Comprehensive Neuroanatomy Course

This chapter introduces the organisation of the central and peripheral nervous systems and shows how lesion level, tract involvement and nerve root anatomy translate into the clinical findings and physiotherapy priorities seen in spinal cord injury (SCI), root and peripheral nerve lesions, and stroke.

Organisation of the Nervous System

The central nervous system (CNS) comprises the brain and spinal cord. The peripheral nervous system (PNS) comprises 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal), the cranial nerves and the peripheral ganglia. Functionally, the PNS is divided into somatic (voluntary motor, conscious sensation) and autonomic divisions. The autonomic division is sympathetic (thoracolumbar outflow, T1–L2) or parasympathetic (cranial and S2–S4 outflow).

The key clinical distinction is between:

• Upper motor neuron (UMN) lesions (cortex, internal capsule, brainstem, cord tracts): spasticity, hyperreflexia, extensor plantar response (Babinski), minimal early atrophy.
• Lower motor neuron (LMN) lesions (anterior horn cell, root, peripheral nerve): flaccid paralysis, hyporeflexia or areflexia, fasciculations, rapid atrophy.

Spinal Cord–Vertebral Relationships

The adult cord ends at the conus medullaris (approximately L1–L2). Below this, the lumbosacral roots form the cauda equina, which is part of the PNS. Cord segments lie above their same-numbered vertebrae, so bony and neurological levels differ.

• Cervical roots C1–C7 exit above their named vertebra. C8 exits between C7 and T1, and all roots below this exit below their vertebra.
• Lumbosacral cord segments (L1–S5) lie behind the T9–L1 vertebrae.
• A T12 vertebral injury therefore damages the conus and upper cauda equina. This produces a mixed UMN/LMN picture with lower limb paralysis and bladder, bowel and sexual dysfunction.

Major Tracts

Because pathways cross at different levels, a hemicord lesion (Brown-Séquard) causes ipsilateral weakness and proprioceptive loss with contralateral pain and temperature loss. A cortical or capsular stroke causes contralateral weakness, because corticospinal fibres have not yet crossed.

Dermatomes and Myotomes

Key myotomes (tested at MRC grade 0–5):

• C5 elbow flexors (biceps), C6 wrist extensors, C7 elbow extensors (triceps), C8 finger flexors (flexor digitorum profundus), T1 small finger abductors.
• L2 hip flexors, L3 knee extensors, L4 ankle dorsiflexors, L5 long toe extensors, S1 ankle plantarflexors.

Key sensory points include:

• C5 lateral antecubital fossa, C6 thumb, C7 middle finger, C8 little finger, T1 medial antecubital fossa.
• T4 nipple line, T6 xiphoid, T10 umbilicus, T12 inguinal ligament midpoint.

Reflexes are graded 0–4+. The biceps reflex tests C5, brachioradialis C6, triceps C7, patellar L3–4 and Achilles S1.

Pathophysiology and Clinical Presentation by Lesion

C4 SCI (tetraplegia). The phrenic nerve arises from C3–C5, so diaphragm function may be partly preserved but is compromised by loss of intercostal and abdominal muscle function. Presentation includes:

• Paralysis of all four limbs and the trunk.
• Reduced vital capacity and an ineffective cough.
• Early risk of respiratory failure and atelectasis.

C5 and C6 root involvement. After a fall onto an outstretched hand, traction injury to the upper trunk of the brachial plexus or a C5 root lesion produces:

• Numbness or paraesthesia over the lateral upper arm (C5).
• Weak shoulder abduction and elbow flexion.
• A reduced biceps reflex.

A C6 lesion affects the thumb and radial forearm, weakens wrist extension and reduces the brachioradialis reflex. A radicular (dermatomal) pattern points to the root. Cord injury at the same level adds bilateral and long-tract signs below the lesion.

T6 SCI. Paraplegia with preserved arm function and impaired trunk control. Sympathetic outflow below the lesion is disconnected from supraspinal control. Lesions at or above T6 therefore carry risk of:

• Autonomic dysreflexia (AD).
• Orthostatic hypotension.
• Impaired thermoregulation.

T12 SCI. Complete loss of lower limb motor function, with preserved upper limbs and trunk. Examination may show a flaccid or mixed picture, depending on conus versus cauda equina involvement. Bladder and bowel dysfunction are expected.

