Clinical Neuroanatomy

Neuroanatomy

Principles of localisation

Clinical neuroanatomy in MRCP is principally lesion localisation: identify whether pathology lies in cortex, subcortical white matter, basal ganglia, brainstem, cerebellum, spinal cord, root, plexus, peripheral nerve, neuromuscular junction or muscle. The key distinction is between upper motor neurone signs—weakness with spasticity, hyperreflexia, clonus, extensor plantar response and loss of fractionated movement—and lower motor neurone signs—weakness with wasting, fasciculation, hypotonia and reduced reflexes. A single lesion above the anterior horn cell produces UMN signs below the level; lesions of anterior horn cell, root, plexus or peripheral nerve produce LMN signs in the affected segmental distribution.

Cortical and subcortical organisation

The primary motor cortex lies in the precentral gyrus, with contralateral somatotopic representation: leg medially in the paracentral lobule, face and hand laterally. Thus anterior cerebral artery infarction preferentially affects contralateral leg, whereas middle cerebral artery infarction affects face and arm, often with dysphasia if dominant hemisphere or neglect if non-dominant. The primary sensory cortex is postcentral and similarly somatotopic. The internal capsule is high-yield: corticospinal fibres descend through the posterior limb; small lacunar infarcts here cause dense contralateral pure motor hemiparesis because fibres are compacted.

Site	Characteristic syndrome	Exam clue
Dominant inferior frontal gyrus	Broca aphasia	Non-fluent speech, preserved comprehension, impaired repetition
Dominant posterior superior temporal gyrus	Wernicke aphasia	Fluent but nonsensical speech, impaired comprehension
Non-dominant parietal cortex	Hemispatial neglect	Extinction, anosognosia, dressing apraxia
Occipital cortex	Homonymous field defect	Macular sparing suggests posterior cerebral artery cortical lesion

Long tracts and spinal cord localisation

The corticospinal tract decussates at the caudal medulla; lesions above decussation cause contralateral UMN weakness, whereas cord lesions cause ipsilateral UMN weakness below the lesion. The dorsal columns carry vibration, proprioception and discriminative touch ipsilaterally in the cord, decussating in the medulla. The spinothalamic tract carries pain and temperature; fibres enter, ascend or descend 1–2 segments in Lissauer’s tract, then decussate via the anterior white commissure. Therefore a hemicord lesion produces ipsilateral UMN weakness and dorsal column loss below the lesion with contralateral pain-temperature loss beginning approximately 1–2 segments below.

In the cord, cervical enlargement spans approximately C5–T1 and lumbosacral enlargement L2–S3 cord segments. Because the adult cord ends at the conus medullaris around L1–L2 vertebral level, vertebral and cord segment levels diverge increasingly caudally. Normal CSF opening pressure is approximately 6–20 cmH₂O in adults; intracranial pressure is usually 5–15 mmHg, with sustained values >20–22 mmHg clinically significant in acute brain injury.

Brainstem: cranial nerve nuclei and crossed signs

The brainstem is organised by modality and level. Midbrain lesions involve cranial nerves III–IV, pons V–VIII, and medulla IX–XII. A brainstem lesion classically produces ipsilateral cranial nerve signs with contralateral long-tract signs. Medial brainstem structures include corticospinal tract, medial lemniscus and motor cranial nerve nuclei; lateral structures include spinothalamic tract, sympathetic fibres, vestibular nuclei, nucleus ambiguus and cerebellar peduncles.

Syndrome	Vessel/region	Key findings
Weber syndrome	Paramedian midbrain	Ipsilateral III palsy with contralateral hemiparesis
Lateral medullary syndrome	PICA/vertebral artery	Dysphagia, hoarseness, ipsilateral facial pain-temperature loss, contralateral body pain-temperature loss, ataxia, Horner syndrome
Medial medullary syndrome	Anterior spinal artery	Contralateral hemiparesis, contralateral dorsal column loss, ipsilateral XII palsy

Basal ganglia, cerebellum and extrapyramidal anatomy

The basal ganglia modulate movement through direct and indirect corticostriatothalamic loops. Dopaminergic input from the substantia nigra pars compacta facilitates movement via D1-mediated direct pathway activation and D2-mediated indirect pathway inhibition. Parkinsonism results from reduced nigrostriatal dopamine, producing bradykinesia, rigidity, rest tremor and postural instability; hemiballismus localises classically to the contralateral subthalamic nucleus. The cerebellum coordinates ipsilateral movement because cerebellar output effectively double-crosses. Midline vermian lesions cause gait and truncal ataxia; hemispheric lesions cause ipsilateral limb ataxia, dysmetria, intention tremor and dysdiadochokinesia. Cerebellar signs are not explained by pyramidal weakness and should prompt posterior fossa localisation, especially if accompanied by nystagmus, dysarthria or vomiting.

