Central Nervous System – Brain Structures and Functions

Central Nervous System (CNS)

• Major anatomical divisions visible in sagittal section: corpus callosum, cerebrum, thalamus, hypothalamus, epithalamus, midbrain, pons, medulla oblongata, cerebellum, spinal cord.
• Functional division: brain stem + diencephalon + cerebellum + cerebrum.
• Practical relevance: localization of damage predicts clinical deficits (e.g., Parkinson’s ↔ substantia nigra).

Brain Stem – General Features

• Comprised of medulla, pons, midbrain (cerebrum & cerebellum removed to expose it).
• Acts as a bidirectional highway: conveys ascending sensory and descending motor signals between spinal cord and higher centers.
• Houses nuclei for 10 of the 12 cranial nerves → crucial for vital reflexes.
• Autonomic control: heart rate, blood pressure, respiration, swallowing, coughing, vomiting.

Medulla Oblongata

• Location: most caudal brain-stem segment, continuous with spinal cord.
• Relay station: cerebrum ↔ spinal cord.
• Vital centers: cardiac, respiratory, vasomotor → maintains heartbeat, respiratory rhythm, systemic blood pressure.
• Somatic reflex centers: swallowing, vomiting, coughing.
• Cranial nerves emerging: \text{CN VIII, IX, X, XI, XII} (vestibulocochlear through hypoglossal).
• Lesion implication: medullary damage can be rapidly fatal (loss of respiratory drive).

Pons

• Dorsal to medulla, ventral to cerebellum; bridges multiple tracts (name = “bridge”).
• Relay pathways: cerebrum ↔ spinal cord; cerebrum ↔ cerebellum (important for motor coordination).
• Respiratory modulation: fine-tunes the medullary rhythm to maintain normal breathing pattern.
• Cranial nerves emerging: \text{CN V, VI, VII} (trigeminal, abducens, facial).

Midbrain

• Most rostral portion of brain stem, directly under diencephalon.
• Conduit: cerebrum ↔ spinal cord; contains cerebral peduncles (motor) & ascending tracts (sensory).
• Reflex centers:
– Superior colliculus ⟶ visual‐startle reflex (turning head toward moving object).
– Inferior colliculus ⟶ auditory‐startle reflex (turning head toward unexpected sound).
• Substantia nigra (ventral midbrain): dopaminergic neurons; degeneration causes Parkinson’s disease (loss of intentional movement smoothing).
• Cranial nerves emerging: \text{CN III, IV} (oculomotor, trochlear).

Cerebellum

• Receives massive proprioceptive input from muscles & joints + copies of motor plans from cerebrum.
• Computes timing, force, coordination; sends corrective feedback to cerebral motor cortex.
• Cognitive functions: recognizes, predicts, and stores sequences of complex movements ("motor patterns").
• Clinical insight: ataxia, intention tremor when damaged.

Motor-Skill Learning: “How to Ride a Bike” Example

1. \text{Brain} issues initial motor plan to muscles + cerebellum.

2. Muscles execute incomplete plan → fall ("result").

3. Sensory feedback: pain + proprioception (muscle ⟶ brain; “OW!”).

4. Simultaneously, proprioception reaches cerebellum.

5. Cerebellum compares “idea” vs. “result,” computes error, advises brain.

6. Brain sends corrected commands; cycle repeats.

7. After multiple loops, stable motor pattern stored in cerebellum → long-term retention ("you never forget how to ride a bike").
   • Educational principle: practice physically engrains cerebellar circuitry.

Diencephalon – Overview

• Three paired structures sitting just above brain stem:

1. Thalamus

2. Hypothalamus

3. Epithalamus (contains pineal gland)
   • Flanked by cerebral hemispheres, capped by corpus callosum.

Thalamus – “Sensory Gateway”

• Bilateral egg-shaped nuclei perched above midbrain; form lateral walls of third ventricle.
• Every ascending sensory input (skin, retina, cochlea, taste buds, visceral organs) synapses in specific thalamic nuclei.
• Functions:
– Edits, sorts, prioritizes sensory info → decides what reaches cerebral cortex.
– “Gate open” vs. “gate closed”: large portions never reach conscious perception.
• Clinical note: damage may cause sensory misperception or central pain syndromes.

Hypothalamus – Visceral Control Center

• Located below & anterior to thalamus; connected inferiorly to pituitary gland via infundibulum → neuro-endocrine axis.
• Functionally heterogeneous; colored map shows nuclei with distinct jobs:

1. Hunger & thirst detection; monitors osmolarity, nutrients; modulates GI tract via \text{CN X}.

2. Pleasure & anger centers; generates emotional responses.

3. Sleep regulation; secretes hypocretin (orexin) – key for wakefulness; imbalance → narcolepsy.

4. Sex drive & reproductive behaviors.

5. Autonomic regulation of heart rate & blood pressure.
   • Practical/ethical angle: manipulating hypothalamic set-points (e.g., appetite) has obesity-treatment potential but raises autonomy concerns.

Pituitary Gland (Hypophysis)

• Endocrine “executive arm” of hypothalamus.
• Hypothalamic releasing/inhibiting hormones travel via portal vessels to anterior pituitary → systemic hormonal control.

Epithalamus & Pineal Gland

• Roof of diencephalon, superior & posterior to thalamus.
• Pineal secretes melatonin – rises in darkness to initiate sleep.
• Circadian biology relevance: blue-light suppression of melatonin explains screen-time insomnia.

Reticular Activating System (RAS)

• Diffuse network of brain-stem neurons.
• Filters incoming sensory stream by relative importance before reaching cortex.
• Keeps cortex alert; inactivation → sleep, pharmacologic depression (anesthesia).
• Safety example: ability to ignore ticking clock yet awaken to baby’s cry (high salience).

Limbic System – “Emotional Brain”

• Primitive ring of structures encircling thalamus & hypothalamus.
• Core components:

1. Amygdala – intuition & fear; assesses facial emotion, triggers fight/flight.

2. Cingulate gyrus – conveys emotion through body language/gestures.

3. Hippocampus – consolidates short-term to long-term memory; chooses cortical storage loci.

4. Olfactory bulb – links odors with memory & emotion (powerful because olfaction projects directly here).
   • Clinical/ethical tie-ins: PTSD involves amygdala hyperactivity; memory manipulation research.

Integrative Connections & Real-World Significance

• Brain-stem cranial nerve nuclei coordinate with limbic and diencephalic circuits for autonomic/emotional expression (e.g., fear ➜ ↑HR via medullary centers).
• Thalamic gating + RAS filtering = attentional spotlight; implications for ADHD therapies.
• Cerebellar motor memory parallels procedural learning (typing, musical instruments), shaping educational practice: repetition, multisensory feedback.
• Dopamine loss in substantia nigra (midbrain) → Parkinson’s; underscores importance of neurotransmitter balance.
• Melatonin & hypocretin highlight neural control of sleep-wake cycle; societal impact of shift work & electronic devices.