A special thanks to The Washington University School of Medicine

SPINAL MOTOR STRUCTURES
 

A. The spinal cord:

Although we usually study the spinal cord as a series of cross sections, it is important to remember that it is in fact a column, with continuous tracts and cell columns. However, the cord can be divided into segments by the nerve roots that come off of it; although the rootlets branch off nearly continuously, they coalesce into about 31 discrete nerves along the cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerves). At each segment, rootlets appear to come out of both the dorsal and ventral halves of the spinal cord, as you see here:
 

In fact, only the ventral roots are coming out of the cord - the dorsal roots are actually going in. Throughout the cord, the dorsal grey matter (dorsal horns) deals with sensory perception, and receives information from the periphery through the dorsal root. The ventral horns contain the a-motor neurons, whose axons exit the cord via the ventral roots and travel directly to the muscles.

Along the dorsal root is a collection of cell bodies called the dorsal root ganglion. Inside the ganglion are the cell bodies of all the receptor neurons that send processes out to the periphery. The free nerve ending in the tip of your finger that feels the paper cut actually has its cell body back in the dorsal root ganglion. As you can see from the picture, the dorsal root ganglion is actually located ventrally, but you can tell that it is part of the dorsal root.
 

B. Levels of the spinal cord:

By this time in the course you have probably noticed that different levels of the cord are different in shape. Could you identify the source of a section if you had nothing to compare it to? In general, you should be able to differentiate cervical from thoracic from lumbar from sacral. Here is a series of cross sections:
 

C. Muscle spindles and the myotatic reflex:

One of the most familiar reflexes is the stretch reflex, also known as the knee-jerk reflex and the myotatic reflex. In its simplest form it is a 2-neuron loop, one afferent neuron and one efferent neuron. The afferent neuron is connected to a muscle spindle, which detects stretch in the muscle. The efferent neuron is the motor neuron, which causes the muscle to twitch.

But there are actually several types of afferents reporting on the status of the muscle. Let's look more closely at the muscle spindle:

The muscle spindle is a small group of muscle fibers walled off from the rest of the muscle by a collagen sheath. The sheath has a spindle or "fusiform" shape, so these fibers are called intrafusal fibers, and are contrasted with the extrafusal fibers, which are the power-generating muscle fibers. There are two types of nerve endings wrapped around this intrafusal fiber, both of which monitor its degree of stretch - as the muscle stretches, so does this capsule within it. These two stretch receptors are sensitive to different time scales, however. The first is called a Ia (that's one-A) fiber; the classification scheme is based on diameter and conduction velocity, so the Ia's are the largest and fastest. The Ia fiber fires like crazy when the muscle is stretching, but it is rapidly adapting. As soon as the muscle stops changing length, and holds a new position, the Ia adapts to the new length and stops firing. But you also need to know the position of your muscle when it is still. The second type of stretch receptor is called a II fiber, and it is slowly adapting. It also responds when the muscle is stretching, but it maintains a firing rate after the muscle has stopped moving (essentially, it is non-adapting). This information is part of what allows you to tell the position of your arm when your eyes are closed.
 

Rule of nomenclature - if there is a Ia, where is Ib? The Ib fibers are not connected to muscle spindles at all. Instead they are embedded in the tendon, and monitor overall muscle tension from there. They are also called Golgi tendon organs (the word "Golgi" is littered throughout neuroanatomy - he was a famous early anatomist).
 

Now, there is a potential problem with the muscle spindle system which may have occurred to you. What happens when the muscle gets shorter? Does the spindle go limp and slack? How can it remain sensitive to stretch at short lengths? This is where the intrafusal fiber comes into play. Like any muscle fiber, it can contract. When it contracts, the entire spindle shortens, remaining taut, and the sensitivity is intact. There are small motor neurons in the ventral horn that innervate the intrafusal muscle fibers and cause them to contract - they are the g-motor neurons. These neurons are excited every time the a-motor neurons fire, so that as the muscle contracts, the intrafusals contract with it.
 

How are they all hooked together? There are two simple rules: 1) When the stretch receptors fire, the a-motor neuron is excited, and the muscle contracts.

2) When the Golgi tendon organ fires, the a-motor neuron is inhibited (via an inhibitory interneuron), and the muscle relaxes. The purpose here is that the stretch receptors tell the muscle when it needs a little more force - that despite intending to contract the muscle is lengthening. This helps you to maintain the correct muscle tone. The Golgi tendon organs, on the other hand, begin to fire when the tension on the tendon is so great that you are in danger of injury. They have a protective function, and therefore they tell the muscle to ease off before it tears.

Occasionally, especially in cases of pyramidal tract damage, these two systems can get stuck in a loop, where they alternately trigger each other, causing the muscle to contract-relax-contract-relax, several times a second. This rapid trembling is called clonus, and can be a sign of pathology or extreme muscle fatigue.
 

D. Multiple motor pathways in the cord:

There are several pathways which innervate the a-motor neurons. They can be roughly grouped into the voluntary motion pathways and the postural pathways. The voluntary pathways include the lateral and anterior corticospinal systems, as covered in the "Basic motor" section. The postural pathways do not originate in cortex; instead their function is to maintain an upright posture against gravity, a task which requires hundreds of little muscular adjustments that we are not aware of. There are three principal pathways in humans: the vestibulospinal, tectospinal, and reticulospinal pathways. Pathways are always named beginning-to-end, so these originate in the vestibular nuclei, tectum (superior colliculi), and reticular formation, respectively. The rubrospinal system (from the red nucleus) is also sometimes included, but in humans it may be insignificant.

Although these pathways do not originate in cortex, they are controlled to some degree by cortical structures. You must be able to turn off selective postural systems to accomplish other movements. This becomes apparent when the cortex is damaged or cut off from the postural pathways, and can no longer control them. If the cortical input is damaged close to its source, (i.e., in the internal capsule), the result is what is called a decorticate posture. Here the postural pathways flex the upper limbs and extend the lower limbs by default, since they are getting no input from cortex.
 

If the damage cuts off not just cortical input but all input from the entire cerebrum, such as with a massive brainstem injury, the result is a decerebrate posture. In the decerebrate position all four limbs are extended and somewhat turned in (pronated). This sort of injury is much more serious than an injury of the internal capsule; the prognosis is usually very poor.
 

E. Injury to the corticospinal tract:

Just as the postural pathways have a "mind of their own" when cortical control is cut off, the spine can also produce some weird behaviors when the corticospinal tract is damaged. All of the spinal reflexes are local - all of the cells involved are contained within one or two segments, and cortex is not necessary. Therefore reflexes would still be present in a transected spinal cord. However, the cortex normally keeps a tight rein on reflexive behavior, so that it doesn't interfere with normal movements. When the cortex is cut off, the spinal cord becomes hyperreflexic. All of the normal reflexes become exaggerated, and some new ones appear. For example, stroking the lateral sole of your foot with a sharp object would normally make your toes curl downward. In a patient with corticospinal damage (also called upper motor neuron damage), the big toe would lift up and the toes would fan out. This is called the Babinski sign, and it is always pathological (with the exception of very young infants).