Autonomic and visual pathway localisation

Sympathetic fibres to the eye descend from hypothalamus to ciliospinal centre of Budge at C8–T2, exit via T1, ascend in the cervical sympathetic chain and travel with the internal carotid artery. Horner syndrome—ptosis, miosis and anhidrosis—therefore localises anywhere along this pathway; painful acute Horner syndrome is carotid dissection until proved otherwise. The visual pathway is similarly localising: optic nerve lesions cause monocular visual loss; chiasmal lesions bitemporal hemianopia; optic tract, radiation or occipital lesions homonymous defects. Meyer’s loop in the temporal lobe carries superior visual field fibres, producing contralateral superior quadrantanopia; parietal radiations carry inferior field fibres.

Dermatomes/Myotomes

Conceptual basis and clinical limitations

A dermatome is the cutaneous territory supplied predominantly by sensory afferents from a single spinal root; a myotome is the group of muscles or movements supplied predominantly by motor efferents from a single spinal root. For examination purposes, root localisation is most reliable when sensory loss, weakness and reflex change converge anatomically. In practice, dermatomes overlap substantially: adjacent roots may share 30–50% of cutaneous innervation, so complete anaesthesia from a single root lesion is uncommon. Pain and paraesthesia often localise better than objective pin-prick loss because ectopic discharges from an irritated dorsal root are perceived in the root distribution before axonal failure produces sensory deficit.

Dermatomal maps differ because they are derived from radiculopathy, herpes zoster, surgical root section and embryological data. MRCP-style questions generally use the conventional clinical map: C5 lateral upper arm, C6 thumb, C7 middle finger, C8 little finger, T4 nipple, T10 umbilicus, L1 inguinal ligament, L4 medial malleolus, L5 dorsum of foot/great toe, S1 lateral foot, and S2–S4 perineum/saddle area.

Dermatomes: high-yield landmarks

Root	Key sensory landmark	Exam relevance
C2	Occiput/posterior scalp	Upper cervical radicular pain may mimic occipital neuralgia; no C1 dermatome of practical clinical value.
C5	Lateral shoulder and upper arm	Often overlaps with axillary nerve territory; correlate with deltoid/biceps weakness and biceps reflex.
C6	Lateral forearm, thumb	Common in C5/6 disc disease; test thumb sensation rather than diffuse radial forearm symptoms alone.
C7	Middle finger	Most common cervical radiculopathy in many series; triceps reflex and wrist/finger extension may be affected.
C8	Little finger, medial forearm	Distinguish from ulnar neuropathy by medial forearm involvement and root-level weakness pattern.
T4	Nipple line	Useful for sensory level in myelopathy; remember dermatomal “levels” are not identical to vertebral levels.
T10	Umbilicus	Classical landmark in thoracic cord/root localisation.
L1	Inguinal region	May be confused with ilioinguinal/genitofemoral neuropathy or hip pathology.
L4	Medial leg and medial malleolus	Often accompanied by reduced knee jerk if L3/L4 fibres affected.
L5	Lateral leg, dorsum of foot, great toe	Classically affected by L4/5 disc prolapse compressing the traversing L5 root.
S1	Lateral foot, sole, posterior calf	Common in L5/S1 disc prolapse; ankle jerk is a key correlate.
S2–S4	Perianal/saddle region	Critical in suspected cauda equina syndrome; assess perianal sensation and anal tone where indicated.

Myotomes and reflex integration

Myotomal testing should focus on movements rather than individual named muscles, because most limb muscles are multisegmentally innervated. A clinically significant radiculopathy usually produces weakness in a myotomal pattern, often with preserved power in muscles supplied by the same peripheral nerve but different roots, or vice versa. Deep tendon reflexes provide an objective segmental marker: biceps C5/6, supinator/brachioradialis C6, triceps C7, knee jerk L3/4, and ankle jerk S1. Reflex loss is particularly useful when pain limits voluntary power testing.

