Movement
Summary: Motor Control and Movement Coordination
The body's well-coordinated muscle contraction and relaxation involve a process initiated by messages from the cerebral cortex, transmitted through the brainstem and spinal cord (CNS), ultimately activating peripheral motor neurons. The peripheral motor neurons are organized to facilitate designated and purposeful movements.
Key Components of Motor Control:
1. Motor Cortex (Pre-rolandic Sulcus):
Initiates and plans voluntary movements.
Plays a crucial role in conscious movement control.
2. Supplementary Motor Cortex:
Contributes to the planning and coordination of complex movements.
Involved in both conscious and subconscious aspects of motor control.
3. Basal Ganglia:
Regulates and refines voluntary motor movements.
Involved in motor learning and the suppression of unwanted movements.
4. Cerebellum:
Coordinates and fine-tunes movements, ensuring accuracy and precision.
Plays a crucial role in motor learning and the maintenance of posture and balance.
5. Motor Movement Control Centers:
Movement initiation, smooth performance, and accurate targeting involve various brain centers working together.
Impairment in any of these systems can lead to a loss of function specific to that system.
These areas collectively contribute to the seamless execution of purposeful movements, both at the conscious and subconscious levels.
Anatomy of Basal Ganglia
Summary: Basal Ganglia and Its Function
The Basal Ganglia is a complex structure consisting of several components, including the striatum (puamen and caudate nucleus), substantial nigra (pars compacta and pars reticularis), globus pallidus (internal and external segments), claustrum, subthalamic nucleus, and amygdala.
Function of Basal Ganglia:
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Plays a major role in controlling posture and movement.
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Responsible for the autonomic execution of learned motor plans, mediating subconscious movements that have been practiced.
Inputs to the Striatum:
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Cortical Projections:
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All parts of the cerebral cortex project to the putamen and caudate in a topographic pattern.
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Frontal cortex projects fibers to the ventral head of the caudate and postal putamen.
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Thalamostriate Projections:
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Intralaminar nuclei, especially the centromedian nucleus, send fibers to the striatum.
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Nigrostriatal Projections:
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Substantia Nigra pars compacta projects fibers to the striatum (caudate, putamen, and globus pallidus).
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Striatal Efferents:
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Majority go to the globus pallidus (internal and external segments).
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Others go to the substantial nigra.
Pallidal Afferents and Efferents:
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Afferents: Globus pallidus receives ascending afferent fibers from the substantial nigra and subthalamus.
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Efferents: Major outflow from globus pallidus is to the ventral anterior (VA) and ventral lateral (VL) nucleus.
Nigral Afferents and Efferents:
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SNpr (pars reticulata):
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Receives fibers from the cerebral cortex, striatum, globus pallidus, and subthalamic nucleus.
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Projects to the VA and VL thalamic nucleus, reticular formation, and superior colliculus.
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SNpc (pars compacta):
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Sends dopaminergic fibers to the caudate nucleus and putamen.
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Excitatory for the striatal neurons in the direct pathway and inhibitory for the striatal neurons in the indirect pathway.
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Direct and Indirect Systems:
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Direct System:
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Striatum receives excitatory input from SNpc and projects inhibitory fibers to GPi and SNpr.
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Disinhibits the VL thalamic nucleus, resulting in cortical excitation.
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Indirect System:
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Striatum receives inhibitory input from SNpc and projects inhibitory fibers to GPext.
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GPext inhibits the subthalamic nucleus, which stimulates GPI, inhibiting the VL thalamic nucleus.
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Results in cortical inhibition.
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Note: The current model may not fully explain some clinical findings, possibly due to its consideration of only neuronal discharge rates rather than more complex firing patterns.
LESIONS OF THE BASAL GANGLIA
Summary: Pathological Processes Affecting Basal Ganglia
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Diffuse Effects:
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Pathological processes often have diffuse effects, impacting neighboring structures like the internal capsule, hypothalamus, and cerebral white matter.
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Correlation between basal ganglia lesions and motor dysfunction can be obscure due to this diffuse nature.
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Specific Lesion Effects:
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Lesions of the subthalamic nucleus lead to contralateral hemiballismus.
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Caudate lesions (small unilateral lesions of the anteroventral portion) cause contralateral choreoathetosis.
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Bilateral pallidal lesions result in akinesia.
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Lesions of the substantia nigra (SN) result in parkinsonism.
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Basal Ganglia Hemorrhage or Infarct:
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Unilateral basal ganglia (pallidal-putamen) hemorrhage or infarct can cause sudden falls to the contralateral side.
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Falls are slow, tilting, and follow a stereotyped lateral or diagonal trajectory ("like a falling log").
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Exacerbated by eye closure but occur with eyes open.
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Behavioral and Movement Disorders:
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Caudate lesions may cause behavioral disorders, especially abulia (loss of initiative and spontaneous thought/emotional responses) or disinhibition.
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Putamen and globus pallidus lesions, particularly involving the putamen, can cause dystonia.
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Bilateral lesions of the putamen or globus pallidus may result in parkinsonism or dystonia-parkinsonism.
