9/2 Central Nervous System: Brain Anatomy, Cortical Organization, and Split-Brain Findings

Central Nervous System: Brain Anatomy, Cortical Organization, and Split-Brain Findings

  • Recap from last class

    • Nervous system built from neurons; humans have about N ext{ neurons} \ N \in [86, \, 100{,}000{,}000{,}000] in the body (brain, spinal cord, and connections to muscles and organs).
    • Neurons operate reflexively at the cellular level; when linked, networks produce reflexes across groups of neurons.
    • Bridging from neurons to the whole nervous system, we focus today on the central nervous system (CNS), with a note that the peripheral nervous system (PNS) will be covered on Thursday.

  • What is the central nervous system (CNS)?

    • Nicely named: CNS = the part of the nervous system in the center, encased in bone (brain within the skull; spinal cord within the vertebral column).
    • The CNS is the seat of our highest-level thinking: reasoning, rational thought, and complex processing.
    • Distinction: CNS vs PNS; today we focus on the brain, with a plan to cover peripheral functions next class.

  • Major theme of today: functional specialization within the brain

    • Different parts of the brain do different things; organization matters for understanding how functions arise.

  • Major subdivision: brain stem vs. cerebral cortex

    • Brain stem
    • Located at the base of the brain, on top of the spinal cord.
    • Evolutionarily older: first to develop; responsible for basic life-sustaining functions (movement, survival).
    • Key components mentioned: medulla, thalamus (below), cerebellum (posterior).
    • Cerebral cortex (outer, newer layers)
    • Evolved more recently; associated with higher-level thinking (language, self-control, inhibition, decision making).
    • Outer layers on the cortex form the four lobes per hemisphere: frontal, parietal, occipital, temporal; total of 8 ext{ lobes} (four per hemisphere).
    • Even within a single person, development is ongoing: basic functions develop earlier; higher-order functions develop later. Brain maturation continues into early adulthood (roughly until age 22).
    • Example of late-developing ability: facial recognition (recognizing identities among similar faces) shows development into the early 20s.

  • The brain stem in more detail (briefly)

    • The brain stem is the first relay point for information entering the brain from the spinal cord.
    • Medulla: controls heartbeat and breathing; considered one of the most basic life-sustaining functions.
    • Thalamus: relays information to various brain regions; acts as a relay or ‘airport’ for information.
    • Airport metaphor: information enters via a common entry (the spinal cord); from the thalamus it’s directed to different destinations (different brain regions).
    • Cerebellum: involved in movement and balance; stores motor memories (the “recipes” for how to perform movements).
    • Pathway summary: cortex plans movement → cerebellum retrieves motor memories and refines → cerebellum signals through the spinal cord to activate muscles; the spinal cord executes the sequence.

  • The cerebral cortex: structure and organization

    • The cortex is divided into left and right hemispheres, separated by a longitudinal fissure. Each hemisphere contains four lobes:
    • Frontal lobe (blue): located at the front; involved in higher-order cognitive functions, planning, and motor control.
    • Parietal lobe (yellow): forms a saddle across the top; contains the somatosensory functions.
    • Occipital lobe (back): primarily visual processing.
    • Temporal lobe (pink): auditory processing and language-related areas.
    • Each hemisphere has a mirror image set of four lobes, yielding 8 ext{ lobes in total}. (Left and right sides mirror each other.)
    • Brain mapping helps us localize functions to regions; the theme is functional specialization across lobes.

