The adult spinal cord ends at the lower border of L1 or upper border of L2 vertebra as the conus medullaris.

Grey matter contains neuronal cell bodies, dendrites, and synapses, giving it the grey appearance.

The lateral horn (intermediolateral cell column) contains sympathetic preganglionic neurons and is present from T1 to L2.

The lateral corticospinal tract (major motor pathway) descends in the lateral column of white matter.

Oligodendrocytes produce myelin in the CNS and can myelinate multiple axons.

These nuclei are relay stations for dorsal column sensory pathways located in the lower medulla.

Pyramidal decussation occurs at the lower medulla-spinal cord junction where 85-90% of corticospinal fibers cross.

Substantia nigra is a pigmented nucleus in the midbrain, part of basal ganglia circuitry, producing dopamine.

The diencephalon includes thalamus, hypothalamus, epithalamus, and subthalamus.

The rhomboid fossa (floor of fourth ventricle) is formed by the posterior surfaces of pons and medulla.

Prosencephalon divides into telencephalon (cerebral hemispheres) and diencephalon (thalamus, hypothalamus).

Rhombencephalon (hindbrain) divides into metencephalon (pons + cerebellum) and myelencephalon (medulla).

The midbrain (mesencephalon) cavity remains narrow and becomes the cerebral aqueduct of Sylvius.

The diencephalon gives rise to thalamus, hypothalamus, epithalamus, and subthalamus.

The rhombencephalon (hindbrain) cavity expands to form the fourth ventricle.

The Circle of Willis surrounds the optic chiasma, pituitary stalk, and interpeduncular fossa.

The anterior cerebral artery supplies the medial surface of cerebral hemispheres including the leg area of motor cortex.

The middle cerebral artery supplies the lateral surface including face and arm areas of motor cortex.

The internal carotid artery enters through the carotid canal in the petrous temporal bone.

Vertebral arteries join at the lower pontine border to form the basilar artery, which runs along the pons.

The lentiform (lenticular) nucleus = putamen + globus pallidus, separated from caudate by internal capsule.

Striatum = caudate nucleus + putamen, connected by grey matter bridges.

The internal capsule is a V-shaped white matter tract between the caudate/thalamus medially and lentiform nucleus laterally.

The posterior limb contains the corticospinal (pyramidal) tract – hence stroke here causes contralateral hemiplegia.

The genu (“knee”) contains corticobulbar fibers to cranial nerve nuclei.

The vermis is the midline structure connecting the two cerebellar hemispheres.

Purkinje cells are large GABAergic neurons that send inhibitory output to deep cerebellar nuclei.

The middle cerebellar peduncle (brachium pontis) is the largest and connects pons to cerebellum, carrying corticopontocerebellar fibers.

The dentate nucleus is the largest deep cerebellar nucleus, located in each cerebellar hemisphere.

The outermost molecular layer contains Purkinje cell dendrites, parallel fibers (granule cell axons), and stellate/basket cells.

Cerebellar damage causes ipsilateral ataxia, intention tremor, dysmetria, and dysdiadochokinesia – incoordination without paralysis.

The lateral cerebellar hemisphere (neocerebellum) controls ipsilateral limb coordination via dentate nucleus.

The choroid plexus in the ventricles produces approximately 500 mL of CSF daily.

Two lateral foramina of Luschka and one median foramen of Magendie allow CSF exit from fourth ventricle.

The interventricular foramen (of Monro) connects each lateral ventricle to the third ventricle.

Arachnoid granulations (villi) project into the superior sagittal sinus and drain CSF into venous blood.

Obstruction within the ventricular system (like aqueduct stenosis) causes non-communicating/obstructive hydrocephalus.

CN VII supplies all muscles of facial expression derived from the second pharyngeal arch.

CN VII exits via the stylomastoid foramen after traversing the facial canal.

LMN facial palsy affects the entire half of the face including forehead (can’t close eye, raise eyebrow).

The facial artery arises from the external carotid in the neck and crosses the mandible to supply the face.

The facial vein (common facial vein) drains into the internal jugular vein.

The area from the corners of mouth to nose bridge has valveless veins connecting to cavernous sinus via ophthalmic veins.

Central lower lip, tip of tongue, and chin drain to submental nodes.

Stensen’s duct (parotid duct) opens on a papilla opposite the upper second molar tooth.

The facial nerve passes through the parotid, dividing it into lobes.

The parotid is purely serous, producing watery secretion rich in amylase.

Masseter, temporalis, and medial and lateral pterygoids are supplied by the mandibular nerve (V3).

The lateral pterygoid protrudes (protracts) the mandible and depresses the chin when acting with digastric.

The TMJ is a synovial joint with a fibrocartilaginous disc dividing it into upper and lower compartments.

The masseter is the primary elevator of the mandible, assisted by temporalis and medial pterygoid.

The upper head of lateral pterygoid attaches to the articular disc and joint capsule, pulling the disc forward during mouth opening.

TMJ dislocation usually involves the condyle moving anterior to the articular eminence during wide mouth opening or yawning.

The TMJ receives sensory supply from auriculotemporal nerve (posterior) and masseteric nerve (anterior) – both from V3.

The anterior triangle boundaries: anterior border of SCM (posteriorly), midline of neck (anteriorly), and inferior border of mandible (superiorly).

The spinal accessory nerve (CN XI) crosses the posterior triangle superficially, making it vulnerable during surgery.

The carotid sheath contains common/internal carotid artery, internal jugular vein, vagus nerve, and deep cervical lymph nodes.

The common carotid bifurcation occurs at the upper border of thyroid cartilage, approximately at C3/C4 level.

EJV forms behind the angle of mandible from posterior auricular vein and posterior division of retromandibular vein.

The cervical plexus arises from C1-C4 ventral rami, supplying neck skin and some muscles.

The phrenic nerve (motor to diaphragm) arises from C3, C4, C5 (mainly C4).

Greater auricular nerve (C2, C3) provides cutaneous sensation to skin over parotid, mastoid, and both surfaces of the ear (mainly lobule).

CN XI supplies motor innervation to SCM and trapezius.

The subclavian artery becomes axillary artery at the outer border of the first rib.

