Premotor Cortex (PMA) & Supplementary Motor Area (SMA) | How Does Your Brain Plan Movements Before You Act?

When learning to play the piano, the PMA and SMA work together. Initially, the PMA is heavily engaged as you rely on external cues—watching your instructor's hands, looking at the sheet music, and finding the correct keys. It guides your finger movements based on this visual information. As you practice, the SMA begins to take over. It starts to internalize the sequence of notes and finger movements, creating a motor program. With enough repetition, you no longer need to look at the keys for every note. The SMA can now run the sequence from memory, allowing you to play smoothly and automatically. This transition from externally-cued (PMA-dominant) to internally-generated (SMA-dominant) action is the essence of motor skill learning.

The Premotor Cortex (PMA), a part of Brodmann area 6, is fundamentally involved in the preparation and guidance of movements, particularly those that are initiated by external cues. Think of it as the brain's navigator for motion. When you see a cup and decide to pick it up, the PMA is activated. It processes sensory information from your eyes—the cup's location, size, and shape—and uses this data to select the appropriate motor plan. It doesn't just command the muscles to move; it specifies *how* they should move in relation to the environment. For example, it calculates the trajectory your hand must take and how your fingers should shape themselves to grasp the cup successfully. This region is crucial for any action that requires interaction with the world around you, translating sensory input into coordinated motor output. It works in close concert with the primary motor cortex, which executes the final command, but the PMA is what ensures the movement is well-aimed and context-appropriate. Therefore, its primary function is sensory-guided motor planning.

The Supplementary Motor Area (SMA), the other key component of Brodmann area 6, specializes in planning and organizing complex sequences of movements that are generated internally, without direct reliance on external stimuli. It is the brain's internal choreographer. The SMA is highly active when you perform a sequence of actions from memory, such as tying your shoelaces, playing a familiar song on the piano, or performing a dance routine. It helps in organizing the temporal structure of the movement, ensuring that each step is executed in the correct order. The SMA is particularly critical for bimanual coordination, where both hands must work together harmoniously. Unlike the PMA, which reacts to the environment, the SMA is proactive, preparing movement sequences based on internal goals and learned experiences. It is essential for self-initiated actions and for transitioning smoothly from one movement to the next in a complex task.

When learning to play the piano, the PMA and SMA work together. Initially, the PMA is heavily engaged as you rely on external cues—watching your instructor's hands, looking at the sheet music, and finding the correct keys. It guides your finger movements based on this visual information. As you practice, the SMA begins to take over. It starts to internalize the sequence of notes and finger movements, creating a motor program. With enough repetition, you no longer need to look at the keys for every note. The SMA can now run the sequence from memory, allowing you to play smoothly and automatically. This transition from externally-cued (PMA-dominant) to internally-generated (SMA-dominant) action is the essence of motor skill learning.

Damage to these areas can lead to a condition called apraxia, which is an inability to perform purposeful, skilled movements, even though there is no muscle weakness or paralysis. If the PMA is damaged, a person might struggle with tasks that require using sensory cues, such as being unable to mimic a gesture someone else makes. If the SMA is damaged, the deficits are more related to self-initiated and sequential movements. The person might have difficulty starting a movement without an external cue or be unable to perform a learned sequence of actions, like buttoning a shirt. They might know what they want to do but are unable to organize and execute the correct motor plan to accomplish it.

Yes, mirror neurons are a specific class of neurons found prominently in the premotor cortex, as well as in other brain regions like the inferior parietal lobule. These neurons are unique because they fire both when an individual performs an action and when they observe another individual performing the same action. For instance, a mirror neuron in your PMA will activate when you pick up a pen, and it will also activate when you watch someone else pick up a pen. This neural mirroring is believed to be a fundamental mechanism for understanding the actions and intentions of other people. It allows us to learn new skills through imitation and is considered a cornerstone of social cognition, empathy, and language development. By simulating others' actions within our own motor system, we can directly understand their goals without needing complex logical reasoning. This makes the premotor cortex not just a center for planning our own movements, but also for interpreting the movements of those around us.