User:Sleasmab/Vestibulospinal Tract

Wikipedia Draft: Vestibulospinal Tract

Presented by: Jianan Shi, Stephen Lorenzen, Brian Sleasman

The vestibulospinal tract is a component of the extrapyramidal system and also can be classified as a component of the medial pathway. The vestibulospinal fibers relay information from nuclei to motor neurons, like other descending motor pathways. Specifically, the vestibular nuclei receive information through the vestibulocochlear nerve about changes in the orientation of the head. The nuclei relay motor commands through the vestibulospinal tract regarding alterations of muscle tone, extension and position to the limbs and head with a goal of supporting posture and maintaining balance of the body.

Classification
The Vestibulospinal tract is part of the "extrapyramidal system" of the central nervous system. In human anatomy, the extrapyramidal system is a neural network located in the brain that is part of the motor system involved in the coordination of movement. The system is called "extrapyramidal" to distinguish it from the tracts of the motor cortex that reach their targets by traveling through the "pyramids" of the medulla. The pyramidal pathways (corticospinal and some corticobulbar tracts) may directly innervate motor neurons of the spinal cord or brainstem (anterior (ventral) horn cells or certain cranial nerve nuclei), whereas the extrapyramidal system centers around the modulation and regulation (indirect control) of anterior (ventral) horn cells. The extrapyramidal subcortical nuclei include the substantia nigra, caudate, putamen, globus pallidus, thalamus, red nucleus and subthalamic nucleus.

It has been thought that the extrapyramidal system operated independently of the pyramidal system. However, more recent research has provided a greater understanding of the integration of motor control. Motor control from both the pyramidal and extrapyramidal systems have extensive feedback loops and are heavily interconnected with each other. Motor nuclei and tracts can be classified by their functions. While the medial pathway helps control gross movements of the proximal limbs and trunk. The lateral pathway helps control precise movement of the distal portion of limbs. The vestibulospinal, as well as tectospinal and reticulospinal tracts are examples of components of the medial pathway.

Sub-Pathways
The vestibulospinal tract is an upper motor neuron tract consists of two sub-pathways:


 * Lateral Vestibulospinal Tract
 * The lateral vestibulospinal tract innervates extension, or antigravity, muscles that are responsible for compensating movement of the body.


 * Medial Vestibulospinal Tract
 * The medial vestibulospinal tract innervates neck muscles that are responsible for stabilizing the head.

Lateral Vestibulospinal Tract
The lateral vestibulospinal tract is a group of descending extrapyramidal motor neurons, or efferent fibers. This tract is found in the lateral funiculus, a bundle of nerve roots in the the spinal cord. The lateral vestibulospinal tract originates in the lateral vestibular nucleus or Deiters’ nucleus in the pons. The Deiters' nucleus extends from pontomedullary junction to the level of Abducens nerve nucleus in the pons.

Lateral vestibulospinal fibers descend uncrossed, or ipsilateral, in the anterior portion of the lateral funiculus of the spinal cord. Fibers run down the total length of the spinal cord and terminate at the interneurons of laminae VII and VIII. Additionally, some neurons terminate directly on the dendrites of alpha motor neurons in the same laminae.

Medial Vestibulospinal Tract
The medial vestibulospinal tract is a group of descending extrapyramidal motor neurons, or efferent fibers found in the anterior funiculus, a bundle of nerve roots in the the spinal cord. The medial vestibulospinal tract originates in the medial vestibular nucleus or Schwalbe's nucleus. The Schwalbe's nucleus extends from the rostral end of the inferior olivary nucleus of the medulla oblongata to the caudal portion of the pons.

Medial vestibulospinal fibers join with the ipsilateral and contralateral medial longitudinal fasciculus, and descend the anterior funiculus of the spinal cord. Fibers run down to the anterior funiculus to the cervical spinal cord segments and terminate on neurons of laminae VII and VIII. Unlike the lateral vestibulospinal tract, the medial vestibulospinal tract innervates muscles that support the head. As a result, medial vestibulospinal fibers run down only to the cervical segments of the cord.

Function
The vestibulospinal tract is part of the vestibular system in the CNS. The primary role of the vestibular system is to maintain head and eye coordination, upright posture and balance, and conscious realization of spatial orientation and motion. The vestibular system is able to respond correctly by recording sensory information from hairs cells in the labyrinth of the inner ear. Then the nuclei receiving these signals project out to the extraocular muscles, spinal cord, and cerebral cortex to execute these functions.

One of these projections, the vestibulospinal tract, is responsible for upright posture and head stabilization. When the vestibular sensory neurons detect small movements of the body, the vestibulospinal tract commands motor signals to specific muscles to counteract these movements and re-stabilize the body. The vestibulospinal tract has two main sections: the medial vestibulospinal tract and the lateral vestibulospinal tract.

