The most important ascending tract is the dorsal column system. This is the pathway the somatosensory receptors take. The neurons of the dorsal column system cross once they reach the medulla, then make their way up to the thalmus (via the ventral posterior lateral nucleus) and reach the sensory cortex through the internal capsule.
The most important descending tract involved in the movement system is the corticospinal pathway. As the name suggests, the projections start in the cortex (premotor area, supplementary motor area, primary motor area, and even the primary sensory area). They bundle up in the midbrain, spread out in the pons, and re-bundle up back in the medulla, where most of the fibers cross at the pyramids.
There are many other descending pathways as well, but they won’t be discussed here.
There are also a couple of loops involved in the movement system.
One such loop involves the basal ganglia. Very generally, the basal ganglia are involved as modifiers of voluntary movement. Problems in the basal ganglia are involved with Huntington’s disease (uncoordinated and jerky movements) and Parkinson’s disease (shaking and slowness of movement).
The other loop is through the cerebellum. The cerebellum has ascending pathways entering it from the sensory system (the same receptors that enter the dorsal column pathway), descending projections from the cortex, and projections from the vestibular system originating in the inner ear.
Very generally, the cerebellum is involved with maintaining upright posture, muscle tone, and balance.
Both ascending and descending pathways are discussed because perturbations happen in the environment all the time.
To avoid movement disruptions by such changes, commands to muscles have to be adjusted. These adjustments are informed by sensory signals reporting that something unexpected has happened.¹
However, nerves are pretty long and the speed at which information travels is relatively slow. Delays are common even for the quickest motor reactions.¹
The complication: basically, the brain always deals with outdated information on the state of the periphery. This information becomes even more outdated by the time it reaches the muscles.¹