Week 8 Flashcards

Question

Secondary Motor Cortex- explaination

Answer 1

• Become active just before initiation of voluntary movement and remain active during movement • Electrical stimulation results in complex movements, typically bilateral • Programming of specific patterns of movement, with input from the dorsolateral prefrontal cortex • PMC/SMA functional distinction – externally or internally guided action • PMC – strong reciprocal connections with posterior parietal cortex – sensory guided actions (catch a ball) • SMA – strong connections with medial frontal cortex – internally guided goals (playing piano)

Answer 2

• Rizzolatti et al. (90’s) studying monkey premotor cortex using single cell recordings • Interested in neurons that respond to complex hand (and mouth) actions – reaching for a toy or reaching for food • Found neurons that fired preferentially when reaching for one type of object but not for different object • Then found that some of these neurons responded identically when reaching for the object and when observing human performing the same action • Mirror Neurons – fire when perform a particular goal directed hand movement or when observing the same

Answer 3

``` • Response to goal directed actions – no response to same action if mimed and no object • Respond to the goal of an action such as the grasping of a piece of food even when this action is performed with different tools (such as normal or reverse pliers) requiring opposite sequences of movements (closing or opening of the fingers) • Transform complex visual input into high level understanding of observed action ```

Answer 4

``` • Don’t need to see the key action if enough clues to create a mental representation • Screen to block and monkey must imagine what is going on • But if first show monkey that no object behind the screen – no response ```

Answer 5

``` • Some mirror neurons in inferior PPC respond to purpose of action rather than the action itself • Fire when food grasped if it was clear it was to be eaten • If repeatedly grasp food to put in bowl – little firing ```

Answer 6

``` • Social cognition: knowledge of perception, ideas, intentions of others • Action understanding: cooperation, teaching/learning • Language • Emotional understanding - empathy ```

Answer 7

``` • Confirmation in humans not as strong (single cell recording) but similar mirror networks (large scale) suggested by fMRI and EEG • Motor imagery – imagine doing an action – PMC, PPC, M1 all become active (imagine observing – weak motor activation – mainly visual) ```

Answer 8

``` • Viewing ballet steps recruits premotor and parietal mirror areas more strongly in expert ballet dancers than in nondancers or in martial-art teachers • Recruitment of motor areas with mirror properties strongly correlates with motor rather than visual expertise • People can improve their ability to judge the goal of an unusual action simply by practising that action themselves; this improvement occurs even when they practise while blindfolded ```

Answer 9

To move … • Need to know where things are – objects in the environment and parts of the body • Need to make a decision to initiate voluntary movement • Posterior parietal cortex (PPC) - spatial information • Dorsolateral prefrontal cortex (dlPC) - decide and initiate

Answer 10

``` • Input from multiple sensory systems (visual, auditory, somatosensory) • Localisation of the body and external objects in space - integrates • Recall PPC in MSI and dorsal pathways • Directs attention • Outputs to secondary motor areas, FEF, and dlPFC • Subregions associated with eye, hand, or arm movements • Damage – apraxia and contralateral neglect ```

Answer 11

• Disorder of voluntary movement but not a simple motor deficit • Difficulty making movements on request, but can make the same movement under natural conditions when not thinking about it • Eg – can hammer a nail but cant demonstrate hammering when asked • Bilateral symptoms but damage usually unilateral – left PPC • Conscious planning of complex coordinated action

Answer 12

• Disturbance of ability to respond to stimuli on side opposite lesion • Usually right lesions and neglect left space • Patients fail to appreciate that they have a problem • Egocentric left – left of body – head tilt doesn’t change (tilt) field of neglect • Also neglect left side of body

Answer 13

• Intrinsic spatial coding: knowing what our own body parts are doing • Intrinsic coding is essential when a body part is obscured from vision at some stage in the movement planning and execution • PJ: 50yo F; head injury when 43yo, 30min loss of consciousness • Jerking of the right arm at 48years, focal seizures, presented for assessment • No visual neglect or extinction, no other visual deficits • Perceives her right arm and leg to drift and fade unless she is able to see them • In bed, loss of knowledge in limb position • In public transport, other passengers tripping over her leg, which had drifted into the aisle • MR: cyst encroaching on the cortex and subcortical white matter of left superior parietal lobe

Answer 14

``` • Input from PPC • Outputs to M1, secondary motor, FEF • Evaluate external stimuli and decide to act – goals based • Decisions to initiate voluntary movement ```

Answer 15

• Complex heterogenous collection of interconnected nuclei • Modulatory function – loops receiving cortical input then back to cortex via thalamus (other nonmotor functions) • Caudate and putamen (striatum), globus pallidus, subthalamic nucleus, substantia nigra • Inputs from all over cortex (sensory, motor, association) to striatum; output from GP and SN to motor and frontal cortex via thalamus • Critical role in selection and initiation of action • Complex, heterogenous interconnected nuclei • Connections excitatory or inhibitory • Initiate selected movement

Answer 16

``` Direct – enhances thalamic output Indirect – inhibits thalamic output But – indirect slightly delayed so brief enhancement then balance SN input – turns up DIRECT and turns down INDIRECT To Enhances overall process ```

Answer 17

• This circuit model is speculative – details are way more complicated • Other inputs, outputs, connections, and complexities not mentioned here • Output (discharge rates) likely not the only important factor • Neuronal firing patterns likely important • Synchrony of higher level activity likely important • Complex coordinated activity of multiple interconnected nuclei – fine balance • Basics of function from disorders

Answer 18

Disorders of motor control: • Parkinson’s Disease – loss of modulatory dopaminergic neurons in the substantia nigra • Huntington’s Disease – loss of inhibitory GABAergic neurons in the striatum Note – these disorders have broader effects than just motor and involve neurodegeneration in multiple regions

Answer 19

• Loss of dopaminergic neurons in the substantia nigra • Increase indirect and decrease direct – decrease thalamic output • Latest – changes in neuronal firing patterns and synchrony rather than reduced discharge rate • Primarily affect indirect pathway • Ultimately - decreased cortical activity • Cardinal features – akinesia (slowed initiation), bradykinesia (slowed movement), muscle rigidity, tremor

Answer 20

• Reduction in spontaneous movement (hypokinesia) • Slow initiation of movement (akinesia); • Progressive slowing or freezing during a movement and reduced range and scale of movement • Micrographia • Slow gait, often with freezing and small steps • Poor arm swing • Postural instability = many falls • Dull, weak voice without inflections (hypophonia) and slow speech • Mask-like, unemotional expression

Answer 21

