Vestibulospinal tracts are descending tracts present in the spinal cord. Like other descending tracts, it is a bundle of nerve fibers that carry information from the higher centers of the brain to the peripheral parts of the body.
It begins with the axons of the vestibular nuclei and terminates by synapsing with the internuncial neurons present in the anterior gray column of the spinal cord. The vestibulospinal tracts consist of a medial vestibulospinal tract and a lateral vestibulospinal tract.
The vestibulospinal tracts are important pathways by which the balance of the body is maintained by higher centers of the brain. They are essential for a number of reflex actions performed by the body. In this article, we will study the location of these tracts in the spinal cord, their initiation, course, and finally their termination.
We will also discuss the major functions performed or assisted by the vestibulospinal tracts. We will conclude our discussion by studying important diseases that affect the functions performed by the vestibulospinal tracts. So, keep reading.
In this section, we will discuss four major anatomical aspects of the vestibulospinal tract; location, anatomical organization, pathway, and termination.
Lateral Vestibulospinal Tract
The lateral vestibulospinal tract is a descending tract present in the white matter of the spinal cord. It occupies the anterior-most part of the white matter. It is present in the anterior white column of the spinal cord just in front of the motor neurons.
In the anterior white column, it is sandwiched between the olivospinal tract and the tectospinal tract. One lateral vestibulospinal tract is present in each half of the spinal cord.
Like all other descending tracts, the anatomical organization of the lateral vestibulospinal tract is discussed under three heading; first-order neurons, second-order neurons, and third-order neurons. We will discuss all these organizational levels in order to have a proper understanding of the vestibulospinal tract.
First Order Neurons
We already know that the first-order neurons have their cell bodies located in the brain. In case of the lateral vestibulospinal tract, the first-order neurons are the vestibular nuclei. The vestibular nuclei are present in the brainstem. Four vestibular nuclei are present in each half of the brainstem. They extend to both pons and medulla oblongata; the two parts of the brainstem.
The vestibular nuclei receive input fibers from the vestibular nerve, that carries information from the inner ear, and from the cerebellum. The axons of only the lateral vestibular nucleus give rise to the lateral vestibulospinal tract. These axons contribute to most of the length of the lateral vestibulospinal tract.
Second Order Neurons
The internuncial neurons of the spinal cord act as second-order neurons of the lateral vestibulospinal tract. They receive input fibers in the form of the axons of the lateral vestibular nucleus. The output fibers of these second-order neurons are very shot and immediately terminate by synapsing with the third-order neurons, the motor neurons present in the immediate vicinity of the internuncial neurons.
Third Order Neurons
The motor neurons present in the anterior gray column of the spinal cord act as third-order neurons. They receive input fibers from the internuncial neurons and send axons that pass through the spinal nerve and innervate the skeletal muscles of the body.
Upper Motor Neurons
The upper motor neurons in case of the lateral vestibulospinal tract are the vestibular nuclei.
Lower Motor Neurons
The lower motor neurons in case of the lateral vestibulospinal tract are the motor neurons of the anterior gray column.
Pathway of Vestibulospinal Tract
After knowing about the anatomical organization of the vestibulospinal tract, it would be easy for us the understand the pathway followed by the vestibulospinal tract.
The vestibulospinal tract begins as a bundle of axons of the lateral vestibular nucleus within the brainstem.
After leaving the vestibular nucleus, the lateral vestibulospinal tract descends through the medulla oblongata without decussation.
After passing through the medulla oblongata, the lateral vestibulospinal tracts enter the spinal cord. Each tract stays on its side of origination and descends in the anterior white column of the spinal cord, one in each half of the spinal cord.
The pathway ends when the fibers of the lateral vestibulospinal tract synapse with the internuncial neurons.
The lateral vestibulospinal tract terminates by synapsing with the internuncial neurons of the spinal cord, present at each level of the spinal cord. These fibers even reach the lowermost segments of the spinal cord.
Medial Vestibulospinal Tract
In addition to the lateral vestibulospinal tract, another spinal tract also originates from the vestibular nuclei. It is known as the medial vestibulospinal tract as it originates from the medial vestibular nucleus.
It is a relatively very small tract that is hardly visible in the sections of the spinal cord. It descends as a component of the medial longitudinal fasciculus. It terminates by synapsing with the motor neurons present in the cervical segment of the spinal cord.
