Clinical Anatomy & Embryology • COMLEX & USMLE Pre-Clerkship

Clinical Anatomy & Embryology

Exhaustive brachial and lumbosacral plexus nerve lesion locators, cranial nerve skull base foramina pathways, pharyngeal arches and pouches, brainstem vascular stroke territories, and retroperitoneal anatomy.

Upper Extremity & Brachial Plexus Injuries

Classic fracture sites, compression syndromes, motor deficits, and sensory loss patterns.

C5-T1 • Posterior Cord Wrist Drop

Radial Nerve

Runs in radial spiral groove of humerus with deep brachial artery.

Mechanism: Midshaft humerus fracture, compression in axilla ("Saturday night palsy", improperly fitted crutches).

Motor Loss: Loss of elbow, wrist, and finger extension (BEST: Brachioradialis, Extensors of wrist/fingers, Supinator, Triceps).

Sensory Loss: Posterior arm and forearm; dorsal surface of hand (1st-3rd digits and webspace).

Triceps reflex (C7) intact if lesion is at spiral groove; absent if injured high in axilla!

C5-T1 • Med/Lat Cords Ape Hand / Popes Blessing

Median Nerve

Runs with brachial artery in cubital fossa, between FDS and FDP.

Proximal Lesion (Supracondylar Fracture): Loss of wrist flexion, pronation, flexion of lateral digits. "Pope's blessing" (hand of benediction) when trying to MAKE A FIST.

Distal Lesion (Carpal Tunnel / Wrist Laceration): Thenar muscle atrophy (loss of thumb opposition → Ape Hand). Sensation lost over palmar aspects of thumb, index, middle, and radial half of ring finger.

Palmar cutaneous branch passes superficial to flexor retinaculum; thenar sensation is SPARED in carpal tunnel syndrome!

C8-T1 • Medial Cord Ulnar Claw

Ulnar Nerve

Passes posterior to medial epicondyle ("funny bone") and through Guyon canal.

Mechanism: Medial epicondyle fracture (proximal) or hook of hamate fracture / cyclist palsy (distal Guyon canal).

Motor Loss: Hypothenar muscles, medial 2 lumbricals, all interossei (PAD/DAB: Palmer adduction, Dorsal abduction). Inability to adduct thumb (Froment sign: compensatorily flexes thumb via FPL).

Deformity: "Ulnar Claw" (hyperextension at MCP, flexion at PIP/DIP of 4th & 5th digits) visible when patient attempts to EXTEND fingers.

C5-C6 Roots

Axillary & Musculocutaneous

Axillary Nerve (C5-C6, Post Cord): Fracture of surgical neck of humerus or anterior shoulder dislocation. Loss of arm abduction beyond 15° (deltoid) and external rotation (teres minor). Numbness over lateral deltoid ("sergeant's patch").

Musculocutaneous Nerve (C5-C7, Lat Cord): Penetrating anterior shoulder/arm trauma. Pierces coracobrachialis. Loss of forearm flexion and supination (biceps, brachialis). Sensory loss on lateral forearm.

Brachial Plexus Trunks

Erb-Duchenne vs Klumpke

Erb Palsy (Upper Trunk C5-C6): Excessive lateral stretch between neck and shoulder (shoulder dystocia at birth or motorcycle fall). Arm hangs medially rotated, adducted, pronated: "Waiter's tip" posture.

Klumpke Palsy (Lower Trunk C8-T1): Upward traction on arm (catching a tree branch while falling, breech delivery). Paralysis of all intrinsic hand muscles → total claw hand. Concomitant Horner syndrome if sympathetic T1 fibers disrupted!

C5-C7 Roots

Long Thoracic Nerve

Courses superficial along lateral thoracic wall to innervate Serratus anterior.

Mechanism: Axillary lymph node dissection (radical mastectomy for breast cancer) or thoracostomy (chest tube insertion).

Deficit: Inability to abduct arm above horizontal plane (90°). Medial border of scapula protrudes posteriorly: Winged Scapula (accentuated when pushing against a wall).

