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🧠 Neuroscience Syllabus - Complete Explanation

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1. Receptors & Pathways → Pyramidal vs. Extrapyramidal (Spinothalamic)

Pyramidal Tract

• Corticospinal tract: controls limb movements
• Corticobulbar tract: controls facial/tongue/throat muscles via cranial nerve nuclei

Extrapyramidal Tract

Spinothalamic Tract (listed as a subtype of sensory extrapyramidal pathways)

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2. Parkinson's Disease

Pathophysiology

• Normal function: dopamine from the substantia nigra activates the direct pathway (promotes movement) and inhibits the indirect pathway (suppresses unwanted movement)
• In PD: dopamine loss tips the balance - the indirect pathway dominates → excessive inhibition of the thalamus → reduced motor output → hypokinesia
• Result: increased tonic and oscillatory GABAergic activity in basal ganglia output nuclei (GPi, SNr), which inhibit the thalamus - Eric Kandel Principles of Neural Science

Classic Tetrad of Symptoms (TRAP)

Treatment

• Levodopa (L-DOPA) - crosses the blood-brain barrier and converts to dopamine in the brain. First-line treatment. - Katzung Pharmacology
• Dopamine agonists, MAO-B inhibitors, COMT inhibitors as adjuncts

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3. Tremors

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4. Glial Cells

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5. Excitatory & Inhibitory Neurotransmitters

Major Excitatory Neurotransmitters

Major Inhibitory Neurotransmitters

Clinical note: Glutamate (excitatory) drives burst firing of dopaminergic neurons; GABA (inhibitory) causes tonic, low-level basal firing. Balance between the two is key to normal brain function. - Goodman & Gilman's

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6. Hemorrhages & Strokes + CT Presentations

Types of Intracranial Hemorrhage

Strokes by Territory

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7. Headaches

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8. Consciousness & Types

1. Arousal (wakefulness) - maintained by the Reticular Activating System (RAS) in the brainstem/thalamus
2. Awareness (content) - maintained by cortical networks

Levels of Impaired Consciousness

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9. Seizures & Epilepsy Types

ILAE Classification (2017):

• Focal aware (consciousness intact) - formerly "simple partial"
• Focal impaired awareness (consciousness altered) - formerly "complex partial"
• Focal to bilateral tonic-clonic

• Bradley & Daroff's Neurology; Lippincott Pharmacology

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10. Vertigo

Peripheral vs. Central Vertigo

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11. Paralysis & Paresis

• Paresis = partial weakness (incomplete loss of motor function)
• Paralysis (plegia) = complete loss of motor function

Patterns:

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12. Stereognosis & Graphesthesia

• Stereognosis = ability to recognize an object by touch alone (e.g., identifying a key placed in the hand with eyes closed). Loss = astereognosis → parietal lobe lesion
• Graphesthesia = ability to recognize numbers/letters traced on the skin. Loss = agraphesthesia → parietal lobe lesion

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13. UMN vs. LMN Lesions - Signs & Tests

• Frameworks for Internal Medicine; Neuroanatomy through Clinical Cases

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14. Cranial Nerves (CN) - Functions

Bell's Palsy vs. Facial Nerve Palsy

• Affects entire ipsilateral face (forehead + lower face) - patient cannot close eye or raise eyebrow
• Compare with UMN lesion (e.g., stroke): spares the forehead (forehead receives bilateral cortical innervation)

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15. EEG Graphs & Waves

• Absence seizure = 3 Hz spike-and-wave discharges (generalized)
• Focal seizures = localized spike activity
• Hypsarrhythmia = chaotic pattern in infantile spasms (West syndrome)

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16. Aphasias

• Eric Kandel Principles of Neural Science

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17. Functions of the Lobes

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18. Horner's Syndrome (Triad)

Classic Triad:

1. Ptosis (drooping eyelid) - from paralysis of superior tarsal muscle (Müller's muscle)
2. Miosis (constricted pupil) - loss of sympathetic mydriasis
3. Anhidrosis (loss of sweating) - on the ipsilateral face (if lesion is pre-ganglionic)

Causes by Level:

• Central (1st order): Stroke (lateral medullary/Wallenberg syndrome), syringomyelia, MS
• Pre-ganglionic (2nd order): Pancoast tumor (lung apex), cervical rib, neck dissection
• Post-ganglionic (3rd order): Carotid artery dissection, cavernous sinus thrombosis

