---

🧠 COMPLETE LECTURE-STYLE EXPLANATION

Neurology - Embryology, Anatomy & Physiology (Pages 500-507)

---

PAGE 500 - NEURAL DEVELOPMENT (EMBRYOLOGY)

---

🔷 What is Embryology?

---

Line 1: "Notochord (precursor to nucleus pulposus of intervertebral discs) induces ectoderm to form neuroectoderm → neural plate"

• Notochord - Think of it as a temporary "backbone rod" that exists in early embryo life (weeks 3-4). It's like a construction scaffolding. In adults, its remnant becomes the nucleus pulposus - the soft jelly-like center of the discs between your spinal bones (vertebrae). When this disc gets damaged and presses on a nerve, doctors call it a "slipped disc" or "disc herniation."
• Induces ectoderm - The notochord sends chemical signals to the outer layer of cells (called ectoderm - "ecto" = outside, "derm" = skin/layer). Think of ectoderm as the topmost "blanket" of cells covering the early embryo.
• Neuroectoderm - Under the chemical influence of the notochord, this outer layer transforms into specialized brain-forming cells called neuroectoderm ("neuro" = nerve).
• Neural plate - The neuroectoderm thickens and flattens into a plate-like structure, like a pancake of future brain cells.

---

Line 2: "Neural plate gives rise to neural tube and neural crest cells"

• Neural tube: The neural plate (that flat pancake) starts to fold upward on both sides, like a taco being folded, and eventually the two edges fuse in the middle to create a hollow tube. This tube will become your entire brain and spinal cord. This is called neurulation.
• Neural crest cells: As the neural tube closes, some cells escape from the edges and scatter throughout the body. These are remarkably versatile cells that become many different things (discussed later). Think of them as "wandering settlers" that colonize different body territories.

---

Line 3: "Lateral walls of neural tube are divided into alar and basal plates"

• Alar plate (dorsal/back half) = becomes sensory pathways - receives information from the body (touch, pain, temperature). "Alar" means wing-like.
• Basal plate (ventral/front half) = becomes motor pathways - sends commands to muscles.

---

Line 4: "Alar plate (dorsal): sensory; induced by bone morphogenetic proteins (BMPs)"

• BMP (Bone Morphogenetic Protein) - Despite the name, this is a signaling chemical (a "molecular messenger") that doesn't just build bones - it also tells cells in the back part of the neural tube to become sensory cells. It's released from a structure called the "roof plate" (top of the neural tube).

---

Line 5: "Basal plate (ventral): motor; induced by sonic hedgehog (SHH)"

• Sonic Hedgehog (SHH) - Yes, it's named after the video game character! It's a signaling protein released from the notochord and the floor plate (bottom of neural tube). It tells cells in the front part of the neural tube to become motor cells.

---

Line 6: "Homeobox (HOX) genes regulate neural tube segmentation, cranial-caudal differentiation"

• HOX genes = master control genes. Think of them as the "blueprint managers" of the embryo. They decide which part of the body becomes head vs. neck vs. chest vs. waist. They are arranged in order on chromosomes and turned on in order from head to tail (cranial to caudal).
• Cranial-caudal differentiation = deciding which end is the head (cranial) and which is the tail end (caudal). HOX genes orchestrate this.

---

Line 7: "Mutations → syndactyly (limbs), hypospadias (urogenital)"

• Syndactyly - fusion of fingers or toes (think "webbed fingers"). "Syn" = together, "dactyly" = digits/fingers.
• Hypospadias - a birth defect where the opening of the urethra (the tube you urinate through) is on the wrong side of the penis (underside instead of the tip).

---

Line 8: "Same orientation as spinal cord"

---

REGIONALIZATION OF THE NEURAL TUBE

"Telencephalon is the 1st part. Diencephalon is the 2nd part. The rest are arranged alphabetically: mesencephalon, metencephalon, myelencephalon"

1. Prosencephalon (Forebrain)
2. Mesencephalon (Midbrain)
3. Rhombencephalon (Hindbrain)

---

CENTRAL AND PERIPHERAL NERVOUS SYSTEM ORIGINS

"Neuroepithelia in neural tube → CNS neurons, CNS glial cells (astrocytes, oligodendrocytes, ependymal cells)"