Mnemonic: "C5-6-7 raise your wings to heaven".

Lower Extremity & Lumbosacral Plexus Injuries

Common peroneal foot drop, femoral nerve reflex loss, and Trendelenburg sign.

L4-S2 Foot Drop

Common Peroneal (Fibular)

Wraps superficially around the neck of the fibula.

Injury: Fibular head/neck fracture, tight lower leg plaster cast, or prolonged crossing of legs.

Motor Deficit: Deep fibular branch → loss of ankle dorsiflexion (Foot Drop, steppage gait). Superficial fibular → loss of eversion.

Sensory Deficit: Anterolateral lower leg and dorsum of foot (webbed space between 1st & 2nd toe = deep fibular).

Mnemonic: "PED = Peroneal Everts and Dorsiflexes; TIP = Tibial Inverts and Plantarflexes".

L4-S3 Inability to Point Toes

Tibial Nerve

Courses through popliteal fossa and tarsal tunnel (posterior to medial malleolus).

Injury: Knee trauma, Baker cyst in popliteal fossa, or compression under flexor retinaculum in tarsal tunnel.

Motor Deficit: Loss of foot plantarflexion (gastrocnemius, soleus), foot inversion (tibialis posterior), and toe flexion. Absent Achilles reflex (S1-S2).

Sensory Deficit: Loss of sensation on the sole (plantar surface) of the foot.

L4-S1 Trendelenburg Sign

Superior Gluteal Nerve

Exits greater sciatic foramen superior to piriformis muscle.

Innervation: Gluteus medius, gluteus minimus, and tensor fasciae latae.

Injury / Deficit: Injections into superomedial quadrant of buttock or hip replacement trauma. Weakness of hip abduction.

Trendelenburg Gait: When standing on the affected (weak) limb, the pelvis DROPS on the contralateral (opposite) side!

Dermatome, Myotome & Deep Tendon Reflex Master Reference (C5–S4)

Spinal root levels, motor muscle actions, sensory test sites, reflex arcs, and discriminating radiculopathy signs.