Although anatomically it is a very small tract that is not even visible independently, the functions performed by the medial vestibulospinal tract are very important.
In this section, we will discuss the important functions of the vestibulospinal tracts.
Head and Eye Coordination
This is a very important function performed by the medial vestibulospinal tract. It synchronizes the movement of the eyes with the movement of the head so that eyes do not lag behind when the head moves to one side. This function is very important for maintaining the balance of the body.
Upright Posture Maintenance
This function is performed by both the medial and the lateral vestibulospinal tracts. The medial tract supplies the muscles of the head and neck whereas the lateral tract supplies the muscles located in other parts of the body.
When the head of the person moves, the signals are sent by these vestibular tracts to specific antigravity muscles. The extensor muscles in the limbs are called the antigravity muscles. These muscles contract and maintain the upright posture of the body.
The vestibulospinal tracts are also responsible for carrying out the vestibulospinal reflexes. A vestibulospinal reflex is the one that uses organs of the vestibular system and the skeletal muscles in order to maintain balance and posture. Take an example of the vestibulospinal reflex when the head is tilted.
When the head tilts, the vestibular organs sense the movement of the head and send impulses to the vestibular nuclei via the vestibular nerve. The vestibular nuclei generate a motor response that is carried by the medial and lateral vestibulospinal tracts to muscles of the neck and other body parts.
This response results in muscle contraction on the side of the neck to which the head bends, and relaxation of muscles on the opposite side.
The vestibulospinal tracts can be injured or damaged by any trauma or disease process which involves the spinal cord. Various reasons that can cause damage to these tracts include;
- Hemisection of Spinal Cord
- Complete Cord Transection
- Anterior Cord Syndrome
Damage to the vestibulospinal tracts results in following clinical conditions;
- Ataxia, the person is unable to maintain his balance, especially when the eyes are closed. It is due to the loss of vestibulospinal reflexes.
- Postural Instability, the person is unable to maintain an upright posture. The patient will not be able to maintain posture while walking and will fall on the affected side.
- Damage to the medial vestibulospinal tract will result in nystagmus.
Vestibulospinal tracts are the descending tracts that originate from the vestibular nuclei of the brainstem. They consist of a medial tract and a lateral tract.
The medial vestibulospinal tract arises from the medial vestibular nucleus.
It descends on the ipsilateral side of the spinal cord.
It terminates by synapsing with the motor neurons in the cervical segments of the spinal cord.
The major function performed by the medial vestibulospinal tract is to synchronize the eye movements with the movement of the eyes.
It also helps in maintaining balance and posture when the head is tilted to one side.
The lateral vestibulospinal tract is the major tract that descends throughout the spinal cord.
This lateral tract originates from the lateral vestibular nuclei, the first-order neurons.
The axons of these neurons form the lateral vestibulospinal tract that descends through medulla oblongata and the spinal cord without decussation.
The lateral tract terminates by synapsing with the internuncial neurons in the spinal cord, the second-order neurons.
The alpha and gamma motor neurons are the third-order neurons that are regulated by the second-order neurons.
The important function performed by the lateral vestibulospinal tract includes postural stability and maintenance of balance.
This tract is also responsible for a number of vestibulospinal reflexes.
The vestibulospinal tracts can be damaged by injury or diseases of the spinal cord.
The major clinical condition that results from damage to these tracts include the following;
- Postural Instability
- Martini, Frederic (2010). Anatomy & Physiology. Benjamin Cummings. ISBN 978-0-321-59713-7.
- Afifi, Adel (1998). Functional Neuroanatomy. McGraw Hill. ISBN 978-0-07-001589-0.
- “Motor Systems”. Retrieved 2 November 2011.
- Voron, Stephen. “The Vestibular System”. University of Utah School of Medicine. Retrieved 1 November 2011.
- Miselis, Dr. Richard. “Laboratory 12 : Tract Systems I”. University of Pennsylvania School of Veterinary Medicine. Retrieved 1 November 2011.
- “VESTIBULAR NUCLEI AND ABDUCENS NUCLEUS”. Medical Neurosciences University of Wisconsin. Archived from the original on November 9, 2011. Retrieved 1 November 2011.
- Bono, Christopher (2010). Spinal Cord Medicine. Demos Medical Publishing. ISBN 978-1-933864-19-8.
- Hain, Timothy. “Postural, Vestibulospinal and Vestibulocollic Reflexes”. Retrieved 1 November 2011.