Muscle & bone ↗Motor networks select a task
Cortical, brainstem, spinal, sensory, and learned motor networks shape which muscles are activated, in what order, and with what timing.
MUSCLE + BONE / 01
Signal becomes calcium.
Calcium becomes force.
Voluntary movement requires a continuous chain from neural command through the neuromuscular junction and sarcomere to force transfer, sensory feedback, and joint control.
A neuron serves multiple fibers
A motor unit contains one motor neuron and the muscle fibers it activates. Recruitment and firing rate help grade force.
The neuromuscular junction transmits the signal
A nerve impulse releases acetylcholine at the motor end plate, producing a muscle-fiber action potential when transmission succeeds.
Activation is not force alone
Neural drive, pain, joint angle, fatigue, motivation, tissue condition, and external load can all affect observed movement.
The signal enters the fiber
The action potential travels along the sarcolemma and into transverse tubules, coordinating activation throughout the muscle fiber.
The sarcoplasmic reticulum releases calcium
Calcium binds troponin and shifts regulatory proteins so actin and myosin can interact briefly and repeatedly.
ATP supports cycling
Myosin cross-bridges bind, pull, detach, and reset using ATP. Force depends on activation, fiber length, shortening velocity, and load.
Calcium is resequestered
Calcium pumps return calcium to the sarcoplasmic reticulum, allowing the fiber to relax and prepare for the next activation.
Muscle force crosses connective tissue
Tendons distribute and transmit force to bone; their elastic behavior can store and return energy during selected movements.
Multiple tissues guide the result
Joint position emerges from bones, cartilage, ligaments, capsule, co-contraction, external load, and movement technique.
Proprioception updates the command
Muscle spindles, tendon organs, skin, vision, vestibular signals, and pain contribute feedback during movement.
Symptoms require context
Weakness, cramping, tremor, pain, or altered movement can arise at many levels and cannot be localized by this map.