More muscle madness (or weakness, as the case may be)
In the last post (missed it? click here) , we discussed 3 causes of muscle weakness: local, segmental and cortical. In that post, we emphasized LOCAL causes of muscle weakness, determining that many of these causes related to direct injury, disease processes or neuromuscular endplate disorders.
This post will look at segmental causes, which as you have probably guessed, are at the spinal cord level. These are usually “electrical” problems, though in uncommon cases (syrinx, ependyoma, dermoid tumors) thay can be due to pathological processes.
We need to understand that the spinal cord is composed of either tracts or nucleii, that’s pretty much it. The tracts are largely on the outside of the cord. Remember that they are the axons of nerves. Since they are myelinated (insulated), they are white in appearence. They have names like the spinocerebellar tracts, dorsal columns, reticulospinal tracts and corticospinal tracts, just to name a few. The nucleii are largely in the central part of the cord, organized in an “H” pattern that is divided up into layers called lamina. They are unmyelinated, so they are grey in appearance. Interspersed with these nucleii are perhaps one of the most important contributors to muscle weakness, the “interneurons” or “interneuronal pool”.
Remember, as impulses travel down from the cortex to control volitional (voluntary)movement, they end here in the internuncial pool and land on, you guessed it, interneurons. To this end, we need to remember that interneurons are 30X more numerous than any of the other cells in the central part of the cord. When a message is on its way down from the cortex, or heading up to it, most of the time, it interacts with an interneuron. In turn, interneurons can be excitatory or inhibitory.
Something you should know about neurons, is that they are all or none. They either fire or they don’t. There is no in between. Same for interneurons. Depending on what they are connected to, the muscles (or autonomics, or other 2nd and 3rd order neurons) fire…or they don’t.
There are 2 types of motor neurons that go to muscle: alpha and gamma. Alpha motor neurons go to “extrafusal” muscle, or muscle we can control voluntarily. Gamma motor neurons go to “intrafusal” muscle, or muscle spindles, which monitor the length and rate of change of length of a muscle.
As you probably guessed, these motor neurons can be either activated directly (few) or through interneurons (many). These motor neurons connect with sensory neurons that monitor things like length, rate of change of length and tension in muscles. These modalities are subserved by receptors you have heard of: muscle spindles and golgi tendon organs. The alpha and gamma motor neurons also interact with axons of neurons that are descending from higher areas of the brain, like the cortex, cerebellum, vestibular nucleus, just to name a few.
It is the interplay of all these inputs and outputs, often referred to as the “central integrated state”, that ultimately determines whether a muscle fires or not, and thus if it is “weak” or “strong”. If excitation wins, then the nerve fires and the muscle contracts; if inhibition wins, then it does not.
What are all the inputs and outputs? Only all the sensory and motor (this includes the autonomics ie sympathetic and parasympathetic) information arriving there from the periphery and central nervous system! Quite a lot!
Segmental causes, just another reason a muscle may test week
Ivo and Shawn: The Gait Guys