Download Functions of the vestibular system Nystagmus Significance of

Functions of the vestibular
system
• Semicircular
The supraspinal control of
movements
canals
• Recognition of angular acceleration of the head
• Activation occurs only at the beginning and at the
end of rotation – dynamic receptor
• do not respond to rotation at constant velocity
• Otolith organ
• Utricle and saccule
• Recognition of the position of the head in space as
well as sensing the gravitation („graviceptor”)
• Recognition of linear acceleration
• Continuously active – static receptor
Nystagmus
The vestibuloocular reflex (VOR)
• When do we see nystagmus?
• Aim
– To maintain retinal fixation, even if the head turns
• Causes nystagmus
• The reflex arch:
1.
2.
3.
4.
5.
Sensor:
Afferent pathway:
Centre:
Efferent pathway:
Effectors:
Semicircular canals
8th cranial nerve
Brain stem
3rd and 6th cranial nerves
medial and lateral rectus
Only at the onset and offset of the rotation, not during the
rotation (unless the eyes are open).
When the rotation stops, opposite nystagmus is observed.
• How to give the direction of the nystagmus?
• Is it possible to evoke nystagmus without
rotating the head?
Yes, e.g. optokinetic nystagmus.
Regulation of the mucle tone and maintaining erect posture
Significance of nystagmus
• Slow component:
– Requires intact brain stem
Cerebral cortex
GP
Cerebellar cortex
• Fast component:
Red nucleus
– Requires intact cortex
• Stimulation of the semicircular canals (e.g.
Caloric stimulation):
– Everything is O.K.: Full nystagmus
– Cortical damage: Only the slow component
– Brain stem damage: No eye movement
Substantia nigra
Cerebellar roof nuclei
Otolith organ
Inhibitory reticular formation
Lateral vestibular nucleus
α motoneurones
γ motoneurones
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Cerebellum
Cerebellar lesions
• Medial part
– Coordination of trunk muscles
• Necessary for walking and maintaining balance
– Coordination of the extraocular muscles
• Lateral part (Hemispheres)
• Medial part:
– Trunk and gait ataxia (swaying towards the
lesion)
– Nystagmus (more severe when the patient
looks towards the side of the lesion)
– Coordination of highly skilled voluntary
movements
(Review the histology of the cerebellar cortex)
Cerebellar lesions 2.
• Hemispheres
– Ataxia
– Dysmetria
– Intention tremor
– Dysarthria (scanning speech)
– Dysdiadochokinesis
– Adiadochokinesis
– Alterations in the muscle tone
– Dyssynergia (Decomposition of movements)
– Rebound phenomenon
Symptoms of basal ganglia
disorders
• Plus (or hyperkinetic) symptoms
– TREMOR
– RIGIDITY
– CHOREA
– ATHETOSIS
– BALLISMUS
• Minus (or hypokinetic) symptoms
– AKINESIA
Significance of the basal ganglia
• Important points:
• Basal ganglia are involved in the planning and
preparation of the movements, as well as in ensuring
the motivation
• Incoming information reaching the basal ganglia
arrives from the cerebral cortex, and after processing
this information, they relay it mainly to the cerebral
cortex
• Their descending projections target the red nucleus,
thus they are involved in the regulation of the muscle
tone, too
Parkinson’s disease (paralysis
agitans)
• Parkinson’s trias:
– Akinesia
– Rigidity
– Tremor
• Reason:
– Damage of the dopaminergic nigrostriatal
pathways
• Treatment:
– Application of L-DOPA
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Huntington’s chorea
• Incidence is 5-10/100,000
• Autosomal dominant with complete
penetrance.
• The mutant gene is found on the 4th
chromosome
• Characterised by the loss of GABA-ergic
neurones
Significance of the cortical motor
areas (1)
• Primary motor cortex
– Actual performance of the motoric tasks
• Premotor cortex
– The activity of this region always preceds that
recorded from the primary motor cortex
– Involved in the „preparation” phase of the
voluntary movements
– Isolated lesion: apraxia (inability to perform
complex motor tasks)
Cortical areas involved in the motor
function
• Primary motor cortex
• Precentral gyrus
• Brodmann’s 4 area
• Motoric „homunculus”
• Praemotor area
• „Non-primary” motor cortex
• Brodmann’s 6 area
• „True” preemotor area
• Supplementary motor area
Significance of the cortical motor
areas (2)
• Supplementary motor cortex
– Organisation of complex motor movements, planning
of the movement
• True premotor area
– It projects to the brain stem and spinal cord mainly
– Particularly important at the beginning of the motor
activity, it is responsible for ensuring and maintaining
the appropriate posture, necessary for completing the
motor required motor activity
The corticospinal (pyramidal) tract
• Composition
–
–
–
–
2×106 axons
~60%: primary motor cortex
~20%: premotor cortex
~20%: somatosensory cortex
• Target
– Grey matter of the spinal cord on the contralateral
side
α-motoneurones
– Direct – monosynaptic
– Indirect – polysynaptic
γ-motoneurones
– polysynaptic
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