The vertebral artery arises from the first part of the subclavian artery, ascends through transverse foramina of C1-C6, and enters the skull via foramen magnum.

The thyroid develops from an endodermal thickening at the foramen cecum, descending via the thyroglossal duct.

Thyroglossal cyst lies in the midline (along thyroglossal duct remnant) and moves upward with swallowing and tongue protrusion (attached to foramen cecum).

Thyroid has spherical follicles lined by simple cuboidal epithelium filled with colloid (thyroglobulin).

Cricothyroid is supplied by the external laryngeal nerve (branch of superior laryngeal nerve).

Posterior cricoarytenoid is the only abductor of vocal cords (opens the glottis for breathing).

First arch forms mandible, maxilla, muscles of mastication, anterior 2/3 tongue mucosa, malleus, incus.

The second pharyngeal arch is innervated by CN VII and forms muscles of facial expression, stapedius, stylohyoid, posterior belly of digastric, and stapes.

CN I (olfactory nerve) transmits smell sensation from nasal mucosa to the olfactory bulb.

Maxillary, anterior ethmoidal, and frontal sinuses drain into the middle meatus.

The nasolacrimal duct drains tears from the eye into the inferior meatus of the nose.

Cleft palate occurs when the palatine shelves (from maxillary processes) fail to fuse with each other and/or the nasal septum.

General sensation to the nasal septum comes from nasopalatine nerve (from V2) and anterior ethmoidal nerve (from V1).

Palatoglossus is supplied by the vagus nerve (CN X) via the pharyngeal plexus (it’s functionally part of the soft palate).

Chorda tympani (branch of CN VII) carries taste from anterior 2/3 of tongue; it travels with the lingual nerve but taste fibers belong to VII.

The submandibular gland is mixed seromucous with serous demilunes capping mucous acini, predominantly serous.

CN IX supplies taste and general sensation to the posterior 1/3 of tongue including circumvallate papillae.

All pharyngeal constrictors are supplied by the pharyngeal plexus (CN X with contribution from CN IX).

CN VI supplies the lateral rectus (abducts eye).

Levator palpebrae superioris is supplied by CN III (oculomotor).

The retina (pigmented and neural layers) develops from the optic cup, an outgrowth of the diencephalon (neuroectoderm).

In conduction deafness, Weber lateralizes to the affected ear and Rinne is negative (bone > air).

Tensor tympani (dampens ossicle vibration) is supplied by CN V3 (from first arch).

The first pharyngeal pouch forms the tubotympanic recess → middle ear cavity and Eustachian tube.

The inferior orbital fissure connects pterygopalatine fossa to the orbit (transmitting infraorbital nerve and vessels).

CN V2 exits middle cranial fossa through foramen rotundum to enter pterygopalatine fossa.

CN X exits through the jugular foramen along with CN IX, CN XI, and internal jugular vein.

CN XII exits through the hypoglossal canal in the occipital bone to supply tongue muscles.

CN XI supplies trapezius (shoulder shrug) and sternocleidomastoid (turns head to opposite side).

Vidian nerve = deep petrosal (sympathetic from internal carotid plexus) + greater petrosal (parasympathetic from CN VII).

The greater petrosal nerve (branch of CN VII) carries preganglionic parasympathetic fibers to the pterygopalatine ganglion, which then supplies lacrimal gland, nasal and palatal glands.

CN IV is unique: only cranial nerve to exit dorsally, only one to fully decussate, and has the longest intracranial course.

Superior oblique (CN IV) depresses the eye when adducted, abducts, and intorts.

The middle meningeal artery passes through foramen spinosum to supply the dura mater.

The subclavian artery has three parts relative to scalenus anterior.

The cornea is avascular and receives nutrition from aqueous humor (posteriorly), tears (anteriorly), and limbal blood vessels (peripherally).

The axon hillock integrates graded potentials because it contains the highest density of voltage-gated sodium channels, making it the trigger zone.

Glycine serves as the primary inhibitory neurotransmitter in the spinal cord and brainstem by opening chloride channels, causing hyperpolarization.

Depolarization of the presynaptic terminal opens voltage-gated calcium channels; the resulting calcium influx directly binds to synaptotagmin, triggering vesicular fusion and neurotransmitter release.

The degeneration of dopaminergic neurons in the substantia nigra pars compacta leads to the hallmark resting tremor, rigidity, and bradykinesia of Parkinson’s disease.

Glutamate mediates the vast majority of fast excitatory synaptic transmission in the brain via AMPA and NMDA receptors, promoting cellular depolarization.

Nicotinic cholinergic receptors are ligand-gated ion channels at the motor endplate that immediately open upon acetylcholine binding, causing rapid muscle depolarization.

Spatial summation occurs when multiple synapses at different spatial locations on the neuron fire simultaneously, combining their potentials to reach the firing threshold.

Acetylcholinesterase rapidly hydrolyzes acetylcholine into acetate and choline, immediately terminating the excitatory signal at the postsynaptic membrane.

Unlike small-molecule transmitters, neuropeptides require ribosomal protein synthesis and packaging in the rough endoplasmic reticulum and Golgi apparatus, located exclusively in the soma.

GABA binds to GABA-A receptors, which are ligand-gated chloride channels; chloride influx hyperpolarizes the postsynaptic membrane, strongly inhibiting the neuron.

A-delta fibers are thinly myelinated and transmit initial, sharp, prickling pain rapidly to the central nervous system to elicit immediate withdrawal reflexes.

When an axon is severed, the distal portion is separated from the cell body’s nutritional support, leading to rapid anterograde axonal and myelin degradation.

Chromatolysis involves cell body swelling, eccentric nuclear displacement, and dispersion of Nissl bodies as the neuron massively upregulates protein synthesis to repair a damaged axon.

Pacinian corpuscles are large, onion-like encapsulated receptors situated deep in the dermis and fascia that adapt very rapidly, making them perfect for detecting high-frequency vibration.

The dorsal column-medial lemniscal (DCML) pathway utilizes highly myelinated, fast-conducting fibers to carry highly localized tactile and proprioceptive signals.

First-order neurons ascend ipsilaterally in the dorsal columns and synapse in the gracile and cuneate nuclei of the medulla, where second-order neurons then cross over as the internal arcuate fibers.

