The Hypothalamus: Homeostasis, Endocrine Control, and Reward Systems
Anatomy and Homeostatic Maintenance:
Located below ("hypo") the thalamus.
Monitors blood chemistry and temperature to coordinate hunger, satiety, osmotic thirst, and sexual arousal.
Neuroendocrine Feedback Loop:
Cortical thoughts regarding sexual stimuli trigger the hypothalamus to secrete releasing hormones.
These hormones prompt the pituitary gland (the endocrine system's "master gland") to direct sex glands (gonads) to release hormones into the bloodstream.
Circulating hormones re-enter the brain and amplify cortical sexual cognitions.
Dynamic principle: The brain influences the endocrine system, which in turn influences the brain.
Discovery of Reward Centers (Olds & Milner, 1954):
Discovered accidentally while trying to place an electrode into a rat's reticular formation (Olds, 1975).
Rats pressed levers to self-stimulate hypothalamic sites over times per hour.
Loci like the nucleus accumbens were confirmed in dolphins and monkeys (Hamid et al., 2016).
Search-and-Rescue Navigation (Talwar et al., 2002):
Researchers steered rats carrying miniature backpacks (receiver, camera, power source) using laptop pulses delivering reward cues to turn, climb trees, and traverse rough terrain.
Human Reward Circuitry:
Stimulated by positive social interactions and affirming messages (Inagaki et al., 2019; Zerubavel et al., 2018).
Human electrical stimulation produces mild pleasure and motivational desire rather than wild rodent-like frenzy (Deutsch, 1972; Hooper & Teresi, 1986).
Wanting vs. Liking: Stimulation triggers dopamine-driven craving/desire rather than hedonic sensory satisfaction (Kringelbach & Berridge, 2012).
Elevating dopamine increases musical "chills"; receptor antagonists diminish musical pleasure (Ferreri et al., 2019).
Reward Deficiency Syndrome: Malfunctions in pleasure circuits drive self-medicating behaviors (substance use, compulsive eating, aggressive outbursts) (Blum et al., 1996, 2014; Chester et al., 2016).
Candace Pert (1986): Evolutionarily wiring essential survival behaviors (eating, sex) as naturally reinforcing guarantees species survival.
Figure Reference (Hypothalamus and Reward Imaging):
The Hippocampus: Explicit Memory Processing and Vulnerability
Structure and Role:
A curved, seahorse-shaped structure within the medial temporal lobes.
Consolidates and indexes explicit, conscious memories of facts (semantic) and life events (episodic).
Damage and Neurotrauma:
Surgical excision or bilateral trauma blocks the formation of new declarative memories (Clark & Maguire, 2016).
Pediatric brain tumor survivors struggle to acquire new declarative memories in adulthood (Jayakar et al., 2015).
Normal aging leads to progressive shrinkage and cognitive decline (O'Callaghan et al., 2019).
Sports Concussions and Chronic Traumatic Encephalopathy (CTE):
Post-mortem neuropathological autopsies on deceased NFL football players revealed that had neurodegenerative damage linked to head trauma (Mez et al., 2017).
NFL player Aaron Hernandez (#81) displayed advanced stage degeneration at autopsy following suicide at age while imprisoned for murder (Kilgore, 2017).
Modern protective strategies involve reinforced helmets and sideline real-time imaging systems (Canadian Press, 2018).
Figure Reference (Hippocampus Studies):
The Cerebral Cortex: Structure, Geometry, and Lobes
Learning Objective (LOQ 6-4): What four lobes make up the cerebral cortex, and what are the functions of the motor cortex, somatosensory cortex, and association areas?
Gross Characteristics:
The cerebrum constitutes of total brain weight, composed of two paired hemispheres that enable conscious perception, speech, and thought.
The cerebral cortex is the thin, wrinkled outer surface of interconnected neural cell bodies covering the white matter like bark on a tree.
