Thursday, 27 October 2011

Week 5 - Movement Disorders

Among many neurological deficits, movement disorders seriously inhibit a person’s day to day life. Movement disorders are caused by damage to the central nervous system (CNS). This includes damage to neurons in subcortical structures or lesions to the cortex. Damage to the two subcortical structures causes the most harm to motor dysfunction. These are; the cerebellar loop, which controls the timings and trajectory of movement using sensory and motor information and; the basal ganglia loop, which controls the excitability of the frontal motor structures and will decide the likelihood of movement and the strength of the movement. The basal ganglia is a collection of nuclei which includes; the caudate, putamen and the globus pallidus. These nuclei are often involved in severe movement disorders such as Huntington’s disease and tourette syndrome, which is caused by excessive motor activity, and Parkinson’s disease which is caused by restricted movement.
This shows that the subcortical structures are extremely important for motor control because any damage or lesion affecting this area leads to some serious inhibitory movement problems.
Below I will discuss movement disorders (including Parkinson’s disease, Huntington disease and tourettes syndrome) and the methods used to treat these disorders.

Parkinson’s disease

The characteristics of Parkinson’s disease include akinesia (general loss of movement) rigidity and tremors when resting; Akinesia involves bradykinesia (a slowness of movement) and hypokinesia, which is a reduction in movement.
Age of onset - Is between 40 and 70 years of age.
Symptoms - Stooped posture, difficulty with speech and trouble turning their bodies, and also trouble with writing.
Treatment - L-dopa (penetrates the blood/brain barrier) Dopamine receptor agonists and MAO B inhibitors.

Huntington’s disease

This is an inherited disease and causes a person to perform involuntary (dance-like) movements and causes dementia.
Age of onset - Is between 40 and 50 years of age.
Symptoms - Difficulty with speech and standing, changes to personality, clumsiness.
Treatment - No long term treatments. Dopamine antagonists relieve a small amount of symptoms for a temporary amount of time. Treatments aim is to restore balance to neurotransmitters in the basal ganglia but in the long run this is not enough to prevent the disease from progressing.

Tourettes syndrome

Has been proposed as the result of dysfunction between the basal ganglia and its connections to the orbito-frontal cortex.
Age of onset - Begins in childhood (typically before the age of 15)
Symptoms - Head jerking, shrugging of the shoulders and the most obvious is the repetition of words and swearing.
Treatment - A drug called haloperidal is administered to patients with tourettes syndrome.

Important revision notes from the article (week 5) - 'Abnormalities in the awareness of action'

- Researchers have proposed that the central nervous system (CNS) holds internal models that interact our own bodies and the external world in order to optimize motor control. The two internal models proposed are; 'The forward model' this predicts the sensory consequences of motor commands. The second model proposed is the 'inverse model' this provides the motor commands to achieve the desired outcome.

- Although we are aware of the goals underlying most of our movement it has been suggested that we are unlikely to have conscious access to all of our motor commands and adjustments that are made in order to carry out a command. This leads to the speculation that we do not have conscious access to the inverse model/motor commands. However this field of research is not conclusive.

- On the other hand the 'forward model' has been proposed as being available to awareness.
-The 'forward model' compares the actual outcome of motor commands to the desired outcome, all of which is made before a movement is made this allows for adjustments to be made before the action is carried out.

- The second job of the 'forward model' is to compare the prediction of the sensory consequences of movement to the actual feedback - this comparison is made after the movement is made. This allows for the body to compensate for the sensory effects of the movements being made.

  • An experiment took place to demonstrate that the motor system can function in the absence of awareness.
  • Goodale etal ran an experiment where participants were asked to point at a visual target. It was reported that during a saccade the target was displaced, this went unnoticed by the participants, however their arm moved in the correct direction.
  • Castiello etal found that participants were only aware of a target jump 200ms after the motor system had made appropriate adjustments.
  • An interesting study was ran by knoblich & Kircher. Participants were instructed to draw circles which they then saw reproduced by dots. The dots were slowly increased therefore making the circle bigger, participants were asked to lift their pen when they saw a change. Results found that participants compensated for the changes well before they were actually aware of the change. This shows that we are not conscious of all the fine adjustments made by the inverse model, and it's only when a change becomes so big that we notice any change. Overall so long as our desired intentions have been met, we are unaware of sensory feedback about the state of our motor systems.
Neurological abnormalities

Optic ataxia

- This is where patients have trouble with grasping objects that they can see before them. Although their actions are carried out in a clumsy manner the movement the patient is making matches their intention. The patient is aware of their problem in reaching and grasping. This condition is caused because the 'inverse model' is not properly tuned.

Anarchic hand sign

- This is where patient’s hands move of their own accord without the will of the patient. It seems that simply seeing an object is enough to trigger a movement. The patient clearly recognises a problem between desired actions and actions that do take place.

Utilization behaviour

- This is where a patient utilizes an object inappropriately, however in contrast to the 'anarchic hand sign' the patient does not see the discrepancy, this is because the patient will rationalize their movement by saying they thought someone wanted them to do it, their actions are involuntary however they feel like the movement was intended.

Phantom limbs

- This occurs after a patient has had a limb amputated, but still feels like they have still got the limb, and report feeling like they can move it or that it feels paralysed. An explanation put forward is that the estimated position of a limb is not entirely based on sensory information but also the stream of motor commands to the limb. The 'forward' model estimates the position of the limb before sensory information has been received. Therefore motor commands may be issued to the limb therefore making the phantom limb feel like its moving. In time the motor control system will adapt to changes therefore many patients report losing the ability to move their phantom limbs.

No comments:

Post a Comment