Supplementary Motor Area plays an important role in the study of neurodegenerative diseases. This page provides comprehensive information about this topic, including its mechanisms, significance in disease processes, and therapeutic implications.
The Supplementary Motor Area (SMA), also known as Brodmann area 6 (medial part), is a critical cortical region located on the medial surface of the frontal lobe, anterior to the primary motor cortex. The SMA plays essential roles in motor planning, sequence learning, the initiation of internally-cued movements, and bimanual coordination. It serves as a higher-order motor area that works in concert with the primary motor cortex and premotor cortex to execute complex motor programs [1][2]. Neurodegenerative diseases, particularly Parkinson's disease (PD) and Alzheimer's disease (AD), significantly affect SMA function, contributing to characteristic motor impairments such as bradykinesia, freezing of gait, and apraxia.
¶ Anatomy and Location
The SMA is located on the medial surface of the frontal lobe, in the paracentral lobule region [3]:
- Anterior boundary: Pre-SMA (caudalmost part of superior frontal gyrus)
- Posterior boundary: Primary motor cortex (Brodmann area 4)
- Superior boundary: Marginal ramus of cingulate sulcus
- Inferior boundary: Cingulate sulcus
The SMA shares similarities with the primary motor cortex but has distinct features:
- Prominent layer V with pyramidal neurons (but smaller than Betz cells)
- Less well-developed layer IV than sensory areas
- Columnar organization
- Higher neuronal density in deep layers
Major inputs:
- Primary motor cortex (M1)
- Premotor cortex (lateral BA6)
- Basal ganglia (via thalamus - ventrolateral nucleus)
- Cerebellum (via thalamus)
- Posterior parietal cortex
- Prefrontal cortex
Major outputs:
- Primary motor cortex (layer V)
- Corticospinal tract (indirect)
- Basal ganglia (via caudate nucleus)
- Brainstem motor nuclei
- Spinal cord (via M1)
The region is functionally divided [1]:
| Region |
Function |
| Pre-SMA |
Motor ideation, abstract sequence planning |
| SMA-proper |
Motor execution, concrete sequence execution |
- Leg representations: Medial (near midline)
- Arm representations: Lateral to leg area
- Face representations: Most lateral
- Extensive overlap between representations
The SMA contains:
- Movement sequences: Ordered sequences rather than single muscles
- Bilateral representations: Both hemispheres control both sides
- Internal sequences: Self-initiated movements
- External sequences: Stimulus-driven movements
Glutamate (excitatory):
- NMDA and AMPA receptors
- Critical for motor learning
- Synaptic plasticity
GABA (inhibitory):
- Local circuit inhibition
- Shapes movement patterns
- Prevents unwanted movements
| Receptor Type |
Function |
| NMDA |
Learning-related plasticity |
| AMPA |
Fast excitation |
| GABA-A |
Fast inhibition |
| Dopamine D1 |
Motor learning, motivation |
| Dopamine D2 |
Motor suppression |
- Dopamine: From ventral tegmental area (VTA)
- Noradrenaline: From locus coeruleus
- Serotonin: From dorsal raphe
- Acetylcholine: From basal forebrain
¶ Function and Motor Control
The SMA is crucial for motor planning [1][4]:
- Sequence representation: Stores motor sequences as abstract representations
- Temporal ordering: Organizes movements in time
- Selection: Chooses among competing motor programs
- Preparation: Prepares motor programs for execution
The SMA is particularly important for learning movement sequences [5]:
- Explicit learning: Conscious awareness of sequences
- Implicit learning: Unconscious acquisition
- Serial reaction time tasks: Classic paradigm
- Finger sequencing: Piano-type tasks
Unlike the primary motor cortex which responds to external cues, the SMA:
- Initiates movements without external triggers
- Supports self-paced movements
- Generates spontaneous motor acts
- Enables mental rehearsal of movements
The SMA coordinates bimanual movements:
- Bilateral representations enable coordination
- Interhemispheric interactions
- Sequential bimanual tasks
- Mirror movements
The SMA is active during motor imagery:
- Mental rehearsal of movements
- No actual movement execution
- Preserved in some neurodegenerative conditions
- Used in rehabilitation
- ** tonic activity**: During motor planning
- ** phasic bursts**: At movement onset
- sequence-specific activity: During learned sequences
- prediction errors: When expectations violated
| Frequency |
State |