• Neuroepithelia = the lining cells inside the neural tube (like cells lining a pipe). These are the original stem cells of the entire nervous system.
• These cells produce all CNS (Central Nervous System) cells:
  • CNS neurons = brain and spinal cord nerve cells
  • Astrocytes = star-shaped support cells (discussed in detail on page 503)
  • Oligodendrocytes = myelin-makers in the brain/spinal cord
  • Ependymal cells = cells lining the fluid-filled cavities of the brain

---

"Neural crest → PNS neurons (dorsal root ganglia, autonomic ganglia [sympathetic, parasympathetic, enteric]), PNS glial cells (Schwann cells, satellite cells), adrenal medulla"

• Dorsal root ganglia = clusters of sensory nerve cell bodies just outside the spinal cord (they relay sensory information from your skin/organs into the spinal cord)
• Autonomic ganglia: Three types of the "automatic" nervous system:
  • Sympathetic = "fight or flight" (heart races when you're scared)
  • Parasympathetic = "rest and digest" (calm state, digestion works)
  • Enteric = the "gut's own brain" - nervous system of the intestines, controls digestion independently
• Schwann cells = myelin-makers in the peripheral nervous system (outside brain/spinal cord). Like plastic insulation around electrical wires.
• Satellite cells = support cells that surround nerve cell bodies in peripheral ganglia (like cushions protecting the cell body)
• Adrenal medulla = the inner part of the adrenal gland (sits on top of kidneys). Releases adrenaline (epinephrine) in stress. It's actually modified neural crest tissue!

---

"Mesoderm → microglia (specialized macrophages)"

• Microglia = the immune defense cells of the brain. Unlike every other brain cell (which comes from neuroectoderm), microglia come from mesoderm (middle embryonic layer) - specifically from bone marrow precursors.
• They are basically macrophages (eat-bacteria/debris cells) that migrated into the brain. Think of them as the brain's own security guards.

---

PAGE 501 - NEURAL TUBE DEFECTS (NTDs)

---

"Failure of the neural tube to close completely by week 4 of development"

---

"Associated with maternal folate deficiency; exposure to teratogens such as valproate and carbamazepine during pregnancy"

• Folate (Folic acid / Vitamin B9) - is needed for DNA synthesis (building new cells). Rapidly dividing cells (like the neural tube) need folate desperately. Without enough folate, the tube cannot seal properly.
• Why does taking folic acid before pregnancy matter? Because the neural tube closes by day 28, often before a woman even knows she's pregnant! That's why all women of reproductive age are advised to take folic acid supplements.
• Valproate and Carbamazepine = anti-epileptic drugs (medications to control seizures). These are teratogens - chemicals that cause birth defects. They interfere with folate metabolism, increasing NTD risk.

---

"Diagnosis: ultrasound, maternal serum AFP and/or amniotic fluid AChE (↑ in open NTDs)"

• AFP (Alpha-fetoprotein) = a protein made by the fetus. Normally stays inside the fetus. If the neural tube is open (like an open wound on the back), AFP leaks out into the mother's blood → high maternal serum AFP = red flag for open NTD.
• AChE (Acetylcholinesterase) = an enzyme from neural tissue. Also leaks into amniotic fluid when there's an open defect. Very specific marker.

---

TYPES OF SPINA BIFIDA:

"Closed NTD: Spina bifida occulta"

• The bony spine (vertebra) fails to close completely, but there's NO herniation (nothing pushes through the gap)
• The meninges (covering membranes) are intact
• Usually seen with a tuft of hair or skin dimple at the lower back over the defect (this is the external clue!)
• Usually asymptomatic (no symptoms, person doesn't even know they have it)
• Most benign form

---

"Open NTD: Meningocele"

• The vertebra is open AND the meninges (the three-layered covering of the spinal cord) herniate (poke out) through the gap
• But the spinal cord itself is still inside - only the coverings protrude
• Forms a fluid-filled sac on the back
• Skin covers it

---

"Open NTD: Myelomeningocele"

• The most severe form of spina bifida
• Both the meninges AND the spinal cord (myelo = cord) herniate through the defect
• The skin is thin or absent over the sac - the neural tissue is exposed
• Often leads to paralysis of legs, bladder/bowel dysfunction
• "Open NTD: Exposed, unfused neural tissue without skin/meningeal covering"

---

"Cranial dysraphism"

• Dysraphism = failure of fusion ("dys" = bad/failed, "raphism" = seam/fusion)
• Cranial = affecting the skull/brain end

• Anencephaly: Failure of rostral (head end) neuropore to close → no forebrain, open calvarium (skull). Often presents with polyhydramnios (too much amniotic fluid) because the baby cannot swallow (fetal swallowing requires a brain). 4 fetal swallowing due to lack of neural control.

