Motor units are fundamental components of muscle function, comprising a motoneuron and the muscle fibers it innervates. Research into these units in domestic cats ( Felis catus ) has unveiled crucial insights into muscle physiology. A detailed study focusing on the triceps surae muscle group of cats has identified two primary types of motor units, categorized by their speed of contraction: fast twitch (F) and slow twitch (S). This classification is based on the time it takes for a muscle twitch to reach its peak tension.
Fast twitch (F) motor units are characterized by a twitch time to peak of 30 milliseconds or less. These units generate significantly greater twitch tensions compared to their slow twitch counterparts. In the study, the median twitch tension for type F units was 18 grams, substantially higher than the 1.6 grams median tension observed in type S units. Furthermore, type F muscle units exhibit much higher tetanus fusion frequencies, with a median of 85 pulses per second, indicating their capacity for rapid, forceful contractions. They also tend to have lower tetanus to twitch tension ratios, suggesting a more efficient force production in response to high-frequency stimulation.
Slow twitch (S) motor units, in contrast, have a twitch time to peak of 40 milliseconds or greater. As mentioned, they produce lower twitch tensions but are capable of sustained contractions. Their median tetanus fusion frequency is notably lower at 25 pulses per second, reflecting their slower contraction speed. However, type S units demonstrate higher tetanus to twitch tension ratios, indicating a greater increase in force with repeated stimulation, which is beneficial for endurance activities.
In terms of muscle distribution, the gastrocnemius muscle in cats contains a mixture of both F and S type motor units, with fast twitch units being approximately three times more prevalent than slow twitch units. Conversely, the soleus muscle is predominantly composed of type S motor units, aligning with its role in maintaining posture and sustained movements. While both gastrocnemius and soleus muscles contain type S motor units, subtle quantitative differences exist between them, suggesting functional specialization even within the same motor unit type.
Interestingly, the properties of the motoneurons themselves correlate with the type of muscle unit they innervate. Motoneurons associated with type F muscle units exhibit faster axonal conduction velocities, shorter post-spike hyperpolarizations, and lower input resistances compared to motoneurons connected to type S units. Despite these correlations, no single motoneuron property alone can definitively distinguish between type F and S motor units, highlighting the complex interplay between neuronal and muscle fiber characteristics in determining motor unit function.
Understanding the different types of motor units in cats provides a valuable framework for comprehending feline muscle physiology and movement. The distinction between fast and slow twitch units, with their unique contractile and neuronal properties, underpins the diverse range of movements cats are capable of, from rapid bursts of activity to sustained, graceful motion.
