What do the parts of the motor cortex do?
(Essay plan notes)Sanjay Manohar (Oxford 2015)
LCST
= 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.M1
1870 Fritsch and Hitzig: anaesthatised dog, stimulation -> contralateral muscle movt AW Campbell - amputees had focal atrophy in parts of motor cx Sherrington 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.SMA
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 executionPMC
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 cortexFunctions
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) posture planning; working memory; task set imagination, simulation (imagery tasks) abstract action/verb representation (mirror neurones)Big picture
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)? Free will Motor systems hard to study - stimulation unnatural; sensory fb.