Comprehensive Neuroanatomy, Cortical Parcellation, and Biophysics Study Notes

Course Logistics and Resources

• Centralized Teaching Assistant Materials:
  • All materials provided by the TA (Piper) are centralized in a single Google Document to streamline access.
  • The document's first tab contains primary administrative details, including office hours and email contact information.
  • Subsequent tabs correspond to specific lecture days and include outline-style notes, structural diagrams, and visual aids.
  • Access pathway: Canvas $\rightarrow$ Modules $\rightarrow$ TA Materials.
• Upcoming Academic Schedule:
  • Wrapping up the neuroanatomy unit, followed by a comprehensive review of all covered structures and functions.
  • The subsequent lecture begins the Methods chapter, which provides critical foundation material for the upcoming Methods Critique assignment.

Overview of the Diencephalon

• Structural Definition:
  • The diencephalon is a major subcortical grouping concept that encompasses both the thalamus and the hypothalamus.
• Thalamus:
  • Sensory Processing Hub: Functions as the central relay station for almost all sensory information (touch, temperature, pressure, vibration) detected throughout the body before it ascends to the cerebral cortex.
  • Active Processing Role: Acts like an active mail carrier. It does not perform mere passive filtering or simple delegation; rather, it engages in active, sophisticated processing of incoming rich sensory signals (e.g., actively processing auditory signals like loud sounds).
  • Key Nuclei:
    • Medial Geniculate Nuclei (MGN): Paired bilateral structures located at the posterior end of the thalamus that process and support the auditory system.
    • Lateral Geniculate Nuclei (LGN): Paired bilateral structures located at the posterior end of the thalamus that process and support the visual system.
• Hypothalamus:
  • Anatomical Position: Situated directly beneath the thalamus and directly connected to the pituitary gland via the pituitary stalk.
  • Homeostatic Regulation: Regulates biological drives that constantly pull the organism back into balance from external disruptions, including feeding/food drive, sleep, sexual behavior, and body temperature regulation.
  • Endocrine Command Center: Controls the hormonal system by synthesizing and secreting neurohormones known as releasing hormones. Exercises oversized physiological influence relative to its small anatomical size.
• Anatomical Orientations: Evaluated through coronal views (cross-sectional front-to-back cut) and sagittal views (side-profile cut dividing left and right hemispheres).

Subcortical Brain Systems

• Basal Ganglia:
  • Constituent Structures: Caudate nucleus, putamen, globus pallidus, nucleus accumbens, and substantia nigra.
  • Spatial Morphology: Visually resembles an over-the-ear headphone or AirPod arrangement wrapping around inner subcortical space.
  • Functional Specializations:
    • Motor Control: The caudate nucleus and putamen coordinate and control voluntary motor systems.
    • Reward and Motivation: The nucleus accumbens plays a primary role in processing reward, reinforcement, motivation, and addiction mechanisms.
• Limbic System:
  • Functional Scope: Subcortical network primary responsible for processing emotional information and integrating emotion with memory.
  • Constituent Structures:
    • Thalamus: Select subcortical relay regions.
    • Cingulate Cortex: A large, C-shaped ribbon of tissue encircling the corpus callosum.
    • Hippocampus: Processes declarative memories and embeds emotional context within memory structures.
    • Amygdala: Core center for processing emotional signals and affective information.
    • Hypothalamus: Coordinates physiological autonomic responses to emotion.
    • Mammillary Bodies: Assist in structuring and formatting declarative memories.

Anatomy of the Cerebral Cortex

• Cerebral Cortex vs. Cerebellar Cortex:
  • Cerebral cortex refers to the outer tissue layer of the brain proper (divided into left and right cerebral hemispheres), which must be distinguished from the cerebellar cortex of the cerebellum.
• Cortical Topography (Surface Landmarks):
  • Gyrus (plural: Gyri): Raised ridges or bumps on the cortical surface.
  • Sulcus (plural: Sulci): Valleys, grooves, or burrows separating gyri.
  • Fissure: An exceptionally deep sulcus or groove.
  • Function of Landmarks: Provide reference coordinates across the cortical terrain to localize functional regions.
• Major Cortical Landmarks:
  • Central Sulcus: A major centrally located valley that physically divides the anterior frontal lobe from the posterior parietal lobe.
  • Sylvian Fissure (Lateral Fissure): A deep horizontal groove running perpendicular to the central sulcus (forming a T-shape). It neatly delineates the temporal lobe (located below the fissure) from the frontal and parietal lobes (located above the fissure).
• Gross Lobar Subdivisions:
  • Frontal Lobe: Situated in the anterior region of the brain.
  • Parietal Lobe: Situated posterior to the central sulcus and superior to the Sylvian fissure.
  • Temporal Lobe: Situated inferior to the Sylvian fissure.
  • Occipital Lobe: Situated in the far posterior region of the cortex.
• Directional Orientation Features:
  • Anterior (Rostral): Toward the front of the head.
  • Posterior (Caudal): Toward the back of the head.
  • Brainstem and Cerebellum: Distinct, tightly folded posterior-inferior landmarks useful for orienting spatial axes.

