In this Neuromechanics weekly, Dr Waerlop Introduces the cerebellum and talks about its importance clinically, since it contains more than ½ of the neurons in the brain! It’s anatomy and inputs from the periphery are discussed. The take home message is the cerebellum is the key to understanding and directing movement, since it receives feedback from most ascending and descending pathways.
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:
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
Just when you thought it was safe to watch a Neuromechanics Weekly episode, Dr Ivo throws a curveball. Check out the interesting clinical asides about myelopathy (pressure on the spinal cord causing ataxic gait) and the importance of which modality to check 1st, when doing an exam.
Keep these things in mind the next time you are evaluating someone’s gait.
The BBC have a new version of The Secret Agent coming starring Toby Jones so here is a photo from the 1992 version starring David Suchet and Peter Capaldi. Peter’s role is played by David Dawson (aka Affers from TTOI) in the new version.