Neuroanatomy – Parietal Lobe, Sensory Systems, Association Areas & Brainstem Comprehensive Study Notes

Parietal Lobe & Somatosensory Processing

• Overall Role
  • Major sensory lobe; specifically somatosensory ("soma" = body → skin-based receptors).
  • Receives end-points of three major ascending tracts:
  • Dorsal Column–Medial Lemniscal (DCML).
  • Spinothalamic (anterolateral).
  • Spinocerebellar (after multiple relays).

Primary Somatosensory Cortex (Post-central Gyrus)

• Destination of 3rd-order neurons from thalamus (DCML & spinothalamic).
• Functions
  • Conscious awareness of stimulus.
  • Localises body part, modality, intensity (e.g., “sharp pain in L foot”).

Somatosensory Association Cortex

• Posterior to post-central gyrus.
• Compares incoming data to prior experiences → higher-level interpretation.
• Sends refined info to frontal lobe for decision making.

Sensory Homunculus (Cortical Map)

• Body mapped medial → lateral just like motor map, but proportions differ.
  • Hands, face, tongue: huge cortical representation ⇒ high receptor density.
  • Feet: larger than motor map ⇒ many nociceptors/pressure receptors.
• Spatial Discrimination
  • Cortex region activated mirrors body part touched.
  • Two-point discrimination test:
  • Finger tips: probes must almost touch before fusion perceived.
  • Upper back: large separation still perceived as one → lower receptor density.

Tract Example (Spinothalamic)

• 1° neuron: receptor (e.g., L foot) → dorsal root ganglion.
• 2°: spinal cord → thalamus (decussates early).
• 3°: thalamus → medial post-central gyrus (foot area).
• Same logic for hand (lateral-superior cortex).

Primary vs Association Areas (Universal Pattern)

• Each sense has:
  • Primary cortex → initial reception (location, intensity).
  • Association cortex → comparison, pattern recognition, context.

Vision (Occipital)

• Primary visual cortex: raw brightness, contrast, retinal map.
• Visual association cortex: colours, shapes, patterns, memory comparison.

Auditory (Temporal – superior lateral)

• Primary auditory: arrival-time difference (\approx 2\,\text{ms}), loudness.
• Auditory association: speech/music patterns, meaning.

Olfactory (Temporal – medial)

• Small in humans; huge in dogs.
• Primary: odour intensity.
• Association: odour identification, emotional ties.

Insula

• Gustatory (taste).
• General visceral sensation (e.g., stomach rumble).
• Vestibular signals (semicircular canals → head motion; nausea when disrupted).

Multimodal Association Areas

Prefrontal Cortex (Frontal, non-motor)

• Executive functions, personality, conscious thought, impulse control.
• Last brain area to fully mature (\approx 25{-}27\,\text{yrs}).
• Stimulus-dependent: early childhood enrichment & lifelong mental activity critical.

Posterior Association Area (Parietal + Temporal + Occipital)

• Integrates diverse sensory input; pattern recognition.
• Contains Wernicke’s Area (dominant hemisphere only).
  • Language comprehension (spoken & written).
  • Injury → fluent but meaningless speech or reading deficits.

Limbic Association Area

• Emotional impact of experiences.
• Key structures:
  • Amygdala → fear & strong emotions.
  • Hippocampus → acquisition & consolidation of memory (case study “H.M.” demonstrates inability to form new declarative memories without hippocampi).
  • Anterior cingulate gyrus (ACC) → emotional expression, last to mature.

Cerebral Lateralisation & White-Matter Tracts

• Left = dominant in \approx 90\% (language, math, logic).
• Right: spatial skills, pattern/face recognition.
• Cutting corpus callosum (commissurotomy) for intractable epilepsy shows hemispheric independence.

Major Fiber Types

• Commissural → connect hemispheres (largest = corpus callosum).
• Association → connect areas within same hemisphere (e.g., post- to pre-central gyri).
• Projection → vertical; cortex ↔ lower CNS.
  • Ascending 3° sensory axons.
  • Descending corticospinal upper motor axons.