Cortical Parcellation and Brodmann Areas

• Parcellation Process:
  • The methodological division of the continuous cortical sheet into distinct units or parcels based on underlying cellular properties.
• Cytoarchitecture:
  • Parcellation relies on mapping structural cellular variations across brain regions, including differences in cortical layer thickness, cell size, and cell packing shape/density.
• Brodmann Areas:
  • The most prevalent cytoarchitectonic parcellation scheme.
  • Brodmann areas segment the brain into numbered regions based strictly on shared anatomical cell structure rather than functional boundaries.

Functional Organization of the Cortex

• Functional Mapping (Modality-Specific Cortices):
  • Alternative to cytoarchitectonic mapping; segments cortex according to the specific processing modality (sensory, motor, visual, auditory) it supports.
• Primary Cortices:
  • Primary Motor Cortex ($M1$ / Precentral Gyrus): Located on the precentral gyrus directly anterior to the central sulcus in the frontal lobe. Executes voluntary motor commands.
  • Primary Somatosensory Cortex ($S1$ / Postcentral Gyrus): Located on the postcentral gyrus directly posterior to the central sulcus in the parietal lobe. Processes body sensations.
  • Primary Visual Cortex ($V1$): Located at the far posterior pole within the occipital lobe. Processes primary visual inputs.
  • Primary Auditory Cortex ($A1$ / Heschl's Gyrus): Located along Heschl's gyrus within the temporal lobe. Processes acoustic inputs.
• Specialized Frontal Regions:
  • Dorsolateral Prefrontal Cortex ($DLPFC$): Positioned on the dorsal and lateral outer surface of the prefrontal cortex. Subserves executive functions, complex decision-making, planning, and future-guided behaviors.
  • Orbitofrontal Cortex ($OFC$): Located on the ventral surface of the frontal lobe above the orbits. Primarily supports decision-making and value evaluation.

Primary Motor Cortex ($M1$)

• Anatomical Identification:
  • Located on the precentral gyrus, directly anterior to the central sulcus.
  • Positioned at the posterior boundary of the frontal lobe.
  • Named precentral gyrus because it is the gyrus immediately preceding the central sulcus.
• Functional Role:
  • Acts as the executive center ("king") of the motor system.
  • Generates and transmits neural commands for voluntary muscle movements and controls fine motor output.
• Organizational Principles of the $M1$ Map:
  • Contralateral Control: The right hemisphere $M1$ controls motor output for the left side of the body; the left hemisphere $M1$ controls the right side of the body.
  • Inverted Map: The dorsal (top) region of $M1$ represents lower body structures (legs, feet), whereas the ventral (bottom) region of $M1$ represents facial and oral structures (jaw, tongue, facial muscles).
  • Distorted Representation (Somatotopic Homunculus):
    • Cortical space is allocated disproportionately based on the need for fine motor control rather than physical body part size.
    • Body parts requiring delicate motor execution (e.g., fingers, hands) receive massive chunks of $M1$ tissue.
    • Body parts with coarse motor control (e.g., back, feet, eyebrows) receive minimal cortical representation.
• Lesion Effects: Localized damage to $M1$ produces muscle weakness or hemiparesis on the contralateral side of the body.