The primary somatosensory cortex (Brodmann areas 3, 1, 2) resides in the postcentral gyrus of the parietal lobe, processing all incoming somatic sensory signals.

The anterolateral system (consisting of the lateral and anterior spinothalamic tracts) carries slower, less localized sensations like pain, thermal changes, and crude touch.

Upon entering the spinal cord, pain and temperature fibers immediately synapse; second-order neurons then cross the midline through the anterior white commissure to ascend contralaterally.

Merkel’s discs are slowly adapting tactile receptors concentrated in the fingertips that continuously fire during sustained pressure, allowing for high-resolution tactile discrimination like reading Braille.

The dorsal and ventral spinocerebellar tracts carry high-speed, unconscious proprioceptive feedback from muscles and joints directly to the cerebellum to constantly refine motor coordination.

All ascending sensory fibers radiating from the thalamus traverse the posterior limb of the internal capsule to successfully reach the postcentral gyrus.

The neospinothalamic tract utilizes fast-conducting A-delta fibers to transmit sharp, immediate pain directly to the VPL nucleus of the thalamus and onto the sensory cortex for precise localization.

The paleospinothalamic tract utilizes unmyelinated C fibers to transmit diffuse, chronic pain, terminating widely in the reticular formation and thalamus to evoke emotional and arousal responses.

Visceral pain fibers from the heart converge on the same T1-T5 spinal cord segments as somatic pain fibers from the left arm and jaw, tricking the brain into perceiving somatic pain.

Enkephalins and endorphins are endogenous opioids that bind to mu-opioid receptors in the dorsal horn, causing presynaptic and postsynaptic inhibition of pain transmission.

Migraines are vascular headaches triggered by the release of vasoactive neuropeptides (like CGRP) that cause painful vasodilation and severe inflammation of the highly sensitive meningeal blood vessels.

The lateral spinothalamic tract exclusively carries pain and temperature sensations, decussating at the spinal cord level immediately upon entry.

Descending pain-control fibers from the nucleus raphe magnus release serotonin in the dorsal horn, which stimulates local interneurons to release pain-blocking enkephalins.

Muscle spindles lie parallel to extrafusal fibers and exquisitely detect changes in muscle length and the rate of stretch, instantly firing to cause reflex muscle contraction.

Golgi tendon organs detect excessive muscle tension and trigger an inverse myotatic reflex via inhibitory interneurons, forcing the contracting muscle to abruptly relax to prevent tendon avulsion.

Gamma motor neurons innervate intrafusal fibers, keeping the muscle spindle taut and exquisitely sensitive to stretch even when the main muscle shortens.

Alpha motor neurons are large, heavily myelinated lower motor neurons that directly innervate the massive extrafusal muscle fibers, providing the actual force for skeletal muscle contraction.

While a painful stimulus causes the ipsilateral leg to rapidly flex and withdraw, the crossed extensor reflex simultaneously contracts the contralateral leg extensors to prevent the person from falling over.

The stretch reflex (e.g., the patellar tendon reflex) is the only reflex where the Ia sensory afferent synapses directly onto the alpha motor neuron without any intervening interneurons.

To allow a joint to move freely during a reflex, collateral branches of the sensory afferent excite inhibitory interneurons (Ia inhibitory interneurons), which actively suppress the antagonist muscle’s motor neurons.

Situated in series with muscle fibers, Golgi tendon organs exclusively monitor the degree of tension or force generated by the contracting muscle.

Severing the lower motor neuron removes all excitatory input to the muscle, causing an immediate loss of muscle tone (flaccidity), absolutely absent reflexes, and rapid profound atrophy.

Upper motor neuron (UMN) lesions remove the cortex’s normal descending inhibitory control over spinal reflexes, leading to exaggerated, hyperactive stretch reflexes and severe spastic muscle tone.

Firmly stroking the sole of the foot normally causes plantar flexion of the toes; an UMN lesion causes a positive Babinski sign, presenting as dorsiflexion of the big toe and fanning of the others.

Fasciculations are spontaneous, visible twitches of individual motor units caused by the spontaneous firing of a sick or dying lower motor neuron before its complete destruction.

Because the spinothalamic tract completely decussates upon entering the spinal cord, cutting half the cord destroys the already-crossed fibers, eliminating pain/temperature on the opposite side below the lesion.

The maculae contain hair cells embedded in a gelatinous matrix loaded with heavy calcium carbonate crystals (otoliths) that shift with gravity or linear acceleration, bending the stereocilia.

When the head rotates, the endolymph fluid inside the semicircular canals lags behind due to inertia, forcefully pushing the cupula and bending hair cells to signal rotational movement.

The flocculonodular lobe of the cerebellum receives direct, massive input from the vestibular nerve, serving as the central computer to instantly calculate and maintain posture and equilibrium.

The medial longitudinal fasciculus (MLF) perfectly links the abducens (CN VI) nucleus of one side with the oculomotor (CN III) nucleus of the other, allowing both eyes to move horizontally together.

The vestibular system constantly drives the vestibulo-ocular reflex; any unilateral damage creates an intense imbalance in vestibular tone, causing the eyes to slowly drift and rapidly snap back (nystagmus).

The precentral gyrus (Brodmann area 4) in the frontal lobe houses the upper motor neurons that meticulously map to specific body parts (motor homunculus) to drive voluntary motor commands.

The cerebral cortex has six layers; Layer V (the internal pyramidal layer) contains the massive Betz cells and other large pyramidal neurons whose long axons form the descending corticospinal tract.

The lateral corticospinal tract bypasses interneurons to synapse directly on alpha motor neurons supplying the hands and fingers, enabling incredible, fractionated manual dexterity.

The profound loss of inhibitory GABAergic medium spiny neurons in the striatum completely removes the brakes on motor activity, leading to the classic hyperkinetic, wildly writhing choreiform movements.

The direct pathway inhibits the globus pallidus internus, preventing it from inhibiting the thalamus.

Destroying the subthalamic nucleus removes its excitatory drive to the globus pallidus internus, destroying the indirect pathway’s “brakes” and resulting in massive, uncontrolled contralateral limb flinging.

The medium spiny neurons of the striatum heavily utilize gamma-aminobutyric acid (GABA) to exert profound inhibitory control over the globus pallidus in both the direct and indirect pathways.

