Brain anatomy

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Brain Anatomy

The human brain weighs about 1.4 kg (3 pounds) and is the most complex structure in the known universe. It is divided into three major parts visible on gross inspection: the cerebrum, the brainstem, and the cerebellum.

1. Overall Lateral View

Lateral view of the brain showing cerebrum, brainstem, cerebellum, and olfactory bulb

Lateral view (life-size). The three major divisions are immediately visible: the large convoluted cerebrum, the stalk-like brainstem, and the rippled cerebellum posteriorly. The small olfactory bulb is visible on the undersurface.

2. Surface Anatomy: Gyri, Sulci, and Key Landmarks

The cerebrum's surface is highly convoluted. The bumps are gyri, the grooves are sulci, and especially deep grooves are fissures.

Gyri, sulci, and fissures of the lateral brain surface

Key landmarks:

Central sulcus - divides frontal from parietal lobe
Precentral gyrus (anterior to central sulcus) - primary motor cortex
Postcentral gyrus (posterior to central sulcus) - primary somatosensory cortex
Lateral (Sylvian) fissure - separates frontal/parietal lobes from the temporal lobe
Superior temporal gyrus - includes primary auditory cortex

3. Cerebral Lobes

By convention the cerebrum is divided into lobes named after the overlying skull bones:

Cerebral lobes - frontal, parietal, temporal, occipital, and insula

Lobe	Location	Key functions
Frontal	Anterior; separated from parietal by central sulcus	Voluntary movement, executive function, language (Broca's area), personality
Parietal	Behind central sulcus	Somatic sensation, spatial processing, integration of sensory input
Temporal	Below Sylvian fissure	Hearing, memory, language comprehension (Wernicke's area)
Occipital	Posterior pole	Vision (primary visual cortex at calcarine fissure)
Insula	Buried deep in Sylvian fissure	Gustatory cortex (taste), interoception, pain processing

4. Cortical Functional Areas (Brodmann's Map)

Cortical functional areas - motor, sensory, and association areas with Brodmann's map

The cortex is organized into functional areas first mapped by Korbinian Brodmann using cytoarchitecture:

Primary motor cortex - area 4 (precentral gyrus)
Premotor / supplementary motor - area 6
Primary somatosensory - areas 3, 1, 2 (postcentral gyrus)
Posterior parietal cortex - areas 5, 7 (integration)
Primary visual cortex - areas 17, 18, 19 (occipital)
Primary auditory cortex - areas 41, 42 (superior temporal gyrus)
Gustatory cortex - area 43 (operculum/insula)
Prefrontal cortex - most of the frontal lobe anterior to motor areas
Inferotemporal cortex - areas 20, 21, 37 (visual object recognition)

5. Medial Surface - Forebrain Structures

Slicing the brain in the midsagittal plane reveals important structures not visible from outside:

Medial surface of brain showing cingulate gyrus, corpus callosum, fornix, optic chiasm, and calcarine fissure

Key structures visible:

Corpus callosum - the largest white-matter commissure; ~300 million axons connecting the two hemispheres
Cingulate gyrus - part of the limbic system; involved in emotion and memory
Fornix - fiber bundle connecting hippocampus to hypothalamus; critical for memory
Optic chiasm - where nasal retinal fibers decussate
Calcarine fissure - primary visual cortex lines its banks

6. Brainstem and Diencephalon (Medial View)

Medial (sagittal) view of brainstem structures: thalamus, hypothalamus, midbrain (tectum/tegmentum), pons, medulla, cerebellum, pineal body

The brainstem (in the broad sense, including diencephalon) consists of:

Diencephalon

Structure	Function
Thalamus	Relay station for nearly all sensory information en route to cortex; also involved in arousal/consciousness
Hypothalamus	Homeostatic regulation (temperature, hunger, thirst, circadian rhythm, autonomic control, pituitary hormone release)
Pineal body	Melatonin secretion; circadian rhythms

Brainstem proper (midbrain, pons, medulla)

Structure	Location	Key roles
Midbrain	Superior; includes tectum (superior/inferior colliculi) and tegmentum	Visual/auditory reflexes, eye movement (CN III, IV), dopamine (substantia nigra, VTA)
Pons	Middle; bulge on ventral surface	Contains nuclei for CN V, VI, VII, VIII; breathing rhythm; sleep regulation
Medulla oblongata	Inferior; merges with spinal cord	Vital autonomic centers (cardiovascular, respiratory); CN IX–XII; pyramidal decussation

7. Cerebellum

Attached posteriorly to the brainstem via three peduncles:

Superior cerebellar peduncle - output to thalamus and red nucleus
Middle cerebellar peduncle - input from pontine nuclei (corticopontocerebellar tract)
Inferior cerebellar peduncle - input/output to spinal cord and medulla

Functions: coordination of voluntary movement, balance, fine motor tuning, and motor learning. The cerebellar cortex has a characteristic three-layer structure (molecular, Purkinje, granular layers).