Primary Somatosensory Cortex ($S1$)

• Anatomical Identification:
  • Located on the postcentral gyrus, directly posterior to the central sulcus in the parietal lobe.
  • Positioned directly adjacent to $M1$, separated only by the central sulcus.
• Functional Role:
  • Serves as the ultimate cortical destination for processing somatosensory inputs: touch, pressure, proprioception (body position awareness), and pain.
• Ascending Somatosensory Pathways:
  1. Pain, Temperature, and Crude Touch: Transmitted via fibers that decussate (cross the midline) immediately upon entering the spinal cord to send a rapid global warning signal.
  2. Fine Touch and Proprioception: Transmitted via fibers that enter the spinal cord and ascend ipsilaterally through the dorsal columns before decussating higher up in the brainstem, preserving high-density spatial localization information.
  3. Thalamic Relay: Both ascending streams pass through the thalamus, which refines the signals and projects them to $S1$
• Organizational Features:
  • Distorted Somatotopic Allocation: Cortical area is devoted proportional to tactile receptor density. Highly sensitive regions with dense arrays of mechanoreceptors (hands, fingertips) occupy expansive cortical territory.
  • Functional Impairments: Damage to $S1$ leads to loss of fine touch discrimination, such as the inability to resolve two distinct tactile points applied close together on the skin (two-point discrimination threshold).
• Cortical Plasticity and Phantom Limb Phenomenon:
  • Following limb amputation, the neural representation map of the missing limb remains intact within $S1$, leaving those cortical neurons expectant of input.
  • Deprived of normal peripheral input, the vacant $S1$ hand territory is encroached upon by adjacent active cortical regions (e.g., the face representation area).
  • Consequently, tactile stimulation of the face triggers mismatched neural activity in the hand region of $S1$, causing the individual to perceive sensory sensations as originating from the non-existent (phantom) hand.

Primary Visual Cortex ($V1$)

• Anatomical Location: Occupies the occipital lobe at the posterior pole of the cerebral cortex.
• Visual Pathway and Mapping Principles:
  • Contralateral Field Processing: Visual input from the right visual field is mapped onto the left $V1$; visual input from the left visual field is mapped onto the right $V1$
  • Retinal Decussation Mechanics:
    • Axons from the nasal hemiretinas cross the midline at the optic chiasm.
    • Axons from the temporal hemiretinas do not cross and project ipsilaterally.
    • Past the optic chiasm, optic tract axons relay through the Lateral Geniculate Nucleus ($LGN$) of the thalamus before projecting to $V1$
  • Inverted Field Mapping: The upper visual field projects to the ventral bank of $V1$; the lower visual field projects to the dorsal bank of $V1$
• Functional Specialization:
  • Operates like the foundational step of an industrial assembly line.
  • $V1$ neurons process elementary visual features, specifically detecting high-contrast orientational light-and-dark bars.
  • Complex visual dimensions (such as color, depth, and spatial contours) are extracted downstream in secondary visual processing streams (e.g., $V3$, $V4$).
  • Damage to $V1$ impairs basic light/dark contrast detection in the corresponding visual field.

Primary Auditory Cortex ($A1$)

• Anatomical Location: Resides within Heschl's gyrus along the superior temporal gyrus of the temporal lobe.
• Functional Scope: Detects and interprets acoustic pressure waves (vibrating atmospheric molecules) across the frequency spectrum.
• Bilateral Projection Architecture:
  • Exhibits both ipsilateral and contralateral projections.
  • Acoustic signals entering a single ear project simultaneously to both the left and right auditory cortices.
• Tonotopic Organization:
  • $A1$ tissue is spatially mapped according to sound wave frequency (pitch).
  • Rostral (Anterior) $A1$: Represents low-frequency sound tones.
  • Caudal (Posterior) and Medial $A1$: Represents progressively higher-frequency sound tones.
• Lesion Dynamics:
  • Unilateral damage to $A1$ does not cause total deafness due to bilateral pathway redundancy.
  • Instead, unilateral damage elevates auditory detection thresholds (increases the volume required to hear sounds) for the contralateral ear.

Olfactory and Gustatory Cortices

• Olfactory System:
  • Transduction: Volatile chemical odorants are detected by olfactory receptors located in the olfactory epithelium of the nasal cavity.
  • Dual Projection Pathways:
    1. Direct Limbic Pathway: Projects directly to limbic structures (amygdala/hippocampus), underlying immediate emotional and memory responses to smells.
    2. Higher-Order Cortical Pathway: Projects to the orbitofrontal cortex ($OFC$) for conscious olfactory processing.
  • Unique Feature: The olfactory system is the only sensory modality that projects exclusively ipsilaterally (right nostril inputs project strictly to the right olfactory cortex) and bypasses an initial obligatory thalamic relay.
• Gustatory System:
  • Transduces taste stimuli via taste bud chemoreceptors distributed across the tongue.
  • Projects taste signals to both the limbic system (for affective processing) and the specialized gustatory cortex.