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19. Anisocoria

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20. Glasgow Coma Scale (GCS)

• GCS ≤ 8 = coma → consider intubation
• GCS 9-12 = moderate head injury
• GCS 13-15 = mild head injury
• Tintinalli's Emergency Medicine; Gray's Anatomy for Students; Rosen's Emergency Medicine

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Summary Map

NEUROSCIENCE SYLLABUS
│
├── Motor Pathways: Pyramidal (voluntary) vs. Extrapyramidal (tone/posture)
│   └── Spinothalamic = sensory (pain/temp)
│
├── Movement Disorders
│   ├── Parkinson's → dopamine loss → basal ganglia dysfunction
│   └── Tremors → resting (PD), intention (cerebellum), postural (essential)
│
├── Support Cells: Glial cells (Astrocytes, Oligodendrocytes, Microglia, Ependymal)
│
├── Neurotransmitters: Glutamate/ACh (excitatory) ↔ GABA/Glycine (inhibitory)
│
├── Vascular Neurology
│   ├── Hemorrhages: Epidural / Subdural / SAH / ICH
│   └── Strokes: ACA (leg), MCA (face+arm), PCA (vision), Lacunar
│       └── Predisposition: Atrial fibrillation → cardioembolic
│
├── Headaches: Migraine, Tension, Cluster, SAH, ICP
│
├── Consciousness: Arousal (RAS) + Awareness (cortex)
│
├── Seizures & Epilepsy: Focal vs. Generalized (absence, tonic-clonic, etc.)
│
├── Vertigo: Peripheral (BPPV, Menière's) vs. Central (stroke)
│
├── Motor Deficits: Paralysis/Paresis → UMN vs. LMN signs
│
├── Cortical Sensation: Stereognosis, Graphesthesia (parietal lobe)
│
├── Cranial Nerves (I-XII) + Bell's palsy (CN VII LMN)
│
├── EEG Waves: Delta/Theta/Alpha/Beta/Gamma
│
├── Aphasias: Broca's (frontal), Wernicke's (temporal), Conduction (arcuate fasciculus)
│
├── Lobe Functions: Frontal/Parietal/Temporal/Occipital
│
├── Horner's Syndrome: Ptosis + Miosis + Anhidrosis (sympathetic chain disruption)
│
├── Anisocoria: Which pupil is abnormal? (light vs. dark test)
│
└── Glasgow Coma Scale: E(4) + M(6) + V(5) = 15 max; ≤8 = coma

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🧠 Sensory Receptors - Complete Guide

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Overview: Classification by Stimulus Type

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Key Concept: Adaptation

• Phasic receptors (rapidly adapting): Fire at onset and offset of stimulus, then go silent. They detect change and movement.
• Tonic receptors (slowly adapting): Keep firing as long as stimulus persists. They encode intensity and duration.

"Phasic receptors detect onset and offset of stimulus. Tonic receptors encode stimulus intensity: the greater the intensity, the larger the depolarizing receptor potential, and the more likely action potentials are to occur." - Costanzo Physiology

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1. Mechanoreceptors

The 6 Cutaneous Mechanoreceptors

• Costanzo Physiology; Neuroscience: Exploring the Brain

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Details on Each

• Largest mechanoreceptor (up to 2 mm long!)
• Onion-like layered capsule surrounding a nerve ending
• Located deep in dermis, subcutaneous tissue, and even inside muscles
• ~2,500 in each human hand
• Detects vibration (best at 200-300 Hz) and rapid changes in pressure
• Because they adapt so rapidly, they fire only at onset AND offset of a stimulus (on-off detectors)

• Encapsulated, located in dermis of non-hairy (glabrous) skin - fingertips, lips, palms
• Only about 1/10th the size of Pacinian corpuscles
• Small receptive fields → excellent two-point discrimination
• Best for detecting texture, tapping, flutter (~5-40 Hz)
• Most dense in fingertips - why fingertips are so sensitive to fine textures

• Networks of nerve fibers wrapping around hair follicles
• Activated when hair is bent or displaced
• Rapidly adapting → detect velocity and direction of stimulus moving across skin
• Even a light breeze over arm hair activates these