8. Ventricular System

The brain's CSF-filled cavities form a connected system:

Ventricle	Location
Lateral ventricles (x2)	Deep in each cerebral hemisphere; bordered by thalamus and basal ganglia medially
3rd ventricle	Midline; flanked by thalami on both sides
Cerebral aqueduct	Through the midbrain; connects 3rd and 4th ventricles
4th ventricle	Between pons/medulla anteriorly and cerebellum posteriorly

CSF exits the 4th ventricle via the foramina of Luschka (lateral) and Magendie (medial) into the subarachnoid space.

9. Deep Structures: Basal Ganglia and Limbic System

Basal ganglia (subcortical nuclei):

Caudate nucleus, putamen (together = striatum), globus pallidus, subthalamic nucleus, substantia nigra
Function: modulation of voluntary movement, habit learning, reward

Limbic system:

Hippocampus - declarative memory formation (long-term memory)
Amygdala - emotional processing (fear, threat detection)
Cingulate gyrus, entorhinal cortex, parahippocampal gyrus

10. Blood Supply (Quick Summary)

The brain is supplied by two systems:

Anterior circulation (internal carotid arteries) - anterior cerebral artery (ACA) and middle cerebral artery (MCA): supply the frontal, parietal, and temporal lobes
Posterior circulation (vertebral arteries joining to form basilar artery) - posterior cerebral artery (PCA): supplies occipital lobe, inferior temporal lobe, thalamus, brainstem, and cerebellum
These two systems anastomose at the Circle of Willis at the base of the brain

Sources: Neuroscience: Exploring the Brain, 5th Edition; Neuroanatomy through Clinical Cases, 3rd Edition; Gray's Anatomy for Students

Motor vs sensory cortex UMN vs LMN Cranial nerves (overview)

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Motor vs Sensory Cortex | UMN vs LMN | Cranial Nerves

Part 1: Motor Cortex vs Sensory Cortex

Location

Both the primary motor and primary somatosensory cortex flank the central sulcus:

Cortex	Location	Brodmann Areas
Primary motor cortex	Precentral gyrus (anterior to central sulcus)	Area 4
Primary somatosensory cortex	Postcentral gyrus (posterior to central sulcus)	Areas 3, 1, 2

The two are adjacent and heavily interconnected - somatosensory feedback from area 3a (muscle spindle input) projects directly to motor cortex and helps regulate precision of muscle contraction.

The Homunculus

Both cortices are somatotopically organized - adjacent body parts are represented by adjacent cortical regions. This is depicted by the famous homunculus ("little man"):

Motor homunculus (right) and somatosensory homunculus (left) mapped onto the dorsal view of the brain - note the disproportionately large hands, lips, tongue, and face reflecting their functional importance

Key somatotopic rule: legs are represented medially (near the midline, tucking into the interhemispheric fissure), while face and hands are represented laterally on the convexity. The area of cortex devoted to a body part reflects its functional importance - hands, lips, and tongue have disproportionately large representations.

Functional Differences

Feature	Primary Motor Cortex (M1)	Primary Somatosensory Cortex (S1)
Location	Precentral gyrus	Postcentral gyrus
Brodmann areas	4	3, 1, 2
Layer IV	Thin (sparse granular layer)	Thick (prominent granular layer)
Output	Commands voluntary movement via corticospinal tract	Processes touch, pressure, pain, temperature, proprioception from contralateral body
Key cell type	Giant Betz cells (layer V) projecting to spinal motor neurons	Stellate cells receiving thalamic (VPL nucleus) relay
Damage result	Contralateral weakness/paralysis	Contralateral loss of discriminative touch, proprioception, two-point discrimination

Other Motor Areas (Frontal Lobe)

Premotor cortex (area 6 lateral) - planning and sequencing movements
Supplementary motor area (area 6 medial) - internally generated movements, bimanual coordination
Broca's area (areas 44, 45) - language production (dominant hemisphere)

Part 2: Upper Motor Neuron (UMN) vs Lower Motor Neuron (LMN)

The Concept

The motor system operates as a two-neuron chain:

Upper motor neuron (UMN): Cell bodies in the motor cortex (and brainstem); axons travel in the corticospinal tract (pyramidal tract) down to the anterior horn of the spinal cord. For cranial nerve muscles, the analogous pathway is the corticobulbar tract projecting to cranial nerve motor nuclei.
Lower motor neuron (LMN): Cell bodies in the anterior horn of the spinal cord (or cranial nerve motor nuclei); axons travel via peripheral nerves directly to skeletal muscle. These are the "final common pathway."