Topography and Complexity:
Wrinkled surface (gyri and fissures) saves cranial volume; flattened out, it would equal the size of a large pizza ( its folded surface area).
Sheath contains to billion neurons and roughly trillion synaptic connections (de Courten-Myers, 2005).
The Four Lobes (FIGURE 6.6):
Frontal Lobes: Behind forehead; executive functions, planning, forward judgment, voluntary movement, emotional inhibition.
Parietal Lobes: At top rear; registers touch, body position, spatial reasoning, and mathematics.
Occipital Lobes: Posterior base of skull; primary visual processing.
Temporal Lobes: Bilateral, above ears; auditory processing, linguistic comprehension, face recognition.
Motor Functions of the Cerebral Cortex
Discovery of the Motor Cortex (Fritsch & Hitzig, 1870):
Applied mild electrical stimulation to dog cortices and elicited muscular movements.
Located at the posterior border of the frontal lobes spanning ear-to-ear, governing contralateral movement.
Topographic Cortical Mapping (Foerster & Penfield, 1930s):
Mapped awake neurosurgical patients (brain tissue lacks pain receptors).
Cortical Allocation: Cortical territory reflects the degree of motor control and precision required, not physical body mass (e.g., fingers, thumbs, and lips occupy enormous cortical areas; the torso or upper arm occupies very little).
José Delgado Demonstration (1969):
Stimulated the hand region of a patient's left motor cortex, forcing the right hand into a fist despite the patient trying to keep it open: "I guess, Doctor, that your electricity is stronger than my will."
Predictive Trajectory and Social Processing:
Neural firing profiles predict monkey arm movement milliseconds before execution (Livi et al., 2019).
Neurons fire differently for social acts (handing an object to an experimenter) versus nonsocial acts (putting an object in a container or mouth) (Coudé et al., 2019).
FIGURE 6.7 — Motor and Somatosensory Homunculus:
Demonstrates that fine control and sensitive skin surfaces have enlarged representations.
Brain-Machine Interfaces and Cognitive Neural Prosthetics
Decoding Thought Plans:
Microelectrode arrays capture cortical electrical patterns, translating motor intentions into external output (Andersen et al., 2010; Rajangam et al., 2016).
Paralyzed patients control television channels, move computer cursors, and play games using an aspirin-sized chip with microelectrodes (Hochberg et al., 2006; Clausen et al., 2017).
Ian Burkhart Case Study (Schwemmer et al., 2018; Wood, 2018):
Paralyzed in all four limbs (quadriplegia) at age .
Implants in his motor cortex paired with machine-learning algorithms decoded hand movement intentions, bypassing his severed spinal cord to stimulate a muscle sleeve on his forearm.
Accomplished grasping bottles, pouring liquids, holding sticks, and playing Guitar Hero.
Robotics and Sensory Feedback:
Adding artificial tactile feedback sensations to robotic limbs improves patient movement precision (Flesher et al., 2021).
Vocal neuroprosthetics decode vocal tract motor planning to generate intelligible synthesized speech (Anumanchipalli et al., 2019).
FIGURE 6.8 — Brain-Machine Interaction:
Microelectrodes in the motor cortex and limb muscles allow a quadriplegic patient to drink coffee (Ajiboye et al., 2017).
Sensory Functions of the Cerebral Cortex
Somatosensory Cortex:
Positioned parallel to and directly behind the motor cortex at the front of the parietal lobes.
Registers touch, temperature, pressure, and proprioception.
Sensitive body zones claim large cortical maps: humans have huge lip areas (kissing instead of touching toes); rodents devote large areas to whiskers; owls devote large areas to hearing.
Visual Cortex (Occipital Lobes):
Located at the rear of the skull; direct stimulation produces flashes of light and color.
Contralateral processing: Damage to the right occipital lobe causes functional blindness across the left visual fields of both eyes.
LSD Neuroimaging (Carhart-Harris et al., 2016): fMRI reveals that LSD induces vivid hallucinations by dramatically increasing communication between the visual cortex and rest of the brain.
