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Robotic Brain Universe

The motor system

How the brain turns intention into movement, from the redrawn motor homunculus to the laws of reaching and the technology that restores it.

Intermediate · about 11 min · updated 2026-10-02 · awaiting clinical review

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Illustrative simulation excitatory inhibitory

The primary motor, premotor and supplementary motor areas and the corticospinal tract; somatotopy and the newly found somato-cognitive action network; directional tuning, population dynamics and mirror neurons; Fitts's law and minimum-jerk reaching; the readiness potential; stroke, the supplementary motor area syndrome, ALS and trainable recovery; and stimulation mapping, precision fMRI and brain–spine interfaces.

Contents
  1. From a thought to a movement
  2. What the motor system is
  3. Why the motor cortex is organised this way
  4. How a movement is made
  5. When: preparation, then action
  6. When movement fails
  7. The mathematics of movement
  8. Technology: mapping and restoring movement
  9. Milestones
  10. Frontiers: the homunculus redrawn
  11. Check yourself

From a thought to a movement

Reaching for a cup takes a fraction of a second, yet it needs a goal, a plan, a command to dozens of muscles and constant correction. In the 1930s Wilder Penfield stimulated the exposed cortex of patients during surgery and found that the strip in front of the central sulcus, the precentral gyrus, moves the opposite side of the body in an orderly map, legs near the top of the head and face low on the side.[1]

That map, the motor homunculus, has been redrawn. In 2023 precision functional MRI showed that it is interrupted by regions that are not tied to any one body part but join up with networks for action planning, arousal and the body's internal state, a newly named somato-cognitive action network.[2]

Movement starts before we know it. In Benjamin Libet's experiments, electrical activity over the brain began several hundred milliseconds before people reported the conscious wish to move. This reading follows a movement from intention to muscle, the equations that describe it, what happens when the system breaks, and the technology that now reads motor intention directly.[3]

What the motor system is

The primary motor cortex lies in the precentral gyrus (Brodmann area 4). Its hand area usually sits at a knob-shaped bend of the gyrus that can be found on axial MRI. In front of it, Brodmann area 6 contains the premotor cortex and, on the medial surface of the superior frontal gyrus, the supplementary motor area.[1,4,5,6]

The main output is the corticospinal tract. It descends from motor and premotor cortex through the corona radiata, the posterior limb of the internal capsule, the cerebral peduncle and the pons to the medullary pyramids. In primates some of its fibres contact spinal motor neurons directly, the cortico-motoneuronal connections, which may serve special functions such as skilled use of the hands.[7,8,9]

Key numbers

Arm-related motor cortex cells studied in detail that were tuned to movement direction
241 of 323 (74.6%)[10]
Lead of brain activity over the reported wish to move (spontaneous acts)
about 350 ms on average[3]
Hand-function test time after constraint-induced therapy, baseline to 12 months
19.3 s to 9.3 s (52% faster)[11]
Effector-specific zones in the redrawn motor map
3 (foot, hand, mouth)[2]

Why the motor cortex is organised this way

Motor control is not a chain of command from brain to spinal cord. It results from operations across the whole motor network, with several descending pathways working in parallel, each able to play many roles.[9]

Organising the cortex around the body serves fine control, while the newly found inter-effector regions serve integration. Gordon and colleagues describe an 'integrate–isolate' pattern: effector-specific regions for the foot, hand and mouth isolate fine motor control, while the somato-cognitive action network integrates goals, physiology and whole-body movement.[2]

Stimulation suggests the motor cortex also maps actions, not just muscles. Half-second trains of microstimulation in monkeys evoked whole coordinated postures, such as the hand closing into a grip and moving to an opening mouth, and the hand positions evoked from different sites formed a map of the space around the body.[12]

How a movement is made

Direction by population. Single motor cortex neurons are broadly tuned: each fires most for one preferred direction of arm movement. The direction of a movement is predicted by the whole population, by adding each cell's preferred direction weighted by its activity (see Brain–computer interfaces).[10,13]

Dynamics, not just codes. Recording many neurons as monkeys reached, Churchland and colleagues found a brief but strong rotational (oscillatory) pattern in the population, surprising for a movement that is not rhythmic. Its amplitude and phase followed from the activity during preparation, suggesting that preparation sets the initial state of a dynamical system that then generates the movement.[14]

