L40. Modulation of Movement Flashcards
What is the role of the motor cortex?
- Initiation of voluntary movement
- Skilled, dextrous movements
- Integration of movement
- Maintenance of postural stability goals
- Response to the physical environment
How is the topography of the periphery generally mapped in the motor cortex?
The motor homunculus
- Lower limbs towards the midline of the motor cortex
- The Upper limbs more superiolaterally
- The face is lateral
Describe what arteries supply the primary motor cortical regions?
The anterior cerebral artery supplies the medial side of the motor cortex (lower limbs) and the rest of the cortex is supplied by the middle cerebral artery
Is there only the one clear set motor homunculus map in the primary motor gyrus?
No.
The motor homunculus is not a clearly distinguishable topography with definite set areas.
There is also probably more than one motor homunculus-like mapping organisation especially within the association areas of the cortex.
The motor homunculus map isn’t the best representation of the primary motor cortex because neurons devoted to one particular part of the body cannot be mapped directly to one specific area of the cortex alone.
Describe a topographic map of movement in the primary cortex that is more appropriate
There are territories devoted to different regions of the body and a distinciton between areas that…
- project directly in the corticospinal tract
- respresent complex patterns in the corticobulbar tract
If a person is asked to perform a simple movement involving a small subset of related muscles… what areas of the brain do you think would be primarily involved?
- Areas of the primary motor cortex involved in innervating that group of muscles
- The somatosensory cortex region that is involved with sensation of that area as a direct consquence of that muscle movement
If a person is then asked to perform a complex movement with a temporal pattern involved what areas of the brain do you think would be primarily involved?
- Areas of the primary motor cortex involved in innervating that group of muscles
- The somatosensory cortex region that is involved with sensation of that area as a direct consquence of that muscle movement
- Recruitment of the motor association area of the anterior cortex due to an ongoing temporal change in movement
If a person is then asked only to think about performing the complex movement with a temporal pattern involved what areas of the brain do you think would be primarily involved?
- Only the motor association would be involved as where the intetntions are put together before signalling the motor cortex to act
Experiment: Continual stimulation of a single area of the monkey’s brain. It lead to the monkey bringing the hand to the face from no matter where the hand was before stimulation.
What inferences can be made about representation in the motor cortex?
It suggests that there is a GOAL DIRECTED organisation of the cortex and motor neurons don’t represent muscles or movement but the end point of a sequence of a movements
Because despite stimulating the same area of the brain:
- There is innervation of different muscles each time
- There is different directions and forces each time
What is another source of evidence to show that the neurons of the motor cortex encode end points and not muscle groups
Measuring the firing rate of a neuron when a person is performing a precision group vs. power grip
- The precision grip - the neuron was highly active in the movement
- The power grip - The same neuron was not involved at all (despite using the exact same muscle groups only adding power)
What is meant by mirror motor neuron activity?
- Performing a task activates a specific set of motor neurons in the cortex
- Simply watching the same task being performed by another person elicis the same motor neurons to be activated.
Are these visually guided motor neurons related to hand actions or the goal?
They are related to the specific hand movements (watching a similar movement but with a different end point elicited the same response). It only fires to the sequence of muscle activity
What about if you are blind folded and produce that same movement, do the visual/mirror motor neurons firs?
Yes they do
The motor units respond by the movement
What is the main significance of these visual/mirror motor neurons
It is believed that this is a major mechanism by which we learn skilled tasks and movements.
We learn a lot by watching other people and copying them.
The brain is recognised to “know” what is happening in other peoples brains when they are performing tasks
What information does the motor cortex require in order to make movements?
- Requires input from sensory systems
- Requires pre-frontal and higher order systems to make a meaningful and decision oriented movements
What makes inputs to the primary motor cortex?
- Motor association areas for planning and sequencing movements
- Somatosensory areas to give sense of space and feedback of movement
- Parietal areas which have an abstract representation of space
What makes inputs to the premotor areas?
From higher order areas like
- the egocentric area to inform place in space
- decision making regions and the purpose of movement
- analysis of the consequences of movement
There are two transcortical pathways that travel to the motor association area of the cortex. What are these pathways and what do they generally encode?
