Movement starts as a plan in the brain and ends as a muscle contraction. Several brain areas work together to make it smooth, strong and accurate.
Motor cortex
The primary motor cortex, just in front of the central sulcus, sends signals down the corticospinal tract. Most fibers cross in the lower brainstem, so the left side of the brain moves the right side of the body.
Upper and lower motor neurons
Upper motor neurons run from the brain to the spinal cord; lower motor neurons run from the spinal cord to the muscle. Lower motor neuron damage gives weakness with wasting, low tone and twitching (fasciculations).
Basal ganglia
These deep nuclei help start wanted movements and stop unwanted ones. Loss of dopamine in Parkinson disease gives slowness, rigidity and a resting tremor.
Cerebellum
The cerebellum compares the planned movement with what actually happens and corrects errors. Damage causes clumsiness, an intention tremor and an unsteady, wide-based gait on the same side as the lesion.
| Feature | Upper motor neuron | Lower motor neuron |
|---|---|---|
| Tone | Increased (spastic) | Decreased (flaccid) |
| Reflexes | Brisk | Reduced or absent |
| Muscle bulk | Mild wasting, late | Marked wasting |
| Fasciculations | Absent | Present |
| Plantar response | Upgoing | Downgoing |
At the bedside
Finger-to-nose, heel-to-shin and rapid alternating movements test the cerebellum. The pattern of weakness, tone and reflexes tells you whether the problem is in the brain, spinal cord, nerve or muscle.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).