Signs Compared

Sign	UMN Lesion	LMN Lesion
Weakness	Yes	Yes
Muscle atrophy	No (mild disuse atrophy possible)	Yes (denervation atrophy)
Fasciculations	No	Yes (spontaneous anterior horn firing)
Tone	Increased (spasticity)*	Decreased (flaccidity)
Reflexes (DTRs)	Increased (hyperreflexia)*	Decreased/absent (hyporeflexia)
Babinski sign	Present (extensor plantar response)	Absent
Clonus	May be present	Absent

*Note: With acute UMN lesions (e.g., acute stroke, spinal cord injury - "spinal shock"), tone and reflexes are initially decreased and only become increased over hours to months as spasticity develops.

Why Spasticity in UMN Lesions?

The corticospinal tract travels alongside descending inhibitory pathways to anterior horn interneurons. Damage to these inhibitory pathways (rather than the corticospinal tract alone) is what causes the disinhibited hyperreflexia and spasticity seen in UMN lesions.

Clinical Examples

Condition	Type	Explanation
Stroke (motor cortex/internal capsule)	UMN	Affects corticospinal tract above anterior horn
Multiple sclerosis	UMN	Demyelination of white matter tracts
Amyotrophic lateral sclerosis (ALS)	Both UMN + LMN	Degenerates both motor neuron populations simultaneously
Polio	LMN	Virus destroys anterior horn cells
Peripheral nerve injury (e.g., radial nerve palsy)	LMN	Damages nerve distal to anterior horn
Guillain-Barre syndrome	LMN	Demyelination of peripheral nerves
Bell's palsy	LMN (CN VII)	Entire face involved ipsilaterally (including forehead)
Stroke affecting CN VII	UMN (CN VII)	Forehead spared (bilateral cortical representation)

Part 3: Cranial Nerves - Overview

There are 12 pairs of cranial nerves, all emerging from the brainstem (except CN I from olfactory epithelium and CN II from retina). Each has a specific foraminal exit from the skull.

Overview diagram of all cranial nerves emerging from the brainstem, showing their connections to target structures with afferent (blue) and efferent (red) fibers labeled for CN I-V

The 12 Cranial Nerves

#	Name	Type	Key Function	Exit Foramen
I	Olfactory	Sensory	Smell	Cribriform plate
II	Optic	Sensory	Vision	Optic canal
III	Oculomotor	Motor	Eye movement (SR, MR, IR, IO, levator palpebrae); pupil constriction & accommodation (parasympathetics)	Superior orbital fissure
IV	Trochlear	Motor	Eye movement (superior oblique - intorts/depresses)	Superior orbital fissure
V	Trigeminal	Mixed	Face/scalp sensation (V1-V3); muscles of mastication (V3 only)	V1: SOF; V2: foramen rotundum; V3: foramen ovale
VI	Abducens	Motor	Eye movement (lateral rectus - abducts)	Superior orbital fissure
VII	Facial	Mixed	Muscles of facial expression; taste (anterior 2/3 tongue); lacrimation, salivation (submandibular/sublingual glands)	Internal acoustic meatus → stylomastoid foramen
VIII	Vestibulocochlear	Sensory	Hearing (cochlear); balance (vestibular)	Internal acoustic meatus
IX	Glossopharyngeal	Mixed	Taste/sensation (posterior 1/3 tongue, oropharynx); stylopharyngeus muscle; parotid secretion; carotid body/sinus	Jugular foramen
X	Vagus	Mixed	Pharynx/larynx sensation and muscle; parasympathetics to thorax/abdomen (heart, lungs, GI to splenic flexure); taste (epiglottis)	Jugular foramen
XI	Accessory	Motor	Sternocleidomastoid; upper trapezius	Jugular foramen
XII	Hypoglossal	Motor	All intrinsic and most extrinsic tongue muscles	Hypoglossal canal

Mnemonic: Type (S=Sensory, M=Motor, B=Both)

"Some Say Marry Money But My Brother Says Big Brains Matter More" S, S, M, M, B, M, B, S, B, B, M, M

Key Cranial Nerve Reflexes

Reflex	Afferent (sensory)	Efferent (motor)
Corneal (blink)	CN V (ophthalmic V1)	CN VII (facial)
Pupillary light	CN II (optic)	CN III (oculomotor - parasympathetics)
Gag	CN IX (glossopharyngeal)	CN X (vagus)
Jaw-jerk	CN V (mandibular V3)	CN V (mandibular V3)

Grouping by Brainstem Level

Midbrain: CN III (oculomotor), CN IV (trochlear - only CN to exit dorsally)
Pons: CN V (trigeminal), CN VI (abducens), CN VII (facial), CN VIII (vestibulocochlear)
Medulla: CN IX, X, XI, XII
Not from brainstem: CN I (olfactory epithelium → olfactory bulb), CN II (retina → optic chiasm)