---

BRAIN MALFORMATIONS

---

"Holoprosencephaly: Failure of forebrain (prosencephalon) to divide into 2 cerebral hemispheres"

• Developmental field defect usually occurring at weeks 3-4
• Associated with SHH mutations - remember SHH (Sonic Hedgehog) was the signal for midline development
• May be seen in Patau syndrome (trisomy 13) - a chromosomal condition with 3 copies of chromosome 13

• Presents with midline defects: monoventricle (one fused ventricle instead of two), fused basal ganglia, cleft lip/palate
• Hypotelorism = eyes too close together (hypo = less, telos = end/distance)
• Cyclopia = single eye in the middle (most extreme form)
• Proboscis = trunk-like nose structure
• ↑ risk for pituitary dysfunction (since pituitary gland development is also affected)
• Also associated with fetal alcohol syndrome

---

"Lissencephaly: Failure of neuronal migration → smooth brain surface that lacks sulci and gyri"

• Neurons fail to migrate from their birthplace (germinal matrix near the ventricles) to their final destination in the cortex
• The brain looks like a smooth ball instead of a walnut
• Presents with: dysphagia (difficulty swallowing), seizures, microcephaly (abnormally small head), facial anomalies

---

PAGE 502 - POSTERIOR FOSSA MALFORMATIONS

---

CHIARI I MALFORMATION

"Downward displacement of cerebellar tonsils through foramen magnum (1 structure)"

• Cerebellum = the brain structure at the back and bottom of the skull, responsible for balance and coordination
• Cerebellar tonsils = small rounded projections at the bottom of the cerebellum
• Foramen magnum = the large hole at the base of the skull where the brainstem connects to the spinal cord ("foramen" = opening, "magnum" = great/large)

• Asymptomatic in childhood, manifests in adulthood with headaches and cerebellar symptoms (balance problems, coordination issues)
• Associated with syringomyelia (spinal cord cavity - discussed below) and non-communicating hydrocephalus (blocked CSF flow)

---

CHIARI II MALFORMATION

"Downward displacement of cerebellum (vermis and tonsils) and medulla through foramen magnum (2 structures)"

• More severe than Chiari I - two structures herniate: the vermis (middle part of cerebellum) AND the medulla (lowest part of brainstem)
• Presents early in life with dysphagia, apnea, stridor (noisy breathing), limb weakness
• Associated with myelomeningocele and non-communicating hydrocephalus (usually lumbosacral myelomeningocele)
• Usually lumbosacral (lower back)

---

DANDY-WALKER MALFORMATION

"Agenesis of cerebellar vermis → cystic enlargement of 4th ventricle"

• Agenesis = failure to form/develop ("a" = without, "genesis" = creation)
• The cerebellar vermis (the worm-shaped midline part of the cerebellum) fails to develop
• This leaves a huge cystic (fluid-filled) space in the posterior fossa
• The 4th ventricle (a fluid-filled cavity at the back of the brainstem) becomes massively enlarged because CSF cannot drain properly
• The posterior fossa (back of the skull) enlarges to accommodate this
• Associated with non-communicating hydrocephalus and enlarged posterior fossa

---

SYRINGOMYELIA

"Fluid-filled, gliosis-lined cavity within spinal cord"

• A syrinx is an abnormal, fluid-filled cavity that forms inside the spinal cord
• Gliosis-lined = surrounded by scar tissue (glia = brain support cells; gliosis = excessive growth of these cells as a response to injury)
• "Syrinx" (Greek) = tube, as in 'syringe'" - the word syringe comes from the same Greek root!

---

"Most lesions occur between C2 and T9"

• The cavity most commonly forms in the cervical and thoracic spinal cord (neck and chest level). C2 = 2nd cervical vertebra, T9 = 9th thoracic vertebra.

