The motor system Medical Assignment Help

THE CORTICOSPINAL TRACTS

The corticospinal tracts originate from the neurones of the fifth layer of the cortex and terminate on the motor nuclei of the cranial nerves and anterior horn cells of the spinal cord. The pathways of importance (Fig. 18.8) in clinical diagnosis decussate in the medulla and pass to the contralateral halves of the cord as the crossed lateral
corticospinal tracts. This is the ‘pyramidal’ system, disease of which causes UMN lesions .

A small proportion of the corticospinal outflow remains uncrossed (the anterior corticospinal tracts).

Characteristics of upper motor neurone lesions Drift of the upper limb 

In the normal individual the outstretched upper limbs are held symmetrically, even when the eyes are closed. In a ‘pyramidal’ lesion, the affected upper limb drifts downwards, medially and tends to pronate and flex slightly.

This important sign may occur early, before weakness is evident.

Drift of upper limb

Weakness with a characteristic distribution Increase in tone of the spastic type
Exaggerated tendon reflexes
An extensor plantar response
Loss of abdominal reflexes
No muscle wasting
Normal electrical excitability of muscle

Weakness

A UMN (‘pyramidal’) lesion above the decussation causes weakness of the contralateral limbs, i.e. a hemiparesis. In an acute, severe lesion such as an infarct of the internal capsule, this weakness will be dense, but in partial lesions there is a characteristic pattern of weakness in the limbs:

in the upper limb the flexors are stronger than the extensors, and in the lower limb the extensors are stronger than the flexors. In the upper limb the weaker movements are thus shoulder abduction and elbow extension; in the forearm and hand, the wrist and finger extensors and abductors are weaker than their antagonists. In the lower limb the weaker movements are flexion and abduction of the hip, flexion of the knee, and dorsiflexion and eversion of the ankle.

When the UMN lesion is below the decussation, the hemiparesis is on the same side as the lesion.

Increase in tone (spasticity) 

An acute lesion of one pyramidal tract causes a flaccid paralysis (and areflexia). An increase in tone follows within several days owing to loss of the inhibitory effect of the corticospinal pathway and an increase in spinal reflex activity. This increase in tone affects all muscle groups on the affected side but is most easily detected in the stronger muscles. It is characterized by changing resistance to passive movement; the change may be sudden- the ‘clasp-knife’ effect. The tendon reflexes are exaggerated and clonus can commonly be demonstrated.

Changes in superficial reflexes 

The plantar response becomes extensor. In a severe lesion (e.g. an infarct of the internal capsule causing hemiplegia) this response can be elicited from a wide area of the affected limb. As recovery occurs the area that is sensitive becomes smaller until only the posterior third of the lateral aspect of the sole is receptive. The stimulus may have to be unpleasant (an orange-stick is the correct instrument to use). For the response to be certainly extensor, the dorsiflexion of the great toe should be accompanied by fanning of the toes.

The abdominal (and cremasteric reflexes) are abolished on the affected side.

The signs of a pyramidal lesion may be minimal. Weakness, spasticity or changes in superficial reflexes may predominate. The absence of one group of signs does not exclude a UMN lesion.

Clinical patterns of upper motor neurone disorders

Two main patterns of clinical features occur in UMN (pyramidal) disorders: hemiparesis and paraparesis. Hemiparesis means weakness of the limbs of one side; it is usually (but by no means always) caused by a lesion within the brain. Paraparesis means weakness of both lower limbs and is characteristically diagnostic of a spinal cord lesion.

The terms hemiplegia and paraplegia strictly indicate total paralysis but the terms are often used loosely to describe severe weakness

Hemiparesis 

The level of a unilateral lesion of the corticospinal tracts may be determined by the accompanying features. MOTOR CORTEX. Weakness localized to one contralaterallimb (monoplegia) or part of a limb is characteristic  of an isolated lesion of the motor cortex. There may be a defect in higher cortical function (e.g. aphasia). Focal epilepsy may occur.