Clinically Important Patterns

Ptosis + dilated pupil + "down and out" eye - CN III palsy (posterior communicating artery aneurysm vs. diabetic microvascular ischemia - the latter is pupil-sparing)
Horizontal diplopia - CN VI palsy (very sensitive to raised ICP due to its long intracranial course)
Vertical diplopia, head tilt - CN IV palsy (most common cause: trauma)
Complete ipsilateral facial weakness (including forehead) - LMN CN VII lesion (Bell's palsy, Lyme disease)
Contralateral lower face weakness, forehead spared - UMN CN VII lesion (stroke) - forehead is spared because it has bilateral cortical representation
Tongue deviates toward lesion on protrusion - CN XII palsy (genioglossus on intact side pushes tongue toward the damaged side)

Sources: Neuroanatomy through Clinical Cases, 3rd Ed.; Neuroscience: Exploring the Brain, 5th Ed.; Gray's Anatomy for Students; Rosen's Emergency Medicine, 9th Ed.

Create a clinical decision tool for UMN vs LMN diagnosis

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mkdir -p /tmp/workspace/clinical-tools && echo "done"

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~/clinical-tools/umn-lmn-tool.html

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</head>
<body>

<header>
  <div>
    <h1>🧠 UMN vs LMN Clinical Decision Tool</h1>
    <p>Upper Motor Neuron &amp; Lower Motor Neuron lesion differentiation — based on clinical signs</p>
  </div>
  <span class="badge">Neurology</span>
</header>

<div class="container">

  <!-- CHECKER -->
  <div class="checker-card">
    <h2>Clinical Sign Checker</h2>
    <p class="subtitle">Select all signs present in your patient. The tool will suggest the most likely lesion type.</p>

    <div class="signs-grid" id="signsGrid">
      <!-- filled by JS -->
    </div>

    <div class="btn-row">
      <button class="btn btn-primary" onclick="assess()">Assess Findings</button>
      <button class="btn btn-reset" onclick="resetAll()">Reset</button>
    </div>

    <div id="result">
      <div class="result-box" id="resultBox">
        <div class="result-title" id="resultTitle"></div>
        <div class="result-score" id="resultScore"></div>
        <div class="result-hints" id="resultHints"></div>
      </div>
      <div class="warning">⚠️ This tool is an educational aid only. Clinical judgment, history, and investigations are required for diagnosis.</div>
    </div>
  </div>

  <!-- MOTOR PATHWAY -->
  <div class="pathway">
    <div class="section-title">⬇️ The Two-Neuron Motor Pathway</div>
    <div class="pathway-flow">
      <div class="p-node">Motor Cortex<br><span class="p-label">(Precentral gyrus)</span></div>
      <span class="p-arrow">→</span>
      <div class="p-node umn-node">UMN Axon<br><span class="p-label">Corticospinal / corticobulbar tract</span></div>
      <span class="p-arrow">→</span>
      <div class="p-node">Anterior Horn<br><span class="p-label">Spinal cord (or CN nucleus)</span></div>
      <span class="p-arrow">→</span>
      <div class="p-node lmn-node">LMN Axon<br><span class="p-label">Peripheral nerve</span></div>
      <span class="p-arrow">→</span>
      <div class="p-node muscle-node">Skeletal Muscle<br><span class="p-label">Neuromuscular junction</span></div>
    </div>
    <p style="text-align:center; font-size:0.8rem; color:#64748b; margin-top:14px;">
      Lesion <strong>above</strong> the anterior horn → <span style="color:var(--umn); font-weight:700;">UMN signs</span> &nbsp;|&nbsp;
      Lesion <strong>at or below</strong> the anterior horn → <span style="color:var(--lmn); font-weight:700;">LMN signs</span>
    </p>
  </div>