---

"Usually associated with Chiari I malformation (red arrow). Less commonly associated with other malformations, infections, tumors, trauma"

• Chiari I is the most common cause because it obstructs CSF flow at the foramen magnum, causing pressure changes that create the syrinx

---

"Fibers crossing in anterior white commissure (spinothalamic tract) are typically damaged first → 'cape-like' loss of pain and temperature sensation in bilateral upper extremities"

---

"As lesion expands → lower motor neuron (LMN) findings"

• LMN signs: muscle weakness, wasting (atrophy), absent reflexes, fasciculations (visible muscle twitching)

---

PAGE 503 - ANATOMY AND PHYSIOLOGY: CELLS OF THE NERVOUS SYSTEM

---

"Neurons and nonneuronal (glial) cells"

1. Neurons = the actual "thinking/signaling" cells
2. Glial cells = support cells (the scaffolding, maintenance crew, and security guards)

---

NEURONS

"Neurons - permanent, signal-transmitting cells of the nervous system, composed of dendrites (receive input), cell bodies, and axons (output)"

• Neurons are permanent - once formed, they generally cannot be replaced (very limited regeneration in adulthood). This is why brain damage is so serious.
• Dendrites ("dendron" = tree in Greek) = branching input cables. They receive signals from other neurons. Look like the branches of a tree.
• Cell body (soma) = the main control center containing the nucleus (DNA command center).
• Axon = the long output cable. Sends electrical signals to the next neuron or to a muscle. Can be up to 1 meter long (e.g., from your spinal cord to your toe)!

---

"Dendrites and cell bodies can be seen on Nissl staining (stains rough endoplasmic reticulum [RER])"

• Nissl staining = a special purple/blue dye (named after Franz Nissl) that stains rough endoplasmic reticulum (RER).
• RER = the cell's protein factory - makes proteins (including neurotransmitters and enzymes)
• Nissl staining highlights the cell body and dendrites but NOT the axon (axons lack RER)
• Not present in axons - this is diagnostically important

---

"Markers: neurofilament protein, synaptophysin"

• Neurofilament protein = structural protein of the neuron's internal skeleton (like rebar in concrete)
• Synaptophysin = protein found in synaptic vesicles (tiny sacs containing neurotransmitters at the synapse)

---

ASTROCYTES

"Physical support, repair, removal of excess neurotransmitters, component of blood-brain barrier, glycogen fuel reserve buffer"

1. Physical support - hold neurons in place, like scaffolding
2. Repair - after brain injury, astrocytes proliferate and form scar tissue ("gliosis")
3. Remove excess neurotransmitters - clean up the synapse after a nerve fires (prevent overstimulation)
4. Blood-brain barrier (BBB) component - their foot processes ("astrocytic feet") wrap around brain blood vessels and help form the barrier (more on BBB in page 506)
5. Glycogen fuel reserve - store energy as glycogen, which they can break down to provide glucose to neurons when blood sugar drops

---

"GFAP ↑ (marker)"

---

OLIGODENDROCYTES

"Myelinate axons in CNS (including CN II). Each myelinates many axons (~30). Predominant type of glial cell in white matter"

• Oligodendrocytes ("oligo" = few, "dendro" = branches) = the myelin-making cells of the CNS (brain and spinal cord)
• Myelin = a fatty insulating sheath wrapped around axons, like plastic insulation on an electrical wire. It dramatically speeds up nerve signal transmission.
• One oligodendrocyte can wrap its processes around ~30 different axons (very efficient!)
• "Fried egg" appearance histologically - the cell body looks like a fried egg under the microscope (round dark nucleus surrounded by clear cytoplasm)
• "Oligodendrocytes" - the text notes this should be written in quotes because of the distinctive appearance
• Injured in multiple sclerosis (MS) - MS is an autoimmune disease where the immune system attacks and destroys myelin in the CNS (demyelination). This causes patchy neurological deficits.
• Injured in progressive multifocal leukoencephalopathy (PML) - a brain infection caused by JC virus, destroying oligodendrocytes

---

"CN II" - Cranial Nerve II (Optic Nerve) is technically part of the CNS (not PNS), so it is myelinated by oligodendrocytes, NOT Schwann cells. This is why MS can cause visual problems (optic neuritis).

