Extrapyramidal system
The extrapyramidal
system is a part of the motor system network causing involuntary actions.
The system is called extrapyramidal
to distinguish it from the tracts of the motor cortex that reach their targets
by traveling through the pyramids of the medulla.
Extrapyramidal tracts are chiefly found in the reticular
formation of the pons and medulla.
The pyramidal tracts (corticospinal tract and corticobulbar
tracts) may directly innervate motor neurons of the spinal cord or brainstem (anterior
(ventral) horn cells or certain cranial nerve nuclei), whereas the
extrapyramidal system centers on the modulation and regulation (indirect
control) of anterior (ventral) horn cells that are involved in reflexes,
locomotion, complex movements, and postural control.
These tracts are in turn modulated by various parts
of the central nervous system, including the nigrostriatal pathway, the basal
ganglia, the cerebellum, the vestibular nuclei, and different sensory areas of
the cerebral cortex. All of these regulatory components can be considered part
of the extrapyramidal system, in that they modulate motor activity without
directly innervating motor neurons.
Compositions
of extrapyramidal system :
Cortical area:
premotor are (brodman’s area 6)
Subcortical
areas : corpus striatum, globus pallidus , red nucleus , reticular
formation , vestibular nucleus and
neocerebellum
Tracts of
extra pyramidal system :
- rubrospinal
tract
- pontine
reticulospinal tract
- medullary
reticulospinal tract
- lateral
vestibulospinal tract
- tectospinal
tract
Functions
:
1. postural
adjustments of the body to maintain balance
2. gross
synergistic voluntary movements in group of muscles affecting proximal joints
of the limbs
3. movement
preformed unconsciously like swings of arms during walking
4. regulatory
influence over the reflex activities
Effects
of lesion of extra-pyramidal system: muscle tone increased
(spasticity)
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.
