Reticulospinal tract is another descending tract present in the white matter of the spinal cord. As the name indicates, the reticulospinal originates in the reticular formation. It consists of bundles of axons that carry information or orders from the reticular formation in the brainstem to the peripheral body parts.
It provides signals to the internuncial neurons present in the spinal cord and terminates there. The reticulospinal tract comprises of the medullary and the pontine reticulospinal tracts.
The reticulospinal tract is an essential component of the CNS by which the motor activities of the peripheral body arts are controlled by the higher centers of the brain. These include both voluntary as well as reflex motor actions performed by the body.
In this article, we will have a detailed discussion on the reticulospinal tract. We will study different anatomical as well as physiological features of the reticulospinal tract. In the end, we will discuss some pathologies that affect the reticulospinal tract. We will conclude our discussion with a summary at the end. So, keep reading.
Like all other descending tracts, we will discuss the anatomical features of the reticulospinal tract under four subheadings; location, anatomical organization, pathway and termination.
The reticulospinal tract comprises of two components;
- Medullary Reticulospinal Tract, arising from the reticular formation resent in medulla
- Pontine Reticulospinal Tract, arising from the reticular formation present in pons
We will discuss the anatomical features of the reticulospinal tract with respect to these two components.
As mentioned earlier, the reticulospinal tract is a bundle of axons present in the white matter of the spinal cord. The medullary reticulospinal tract is present in the lateral white column of the spinal cord.
The pontine reticulospinal tract is present in the anterior white column of the spinal cord. Both these tracts are present bilaterally, in the two halves of the spinal cord.
In this part of the article, we will discuss the anatomical organization of the medullary as well as the pontine reticulospinal tract. The anatomical organization is discussed at the three levels of organization i.e. first-order neurons, second-order neurons, and third-order neurons.
First Order Neurons
The first-order neurons have cell bodies located in the higher centers of the brain or brain stem. In case of the reticulospinal tracts, the first-order neurons are the nerve cells present in the nuclei of the reticular formation.
The reticular formation on each side of the brain stem consists of three columns of nerve cells or neurons. These neurons receive input fibers from different areas of the brain. The axons of these neurons give rise to the reticulospinal tract.
- The medullary reticulospinal tract arises from the nuclei of reticular formation located in the medulla of the brainstem.
- The pontine reticulospinal tract arises from those nuclei of reticular formation which are present in pons.
Second Order Neurons
The second-order neurons in case of reticulospinal tract are the internuncial neurons or interneurons of the spinal cord. These are the inhibitory neurons present within the anterior gray matter of the spinal cord.
The medullary as well pontine reticulospinal tracts synapse on these inhibitory neurons. The axons of these inhibitory neurons in turn synapse onto the alpha and gamma motor neurons.
Third Order Neurons
The third-order neurons are the alpha and gamma motor neurons present in the anterior gray matter of the spinal cord. These motor neurons directly supply the muscles of the body. These motor neurons can be excited or inhibited by the impulses coming from the higher centers of the brain. In some cases, the local response originated in the spinal cord can also control the activity of these motor neurons.
Upper Motor Neurons
The upper motor neurons for this tract are the neurons present in the three columns of reticular formation, medial, middle and lateral columns.
- The upper motor neurons of the medullary tract are present in the medulla.
- The upper motor neurons of the pontine tract are present in the pons.
Lower Motor Neurons
The lower motor neurons in case of the medullary as well as the pontine reticulospinal tracts are the alpha and gamma motor neurons present in the anterior gray matter of the spinal cord.
Pathway of Reticulospinal Tract
As we have studied the anatomical organization of the reticulospinal tract, we can now easily understand the pathway followed by the reticulospinal tract.
Pontine Reticulospinal Tract
The reticular formation in the pons gives rise to the pontine reticulospinal tract.
These fibers descend uncrossed through the medulla.
After passing through the medulla, the fibers of the pontine reticulospinal tract enter the anterior white column of the spinal cord.
These fibers descend uncrossed throughout the spinal cord.