• Elongated, spindle-shaped encapsulated receptor
• Found in both hairy and non-hairy skin, and in joint capsules
• Large receptive fields - a stimulus from a distance can still activate them
• Slowly adapting → detects sustained skin stretch, joint rotation and angle
• Important for hand grip and knowing joint position

• A nerve terminal ending + a specialized epithelial cell (the Merkel cell)
• In the epidermis of non-hairy skin
• Small receptive field → good spatial resolution
• Slowly adapting → fires continuously with sustained pressure
• Best for fine spatial detail and texture discrimination (e.g., reading braille)

• Similar to Merkel receptors but found in hairy skin

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Two-Point Discrimination

• Fingertip: ~2 mm
• Lip: ~3 mm
• Back: ~40-70 mm (fewest fine receptors)

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2. Thermoreceptors

• At ~36°C both types overlap in activity
• Above 45°C: warm receptors shut off → nociceptors (TRPV1) take over - this is when heat becomes painful
• Below freezing: cold receptors shut off → nociceptors take over - freezing cold = painful
• TRP channels = Transient Receptor Potential channels - ion channels that open in response to physical/chemical stimuli

"Transduction of cold temperatures involves TRPM8, which is also opened by menthol (which gives a cold sensation)." - Costanzo Physiology

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3. Nociceptors (Pain Receptors)

Two Major Types

• Aδ fibers carry the "first pain" - the immediate, sharp sensation you feel right when you stub your toe
• C fibers carry the "second pain" - the throbbing, burning pain that lingers afterward

Inflammatory Sensitization (Hyperalgesia)

• Bradykinin - directly activates nociceptors
• Prostaglandins - lower pain threshold (why NSAIDs/aspirin/ibuprofen work - they block prostaglandin synthesis)
• Substance P - released by nociceptors themselves; causes vasodilation and mast cell degranulation
• Histamine (from mast cells) - directly activates nociceptors
• K⁺ and H⁺ (from damaged cells) - depolarize nociceptor membranes

"Damaged skin releases a variety of chemicals including bradykinin, prostaglandins, substance P, K+, and H+ which initiate the inflammatory response... Mast cells release histamine, which directly activates nociceptors." - Costanzo Physiology

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4. Proprioceptors (Position Sense)

Muscle Spindle

• Located inside skeletal muscle (intrafusal fibers), parallel to regular muscle fibers (extrafusal fibers)
• Detects change in muscle length (stretch)
• Afferent fibers:
  • Ia (primary): wraps around nuclear bag fibers → detects rate of change (velocity) of stretch → phasic response
  • II (secondary): wraps around nuclear chain fibers → detects sustained stretch → tonic response
• Controls the stretch reflex (myotatic reflex): muscle stretches → spindle fires → Ia fibers → alpha motor neuron → muscle contracts back
• Gamma motor neurons set the sensitivity of the spindle by contracting the intrafusal fibers

Golgi Tendon Organ (GTO)

• Located at the muscle-tendon junction, in series with muscle fibers
• Detects muscle tension (force, not length)
• Afferent fiber: Ib
• When tension is too high → Ib fibers → inhibitory interneuron → motor neuron inhibited → muscle relaxes
• This is the inverse stretch reflex (protective mechanism to prevent tendon/muscle tear)
• Ganong's Physiology

Joint Receptors

• Free nerve endings and Ruffini-like endings in joint capsules and ligaments
• Detect joint angle, velocity of joint movement, pressure

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5. Chemoreceptors

Peripheral Chemoreceptors

• Carotid body (at carotid bifurcation) and aortic bodies: detect ↓PO₂, ↑PCO₂, ↓pH in blood → stimulate breathing
• Afferent via CN IX (glossopharyngeal) for carotid body, CN X (vagus) for aortic bodies

Central Chemoreceptors

• In the medulla oblongata: detect ↑PCO₂ and ↓pH in CSF → most powerful drive for ventilation

Taste (Gustatory) Receptors

• On taste buds (tongue, palate, epiglottis)
• 5 basic tastes: sweet, salty, sour, bitter, umami
• Receptor cells are modified epithelial cells, not neurons

