Extrapyramidal System
Introduction
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Gray matter - ventral view
The extrapyramidal pathway or system is an important part of the motor system of the body and can also be described as the descending motor pathway, whose fibres pass through the tegmentum rather than the medullary pyramid. The extrapyramidal pathway is actively involved in the initiation and selective activation of movements, along with their coordination. Major contribution in the extrapyramidal motor system are the nuclei of the basal ganglia. Other structures which are involved include substantia nigra, red nucleus, subthalamic nucleus, mesencephalic reticular formation and the cerebellum. The extrapyramidal system is also often described as the motor-modulation system. The term “extrapyramidal” is to distinguish between the effects of basal ganglia diseases and those of damage to the “pyramidal” system, even though there is an intertwine of a functional relationship between the two systems. Extrapyramidal system is polysynaptic in nature with many synapses within the brainstem.
Components
Most of the descending pathways running from the cerebrum, cerebellum and brainstem towards the spinal cord, without coursing through the pyramids of the medulla, are considered part of the extrapyramidal system. Such tracts include:
- Parts of the rubrospinal tracts
- Tectospinal tracts
- Reticulospinal tracts (lateral and medial)
- Lateral and medial vestibulospinal tracts
Rubrospinal tracts
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Midbrain - medial view
Tectospinal tracts
Another collection of extrapyramidal tracts originates from the superior colliculus. These tracts are referred to as the tectospinal tracts and cross in the posterior tegmental decussation of the midbrain. Fibres of the tract synapse with motor neurons of the cervical muscles and mediate reflex postural movements. They follow the same course of the rubrospinal tracts by passing through the pons and medulla but terminate in the anterior funiculus of the spinal cord.
Reticulospinal tracts
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Pons - lateral left view
In the medial parts of the reticular formations of the pons and medulla, arise the medial reticulospinal tracts. Pontine fibres of these tracts travel in the anterior funiculus and are mainly uncrossed. Medullary fibres travel in the anterior and lateral funiculus as crossed and uncrossed. This tract is mainly concerned with posture.
Vestibulospinal tracts
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Medial vestibular nucleus - ventral view
On the other hand, most of the fibres of the medial vestibulospinal tract decussate as the tracts descend through the medulla to terminate in the anterior funiculus within the cervical region of the spinal cord. Thier axons inhibit motor neurons of neck axial muscles.
Clinical relevance
Damage to the extrapyramidal system leads to different forms of movement and cognitive disorders. The majority of such movement disorders are classically described as dyskinesia. Different types of dyskinesia include myoclonus, tics, chorea, athetosis and tremors. Injury to the pyramidal system induces paralysis, whereas extrapyramidal tract disorders result in involuntary movements, muscle rigidity and immobility without paralysis. In some cases, both pyramidal and extrapyramidal systems are affected, such as spastic-athetoid cerebral palsy. Disorders which are seen with many types of extrapyramidal disease are:
- Parkinson’s disease, which is linked to dopamine deficiency in the striatum of basal ganglia. It is characterised by rigidity (increased muscle tone), bradykinesia (slowing of movements) and tremors.
- Non-spastic cerebral palsy, which could be dyskinetic or ataxic. It is caused by damage to the nerve cells outside the pyramidal tracts in the basal ganglia or in the cerebellum.
- Huntington’s disease, which is a choreiform disorder. It is caused by the degenerative process in the striatum.
Extrapyramidal symptoms including acute and tardive dyskinesias and dystonic reactions are sometimes serious side effects of antipsychotic and other drugs.
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