Final answer:
Skeletal muscle tone is typically preserved during NREM sleep, allowing muscles to maintain posture and stabilize joints.
Step-by-step explanation:
Skeletal muscle tone in NREM (Non-Rapid Eye Movement) sleep is typically preserved. During NREM sleep, especially in the deeper stages, muscles retain some tone and are not completely relaxed. This is contrary to REM sleep, where there is generally a characteristic paralysis of voluntary muscles, with the notable exceptions of those muscles responsible for circulation and respiration. Muscle tone is important as it allows for the continuous stabilization of joints and maintenance of posture even during sleep.
Muscle tone is achieved through a complex interaction between the nervous system and skeletal muscles, activating a few motor units at a time in a cyclical manner. This process ensures that muscles do not fatigue completely, allowing some motor units to rest while others are active. An absence of muscle tone can indicate potential issues, such as hypotonia or flaccidity, which may be due to damage in parts of the central nervous system or to lower motor neuron problems.