Association Cortices

• Definition: Expansive regions of the cerebral cortex that integrate multi-sensory inputs and subserve higher-order cognitive operations.
• Lobar Functional Associations:
  • Frontal Association Cortex: Primarily associated with motor planning, behavioral inhibition, executive control, and decision-making.
  • Temporal Association Cortex: Primarily associated with memory storage/retrieval, emotional processing, and auditory comprehension.
  • Parietal Association Cortex: Primarily associated with spatial processing, spatial attention, and somatosensory integration.

Biophysics of Neuronal Communication and Action Potentials

• Modes of Neural Communication:
  1. Electrical Signaling: Intracellular transmission via action potentials traversing the axon.
  2. Chemical Signaling: Intercellular transmission via neurotransmitter release across the synaptic cleft.
• Resting Membrane Potential:
  • The baseline electrical charge across the neuronal membrane at rest is approximately $\text{-70}\,mV$ (microvolts/millivolts; internal environment is net negative relative to external fluid).
  • Ion Distribution Mechanism:
    • Maintained by the sodium-potassium pump ($Na^+/K^+$ ATPase).
    • The pump actively extrudes $3\,Na^+$ ions out of the cell for every $2\,K^+$ ions it pumps inside.
    • Establishes high extracellular concentration of $Na^+$ and high intracellular concentration of $K^+$, maintaining a net negative resting state.
• Phases of the Action Potential:
  1. Threshold of Excitation: Local depolarization forces the membrane potential to threshold.
  2. Depolarization (Rising Phase): Voltage-gated $Na^+$ channels open rapidly, allowing an explosive influx of $Na^+$ driven by concentration and electrostatic gradients, creating a sharp positive voltage deflection.
  3. Repolarization (Falling Phase): Voltage-gated $Na^+$ channels become inactivated. Delayed voltage-gated $K^+$ channels open fully, driving a massive efflux of $K^+$ out of the cell to restore internal negativity.
  4. Hyperpolarization / Restoration: The membrane briefly dips below baseline resting potential before returning to $\text{-70}\,mV$
• Synaptic Transmission Events:
  1. The action potential arrives at the presynaptic axon terminal.
  2. Depolarization triggers the opening of voltage-gated calcium channels, causing an influx of calcium ($Ca^{2+}$) into the terminal.
  3. $Ca^{2+}$ influx acts as a signal that triggers synaptic vesicles containing neurotransmitters to migrate toward and fuse with the presynaptic membrane.
  4. Neurotransmitters are released into the synaptic cleft via exocytosis, where they cross the gap to bind to receptors on the postsynaptic membrane.

Questions & Discussion

• Exam Structural Demands:
  • Question: Will exams require structural identification of cortical regions?
  • Response: Yes. Exams require memorizing structural positions on diagrams and mastering the underlying functional roles of each area.
• Brodmann Areas vs. Functional Classification:
  • Question: Clarification on how Brodmann areas differ from functional cortical designations.
  • Response: Brodmann areas are defined strictly by cytoarchitecture (homogenous cell size, layer thickness, cellular shape), whereas functional areas (like $M1$ or $S1$) are classified according to the specific operational tasks they perform.
• Differential Spinal Pathways for Touch vs. Pain:
  • Question: Why do fine touch and pain signals use different spinal pathways?
  • Response: Fine touch and proprioception ascend ipsilaterally in the dorsal columns before decussating to preserve high-density spatial localization coordinates. Pain and temperature signals decussate immediately upon entering the spinal cord to send a rapid, global threat signal to the brain.
• Phantom Limb Sensations in Congenital Limb Differences:
  • Question: Does phantom limb sensation occur in individuals born with congenital limb differences?
  • Response: The fundamental baseline representation map of $S1$ exists, but over developmental time, adjacent active cortical regions encroach upon un-stimulated cortical territory, altering sensory responses.
• Review Session Diagnostic Exercises:
  • Dividing landmark between frontal and parietal lobes: Central sulcus.
  • Dividing landmark between precentral and postcentral gyri: Central sulcus.
  • Dividing landmark between temporal lobe and overlying frontal/parietal lobes: Sylvian fissure (Lateral fissure).
  • Term for cortical representation expansion based on receptor density: Map distortion (distorted map).
  • Neurons specialized for light/dark contrast in occipital lobe: Primary visual cortex ($V1$) neurons.
  • Term for organization of auditory cortex by pitch frequency: Tonotopic organization.
  • Three distinct regions within the frontal lobe: Precentral gyrus ($M1$), Orbitofrontal cortex ($OFC$), and Dorsolateral prefrontal cortex ($DLPFC$).