``` • Dopamine agonist: L-Dopa (precursor) • Increase dopamine generally • Efficacy drops with usage • Numerous side effects • Chronic high frequency deep brain stimulation (DBS) • Implanted device to stimulate STN (usually) • High frequency disrupts activity • Not sure how/why it works • Reduce inhibition of thalamus? • Replace irregular BG output to cortex with a regular, better tolerated pattern? • Disrupt abnormal frequencies? ```

Answer 22

• Genetic neurodegenerative disease – manifests in adulthood (35-55); life expectancy 10-25 years post onset • Autosomal dominant with complete lifetime penetrance, chromosome 4 – excessive build up of Huntingtin protein • Destruction of GABAergic neurons in striatum (caudate and putamen) – primarily affecting indirect pathway • Progressive striatal atrophy: medial caudate first (small spiny neurones), then putamen, then tail of caudate • Reduced (indirect) basal ganglia output

Answer 23

``` Direct – enhances thalamic output Indirect – inhibits thalamic output Enhancement of direct versus indirect reduced Decreased thalamic output ```

Answer 24

``` Direct – enhances thalamic output Indirect – inhibits thalamic output Thalamic inhibition reduced – increased thalamic excitation of motor cortex ```

Answer 25

• First signs usually affective: depression, anxiety, irritability, impulsivity, aggression • Followed by: restlessness, clumsiness, poor coordination, forgetfulness and personality changes • Characteristic - athetosis (writhing movement) and chorea (jerky movement) • Poor motor dexterity, unsteadiness, reduced speed • Altered speech and writing, saccadic changes • They involve multiple joints and thus resemble voluntary action • They are briefly suppressible, and decrease during sleep • They increase with stress and with voluntary movements like walking

Answer 26

• Modulatory – fine-tuning and learning (other non-motor functions) • Inputs from cortex (M1 and secondary motor), descending motor signals from brain stem nuclei, somatosensory and vestibular feedback • Compare signals with feedback to correct ongoing movement that deviates from intended • Project to brainstem nuclei and cortex (M1 and secondary) via thalamus • Role in motor learning especially when sequence timing critical • Diffuse damage – lose precise control of direction, force, velocity and amplitude of movement; lose ability to adapt movement to changing conditions, disturbances in balance, gait, speech, eye movement • Cells in cerebellum that project to spinal cord especially sensitive to effects of alcohol – unsteady gait, disturbance of balance

Answer 27

• Loss of sensory co-ordination of distal limbs disrupting fine coordination – finger to nose test • Lack of muscle control or coordination of voluntary movements, such as walking or picking up objects. • Speech impacts • Alcohol abuse, certain medications, stroke, tumour, cerebral palsy, brain degeneration and multiple sclerosis

Answer 28

• Described voluntary behaviour as wilful – intentionally initiated following a decision to act (including rejection of the alternative of doing nothing) • Subjective experience – ownership of action, agency, conscious control – ‘I’ did something on purpose • But ….

Answer 29

• Often act unconsciously or with minimal conscious control • Split brain patients – dissociation of action and awareness • Confabulation – plausible (but incorrect) reasoning about decisions to act with feeling of agency • People are subject to other illusions of ownership • People are subject to other illusions of control

Answer 30

• A bilateral slowly increasing negativity (EEG of SMA) (termed the ‘readiness potential’) was shown to precede voluntary action by Kornhuber and Deeke (1965) • Libet – have subjects perform a simple motor at random act while looking at a fast moving ‘clock’ – note the position of the clock when first aware of the intention to act • Onset of the readiness potential hundreds of ms before awareness of intention to act • “the brain evidently 'decides’ to initiate or, at the least, prepare to initiate the act at a time before there is any reportable subjective awareness that such a decision has taken place”

Answer 31

• Soon et al (2008) – similar approach but using fMRI • Look at a letter stream – freely press one of two buttons – recall letter when motor decision was consciously made • Intentions mostly formed 1000ms before press • 2 brain regions encoded left vs right decision early – frontopolar cortex and parietal • Over 10 seconds before the decision to act (7 seconds on fMRI signal) • Timing of decision predicted in preSMA and SMA • 5 seconds before the decision to act • Dissociation between outcome of motor decision and timing

Answer 32

• Hierarchical, functional segregation, parallel, feedback • Simple spinal circuits control some movement • 4 descending pathways dorsolateral for fine control of limbs, ventromedial for axial muscles/ whole body movement (direct and indirect for each) • MND – neurodegeneration of upper and/or lower motor neurons • Motor cortex – primary (M1) and several secondary • Stimulate M1 – complex unilateral movement – motor homunculus

Answer 33

• Stimulate secondary – complex bilateral movements • Secondary – functional distinction – internally or externally guided action • Mirror neurons – goal directed actions • Association cortex – functional distinction - spatial information and decide/initiate • Basal ganglia – selection and initiation of action – PD and HD • Cerebellum – fine tuning and learning – cerebellar ataxia • ‘Voluntary’ action