Final answer:
Planned motor movement is a complex process that involves regions of the brain planning and executing voluntary movements, with coordination between the premotor cortex, supplemental motor area, and cerebellum, and signals transmitted through the corticospinal tract to motor neurons that cause muscle contraction.
Step-by-step explanation:
Planned motor movement begins in the brain where various regions work together to coordinate and execute voluntary actions. When you make a decision to move, the premotor cortex and the supplemental motor area adjacent to the primary motor cortex become active. The premotor area is involved with controlling core muscles for posture during movement, while the supplemental motor area manages sequential movements learnt from experience.
The process continues when a neuron in the cerebral cortex fires a nerve impulse that travels down to a neuron in the spinal cord, which in turn, signals a muscle cell to contract, producing movement. This pathway involves the corticospinal tract, where neurons transmit commands from the primary motor cortex. These fibers descend from the cerebrum through the brainstem and spinal cord, eventually synapsing on motor neurons in the ventral horn of the spinal cord, comprising the upper motor neuron (UMN) and the lower motor neuron (LMN).
The coordinated action of the UMN and LMN is essential for voluntary movement. The cerebellum also contributes, especially when rapid, alternating movements are involved, by ensuring that movement commands are properly coordinated.
The signal for action is completed when acetylcholine is released from motor neurons at the neuromuscular junction, causing a muscle to contract in response. This intricate network of neural pathways and processes culminate in the planned movement, demonstrating the complexity of motor control in the human body.