jaras2 sensorik dan motorik

7/28/2019 Jaras2 Sensorik Dan Motorik

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Fig. 249The long ascending pathways of the dorsal columns (yellow lines) and

spinothalamic tracts (red lines).


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Fig. 250The long descending pathway of the pyramidal tract.


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Fig. 237The location of the important spinal tracts. (The descending tracts are

shown on the left, the ascending tracts on the right.)

Clinical features

1Complete transection of the cord is followed by total loss of sensation

in the regions supplied by the cord segments below the level of

injury together with flaccid muscle paralysis. As the cord distal to the

section recovers from a period of spinal shock, the paralysis becomes

spastic, with exaggerated reflexes. Voluntary sphincter control is lost

but reflex emptying of bladder and rectum subsequently return, provided

that the cord centres situated in the sacral zone of the cord are not

destroyed.

2Destruction of the centre of the cord, as occurs in syringomyelia and in

some intramedullary tumours, first involves the decussating spinothalamic

fibres so that initially there is bilateral loss of pain and temperature sensebelow the

lesion; proprioception and fine touch are preserved till late in the

uncrossed posterior columns.


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3Hemisection of the cord is followed by theBrown-Squard syndrome;

there is paralysis on the affected side below the lesion (pyramidal tract) and

also loss of proprioception and fine discrimination (dorsal columns). Pain

and temperature senses are lost on the opposite side below the lesion,

because the affected spinothalamic tract carries fibres which have decussated

below the level of cord hemisection.

4Tabes dorsalis, which is a syphilitic degenerative lesion of the posterior

columns and posterior nerve roots, is characterized by loss of proprioception;

the patient becomes ataxic, particularly if he closes his eyes, because

he has lost his position sense for which he can partially compensate byvisual knowledge of his spatial relationship (Rombergs sign).

5Intractable pain can be treated in selected cases by cutting the appropriate

posterior nerve roots (posterior rhizotomy) or by division of the

spinothalamic tract on the side opposite the pain (cordotomy).

The long ascending and descending pathways

The somatic afferent pathways (Fig. 249)

1Proprioceptive and tactile impulses pass uninterruptedly through the

posterior root ganglia, through the ipsilateralposterior columns of the spinal

cord to thegracile and cuneate nuclei in the lower part of the medulla. In the

posterior columns there is a fairly precise organization of the afferent fibres;

those from sacral and lumbar segments are situated medially in the tracts

while fibres from thoracic and cervical levels are successively added to their

lateral aspect. This arrangement according to body segments is maintained

in the gracile and cuneate nuclei and in the efferents from these nuclei to the

contralateral thalamus. The fibres arising from the gracile and cuneate


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nuclei immediately cross over to the opposite side in thesensory decussation

of the medulla (Fig. 241) and continue up to the thalamus as a compact

contralateral bundlethe medial lemniscus.

2Dorsal root fibres subserving pain and temperature, together with some

tactile afferents, end ipsilaterally in thesubstantia gelatinosa of the posterior

horn. They then synapse and cross to the contralateral anterior lateral

columns of the cord and are relayed to the contralateral thalamus. The fibre

crossing occurs in the anterior white commissure of the spinal cord. In the

brainstem these fibres come to lie immediately lateral to the medial lemniscus

and are sometimes known as thespinal lemniscus (see Figs 249, 258).They terminate in the thalamus.

These somatic afferents are relayed from the thalamus, through the posterior

limb of the internal capsule (Fig. 248) to the somatic sensory cortex of

thepostcentral gyrus. In the internal capsule the fibres are arranged in the

sequence face, arm, trunk and leg from before backwards, and this segregation

persists in the sensory cortex, where the leg is represented on the dorsal and medialpart of the cortex, the trunk and arm in its middle portion and the face most

inferiorly. Since the size of the area of cortical representation reflects the density

of the peripheral innervation and hence

complexity of the function being performed rather than the area of the

receptive field, there is a good deal of distortion of the body image in the

cortex, the cortical representation of the face and hand being much greater

than that of the limbs and trunk.


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Clinical features

1Lesions of the sensory pathway most commonly occur in the internal

capsule following some form of cerebrovascular accident. If complete,

these result in a total hemianaesthesia of the opposite side of the body. In

partial lesions the area of sensory loss will be determined by the site of the

injury in the internal capsule and, from a knowledge of the sensory (and

motor) loss, it is usually possible to determine with some degree of accuracy

the site of a lesion in the capsule.

