Brain, Cranial Nerves, and Spinal Cord Lecture

Diencephalon

• Mostly gray matter. 3 parts:
  • Thalamus (≈80\%) – sensory relay/edit to cortex.
  • Hypothalamus – master autonomic & endocrine control (ANS, temp, hunger, thirst, circadian, emotions).
  • Epithalamus – includes pineal gland → melatonin.

• Key landmarks: optic chiasma (CN II cross), pituitary on infundibulum in sella turcica.

Brainstem

• Mainly white matter; nuclei = survival centres. 3 parts:
  • Midbrain – visual/auditory reflexes; cerebral aqueduct; tectal plate (sup./inf. colliculi).
  • Pons – bridge cortex↔cerebellum; pontine respiratory centre.
  • Medulla oblongata – cardiac, vasomotor, respiratory centres; coughing/sneezing; decussation of pyramids (motor-sensory crossover).

Cerebellum

• Inputs: \text{PMC \,/\, premotor}, vestibular apparatus, proprioceptors.

• Coordinates smooth, balanced, accurate movement; ipsilateral mapping.

• Lobes: anterior (superior), posterior (inferior); white arbor vitae.

Functional Brain Systems

• Limbic system (emotional brain): cingulate gyrus (expression/conflict), amygdala (emotional memory/fear).

• Reticular formation / RAS:
  • Filters sensory flood.
  • Maintains wakefulness (damage → coma).
  • Over/under-filtering → sensory avoider/seeker behaviors.

Cranial Nerve Essentials

Mnemonic (names): On Occasion Our Trusty Truck Acts Funny—Very Good Vehicle Any How.
Mnemonic (type S/M/B): Some Say Money Matters, But My Brother Says Big Brains Matter More.

CN I – VI

1 Olfactory – smell (S).
2 Optic – vision (S).
3 Oculomotor – 4/6 extrinsic eye mm., iris, lens (M).
4 Trochlear – sup. oblique m. (M).
5 Trigeminal – facial somatic sensation; mastication mm. (B).
6 Abducens – lateral rectus m. (M).

CN VII – XII

7 Facial – taste ant. 2/3 tongue; facial exp. mm., salivary glands (B).
8 Vestibulocochlear – hearing & equilibrium (S).
9 Glossopharyngeal – taste post. 1/3, pharynx; parotid gland, 1 pharyngeal m. (B).
10 Vagus – visceral sensory/motor neck → thoracoabdominal organs (B).
11 Accessory – SCM & trapezius (M).
12 Hypoglossal – tongue muscles (M).

Spinal Cord Overview

• Continuation of medulla to L1.

• Enlargements: cervical; lumbosacral.

• End cone = conus medullaris; nerves below = cauda equina.

• 31 pairs spinal nerves (C1–C8, T1–T12, L1–L5, S1–S5, Co1).

Cross-Section Anatomy

• Meninges: dura (epidural fat), arachnoid (CSF in sub-space), pia.

• Gray horns: posterior (sensory), lateral (ANS), anterior (motor); joined by gray commissure.

• White funiculi: posterior, lateral, anterior; white commissure.

• Central canal → CSF.

Tract Basics

Ascending = sensory (spinothalamic, spinocerebellar, dorsal columns).
Descending = motor (corticospinal, vestibulospinal…).
Shared rules: decussate, 2–3 neuron relay, somatotopy, bilateral symmetry.
Spinocerebellar = ipsilateral exception.

Spinal Nerve Roots & Rami

• Posterior root = sensory; dorsal root ganglion = somas.

• Anterior root = motor.

• Merge → spinal nerve → splits:
  • Anterior ramus (largest; forms plexuses).
  • Posterior ramus (local back).
  • Meningeal br. (to meninges).
  • Rami communicantes (ANS → sympathetic trunk).

Dermatomes

• Skin map by spinal nerve; horizontal on trunk, longitudinal on limbs.

• Clinically locate cord injury.

Plexus 1 – Cervical

• Roots C1{-}C5.

• Major branch: phrenic n. → diaphragm (sole motor supply).

Plexus 2 – Brachial (Upper Limb)

Roots C5{-}T1. Key nerves & regions:

• Axillary – deltoid, teres minor; skin shoulder.

• Musculocutaneous – arm flexors (biceps, brachialis); skin lat. forearm.

• Radial – arm & forearm extensors; skin post. limb/hand.

• Median – lat. forearm flexors; skin lat. palm/thumb.

• Ulnar – med. forearm flexors; skin med. hand (funny-bone hit = referred pain).