The medial spinal tract projects bilaterally from the medial vestibular nucleus within the medial longitudinal fasciculus to the ventral horns in the upper cervical cord (T6 vertebra). It promotes stabilization of head position by innervating the neck muscles, which helps with head coordination and eye movement.

The lateral vestibulospinal tract is an ipsilaterally descending tract from the vestibular system through the anterolateral white matter. It that provides excitatory signals to interneurons, which relay the signal to the motor neurons in antigravity muscles. These antigravity muscles are extensor muscles in the legs that help maintain upright and balanced posture.

Reflexes
The vestibulospinal reflex uses the vestibular organs as well as skeletal muscle in order to maintain balance, posture, and stability in an environment with gravity. These reflexes can be further broken down by timing into a dynamic reflex, static reflex or tonic reflex. It can also be categorized by the sensory input as either canal, otolith, or both. The term vesitbulospinal reflex, is most commonly used when the sensory input evokes a response from the muscular system below the neck. These reflexes are important in the maintenance of homeostasis.

Example of Vestibulospinal Reflex

 * 1) The head is tilted to one side which stimulates both the canals and the otoliths
 * 2) This movement stimulates the vestibular nerve as well as the vestibular nucleus.
 * 3) These impulses are transmitted down both the lateral and medial vestibulospinal tracts to the spinal cord.
 * 4) The spinal cord induces extensor effects in the muscle on the side of the neck to which the head is bent, and flexor effects in the muscle in the side of the neck away from the direction of the displaced head.

Tonic Labyrinthine Reflex
The tonic labyrinthine reflex is a reflex that is present in newborn babies directly after birth and should be fully inhibited by 3.5 years. This reflex helps the baby master head and neck movements outside of the womb as well as the concept of gravity. Increase muscle tone, development of the the proprioceptive and vestibular senses and opportunities to practice with balance are all consequences of this reflex. During early childhood, the TLR matures into more developed vestibulospinal reflexes to help with posture, head alignment and balance.

The tonic labyrinthine reflex is found in two forms.
 * 1) Forward: When the head bends forward, the whole body, arms, legs and torso curl together to form the fetal position.
 * 2) Backwards: When the head is bent backward, the whole body, arms, legs and torso straighten and extend.

Righting Reflex
The righting reflex is another type of reflex. This reflex positions the head or body back into its "normal" position, in response to a change in head or body position. A common example of this reflex is the cat righting reflex, which allows them orient themselves in order to land on their feet. This reflex is initiated by sensory information from the vestibular, visual, and the somatosensory systems and is therefore not only a vestibulospinal reflex.

Central Nervous System Development
During the gastrulation stage of vertibral development, the blastula divides into three distinct germ layers. These three layers are the endoderm, mesoderm, and ectoderm. The ectoderm is the outermost of these layers and eventually becomes the nervous system, the epidermis, and the lining of various external orifices. The next stage in the development of the nervous system is neurulation which is the name for organogenisis of the nervous system. This stage begins with the formation of the notochord, a thin layer of mesodermal cells in the most dorsal portion of the embryo. The notochord signals the ectodermal cells above it to form the neural tube. The formation of the neural tube is done specifically by the folding of the neural plate into a circle which is accomplished with the help of the medial and dorsolateral hinge point cells. This sturcture, the neural tube, gives rise to the brain and spinal cord.

Development of the Vestibulospinal Tract During Spinal Cord Formation
When the neural tube begins to form into spinal cord, there are two different plates, the alar and basil plates. These two plates are separated by the sulcus limitans. The alar plate will turn into the dorsal horn, consisting of sensory neurons and the basil plate will turn into the ventral horn consisting of motor neurons. The formation of the ventral horn is achieved by the secretion of sonic hedgehog, or SHH from the notochord during neural tube development. This induces the floor plate to produce more SHH. The increased amount of SHH is what makes the basil plate to from motor neurons. The vestibulospinal tract is one of the four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons.

Injury and Lesions
A typical person sways from side to side when the eyes are closed. This is the result of the vestibulospinal reflex working correctly. When an individual sways to the left side, the left lateral vestibulospinal tract is activated to bring the body back to midline. If the vestibulocochlear nerve, lateral vestibular nucleus, semicircular canals or lateral vestibulospinal tract are damaged, the person will likely sway to that side and fall when walking. Injuries to the lateral vestibulospinal tract are most likely masked by more serious injuries to the lateral corticospinal tract.

Resulting lesions on either the left or right vestibulocochlear nerve, lateral vestibular nucleus, semicircular canals or lateral vestibulospinal tract will cause an imbalance. The healthy side "over powers" the weak side in a way that will cause the the person to veer and fall towards the injured side. Potential early onset of damage can be witnessed through a positive Romberg's test.