The pontine reticulospinal tract terminates by entering the anterior gray column of the spinal cord and synapsing with the second-order neurons.
Medullary Reticulospinal Tract
The reticular formation in medulla gives rise to the medullary reticulospinal tract fibers.
One half of these fibers cross to the opposite side while the other half descends through the medulla uncrossed.
After passing the medulla, both the uncrossed fibers enter the lateral white column of the spinal cord present on the same side.
On the other hand, the crossed fibers enter the lateral white column of the opposite half of the spinal cord.
Both the crossed as well as uncrossed fibers travel throughout the spinal cord and terminate by synapsing with the third-order neurons.
The fibers of the medullary as well as pontine reticulospinal tracts enter the anterior gray column of the spinal cord. Here, these fibers synapse with the alpha and gamma motor neurons. These tracts terminate by synapsing with the third-order neurons present in the anterior gray column of the spinal cord.
In this section of the article, we will discuss important physiological functions performed by the reticulospinal tract
Control of Motor Activity
The reticulospinal tract is considered to be one of the most important extra-pyramidal tracts for controlling the activity of lower motor neurons. It can influence the activities of the alpha and gamma motor neurons through internuncial neurons. These internuncial neurons are the inhibitory neurons.
These neurons inhibit the activity of alpha and gamma motor neurons. The reticulospinal tract can inhibit or excite these internuncial neurons. Thus, the fibers of the reticulospinal tract can inhibit or stimulate motor activity.
Control of Autonomic Functions
The reticulospinal tract also consists of the descending autonomic fibers. These autonomic fibers are responsible for controlling some autonomic functions such as heart rate, circulation, breathing, respiratory rate, etc.
Control of Sympathetic and Parasympathetic Outflow
The autonomic fibers in the reticulospinal tract also control the sympathetic outflow as well as the sacral parasympathetic outflow.
The reticulospinal tract is essential for maintaining the posture of the body.
Assists in case of Corticospinal tract lesions
This is the major benefit of the reticulospinal tract. Although it is considered to be an inhibitory pathway, the reticulospinal tract also contains some excitatory fibers. It can assist in the contraction of the upper limb following a lesion of the corticospinal tract. This important feature of the reticulospinal tract helps in early recovery of upper limbs after lesions of the corticospinal tract.
In this section, we will study some pathologies or lesions of the reticulospinal tract.
Lesions of the reticulospinal tract will affect all the functions performed by this tract. The major sign of reticulospinal tract lesion is postural abnormality. Once the reticulospinal tract is affected, the person is unable to control the motor activities. This results in an abnormal posture of the body.
Hypotonia and Hypertonia
We already know that the reticulospinal tract includes both inhibitory as well as excitatory fibers. If the lesions affect the inhibitory fibers, this will result in hypertonia and continuous contraction of muscles of the body. On the other hand, if the lesions affect only the excitatory fibers, this will result in hypotonia and flaccidity.
Reticulospinal tract is a descending tract present in the white matter of the spinal cord.
On the basis of its origin, it comprises of two tracts;
- Medullary Reticulospinal Tract
- Pontine Reticulospinal Tract
The first-order neurons of both of these tracts are present in the reticular formation of medulla and pons, respectively.
The second-order neurons are the internuncial neurons of the spinal cord.
The third-order neurons are the alpha and gamma motor neurons, the lower motor neurons.
The fibers of the pontine reticulospinal tract descend uncrossed.
Some of the fibers of the medullary reticulospinal tract cross to the opposite side while other descend uncrossed.
These tracts terminate by entering the anterior gray column of the spinal cord and synapsing with the internuncial neurons.
The reticulospinal tract contains both excitatory as well as inhibitory fibers. The functions performed by the reticulospinal tract include;
- Control of Motor Activity
- Control of Autonomic Functions
- Control of Sympathetic as well as Parasympathetic outflow
- Postural Maintenance
- Upper limb Recovery after Lesion of Corticospinal Tract
The lesions of the reticulospinal tract can result in;
- Postural abnormalities