- There is a ventral pathway that encodes where we seeing something. It is very important in influencing our reaching movements (where the limb has to go in order to get to it)
- There is also a dorsal pathway that encodes what our eyes are seeing (primary visual cortex). It impacts on our grasping and informs what shape our hands need to be and with what force our movements should be to get it.
How are the basal ganglia related to motor control?
The basal ganglia is intimately related to the motor cortex with all areas of the motor cortex having some interconnection with the basal ganglia
The basal ganglia helps movement be “better”
What are the functions of the basal ganglia in movement? [3]
- Allow the selection of complex patterns of voluntary movements
- Evaluates the success of each complex pattern of vountary movement
- Initating movement
How is the basal ganglia involved in learning movements?
When a person is first learning a movement pattern, it is wholly cerebral and requires their full attention.
Eventually, the movement becomes automatic and the basal ganglia becomes more in control of selecting, intiating and evalutating the movement by its connections to the motor cortex
What are the [5] structures that make up the basal ganglia? At what general level of the brain are they located?
Sub-cortex:
- Caudate Nucleus
- Putamen
- Globus pallidus
Diencephalon:
- Subthalamic nucleus
Midbrain (Brainstem):
- Substantia Nigra
Is the basal ganglia solely involved in motor control?
No
What the basal ganglia does for motor control, it also does for other areas including the limbic centre (emotion), the frontal eye fields (movement of the eyes) and for the pre-frontal circuit (planning and salience)
It is a major modifier of the action of the cerebral cortex
What is seen in terms of the basal ganglia in parkinson’s disease?
A substantial loss in the neurons of the substantia nigra
What is seen in terms of the basal ganglia in Huntington’s disease?
Atrophy of the caudate nucleus, globus pallidus and the putamen
What are the two pathways in which the basal ganglia influences the cortical circuits?
There is one direct pathway that tells the motor neurons to elicit a specific action and there is also an indirect pathway that selectively inhibits other movements from occuring
Describe in more detail the concept of contrast enhancement activity of the basal ganglia
The basal ganglia dampens down movements that it doesn’t want and makes a more pronounced set of single movements it does want.
Centre-surround functional organisation
What specific regions of the basal ganglia are involved in each direct/indirect pathway?
- The direct pathway acts via the caudate and the putamen
- The indirect pathway works via the nucleus
They both converge onto the globus pallidus and thalamus
How does the cerebellum impact movement?
It optimizes movement. It smoothens and prevents the clumsiness of movement
Describe the function of the cerebellum [4]
- Coordination of the timing and sequence of muscle actions and movements
- Maintenance of muscle tone
- Motor learning: onset/offset optimization (but on a shorter time scale) – getting better at more complicated things rather than simple actions
- Planning sequences of muscle activation for complex movements
Describe the gross structure of the cerebellum
- Tightly folded
- Three main divisions (lobes)
- Attached to the pons by the cerebellar peduncles which carry fibrous connections with the brainstem
Describe the histology of the cerebellum
Has an outer cortex and a sub-cortical white matter
Subcortical grey matter (deep nuclei)
Is there functional organisation of the cerebellum?
Yes
Approximately medial to lateral in a fragmented somatotopy
Medial control axial muscles, Lateral controls the limbs and the most lateral are motor planning and motor cortex connections
What would a deficit in the cerebellum lead to?
Inability to appropriately integrate and coordinate movement (ataxia). Unilateral lesions are generally ipsilateral
Describe a specific cerebellar anterior lobe syndrome
- Ataxic gait “drunken sailor”
- Loss of interlimb coordination
Describe a specific cerebellar posterior lobe syndrome
- Dysmetria (over shoot precision reach)
- Dysdiadochokinesia (can’t perform rapidly alternate movements)
- Speech abnormalities (los of rhythm)
What is flocculonodular lobe syndrome of the cerebellum?
Truncal ataxia
How does the cerebellum communicate with the cerebral cortex
The cerebellum controls the contralateral cerebral hemisphere. It does not have an direct projections down to the spinal cord neurons.
Lateral parts control the motor cortex
What is meant by the double crossing of the motor fibres of cerebellar control?
The cerebellum sends up contralateral fibres to the cortex. The cortex also sends contralateral fibres to the motor neurons. Hence this double crossing means that the cerebellum has an ipsilateral influence on the muscles.
(thus lesions present ipsilaterally)