---

EPENDYMAL CELLS

"Ciliated simple columnar glial cells lining ventricles and central canal of spinal cord"

• Ependymal cells = cells that line the hollow, fluid-filled cavities of the brain (ventricles) and spinal cord (central canal)
• They are columnar (taller than wide, like columns), ciliated (have tiny hair-like projections called cilia that beat rhythmically), and arranged in a simple single layer
• The cilia help circulate CSF (cerebrospinal fluid) through the ventricles

---

"Apical surfaces covered with cilia (circulate CSF) and microvilli (help with CSF absorption)"

• Microvilli = tiny finger-like projections (like the villi of the intestine) that increase surface area for absorbing CSF

---

"Specialized ependymal cells (choroid plexus) produce CSF"

• Choroid plexus = specialized clusters of modified ependymal cells in the ventricles that produce CSF (cerebrospinal fluid)
• CSF is the clear fluid that bathes the brain and spinal cord, acting as a shock absorber and nutrient delivery system

---

MICROGLIA

"Phagocytic scavenger cells of CNS. HIV-infected microglia fuse to form multinucleated giant cells in CNS in HIV-associated dementia"

• Microglia = brain immune cells. "Phagocytic" = they engulf and destroy debris, bacteria, dead cells (like Pac-Man)
• They are the macrophages of the CNS
• In HIV infection: The virus infects microglia. Infected microglia fuse together forming giant cells with multiple nuclei. This causes HIV-associated dementia (cognitive decline in AIDS patients)
• Activation response: When microglia sense injury, they release inflammatory mediators like nitric oxide and glutamate
• "Not readily discernible by Nissl stain" - they don't stain prominently, making them hard to see without special stains

---

PERIPHERAL NERVOUS SYSTEM (PNS) GLIAL CELLS

SATELLITE CELLS

"Surround neuronal cell bodies in ganglia. Similar supportive role to astrocytes"

• Satellite cells surround the cell bodies of peripheral ganglia (clusters of nerve cells outside the CNS)
• Think of them as the astrocytes of the PNS - same supportive function but in peripheral ganglia

---

SCHWANN CELLS

"Myelinate axons in PNS (including CN III-XII). Each myelinates a single axon"

• Schwann cells = the myelin-makers of the peripheral nervous system
• KEY DIFFERENCE from oligodendrocytes: Each Schwann cell myelinates only ONE axon (vs. oligodendrocytes wrapping ~30 axons)
• "Schwone" - each myelinates a single ("one") axon - this is the memory cue
• Injured in Guillain-Barré syndrome (GBS) - an autoimmune condition where the immune system attacks peripheral nerve myelin, causing ascending paralysis (weakness starting from feet, moving upward). Often triggered by Campylobacter infection.
• They myelinate cranial nerves III through XII (CN III-XII are PNS nerves; CN II is CNS)

---

PAGE 504 - NEURON ACTION POTENTIAL

---

"Resting membrane potential: membrane is more permeable to K⁺ than Na⁺ at rest. Voltage-gated Na⁺ and K⁺ channels are closed"

• At rest, the inside of the neuron is NEGATIVE (-70 to -75 mV) relative to outside
• The membrane is more leaky to potassium (K⁺) than sodium (Na⁺) at rest, because more K⁺ leak channels are open
• Voltage-gated channels are special channels that open or close depending on the electrical charge (voltage) across the membrane. At rest, they are all closed.

---

Step 1: "Membrane depolarization: Na⁺ activation gate opens → Na⁺ flows inward"

• Voltage-gated Na⁺ channels OPEN (specifically the activation gate opens)
• Na⁺ rushes INTO the cell (down its concentration gradient AND electrical gradient)
• This makes the inside MORE POSITIVE (from -75 to +40 mV) = depolarization (losing the negative charge)
• This is the rising phase of the action potential graph

---

Step 2: "Membrane repolarization: Na⁺ inactivation gate closes → K⁺ activation gates open → K⁺ flows outward"

• Once the peak (+40 mV) is reached, the Na⁺ inactivation gate CLOSES (a second gate on the same channel - different from the activation gate)
• Simultaneously, K⁺ channels open - potassium rushes OUT
• K⁺ leaving takes positive charge out = inside becomes negative again = repolarization (restoring negative charge)
• This is the falling phase of the action potential graph

---

Step 3: "K⁺ activation gates are slow to close → excess K⁺ efflux and brief period of hyperpolarization"

• The K⁺ channels are slow to close, so K⁺ keeps flowing out PAST the resting potential
• This makes the inside more negative than normal = hyperpolarization (overshoot below resting)
• On the graph, this appears as the undershoot (dipping below -75 mV)
• During hyperpolarization = absolute refractory period (temporarily cannot fire another action potential)

---

Step 4: "Voltage-gated Na⁺ channels switch back to resting state. Na⁺/K⁺ pump restores ion concentration"