Olfactory Receptors

• Located in the olfactory epithelium of the nasal roof
• Only sensory neurons that directly regenerate and have direct access to the brain (CN I → olfactory bulb → piriform cortex)
• ~400 types of receptors → can distinguish thousands of odors by combination

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6. Photoreceptors (Retinal Receptors)

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7. Baroreceptors & Osmoreceptors

Baroreceptors (Stretch Receptors in Blood Vessels)

• Carotid sinus baroreceptors: in the wall of internal carotid artery → detect blood pressure via vessel wall stretch
• Aortic arch baroreceptors: similar function
• When BP rises → baroreceptors fire more → via CN IX/X → brainstem (nucleus tractus solitarius) → reflex bradycardia + vasodilation to lower BP
• Bainbridge reflex: atrial stretch → ↑ heart rate

Osmoreceptors

• In the anterior hypothalamus (organum vasculosum of lamina terminalis, OVLT)
• Detect ↑ plasma osmolarity → stimulate ADH (vasopressin) release + thirst

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Sensory Pathways: How Receptor Signals Reach the Brain

1st Order Neuron → 2nd Order Neuron → 3rd Order Neuron → Cortex
(DRG/cranial ganglion)  (spinal cord/brainstem)  (Thalamus, VPL)

Two Major Pathways:

Clinical pearl: A right-sided spinal cord lesion → right side loses fine touch/proprioception but left side loses pain/temperature (crossed at different levels). This pattern is called Brown-Séquard syndrome.

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Quick Summary Table

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⚡ Excitatory & Inhibitory Neurotransmitters

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The Basic Concept

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Classification of Neurotransmitters

• Costanzo Physiology; Ganong's Review of Medical Physiology

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🟢 EXCITATORY NEUROTRANSMITTERS

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1. Glutamate

Receptors - 4 Subtypes:

• mGluR1-8: Modulate synaptic transmission; can be excitatory or inhibitory depending on subtype

The NMDA Receptor - Most Important

1. Ligand binding (glutamate + glycine as co-agonist)
2. Membrane depolarization (to remove the Mg²⁺ block from the channel)

Clinical Relevance of Glutamate:

• Kaplan & Sadock's Comprehensive Textbook of Psychiatry

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2. Acetylcholine (ACh)

Synthesis & Degradation:

• Synthesis: Choline + Acetyl CoA → ACh (enzyme: choline acetyltransferase, ChAT)
• Breakdown: ACh → choline + acetate (enzyme: acetylcholinesterase, AChE on the postsynaptic membrane)
• Choline is recycled (50%) back into the presynaptic terminal

Receptors - 2 Types:

5 Muscarinic Subtypes:

• M1: CNS, memory, cognition
• M2: Heart → ↓ heart rate (Gi-coupled)
• M3: Smooth muscle (contraction), glands (secretion)
• M4/M5: CNS

ACh Brain Pathways:

• Nucleus basalis of Meynert → cerebral cortex: attention, memory
• Septum → hippocampus: memory consolidation
• Pedunculopontine nucleus → thalamus: arousal, REM sleep

Clinical Relevance:

• Lippincott Pharmacology; Katzung's Pharmacology

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3. Norepinephrine (NE) / Noradrenaline

Synthesis Pathway (from the diagram above):

• Tyrosine hydroxylase (rate-limiting step): Tyrosine → L-DOPA
• Dopamine β-hydroxylase: Dopamine → NE
• PNMT (in adrenal medulla only): NE → Epinephrine

Degradation:

• MAO (monoamine oxidase) - in presynaptic terminals
• COMT (catechol-O-methyltransferase) - in other tissues
• Major metabolite of NE: Normetanephrine and VMA (vanillylmandelic acid)

Receptors:

Brain NE Pathway:

• Locus coeruleus (pons) → widespread cortex, limbic, spinal cord
• Functions: arousal, attention, fight-or-flight, mood
• Low NE → depression

Clinical Relevance:

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4. Dopamine (DA)

Synthesis: Same pathway as NE but stops at dopamine (no dopamine β-hydroxylase in dopaminergic neurons)

• Tyrosine → L-DOPA → Dopamine
• Major metabolite: HVA (homovanillic acid)

4 Major Dopamine Pathways in the Brain:

Receptors (D1-D5):

Clinical Relevance:

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5. Serotonin (5-HT - 5-hydroxytryptamine)