  • Sensory and motor maps in the cortex

    • Visual processing
    • Occipital lobe (visual cortex) at the back of the brain processes visual information (colors, shapes, forms, motion).
    • Visual input travels from the retina through the optic pathways to the thalamus and then to the occipital cortex for processing.
    • If the occipital lobe is damaged, vision can be impaired (e.g., blindness), illustrating the role of this area in sight.
    • Hearing and language
    • Auditory cortex located near the temporal lobe processes sounds and language; language areas are adjacent to auditory areas in the left hemisphere.
    • Smell and taste
    • Olfactory (smell) and gustatory (taste) areas are anatomically close, helping explain why smell influences perceived taste.
    • Touch and movement (somatosensory and motor cortices)
    • A strip along the front edge of the parietal lobe is the somatosensory cortex (often called the sensory cortex).
    • Adjacent to it, along the precentral gyrus of the frontal lobe, lies the motor cortex.
    • The arrangement preserves the body’s layout in the cortex: the face and hands take disproportionately large cortical real estate due to high sensitivity and dexterity.
    • The mapping is called the homunculus (little man) due to the body layout representation in the cortex.
    • Cortical organization and body maps
    • In the sensory cortex (somatosensory), the face has a large representation; the hands have a substantial representation; the trunk and legs occupy smaller regions.
    • In the motor cortex, similar organization exists; these maps preserve the body’s layout, emphasizing the link between body sensitivity and cortical area.

  • Lateralization and contralateral control

    • Each hemisphere primarily controls the opposite side of the body (left brain controls the right side; right brain controls the left side).
    • Vision is sometimes discussed in terms of visual fields rather than eyes: the right visual field projects to the left hemisphere, and the left visual field projects to the right hemisphere.
    • Language and sequential/verbal processing are typically left-lateralized; pattern recognition and some visuospatial tasks are more right-lateralized.
    • Corpus callosum: a major bundle of neurons that coordinates information between the two hemispheres; ensures left and right hemispheres communicate.
    • Example linked to skills and development: children who learn piano before age 8 often develop a larger corpus callosum, improving left-right coordination; when the corpus callosum is severed in extreme epilepsy cases, left and right hemispheres operate more independently.

  • Split-brain findings and methods

    • Split-brain concept: severing the corpus callosum to prevent seizure activity from spreading between hemispheres.
    • Why this matters: this disruption reveals functional specialization, as information can be processed independently by each hemisphere with limited cross-hemispheric communication.
    • Classic split-brain observations (in patients) show that:
    • The left hemisphere tends to be better at language and verbal reports.
    • The right hemisphere tends to excel at nonverbal, visual-spatial tasks and pattern recognition.
    • Experimental approach in split-brain studies (conceptual):
    • A patient is presented with items flashed to the left or right visual field (or shown in each hemisphere via lateralized presentation) and asked to report or respond.
    • If an image is shown to the right visual field (processed by the left hemisphere), the patient can verbalize what was seen; if shown to the left visual field (processed by the right hemisphere), verbal reporting is often not possible, but the patient may express recognition nonverbally.
    • In the classroom demonstration, the instructor simulated split-brain tasks with intact individuals to illustrate hemispheric differences.

  • In-class demonstration (split-brain-like manipulation in intact individuals)

    • Setup: four volunteers participate in quick tasks involving reading lists of color words and naming colors.
    • Task 1 (word reading): read aloud a list of color words (e.g., black, red, blue, green) as fast as possible.
    • Measured times: e.g., t_1 = 11.43 \, ext{s} for the first trial.
    • Task 2 (color naming with ink): read colors of colored rectangles (only the ink color, not the word).
    • Measured times: e.g., t_2 = 10.57 \, ext{s} for the second trial.
    • Task 3 (interference condition): colored words printed in mismatched ink color (e.g., the word “blue” printed in green ink).
    • This condition produced the largest delay and the most errors, illustrating interference between automatic reading (left-hemisphere processing) and color naming (often right-hemisphere processing needing cross-hemispheric transfer).
    • Explanation of the results
    • The fastest condition (Task 1) involves reading the word and generating the spoken word via the left hemisphere directly.
    • Task 2 is often slightly slower due to color processing in the right hemisphere requiring a callosal transfer to produce the spoken word.
    • Task 3 shows the strongest interference: the automatic processing of the word (left hemisphere) competes with the color naming (right hemisphere) and must be transferred across the corpus callosum to produce the correct spoken response; the conflict yields mistakes and slower responses.
    • Takeaway: demonstrations in intact brains mirror split-brain findings, illustrating hemispheric specialization and interhemispheric communication.

