The vestibulospinal tract is a nerve tract in the central nervous system. Specifically, it is a component of the extrapyramidal system and is classified as a component of the medial pathway. Like other descending motor pathways, the vestibulospinal fibers of the tract relay information from nuclei to motor neurons. 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. 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.
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, such as corticospinal and some corticobulbar tracts, may directly innervate motor neurons of the spinal cord or brainstem. This is seen in anterior (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. Motor control from both the pyramidal and extrapyramidal systems have extensive feedback loops and are heavily interconnected with each other. 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. The vestibulospinal tract, as well as tectospinal and reticulospinal tracts are examples of components of the medial pathway.
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 is an upper motor neuron tract consisting of two sub-pathways:
The medial vestibulospinal tract projects bilaterally from the medial vestibular nucleus within the medial longitudinal fasciculus to the ventral horns in the upper cervical cord (C6 vertebra). It promotes stabilization of head position by innervating the neck muscles, which helps with head coordination and eye movement. Its function is similar to that of the tectospinal tract. The lateral vestibulospinal tract 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.
Anatomy
Lateral vestibulospinal tract The lateral vestibulospinal tract is a group of descending extrapyramidal motor neurons, or efferent nerve fibers. This tract is found in the lateral funiculus, a bundle of nerve roots in the spinal cord. The lateral vestibulospinal tract originates in the lateral vestibular nucleus (Deiters’ nucleus) in the pons. Deiters' nucleus extends from the pontomedullary junction to the level of the 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 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 in 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.
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 canals, otolith, or both. The term vestibulospinal 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.
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