From intention to musclePremotor and supplementarymotor areasgoal and planPrimary motor cortexprecentral gyrus, somatotopicInternal capsuleposterior limbPeduncle, pons, pyramidsthe tract descends through thebrainstemSpinal motor neuronssome contacted directlyMusclesforce and movementSensory feedbackposition and touchcorticospinal tractcorrection
From intention to muscle. The corticospinal route from plan to muscle. Damage anywhere along the tract above the decussation in the lower medulla causes weakness on the opposite side of the body.[6,7,8,9]
Text version of the diagram
  1. Premotor and supplementary motor areas: goal and plan. Leads to Primary motor cortex.
  2. Primary motor cortex: precentral gyrus, somatotopic. Leads to Internal capsule.
  3. Internal capsule: posterior limb. Leads to Peduncle, pons, pyramids.
  4. Peduncle, pons, pyramids: the tract descends through the brainstem. Leads to Spinal motor neurons (corticospinal tract).
  5. Spinal motor neurons: some contacted directly. Leads to Muscles.
  6. Muscles: force and movement. Leads to Sensory feedback.
  7. Sensory feedback: position and touch. Leads to Primary motor cortex (correction).

Smoothness. Unconstrained reaching movements are roughly straight with a bell-shaped speed profile. Flash and Hogan showed that this is exactly the movement that minimises the integrated squared jerk (the rate of change of acceleration) of the hand, as if the brain aimed for the smoothest possible path.[15]

Watching and doing. In area F5 of the monkey premotor cortex, 'mirror' neurons fire both when the monkey performs an action and when it watches another individual perform a similar one, which may link understanding actions to producing them.[16]

When: preparation, then action

Motor cortex activity changes well before a movement starts. Directional tuning was already present during the reaction time, including the period roughly 80 ms before the first change in muscle activity.[10]

Before a spontaneous, self-chosen movement, a slow 'readiness potential' over the brain began on average about 350 ms (and at least 150 ms) before the time people reported first wanting to move. Libet concluded that a voluntary act can begin unconsciously, a finding still debated.[3]

The organisation is present early in life. Precision fMRI in newborns, infants and children found likely developmental precursors of the inter-effector system, and scans of macaques suggested counterparts in other primates.[2]

When movement fails

Stroke and injury. Damage to the motor cortex causes weakness of the opposite side whose distribution follows the map, face and hand with lateral lesions, leg with medial ones; damage anywhere along the corticospinal tract above the pyramidal decussation also causes weakness on the opposite side.[1,7,8,17]

The supplementary motor area. Removing it can cause a striking syndrome of contralateral akinesia, and reduced speech output when the dominant hemisphere (usually left) is involved, which usually recovers, unlike damage to primary motor cortex or the corticospinal tract.[5,6,18]

Motor neuron disease. Amyotrophic lateral sclerosis is a progressive degeneration of the upper motor neurons in the cortex and the lower motor neurons in the brainstem and spinal cord, leading to weakness and eventually paralysis.[19]

Recovery is trainable. After a small stroke in the hand area of monkey motor cortex, retraining skilled hand use prevented the further loss of hand territory in the undamaged cortex next to the lesion, and sometimes the hand map expanded into the elbow and shoulder areas. In people, a two-week programme of constraint-induced movement therapy, restraining the less-affected hand while practising with the weak one, roughly halved the time taken on a hand-function test a year later, against a 26% improvement with usual care.[11,20]

The mathematics of movement

Movement has some of the cleanest laws in neuroscience: how long it takes, what path it follows and how neurons encode it.[15,22]

Fitts's law[22]
MT=a+b log⁡2 ⁣2DWMT = a + b\,\log_2\!\frac{2D}{W}

The time to move to a target grows with the logarithm of distance over target width. Fitts called the logarithm the index of difficulty, in bits, and treated the motor system as a channel with a limited information capacity: doubling the distance or halving the target adds the same time.

Symbols in Fitts's law
SymbolMeaningUnit
MTMTmovement times
DDdistance to the targetm
WWwidth of the targetm
a,ba, bconstants fitted for a person and a tasks, s/bit
Minimum-jerk cost[15]
J=12∫0T∥d3xdt3∥2dtJ = \frac{1}{2}\int_{0}^{T} \left\lVert \frac{d^{3}\mathbf{x}}{dt^{3}} \right\rVert^{2} dt

The smoothness cost that Flash and Hogan proposed the brain minimises: the squared jerk of the hand summed over the movement. Among all paths between two points in a given time, the one with the least jerk is straight and has a bell-shaped speed profile, as real reaches do.