The parasympathetic nervous system is anatomically defined by its craniosacral outflow, originating from brainstem cranial nerve nuclei (III, VII, IX, X) and sacral segments S2-S4.

Acetylcholine is the universal preganglionic neurotransmitter, crossing the autonomic ganglia to rapidly bind and activate nicotinic cholinergic receptors on the postganglionic neurons.

Stimulated directly by preganglionic sympathetic fibers, chromaffin cells in the adrenal medulla dump massive amounts of epinephrine (80%) and norepinephrine (20%) directly into the bloodstream for a global fight-or-flight response.

The GCS maxes out at 15 points by assessing Eyes (4), Verbal (5), and Motor (6) responses to objectively grade the depth of a patient’s coma or altered mental status.

The medulla oblongata houses the solitary tract nucleus, dorsal/ventral respiratory groups, and vasomotor centers, ensuring continuous, automatic regulation of blood pressure, heart rate, and breathing.

The left cerebral hemisphere almost universally houses Broca’s and Wernicke’s areas, making it the absolute dominant hemisphere for speech production and language comprehension in nearly all humans.

Located in the superior temporal gyrus of the dominant hemisphere, Wernicke’s area decodes spoken and written words to extract their exact meaning.

Located in the inferior frontal gyrus, Broca’s area physically orchestrates the motor programs required to speak; damage here causes non-fluent, halting, incredibly frustrated speech, though comprehension remains perfectly intact.

The hippocampus critically processes and encodes new facts, events, and spatial maps, slowly transferring these explicit memories to the cortex for permanent long-term storage.

The amygdala assigns intense emotional valence (especially fear) to sensory inputs, triggering instant autonomic fight-or-flight responses and encoding fearful memories to ensure future survival.

Alpha waves are the classic, perfectly synchronized resting rhythm of the occipital cortex, immediately vanishing and being replaced by fast beta waves the moment the person opens their eyes or concentrates.

During REM sleep, the cortex is highly active (beta waves, dreaming), but descending brainstem pathways completely paralyze all voluntary muscles (except the eyes and diaphragm) to prevent acting out the dreams.

Absence seizures involve abrupt, brief lapses of consciousness (staring spells) without losing postural tone, perfectly synchronized with a 3-Hz spike-and-wave EEG discharge generated by the thalamus.

A grand mal seizure involves a sudden, terrifying loss of consciousness followed by rigid muscle contraction (tonic phase) and violently rhythmic jerking (clonic phase) across the entire body.

The RAS is a diffuse network of highly excitable neurons in the brainstem that constantly bombards the thalamus and cortex with excitatory signals to keep the brain completely awake and conscious.

The choroid plexus is a tuft of specialized ependymal cells and capillaries present in all ventricles that actively filters blood plasma to continuously produce roughly 500 mL of pristine CSF daily.

Arachnoid villi act as strictly one-way pressure valves, protruding directly into the superior sagittal sinus to continuously dump old CSF back into the venous bloodstream.

In communicating hydrocephalus, all ventricular pathways are perfectly open and communicating, but the arachnoid villi fail to reabsorb the fluid (often due to post-meningitis scarring), massively dilating all four ventricles simultaneously.

The tiny axons of olfactory neurons must physically pass through the porous cribriform plate to reach the olfactory bulb resting on top of it.

The olfactory nerve (CN I) is dedicated entirely to transmitting odorant signals straight to the olfactory cortex without first relaying through the thalamus, unique among all sensory systems.

While all taste qualities can be perceived everywhere to some degree, the fungiform papillae packed on the anterior tip of the tongue are exceptionally sensitive to sugars and sweet stimuli.

The chorda tympani branch of the Facial nerve (CN VII) carries all taste sensation from the anterior two-thirds of the tongue directly to the solitary tract nucleus in the medulla.

Contracting the ciliary muscle actually releases the tension on the suspensory ligaments, allowing the elastic lens to bulge and become rounder, massively increasing its refractive power to focus on close items.

Rhodopsin (visual purple) is incredibly sensitive to light; capturing a single photon causes it to violently break down into opsin and all-trans retinal, initiating the phototransduction cascade.

Retinal ganglion cells are the final output neurons of the entire retina; their long axons traverse the retinal surface and exit cleanly at the optic disc to form the optic nerve.

The central optic chiasm contains exclusively the decussating fibers from the nasal halves of both retinas; severing them completely blinds the patient to the outer (temporal) halves of their visual fields.

The primary visual cortex (V1 or Brodmann area 17) resides purely in the occipital lobe, firmly anchored around the deep calcarine fissure, meticulously processing all raw visual inputs from the LGN.

Sound traveling from low-impedance air to high-impedance cochlear fluid would normally bounce off; the ossicles massively amplify the pressure to forcefully drive the sound into the fluid, matching the impedance.

The basilar membrane is incredibly stiff, narrow, and tight at the base of the cochlea (near the oval window), making it perfectly tuned to vividly resonate at only very high frequencies.

The Organ of Corti sits squarely on the basilar membrane, housing the inner and outer hair cells whose stereocilia violently bend against the tectorial membrane to generate auditory action potentials.

The medial geniculate nucleus (MGN) of the thalamus is the mandatory final relay station for all auditory information before it is sent to the primary auditory cortex in the temporal lobe.

Serotonin (5-hydroxytryptamine) is synthesized from tryptophan via tryptophan hydroxylase and aromatic L-amino acid decarboxylase.

Tyrosine hydroxylase converts tyrosine to DOPA and is the rate-limiting enzyme in catecholamine biosynthesis.

GABA is synthesized from glutamate by the enzyme glutamate decarboxylase (GAD), which requires pyridoxal phosphate (vitamin B6).

Acetylcholinesterase specifically hydrolyzes acetylcholine into acetate and choline at synaptic junctions.

Parkinson’s disease results from degeneration of dopaminergic neurons in the substantia nigra.

The brain primarily uses glucose as its main fuel source because fatty acids cannot cross the blood-brain barrier efficiently.

During prolonged fasting/starvation, ketone bodies (acetoacetate and β-hydroxybutyrate) become the major alternative fuel for the brain.