Spinal Level	Primary Myotome / Muscle Action	High-Yield Sensory Landmark	Reflex Arc	Discriminating Radiculopathy Pearls
C5 Root	Deltoid (shoulder abduction 15°-90°), Biceps (elbow flexion)	Lateral upper arm over deltoid insertion ("regimental badge")	Biceps reflex (C5, C6)	C4-C5 posterolateral disc herniation. Weakness in shoulder abduction and diminished biceps reflex.
C6 Root	Extensor carpi radialis (wrist extension), Biceps / Brachioradialis	Lateral forearm, thumb, and index finger	Brachioradialis reflex (C5, C6)	C5-C6 herniation. Weak wrist extension; numbness in thumb/index finger; lost brachioradialis jerk.
C7 Root	Triceps (elbow extension), Flexor carpi radialis (wrist flexion), Finger extensors	Middle finger (palmar and dorsal surfaces)	Triceps reflex (C7, C8)	Most common cervical radiculopathy! C6-C7 disc herniation. Triceps weakness, lost triceps jerk, numbness in middle finger.
C8 Root	Flexor digitorum profundus (finger flexion / hand grip strength)	Medial forearm, ring finger, and little (5th) finger	None standard (Finger jerk)	C7-T1 disc herniation. Weakness of finger flexors and intrinsic grasp; sensory loss over 5th digit.
T1 Root	Dorsal and palmar interossei (finger abduction [DAB] and adduction [PAD])	Medial upper arm / axilla	None standard	Thoracic outlet syndrome / cervical rib compression. Klumpke palsy involves C8-T1 → intrinsic claw hand + Horner syndrome.
T4 Landmark	Intercostal muscles (respiration)	Nipple line (dermatome T4)	None	Mnemonic: "T4 at the teat-pore". Sensory level for high thoracic cord transection.
T7 Landmark	Intercostal muscles	Xiphoid process (dermatome T7)	None	Inferior tip of the sternum / epigastrium.
T10 Landmark	Abdominal wall musculature	Umbilicus / Belly Button (dermatome T10)	Abdominal reflex (T8-T12)	Mnemonic: "T10 at the belly-but-ten". Early acute appendicitis visceral pain refers to T10 umbilicus before shifting to RLQ!
L1 Landmark	Iliopsoas (hip flexion initiation)	Inguinal ligament & groin crease	Cremasteric reflex (L1-L2)	Mnemonic: "L1 at the Inguinal Ligament (L-1 = 1-L)". Cremasteric reflex absent in testicular torsion!
L4 Root	Quadriceps femoris (knee extension), Tibialis anterior (foot inversion)	Medial lower leg, medial malleolus, medial arch of foot	Patellar tendon reflex (L3, L4)	L3-L4 disc herniation. Weak knee extension (difficulty climbing stairs / knee gives out); diminished patellar jerk.
L5 Root	Extensor hallucis longus (great toe dorsiflexion), Gluteus medius (hip abduction)	Dorsum of foot and first webspace (between 1st and 2nd toes)	None standard (Hamstrings / Tibialis posterior)	Most common lumbar radiculopathy! L4-L5 disc herniation. Foot drop, inability to heel walk, positive Trendelenburg sign. Reflexes are normal!
S1 Root	Gastrocnemius & Soleus (foot plantarflexion), Gluteus maximus (hip extension)	Lateral foot, small (5th) toe, sole of foot, posterior calf	Achilles tendon reflex (S1, S2)	L5-S1 disc herniation. Inability to toe walk, diminished or absent Achilles tendon jerk, pain radiating down posterior leg to lateral foot.
S2–S4 Roots	External anal sphincter, pelvic floor / levator ani, bladder sphincters	Perianal region, buttocks, inner thighs ("saddle anesthesia")	Anal wink reflex (S3, S4), Bulbocavernosus	Cauda Equina Syndrome: Surgical emergency! Massive central disc herniation causes bilateral saddle anesthesia, bowel/bladder incontinence, lost anal wink. Mnemonic: "S2, 3, 4 keep the poop off the floor".

Skull Base Foramina

Cranial Nerves (I – XII) & Foramina Pathways

Exits, brainstem nuclei levels, autonomic components, and high-yield clinical neuropathies.