Significance |
| Beta (15-30 Hz) |
Resting |
Movement suppression |
| Gamma (40-100 Hz) |
Movement |
Movement initiation |
| Alpha (8-12 Hz) |
Idling |
May inhibit motor areas |
- Readiness potential (Bereitschaftspotential): Precedes self-initiated movements
- NS' (negative slope): In pre-SMA
- Motor potential: In SMA-proper
The SMA is profoundly affected in PD [2][6]:
Hypoactivity:
- Reduced SMA activity during internally-cued movements
- Correlates with bradykinesia
- Linked to dopaminergic deficiency
Freezing of Gait:
- SMA dysfunction contributes to freezing
- Impaired motor planning
- Failure to generate gait sequences
Sequence Learning:
- Impaired implicit sequence learning
- Reduced practice effects
- Dopamine-dependent
Treatment Effects:
- Levodopa increases SMA activity
- Deep brain stimulation modulates SMA
- Rehabilitation can improve function
SMA involvement in AD [7]:
Motor Planning Deficits:
- Impaired motor sequence learning
- Reduced anticipatory activity
- Early indicator of cortical dysfunction
Apraxia:
- Loss of learned motor programs
- Ideomotor apraxia
- Related to parietal and frontal connectivity
Neuroimaging:
- Reduced glucose metabolism
- Cortical atrophy
- White matter disconnection
- Impaired sequence learning
- Reduced SMA activation
- Motor timing deficits
- Chorea may involve SMA dysfunction
- SMA hypometabolism
- Gait freezing
- Autonomic-SMA interactions
- SMA lesions cause limb kinetic apraxia
- Impaired motor planning
- Recovery involves SMA reorganization
- Target for rehabilitation
The SMA shows experience-dependent plasticity [5]:
- Practice-induced activation changes
- Representational map reorganization
- Synaptic strength modifications
- NMDA receptor-dependent
Motor sequence memory consolidates in SMA:
- Initial learning: Sensorimotor cortex
- Consolidation: SMA
- Long-term storage: SMA
Learning in SMA enables transfer:
- New sequences become automated
- Applicable to novel situations
- Basis for motor expertise
| Method |
Application |
| fMRI |
Localize activity during tasks |
| PET |
Glucose metabolism |
| EEG |
Temporal dynamics |
| MEG |
Magnetic fields |
| TMS |
Causal testing |
- Serial reaction time task: Implicit sequence learning
- Finger sequencing: Explicit learning
- Self-paced movements: Internally-cued
- Motor imagery: Mental rehearsal
- Motor sequence tasks: Assess learning
- Timed up and go: Gait initiation
- Freezing of gait assessment: PD-specific
- Apraxia testing: AD assessment
- Levodopa: Improves SMA function in PD
- Dopamine agonists: Motor learning enhancement
- Acetylcholinesterase inhibitors: May improve cortical function in AD
- Deep brain stimulation: STN stimulation affects SMA
- Motor cortex stimulation: For PD
- Stroke rehabilitation: May target SMA
- Sequence training: Explicit motor learning
- Mental practice: Motor imagery
- tDCS: Modulate SMA excitability
- Physical therapy: Gait and movement training
- Brain-computer interfaces: Neural feedback
- Robotic training: Intensive practice
- Virtual reality: Enriched motor environments
- Gene therapy: Neuroprotection
[1] Nachev et al., Supplementary motor area (2008)
[2] Jenkins et al., Impaired SMA activation in PD (1992)
[3] Picard & Strick, Motor areas of the medial wall (1996)
[4] Shima & Tanji, Role for SMA in movement selection (2000)
[5] Hikosaka et al., Motor learning in SMA (2002)
[6] Sabatini et al., SMA and freezing of gait in PD (2000)
[7] Morrison & Hof, Motor cortex changes in AD (2007)
Supplementary Motor Area plays an important role in the study of neurodegenerative diseases. This page provides comprehensive information about this topic, including its mechanisms, significance in disease processes, and therapeutic implications.
The study of Supplementary Motor Area has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
- Tanji J, Shima K. Role for supplementary motor area cells in planning several movements. Nature. 1984;311(5985):414-417. PMID:6483109
- Matsuzaka Y, et al. Skill learning in the motor cortex. Neurosci Res. 1992;13(4):279-289. PMID:1375283
- Picard N, Strick PL. Motor areas of the medial wall: a review of their location and functional activation. Cereb Cortex. 1996;6(3):342-353. PMID:8670662
- Shima K, Tanji J. Both supplementary and premotor areas encode the timing of sequential movements. J Neurophysiol. 1998;80(3):1207-1220. PMID:9744928
- Lee KM, Chang KH. Neural correlates of motor memory. Exp Brain Res. 2001;136(4):431-438. PMID:11243430