Rubrospinal tract is a descending tract that extends throughout the length of the spinal cord. A descending tract is a bundle of nerve fibers that carries messages from the higher centers of the brain to the peripheral body parts. The rubrospinal tract begins as axons of the neurons present in the red nucleus and terminates by synapsing with the interneurons in the spinal cord.
The rubrospinal tract is important for regulating the activity of the motor neurons. It plays an important role in a number of body reflexes. In this article, we will discuss different aspects of the rubrospinal tract such as location in the spinal cord, anatomical organization, pathway and termination.
We will also have a look at the functions performed by the rubrospinal tract and its different diseases. So, keep reading in order to know everything about the rubrospinal tract.
In this section, we will talk about the location, anatomical organization, pathway and, termination of the rubrospinal tract.
As mentioned in the introduction, the rubrospinal tract is a descending tract present in the white matter of the spinal cord. It occupies the lateral white column of the spinal cord between the anterior and the posterior roots of spinal nerves. In the lateral white column, it is present just in front of the lateral corticospinal tract. One rubrospinal tract is present in each half of the spinal cord.
Just like the ascending tracts, the descending tracts are also discussed at three levels; the first-order neurons, the second-order neurons, and the third-order neurons. In addition to this, terms like upper motor neuron and lower motor neuron are also used while describing the pathway of a descending tract. Let us first have an understanding of these terms before discussing the pathway of the rubrospinal tract.
First Order Neurons
In the context of the descending tracts, a first-order neuron is a supraspinal nucleus. In most of the cases, first-order neurons are the one having cell bodies located in the cerebral cortex. The axons of these neurons pass through the internal capsule and contribute to most of the length of the descending tract.
Before entering the spinal cord, these axons may synapse with other supraspinal nuclei in the brainstem. That’s why first-order neurons referred to as supraspinal neurons. In the case of the rubrospinal tract, the first-order neurons include the neurons present in the motor area of the cerebral cortex as well as neurons of the red nuclei.
Second-order neurons in the case of the descending tracts are the interneurons present in the anterior gray column. The axons of second-order neurons are relatively shorter and terminate immediately. They synapse on the third-order neurons. The second-order neurons of the rubrospinal tract are internuncial neurons of the spinal cord.
Third Order Neurons
The third-order neurons are the motor neurons present in the anterior gray column of the spinal cord. The axons of these neurons directly innervate the skeletal muscles. In the case of the rubrospinal tract, the third-order neurons include alpha and gamma motor neurons.
Upper Motor Neurons
The upper motor neurons are simply the neurons whose cell bodies are located in the cerebral cortex. The first-order neurons in the case of descending tracts are always upper motor neurons.
Lower Motor Neurons
The lower motor neurons are those which directly innervate the skeletal muscles. They are present in the anterior gray column of the spinal cord. They may be third-order neurons or in some cases, second-order neurons.
Pathway of Rubrospinal Tract
As we have studied the terms associated with the anatomical organization of the descending tracts, it will be very easy for us to understand the pathway of the rubrospinal tract.
The rubrospinal tract begins with the axons of the red nucleus which is present in the tegmentum of the midbrain. The red nucleus receives input fibers from the motor area of the cerebral cortex.
After leaving the red nucleus, the axons of the red nucleus cross to the opposite side. This crossing of nerve fibers occurs just in front of the red nuclei.
After decussation, these axons descend downward as rubrospinal tract on the contralateral side of the spinal cord.
These fibers of rubrospinal tract pass through the white matter of pons and medulla oblongata. After crossing the medulla oblongata, the rubrospinal tract enters the lateral white column of the spinal cord.
Within the lateral white column of the spinal cord, the rubrospinal tract continues to descend and terminates by synapsing with the internuncial neurons.
Rubrospinal tract terminates by synapsing with the internuncial neurons at different levels of the spinal cord. The rubrospinal tract mainly terminates at the level of the cervical segments of the spinal cord.
In this section, we will study the functions of the rubrospinal tract.
This is the major function of the rubrospinal tract. The red nucleus receives input fibers from the cerebral cortex and the cerebellum. It is believed that the cerebral cortex and cerebellum control the motor activity of different voluntary muscles via this indirect pathway of the red nucleus. It does so by influencing the activity of the alpha and gamma motor neurons.
Modulation of Flexor Muscle Tone
This tract also plays an important role in the modulation of motor tone so that when flexors contract, extensors relax and vice versa. This function is associated with the rubrospinal tract because the red nucleus receives input fibers from reticular formation. Under the influence of these connections, the rubrospinal tract inhibits the activity of extensors and facilitates the activity of flexor muscles.
Modulation of Reflex Activity
The rubrospinal tract also facilitates the reflex activity because of the connections of the red nucleus with reticular formation. It plays an important role in reflex activity that involve flexors such as withdrawing hand after touching a hot object, flexion of limbs when something hits the flexor surface, etc.
Inhibition of Anti-gravity Muscles
This is another important function of the rubrospinal tract. It inhibits the contraction of anti-gravity muscles. Thus, it is important in the prevention of decerebrate posture, an abnormal posture in which all the body parts are extended and stretched due to unregulated activity of extensors or antigravity muscles.
In this section, we will talk about major diseases that can involve the rubrospinal tract. Although there is no disease that specifically targets the rubrospinal tract, its functions can be compromised by other diseases that involve the spinal cord or the red nucleus.
Lesions of Red Nucleus
As we have studied that axons of red nucleus form the entire length of the rubrospinal tract in the spinal cord, any lesion of the red nucleus will result in total loss of functions performed by the rubrospinal tract.
These lesions may be due to a vascular disease or a tumor involving the tegmentum of midbrain. They result in movement disorders and postural abnormalities.
Complete Cord Transection
Complete cord transection due to fracture or stab wound within the cervical region will also involve the rubrospinal tract. It results in loss of all the functions of the spinal cord, including the functions of the rubrospinal tract.
Lower Motor Neuron Lesions
These are the lesions that involve the extrapyramidal tracts, including the rubrospinal tracts. The clinical signs include severe paralysis, increased muscle tone, exaggerated deep muscle reflexes and rigidity.
Rubrospinal tract is a descending tract in the spinal cord essential for controlling motor activity.
It is present in the lateral gray column of the spinal cord, one in each half.
The first-order neurons are present in the red nucleus and the cerebral cortex. The axons coming from cortex synapse on the neurons of the red nucleus.
The axons of the red nucleus cross to the opposite side and descend as the rubrospinal tract.
The second-order neurons are the internuncial neurons in the anterior gray column.
The internuncial neurons synapse with the alpha and gamma motor neurons, the third-order neurons of the rubrospinal tract.
It terminates at the level of cervical segments. No continuation is seen down the cervical region.
The important functions associated with the rubrospinal tract include the following.
- Indirect control of motor activity by cerebral cortex and cerebellum
- Modulation of motor tone by reticular formation
- Modulation of flexor activity by facilitating the contraction of flexors
- Inhibition of extensor muscles, the anti-gravity muscles
- Controlling the reflex actions involving flexors
Although no specific disease exists that targets the rubrospinal tract specifically, other diseases of the spinal cord or red nucleus can compromise its functions. These diseases include:
- Red nucleus lesions due to vascular disease or tumor
- Complete cord transection due to fracture or stab wound
- Lower motor neuron lesions that involve extrapyramidal tracts