9. Control of movement 2/ Voluntary motor output Flashcards
Name 4 characteristics of voluntary movements
- PURPOSEFUL, goal directed
- TRIGGERED either by imagination or by a wilful decision
- (Where the goal could be achieved by different strategies there is MOTOR EQUIVALENCE. e.g. writing on a small piece of paper or writing on a blackboard. This illustrates multiple levels of control of movement
- Often is LEARNED. So may initially need concentration but with practice the movement can be achieved fluently. Efficiency and accuracy improves
What is the 3 steps that structure a voluntary movement?
- Sensory integration:
Target identification; goal needs to be identified as such and its location understood - Planning;
Movements required to achieve goal must be ascertained. Relative positions of object and self need to be computed so movement can be planned - Execution
Commands from cortical and brainstem centres need to be ordered and initiated in order to provide the correct strength grip and to smoothly move the object
What areas have the greater cortical representation?
Hands, lips and tongue
What is meant by cortical magnification?
This ‘cortical magnification’ reflects the relative ability for precise movement in that part of the body
Comparison between premotor and association areas to the primary motor area?
- Stimulation of both produce movement
- Premotor + association require STRONG AND MORE PROLONGED STIMULATION
- PM and ass areas produce more complex movement, can involve 1+ joints and be bilateral
How is the premotor area functionally subdivided>
Premotor cortex
Supplementary or secondary motor cortex
How does damage to the primary motor area present differently to the premotor area?
Primary motor AREA destruction –>
- Initially electrical stimulation of either premotor areas has no effect
- So their principal actions are mediated through the primary motor CORTEX
Premotor area damage –> APRAXIA
Cannot perform tasks that involve a complex sequence of movements (like brushing one’s hair or drawing)
One role of the supplementary motor area has deen demonstrated by measuring cerebral blood flow (fMRI)
Rehearsal of complex movements stimulates the…
Supplementary cortex
During simple finger flexion: primary motor area and somatic sensory cortex stimulated
In mental rehearsal of sequence of finger movements: Supplementary motor area stimulations
During moving sequence of fingers: Supplementary motor area, primary motor area and somatic sensory cortex ALL stimulated
The premotor cortex:
Projects fibers to?
Function?
It projects fibers to:
- Brainstem motor nuclei
- Spinal circuits controlling proximal and axial muscles (mainly postural)
Function: (not confirmed)
- Involved in plasticity of complex sequences of movements based on prior experience. Influenced by memory and limbic system
- Own neurones begins to fire in primates prior to movment, but only when a stimulus requiring movement is detected. Shows an intent to move.
- Orienting the body in preparation for a voluntary movement
Name the 5 cortical areas involved in movement?
- Parietal cortex: Receives information from the somatosensory cortex and visual cortex
- Somatosensory cortex
- Premotor area: Complex movement
- Supplementary motor area: Mental rehearsal of an action
- Primary motor area
The parietal cortex and the prefrontal cortex both send projections to the pre-motor area, and the supplementary motor cortex
Function of the parietal cortex?
Spatial processing of visual stimuli. This info is integrating in the planning of a sequence of motor outputs
Which modalities provide information to be integrated by the motor system?
- somatic sensory area (about limb position)
- vestibular system (about head position)
- premotor areas (about motor plans)
- visual system
- limbic cortex (about motivational state)
What is the digital code used by UMN to control LMN to determine force and direction?
FORCE: During a movement the impulse FREQUENCY OF NEURONS in the primary motor cortex is directly related to the force required.
On their own they DO NOT code for the distance of the movement .
DIRECTION: is coded for by different populations of neurons
However individual neurons can be active for a range of angles of movement
-system works by accurate coordination of the activation of motor neurons
What are the inputs to the cortex that determine the output for fine tune movement via the corticospinal tract?
- Sensory receptors
Either direct to primary motor area or indirectly via the somatic sensory area, the premotor areas or the posterior parietal association cortex
2. From the cerebellum Both planning (feed forward) movement and corrective feedback from proprioception etc.
- From the basal ganglia
Both initiating complex movement and correction from the reward system. Overlay of emotional component from limbic circuits.
**
N.B. All movement-related inputs to the cortex coming from other parts of the brain and spinal cord also pass though the Thalamus
How are the VOLUNTARY movement CORTICAL TRACTS organised?
Contents?
Decussation?
The primary spinal motor control routes are the Corticospinal tracts.
CONTENTS: These contain about 1,000,000 nerve fibres, about 1/3 of which come from the PRIMARY MOTOR CORTEX. Most of the remainder come from pre-motor areas and association areas
DECUSSATION: Most axons in the corticospinal tract DECUSSATE at the ventral pyramids in the brainstem (lateral corticospinal tract).
10% remain ipsilateral until they finally innervate bilaterally at the level of their ventral root (anterior/medial corticospinal)
The axons in the lateral tract are excitatory and control distal muscle (for example digits), but have collateral branches which feed onto the upper motor nuclei in the brainstem (predictive postural set)
Lesions of corticospinal neurons: Why common? Type of neurone? Most common cause? Other causes?
Relatively common as these axons are very long (therefore vulnerable)
These neurons are Upper motor neurons
Most commonly caused by infarcts related to cerebrovascular incidents. The middle cerebral artery (MCA, supplying the lateral surface of the hemispheres and the internal capsule) is particularly vulnerable
Middle cerebral artery
Other causes: Trauma, tumors and demyelinating diseases can also damage the corticospinal system
Lesions of corticospinal neurons:
Presentation?
Positive signs:
Appearance of abnormal response. Extensor Planter reflex (Babinski sign) is an example of a positive sign seen following corticospinal lesions i.e. Fanning of toes
Negative signs:
Negative is a loss of function, (weakness or paralysis) Note: the site of weakness or paralysis provides a good diagnostic tool for locating the possible site of lesion
Difference in presentation of UpperMN and LowerMN syndrome?
UMN
- Pyramidal muscle weakness (Caveman escape)
- No muscle atrophy
- No fasciculation
- Increased muscle tone (spasticity). Due to enhanced gamma activity when muscle moving.
- Increased stretch reflexes. Due to increased tension in muscle spindles
- Abnormal reflexes (Babinski)
LMN
- Muscle weakness (lesion dependant)
- Muscle atrophy. LMN provide continuous tension, syndrome leads to no tension –> atrophy.
- Fasciculation
- Reduced muscle tone
- Reduced stretch reflexes
What is ALS stand for?
Amyotrophic lateral sclerosis
What is ALS?
Progression?
What is spared?
Affects both upper and lower MN.
Lower MNs affected are in the anterior
horn and the brainstem nuclei. Upper MNs affected are in the cerebral cortex
PROGRESSION:
With ALS, neurons die at the periphery first and move inwards.
As lower MNs die, muscles atrophy (amyotrophic).
As the axons in descending tracts die, the resultant hard scar tissue forms the sclerosis.
Spared control: Ocular motility Sacral parasympathetic neurons (bladder amongst others) Motor coordination systems Intellectual capabilities
