July 1 Part 3 Neuro Lecture – Histology, Physiology & Embryology

Course Logistics

Remaining class time
- "Tomorrow and three more days" of lectures left before the final exam.
- Instructor plans an in-class review 1–2 days before the test, not the same morning.
- Evening review sessions expected; no separate homework-style review packet.

3 functional layers covered in the course
- Neuro-histology (cell types & stains)
- Basic neuro-physiology (electrical properties, conduction)
- Neuropathology (disease correlations)
Major divisions
- Central Nervous System (CNS) – brain & spinal cord
- Peripheral Nervous System (PNS) – cranial & spinal nerves
- Enteric Nervous System – GI tract; intrinsically autonomous but modulated by ANS
Autonomic vs Somatic
- Somatic = voluntary, skeletal muscle, conscious sensation
- Autonomic = involuntary, visceral regulation; maintains homeostasis
  • Sympathetic (thoraco-lumbar) – "fight/flight"
  • Parasympathetic (cranio-sacral) – "rest/digest"

Brain receives 40–50\;\text{mL} / 100\,\text{g} tissue of blood flow at rest.
Delivered via Internal Carotids & Vertebral arteries → Basilar → Circle of Willis.
Regional cerebral blood flow varies with activity.

Motion sickness
- Eye sees static interior of car while semicircular canals sense acceleration → sensory mismatch → nausea.
- Drug: Scopolamine (antimuscarinic) used for motion sickness; historical use as pre-anesthetic to induce amnesia during labor.
Balance demo #1 (head tilt)
- Head tipped 45–60° → no corrective movement.
- Head/torso tipped ≈30° → leg extension via vestibulo-spinal reflex.
- Shows integration of semicircular canals & cerebellum with gamma motor loop.
Balance demo #2 (eyes closed sway)
- Vision removed → reliance on slower somatic proprioceptors → observable postural sway.

Sympathetic reactions when “running from a mugger”
- ↑ Heart rate, ↑ respiration, ↑ skeletal muscle blood flow, pupillary dilation, lens flattens for distance focus.
Parasympathetic = “PIG” / “SLUG”
- Secretion, Lacrimation, Urination, Defecation, plus digestion, reading, studying.
- Sexual arousal = parasympathetic; male ejaculation = sympathetic.

Time terminology
- Gestational (menstrual) age: last menstrual period → term = 40\,\text{weeks}.
- Conceptual (fertilization) age: ~14 days shorter → 38\,\text{weeks}.
Carnegie stages map daily events during first 8 weeks (embryonic period).
Early cleavage
- 2-, 4-, 8-cell stages → morula (solid) → compaction → blastocyst (inner cell mass + trophoblast) → implantation day 6.
Gastrulation (primitive streak)
- Epiblast cells invaginate → 3 germ layers: ectoderm, mesoderm, endoderm.
- Birthday-cake analogy: finger poke = future anus; frosting scraped forward = neural plate/brain; rest of frosting = germ layers.
- Failure of streak process → body “twinning” or organ inversion.
Neurulation
- Neural tube formation crucial for CNS organization.
- HOX genes orchestrate sequential development.
Organogenesis vs Fetal growth
- Organs form in weeks 1–8; then enlarge & mature.
- CNS extremely sensitive to teratogens during this window.

Neural-tube defects, cardiac septal defects, limb defects (amelia) occur when teratogen exposure coincides with organ’s critical window.
Thalidomide tragedy (1950s–60s)
- Given for morning sickness; a 14-day sensitive window caused phocomelia (absent/short limbs).
- Safe before and after that window → underscores timing + drug effect.
Other defects listed: deafness, glaucoma, cleft palate, etc.

3 primary vesicles:
- Prosencephalon (forebrain)
- Mesencephalon (midbrain) → auditory reflex center.
- Rhombencephalon (hindbrain)
5 secondary vesicles:
- Telencephalon → cerebral hemispheres
- Diencephalon → thalamus, hypothalamus, epithalamus
- Mesencephalon (remains midbrain)
- Metencephalon → pons + cerebellum
- Myelencephalon → medulla oblongata

Electrical principles: Voltage (V), Current (I), Resistance (R) – Ohm’s Law V = I \times R.
Action potentials, graded potentials, saltatory conduction via myelin.
Ion channels (voltage & ligand-gated) dictate excitability.

Neurons (ectoderm, neural crest origin)
- Anatomical classifications: multipolar, bipolar, unipolar (embryonic/invertebrate), pseudo-unipolar.
- Functional: sensory (afferent), motor (efferent), interneurons.
Glia (support “glue”)
- CNS: astrocytes, oligodendrocytes, ependymal cells, microglia.
- PNS: Schwann cells (myelin), satellite cells (support).
- Astrocytes supply nutrition, maintain BBB, buffer ions, inhibit CNS axon regeneration via scar tissue.
Axonal transport supported by microtubules & mitochondria; lipofuscin accumulates with age.
Nissl bodies = RER; distribution pattern diagnostic in pathology.
- Absent in axon hillock; clear triangle marks initial segment where AP begins.
Giant Purkinje cells of cerebellum ≈100 k dendritic spines.

Alpha motor neurons → extrafusal skeletal fibers.
Gamma motor neurons → intrafusal spindle fibers; adjust sensitivity; crucial for balance reflexes.
Golgi Type I (long-range) vs Golgi Type II (local/stellate) interneurons distinguished.

All pre-ganglionic fibers (both symp & parasymp) release acetylcholine at nicotinic receptors.
Post-ganglionic
- Sympathetic: usually norepinephrine (adrenergic); exceptions (e.g., sweat glands) are cholinergic.
- Parasympathetic: acetylcholine at muscarinic receptors.

Four stairwells identified (A, B front; C, D rear).
Doors are fireproof; use phone flashlight if lights fail; keep right on descent; “make sure I’m first” (instructor’s joke).

Nervous system integrates vast multi-sensory data; interpretation depends on context (e.g., vision vs vestibular inputs).
Early embryonic weeks are “all-or-nothing” for many malformations; timing matters more than dose.
Cell morphology (Nissl pattern, axon hillock clarity) is diagnostically rich.
Autonomic pharmacology hinges on location & receptor type, not just neurotransmitter.
Balance, nausea, and reflexes provide everyday demonstrations of foundational neurophysiologic principles.
Always know your exits—both for exams and actual fires!