• The Na⁺/K⁺ ATPase pump restores the original ion concentrations:
  • Pumps 3 Na⁺ OUT and 2 K⁺ IN per cycle (uses ATP energy)
  • This is electrogenic (moves more positive charges out than in) - slightly contributes to maintaining negative inside

---

SENSORY RECEPTORS TABLE

---

"Free nerve endings - Aδ fast, myelinated; C slow, unmyelinated. A Delta plane is fast, but a taxC is slow"

• Free nerve endings = the simplest type, no special structure, just bare nerve endings
• Found in all tissues except cartilage, eye lens, and corneous in skin
• Detect pain and temperature

• Aδ (A-delta) fibers = myelinated (insulated), large, fast conduction. Carry SHARP, immediate pain ("first pain" - the "ouch!" when you stub your toe)
• C fibers = unmyelinated, small, slow conduction. Carry dull, burning, aching pain ("second pain" - the throbbing afterward)

---

"Meissner corpuscles - Large, myelinated fibers; adapt quickly - Glabrous skin (hairless) - Dynamic, fine/light touch, low-frequency vibration, indentation"

• Meissner corpuscles = encapsulated touch receptors found in glabrous skin (skin without hair - fingertips, palms, soles, lips)
• Detect light touch and low-frequency vibration (e.g., feeling texture, reading Braille)
• "Adapt quickly" = they stop firing if a stimulus remains constant (adapt = get used to it). This is why you stop feeling your clothes after a while.
• Found in glabrous skin (hairless skin)

---

"Pacinian corpuscles - Large, myelinated; adapt quickly - Deep skin layers, ligaments, joints - High-frequency vibration, pressure cooker"

• Pacinian corpuscles = large, onion-shaped receptors deep in skin and joints
• Detect high-frequency vibration (like a tuning fork vibration) and deep pressure
• Also adapt quickly
• Found deep in skin, ligaments, joints

---

"Merkel discs - Large, myelinated; adapt slowly - Finger tips, superficial skin - Pressure, deep static touch and shapes, edges"

• Merkel discs = flat disc-like receptors in superficial skin layers
• Detect sustained/static pressure, shapes, edges
• "Adapt slowly" = keep firing as long as the stimulus is present. Essential for knowing if you're holding something without looking at your hand.
• Important for texture discrimination

---

"Ruffini corpuscles - Large, myelinated fiber intertwined among collagen fiber bundles; adapt slowly - Finger tips, joints - Stretch, joint angle change"

• Ruffini corpuscles = elongated receptors in deep skin and joints
• Detect skin stretch and joint position/angle (proprioception)
• Adapt slowly - important for maintaining awareness of body position

---

PAGE 505 - PERIPHERAL NERVE STRUCTURE + INJURY RESPONSE

---

PERIPHERAL NERVE COVERINGS

"Endoneurium - thin, surrounds and supports individual myelinated nerve fibers. Guillain-Barré syndrome"

• Endoneurium = the innermost connective tissue sheath - wraps around individual axons. "Endo" = inner
• In Guillain-Barré syndrome, this layer is affected

---

"Perineurium - surrounds a fascicle; blood-nerve barrier"

• Perineurium = wraps around a fascicle (a bundle of nerve fibers). "Peri" = around
• Forms the blood-nerve barrier (similar to blood-brain barrier, prevents harmful substances from reaching the nerve fibers)
• "May be affected in epineural injections"

---

"Epineurium - surrounds entire nerve (fascicles and blood vessels)"

• Epineurium = the outermost layer, enclosing the whole nerve trunk. "Epi" = outer/upon

---

NEURONAL RESPONSE TO INJURY

"Chromatolysis - dispersion of Nissl substance throughout cytoplasm"

• When an axon is cut or severely damaged, the cell body reacts:
  • Nissl substance (RER) disperses throughout the cytoplasm (breaks down and spreads)
  • Cell body swells (round cellular swelling)
  • Nucleus moves to periphery (eccentric nucleus) - moves away from its normal central position
• This represents the cell body increasing protein synthesis (ramping up the factory) to try to repair the damaged axon

---

"Axonal retraction - proximal axon segment retracts and sprouts new protrusions for potential reinnervation"

• After injury, the axon breaks. The proximal stump (part still connected to cell body) retracts slightly, then sprouts multiple new growth tips called "growth cones"
• These growth cones try to find their way to reinnervate (reconnect with) the target

