What do the parts of the motor cortex do?
(Essay plan notes)
Sanjay Manohar (Oxford 2015)
= 10^6 axons. 30% from M1. 30% BA6. 40% parietal. medium size 0.1mm
pyramids 80% decussation in man, 100% in dog.
lesion - slow inaccurate hand movt, but ok posture.
improvement, but flexor weakness and no independent finger movt.
1870 Fritsch and Hitzig: anaesthatised dog, stimulation -> contralateral muscle movt
AW Campbell - amputees had focal atrophy in parts of motor cx
1874 Betz - cells exclusive to M1 up to 0.1mm
1870s Hughlings Jackson: seizures march; focal strokes
1952 Penfield and Rasmussen: stimulation in epileptic patients. Somatotopy.
Fetz and Cheney: reliable EMG when stimulating single betz cell
Hypothesis: one cell for one motor unit?
2007 Graziano and Aflalo: longer stimulation of macacque M1 neurones elicit complex movements
1991 Humphrey and Tanji: Many different neurones cause same muscle contraction of deltoid
Homunculus. Plasticity. Interindividual differences
Georgopoulos 1980s: population vector. Explains homunculus?
Donoghue & Sanes: M1 microstimulation whiker area,
- cut motor nerve to snout -> elicits forelimb movt
- Plasticity = expansion of other regions
stroke: decoding and robotic control
input from VLc/cerebellum.
1960 Evarts: single cell recordings from SMA: active 1s before voluntary movt
happens for both sides - corpus callosum
lesion: Loss of bimanual coordination, instead mirror movements
1995 Chen: monkeys learned to perform sequence. Lesion abolishes performance
1998 Shima and Tanji: inactivation SMA with muscimol - sequential but not single movts impaired.
2000 Shima and Tanji: SMA Recording = different neurones active in different stages of sequence
1997 Gerloff: Human TMS during piano sequences:
delay in ability to carry out sequence => planning not execution
i: CBM, parietal. o: medial tracts eg reticulospinal, M1
1991 Mushiake: greater PMC activation before visually guided, SMA before internally guided movt
1982 Weinrich and Wise: monkey direction cued movements.
signle cell recording in cue-movement interval: direction selectivity
Kakei + Strick: postures not directions
Rizzolatti & Gallese: ventral PMC also responds to observation
"mirror neurons", useful in mimickry, learning, higher cognition
Kohler 2002 - responds to recognisable sounds too
2005 Iacobani et al: video of arm movement, isolated or with intention-context
mirror neuron encodes intention?
Per Roland: PET, finger movt sequence, performance=4+6; rehearsal=6 only
Roth 1996: similar fMRI
Anatomy: PMA input from cerebellum, output to M1, spinal cord
1994 Kurata & Hoffman: muscimol inactivation
abolishes endogenously cued movements but not exogenous ones
pre-SMA: (anterior SMA) volition; Libet task
CMZ; midcingulate cortex
computing muscle synergies
sequencing steps (SMA and PMC)
bimanual coordination (SMA bilateral receptive fields)
endogenous volition (SMA bereitschaftspotential)
integration of sensory information (sensory RFs in M1)
transcortical reflexes and force-setting
decision making (competition in M1, conflict in SMA)
planning; working memory; task set
imagination, simulation (imagery tasks)
abstract action/verb representation (mirror neurones)
Is plasticity due to unmasking or new synapses?
Corticospinal projection from parietal cortex - is it also motor?
Is the whole brain motor? Where is the distinction?
Is motor cortex actually quite sensory?
1972 Asanuma and Rosen: precise wiring from thumb extensor M1 to tactile thumb tip S1
"closed loop circuits"? damaging dorsal column and S1 causes paralysis
Alternatively, does motor cortex do other cognitive functions?
Working memory? Task set (Evarts)?
Decision-making (multiple motor plans competeing)?
Imagination (motor imagery)?
Motor systems hard to study - stimulation unnatural; sensory fb.