The LMN is the motor pathway from anterior horn cell (or cranial nerve nucleus) via peripheral nerve to motor endplate.

The motor unit consists of a single anterior horn cell, the single fast-conducting motor nerve fibre that leaves the spinal cord via the anterior root, and the group of muscle fibres (100-2000) being supplied via the mixed peripheral nerve. Anterior horn cell activity is modulated by impulses from:

• corticospinal tracts
• extrapyramidal system
• cerebellum
• afferent fibres from posterior roots.

Signs of lower motor neurone lesion

These are seen in voluntary muscles, which depend upon an intact nerve supply to produce movement and for metabolic integrity. Signs follow rapidly if the LMN is interrupted (Table 21.16). Muscle wasting appears within 3 weeks. Fasciculation (visible twitching) occurs – due to contractions of denervated single motor units. Fibrillation potentials are seen when denervated muscle is sampled electrically.

Causes

Examples of LMN lesions at various levels are:

• cranial nerve nuclei and anterior horn cell – Bell’s palsy, motor neurone disease, poliomyelitis
• spinal root – cervical and lumbar disc protrusion, neuralgic amyotrophy
• peripheral (or cranial) nerve – nerve trauma or entrapment, mononeuritis multiplex .

Table 21-16. Signs of a lower motor neurone lesion

Weakness
Wasting
Hypotonia
Reflex loss
Fasciculation	long term effects
Contractures of muscle
‘Trophic’ changes in skin and nails

NB: Fibrillation potentials can be detected electromyographically.

Expanding mass lesions within the cerebellum obstruct the aqueduct to cause hydrocephalus, with severe pressure headaches, vomiting and papilloedema. Coning of the cerebellar tonsils through the foramen magnum and respiratory arrest occur, often within hours. Rarely, tonic seizures (sudden attacks of limb stiffness) occur.

Lateral cerebellar lobes

A lesion within one cerebellar lobe (e.g. a tumour or infarction) causes disruption of the normal sequence of movements (dyssynergia) on the same side. Ataxia and other signs develop.
Neurotransmitter changes in cerebellar disease are poorly understood.

Posture and gait.

The outstretched arm is held still in the early stages of a cerebellar lesion (cf. the drift of a pyramidal lesion) but there is rebound upward overshoot when the limb is pressed downwards and released. Gait becomes broad and ataxic; the patient falters towards the lesion.

Tremor and ataxia.

Movement is imprecise in direction, in force and in distance (dysmetria). Rapid alternating movements (tapping, clapping or rotary hand movements) are clumsy and disorganized (dysdiadochokinesis). Intention tremor (action tremor, with past-pointing) is seen. Speed of movement is preserved, cf. extrapyramidal disease.

Nystagmus.

Coarse horizontal nystagmus develops with lateral cerebellar lobe lesions – the fast component towards the lesion.

Dysarthria.

A halting, jerking dysarthria occurs – the scanning speech of cerebellar lesions (usually bilateral).

Other signs.

Table 21-15. Principal causes of cerebellar syndromes

Tumours	Haemangioblastoma
	Medulloblastoma
	Secondary neoplasm
	Compression by acoustic neuroma
Vascular lesions	Haemorrhage
	Infarction
	Arteriovenous malformation
Infection	Abscess
	HIV
	Kuru
Developmental	Arnold-Chiari malformation
	Basilar invagination
	Cerebral palsy
Toxic and metabolic	Anticonvulsant drugs
	Chronic alcohol abuse
	Following carbon monoxide poisoning
	Lead poisoning
	Solvent abuse
Inherited	Friedreich’s ataxia
	Ataxia telangiectasia
	Essential tremor
Miscellaneous	Multiple sclerosis
	Hydrocephalus
	Postinfective cerebellar syndrome of childhood
	Hypothyroidism
	Non-metastatic manifestation of malignancy
	Cerebral oedema of chronic hypoxia

Titubation – rhythmic head tremor in whether to and fro (yes and yes) signs or rotary (no and no) signs – also occurs, mainly when cerebellar connections are involved (e.g. in essential tremor and MS. Hypotonia (floppy limbs) and depression of reflexes (and slow, pendular reflexes) are also sometimes seen with cerebellar disease, though of little value as localizing signs.

Midline cerebellar lesions

Midline cerebellar vermis lesions have a dramatic effect on trunk and axial musculature-difficulty standing and sitting unsupported, with a rolling, broad, ataxic gait (truncal ataxia). Lesions of the flocculonodular region of the cerebellum cause vertigo, vomiting and gait ataxia if they extend to the roof of the fourth ventricle.

Table 21.15 summarizes the main causes of cerebellar disease.

The third system of motor control modulates coordination, rather than speed. Ataxia, i.e. unsteadiness, is characteristic when it malfunctions.

The cerebellum receives afferent fibres from:

• proprioceptive receptors in joints and muscles
• vestibular nuclei
• basal ganglia
• the corticospinal system
• olivary nuclei.

Efferent fibres pass from the cerebellum to:

• each red nucleus
• vestibular nuclei
• basal ganglia
• corticospinal system.

Each lateral cerebellar lobe coordinates movement of the ipsilateral limbs. The vermis (a midline structure) is concerned with maintenance of axial (midline) posture and balance.

For clinical purposes, and particularly in general medicine, the extreme complexity of neuroanatomy must be reduced to its core elements. The following sections cover:

• cranial nerves
• three systems of motor control:
• corticospinal or pyramidal system
• extrapyramidal system
• cerebellum
• motor unit
• reflex arc
• sensory pathways and pain
• control of the bladder and sexual function.

Table 21-4. Causes of the amnestic syndrome

Alcohol (Wernicke-Korsakoff syndrome)

Head injury (severe)

Anoxia

Posterior cerebral artery occlusion (bilateral)

Herpes simplex encephalitis

Chronic sedative and solvent abuse

Bilateral invasive tumours

Arsenic poisoning

Following hypoglycaemia