Peripheral nerve injury (forearm and hand). Grade according to Seddon (neurapraxia, axonotmesis, neurotmesis) or Sunderland (I–V). Typical patterns are:

• Median nerve: weak thenar muscles, loss of pinch, "ape hand".
• Ulnar nerve: claw hand, weak finger abduction, Froment's sign.
• Radial nerve: wrist drop.

The LMN pattern is flaccidity, atrophy and a nerve-specific sensory loss. Axonal regrowth proceeds at about 1 mm/day.

Stroke. Middle cerebral artery (MCA) infarction typically produces:

• Contralateral hemiparesis, with the face and arm affected more than the leg.
• Sensory loss on the same side.
• Broca's (non-fluent) aphasia or dysarthria with a dominant-hemisphere lesion.

An anterior cerebral artery stroke affects the leg more than the arm. Spasticity develops after an initial period of flaccidity.

Assessment

SCI: ISNCSCI (ASIA) examination

• Test 10 key myotomes bilaterally, graded 0–5.
• Test 28 dermatomes for light touch and pinprick, scored 0–2.
• Perform deep anal pressure and voluntary anal contraction.
• Derive the neurological level and the AIS grade: A (complete, no sacral sparing), B (sensory incomplete), C (motor incomplete, more than half of key muscles below the level <3), D (motor incomplete, at least half ≥3), E (normal).

SCI: other measures

• Spirometry (vital capacity) and peak cough flow. Assisted cough is typically needed if peak cough flow is below 270 L/min.
• Seated blood pressure. Baseline systolic is often 90–110 mmHg in cervical and high thoracic injuries.
• Modified Ashworth Scale (0–4, including 1+).
• Spinal Cord Independence Measure (SCIM III).
• Walking Index for SCI (WISCI II) and the 10-metre walk test.

Root and peripheral nerve

• Dermatomal and myotomal mapping.
• Spurling's test and the upper limb tension test.
• Tinel's sign over the lesion site.
• Two-point discrimination (normal <6 mm at the fingertip).
• Grip dynamometry.
• Serial MRC grading, with electrodiagnostic studies at ≥3 weeks.

Stroke

• NIHSS (0–42).
• Fugl-Meyer Assessment (motor total 100, upper extremity 66).
• Berg Balance Scale (0–56; <45 suggests increased fall risk).
• 10-metre walk test: <0.4 m/s is household ambulation, 0.4–0.8 m/s is limited community, and >0.8 m/s is community ambulation.
• Modified Ashworth Scale.
• Screening for aphasia, dysphagia and neglect.

Physiotherapy Management: Spinal Cord Injury

Acute phase (0–6 weeks, spinal precautions until stability is confirmed)

• Respiratory care for cervical and high thoracic lesions: positioning, manually assisted cough, abdominal binder, incentive spirometry and inspiratory muscle training.
• Passive ROM 1–2 times daily, with care to avoid overstretching.
• Pressure care with 2-hourly turns.
• DVT prophylaxis in line with medical orders.

Subacute and rehabilitation phase

• Strengthen innervated muscle (8–12 repetitions, 2–3 sets, 3 sessions/week).
• Provide trunk and sitting balance training, pressure-relief techniques, transfer practice and wheelchair skills.
• Provide a tilt table or standing frame for graded orthostatic training.
• Use body-weight-supported treadmill training or functional electrical stimulation for incomplete injuries (AIS C/D).

Functional goals by level

• C4: power wheelchair with head-, chin- or sip-and-puff control, and dependence for personal care.
• T6: independent manual wheelchair, transfers and, usually, independent self-care.
• T12: independent mobility, with possible standing or walking using KAFOs or similar orthoses.

Long-term: At least 2 sessions/week of 20 minutes of moderate to vigorous aerobic exercise, plus strength training 2×/week (3 sets of 8–10 repetitions).

Physiotherapy Management: Peripheral Nerve and Stroke

Peripheral nerve injury and root lesions

• Protect the denervated muscle with splinting (for example, a cock-up splint for wrist drop).
• Keep joints mobile and manage oedema.
• Introduce gentle isometric exercise, progressing as reinnervation occurs.
• Provide sensory re-education and desensitisation.
• Avoid overstretching or fatiguing weak muscle.
• Refer for surgical opinion if there is no recovery at 3 months after a suspected neurotmesis.