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Subcortical Aphasias:
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Damage to the frontal-caudate functional system likely underlies aphasias resulting from subcortical lesions affecting the frontal and paraventricular white matter.
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Chorea
Summary: Chorea and Related Disorders
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Chorea Characteristics:
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Sudden, brief, involuntary, continuous, irregular jerks of appendicular, facial, or truncal muscles.
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Truncal involvement in Huntington's disease, distal appendicular involvement in Sydenham's chorea.
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Intensifies during stress, may be camouflaged by purposeful acts (parakinesia).
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Associated Features:
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Changing muscle tone: Hypotonia in Sydenham's chorea, rigidity and hypokinesia in Westphal variant of Huntington's disease (HD), dystonia in juvenile variant of HD.
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Huntington's Disease (HD): Onset in third to fifth decades, progressive chorea, hypotonia, eye movement abnormalities, behavioral issues, and progressive dementia.
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Causes of Chorea:
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Autosomal recessive cause with basal ganglia degeneration and acanthocytosis.
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Sydenham's chorea: Unilateral, may result in flaccid paralysis, occurs in childhood/adolescence.
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Drug-induced chorea: Cocaine, crack dancing.
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Other causes: SLE, hyperthyroidism, pregnancy (chorea gravidarum), oral contraceptives, antiphospholipid antibody syndrome.
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Neuroacanthocytosis: Familial/nonfamilial disorder with chorea, orofacial-lingual dystonic movements, dysphagia, dysarthria, cognitive impairment, psychiatric features, seizures, axonal neuropathy.
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Neuroacanthocytosis:
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Onset around 32 years, acanthocytosis in blood smear, orofacial-lingual dystonia, dysphagia, dysarthria, cognitive impairment, psychiatric features, seizures, axonal neuropathy.
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Pathogenesis:
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Unknown, caudate and putamen pathology inconclusive in many disorders, including Huntington's disease.
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Hemichorea:
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Associated with infarction, hemorrhage, complications of thalamotomy, or rarely neoplasm.
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Athetosis
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Characteristics:
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Slow, writhing movements of wide amplitude.
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Predominantly affects distal appendicular musculature, especially upper extremities.
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Facial and axial muscles may also be involved.
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Movements associated with episodic muscular hypertonia, interfering with daily living.
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Associated Conditions:
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Seen in degenerative disorders like Wilson's disease, kernicterus, status marmoratus, and perinatal anoxia.
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Involves widespread cerebral structures: Globus pallidus, subthalamus, red nucleus, midbrain tegmentum.
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Differential Diagnosis:
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Pseudoathetosis: Occurs during attempts to maintain posture (e.g., extending the arms) due to faulty proprioception.
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Associated with lesions affecting large peripheral nerve fibers, posterior columns, and parietal lobes.
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Athetosis involves slow, involuntary movements with a wide range, primarily affecting distal appendicular muscles, and is often associated with underlying degenerative disorders affecting various cerebral structures. It should be differentiated from pseudoathetosis, which is related to posture maintenance issues due to proprioceptive faults and is associated with specific lesions.
Dystonia
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Characteristics:
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Slow, long-sustained, contouring movements and posture.
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Involves proximal appendicular and axial muscles.
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Characteristic Features:
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Excessive contractions of antagonist muscles during involuntary movement.
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Overflow of contraction to remote muscles not normally involved.
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Spontaneous spasms and contractions.
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Types of Dystonia:
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Generalized:
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Idiopathic or symptomatic (drug-induced, Wilson's disease, GM1, and GM2 gangliosidosis).
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Segmental:
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Idiopathic forms: Spasmodic torticollis, writer's cramp, musician's cramp, spasmodic dysphonia, blepharospasm, orofacial dyskinesia.
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Symptomatic forms: Posthemiplegic dystonia.
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Hemidystonia:
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Follows lesions in contralateral caudate or putamen or thalamus.
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Paroxysmal dystonia may occur with contralateral midbrain lesions.
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DOPA Responsive Dystonia (Segawa Variant):
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Autosomal dominant disorder presenting in childhood.
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Remarkably responsive to low doses of levodopa.
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May present with a combination of dystonia and features of parkinsonism in children.
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Torticollis (Wryneck)
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Definition:
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Hyperkinesia, slightly more frequent in women.
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Characterized by tonic or clonic contraction of neck musculature, especially sternocleidomastoid and trapezius muscles.
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Characteristics:
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Usually unilateral.
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Ceases during sleep.
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Increases in stress and anxiety.
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Relieved by sensory tricks (geste antagonist), like a slight touch to the face.
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Associated Symptoms:
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Majority report local pain.
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10% may have oral, mandibular, or hand-arm dystonia or blepharospasm.
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Head tremor present in about 60% of patients.
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Types:
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Congenital: Present from birth.
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Tardive: Developing as a side effect of certain medications.
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Secondary: Resulting from acquired cervical spine abnormalities (cervical spondylosis, subluxation, inflammatory disorders).
Torticollis can be a complex condition with various contributing factors, including both genetic and acquired elements. associated with specific lesions.
Orofacial Dyskinesias
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Definition:
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Abnormal movements of facial musculature, lips, and tongue.