Symbols in Minimum-jerk cost
SymbolMeaningUnit
x(t)\mathbf{x}(t)hand position over timem
TTmovement durations
Minimum-jerk trajectory[15]
x(t)=x0+(xf−x0)(10τ3−15τ4+6τ5),τ=t/Tx(t) = x_0 + (x_f - x_0)\left(10\tau^{3} - 15\tau^{4} + 6\tau^{5}\right), \qquad \tau = t/T

The solution of the minimum-jerk problem for a point-to-point reach that starts and ends at rest. Its speed is zero at both ends and peaks halfway, giving the bell-shaped profile.

Symbols in Minimum-jerk trajectory
SymbolMeaningUnit
x0,xfx_0, x_fstart and end positionsm
τ\taunormalised time, from 0 to 1—
Cosine tuning[10]
f(θ)=b0+c1cos⁡(θ−θ0)f(\theta) = b_0 + c_1\cos(\theta - \theta_0)

A motor cortex neuron's firing rate as a function of movement direction: highest at its preferred direction and falling off smoothly, a relation found in about three quarters of directionally tuned cells.

Symbols in Cosine tuning
SymbolMeaningUnit
θ\thetamovement direction°
θ0\theta_0preferred direction°
b0,c1b_0, c_1baseline rate and modulation depthspikes/s
Rotational population dynamics[14]
x˙(t)=M x(t),M=−M⊤\dot{\mathbf{x}}(t) = M\,\mathbf{x}(t), \qquad M = -M^{\top}

Churchland and colleagues looked for the plane in which population activity is best described by a linear dynamical system whose matrix is skew-symmetric; such a system can only rotate, and motor cortex activity during reaching rotated in this way, starting from the state set during preparation.

Symbols in Rotational population dynamics
SymbolMeaningUnit
x(t)\mathbf{x}(t)population activity projected into a low-dimensional space—
MMskew-symmetric matrix fitted to the data—

Technology: mapping and restoring movement

Stimulation mapping and tractography. During surgery near motor areas, stimulating the cortex and the white matter beneath it identifies tissue that must be spared; tractography estimates of the corticospinal tract have been compared against this reference.[17,21]

Precision imaging. Precision functional MRI, which maps each individual brain in fine detail, revealed the inter-effector regions of motor cortex, and the finding was verified in the three largest fMRI datasets.[2]

Reading and bridging. Because motor cortex keeps producing movement signals after paralysis, implanted interfaces can decode them: attempted speech has been decoded from motor cortex at 62 words per minute, and a brain–spine interface links cortical signals to stimulation of the spinal cord so that a man with tetraplegia can stand and walk.[23,24]

How the motor system is studied[1,2,10,12,13,14]
MethodWhat it showed
Cortical stimulation in surgeryThe somatotopic map of the precentral gyrus
Single-neuron recordingDirectional tuning and population coding
Population recordingRotational dynamics after preparation
Long-train microstimulationComplex postures mapped around the body
Precision fMRIInter-effector regions of the somato-cognitive action network

Milestones

Mapping movement

  1. 1937Penfield and Boldrey map motor and sensory representation by stimulating the human cortex.[1]
  2. 1954Fitts describes the information capacity of the motor system.[22]
  3. 1977The supplementary motor area syndrome is described after corticectomies.[5]
  4. 1982Motor cortex neurons are found to be tuned to movement direction.[10]
  5. 1983The readiness potential is shown to precede the reported intention to move.[3]
  6. 1985The minimum-jerk model predicts the shape of reaching movements.[15]
  7. 1986The population vector predicts movement direction.[13]
  8. 1996Mirror neurons are described in premotor cortex; rehabilitative training is shown to shape cortical reorganisation after infarcts.[16,20]
  9. 1997The hand knob is identified as an MRI landmark of the motor hand area.[4]
  10. 2002Long stimulation trains evoke complex, goal-like postures.[12]
  11. 2006The EXCITE trial tests constraint-induced movement therapy after stroke.[11]
  12. 2012Rotational population dynamics are found during reaching.[14]
  13. 2023The somato-cognitive action network is found interleaved with the motor map; a brain–spine interface restores walking.[2,24]

