Somatic Motor and Sensory Pathways

Somatic Motor Pathways

The somatic motor pathways of the brain and spinal cord are divided into pyramidal and extrapyramidal systems. Both these systems control the motor activities of body through lower motor neurons. The pyramidal system has a direct route to the lower motor neurons, while the extrapyramidal system has an indirect, tortuous route to these neurons. The lesions of somatic motor pathways lead to paralysis.

Clinical Correlation

Paralysis

The term paralysis denotes abolition of function, either motor or sensory, but in common clinical practice, this term is used to denote only loss of motor function.

Types of Paralysis

• Hemiplegia is the paralysis of one half of the body and usually involves upper and lower limbs. If the paralysis is partial, the term hemiparesis is used.

• Monoplegia is the paralysis of one limb only, i.e. the paralysis is restricted to a single limb.

• Diplegia is the bilateral paralysis of the corresponding parts, viz. both upper limbs or both lower limbs (paraplegia), or bilateral facial paralysis (facial diplegia).

• Paraplegia is the paralysis of two upper or two lower limbs but generally, the term paraplegia is used for the paralysis of the two lower limbs.

• Quadriplegia is the paralysis of all the four limbs.

Table 17.1

Arterial supply of the different parts of brain and spinal cord containing pyramidal tract

Parts	Arterial supply
Motor cortex
• Leg area	Anterior cerebral artery
• Face, trunk and arm areas	Middle cerebral artery arm areas
Internal capsule	Branches of middle cerebral artery
Midbrain (cms cerebri)	Posterior cerebral artery
Pons	Pontine branches of basilar artery
Medulla	Medullary branches of vertebral artery
Spinal cord	Segmental branches of anterior spinal artery

N.B. The pyramidal tract is most frequently involved in cerebrovascular accident where it passes through the internal capsule.

Clinical Correlation

Effects of the lesions of corticospinal tracts

The lesions of corticospinal tracts/upper motor neuron lesions result in hemiplegia with or without involvement of cranial nerves. The classical signs are as follows:

• Spastic paralysis, due to involvement of upper motor neurons (UMN). Normally the lower motor neurons (LMNs) are under control of UMNs. Once the UMNs are damaged, they have no control on LMNs. Consequently, LMNs become hyperactive causing hypertonia or spasticity of muscles and exaggerated tendon reflexes.

– Babinski’s sign is present, i.e. great toe becomes dorsiflexed and other toes fan outward when the skin along the lateral aspect of the sole of the foot is scratched with a blunt object.

– Superficial abdominal reflexes are absent, i.e. abdominal muscles fail to contract when the skin of the abdomen is scratched,

– Cremasteric reflex is absent, i.e. cremasteric muscle fails to contract when the skin on the medial side of the upper part of thigh is stroked.

– Loss of performance of fine skilled voluntary movements.

The signs and symptoms of hemiplegia differ according to the site of lesion (Table 17.2).

Table 17.2

Clinical features of hemiplegia

N.B. The lesions of corticospinal tract above the pyramids produce contralateral paralysis, whereas lesions below the pyramids cause ipsilateral paralysis.

N.B. The most important characteristic feature of unilateral brainstem lesion (haemorrhage, tumour) is ‘alternating hemiplegia’ which is characterised by:

(a) ipsilateral cranial nerve palsy (at the level of lesion), and

(b) contralateral hemiplegia, i.e. loss of motor power and sensations in the limbs (below the level of lesion).

Extrapyramidal System

Phylogenetically, it is an older system than the pyramidal system. It consists of all the motor tracts of the brain and spinal cord which do not pass through the medullary pyramids. The extrapyramidal system works hand in hand with the pyramidal system to perform voluntary movements (Flowcharts 17.1 and 17.2).

Flowchart 17.1 Indirect motor pathways through which the corpus striatum influences the spinal cord.

Flowchart 17.2 Indirect motor pathways through which the cerebral cortex influences the spinal cord. These are generally described as extrapyramidal tracts.

Components of extrapyramidal system

The extrapyramidal system includes subcortical centres such as corpus striatum, globus pallidus, tectum, red nucleus, reticular formation, vestibular nuclei and neocerebellum.

The corpus striatum influences descending pathways principally by its cortical connections. The other subcorti-cal centres influence the lower motor neurons in the spinal cord directly through rubrospinal, reticulospinal, tectospinal, vestibulospinal, and olivospinal tracts (Flowchart 17.2).

Functions of extrapyramidal system

• Postural adjustments of the body to maintain balance.

• Gross synergistic voluntary movements in group of muscles affecting proximal joints of the limbs.

• Movements performed unconsciously, like swinging of arms during walking.

• Regulatory influence over the reflex activities.

The differences between the pyramidal and extrapyrami-dal systems are given in Table 17.3.

Table 17.3

Differences between the pyramidal and extrapyramidal systems

	Pyramidal system	Extrapyramidal system
Phylogeny	Phylogenetically recent in acquisition, present only in mammals and achieving its greatest development in man	Phylogenetically older than pyramidal system
Function	Responsible for non-postural, precise movements of small muscles involved in skilful activity	Responsible for gross postural (stereotyped) movements involving large groups of muscles
Pathways	Connected directly to the lower motor neurons. Therefore impulses reach the LMNs, through a direct route	Connected indirectly (polysynaptic pathway) to lower motor neurons. Therefore, impulses reach the LMNs through a circuitous route
Effects of lesion	No increase in muscle tone	Muscle tone increased (spasticity)
Cortical fibres	Arise predominantly in primary motor area (Brodmann’s area 4)	Arise predominantly in premotor area (Brodmann’s area 6)
Subcortical centres and basal ganglia	Play no role in pyramidal system	Play a key role in extrapyramidal system

N.B. Naturally occurring lesions in man rarely, if ever involve pyramidal pathway without simultaneous involvement of extrapyramidal pathways therefore the division of motor pathways into pyramidal and extrapyramidal systems is of little or no clinical relevance.

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