Hold my beer...
Assume an experienced rider, as learning is different.
Intention is set, requiring the basal ganglia and fore brain and either a notion of free will, determinism, or whatever you fancy.
The area ahead is scanned and mapped for a clear path via retina- optic nerve - visual cortex and particularly the dorsal parietal pathway.
Initial organised motor signal sequences originate in pre and pre pre motor areas, hitting motor strip of the brain, particularly those homoncular areas corresponding to legs, arms and torso. Basal ganglia loops prime these circuits into action and help maintain their engagement.
Activated motor strip neurons pass through the internal capsule, down the pyramidal tracts, the spinal cord, and meet a lower motor neuron in the anterior horn, which then carries the baton and traverses out if the cord (still the central nervous system) into the body and to the final destination: a muscle. Electrical depolarisation along the axon hops rapidly between nodes of ranvier, enabled by insulation myelination. At the terrminal synaptic bouton lower motor neurons branch across a muscle body. Each neurons innervated patch is a motor unit; multiple combine into a motor pool. Depolarisation triggers fusion of vesicles to the membrane endplate and release of acetylcholine into the thin synapse. Rapid diffusion moves thr snall molecukes to the muscle membrane, the sarcolemma, who then bind to the membrane spanning Nm nicotinic receptors which open and allow a rapid flooding of sodium into the muscle cell syncitium and efflux of potassiun into the extra cellular matrix. Depolarisation of that muscle allows further calcium released from the sarcoplasmic reticulum to activate protein machinery; myosin and actin run across each other and fibres contract. With enough activity concentric movement is achieved across the associated joint.
In a manner similar to walking, various spinal reflexes and the spinal locomotor pattern generator create a local, fast framework for actualisation of the impulses.
Feed back on state of the musculature ascends the spine via dorsal root ganglia and the dorsal horn. Amongst these are proprioceptive afferents, rapidly feeding back state of tension in muscle fibres from golgi tendon organs along highly myelinated type 1a fibres. These signals pass into the cerebellum where they are co processed with signals from the eyes and vestibular system.
The cerebellum modulates
the intensity of descending motor activity by comparing expected to perceived muscle state. It also orchestrates balance by integrating general body state, visual cues and vestibular information. In this way the small and large oscillations of riding the bike are maintained and constrained into an orderly process.
Experienced riders can dedicate higher function, i.e Executive frontal areas to other tasks, or to refined modulation of thr task to overcome specific issues.
Beginners must use all their frontal powers to focus attention on the task, painstakingly sequence actions, and reflect on the numerous errors and their consequences. Learning is slow, multi system, and largely independent of autobiographical memory.