Questions: Motor Planning: Premotor and Supplementary Motor Cortex

Question 1

A patient sustains a stroke affecting premotor areas. She has full muscle strength, intact reflexes, and normal M1 function — yet she cannot pantomime brushing her teeth when asked. Which best explains this?

• A. M1 damage has disrupted the somatotopic map, preventing muscle recruitment
• B. Premotor/SMA damage has disrupted the planning system that sequences learned motor acts, even though execution capacity is intact
• C. Cerebellar damage has impaired coordination of her hand movements
• D. Spinal cord involvement has blocked the efferent signal to her arm muscles

Answer: (B)

This scenario describes apraxia — the hallmark of premotor damage. Because M1 and the muscles are intact, the patient can move her arm; what she cannot do is assemble and dispatch a purposeful, learned sequence. This dissociation is the clearest evidence that planning and execution are anatomically separable: PMC/SMA builds the motor program and sends it to M1; M1 executes it. Damage at the planning stage produces intact movement capacity but broken sequencing.

Question 2

The readiness potential (Bereitschaftspotential) is a slow buildup of neural activity that appears up to a second before a voluntary movement begins. Which region is its primary source, and what does this tell us?

• A. Primary motor cortex (M1) — it starts activating well before the movement to allow muscle warm-up
• B. Supplementary motor area (SMA) — it reflects preparation for self-initiated sequences before M1 begins firing
• C. Dorsolateral prefrontal cortex (DLPFC) — it generates goals that must precede any motor output
• D. Cerebellum — it predicts the sensory consequences of movement before it occurs

Answer: (B)

The SMA is particularly active for internally-initiated, self-generated movements. The readiness potential originates largely in SMA (and to a lesser degree premotor cortex), building for hundreds of milliseconds before M1 begins firing and before the muscle contracts. This proves that the 'decision' to move and the preparation of its sequence happen upstream of execution — in the SMA — not simultaneously with it.

Question 3

Apraxia results from damage to the primary motor cortex and is characterized by muscle weakness.

• T. True
• F. False

Answer: False

Apraxia results from premotor or SMA damage, not M1 damage. By definition, a patient with apraxia has intact M1 function and normal muscular strength — they can move, but they cannot execute purposeful learned sequences. M1 damage produces paresis (weakness or paralysis). The distinction between these syndromes maps directly onto the planning-versus-execution architecture: apraxia = broken plan; paresis = broken execution.

Question 4

The supplementary motor area (SMA) shows increased neural activity before self-initiated movements begin, even before primary motor cortex activation.

• T. True
• F. False

Answer: True

This is well-established by both recording studies and the readiness potential. SMA activity ramps up hundreds of milliseconds before M1 fires and before any overt movement, reflecting its role in preparing and initiating internally generated motor sequences. PMC, by contrast, is more active for externally cued movements.

Question 5

Explain why the premotor cortex and SMA are necessary for skilled voluntary movement, given that the primary motor cortex already contains a complete somatotopic map of the body.

Model answer: M1's somatotopic map tells the motor system *how* to activate individual muscles, but not *which* muscles to recruit, *in what order*, or *toward what goal*. PMC and SMA translate higher-level intentions and goal representations (passed down from DLPFC) into organized motor sequences that are then sent to M1 for execution. Without this planning stage, M1 would have no coherent instruction to execute — individual muscle movements would be possible but purposeful sequencing would be impossible.

The motor hierarchy (DLPFC → PMC/SMA → M1 → spinal cord) reflects successive levels of abstraction: goals become sequences, sequences become muscle commands. M1 alone is like a piano that can play any note but has no score. PMC/SMA is the conductor who selects the notes, their order, and their timing. Apraxia is what happens when the conductor is incapacitated but the piano is still functional.