  • Methods historically used to map brain function

    • Lesion studies (historical): observe deficits following brain damage or removal to deduce function of affected areas. Limitation: not ethical to deliberately lesion healthy brains; relies on accidents or war injuries.
    • Electrical stimulation (historical): stimulate a brain region and observe the resulting sensory or motor experience; used intraoperatively to map functions around tumors or lesions.
    • Imaging and monitoring (modern, noninvasive):
    • Positron Emission Tomography (PET)
    • Computed Tomography (CT or CAT scans)
    • Electroencephalography (EEG)
    • Functional Magnetic Resonance Imaging (fMRI)
    • These technologies measure blood flow, oxygen usage, magnetic fields, or electrical activity to infer which brain areas are active during tasks.
    • Summary: techniques have evolved from invasive lesioning to noninvasive imaging, allowing us to map function with increasing precision and safety.

  • Integrated view: connections to previous lectures and real-world relevance

    • Previous topics (nervous system basics and neurons): this lecture builds from neurons to networks and then to whole-brain organization.
    • Real-world relevance:
    • Understanding which brain regions support vision, hearing, touch, and movement helps explain everyday experiences (e.g., why head injury can cause blindness).
    • Knowledge about cortical maps informs education and rehabilitation after injury.
    • Lateralization insights explain language development, reading, and certain cognitive strengths and weaknesses.

  • Ethical, philosophical, and practical implications

    • Historical lesion studies raise ethical concerns about harming patients; modern research emphasizes noninvasive methods and patient safety.
    • Surgical corpus callosum severing for epilepsy highlights a trade-off: reducing seizures at the cost of interhemispheric coordination in some tasks; patients can often function well in daily life but may show deficits in controlled lab contexts.
    • The idea of modular, specialized brain regions invites reflection on determinism vs. plasticity: development continues into the early 20s, and training (e.g., music) can influence brain structure (e.g., corpus callosum size).

  • Mathematical and conceptual recap (LaTeX-formatted)

    • Number of neurons in the CNS: N ext{ neurons} \ N \in [86, \, 100{,}000{,}000{,}000]
    • Hemispheric control relationships: Left_hemisphere \rightarrow Right_side_of_body, \quad Right_hemisphere \rightarrow Left_side_of_body.
    • Visual field mapping (split-brain context): Right_visual_field \rightarrow Left_hemisphere, \quad Left_visual_field \rightarrow Right_hemisphere.
    • Cortical lobe count: 8 \text{ lobes in total} \quad (4 \text{ per hemisphere}).
    • Visual/occipital processing: occipital lobe processes vision; damage -> blindness; stimulation -> visual experiences.
    • Sensorimotor cortical maps: somatosensory cortex (front of parietal lobe) and motor cortex (back of frontal lobe) are topographically organized; body layout preserved in these maps (the homunculus).
    • Split-brain experimental times (class demonstration):
    • t1 = 11.43\,\mathrm{s}}, \ t2 = 10.57\,\mathrm{s}, \ t_3 = 15.89\,\mathrm{s}.

  • Quick synthesis: big takeaways

    • The CNS comprises brain and spinal cord; the brain’s architecture (brain stem vs cortex) reflects an evolutionary sequence and functional hierarchy.
    • The cortex is organized into two hemispheres with four lobes each; different lobes support distinct functions, and sensory/motor maps preserve body layout (homunculus).
    • Lateralization provides division of labor between hemispheres (language vs pattern/visual thinking), with interhemispheric communication via the corpus callosum.
    • Studying brain function has progressed from lesion studies to electrical stimulation to noninvasive imaging, enabling nuanced understanding of how the brain enables perception, action, and thought.

  • Next class note

    • Thursday’s session will cover the peripheral nervous system and its important functions, building on today’s CNS-focused concepts.