Neurophysiology of systems

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Aron Wilcox
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1 Neurophysiology of systems Motor cortex (voluntary movements) Dana Cohen, Room 410, tel: 7138

2 Voluntary movements vs. reflexes Same stimulus yields a different movement depending on context Generated internally Determine whether or not to respond to a given stimulus Effectiveness of voluntary movements improves with experience and learning

3 Electrical stimulations 1870 electrical stimulation of the frontal lobe produces movements of muscles on the contralateral side of the body Generation of motor maps! Primary motor cortex (Brodmann s area 4) is defined as the area with the lowest intensity stimulationelicited movements Betz cells - layer 5 pyramidal neurons

4 Magnetic stimulation of motor cortex

5 The motor Homunculus

6 Microstimulation maps Distal in core, proximal around it Shoulder Elbow Wrist Finger Kwan et al 1978 (taken from chapter by Dum and Strick 2005)

7 Microstimulation Eliciting complex purposeful responses* Redundancy in muscle representation allows combining proximal and distal muscles in different tasks 1.Hand+arm: reach & grasp 2.Defensive Bimodal response to Visual stimulus +stimulation * Stimuli are suprathreshold and long lasting arm defensive Hand+arm Hand to mouth

8 Premotor areas Medial wall 1. SMA - Supplementary motor area (gives rise to bilateral movements) 2. CMA - Cingulate motor area d 3. CMA - Cingulate motor area v Lateral surface: 5. PMd dorsal premotor area (multiple joint movements) 6. PMv ventral premotor area These areas are identified by: Smaller pyramidal neurons in layer 5 Project to M1 and to the spinal cord Motor maps exist in all areas Evoke complex movements Dum and Strick 2005

9 Main cortical areas participating in the planning and execution of movements 1.Posterior Parietal areas at least 2 2.Primary motor cortex 3.Premotor cortex PMv and PMd 4.Supplementary motor area (SMA) SMA-proper Pre-SMA 5. Prefrontal FEF and more.. 6.Cingulate cortical fields at least 2-3

10 Organization of the motor system: Anatomy of Upper levels loops

11 Primary motor cortex Inputs Somatosensory cortex (receptive fields) Posterior parietal cortex (integration of multiple sensory modalities) Premotor areas Thalamus (basal ganglia and cerebellum) Outputs Spinal cord (α motor neurons, interneurons) Cortico-cortical projections

12 Premotor areas Inputs Posterior parietal cortex Area 46 (working memory) Dense premotor areas connections Thalamus (basal ganglia and cerebellum) Outputs Primary motor cortex Spinal cord (α motor neurons, interneurons) Cortico-cortical projections Highly similar and differ in the number of projections to each area!

13 Organization of the motor system: Gross Anatomy Upper motorneuron Basal Ganglia Brain stem PT neurons Lower motorneuron Inter Neurons Motor Neurons muscles

14 Descending Pathways (brief) 1. Cortico-spinal Descending Pathways Lateral Cortico-spinal tract: Splits to two at medulla - 1. lateral (90%, contralateral) and medial (10%, ipsilaterl). Mainly voluntary movements (more distal like fingers) Ventral cortico-spinal tract Mainly axial, neck 2. Cortico-bulbar tract: Mainly cranial nuclei controlling facial expressions, speech, eyes 3. Brain-stem Descending Pathways Tecto-Spinal Tract: Mainly proximal and axial Vestibulo-spinal tract: Mainly balance and Head movements Reticulo-Spinal Tract: Mainly Proximal; Automatic, posture, Various respiratory functions Rubro-Spinal Tract: Mainly lateral - distal- voluntary limb movements

15 Lateral and Ventral Corticospinal Tracts Lateral corticospinal Ventral corticospinal Medial brain stem pathways Ventral corticospinal Red nucleus Pyramidal decussation Lateral corticospinal

16 Medial and lateral Brain-stem descending pathways Medial Lateral Tecto-spinal Vestibular Nuclei Vestibular tract Tectum Medial reticular formation Reticulo-spinal tract Rubro-spinal tract Red nucleus

17 Anatomical support: Divergent connectivity of PT neurons Shinoda et al 1980

18 Segregation of Distal and Proximal Projections

19 Three Principles describe the organization of the motor system: 1. Three levels: frontal cortex, brain stem and spinal cord 2. Hierarchical organization with parallel pathways 3. Feedback loops at all levels

20 Organization of the motor system: block Diagram Cerebral cortex motor areas Level 3 Basal Ganglia Thalamus Brain Stem Level 2 Cerebellu m Spinal Cord Sensory receptors Muscles Contraction Level 1

21 Investigating cortical functions by recording neuronal activity

22 A.B. Schwartz, Pittsburg, USA

23 Evarts Experiment: (1968) Conclusion: The activity is related to force - not to displacement

24 Set-related activity during preparation of movement

25 Preparation of movement set-related activity in premotor cortex Instruction Go! Instruction Go!

26 Cell activity in motor cortex depends on whether a sequence of movements is guided by visual cues or prior training Primary motor cortex Premotor cortex Supplementary motor cortex Visual cue Prior training

27 A cell in PMv that is active whether or not the monkey performs the movement Another monkey A human The monkey

28 What do single neurons encode? Muscle activation (upper motor neuron) code for dynamics Movement preparation. What about kinematics? Such as movement direction, velocity, distance!

29 The 8 direction center-out reaching task

30 Single neurons in MI are directionally tuned. P.D. Movement onset (Georgopoulos et al 1982)

31 Neurons in motor cortex are broadly tuned to the direction of arm movements Firing rate (Spikes/second) Direction of Movement Firing rate (M) Cos (θ PD-M ) Then how do we make accurate movements? (Georgopoulos et al 1982)

32 The Population Vector Assumptions: 1. Single units have symmetrical directional tuning curves 2. The preferred directions of different cells in MI are evenly distributed 3. The parameters b, and k that specify the firing rate of the neurons according to the following equation are evenly distributed for all movement directions: d i ( M ) = bi + ki cos( θpdi,m Then: A vector P(M) exists which specifies the movement direction as: P ( M ) = Wi ( M N i = 1 ) ) PDi Where: PD i is the preferred direction of the i-th cell. : Wi is the weighting function. W i d i ( M )

33 The First requirement for existence of PV: Tuning curves Many cells show symmetric tuning (numbers change between 40% to 80%) Firing rate (Spikes/second) Direction of Movement

34 Testing the second requirement for existence of PV 2. Distribution of PDs The preferred direction of different cells in MI are evenly distributed Some scientists disagree!

35 Testing the Third requirement for existence of PV: Distribution of the model parameters The value of the firing rate parameters (K and b) do not depend on the preferred direction ( M) = b + cos ( ) di k i θ i PDi, M

36 Different cells push the arm in different directions P ( M) = Wi ( M) PDi P.D. Where: P(M) : The population vector for movement M Wi : Weight function for neuron i. proportional to the firing rate of neuron i in movement M PDi : The preferred direction of neuron i

37 Population of neurons in MI accurately represent the direction of movements P ( M ) = Wi( M ) PDi

38 Activity of ONE cell during drawing movements: experimenter s View A.B. Schwartz, Pittsburg, USA

39 Time Evolution of population vector Assumption: The preferred direction of a cell does not change in time! di Movement onset The general form for the population vector P ( t ) N = i 1 W ( t ) i PD i

40 Population vector reconstruction of drawing movements

Brain Stem and cortical control of motor function. Dr Z Akbari

Brain Stem and cortical control of motor function Dr Z Akbari Brain stem control of movement BS nuclear groups give rise to descending motor tracts that influence motor neurons and their associated interneurons