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Causes:
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Spontaneous occurrence in the elderly.
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Associated with conditions like Huntington's disease or Wilson's disease.
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Prolonged neuroleptic therapy may induce it, suggesting denervation supersensitivity of the striatum.
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Clinical Observations:
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Orofacial dyskinesias may manifest spontaneously in the elderly or be linked to specific neurodegenerative disorders.
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Prolonged use of neuroleptic medications can contribute to their development, indicating changes in striatal sensitivity.
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Lesions and Dyskinesia:
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Unilateral striatonigral lesion may lead to bilateral orofacial-lingual dyskinesia along with contralateral dystonia.
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Suggests that the basal ganglia of one hemisphere may have bilateral control over orofacial-lingual motor functions.
Orofacial dyskinesias can have diverse origins, ranging from spontaneous occurrences to associations with specific neurological conditions and medication-induced effects.
Myoclonus
Myoclonus and Related Conditions
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Myoclonus Overview:
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Characterized by unexpected, brisk, repetitive contractions of muscles.
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Can occur with dystonia (myoclonus dystonia).
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May be focal, multifocal, or generalized.
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Types of Myoclonus:
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Spinal Myoclonus:
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Simple segmental: Confined to one or more adjacent spinal segments.
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Proprioceptive: Axial, involving many spinal segments, triggered by recumbent position.
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Peripheral Myoclonus:
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Rhythmic or semi-rhythmic jerks due to plexus, root lesions, or anterior horn cell disease.
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Palatal Myoclonus:
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Involving palatal and pharyngeal structures, often related to lesions affecting specific pathways.
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Stimulus-Sensitive Myoclonus:
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Triggered by external events like noise, movement, and light.
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Sleep Myoclonus:
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Hypnic jerks occurring during sleep.
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Essential Myoclonus:
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Spontaneous, may be inherited, and associated with essential tremor or dystonia.
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Action Myoclonus:
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May occur due to hypoxic-anoxic brain injury, involving arms, legs, face, and can be disabling.
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Cortical Myoclonus:
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Originates in the cerebral cortex, affecting distal upper limbs and face.
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Often focal but can be multifocal, bilateral, or generalized.
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Triggered by voluntary action and may affect speech and gait.
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Examples include post-hypoxic myoclonus, progressive myoclonic epilepsies.
5. Negative Myoclonus (Asterixis):
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Sudden interruption in ongoing muscle contraction
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Time-locked with an interruption in EEG activity.
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Example: Subcortical negative myoclonus.
6. Subcortical Myoclonus:
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Startle-Hyperexplexia:
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Pathological exaggeration of normal startle response, may be familial or idiopathic.
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Reticular Reflex Myoclonus:
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Rare generalized myoclonus, distinguished from hyperexplexia by spontaneous myoclonus.
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Sensitivity to somatosensory stimuli, seen in various neurological conditions.
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7. Epileptic Myoclonus:
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Examples include Juvenile Myoclonic Epilepsy (JME), Myoclonic Astatic Epilepsy (Lennox-Gastaut syndrome), and Progressive Myoclonic Epilepsies (PNE).
8. Summary:
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Myoclonus encompasses various types, originating from different neurological structures.
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Clinical presentations and triggers vary, and underlying causes may range from structural lesions to genetic factors.
Pathophysiology Parkinson's disease
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Basal Ganglia Function:
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Reciprocal connections with the motor cortex for speed control and fine regulation of movements.
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Dopamine Deficiency State:
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Associated with increased GABAergic output in basal ganglia nuclei: globus pallidus internus (GPi) and pars reticulata of the substantia nigra (SNpr).
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Reduced inhibition via a direct GABAergic pathway from the striatum.
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Excessive excitation through an indirect pathway: subthalamic nuclei excite GPi and SNpr through glutamate.
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Dopamine Receptors in Striatum:
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GABAergic output neurons to GPi and SNpr have a predominance of D1 receptors.
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Neurons projecting to the globus pallidus externus (GPe) contain a predominance of D2 receptors.
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Dopamine has different effects on D1 and D2 receptors.
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Dopamine Deficiency Effects:
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Causes overactivity of the indirect pathway, leading to excessive glutaminergic drive to GPi and SNpr.
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Reduces activity of the inhibitory GABAergic direct pathway, further disinhibiting the activity of GPi.
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Excessive thalamic inhibition results in suppression of the cortical motor system, contributing to akinesia, rigidity, and tremor.
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Inhibitory descending projections to brainstem locomotor areas may contribute to gait and posture abnormalities.
5. Experimental Observations:
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Dopamine deficiency may reduce excessive inhibitory outflow from basal ganglia output nuclei.
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High firing rate of GPi diminishes with the administration of potent dopamine-receptor agonists.
6. Model Deficiencies:
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Does not consider certain projections and roles of dopamine outside the striatum.
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Explanation for rigidity and tremor is less clear.
7. Surgical Intervention:
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Thalamotomy is effective in controlling tremors in Parkinson's disease, contrary to the model's prediction.
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Pallidotomy eliminates levodopa-induced dyskinesia, rather than producing hemiballism, as predicted by the model.