2Since there is modality segregation below the decussation of the

medial lemniscus, lesions of the sensory pathways at cord level result indissociation of sensation, with an area of analgesia contralaterally together

with impairment of tactile sensibility ipsilaterally (for further details, see

pages 3667).

The auditory, visual and olfactory pathways are dealt with later under

the appropriate cranial nerves.

The motor pathways (Fig. 250)

It is customary to divide the motor pathways of the brain and spinal cord

into pyramidal and extrapyramidal systems. Although the latter is an

imprecise concept, it provides a useful collective term for the many motor

structures not confined to the pyramidal tracts in the medulla.

The pyramidal tract

The pyramidal system is the main voluntary motor pathway and derives

its name from the fact that projections to the motor neurons in the spinal

cord are grouped together in the medullary pyramids. The fibres in this

pathway arise from a wide area of the cerebral cortex. About two-thirds


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derive from the motor and premotor cortex of the frontal lobes; however,

about one-third arises from the primary somatosensory cortex. In both the

motor and premotor cortex there is an organization comparable to that seen

in the sensory area. Again, the body is inverted so that the leg area is situated

in the dorsomedial part of the precentral gyrus encroaching on the

medial surface of the hemisphere, supplied by the anterior cerebral artery.

The face area is near the lateral sulcus, while the arm area occupies a

central position, both supplied by the middle cerebral artery. Again, the

body image is greatly distorted; the area representing the hand, lips, eyes

and foot are exaggerated out of proportion to the rest of the body and in

accordance with the complexity of the tasks they perform.

From the cortex, the motor fibres pass through the posterior limb of the

internal capsule (Fig. 248) where they are again organized in the sequence

of face, arm, leg, anteroposteriorly. From the internal capsule the fibres

form a compact bundle which occupies the central third of the cerebral

peduncle. Hence they pass through the ventral pons, where they are brokenup into a number of small bundles between the cells of the pontine nuclei

and the transversely disposed pontocerebellar fibres. Near the lower end of

the pons they again collect to form a single bundle which comes to lie on the

ventral surface of the medulla and forms the elevation known as the

pyramid. As it passes through the brainstem, the pyramidal system gives

off, at regular intervals, contributions to the somatic and branchial arch

efferent nuclei of the cranial nerves. Most of these corticobulbar fibres cross

over in the brainstem, but many of the cranial nerve nuclei are bilaterally

innervated.

Near the lower end of the medulla the great majority of the pyramidal


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tract fibres cross over to the opposite side and come to occupy a central

position in the lateral white column of the spinal cord. This is the so-called

crossed pyramidal tract shown in Fig. 237. Asmall proportion of the fibres

of the medullary pyramid, however, remain uncrossed until they reach the

segmental level at which they finally terminate. This is the director

uncrossed pyramidal tract, which runs downwards close to the anteromedian

fissure of the cord, with fibres passing from it at each segment to the opposite

side.

In view of the frequent involvement of the pyramidal tract in cerebrovascular

accidents, its blood supply is listed here in some detail:

motor cortexleg area: anterior cerebral artery; face and arm areas:

middle cerebral artery;

internal capsulebranches of the middle cerebral artery;

cerebral peduncleposterior cerebral artery;

ponspontine branches of basilar artery;

medullaanterior spinal branches of vertebral artery;

spinal cordsegmental branches of anterior and posterior spinal

arteries.

Clinical features

1It is important to remember that, in the motor cortex, movements are

represented rather than individual muscles; lesions of this pathway result

in paralysis of voluntary movement on the opposite side of the body

although the muscles themselves are not paralysed and may cause involuntary

movements. This is the essential difference between an upper motor

neuron lesion (i.e. a lesion of the central motor pathway) and a lower

motor neuron lesion (i.e. a lesion affecting the cranial nerve nuclei, or the


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anterior horn cells or their axons). In both types of lesion muscular paralysis

results; in the latter, reflex activity is abolished, flaccidity and muscular

atrophy follow, whereas, in pyramidal lesions, there is spasticity, increased

tendon reflexes and an extensor plantar response.