FIGURE 6.9 & 6.10 — Seeing Without Eyes and Cortical Receiving Areas:
Auditory Cortex (Temporal Lobes):
Sits above the ears in the superior temporal lobes, receiving sound signals primarily from the contralateral ear.
Schizophrenia Hallucinations: Auditory hallucinations activate temporal lobe auditory zones in fMRI scans (Lennox et al., 1999).
Tinnitus: Phantom ear ringing correlates with compensatory hyperactivity in the contralateral temporal lobe (Muhlnickel, 1998).
Association Areas and Higher Mental Functions
Learning Objective (LOQ 6-5): Is it true that of our brain isn't really used?
The Ten Percent Myth Debunked:
Primary motor and sensory zones occupy roughly of the cerebral cortex.
The remaining comprises association areas responsible for synthesizing memories, attention, and executive reasoning.
Probing them does not yield simple twitches or flashes, misleading public surveys into assuming that of brain tissue is dormant (Furnham, 2018; Macdonald et al., 2017).
Prefrontal Cortex (Frontal Lobe Association Areas):
Directs forward thinking, planning, executive control, and social judgment (Panichello & Buschman, 2021).
Damage preserves basic intelligence scores and cooking steps, but abolishes the ability to plan ahead to bake or learn new recipes, leaving people indifferent to social commitments (MacPherson et al., 2016; Bault et al., 2019).
Frontal Lobe Lesions, Personality, and Morality:
Phineas Gage Case (1848; FIGURE 6.11): An explosive blasting accident propelled an iron tamping rod through his left cheek and frontal lobes (Van Horn et al., 2012).
Survived and spoke immediately, but his personality shifted from polite and efficient to profane, irritable, and dishonest ("no longer Gage") (Thiebaut de Schotten et al., 2015).
Later adapted to work as an international stagecoach driver (Macmillan & Lena, 2010).
Cecil Clayton Case (1972/2015): Lost of his left frontal lobe in a sawmill accident; IQ dropped to elementary levels with severe impulsivity; fatally shot a deputy sheriff in 1996 and was executed in 2015 at age 74 (Williams, 2015).
Loss of Moral Brakes: The frontal lobes steer behavior toward empathy and away from violence (Achterberg et al., 2020; Lieberman et al., 2019).
Parietal Lobe Association Areas:
Coordinates spatial reasoning and mathematical logic (Amalric & Dehaene, 2019; Wilkey et al., 2018).
Albert Einstein's brain showed exceptionally large, unique parietal association structures (Sandra Witelson, 2011).
Intention vs. Movement (Desmurget et al., 2009): Stimulating parietal association areas made awake patients feel an intense urge to move or falsely believe they had moved; stimulating motor areas elicited physical movements without conscious awareness.
Temporal Lobe Association Areas and Facial Recognition:
Located on the underside of the right temporal lobe (fusiform area) (Retter et al., 2020).
Prosopagnosia: Damage spares the ability to describe features, age, or gender, but prevents recognizing who the face belongs to (e.g., family members, celebrities).
Functional Connectivity vs. Modern Phrenology:
Complex mental operations are emergent properties of coordinated functional connectivity across distributed neural networks, not localized phrenological "hot spots" (Chein & Schneider, 2012; Bassett et al., 2018).
Disrupted network communication underpins many psychological disorders (J. Baker et al., 2019; Zhang et al., 2019).
Key Takeaways Explained Simply
The Three Main Levels:
The hindbrain and brainstem keep you alive (heartbeat, breathing, basic reflexes).
The limbic system keeps you motivated (feelings, fear, drives, rewards, and memories).
The cerebral cortex lets you think, speak, plan, and feel complex sensations.
Contralateral Control: The left side of the brain controls and feels the right side of the body, and vice versa, because nerve tracts cross over in the brainstem.