Nerve	Skull Base Foramen	Fiber Type & Function	Clinical Lesion & Testing Pearls
CN I • Olfactory	Cribriform plate	Sensory: Smell	Anosmia from cribriform plate fracture (head trauma) or olfactory groove meningioma; early sign of Kallmann syndrome.
CN II • Optic	Optic canal	Sensory: Vision & pupillary light reflex afferent	Optic chiasm compression (pituitary adenoma) → bitemporal hemianopia. Optic neuritis (multiple sclerosis). Marcus Gunn afferent pupillary defect.
CN III • Oculomotor	Superior orbital fissure	Motor: SR, IR, MR, IO, Levator palpebrae; Parasympathetic: Ciliary ganglion (miosis, near focus)	"Down and out" pupil with ptosis. Parasympathetic pupillary fibers are superficial (compressive lesions like PCom aneurysm blown pupil early; ischemic diabetic neuropathy spares pupil).
CN IV • Trochlear	Superior orbital fissure	Motor: Superior oblique (SO4 → intorsion, depression in adducted eye)	Diplopia when looking down and in (walking down stairs or reading). Patient tilts head AWAY from affected side to compensate. Only CN exiting dorsal brainstem!
CN V • Trigeminal (V1, V2, V3)	V1: Sup orbital fissure V2: Foramen rotundum V3: Foramen ovale	Sensory: Face, scalp, cornea (V1), teeth; Motor: Muscles of mastication (V3), mylohyoid, anterior digastric, tensor tympani	Trigeminal neuralgia (tic douloureux → Carbamazepine). Jaw deviates TOWARD side of lesion (due to unopposed contralateral pterygoid). Absent corneal reflex afferent (V1).
CN VI • Abducens	Superior orbital fissure	Motor: Lateral rectus (LR6 → abducts eye)	Medial strabismus with horizontal diplopia on lateral gaze. Long intracranial course → highly sensitive to elevated intracranial pressure (false localizing sign).
CN VII • Facial	Internal acoustic meatus → Stylomastoid foramen	Motor: Facial expression, stapedius; Sensory: Taste ant 2/3 tongue (chorda tympani); Parasympathetic: Lacrimal, submandibular, sublingual glands	Bell Palsy: LMN lesion paralyzes ENTIRE ipsilateral face (forehead included). UMN stroke spares forehead (bilateral cortical innervation). Hyperacusis (loss of stapedius).
CN VIII • Vestibulocochlear	Internal acoustic meatus	Sensory: Hearing & balance	Sensorineural hearing loss, vertigo, nystagmus. Vestibular schwannoma (acoustic neuroma at cerebellopontine angle; bilateral in Neurofibromatosis Type 2).
CN IX • Glossopharyngeal	Jugular foramen	Sensory: Taste post 1/3 tongue, gag afferent, carotid sinus/body chemo/baroreceptors; Parasympathetic: Otic ganglion to parotid	Loss of gag reflex afferent. Glossopharyngeal neuralgia (stabbing throat pain triggered by swallowing). Carotid sinus syncope.
CN X • Vagus	Jugular foramen	Motor: Palate, pharynx, larynx, gag efferent; Parasympathetic: Heart, lungs, GI tract to splenic flexure	Uvula deviates AWAY from side of lesion. Hoarseness (recurrent laryngeal nerve injury during thyroidectomy). Dysphagia, loss of gag reflex efferent.
CN XI • Accessory	Jugular foramen	Motor: SCM & Trapezius	Weakness turning head to OPPOSITE side (SCM). Shoulder droop and impaired arm abduction above horizontal (trapezius). Injured in posterior neck triangle surgery.
CN XII • Hypoglossal	Hypoglossal canal	Motor: All intrinsic/extrinsic tongue muscles (except palatoglossus, CN X)	Tongue deviates TOWARD the side of the lesion on protrusion (due to unopposed contralateral genioglossus muscle). LMN lesion produces tongue fasciculations and atrophy.

Brainstem Vascular Stroke Syndromes

Lateral vs Medial Medullary, Lateral Pontine, and Midbrain strokes.

PICA • Lateral Medullary

Wallenberg Syndrome

Posterior Inferior Cerebellar Artery (PICA) occlusion.

Nucleus Ambiguus (CN IX, X, XI): Dysphagia, hoarseness, decreased gag reflex.

Vestibular Nuclei: Vertigo, nystagmus, nausea/vomiting.

Spinothalamic Tract: Contralateral loss of pain and temperature on BODY.

Spinal Trigeminal Nucleus: Ipsilateral loss of pain and temperature on FACE.

Sympathetic Fibers: Ipsilateral Horner syndrome (ptosis, miosis, anhidrosis).

Mnemonic: "Don't PICA (pick a) horse that can't swallow (Horner, dysphagia)".

AICA • Lateral Pontine

AICA Occlusion

Anterior Inferior Cerebellar Artery (AICA) occlusion.

Facial Nucleus (CN VII): Ipsilateral facial paralysis (LMN Bell-like deficit), loss of taste anterior 2/3 tongue, decreased lacrimation/salivation.

Vestibular & Cochlear Nuclei: Sensorineural hearing loss, vertigo, tinnitus.

Spinothalamic & Spinal Trigeminal: Crossed pain/temperature loss (ipsilateral face, contralateral body).

Sympathetic Fibers: Ipsilateral Horner syndrome.

Mnemonic: "Facial droop means AICA's pooped".

ASA • Medial Medullary

Medial Medullary Syndrome

Anterior Spinal Artery (ASA) or vertebral branch occlusion.