Tay-Sachs disease results from deficiency of hexosaminidase A, causing accumulation of GM2 ganglioside in neurons.

Astrocytes, along with endothelial tight junctions, form the blood-brain barrier that regulates molecular exchange.

Vitamin B12 (cobalamin) is essential for myelin synthesis and its deficiency causes subacute combined degeneration of the spinal cord.

Sphingomyelin is the most abundant phosphosphingolipid in myelin sheaths and cell membranes.

Sphingolipids have sphingosine (an amino alcohol) as their backbone, unlike glycerophospholipids which have glycerol.

Niemann-Pick disease (Type A and B) results from sphingomyelinase deficiency, causing sphingomyelin accumulation.

Sphingomyelin is formed when phosphocholine is attached to ceramide.

Gaucher disease results from glucocerebrosidase deficiency, causing glucocerebroside accumulation.

The choroid plexus, located in the ventricles, produces approximately 70% of CSF.

Normal CSF glucose is approximately 60-70% (two-thirds) of blood glucose levels.

Bacterial meningitis characteristically shows decreased CSF glucose due to bacterial consumption and impaired transport.

Normal CSF protein is 15-45 mg/dL, much lower than plasma due to the blood-brain barrier.

Oligoclonal bands (immunoglobulins) in CSF are highly suggestive of multiple sclerosis, indicating intrathecal antibody production.

Prostaglandins are synthesized from arachidonic acid (a 20-carbon polyunsaturated fatty acid) via the cyclooxygenase pathway.

Aspirin irreversibly inhibits cyclooxygenase (COX-1 and COX-2), reducing prostaglandin and thromboxane synthesis.

PGE2 protects gastric mucosa by stimulating mucus and bicarbonate secretion and reducing acid secretion.

Platelets produce thromboxane A2 (TXA2), which promotes platelet aggregation and vasoconstriction.

Phospholipase A2 cleaves arachidonic acid from the sn-2 position of membrane phospholipids, initiating eicosanoid synthesis.

Restriction endonucleases (restriction enzymes) cut DNA at specific palindromic sequences.

DNA ligase joins DNA fragments by forming phosphodiester bonds between them.

A vector (plasmid, phage, cosmid) carries foreign DNA into a host cell for replication.

Escherichia coli is the most commonly used host due to its rapid growth, well-understood genetics, and ease of manipulation.

CRISPR-Cas9 is a revolutionary gene-editing technology that allows precise modification of DNA sequences.

Lesch-Nyhan syndrome results from HGPRT deficiency, causing hyperuricemia, gout, and self-mutilation behavior.

Humans lack uricase, so uric acid is the final product of purine catabolism.

Allopurinol inhibits xanthine oxidase, reducing uric acid production.

Pyrimidine de novo synthesis begins with carbamoyl phosphate (formed in cytoplasm by carbamoyl phosphate synthetase II).

Uracil is found in RNA instead of thymine (found in DNA).

Hydrogen bonds form between complementary base pairs (A-T has 2; G-C has 3).

Semiconservative replication means each daughter DNA molecule contains one parental (old) strand and one newly synthesized strand, as demonstrated by Meselson and Stahl.

Primase synthesizes short RNA primers that provide the 3′-OH group needed for DNA polymerase to begin synthesis.

Okazaki fragments are short DNA segments synthesized discontinuously on the lagging strand (3′ to 5′ template direction).

DNA polymerase I has 5′ to 3′ exonuclease activity that removes RNA primers and fills the gaps with DNA.

AUG is the universal start codon and codes for methionine in eukaryotes (N-formylmethionine in prokaryotes).

Transfer RNA (tRNA) carries specific amino acids to the ribosome, matching them to mRNA codons via anticodons. mRNA carries genetic information. rRNA forms ribosomes. snRNA is involved in splicing.

The wobble hypothesis (by Crick) explains that non-standard base pairing can occur at the third codon position, allowing one tRNA to recognize multiple codons.

The P (peptidyl) site holds the tRNA with the growing polypeptide chain.

Splicing removes introns and joins exons, performed by spliceosomes.

Taq DNA polymerase (from Thermus aquaticus) is heat-stable and essential for PCR.

Denaturation occurs at 94-98°C to separate DNA strands.

Primers are short oligonucleotides that anneal to template DNA and provide a 3′-OH group for Taq polymerase to extend.

PCR amplification is exponential: 2^n copies after n cycles.

Annealing (55-65°C) allows primers to hydrogen bond to complementary sequences on template DNA.

The lateral spinothalamic tract carries pain and temperature sensations and crosses at the spinal cord level within 1-2 segments of entry.

The caudal neuropore closes by day 28 and forms the spinal cord.

Alpha motor neurons are large multipolar cells in the anterior horn with prominent Nissl bodies that directly innervate extrafusal skeletal muscle fibers.

The posterior funiculus contains fasciculus gracilis (lower limb) and fasciculus cuneatus (upper limb), carrying discriminative touch, vibration sense, and conscious proprioception.

Astrocytes have perivascular foot processes that surround capillaries and contribute to the blood-brain/spinal cord barrier.

This is lateral medullary syndrome (Wallenberg syndrome) caused by PICA occlusion.

The tongue deviates toward the side of a lower motor neuron lesion (hypoglossal nucleus or nerve).

The facial colliculus is formed by facial nerve fibers wrapping around the abducens nucleus in the pons before exiting.

Weber syndrome results from lesion of the ventral midbrain affecting the oculomotor nerve (causing ipsilateral CN III palsy) and cerebral peduncle (causing contralateral hemiplegia due to corticospinal tract damage).

The substantia nigra pars compacta contains dopaminergic neurons that form the nigrostriatal pathway, projecting to the striatum (caudate and putamen).

The superior cerebellar peduncle (brachium conjunctivum) primarily carries efferent fibers from the dentate nucleus to the red nucleus and thalamus.

The inferior cerebellar peduncle (restiform body) carries vestibular afferents from the vestibular nuclei to the flocculonodular lobe (vestibulocerebellum), which coordinates balance and eye movements.

The VPL (body sensation) and VPM (face sensation) nuclei receive all somatosensory information and relay it to the sensory cortex.

The telencephalon develops into cerebral hemispheres, cerebral cortex, basal ganglia, hippocampus, and olfactory structures.