  <!-- COMPARISON TABLE -->
  <div class="checker-card" style="margin-bottom:28px;">
    <div class="section-title">📋 Full Comparison Table</div>
    <table class="comp">
      <thead>
        <tr>
          <th>Sign / Feature</th>
          <th>UMN Lesion</th>
          <th>LMN Lesion</th>
          <th>Notes</th>
        </tr>
      </thead>
      <tbody>
        <tr><td><strong>Weakness</strong></td><td class="umn-cell">Yes</td><td class="lmn-cell">Yes</td><td>Both cause weakness — not discriminating alone</td></tr>
        <tr><td><strong>Muscle tone</strong></td><td class="umn-cell">↑ Spasticity</td><td class="lmn-cell">↓ Flaccidity</td><td>Acutely, UMN may show flaccid paralysis ("spinal shock")</td></tr>
        <tr><td><strong>Deep tendon reflexes</strong></td><td class="umn-cell">↑ Hyperreflexia</td><td class="lmn-cell">↓ Hyporeflexia / absent</td><td>Acutely, UMN reflexes may be reduced</td></tr>
        <tr><td><strong>Babinski sign</strong></td><td class="umn-cell">Present (extensor plantar)</td><td class="lmn-cell">Absent</td><td>Most specific UMN sign</td></tr>
        <tr><td><strong>Clonus</strong></td><td class="umn-cell">May be present</td><td class="lmn-cell">Absent</td><td>Sustained clonus strongly suggests UMN</td></tr>
        <tr><td><strong>Muscle atrophy</strong></td><td class="umn-cell">Absent (mild disuse atrophy possible)</td><td class="lmn-cell">Present (denervation)</td><td>LMN atrophy is more rapid and prominent</td></tr>
        <tr><td><strong>Fasciculations</strong></td><td class="umn-cell">Absent</td><td class="lmn-cell">Present</td><td>Spontaneous anterior horn cell firing</td></tr>
        <tr><td><strong>Distribution</strong></td><td class="umn-cell">Pyramidal pattern: arm flexors &gt; extensors; leg extensors &gt; flexors</td><td class="lmn-cell">Follows myotome or peripheral nerve territory</td><td>Key for localization</td></tr>
        <tr><td><strong>Pattern (face CN VII)</strong></td><td class="umn-cell">Lower face only (forehead spared — bilateral UMN supply)</td><td class="lmn-cell">Entire face (including forehead)</td><td>Important stroke vs Bell's palsy distinction</td></tr>
        <tr><td><strong>Hoffmann's sign</strong></td><td class="umn-cell">Present</td><td class="lmn-cell">Absent</td><td>Upper limb equivalent of Babinski</td></tr>
        <tr><td><strong>Coordination</strong></td><td class="umn-cell">Impaired (if cerebellar tracts also affected)</td><td class="lmn-cell">Impaired by weakness, not cerebellar</td><td></td></tr>
        <tr><td><strong>Location of lesion</strong></td><td class="umn-cell">Brain, brainstem, or spinal cord (above anterior horn)</td><td class="lmn-cell">Anterior horn, nerve root, peripheral nerve, NMJ</td><td></td></tr>
      </tbody>
    </table>
  </div>

  <!-- EXAMPLES -->
  <div class="two-cols">
    <div class="info-card">
      <h3 style="color:var(--umn);">🔵 UMN Conditions</h3>
      <ul>
        <li><span class="dot dot-umn"></span><div><strong>Stroke</strong> — MCA territory most common; contralateral hemiplegia</div></li>
        <li><span class="dot dot-umn"></span><div><strong>Multiple Sclerosis</strong> — white matter demyelination; relapsing-remitting</div></li>
        <li><span class="dot dot-umn"></span><div><strong>Spinal cord compression</strong> — bilateral UMN below lesion level</div></li>
        <li><span class="dot dot-umn"></span><div><strong>Cerebral palsy</strong> — perinatal corticospinal tract injury</div></li>
        <li><span class="dot dot-umn"></span><div><strong>Primary lateral sclerosis</strong> — pure UMN variant of MND</div></li>
        <li><span class="dot dot-umn"></span><div><strong>Syringomyelia</strong> — UMN below syrinx level</div></li>
        <li><span class="dot dot-both"></span><div><strong>ALS</strong> — both UMN + LMN signs simultaneously</div></li>
      </ul>
    </div>
    <div class="info-card">
      <h3 style="color:var(--lmn);">🔴 LMN Conditions</h3>
      <ul>
        <li><span class="dot dot-lmn"></span><div><strong>Bell's palsy</strong> — CN VII; complete ipsilateral facial weakness incl. forehead</div></li>
        <li><span class="dot dot-lmn"></span><div><strong>Peripheral neuropathy</strong> — GBS, diabetic, toxic</div></li>
        <li><span class="dot dot-lmn"></span><div><strong>Poliomyelitis</strong> — anterior horn cell destruction; no sensory loss</div></li>
        <li><span class="dot dot-lmn"></span><div><strong>Radiculopathy</strong> — nerve root compression (disc herniation)</div></li>
        <li><span class="dot dot-lmn"></span><div><strong>Spinal muscular atrophy (SMA)</strong> — anterior horn cell degeneration</div></li>
        <li><span class="dot dot-lmn"></span><div><strong>Mononeuropathy</strong> — radial nerve palsy, carpal tunnel</div></li>
        <li><span class="dot dot-lmn"></span><div><strong>Myasthenia gravis</strong> — NMJ (fatigable weakness, no atrophy/fasciculations)</div></li>
      </ul>
    </div>
  </div>