Frontiers: the homunculus redrawn

The classic homunculus runs from foot to face down the precentral gyrus. Gordon and colleagues found instead concentric zones for the foot, hand and mouth separated by inter-effector regions that are thinner, strongly connected to each other and to the cingulo-opercular network involved in action, arousal, errors and pain, and active during action planning and movements of the trunk and face rather than any one limb.[2]

Motor cortex is increasingly studied as a dynamical system: preparation sets an initial state from which population activity unfolds in a lawful, partly rotational way, and this structure explains many puzzling features of single-neuron responses.[14]

Check yourself

Check yourself

  1. Where is the hand area of the primary motor cortex usually found on MRI?
    Show answer

    At a knob-shaped bend of the precentral gyrus seen on axial images.

  2. Trace the corticospinal tract from cortex to the medulla.
    Show answer

    Corona radiata, posterior limb of the internal capsule, cerebral peduncle, pons, medullary pyramids.

  3. What is the somato-cognitive action network?
    Show answer

    Inter-effector regions interleaved with the foot, hand and mouth areas of motor cortex, connected to action-control networks and active in planning and whole-body movement.

  4. According to Fitts's law, what happens to movement time if you halve the width of the target?
    Show answer

    It increases by a fixed amount, b, because the index of difficulty rises by one bit.

  5. What does the minimum-jerk model predict about a reach?
    Show answer

    A roughly straight path with a bell-shaped speed profile.

  6. What did Libet find about the readiness potential?
    Show answer

    It began several hundred milliseconds before people reported the conscious wish to move.

  7. How can rehabilitation change the motor cortex after a stroke?
    Show answer

    Training can prevent loss of the hand map next to the lesion and even expand it, alongside recovery of hand function.

Glossary[1,3,9,11,15,16,22]

Primary motor cortex
Brodmann area 4 in the precentral gyrus, the main source of commands for voluntary movement.
Supplementary motor area
Part of area 6 on the medial superior frontal gyrus, involved in planning and initiating movement.
Somatotopy
An orderly map of the body across a region of cortex.
Corticospinal tract
The pathway from motor cortex to the spinal cord, crossing in the lower medulla.
Cortico-motoneuronal connection
A direct synapse from a corticospinal neuron onto a spinal motor neuron.
Readiness potential
A slow electrical change over the brain that precedes voluntary movement.
Mirror neuron
A neuron that fires both when performing an action and when observing it.
Jerk
The rate of change of acceleration.
Index of difficulty
In Fitts's law, the base-2 logarithm of twice the distance over the target width, in bits.
Constraint-induced movement therapy
Rehabilitation that restrains the stronger limb to force practice with the weaker one.