---

"Wallerian degeneration - distal axon segment and associated myelin sheath disintegrates with macrophages removing debris"

• The distal segment (cut off from the cell body, its source of nutrients) breaks down
• Macrophages (immune cells) come in and eat the debris
• The myelin sheath also breaks down

---

NEUROTRANSMITTER CHANGES WITH DISEASE TABLE

• Loss in Parkinson's (substantia nigra neurons die) = tremor, rigidity, slowness
• Excess in certain brain areas = Schizophrenia hallucinations/delusions
• Low in Depression = lack of pleasure/motivation

---

PAGE 506 - MENINGES + BLOOD-BRAIN BARRIER + VOMITING CENTER

---

MENINGES

"Three membranes that surround and protect the brain and spinal cord. Derived from both neural crest and mesoderm"

• Meninges = three-layered protective wrapping around the brain and spinal cord, like a layered bubble wrap

1. Dura mater = thick outer layer closest to skull
   • "Dura" = tough, "mater" = mother (Latin)
   • Fibrous, tough
   • Two layers in the skull; one in spinal cord
   • Contains venous sinuses (blood channels)
   • Derived from neural crest and mesoderm
2. Arachnoid mater = middle layer, contains weblike connections
   • "Arachnoid" = spider-like (cobweb appearance)
   • The subarachnoid space (between arachnoid and pia) contains CSF
   • Contains CSF
3. Pia mater = thin, fibrous inner layer that firmly adheres to brain/cord surface
   • "Pia" = tender/gentle (Latin)
   • Follows every fold of the brain

• Epidural space = potential space between dura and skull/vertebral column. "Epidural hematoma" = blood between skull and dura (from middle meningeal artery rupture)
• Subdural space = between dura and arachnoid. "Subdural hematoma" = blood here (from bridging vein rupture)

---

BLOOD-BRAIN BARRIER (BBB)

"Prevents circulating blood substances (eg, bacteria, drugs) from reaching the CSF"

• Many drugs don't work in the brain
• Many infections cannot invade the brain
• Brain is an "immunologically privileged" site

• Tight junctions between non-fenestrated capillary endothelial cells - unlike blood vessels elsewhere, brain capillaries have NO gaps between their cells (non-fenestrated = no windows/holes)
• Pericytes = cells that wrap around the capillaries
• Basement membrane = structural support layer
• Astrocyte foot processes = astrocyte extensions that wrap around the outside of the capillary

---

What PASSES the BBB?

• "Nonpolar/lipid-soluble substances cross rapidly" = fat-soluble things cross easily (alcohol, many drugs, CO₂, O₂, steroids). Think: if it dissolves in fat, it gets into the brain.
• "Glucose and amino acids cross slowly by carrier-mediated transport mechanisms" = glucose needs a special GLUT1 transporter to cross (it's water-soluble)

---

What BYPASSES the BBB?

• Area postrema = the vomiting trigger zone in the medulla. It can detect poisons/drugs in the blood and trigger vomiting. This is why chemotherapy causes nausea - the drugs reach the area postrema.
• OVLT = detects blood osmolality (salt concentration) to regulate water balance

---

"BBB disruption (eg, stroke) → vasogenic edema"

• In conditions like stroke, tumors, or infections, the BBB breaks down
• Vasogenic edema = fluid from blood leaks into the brain tissue, causing swelling (edema)
• "Hyperosmolar agents (eg, mannitol) can draw fluid" → ↑ permeability of medications

---

VOMITING CENTER

"Coordinated by NTS in the medulla, which receives information from the chemoreceptor trigger zone (CTZ), located within area postrema..."

• NTS (Nucleus Tractus Solitarius) = the coordination hub for vomiting in the medulla
• Chemoreceptor Trigger Zone (CTZ) = located in area postrema at base of 4th ventricle. Detects chemicals in blood (drugs, toxins) and signals NTS to initiate vomiting.
• 5 major receptor inputs to the vomiting center: muscarinic (M₁), neurokinin (NK-1), dopamine (D₂), and serotonin (5-HT₃)

• H₁ and M₁ antagonists → treat motion sickness (antihistamines like dimenhydrinate)
• H₁ antagonists → treat hyperemesis gravidarum (pregnancy vomiting)
• 5-HT₃, D₂, and NK-1 antagonists → treat chemotherapy-induced vomiting (ondansetron blocks 5-HT₃; haloperidol/metoclopramide blocks D₂; aprepitant blocks NK-1)

