Electrophysiology of the Normal and Pathologic Muscle Activation

Motorneurons are Central Processing Units able to generate sequences of action potentials whose frequency and duration are the result of algebraic sum of inward and outward currents crossing their membranes. On a macroscopic scale the activity of these Units will result in muscle contraction. This lecture starts by focusing on the action potential, the motor unit and some integrative aspects of motor orders directed to spinal level. In fact, muscle contraction is subjected to multiple controls which take place at peripheral (proprioceptors), spinal (local circuits) and supraspinal (higher motor centers) levels. The excitation-contraction coupling, consisting in the switching-on of the neuromuscular synapsis, triggers the consequent mechanical event into the sarcomere. Exaggerated inward current in the motorneuron will produce hyperexcitability with consequent muscular rigidity (spasticity). Examples of incoming pathologic inputs to the motorneuron are analyzed and discussed.