References

  1. Penfield W, Boldrey E. Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 1937;60(4):389-443. doi:10.1093/brain/60.4.389
  2. Gordon EM, Chauvin RJ, Van AN, Rajesh A, Nielsen A, Newbold DJ, et al.. A somato-cognitive action network alternates with effector regions in motor cortex. Nature 2023;617(7960):351-359. doi:10.1038/s41586-023-05964-2
  3. Libet B, Gleason CA, Wright EW, Pearl DK. Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). Brain 1983;106(3):623-642. doi:10.1093/brain/106.3.623
  4. Yousry T. Localization of the motor hand area to a knob on the precentral gyrus. A new landmark. Brain 1997;120(1):141-157. doi:10.1093/brain/120.1.141
  5. Laplane D, Talairach J, Meininger V, Bancaud J, Orgogozo JM. Clinical consequences of corticectomies involving the supplementary motor area in man. Journal of the Neurological Sciences 1977;34(3):301-314. doi:10.1016/0022-510X(77)90148-4
  6. Fontaine D, Capelle L, Duffau H. Somatotopy of the Supplementary Motor Area: Evidence from Correlation of the Extent of Surgical Resection with the Clinical Patterns of Deficit. Neurosurgery 2002;50(2):297-305. doi:10.1097/00006123-200202000-00011
  7. Catani M, Thiebautdeschotten M. A diffusion tensor imaging tractography atlas for virtual in vivo dissections. Cortex 2008;44(8):1105-1132. doi:10.1016/j.cortex.2008.05.004
  8. Schmahmann JD, Pandya DN. Fiber Pathways of the Brain. Oxford University Press 2006. doi:10.1093/acprof:oso/9780195104233.001.0001
  9. Lemon RN. Descending pathways in motor control. Annual Review of Neuroscience 2008;31:195-218. doi:10.1146/annurev.neuro.31.060407.125547
  10. Georgopoulos AP, Kalaska JF, Caminiti R, Massey JT. On the relations between the direction of two-dimensional arm movements and cell discharge in primate motor cortex. The Journal of Neuroscience 1982;2(11):1527-1537. doi:10.1523/JNEUROSCI.02-11-01527.1982
  11. Wolf SL, Winstein CJ, Miller JP, Taub E, Uswatte G, Morris D, et al.. Effect of constraint-induced movement therapy on upper extremity function 3 to 9 months after stroke. JAMA 2006;296(17):2095-2104. doi:10.1001/jama.296.17.2095
  12. Graziano MSA, Taylor CSR, Moore T. Complex movements evoked by microstimulation of precentral cortex. Neuron 2002;34(5):841-851. doi:10.1016/S0896-6273(02)00698-0
  13. Georgopoulos AP, Schwartz AB, Kettner RE. Neuronal population coding of movement direction. Science 1986;233(4771):1416-1419. doi:10.1126/science.3749885
  14. Churchland MM, Cunningham JP, Kaufman MT, Foster JD, Nuyujukian P, Ryu SI, Shenoy KV. Neural population dynamics during reaching. Nature 2012;487(7405):51-56. doi:10.1038/nature11129
  15. Flash T, Hogan N. The coordination of arm movements: an experimentally confirmed mathematical model. The Journal of Neuroscience 1985;5(7):1688-1703. doi:10.1523/JNEUROSCI.05-07-01688.1985
  16. Rizzolatti G, Fadiga L, Gallese V, Fogassi L. Premotor cortex and the recognition of motor actions. Cognitive Brain Research 1996;3(2):131-141. doi:10.1016/0926-6410(95)00038-0
  17. Keles GE, Lundin DA, Lamborn KR, Chang EF, Ojemann G, Berger MS. Intraoperative subcortical stimulation mapping for hemispheric perirolandic gliomas located within or adjacent to the descending motor pathways: evaluation of morbidity and assessment of functional outcome in 294 patients. Journal of Neurosurgery 2004;100(3):369-375. doi:10.3171/jns.2004.100.3.0369
  18. Krainik A, Lehéricy S, Duffau H, Capelle L, Chainay H, Cornu P, et al.. Postoperative speech disorder after medial frontal surgery. Neurology 2003;60(4):587-594. doi:10.1212/01.WNL.0000048206.07837.59
  19. Brown RH, Al-Chalabi A. Amyotrophic lateral sclerosis. New England Journal of Medicine 2017;377(2):162-172. doi:10.1056/NEJMra1603471
  20. Nudo RJ, Wise BM, SiFuentes F, Milliken GW. Neural substrates for the effects of rehabilitative training on motor recovery after ischemic infarct. Science 1996;272(5269):1791-1794. doi:10.1126/science.272.5269.1791
  21. Berman JI, Berger MS, Mukherjee P, Henry RG. Diffusion-tensor imaging-guided tracking of fibers of the pyramidal tract combined with intraoperative cortical stimulation mapping in patients with gliomas. Journal of Neurosurgery 2004;101(1):66-72. doi:10.3171/jns.2004.101.1.0066
  22. Fitts PM. The information capacity of the human motor system in controlling the amplitude of movement. Journal of Experimental Psychology 1954;47(6):381-391. doi:10.1037/h0055392
  23. Willett FR, Kunz EM, Fan C, Avansino DT, Wilson GH, Choi EY, et al.. A high-performance speech neuroprosthesis. Nature 2023;620(7976):1031-1036. doi:10.1038/s41586-023-06377-x
  24. Lorach H, Galvez A, Spagnolo V, Martel F, Karakas S, Intering N, et al.. Walking naturally after spinal cord injury using a brain–spine interface. Nature 2023;618(7963):126-133. doi:10.1038/s41586-023-06094-5

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