Root	Principal myotomal movement	Reflex association	Clinical testing point
C5	Shoulder abduction; elbow flexion contribution	Biceps C5/6	Test deltoid initiation of abduction and biceps against resistance.
C6	Wrist extension; elbow flexion contribution	Brachioradialis C6	Wrist extension weakness with thumb sensory symptoms suggests C6 radiculopathy.
C7	Elbow extension; wrist flexion/finger extension contribution	Triceps C7	Triceps weakness and reduced triceps reflex are high-yield localisers.
C8	Finger flexion; distal hand grip	No reliable routine reflex	Test finger flexion and thumb/finger distal flexors; compare with ulnar/median distributions.
T1	Finger abduction/adduction	No reliable routine reflex	Intrinsic hand weakness may reflect T1 root, lower trunk plexopathy or ulnar neuropathy.
L2	Hip flexion	None routinely reliable	Test iliopsoas seated; consider pain inhibition from hip disease.
L3	Knee extension contribution	Knee jerk L3/4	Quadriceps weakness with anterior thigh sensory symptoms suggests upper lumbar root disease.
L4	Knee extension; ankle dorsiflexion contribution	Knee jerk L3/4	Medial malleolar sensory loss and reduced patellar reflex support L4 involvement.
L5	Great toe extension; ankle dorsiflexion/eversion	No dependable tendon reflex	Extensor hallucis longus weakness is the classic bedside test.
S1	Ankle plantarflexion; foot eversion	Ankle jerk S1	Test repeated single-leg heel raises; subtle S1 weakness may be missed on static testing.

Applied localisation principles

Root symptoms typically follow radicular pain radiating from spine to distal dermatome, exacerbated by manoeuvres increasing foraminal or intraspinal pressure, such as coughing, sneezing or Valsalva. Sensory findings alone are insufficient because peripheral nerves, plexus lesions and entrapment neuropathies may mimic dermatomes. Conversely, a spinal cord lesion produces a sensory level rather than single-root dermatomal loss and is accompanied by upper motor neurone signs below the lesion.

The anatomical relation of disc disease is frequently tested. In the cervical spine, roots exit above their corresponding vertebrae until C8 exits between C7 and T1; therefore a C5/6 posterolateral disc typically affects the C6 root. In the lumbar spine, roots exit below their corresponding vertebrae, and posterolateral disc prolapse usually compresses the traversing root: L4/5 affects L5, and L5/S1 affects S1. Far-lateral disc prolapse compresses the exiting root, an important exception.

In examinations, the safest approach is a triangulation: identify the pain trajectory, map objective sensory loss using pin-prick/light touch, test the relevant myotomal movement against resistance, and check the corresponding reflex. A coherent pattern across these domains is much more diagnostically powerful than memorising isolated dermatome diagrams.

Limb Nerve/Root Lesions

Limb weakness and sensory loss should be localised by asking whether the pattern fits a root, plexus, peripheral nerve, mononeuritis multiplex, or length-dependent polyneuropathy. In MRCP questions, the decisive clues are: pain radiating in a dermatomal distribution for radiculopathy; weakness across multiple named nerves but within one plexus territory for plexopathy; motor and sensory loss conforming to a single named nerve for mononeuropathy; preserved sensory nerve action potentials in pre-ganglionic root lesions; and autonomic or systemic features suggesting vasculitic, diabetic, neoplastic, or inflammatory causes.

Pathophysiological classification of nerve injury

Classification	Pathology	Clinical implication
Neurapraxia	Conduction block from focal demyelination; axon intact	Recovery usually days to 12 weeks; no denervation fibrillations if axon preserved
Axonotmesis	Axonal disruption with intact endoneurial tubes	Wallerian degeneration distal to lesion; regeneration approximately 1–3 mm/day
Neurotmesis	Complete nerve transection or severe disruption	Poor spontaneous recovery; surgical exploration/repair often required

Sunderland grades refine this: grade I is neurapraxia; grade II axon loss with intact endoneurium; grade III endoneurial disruption; grade IV perineurial disruption; grade V complete transection. Neurophysiology is time-dependent: conduction block may be evident immediately, but denervation potentials on EMG typically appear after 10–14 days proximally and 2–3 weeks distally. Sensory nerve action potentials are usually normal in radiculopathy because the dorsal root ganglion lies distal to the lesion, but reduced in plexus or peripheral nerve lesions.

Root lesions: key clinical patterns

Cervical radiculopathy commonly results from disc prolapse in younger patients and foraminal spondylosis in older patients. Pain is neck-to-arm, worsened by foraminal compression; motor, reflex, and sensory signs map to a root rather than a named nerve. Important pairings are: C5 shoulder abduction/elbow flexion with reduced biceps reflex; C6 wrist extension and thumb sensation with reduced brachioradialis reflex; C7 elbow extension, wrist flexion/finger extension, middle finger sensation, reduced triceps reflex; C8/T1 intrinsic hand weakness, finger flexion/abduction, medial forearm/hand sensory loss.