  <!-- LOCALIZATION TIP -->
  <div class="checker-card">
    <div class="section-title">📍 Lesion Localization Guide</div>
    <table class="comp">
      <thead>
        <tr><th>Pattern</th><th>Likely Location</th><th>Key Features</th></tr>
      </thead>
      <tbody>
        <tr><td>Contralateral face + arm + leg UMN</td><td>Internal capsule / corona radiata (stroke)</td><td>Contralateral hemisensory loss if posterior limb involved</td></tr>
        <tr><td>Ipsilateral CN palsy + contralateral limb UMN</td><td>Brainstem (crossed syndrome)</td><td>e.g., Weber's syndrome (ipsilateral CN III + contralateral hemiplegia)</td></tr>
        <tr><td>Bilateral UMN below a level</td><td>Spinal cord</td><td>Paraplegia/quadriplegia; sensory level; bladder/bowel involvement</td></tr>
        <tr><td>UMN + LMN at same level</td><td>Anterior horn + corticospinal tract</td><td>Think ALS; also consider cervical myelopathy with radiculopathy</td></tr>
        <tr><td>Dermatomal sensory loss + LMN in myotome</td><td>Nerve root (radiculopathy)</td><td>Disc herniation, foraminal stenosis</td></tr>
        <tr><td>LMN following peripheral nerve distribution</td><td>Peripheral nerve (mononeuropathy)</td><td>Trauma, compression, entrapment</td></tr>
        <tr><td>Distal &gt; proximal LMN; stocking-glove sensory loss</td><td>Peripheral neuropathy (polyneuropathy)</td><td>Diabetes, alcohol, GBS (ascending)</td></tr>
        <tr><td>Complete unilateral facial weakness (incl. forehead)</td><td>LMN CN VII (facial nerve)</td><td>Bell's palsy, Lyme disease, parotid tumor</td></tr>
        <tr><td>Unilateral lower facial weakness (forehead spared)</td><td>UMN CN VII (contralateral hemisphere)</td><td>Stroke, tumor</td></tr>
      </tbody>
    </table>
  </div>

</div>

<footer>
  Clinical Decision Tool — For educational use. Based on <em>Neuroanatomy through Clinical Cases, 3rd Ed.</em> &amp; <em>Ganong's Review of Medical Physiology</em>. Not a substitute for clinical judgment.
</footer>

<script>
const signs = [
  { id: 'spasticity',     label: 'Increased tone (spasticity)',      type: 'umn' },
  { id: 'hyperreflexia',  label: 'Hyperreflexia (brisk DTRs)',       type: 'umn' },
  { id: 'babinski',       label: 'Babinski sign present',            type: 'umn' },
  { id: 'clonus',         label: 'Clonus',                           type: 'umn' },
  { id: 'hoffmann',       label: "Hoffmann's sign",                  type: 'umn' },
  { id: 'forehead_spare', label: 'Forehead spared (facial)',         type: 'umn' },
  { id: 'pyramidal',      label: 'Pyramidal weakness pattern',       type: 'umn' },
  { id: 'flaccid',        label: 'Decreased tone (flaccidity)',      type: 'lmn' },
  { id: 'hyporeflexia',   label: 'Hyporeflexia / absent reflexes',   type: 'lmn' },
  { id: 'atrophy',        label: 'Muscle atrophy',                   type: 'lmn' },
  { id: 'fasciculations', label: 'Fasciculations',                   type: 'lmn' },
  { id: 'forehead_all',   label: 'Full face weak (incl. forehead)',  type: 'lmn' },
  { id: 'nerve_dist',     label: 'Nerve/dermatomal distribution',    type: 'lmn' },
  { id: 'weakness',       label: 'Weakness (non-specific)',          type: 'both' },
];

const grid = document.getElementById('signsGrid');
signs.forEach(s => {
  const div = document.createElement('div');
  div.className = 'sign-item';
  div.id = 'item_' + s.id;
  const tagClass = s.type === 'umn' ? 'tag-umn' : s.type === 'lmn' ? 'tag-lmn' : 'tag-both';
  const tagLabel = s.type === 'umn' ? 'UMN' : s.type === 'lmn' ? 'LMN' : 'Both';
  div.innerHTML = `
    <input type="checkbox" id="chk_${s.id}">
    <label for="chk_${s.id}">${s.label}</label>
    <span class="sign-tag ${tagClass}">${tagLabel}</span>`;
  div.querySelector('input').addEventListener('change', function() {
    div.classList.remove('selected-umn','selected-lmn');
    if (this.checked) {
      div.classList.add(s.type === 'lmn' ? 'selected-lmn' : 'selected-umn');
    }
  });
  grid.appendChild(div);
});