The lenticulostriate arteries (branches of the middle cerebral artery) supply the posterior limb of the internal capsule, where the corticospinal tract passes.

Bitemporal hemianopia results from compression of crossing fibers at the optic chiasm, typically by pituitary tumors.

The suprachiasmatic nucleus receives direct input from the retina (retinohypothalamic tract) and acts as the master circadian pacemaker, regulating sleep-wake cycles.

Watershed infarcts occur at boundary zones between major arterial territories (ACA-MCA or MCA-PCA) and are most commonly caused by severe systemic hypotension or cardiac arrest, where distal territories receive least perfusion.

Layer V (internal pyramidal layer) contains large pyramidal neurons, including Betz cells in the primary motor cortex.

In the direct pathway, striatal medium spiny neurons (expressing D1 receptors) send GABAergic inhibitory projections directly to the globus pallidus interna (GPi), which then reduces its inhibition on the thalamus, facilitating movement.

Huntington’s disease causes degeneration of GABAergic medium spiny neurons in the striatum, particularly those of the indirect pathway.

The claustrum has widespread reciprocal connections with virtually all cortical areas and is hypothesized to integrate sensory information and contribute to consciousness and attention.

The subthalamic nucleus (STN) excites the GPi via glutamate in the indirect pathway.

The flocculonodular lobe (vestibulocerebellum) and vermis receive vestibular input through the inferior cerebellar peduncle (restiform body) and maintain balance and posture.

Purkinje cells are large GABAergic neurons arranged in a single layer between molecular and granular layers.

The lateral cerebellar hemisphere (cerebrocerebellum/neocerebellum) projects to the dentate nucleus, the largest and most lateral deep cerebellar nucleus.

Granule cell axons ascend to the molecular layer and bifurcate to form parallel fibers, which run perpendicular to Purkinje cell dendritic trees.

The inferior olivary nucleus sends climbing fibers through the inferior cerebellar peduncle to synapse directly on Purkinje cell dendrites.

Dilated lateral and third ventricles with a normal fourth ventricle indicate obstruction at the cerebral aqueduct (of Sylvius), the narrowest point of the ventricular system.

Choroid plexus is present in the lateral ventricles (body and inferior horn), third ventricle (roof), and fourth ventricle (roof).

Arachnoid granulations (villi) project into the superior sagittal sinus and allow one-way drainage of CSF into venous blood.

In lumbar puncture, the needle passes through: skin → subcutaneous tissue → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura mater → arachnoid mater → subarachnoid space.

Hyperacusis indicates loss of the stapedius muscle function (nerve to stapedius).

The inferior alveolar nerve is a branch of V3 (mandibular division).

The inferior head of lateral pterygoid inserts on the neck of the mandible and protrudes the mandible during opening.

The lateral pterygoid muscles work together to protrude the mandible and open the mouth.

The danger area of the face (medial angle, nose, upper lip) has venous drainage through the facial vein, which connects to the superior ophthalmic vein via the angular vein.

The facial artery crosses the inferior border of the mandible anterior to the masseter muscle, where its pulse can be palpated.

The central lower lip and chin drain to submental lymph nodes (below the chin between the anterior bellies of digastric).

The forehead, lateral face, eyelids, and anterior scalp drain to preauricular (superficial parotid) lymph nodes located just anterior to the tragus of the ear.

The TMJ is a synovial joint combining hinge and gliding movements.

The upper compartment allows translation (gliding) of the disc-condyle complex on the articular eminence during wide opening and protrusion.

The TMJ is innervated primarily by the auriculotemporal nerve (from V3) and masseteric nerve.

Immediately after exiting the stylomastoid foramen, CN VII gives motor branches to the posterior belly of digastric and stylohyoid muscles.

The marginal mandibular branch courses along the lower border of the mandible and supplies muscles of the lower lip including depressor anguli oris, depressor labii inferioris, and mentalis.

The parotid gland is composed almost entirely of serous acini with dark-staining cells containing zymogen granules.

Stensen’s duct runs forward over the masseter, turns medially at its anterior border, pierces the buccinator muscle, and opens into the oral vestibule opposite the upper second molar tooth as a small papilla.

The thyroid develops from the foramen cecum at the base of the tongue and descends anterior to the hyoid bone and laryngeal cartilages to reach its final pretracheal position.

Most branchial cysts (90%) arise from second pharyngeal apparatus remnants.

The accessory nerve (CN XI) crosses the posterior triangle superficially, emerging from behind SCM at Erb’s point and descending to enter trapezius.

The carotid triangle is bounded by SCM (posteriorly), superior belly of omohyoid (anteroinferiorly), and posterior belly of digastric (superiorly).

The common carotid bifurcates at the upper border of thyroid cartilage (C3-4 level).

The internal jugular vein joins the subclavian vein behind the sternoclavicular joint to form the brachiocephalic vein.

The cervical plexus is formed by anterior rami of C1-C4.

The accessory nerve (CN XI) passes through the posterior triangle and supplies sternocleidomastoid (turns head to opposite side) and trapezius (shoulder shrugging and arm elevation).

The second pharyngeal arch (hyoid arch) gives rise to muscles of facial expression, stapedius, stylohyoid, and posterior belly of digastric.

The posterior third of the tongue develops from the third pharyngeal arch (copula and hypobranchial eminence) and receives both general and taste sensation from CN IX (glossopharyngeal).

Superior parathyroid glands develop from the fourth pharyngeal pouch and undergo short descent, so they usually end up near or behind the upper thyroid.

The cervical sinus forms when the second pharyngeal arch grows over the 2nd, 3rd, and 4th clefts.

Thyroid follicles are lined by follicular (principal) cells that synthesize thyroglobulin, which is stored in the colloid.

The right recurrent laryngeal nerve loops around the right subclavian artery and ascends in the tracheoesophageal groove posterior to the thyroid lobe.

The superior thyroid artery is the first branch of the external carotid artery.

Olfactory nerve fibers (CN I) pass through multiple small foramina in the cribriform plate of the ethmoid bone to reach the nasal cavity.

The floor of the maxillary sinus is formed by the alveolar process of maxilla and lies close to the roots of maxillary premolars and molars.