Lumbosacral radiculopathy is most often L4/5 or L5/S1 disc disease. L4 affects knee extension and patellar reflex with medial leg sensory loss; L5 causes dorsiflexion, toe extension, hip abduction weakness and dorsum-foot sensory symptoms; S1 causes plantarflexion weakness, reduced ankle jerk and lateral-foot sensory loss. A central disc causing cauda equina syndrome produces bilateral sciatica, saddle anaesthesia, urinary retention or overflow, faecal incontinence, and reduced anal tone; this requires urgent MRI and neurosurgical discussion.

Upper limb peripheral nerve lesions

Nerve	Typical lesion	Motor deficit	Sensory clue
Axillary C5–C6	Surgical neck fracture, anterior shoulder dislocation	Deltoid weakness; impaired abduction from 15–90°	Regimental badge area
Radial C5–T1	Humeral shaft fracture, Saturday-night palsy, posterior interosseous entrapment	Wrist/finger drop; triceps spared if lesion below spiral groove	Dorsal first web space; absent in pure posterior interosseous palsy
Median C6–T1	Carpal tunnel, supracondylar fracture, pronator syndrome	Thenar wasting, weak thumb abduction/opposition; anterior interosseous palsy causes impaired “OK” sign	Palmar thumb, index, middle and radial half ring finger; thenar eminence spared in carpal tunnel due to palmar cutaneous branch
Ulnar C8–T1	Medial epicondyle injury, cubital tunnel, Guyon canal	Interossei weakness, finger ab/adduction loss, Froment sign; clawing worse with distal lesions	Little finger and ulnar half ring finger

Distinguish C8/T1 radiculopathy from ulnar neuropathy by testing median-innervated T1 muscles such as abductor pollicis brevis and lateral lumbricals. In true neurogenic thoracic outlet syndrome, lower trunk involvement causes thenar/intrinsic wasting and medial forearm sensory loss; vascular symptoms alone do not prove plexopathy.

Lower limb peripheral nerve lesions

Nerve	Common site/cause	Motor pattern	Localising sensory feature
Femoral L2–L4	Retroperitoneal haematoma, pelvic surgery, diabetes	Weak hip flexion and knee extension; reduced patellar reflex	Anterior thigh and medial leg via saphenous nerve
Obturator L2–L4	Pelvic malignancy/surgery	Weak thigh adduction	Medial thigh
Common peroneal L4–S2	Fibular neck compression, weight loss, casts, leg crossing	Foot drop with weak dorsiflexion and eversion; inversion relatively preserved	Dorsum of foot; first web space in deep peroneal lesion
Tibial L4–S3	Popliteal fossa trauma, tarsal tunnel	Weak plantarflexion/inversion; toe flexion weakness	Sole of foot
Sciatic L4–S3	Hip dislocation, pelvic fracture, injection injury	Hamstrings plus peroneal/tibial components; peroneal division often more affected	Below-knee sensory loss except medial leg

A classic examination trap is foot drop: L5 radiculopathy weakens dorsiflexion, toe extension, inversion and hip abduction; common peroneal neuropathy weakens dorsiflexion and eversion with preserved inversion; sciatic neuropathy adds hamstring weakness and tibial deficits. Painful asymmetric proximal lower-limb weakness in diabetes suggests diabetic lumbosacral radiculoplexus neuropathy, often with weight loss and autonomic features.

Investigation and management principles

MRI is preferred for suspected compressive radiculopathy with progressive deficit, myelopathy, cancer/infection red flags, or cauda equina features. Nerve conduction studies and EMG localise lesions, assess severity, and distinguish demyelinating conduction block from axonal loss; they are most informative after approximately 3 weeks if axonal injury is suspected. Management is cause-specific: splinting and decompression for entrapment, urgent surgical input for transection or severe compressive syndromes, and immunotherapy where inflammatory neuropathy is proven. Neuropathic pain options used in UK practice include amitriptyline 10–25 mg nocte titrated to 50–75 mg, duloxetine 30 mg daily increasing to 60 mg, pregabalin 75 mg twice daily titrated to 300 mg twice daily, or gabapentin titrated up to 1.2 g three times daily, adjusted for renal function.