2Experimental lesions strictly confined to the pyramidal tract are not followed

by increased muscular tone in the affected part (spasticity), but clinically

this is a feature of upper motor neuron lesions; it is attributable tohe central

nervous system

concomitant involvement of the extrapyramidal system, hence demonstrating

the over simplification of the pyramidal and extrapyramidalconcept.

3The pyramidal tract is most frequently involved in cerebrovascular accidents

where it passes through the internal capsule. Indeed, the artery supplying

this areathe largest of the perforating branches of the middle

cerebral arteryhas been termed the artery of cerebral haemorrhage.

4A list of the more important related signs is given here for involvement

of the pyramidal tract at each level.

Cortexisolated lesions may occur here, resulting in loss of voluntary

movement in, say, only one contralateral limb, but often the sensory cortex

is also involved. Aphasia in dominant hemisphere lesions, (usually left),

involving Broca and Wernickes areas and the cortex between them, is not

uncommon.

Internal capsuleusually all parts of the tract are involved, giving a complete

contralateral hemiplegia with associated sensory loss. The lesion may

extend back to involve the visual radiation, giving a contralateral homonymous

field defect (hemianopia).


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Cerebral peduncle and midbrainthe fibres from the 3rd nerve are often

concomitantly involved so that there are the associated signs of a 3rd nerve

palsy.

Ponshere the 4th nerve is often involved, alone or together with VII.

There may then be a hemiplegia affecting the arm and leg of the opposite

side and an abducens and a facial palsy of the lower motor neuron type on

the same side as the lesion.

Medullabecause of the proximity of the pyramids to one another,

medullary lesions often affect both sides of the body. Paralysis of the tongue

on the side of the lesion is due to involvement of the 12th nerve or itsnucleus. The respiratory, vasomotor and swallowing centres may also be

affected.

Spinal cordthe paralysis following lesions of the spinal cord is ipsilateral

and accurately depends on the level at which the pyramidal tract is

involved. Lower motor neurone lesion signs can be detected at the level of

the spinal trauma (direct injury) and upper motor neurone lesion signs

below. The proximity of the pyramidal tracts to the ascending sensory pathways

accounts for the concomitant sensory changes which are usually

found.

The extrapyramidal system

The extrapyramidal motor system should, by definition, include all those

motor projections which do not pass physically through the medullary

pyramids. It was once thought to control movement in parallel with and,

to a large extent, independently of the pyramidal motor system and the

pyramidal/extrapyramidal division was used clinically to distinguish

between two motor syndromes: one characterized by spasticity and paralysis


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whereas the other involved involuntary movements, or immobility

without paralysis. It is now clear that many extrapyramidal structures,

The brain 359

particularly the basal ganglia, actually control movement by altering activity

in the premotor cortex and, thus, the pyramidal motor projections. This

clearly emphasizes the blur between the two systems.

Components of the extrapyramidal system include the red nuclei,

vestibular nuclei, superior colliculus and reticular formation in the brain

stem, all of which project via discrete pathways to influence spinal cord

motor neurons. Cerebellar projections (see page 344) are also included

since they influence not only these brainstem motor pathways, but also

the motor cortex itself via the dentatothalamic projection.

Perhaps the most important structures to retain an extrapyramidal definition

are the basal ganglia (see pages 353 and 354). The neostriatum

(caudate and putamen) receives widespread cortical afferents, including

those from high order sensory association and motor areas, and projects

mainly to the globus pallidus. The latter nucleus is the major outflow for the

basal ganglia and, via the ventral anterior thalamus, exerts its major influence

on premotor and hence the motor cortices. This pattern of connections

suggests that the basal ganglia are involved in complex aspects of motor

control, including motor planning and the initiation of movement.

A variety of motor disorders are associated with basal ganglia pathology

and, in some instances, neuroanatomically discrete deficits in specific

neurotransmitters. For example, Parkinsons disease involves the degeneration

of dopaminergic neurons in the substantia nigra in the midbrain. This

pigmented nucleus provides the neostriatum with a dense dopaminergic

innervation which may be completely lost in severe cases of Parkinsonism.


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Knowledge of this selective chemical neuropathology has resulted in the

development of a treatment of the disease which involves the oral administration

of the dopamine precursor l-dopa.

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