function assess() {
  let umnScore = 0, lmnScore = 0;
  const umnSigns = [], lmnSigns = [];
  signs.forEach(s => {
    const chk = document.getElementById('chk_' + s.id);
    if (chk && chk.checked) {
      if (s.type === 'umn') { umnScore += 2; umnSigns.push(s.label); }
      else if (s.type === 'lmn') { lmnScore += 2; lmnSigns.push(s.label); }
      else { umnScore += 0.5; lmnScore += 0.5; }
    }
  });

  const resultDiv = document.getElementById('result');
  const box = document.getElementById('resultBox');
  const title = document.getElementById('resultTitle');
  const score = document.getElementById('resultScore');
  const hints = document.getElementById('resultHints');

  resultDiv.style.display = 'block';
  box.className = 'result-box';

  if (umnScore === 0 && lmnScore === 0) {
    box.classList.add('unclear');
    title.innerHTML = '⬜ No discriminating signs selected';
    score.textContent = 'Select clinical signs above to see an assessment.';
    hints.innerHTML = '';
    return;
  }

  let verdict, cls, icon, nextSteps, differential;

  if (umnScore > 0 && lmnScore > 0 && Math.abs(umnScore - lmnScore) <= 2) {
    cls = 'mixed'; icon = '🟣';
    verdict = 'Mixed UMN + LMN Pattern';
    nextSteps = 'Consider ALS / MND, cervical myelopathy with radiculopathy, or a multi-level lesion.';
    differential = 'ALS, cervical spondylotic myelopathy, subacute combined degeneration, syringomyelia with cord expansion.';
  } else if (umnScore > lmnScore) {
    cls = 'umn'; icon = '🔵';
    verdict = 'Likely Upper Motor Neuron (UMN) Lesion';
    nextSteps = 'Localise above the anterior horn: brain (cortex, internal capsule, brainstem) or spinal cord. MRI brain ± spine.';
    differential = 'Stroke, MS, spinal cord compression, cerebral palsy, tumour, TBI.';
  } else {
    cls = 'lmn'; icon = '🔴';
    verdict = 'Likely Lower Motor Neuron (LMN) Lesion';
    nextSteps = 'Localise at/below anterior horn: nerve root, plexus, peripheral nerve, NMJ, or muscle. EMG/NCS often indicated.';
    differential = 'Radiculopathy, peripheral neuropathy, GBS, polio, MND (LMN variant), Bell\'s palsy.';
  }

  box.classList.add(cls);
  title.innerHTML = `${icon} ${verdict}`;
  score.innerHTML = `UMN signs: <strong>${umnScore > 0 ? umnSigns.length : 0}</strong> &nbsp;|&nbsp; LMN signs: <strong>${lmnScore > 0 ? lmnSigns.length : 0}</strong>`;
  hints.innerHTML = `
    <div class="hint"><strong>Next steps</strong>${nextSteps}</div>
    <div class="hint"><strong>Differential diagnosis</strong>${differential}</div>`;
}

function resetAll() {
  signs.forEach(s => {
    const chk = document.getElementById('chk_' + s.id);
    if (chk) chk.checked = false;
    const item = document.getElementById('item_' + s.id);
    if (item) item.classList.remove('selected-umn','selected-lmn');
  });
  document.getElementById('result').style.display = 'none';
}
</script>
</body>
</html>

Finding Sources

Reading File

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Reading File

Finding Sources

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UMN vs LMN Clinical Decision Tool

Here is the interactive tool - open it in your browser:

umn lmn tool

HTML File · HTML

The tool includes:

Sign checker - tick present signs, get an instant UMN / LMN / Mixed assessment with differential diagnosis
Two-neuron motor pathway diagram
Full comparison table (tone, reflexes, atrophy, fasciculations, Babinski, Hoffmann's, distribution pattern)
Localization guide - maps sign patterns to lesion locations (internal capsule, brainstem, spinal cord, nerve root, peripheral nerve)
Clinical examples for both UMN and LMN conditions

Circle of Willis

The Circle of Willis is the anastomotic ring at the base of the brain where the anterior (carotid) and posterior (vertebrobasilar) circulations meet. It provides collateral flow capacity - though a complete, full-caliber ring is present in only about 34% of individuals.

Overview: Anterior vs Posterior Circulation

Anterior and posterior cerebral circulations - lateral and posterior views showing ICA segments (cervical, petrous, cavernous, supraclinoid), ophthalmic artery, vertebral arteries ascending through foramina transversaria, and basilar artery

Circulation	Origin	Main vessels
Anterior	Internal carotid arteries (ICA)	ACA, MCA
Posterior	Vertebral arteries (from subclavian) → basilar artery	PCA, SCA, AICA, PICA

The two vertebral arteries ascend through the foramina transversaria of C1-C6, enter the foramen magnum, then join at the pontomedullary junction to form the basilar artery.