• INTERNAL CAPSULE. Since the corticospinal fibres are tightly packed in the internal capsule, occupying about 1 em”, a small lesion causes a large deficit. For example, an infarct of a small branch of the middle cerebral artery (see p. 906) causes a sudden, dense, contralateral hemiplegia that includes the face.

PONS. A pontine lesion is rarely confined to the corticospinal tract alone. Adjacent structures such as cranial nerve nuclei (e.g. of the sixth or seventh nerve) are involved, causing ipsilateral cranial nerve lesions with contralateral hemiparesis.

SPINAL CORD. A lesion of one lateral corticospinal tract in the cord causes an ipsilateral UM lesion and is indicated by a reflex level in the upper limbs (e.g. absent biceps jerk) or the presence of a Brown-Sequard syndrome. A spinal cord cause of a hemiparesis is unusual.

Paraparesis

Paraparesis (or tetraparesis, when the four limbs are involved) indicates bilateral damage to the corticospinal tracts. Spinal cord disease is the usual cause, but bilateral cerebral lesions can cause a similar picture.

THE EXTRAPYRAMIDAL SYSTEM AND THE CONTROL OF MOVEMENT 

The extrapyramidal system is a general term for the basal ganglia. In disorders of this system, the commonest of which is Parkinson’s disease, there is a combination of:

REDUCTION IN MOVEMENT, i.e. bradykinesia (slow movement) or akinesia (no movement) INVOLUNTARY MOVEMENTS (tremor, chorea, dystonia or athetosis)  RIGIDITY

Anatomy and physiology 

Structure 

The corpus striatum, consisting of the caudate nucleus, globus pallidus and putamen (the latter two forming the lentiform nucleus) lies close to the substantia nigra, thalami and subthalamic nuclei. There are interconnections between these structures and the cerebral cortex, the cerebellum and the reticular formation, the cranial nerve nuclei (particularly the vestibular nerve) and the spinal cord.

The overall function of this complex system is the initiation and modulation of movement. The system modulates cortical motor activity by a series of hypothetical servo loops.

It is now clear that in many basal ganglia disorders there are substantial and specific changes in neurotransmitter profile rather than discrete anatomical lesions.

In Parkinson’s disease there is reduction of: DOPAMINE (to 10% of normal) in the putamen and substantia nigra.

NORADRENALINE AND 5-HYDROXYTRYPTAMINE (to 40% of normal) in the putamen.

GLUTAMIC ACID DECARBOXYLASE(GAD) in substantia nigra and cerebral cortex. GAD is the enzyme responsible for synthesizing GABA.

Cholinergic activity is relatively well preserved in Parkinson’s disease.

Changes in the neurotransmitter profile are associated with characteristic clinical patterns. For example:

IN PARKINSON’S DISEASE an increase in dopamine activity (due to levodopa therapy) relieves rigidity. In excess (in both normal people and those with Parkinson’s disease), levodopa therapy causes chorea.

IN HUNTINGTON’S DISEASE (chorea) there is a marked reduction in acetylcholine and GAB A activity in the striatum. Dopamine activity is normal.

IN NORMAL people an increase in acetylcholine activity or a decrease in dopamine activity causes rigidity and  bradykinesia (parkinsonism). For example, reserpine (which depletes neurones of dopamine) and phenothiazines or butyrophenones (which block dopaminergic neurones) cause or exacerbate parkinsonism.