Brain Real Estate: The brain does not allocate territory based on how big a body part is. It gives the most space to areas needing delicate precision (fingers, mouth) and sharp sensitivity (lips).
No 10% Myth: You use of your brain. The three-quarters called "association areas" don't cause muscle twitches when touched, but they are constantly busy organizing thoughts, memories, language, and executive plans.
Basic Definitions Glossary
Hindbrain: The lower, evolutionarily oldest part of the brain that controls automatic survival jobs like breathing, sleep regulation, heartbeat, balance, and motor coordination.
Midbrain: The small central section resting on top of the brainstem that bridges the lower hindbrain with the forebrain, helping integrate sensory signals (vision and hearing) and basic movement.
Forebrain: The highest and largest brain division, including the thalamus, limbic structures, and cerebrum, responsible for complex thought, conscious emotion, sensation, and voluntary action.
Brainstem: The central structural stalk extending up from the spinal cord into the skull; made up of the medulla and pons, it runs essential vegetative life-support operations below conscious awareness.
Medulla [muh-DUL-uh]: The slight swelling at the base of the brainstem continuous with the spinal cord; directly regulates life-sustaining reflexes, specifically breathing rhythm and heart rate.
Thalamus [THAL-uh-muss]: The forebrain's egg-shaped sensory routing switchboard atop the brainstem; directs incoming sensory inputs (except smell) to cortical processing centers and passes feedback downward.
Reticular Formation: A netlike nerve network traveling up through the brainstem into the thalamus; acts as a sensory filter and regulates alert wakefulness and conscious arousal.
Cerebellum [sehr-uh-BELL-um]: The baseball-sized "little brain" at the rear of the brainstem; directs balance, dynamic movement coordination, posture, and nonverbal skill memories.
Limbic System: A donut-shaped group of subcortical structures (amygdala, hypothalamus, hippocampus) lying between the brainstem and cortex; mediates emotions, basic drives, and memory formation.
Amygdala [uh-MIG-duh-la]: Two almond-shaped clusters located deep in the temporal lobes; regulates threat detection, fear conditioning, vigilance, and aggressive defense reactions.
Hypothalamus [hi-po-THAL-uh-muss]: A limbic command hub located just below the thalamus; monitors internal body chemistry to regulate homeostasis (eating, drinking, temperature, sex) and instructs the pituitary gland.
Hippocampus: A curved, seahorse-shaped limbic structure in the temporal lobes; essential for processing, encoding, and consolidating conscious, explicit (fact and event) memories.
Cerebral [seh-REE-bruhl] Cortex: The thin, wrinkled outer gray-matter mantle covering the cerebral hemispheres; the control and information-processing center responsible for higher mental functions, speech, and perception.
Frontal Lobes: The anterior cortical lobes situated behind the forehead; responsible for motor control, expressive speech, forward planning, abstract reasoning, and impulse restraint.
Parietal [puh-RYE-uh-tuhl] Lobes: The top-rear cortical lobes lying behind the frontal lobes; house the somatosensory cortex and manage spatial navigation, mathematical thinking, and body positioning.
Occipital [ahk-SIP-uh-tuhl] Lobes: The lobes located at the back of the skull; house the primary visual cortex dedicated to receiving and processing visual information.
Temporal Lobes: The lobes positioned on the sides of the head above the ears; house the auditory cortex for hearing and specialized areas for language comprehension and facial recognition.
Motor Cortex: An arch-shaped strip of tissue at the back edge of the frontal lobes; directs contralateral voluntary muscle movements, allotting the most space to areas requiring fine motor precision.
Somatosensory Cortex: A cortical strip at the front edge of the parietal lobes, running parallel to the motor cortex; registers contralateral bodily touch, temperature, pressure, and movement.
Association Areas: Expansive regions occupying roughly three-quarters of the cerebral cortex that integrate sensory inputs with stored memories to power higher-order executive thinking, reasoning, and personality.