The hiatus semilunaris is a curved groove in the middle meatus that receives drainage from the frontal sinus (via frontonasal duct), maxillary sinus (via maxillary ostium), and anterior ethmoidal air cells.

Little’s area (Kiesselbach’s plexus) is located on the anteroinferior nasal septum and receives sensory innervation from both the anterior ethmoidal nerve (branch of V1 via nasociliary) and nasopalatine nerve (branch of V2 via pterygopalatine ganglion).

The secondary palate forms from bilateral lateral palatine shelves (from maxillary prominences) that grow medially, elevate above the tongue, and fuse in the midline with each other and with the primary palate anteriorly.

Tensor veli palatini is innervated by the mandibular nerve (V3), the only palatine muscle not supplied by the pharyngeal plexus (CN X and XI).

Taste from the anterior two-thirds of the tongue is carried by the chorda tympani, a branch of the facial nerve (CN VII), which joins the lingual nerve (V3) in the infratemporal fossa.

Palatoglossus is anatomically a tongue muscle but embryologically develops from pharyngeal arch mesoderm and is supplied by the pharyngeal plexus (CN X and XI, primarily vagus).

Genioglossus is the main protruder of the tongue.

The submandibular gland wraps around the posterior free edge of mylohyoid muscle (superficial and deep lobes).

The submandibular gland is a mixed gland with both serous and mucous acini.

The sublingual gland has 8-20 small ducts (ducts of Rivinus) that open along the sublingual fold (plica sublingualis) on the floor of the mouth.

The vagus nerve supplies all palate muscles except tensor veli palatini.

CN IX exits through the jugular foramen (with CN X and XI).

The vagus nerve (via pharyngeal plexus) supplies levator veli palatini, which is the main elevator of the soft palate.

CN XII descends between IJV and ICA, then loops around the occipital artery and passes superficial to the external carotid artery and lingual artery before entering the tongue deep to the mylohyoid muscle with the lingual artery.

The trochlear nerve (CN IV) supplies the superior oblique muscle, which depresses the eye when adducted (looking down and medially) and intorts the eye.

CN III (oculomotor) supplies all extraocular muscles except superior oblique (IV) and lateral rectus (VI).

Sensory innervation to the eyeball comes from the nasociliary nerve, a branch of the ophthalmic division (V1) of the trigeminal nerve.

Parasympathetic fibers run superficially in CN III, making them vulnerable to external compression (aneurysm, uncal herniation).

The outer nuclear layer contains the cell bodies (nuclei) of rods and cones.

The fovea centralis contains only cones (especially in the foveola) for high-acuity color vision.

Zonular fibers attach the lens to the ciliary body.

Conductive hearing loss is caused by problems in sound transmission through the external ear, tympanic membrane, or middle ear ossicles.

Ménière’s disease is associated with endolymphatic hydrops–excess endolymph in the membranous labyrinth causing distension.

The external surface of the tympanic membrane is primarily innervated by the auriculotemporal nerve (V3) with contributions from the auricular branch of vagus (X) and facial nerve (VII).

The stapes footplate articulates with the oval window (fenestra vestibuli), and its vibration creates pressure waves in the perilymph of the scala vestibuli.

Hair cell stereocilia are embedded in the overlying tectorial membrane.

The posterior cricoarytenoid is the ONLY abductor of the vocal cords, supplied by the recurrent laryngeal nerve (RLN).

The cricothyroid muscle tilts the thyroid cartilage forward on the cricoid cartilage, lengthening and tensing the vocal cords.

During forced inspiration, the posterior cricoarytenoid muscles contract bilaterally, rotating the arytenoid cartilages laterally and fully abducting the vocal cords.

The pterygopalatine fossa communicates with the nasal cavity through the sphenopalatine foramen.

V2 enters the pterygopalatine fossa through foramen rotundum from the middle cranial fossa.

The lesser occipital nerve (C2) ascends along the posterior border of SCM to supply skin behind the ear and posterior scalp.

The internal branch of the superior laryngeal nerve (from vagus) pierces the thyrohyoid membrane along with the superior laryngeal artery to enter the larynx.

Cleft lip results from failure of fusion between the maxillary prominence and the medial nasal prominence on one or both sides.

Tensor tympani is supplied by the mandibular nerve (V3) and contracts in response to loud sounds (acoustic reflex) to tense the tympanic membrane and reduce sound transmission, protecting the cochlea from acoustic trauma.

While the spinal accessory nerve (CN XI) provides the main motor supply to trapezius, the ventral rami of C3 and C4 provide proprioceptive fibers and some motor contribution (debated but clinically observed).

The central nervous system (CNS) consists only of the brain and spinal cord.

The somatic nervous system is a functional division of the PNS, carrying sensory input from skin and voluntary motor output to skeletal muscles.

Arrival of an action potential opens voltage-gated calcium channels in the presynaptic terminal.

GABA is the major inhibitory neurotransmitter in the adult CNS, especially in the brain.

Glutamate is the principal excitatory neurotransmitter in the CNS.

At rest, neuronal membranes are most permeable to potassium, mainly due to potassium leak channels.

The rapid upstroke of the neuronal action potential is due to opening of voltage-gated sodium channels, allowing a sudden influx of sodium and causing depolarization.

Temporal summation occurs when repeated impulses from the same presynaptic neuron arrive in quick succession, so their postsynaptic potentials add together.

The Pacinian corpuscle is a classic mechanoreceptor that responds to deep pressure and vibration.

A receptor potential is a graded local potential, meaning its amplitude varies with stimulus strength.

Lateral inhibition increases the contrast between strongly stimulated and neighboring less stimulated pathways, improving two-point discrimination and sensory localization.

A-alpha fibers are the largest and most heavily myelinated somatic fibers, so they conduct fastest.

Wallerian degeneration is degeneration of the axon and myelin sheath distal to the site of injury.

Schwann cells in the PNS form regeneration tubes and secrete factors that promote axonal regrowth.

The dorsal column-medial lemniscal pathway carries fine touch, vibration, and conscious proprioception from the body.

First-order fibers of the DCML pathway ascend ipsilaterally and synapse in the gracile and cuneate nuclei of the medulla.

The dorsal column fibers ascend ipsilaterally in the spinal cord and cross only in the medulla.

