| Vestibulospinal tract | |
|---|---|
 Vestibulospinal tract is labeled, in red at bottom left. | |
 Diagram of the principal fasciculi of the spinal cord. (Vestibulospinal fasciculus labeled at bottom right.) | |
| Details | |
| Identifiers | |
| Latin | tractus vestibulospinalis |
| NeuroLex ID | birnlex_1643 |
| FMA | 72646 |
| Anatomical terms of neuroanatomy | |
Thevestibulospinal tract is anerve tract in thecentral nervous system. Specifically, it is a component of theextrapyramidal system and is classified as a component of the medial pathway. Like other descending motor pathways, the vestibulospinal fibers of the tract relay information fromnuclei to motor neurons.[1] Thevestibular nuclei receive information through thevestibulocochlear nerve about changes in the orientation of the head. The nuclei relay motor commands through the vestibulospinal tract. The function of these motor commands is to alter muscle tone, extend, and change the position of the limbs and head with the goal of supporting posture and maintaining balance of the body and head.[1]
The vestibulospinal tract is part of the "extrapyramidal system" of the central nervous system. Inhuman anatomy, the extrapyramidal system is aneural network located in thebrain that is part of themotor system involved in the coordination of movement.[2] 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 themedulla. Thepyramidal pathways, such ascorticospinal and somecorticobulbar tracts, may directly innervate motor neurons of the spinal cord or brainstem. This is seen inanterior (ventral) horn cells or certain cranial nerve nuclei. Whereas the extrapyramidal system centers around the modulation and regulation through 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.[3]
Motor control from both the pyramidal and extrapyramidal systems have extensive feedback loops and are heavily interconnected with each other.[1] An appropriate classification of motor nuclei and tracts would be by their functions. When broken down by function there are two major pathways: medial and lateral. 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.[1] The vestibulospinal tract, as well astectospinal andreticulospinal tracts are examples of components of the medial pathway.[1]
The vestibulospinal tract is part of thevestibular system in theCNS. 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 thelabyrinth of the inner ear. Then the nuclei receiving these signals project out to theextraocular muscles, spinal cord, and cerebral cortex to execute these functions.[4]
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 is anupper motor neuron tract consisting of two sub-pathways:
| Spinal cord | |
|---|---|
 | |
| Details | |
| Identifiers | |
| Latin | medulla spinalis |
| NeuroLex ID | birnlex_1643 |
| FMA | 72646 |
| Anatomical terminology | |
The lateral vestibulospinal tract is a group of descending extrapyramidal motor neurons, orefferent nerve fibers.[2] This tract is found in thelateral funiculus, a bundle ofnerve roots in thespinal cord. Thelateral vestibulospinal tract originates in thelateral vestibular nucleus (Deiters’ nucleus) in thepons.[2] Deiters' nucleus extends from thepontomedullary junction to the level of theabducens nerve nucleus in thepons.[2]
Lateral vestibulospinal fibers descend uncrossed, oripsilateral, in the anterior portion of thelateral funiculus of the spinal cord.[2][7] Fibers run down the total length of the spinal cord and terminate at theinterneurons of laminae VII and VIII. Additionally, some neurons terminate directly on the dendrites ofalpha motor neurons in the same laminae.[2]
The medial vestibulospinal tract is a group of descending extrapyramidal motor neurons, or efferent fibers found in theanterior funiculus, a bundle of nerve roots in the spinal cord. The medial vestibulospinal tract originates in themedial vestibular nucleus or Schwalbe's nucleus.[2] The Schwalbe's nucleus extends from the rostral end of theinferior olivary nucleus of the medulla oblongata to the caudal portion of the pons.[2]
Medial vestibulospinal fibers join with the ipsilateral andcontralateralmedial longitudinal fasciculus, and descend in the anterior funiculus of the spinal cord.[2][7] Fibers run down to the anterior funiculus to the cervical spinal cord segments and terminate onneurons 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.[2]
The vestibulospinalreflex 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 eithercanals,otolith, or both. The term vestibulospinal reflex, is most commonly used when the sensory input evokes a response from themuscular system below the neck. These reflexes are important in the maintenance ofhomeostasis.[8]
Thetonic labyrinthine reflex (TLR) is a reflex that is present in newborn babies directly after birth and should be fully inhibited by 3.5 years.[9] This reflex helps the baby master head and neck movements outside of thewomb as well as the concept ofgravity. Increased muscle tone, development of theproprioceptive 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.[10]
Thetonic labyrinthine reflex is found in two forms.
Therighting 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 thecat righting reflex, which allows them to orient themselves in order to land on their feet. This reflex is initiated by sensory information from the vestibular,visual, and thesomatosensory systems and is therefore not only a vestibulospinal reflex.[8]
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.[7] Generally damage to the vestibulospinal system results inataxia and postural instability.[11] For example, if unilateral damage occurs to thevestibulocochlear nerve,lateral vestibular nucleus,semicircular canals orlateral vestibulospinal tract, the person will likely sway to that side and fall when walking. This occurs because the healthy side "over powers" the weak side in a way that will cause the person to veer and fall towards the injured side.[6] Potential early onset of damage can be witnessed through a positiveRomberg's test.[6] Patients with bilateral or unilateral vestibular system damage will likely regain postural stability over weeks and months through a process called vestibular compensation.[11] This process is likely related to a greater reliance on other sensory information.