Ah yes, the Ia and type II afferents.
One of our favorites! Acting as a sentinel from the muscle spindle, concentrated in the antigravity and extensor musculature, Ia and type II afferents live in the belly of the muscle and send information regarding length and rate of change of length to the CNS via the spino cerebellar and inferior olivary pathways. In more simpler terms, think of muscle spindles as small computer chips embedded in the muscle and using la and type II afferents the team act as volume controls helping to set the tone of the muscle and it responsiveness to stretch. If they are active, they make a muscle more sensitive to stretch.
So what does that mean? Muscle spindles turn up the volume or sensitivity of the muscles response to stretch. Remember when we stretch a muscle, it’s response is to contract. Think about when a doctor tests your reflexes. What makes them more or less reactive? You guessed it, the muscle spindle; which is a reflection of what is going on in the higher centers of the brain. The muscle spindles level of excitation is based on the sum total of all information acting on the gamma motor neuron (ie the neuron going to the muscle spindle) in the spinal cord. That includes all the afferent (ie. sensory) information coming in (things like pain can make it more or less active) as well as information descending from higher centers (like the brain, brainstem and cerebellum) which will again influence it at the spinal cord level.
So we found this cool study that looks at spindles and supports their actions:
J Physiol. 2009 Jul 1;587(Pt 13):3375-82. Epub 2009 May 18.
Mechanical and neural stretch responses of the human soleus muscle at different walking speeds.
At increased speeds of walking, the muscles themselves (particularly the soleus in this study) become stiffer due to changes in spindle responsiveness. The decline in amplitude and velocity of stretch of the soleus muscle fasicles with increasing walking speeds was NOT accompanied by a change in muscle spindle amplitude, as was hypothesized.
Clinically, this means that the spindles were STILL RESPONSIVE to stretch, even though the characteristics of the muscle changed with greater speeds of action. This may be one of the reasons you may injure yourself when moving or running quickly; the muscle becomes stiffer and the spindle action remains constant (the volume is UP).
Thankfully, we have another system that can intervene (sometimes) when the system is overloaded, and take the stress of the muscle. This is due to the golgi tendon organ; but that is a post for another day…
Geeking out and exploring the subtleties of the neurology as it relates to the system, we remain…The Gait Guys