The primary somatosensory cortex lies in the postcentral gyrus of the parietal lobe, corresponding to Brodmann areas 3, 1, and 2.

The lateral spinothalamic tract carries pain and temperature sensation.

Pain and temperature fibers cross within a few segments of entering the spinal cord.

Lower motor neuron (LMN) lesions cause fasciculations, flaccid paralysis, hypotonia, hyporeflexia, and muscle atrophy.

The muscle spindle detects muscle stretch and initiates the stretch reflex.

Golgi tendon organs send Ib afferents that activate inhibitory interneurons in the spinal cord, producing inhibition of alpha motor neurons of the same muscle.

When a painful stimulus causes withdrawal of one limb, the crossed extensor reflex extends the opposite limb to support body weight and maintain posture.

The semicircular canals detect angular or rotational acceleration of the head.

The vestibulocerebellum (mainly the flocculonodular lobe) is primarily concerned with equilibrium, eye movements, and balance.

UMN lesions typically cause hyperreflexia, spasticity, increased tone, and extensor plantar response.

The reticular activating system (RAS) in the brainstem is essential for arousal and wakefulness.

In brain death, all brainstem reflexes, including the pupillary light reflex, are absent.

Most postganglionic sympathetic fibers release norepinephrine.

All preganglionic autonomic fibers, both sympathetic and parasympathetic, release acetylcholine onto nicotinic receptors in autonomic ganglia.

Parasympathetic stimulation causes miosis by contracting the sphincter pupillae muscle through the oculomotor nerve and ciliary ganglion.

The primary motor cortex is located in the precentral gyrus of the frontal lobe, corresponding mainly to Brodmann area 4.

The lateral corticospinal tract is essential for fractionated, skilled voluntary movements, especially of distal limb muscles such as the hand and fingers.

The direct pathway of the basal ganglia facilitates movement by reducing inhibitory output from the globus pallidus internus to the thalamus, thereby enhancing cortical motor activity.

Dopamine from the substantia nigra pars compacta facilitates movement overall by stimulating the direct pathway via D1 receptors and inhibiting the indirect pathway via D2 receptors.

C fibers are unmyelinated and conduct slow, dull, aching, burning pain.

Referred pain occurs because visceral afferent fibers and somatic afferent fibers converge on the same second-order neurons in the spinal cord or CNS.

The meninges, especially the dura and large vessels, are pain-sensitive and common sources of headache.

Due to differential growth of the vertebral column and spinal cord, the adult spinal cord terminates at L1-L2 vertebral level (conus medullaris).

During lumbar puncture, the needle traverses skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura mater, and finally arachnoid mater before entering the subarachnoid space.

Syringomyelia causes expansion of the central canal, first damaging the anterior white commissure where lateral spinothalamic tract fibers decussate, resulting in bilateral “cape-like” loss of pain and temperature (dissociated sensory loss).

Lamina IX of Rexed contains alpha motor neurons (lower motor neurons) that innervate skeletal muscles.

Astrocytes are star-shaped glial cells with elongated nuclei that provide structural support, maintain the blood-brain barrier, regulate ion/neurotransmitter concentrations, and form glial scars after injury.

This presentation describes medial medullary syndrome with lateral medullary features.

The alar plate (dorsal/posterior) gives rise to sensory nuclei including general somatic afferent (e.g., trigeminal sensory nuclei), general visceral afferent, and special sensory nuclei.

Lateral striate arteries (lenticulostriate arteries) arise from the M1 segment of the middle cerebral artery and supply the putamen, lateral globus pallidus, and posterior limb of the internal capsule.

Arachnoid granulations (villi) are protrusions of arachnoid mater into dural venous sinuses (especially superior sagittal sinus) that allow one-way drainage of CSF into venous blood via pressure-dependent bulk flow.

Aqueductal stenosis (obstruction of the cerebral aqueduct of Sylvius connecting third and fourth ventricles) causes dilation of lateral and third ventricles with a normal fourth ventricle–a classic non-communicating hydrocephalus pattern.

Skeletal muscles of the limbs and trunk derive from the myotome portion of somites (paraxial mesoderm).

The nucleus ambiguus contains motor neurons for CN IX, X, and XI that innervate pharyngeal and laryngeal muscles.

Medial medullary syndrome (Dejerine syndrome) results from anterior spinal artery or vertebral artery branch occlusion affecting the pyramid (contralateral hemiplegia), medial lemniscus (contralateral sensory loss), and hypoglossal nerve/nucleus (ipsilateral tongue weakness/atrophy).

In the pons, facial nerve fibers loop around the abducens nucleus (forming the facial colliculus) before exiting.

Weber syndrome involves midbrain damage affecting CN III fibers (ipsilateral oculomotor palsy with ptosis, dilated pupil, “down and out” eye) and cerebral peduncle (contralateral hemiplegia).

Purkinje cells are large flask-shaped neurons in the middle (Purkinje) layer of cerebellar cortex.

Alcoholic cerebellar degeneration predominantly affects the anterior vermis, causing truncal ataxia and gait disturbance while sparing limb coordination (hemispheric function).

The juxtarestiform body is the portion of the inferior cerebellar peduncle that carries reciprocal connections between the vestibular nuclei and the flocculonodular lobe (vestibulocerebellum).

The globus pallidus internus (GPi) and substantia nigra pars reticulata are the primary output nuclei of the basal ganglia, sending inhibitory GABAergic projections to the thalamus.

Huntington’s disease causes selective degeneration of GABAergic medium spiny neurons in the striatum, particularly those of the indirect pathway projecting to the external globus pallidus.

Loss of taste over the anterior two-thirds of the tongue indicates chorda tympani involvement.

Hyperacusis (stapedius paralysis) with intact taste (chorda tympani spared) localizes the lesion between stapedius branch and chorda tympani origin.

The facial nerve trunk and its branches divide the parotid gland into superficial and deep lobes.

The parotid gland is a purely serous gland with serous acini only, producing watery, enzyme-rich (amylase) secretions.

The parotid duct (Stensen’s duct) runs horizontally across the masseter, one fingerbreadth below the zygomatic arch.

The lateral pterygoid muscle protrudes the jaw and deviates it to the opposite side.