Corticospinal Tract: Contralateral hemiparesis of upper and lower extremities.

Medial Lemniscus: Contralateral loss of proprioception and vibration sensation.

Hypoglossal Nerve (CN XII): Ipsilateral flaccid tongue deviation TOWARD the side of the lesion.

SAD

Retroperitoneal Organs • SAD PUCKER

Injuries produce blood or gas in retroperitoneal space without free intraperitoneal fluid.

S Suprarenal

Adrenal glands

A Aorta & IVC

Abdominal vessels

D Duodenum

Parts 2, 3, 4 (not 1st)

P Pancreas

Head & body (not tail!)

U Ureters

Bilateral ureters

C Colon

Ascending & Descending

K Kidneys

Perirenal fat

E Esophagus

Lower 2/3 abdominal

R Rectum

Middle/lower parts

Cortical Aphasia Localization & Lacunar Stroke Syndromes

Language localization algorithms (Fluency, Comprehension, Repetition), vascular territories, and subcortical ischemic syndromes.

Aphasia Type	Fluency	Comprehension	Repetition	Anatomic Localization & Vascular Supply	Clinical Features & Patient Behavior
Broca (Expressive / Motor)	Non-fluent (effortful, halting, telegraphic)	Intact (good)	Impaired (poor)	Left inferior frontal gyrus (Brodmann 44/45); Superior division of Left MCA	Patient has insight into deficit and is extremely frustrated. Often accompanied by right-sided facial weakness and right hemiparesis (arm > leg).
Wernicke (Receptive / Sensory)	Fluent (effortless, voluminous word salad)	Impaired (severely poor)	Impaired (poor)	Left superior temporal gyrus (Brodmann 22); Inferior division of Left MCA	Patient is unaware of deficit (anosognosia) and not frustrated. Speech is meaningless neologisms and paraphasias. Associated with right superior quadrantanopia ("pie in the sky").
Conduction Aphasia	Fluent (normal rate and melody)	Intact (good)	Markedly Impaired (disproportionately poor)	Arcuate fasciculus (white matter pathway connecting Broca and Wernicke)	Patient understands everything said and speaks fluently, but cannot repeat simple phrases ("no ifs, ands, or buts"). Frequent phonemic paraphasic self-corrections.
Global Aphasia	Non-fluent (mute or stereotyped syllables)	Impaired (poor)	Impaired (poor)	Extensive perisylvian damage involving both Broca and Wernicke; Left MCA trunk occlusion	Profound inability to produce or understand language. Severe right hemiplegia, right hemisensory loss, and homonymous hemianopia.
Transcortical Aphasias (Motor, Sensory, Mixed)	Motor: Non-fluent Sensory: Fluent	Motor: Intact Sensory: Impaired	INTACT / PRESERVED (striking repetition!)	Watershed / borderzone infarcts (ACA-MCA or MCA-PCA border zones following severe systemic hypotension / cardiac arrest)	Hallmark: Repetition is completely spared! Patients can parrot back complex sentences. Transcortical sensory patients often exhibit compulsive echolalia.

Pure Motor Hemiparesis

Posterior Limb of Internal Capsule

Contralateral hemiparesis/paralysis of face, arm, and leg equally without sensory loss, visual defect, or aphasia. Caused by lipohyalinosis of lenticulostriate arteries from chronic hypertension.

Pure Sensory Stroke

Ventral Posterolateral (VPL) / VPM Thalamus

Contralateral numbness and sensory loss of face and body equally. Motor function, speech, and vision completely preserved. Caused by occlusion of thalamoperforating arteries.

Ataxic Hemiparesis

Base of Pons or Internal Capsule

Weakness and cerebellar ataxia on the same side of the body, disproportionate to weakness (ataxia is prominent). Leg weakness often > arm weakness.

Dysarthria-Clumsy Hand

Genu of Internal Capsule or Basal Pons

Severe dysarthria (slurred speech) with clumsiness, intentional tremor, and fine motor loss of the contralateral hand (e.g. inability to write or button shirt).