Basal Nuclei (Basal Ganglia)

• Deep cerebral nuclei (caudate, putamen, globus pallidus) + midbrain substantia nigra.
• Functions
  • Refine & smooth motor patterns, especially rhythmic/repetitive actions (arm swing).
• Parkinson’s Disease = degeneration of dopaminergic neurons in substantia nigra → tremor, rigidity.

Diencephalon

Thalamus

• Bilateral “relay station”; multiple nuclei (jelly-beans).
• Filters, routes nearly all sensory input to appropriate cortical areas.
• Candidate site of consciousness generation.

Hypothalamus (below thalamus)

• Neuro-endocrine nexus; controls pituitary gland.
• Major roles:
  • Autonomic nervous system regulation.
  • Limbic interaction → emotional responses.
  • Circadian wake drive.
  • Thermoregulation, hunger & thirst.

Epithalamus / Pineal Gland

• Secretes melatonin → promotes sleep onset.
• Light suppresses melatonin (basis of bright-light therapy for shift workers).
• Chronic supplementation can down-regulate endogenous production.

Brainstem

Midbrain

• Anterior cerebral peduncles = corticospinal upper motor axons.
• Posterior corpora quadrigemina (tectum):
  • Superior colliculi → visual reflexes (tracking motion).
  • Inferior colliculi → auditory reflexes (turn head toward sound).
• Superior cerebellar peduncle: cerebellum → thalamus/basal nuclei output.

Pons

• Houses respiration-modifying nuclei.
• Contains middle cerebellar peduncle (massive): cortex/pons → cerebellum.
• Traversed by corticospinal fibers ("pyramidal tract").
• Part of medial lemniscus (DCML pathway).
• Part of reticular formation begins here.

Medulla Oblongata

• Pyramids: continuation of corticospinal tract; decussation of the pyramids (crossing over).
• Inferior cerebellar peduncle: spinal cord/vestibular nuclei → cerebellum.
• Vital autonomic centers:
  • Cardiovascular (heart rate, vessel diameter → \text{BP}).
  • Respiratory rhythm generators.
  • Reflexes: vomiting, swallowing, coughing, sneezing.

Cerebellum

• ~11\% of brain mass; subconscious.
• Anatomy
  • Surface gray cortex; internal white arbor vitae.
  • Three peduncles: inferior & middle (input), superior (output).
• Functions
  • Proprioception; balance; monitors & fine-tunes movements in real time.
  • Updates motor programs via feedback to premotor cortex.
  • Cognitive roles: prediction, planning, problem solving.
• Spinocerebellar pathways remain ipsilateral; crossing occurs after cerebellar processing when signals ascend to thalamus.

Limbic System (Extended)

• Emotional brain; overlaps limbic association area.
• Structures: amygdala, hippocampus, anterior cingulate, parts of thalamus & hypothalamus, fornix, etc.
• Governs emotional behaviour, motivation, memory storage & retrieval, facial expression interpretation.

Reticular Formation & Reticular Activating System (RAS)

• Network of nuclei in pons & medulla.
• Functions
  • Maintains cortical alertness (RAS → widespread thalamic/cortical projections).
  • Filters weak/irrelevant stimuli.
  • Autonomic integration.
• Damage → loss of consciousness, potential brain-death states.

CNS Protection

Bone

• Skull absorbs & redistributes impact energy (fracture preferable to direct neural disruption).

Meninges (Superficial → Deep)

1. Dura Mater (tough)
   • Periosteal layer (fuses with skull’s periosteum) + meningeal layer.
2. Arachnoid Mater (web-like).
   • Sub-dural & sub-arachnoid spaces present.
3. Pia Mater (delicate, adherent; follows sulci/gyri).

• Subarachnoid Space → circulates cerebrospinal fluid (CSF).

Blood Brain Barrier (mentioned briefly)

• Specialized capillaries + astrocyte end-feet regulate chemical environment of brain.

Numerical & Misc. References

• Auditory localisation threshold ≈ 2\,\text{ms} inter-aural arrival difference.
• Cerebellum ≈ 11\% of brain mass.
• Prefrontal full maturation \approx 25{-}27\,\text{years}.
• Cortical lateralisation: \approx 90\% right-handed → left-dominant hemisphere.