Motor Cortex, Basal Ganglia and Cerebellum Flashcards

1
Q

What has traditionally been thought as being represented in the motor cortical regions?

A

The motor cortex is involved in the initiation of voluntary movement, skilled-dextrous movements and the integration of movement with postural stability goals and the physical environment.

The primary motor cortex is located in the pre-central sulcus. Secondary/supplementary motor areas are located rostrally to this and assist in coordinating complex movements

Most of the knowledge we have of the motor cortex is from 1) electrode stimulation of conscious patients or 2) lesion studies.

Traditionally, the organisation of the motor cortex was thought to be a homunculus representing the topographical arrangement of the motor (and sensory) cortex to their functional and contralateral anatomy in the body. This principle is, however, over simplified and over sold

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2
Q

Recent research has lead to the belief that the organisation of the motor cortex differs from the homunculus theory. Discuss this.

A

It is now thought that there is a clustering of functionally relevant movements within the motor cortex as opposed to different body parts.

For example: neurons relevant to the motor function of reaching are clustered together; likewise defensive behaviour, hand to mouth movements and climbing.

Stimulation via an electrode to a specific area of the motor cortex in monkeys produced the same functional end point of the hand - no matter where the hand position started. i.e hand to face despite hand starting at feet, knee, back etc.

Suggest less of an anatomical homunulus (different muscle groups used from different starting positions) and more of a functionally clustered arrangement.

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3
Q

Discuss the concept of mirror motor neurons

A

**Mirror motor neurons **are visually guided motor neurons located in the lateral pre-motor cortex.

They are responsible for the recognition of how to do a particular task and also the execution of that task.

The sensation that “you think you can do something” when you see it is afforded by these mirror motor neurons.

These same mirror neurons are then capable of modelling the task and executing it themselves.

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4
Q

What information flows to the motor cortex?

A

Inputs to the primary motor cortex

  • Posterior parietal areas
    • provide input of egocentric spatial sensory information to provide a model of the environment that a person must navigate.

Inputs to premotor areas

  • Prefrontal cortex areas
    • provide input of planning, rules of movement etc. to best coordinate movement with desired goals
  • Primary visual cortex
    • _​_input of visual environment with seperate tracts to different pre-frontal areas containing information regarding where an object is (e.g info to determine motor movements to interact with object / hand reaching) or what an object is (e.g determine how to interact with the object itself / hand grasping-grip)
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5
Q

What do basal ganglia contribute to motor control?

A

Functions of the basal ganglia:

  1. Allow the selection of complex patterns of voluntary movements
  2. **Evaluating the success of actions in achieving goals **
    * implicit learning of successful actions
  3. Initiating movements

**Note: **there are many connections between the basal ganglia and cortical areas; not all of these are motor. It is involved in many other domains of CNS function.

The basal ganglia is collection of nuclei including:

  • caudate nucleus, putamen, globus pallidus (external and internal segment), subthalamic nucleus and substantia nigra

It is thought that there are two pathways through the basal ganglia-thalamus to the supplimentary motor cortex:

  • Direct pathway: facilitates the initiation and execution of voluntary movement
  • Indirect pathway: inhibits the initiation and execution of voluntary pathways.

In Parkinson’s disease, the substantia nigra is degenerated due to the loss of dopaminergic neurons. These neurons also produce melanin that give them black pigmentation

In Huntington’s disease, neuronal death of the caudate results in the expansion of the adjacent ventricles - as well as degeneration in other areas.

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6
Q

What does the cerebellum contribute to motor control?

A

The cerebellum (‘little brain’) is made up of as many neurons as the rest of the body combined (small neurons).

The functions of the cerebellum include:

  1. coordinating the timing and sequence of muscle actions and movements
    * optimises movements by coordinating fluidity and efficiency of movement
  2. Maintainance of muscle tone
  3. Motor learning
    * not as much as the basal ganglia
  4. Planning sequences of muscle activation in complex movements

The cerebellum provides contralateral supply to the motor cortex and the motor cortex provides contralateral supply to muscles. Thus there is a double cross of cerebellar output to muscles.

Cerebellar lesions are ipsilateral to deficits

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7
Q

Discuss the anatomy of the cerebellum

A

The cerebellum is tightly folded, with three main lobes. Cerebellar peduncles attach the cerebellum to the brainstem and house fibres to the medulla, pons and midbrain - but most are to the pons.

The three lobes are the anterior, posterior and flocculonodular lobes

Histologically, has a cortex (white matter) with sub-cortical white matter and grey matter (most deep).

Functionally, topographical organisation exists. Medial neurons control medial muscles and lateral neurons control lateral muscles.

Deficits include the inability to appropriately integrate/coordinate movement = ataxia

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8
Q

Describe the deficits characterised by pathology to specific lobes of the cerebellum

A

Anterior Lobe Syndrome

  • ataxic gait (drunken sailor)
  • loss of inter-limb coordination (fails slide heel down opposite shin neurological exam)
  • with chronic alcohol toxicity, anterior purkinje fibres are preferentially lost (lower limb) -> progression posteriorly involves upper limb and facial muscles (ataxic speech)

Posterior Lobe Syndrome

  • Dysmetria (overshoot in precision reach)
  • Dysdiadochokinesia (inability to rapidly alternate movements)
  • Speech abnormality (loss of natural rhythm, slurring, compensatory explosive speech)

Flocculonodular Lobe Syndrome

  • Truncal ataxia (difficulty walking or even standing/sitting (in severe)
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9
Q

Briefly list the different structures of the CNS that contribute to motor function control

A
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10
Q
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