Stroke (staged)

• Acute (0–7 days): early mobilisation once medically stable, but not very early high-dose sessions within 24 hours (see AVERT trial). Provide positioning and prevent contracture.
• Subacute (1 week–6 months): high-repetition task-specific training with intensity as tolerated. Include treadmill gait training and sit-to-stand practice.
• Upper limb: constraint-induced movement therapy (CIMT) for patients with at least 10° of active finger extension and 20° of wrist extension.
• Chronic: community exercise and fall prevention.
• Manage spasticity with stretching, positioning, splinting and referral for botulinum toxin if appropriate.
• Cooperate with the speech-language therapist.

Red Flags and Contraindications

• Autonomic dysreflexia (SCI at T6 or above): a rise in systolic BP of 20–40 mmHg above baseline, headache, flushing and sweating above the lesion, and bradycardia. Sit the patient upright, loosen clothing and binders, check for bladder distension (blocked catheter) and bowel impaction, and call for medical help if BP stays elevated. It is a medical emergency.
• Spinal instability: respect precautions until fixation or clearance.
• Orthostatic hypotension: change posture gradually.
• DVT: do not mobilise a limb with suspected DVT. Look for unilateral swelling and calf warmth.
• Heterotopic ossification (loss of ROM, warmth, swelling around hip or knee): avoid forceful ROM.
• Progressive neurological deficit, saddle anaesthesia or new sphincter loss: urgent referral (cauda equina syndrome).
• Fall onto an outstretched hand: exclude fracture, plexus avulsion (Horner's sign) and vascular injury before treating.
• Stroke: monitor BP during exercise, and be aware of shoulder subluxation (avoid pulling the hemiplegic arm), aspiration risk and recurrent stroke signs.

Evidence and Clinical Pearls

• Always specify whether a level refers to the neurological, motor or vertebral level. They often differ.
• Most spontaneous motor recovery after SCI occurs in the first 3–6 months, and AIS grade at 72 hours to 1 month is the best predictor of ambulation.
• The most useful bedside question for localisation is: UMN or LMN? Then: root, plexus, nerve or cord?
• Dermatomes overlap, so a loss confined to one dermatome is often clinically subtle. Use autonomous zones for peripheral nerves.
• Intensity and specificity of task practice are the dominant drivers of motor relearning after stroke (AHA/ASA guidelines).
• Check C4 patients' vital capacity: a fall below about 1 L suggests a risk of ventilatory failure.

Summary

Neuroanatomy underpins rehabilitation. Cord segment, tract and nerve root anatomy determine the pattern of weakness, sensory loss and autonomic dysfunction, from C4 tetraplegia to T12 paraplegia, root lesions at C5 and C6, peripheral nerve injury and stroke. Standardised assessment (ISNCSCI/AIS, MRC grading, NIHSS, Fugl-Meyer, Modified Ashworth Scale), a clear UMN/LMN distinction and attention to red flags such as autonomic dysreflexia guide safe, staged and goal-directed physiotherapy.

References

1. Rupp R, Biering-Sørensen F, Burns SP, et al. International Standards for Neurological Classification of Spinal Cord Injury: Revised 2019. Top Spinal Cord Inj Rehabil. 2021;27(2):1–22.
2. Winstein CJ, Stein J, Arena R, et al. Guidelines for adult stroke rehabilitation and recovery: a guideline for healthcare professionals from the AHA/ASA. Stroke. 2016;47(6):e98–e169.
3. Martin Ginis KA, van der Scheer JW, Latimer-Cheung AE, et al. Evidence-based scientific exercise guidelines for adults with spinal cord injury: an update and a new guideline. Spinal Cord. 2018;56(4):308–321.
4. AVERT Trial Collaboration Group. Efficacy and safety of very early mobilisation within 24 h of stroke onset (AVERT): a randomised controlled trial. Lancet. 2015;386(9988):46–55.
5. Wolf SL, Winstein CJ, Miller JP, et al. Effect of constraint-induced movement therapy on upper extremity function 3 to 9 months after stroke: the EXCITE randomized clinical trial. JAMA. 2006;296(17):2095–2104.
6. Consortium for Spinal Cord Medicine. Early acute management in adults with spinal cord injury: a clinical practice guideline for health-care professionals. J Spinal Cord Med. 2008;31(4):403–479.
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8. Kandel ER, Koester JD, Mack SH, Siegelbaum SA. Principles of Neural Science. 6th ed. McGraw-Hill; 2021.