Synthesis:

• Rate-limiting enzyme: Tryptophan hydroxylase
• Degraded by MAO → 5-HIAA (5-hydroxyindoleacetic acid) (measured in urine)
• In the pineal gland, serotonin → melatonin (sleep regulation)
• 90% of body's serotonin is in the GI tract (enterochromaffin cells); only 10% in brain

Brain Serotonin Pathway:

• Raphe nuclei (brainstem) → widespread cortex, limbic, spinal cord
• Functions: mood, sleep, appetite, pain modulation, temperature regulation

Receptors (7 families, 5-HT1 through 5-HT7):

Note: Serotonin can be excitatory OR inhibitory depending on which receptor it binds to and which cell type - Stahl's Essential Psychopharmacology

Clinical Relevance:

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🔴 INHIBITORY NEUROTRANSMITTERS

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1. GABA (γ-Aminobutyric Acid)

Synthesis & Degradation:

• Synthesis: Glutamate → GABA (enzyme: Glutamic Acid Decarboxylase, GAD - requires Vitamin B6)
  • GAD is unique to GABAergic neurons
• Degradation: GABA → Succinate semialdehyde → Citric acid cycle (by GABA-transaminase)
• GABA does NOT have any metabolic functions outside neurotransmission (unlike glutamate and glycine which are incorporated into proteins)

2 Receptor Types:

GABA-A Receptor in Detail:

• GABA binds → opens Cl⁻ channel → Cl⁻ flows IN → neuron hyperpolarizes → inhibited
• Benzodiazepines (e.g., diazepam) bind at the α-γ interface → increase frequency of Cl⁻ channel opening
• Barbiturates (e.g., phenobarbital) bind at the β subunit → increase duration of Cl⁻ channel opening
• Both drugs potentiate GABA but do NOT work without GABA present (except barbiturates at high doses)

"GABA is the principal inhibitory neurotransmitter in the mammalian CNS, and the GABA-A receptor is the only site proven to be involved in barbiturate-induced anesthesia." - Miller's Anesthesia

Clinical Relevance:

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2. Glycine

• Synthesis: Serine → Glycine (by serine hydroxymethyltransferase, requires folate)
• Mechanism: Glycine receptor → Cl⁻ channel opens → hyperpolarization (same mechanism as GABA-A)
• Also acts as a co-agonist at NMDA receptors (glycine binds the GluN1 subunit to allow NMDA receptor activation) - so glycine can be both inhibitory (own receptor) and facilitatory (at NMDA)

Glycine Receptor:

• Pentameric Cl⁻ channel (similar structure to GABA-A)
• Blocked by strychnine (convulsant rat poison) → without glycine inhibition → uncontrolled motor activity → tetanic convulsions
• Location: spinal cord, brainstem, thalamus, hippocampus, cortex

"Glycine is the major inhibitory neurotransmitter in the spinal cord... its mechanism of action is to increase Cl⁻ conductance of the postsynaptic cell membrane." - Costanzo Physiology

Clinical Relevance:

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3. Nitric Oxide (NO)

• Synthesis: Arginine → Citrulline + NO (enzyme: NO synthase, NOS)
• Unique because it is a gas - it simply diffuses from the presynaptic terminal to target cells (no vesicle packaging, no exocytosis)
• Acts on guanylyl cyclase → ↑cGMP → smooth muscle relaxation
• Short-lived (seconds) - rapidly inactivated by reacting with hemoglobin

Clinical Relevance:

• Erectile dysfunction → sildenafil (Viagra) inhibits phosphodiesterase-5 → prevents cGMP breakdown → prolongs vasodilation
• Septic shock → excess NO → massive vasodilation

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Quick Summary Comparison Table

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The Balance Concept

Excitatory (Glutamate) ←→ Inhibitory (GABA)
         ↑                      ↓
   Action Potentials       Silence/Damping

E/I imbalance:
- Too much Excitation → SEIZURES, EXCITOTOXICITY
- Too much Inhibition → COMA, SEDATION, ANESTHESIA

"Glutamate inputs enable burst-like firing of dopaminergic neurons. GABA inhibition causes tonic, basal level firing. The balance between the two is key to normal brain function." - Goodman & Gilman's Pharmacology

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