---

PAGE 507 - SLEEP PHYSIOLOGY

---

"Sleep occurs in 4-6 cycles per night, each lasting ~90 mins and consisting of 2 main stages: Non-rapid eye movement (NREM) sleep and Rapid-eye movement (REM) sleep"

• NREM = 75% of the night
• REM = 25% of the night

---

EEG Waveforms (Brain waves during different states)

• Beta = awake, alert, active thinking (highest frequency)
• Alpha = awake, eyes closed, relaxed
• Theta = light sleep (Stage N1)
• Sleep spindles + K complexes = Stage N2
• Delta = deep sleep (Stage N3)
• At night Blood isn't needed - this is just to complete the mnemonic

---

"SCN activity → NE from superior cervical ganglion → ↑ SCN → circadian rhythm..."

• SCN (Suprachiasmatic Nucleus) = the "master clock" of the body, located in the hypothalamus. It controls the circadian rhythm (24-hour body clock).
• Regulated by light input from the retina
• "↑ melatonin from pineal gland" = at night, the pineal gland (a tiny gland deep in the brain) releases melatonin, which promotes sleep

---

NREM SLEEP STAGES:

"Stage N1 (5%): Light sleep, theta waves"

• Brief transitional stage
• Easy to wake from
• May experience hypnic jerks (sudden muscle twitches as you fall asleep)

"Stage N2 (45%): Deeper sleep; sleep spindles and K complexes"

• Sleep spindles = bursts of rapid brain activity (12-14 Hz) - appear as wavy spikes on EEG
• K complexes = large sharp waves on EEG, thought to suppress arousal
• Bruxism (teeth grinding) occurs here
• Accounts for the largest portion of sleep (45%)

"Stage N3 (25%): Deepest NREM sleep. 'Two'll' - delta waves (slow-wave sleep)"

• Delta waves = slow, high-amplitude waves
• Most restorative sleep (growth hormone released here)
• Bedwetting, sleepwalking, and night terrors occur in N3!
  • Why? Because these are behaviors that emerge when someone is partially roused from deep sleep but not fully awake
• Very hard to wake from

---

REM SLEEP (25%):

"Loss of muscle tone (atonia) except in diaphragm and extraocular muscles, ↑ brain O₂ use, variable pulse/BP"

• REM = Rapid Eye Movement sleep - eyes dart back and forth
• Muscle atonia = total loss of muscle tone (except diaphragm for breathing, and eye muscles)
  • This is a protective mechanism - prevents you from physically acting out your dreams!
  • If this atonia fails = REM Sleep Behavior Disorder (person physically acts out dreams, sometimes violently)
• Brain is very active (↑ O₂ use, variable heart rate and blood pressure) - similar to being awake
• Dreams occur in REM ("REMember" your dreams)
  • Also: clitoral tumescence (engorgement) and penile erection occur during REM

---

"REM sleep behavior disorder: Loss of atonia leading to dream enactment (often violent) and vocalization. Most commonly associated with Lewy body dementia and Parkinson's disease"

• The normal muscle paralysis during REM is lost
• Person acts out their dreams - kicking, punching, yelling while asleep
• Strongly associated with Parkinson's disease and Lewy body dementia - RBD can precede Parkinson's by YEARS (prodromal Parkinson's)
• Lewy bodies = abnormal protein aggregates (clumps) found in neurons in Parkinson's and Lewy body dementia

---

FACTORS AFFECTING SLEEP ARCHITECTURE:

• Alcohol, benzodiazepines, barbiturates: ↑ N3 and REM sleep (suppress deep and dream sleep). "Benzodiazepines are useful for sleepwalking and night terrors" (by suppressing N3 where they occur).
• Aging: ↓ N3 and REM sleep, ↑ sleep-onset latency, early morning awakenings. Older people sleep less deeply and wake more easily.
• Depression: ↓ N3 sleep, ↑ REM sleep, ↑ REM latency (REM starts EARLIER in the night = "early onset REM"), repeated nighttime awakenings, early morning awakening. "Early morning awakening" is a classic symptom of depression.
• Narcolepsy: ↓ REM latency (people go straight into REM sleep = "sleep attacks" with sudden onset REM)

---

SUMMARY TABLE: Key Memory Pegs

---