Clinically, extrapyramidal disorders are classified broadly into the akinetic-rigid syndromes (see p. 918), in which poverty of movement predominates, and the dyskinesias, in which there are a variety of involuntary movements. Spinal lesions Spinal cord compression

Multiple sclerosis
Myelitis (e.g. VZV)
Motor neurone disease
Subacute combined degeneration of the cord
Syringomyelia
Syphilis
Familial or sporadic paraparesis
Vascular disease of the cord
Non-metastatic manifestation of malignancy
Tropical spastic paraparesis (HTLV-1)
Cerebral lesions’
Parasagittal cortical lesions
Meningioma
Venous sinus thrombosis
Hydrocephalus
Multiple cerebral infarction

THE CEREBELLUM

The cerebellum receives afferent fibres from:

• Proprioceptive organs in joints and muscles
• The vestibular nuclei
• The basal ganglia
• The corticospinal system

Efferent fibres pass from the cerebellum to:

• Each red nucleus
• The vestibular nuclei
• The basal ganglia
• The anterior horn cells

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

Cerebellar lesions 

Expanding mass lesions within the cerebellum produce hydrocephalus, causing severe headaches, vomiting and papilloedema. ‘Coning’ of the cerebellar tonsils through the foramen magnum and respiratory arrest occur, often within hours. Very rarely ‘tonic seizures’ of the limbs occur with cerebellar masses

Lateral cerebellar lob

A lesion within one cerebellar lobe (e.g. a tumour or infarction) causes disruption of the normal sequence of movements (dyssynergia) on the side of the lesion.

POSTURE AND GAIT. The outstretched arm is held still in the early stages of a cerebellar lesion but there is rebound overshoot when the limb is pressed downwards by the examiner and released. Gait is ataxic with a broad base; the patient falters towards the side of the lesion.

TREMOR AND ATAXIA. Movement is imprecise in direction, in force and in distance (dysmetria). Rapid alternating movements (tapping, clapping or rotary movements of the hand) are clumsy and disorganized (dysdiadochokinesis).

‘Intention tremor’ (action tremor, with past-pointing) is seen when the ‘finger-nose-finger’ and ‘heel-shin’ tests are performed.

NYSTAGMUS. Coarse horizontal nystagmus appears with cerebellar lobar lesions. Its direction is towards the side of the lesion.DYSARTHRIA. Speech is affected (usually with bilateral lesions). A halting, jerking dysarthria results-the ‘scanning
speech’ o f cerebellar lesions.

OTHER SIGNS. Titubation-rhythmic tremor of the head in either to and fro (,yes-yes’) movements or rotary (‘no-no’) movements-also occurs, mainly when cerebellar connections are involved (e.g. in essential tremor and MS).

Hypotonia and depression of reflexes are also sometimes seen with cerebellar disease, but are usually of little value as localizing signs. ‘Pendular’, i.e. slow, reflexes also occur.

Midline cerebellar lesions 

Lesions of the cerebellar vermis have a dramatic effect on the equilibrium of the trunk and axial musculature. Truncal ataxia causes difficulty in standing and sitting unsupported, with a rolling, broad, ataxic gait. Lesions of the flocculonodular region cause vertigo, vomiting and ataxia of gait if they extend to the roof of the fourth ventricle.

Table 18.17 summarizes the main causes of cerebellar disease.

TREMOR

Tremor is an oscillation, regular and sinusoidal, of a part or parts of the body. Different varieties are outlined below.

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Postural tremor

Everyone has a physiological tremor of the outstretched hands at 8-12 Hz. This is increased with anxiety, hyperthyroidism and certain drugs (lithium, sodium valproate and sympathomimetics) or in mercury poisoning. A coarse, postural tremor is seen in chronic alcohol abusers and in benign essential tremor (usually at 5-8 Hz). Postural tremor does not worsen on movement.

Intention tremor

Tremor that is exacerbated by action, with past-pointing and accompanying slowness and incoordination of rapid alternating movement (dysdiadochokinesis), occurs in cerebellar lobe disease and with lesions of cerebellar connections.

Titubation (tremor of the head) and nystagmus may be present.

Rest tremor 

This is present at rest, is between 4 and 7 Hz and is not made worse by action. It occurs primarily in Parkinson’s disease and is sometimes described as ‘pill-rolling’ between the thumb and index finger.

Other tremors 

Tremor is seen following lesions of the red nucleus (e.g. infarction, demyelination) and rarely with frontal-lobe lesions.

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