The Circle of Willis - Structure

Circle of Willis and its branches - ventral view of the brain showing all named vessels including ACA, AComm, ICA, MCA, PComm, anterior choroidal artery, PCA, SCA, basilar, AICA, PICA, vertebral, and anterior spinal arteries

Schematic diagram of the full cerebral arterial supply from aorta to Circle of Willis, labeling all named vessels

Components of the Circle

The circle is formed by 6 vessels connected by 2 communicating arteries:

Vessel	Part of circle	Origin
Anterior communicating artery (AComm)	Anterior bridge	Connects the two ACAs
Anterior cerebral arteries x2 (ACA)	Anterior arms	Terminal branch of ICA
Internal carotid arteries x2 (ICA)	Lateral pillars	Carotid bifurcation
Posterior communicating arteries x2 (PComm)	Lateral bridges	ICA → PCA; joins anterior to posterior
Posterior cerebral arteries x2 (PCA)	Posterior arms	Top of basilar artery

The Middle cerebral artery (MCA) is the other major terminal branch of the ICA but is not strictly part of the circle itself - it exits laterally into the Sylvian fissure.

The Three Main Cerebral Arteries and Their Territories

Artery	Territory	Key structures supplied	Stroke deficit
ACA (Anterior cerebral)	Medial surface; anterior 2/3	Medial sensorimotor cortex (leg area), frontal lobe medial, anterior corpus callosum	Contralateral leg > arm weakness/sensory loss; incontinence; abulia
MCA (Middle cerebral)	Lateral convexity	Lateral sensorimotor cortex (face/arm), Broca's area (dominant), Wernicke's area (dominant), deep via lenticulostriate to internal capsule + basal ganglia	Contralateral face + arm > leg weakness; aphasia (dominant) or neglect (non-dominant)
PCA (Posterior cerebral)	Occipital lobe; inferior temporal; thalamus, midbrain	Visual cortex, thalamus, posterior limb of internal capsule, hippocampus	Contralateral homonymous hemianopia; thalamic pain; memory loss; alexia without agraphia

Deep Penetrating Branches (Clinically Important)

Branch	Origin	Supplies	Clinical significance
Lenticulostriate arteries	Proximal MCA (M1)	Putamen, caudate, internal capsule (posterior limb)	"Charcot's artery of cerebral hemorrhage" - common site of hypertensive bleed; also causes lacunar infarcts
Anterior choroidal artery	ICA (after PComm)	Posterior limb internal capsule, optic tract, hippocampus, choroid plexus	Occlusion: contralateral hemiplegia + hemianesthesia + hemianopia ("three H's")
Perforating branches of basilar	Basilar artery	Pons, midbrain	Lacunar infarcts in pons; locked-in syndrome (basilar occlusion)

Cerebellar Arteries

Three paired cerebellar arteries arise from the posterior circulation:

Artery	Origin	Territory	Key syndrome on occlusion
PICA (Posterior inferior cerebellar)	Vertebral artery	Lateral medulla + inferior cerebellum	Wallenberg (lateral medullary) syndrome - ipsilateral face, contralateral body pain/temp loss; dysphagia, Horner's, ataxia
AICA (Anterior inferior cerebellar)	Basilar artery	Lateral pons + anterior inferior cerebellum	Ipsilateral CN VII/VIII involvement + cerebellar signs
SCA (Superior cerebellar)	Basilar artery (just before bifurcation)	Superior cerebellum	Cerebellar ataxia; ipsilateral Horner's

ICA Segments - Mnemonic: OPAAM

The main branches of the supraclinoid ICA (after it pierces the dura):

O - Ophthalmic artery (retinal supply - "window to brain vasculature")
P - Posterior communicating artery (PComm)
A - Anterior choroidal artery
A - Anterior cerebral artery (ACA)
M - Middle cerebral artery (MCA)

Clinical Pearls

PComm aneurysm compresses CN III as it runs alongside it → CN III palsy (dilated pupil, ptosis, "down and out" eye) - a neurosurgical emergency
The AComm is the most common site for berry aneurysm rupture (subarachnoid hemorrhage)
MCA is the most commonly occluded artery in stroke - lenticulostriate branches are "end arteries" with no collateral supply
Collateral flow via the Circle of Willis allows ICA stenosis/occlusion to sometimes be tolerated if other vessels are patent
Posterior circulation strokes often cause crossed deficits (ipsilateral cranial nerve + contralateral limb) and vertigo/ataxia - the vertebrobasilar territory is not just the "back of the brain"

Sources: Neuroanatomy through Clinical Cases, 3rd Ed.; Medical Physiology (Boron & Boulpaep); Neuroscience: Exploring the Brain, 5th Ed.

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