The Neural Control of Movement Flashcards
what is the hierarchical organisation of movement
- Lower motoneurons (LMN)
- Upper motoneurons (UMN)
- Cerebellum
- Basal ganglia
what are LMNs
- these are alpha motoneurons that constitute the final common pathway of all motor commands
- they are part of a motor unit
what do the LMNs innervate
Directly innervate skeletal muscles to produce muscle contractions
where are the cell bodies of the LMNs
Cell bodies in the ventral horn of the spinal cord and exit via ventral roots
what are the 4 anatomical points where the LMN injury can occur
- Ventral horn
- Ventral root
- The NMJ
- Skeletal muscle itself
what does Lower motor neurone injury arise from
- arises from damage to alpha motoneurons
- for example diseases that selectively affect alpha motor neurones such as polio or localised lesions near the spinal cord
what are the characteristics set of symptoms symptoms of LMN injury
- Muscle atrophy
- Hypotonia (reduced muscle tone) – because alpha motor neurons are the only way to stimulate extrafusal muscle fibres, the loss of these neurons causes a decrease in muscle tone
- Hyporeflexia (reduced or absent myotactic /stretch reflexes) this is because the alpha motor neuornes that cause muscle contraction are damaged
- Flaccid paralysis / muscle weakness
- Fasciculations- damaged alpha motor neurons produce spontaneous action potentials visible twitches of affected muscle(s).
- Fibrillations
what are Upper motor neurones
All motor system neurons higher in the hierarchy are referred to as UMNs
what do UMN do
these carry voluntary motor commands to LMNs
- they directly or indirectly synapse onto the LMNs
- there are 3 functionally distinct motor tracts
where are the cell bodies of the UMNs
Cell bodies in the cerebral cortex and brainstem
what are the upper motor neurone tracts
Corticospinal (lateral & anterior/ventral)
Reticulospinal & Vestibulospinal
Rubrospinal
what does the corticospinal tract do
precise motor movements
what do the reticulospinal and vestibulospinal tract do
Gross movements
Facilitates flexor movement
what does the rubrospinal tract do
posture and balance
what are the 3 main synaptic inputs into the LMNs
- UMN of descending tracts (corticospinal, reticulospinal, vestibulospinal, rubrospinal)
- interneurons locally
- Local sensory nerve fibres (part of reflex arcs)
where does the corticobulbopsinal tract run
From the motor cortex
To the brainstem
Courses through the internal capsule (a bundle of axons from the motor cortex)
what are the 2 components of the corticokbulbospinal tract
- corticobulbal fibres
- corticospinal tract
where do the corticobulbal fibres decussate
- they decussate at the midbrain
- synapse onto LMNs destined for the face
- will synapse onto motor fibres of the CNV
where do the corticospinal tract decussate
motor fibres destined for the arms & legs continue down and decussate at the lower medulla
what do the corticospinal tract form
- lateral corticospinal tract and small anterior corticospinal tract
what does the lateral corticospinal tract innervate
innervate LMNs going to muscles of the limbs (hands & thumbs)
what does the anterior corticospinal tract innervate
innervate LMNs going to shoulder and neck muscles (axial muscles)
what makes up the extrapyramidal system and where does it originate from
- they originate from the brainstem as the uMN cell body is in the brainstem
Reticulospinal – originates from the reticular formation of pons
Vestibulospinal – originates from the vestibular nuclei in the upper medulla/lower pons
Rubrospinal – originates from the red nucleus in the midbrain
what is tension in the muscles due to
Tension in muscles due to small amounts of contraction (this produces slight resistance to passive movement)
how is Tone maintained
Tone is maintained reflexively by stretch & gamma motoneurons, and is adjusted to the needs of posture and movement
- it is also regulated by descending motor pathways
how do you maintain posture
- To maintain posture, muscle tone is constantly finely adjusted (happens unconsciously)
- Input from higher levels of the nervous system
define spasticity
Increased muscle tone due to loss of inhibition of gamma () motoneurons
define rigidity
Increased muscle tone due to loss of inhibition of α motoneurons
why does spasticity occur
- The strength of the stretch reflex is controlled by the gamma motoneurons.
- Normally, gamma motoneurons receive both excitatory & inhibitory input in balance.
However when there is increased excitation or decreased inhibition of gamma motoneurons this increases the tension of intrafusal muscle fibres which cause the 1a muscle spindle afferent to become more sensitive to muscle stretch. This results in spasticity.
what can upper motor neurone be due to
due to stroke, tumors, blunt trauma
what are the characteristic symptoms of upper motor neurone injury
Spasticity Hyperreflexia Cog wheel rigidity Clonus/ clonic reflexes Babinski sign
what are the regions of the cortex involved in the motor control
A – Prefrontal cortex B – Frontal eye fields C – Premotor cortex & Supplementary cortex D – Primary motor cortex (Broca’s)
what makes up the motor cortex
- posterior parietal cortex
2, association motor cortex - primary motor cortex
what does the posterior parietal cortex do
Areas 5&7
Decide to move (READY)
what does the association motor cortex do
Premotor & supplementary cortex
Area 6 (& 8)
Planning to perform complex tasks or movements (SET)
what does the primary motor cortex do
Area 4
Activation of descending motor pathways to bring about movement (GO!)
what does damage to the association cortex result in
apraxia
What is apraxia
Apraxia is the impairment in motor planning and strategies, and the inability to perform complex learned motor tasks
How do you test for Apraxia
get patient to arrange a set of blocks to match an example block-structure in front of them. They can move the blocks individually, but cannot come up with a motor plan to arrange them properly.
what is a neuromuscular disorder
A condition affecting one or more of the following:
- Muscles
- Nerves
- NMJ
- Motor or sensory nerve cell body
what is myasthenia gravis
- this is a chronic and autoimmune condition and a disorder of the NMJ
what are the symptoms of NMJ
Weakness in skeletal muscles
Fatiguability
Worsens after periods of activity and improves after periods of rest.
Muscles that control eye and eyelid movement, facial expression, chewing, talkingand swallowing are most commonly affected, although muscles that control breathing, neck and limb movements may also be affected.
what causes myasthenia gravis
In myasthenia gravis, antibodies (immune proteins) block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction, which prevents the muscle from contracting.
what is Gower’s Sign
indicates weakness of proximal muscles in the lower limbs.
Patient has to use their arms and hands to ‘walk’ up their own body from a squatting position due to lack of hip and thigh muscle strength.
what are primary motor muscle disorders
- Gower’s Sign
- inflammatory myopathies (polymyositis)
- congenital myopathies,
- Duchenne muscular dystrophy
what are the 3 lobes of the cerebellum
anterior, posterior and flocculonodular
what are the 3 clinically important function zones of the cerebellum
Vestibulocerebellum
Spinocerebellum
Cerebrocerebellum
what does the vestibulocerebellum do
Balance & posture Eye movements (VOR)
what does the spinocerebellum do
Locomotion
Voluntary movements of arms & legs
what does the cerebrocerebellum do
skilled motor tasks e.g. speech, hand-eye co-ordination, and cognitive eye movements
what is the role of the basal ganglia
Initiation and sequencing/maintenance of
voluntary movements
Enables motor program switch
Regulates intensity of slow and stereotyped movements
Inhibits unnecessary movements
- basically it controls the selection, start and stop posts of motor programs
what is the basal ganglia made out of
- Caudate (head and tail)
- Putamen
- Globus pallidus (internal and external)
- Substantia nigra (pars compacta and pars reticulata)
- Subthalamic nucleus
