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The reticulospinal tracts (also known as descending or anterior reticulospinal tracts) are extrapyramidal motor tracts that descend from the reticular formation [1] in two tracts to act on the motor neurons supplying the trunk and proximal limb flexors and extensors. The reticulospinal tracts are involved mainly in locomotion and postural control, although they do have other functions as well. [2]
The reticulospinal tracts are one of four major cortical pathways to the spinal cord for musculoskeletal activity. The reticulospinal tracts work with the other three pathways to give a coordinated control of movement, including fine movement. The four pathways can be grouped into two main system pathways – a medial system and a lateral system. The medial system includes the reticulospinal pathway and the vestibulospinal pathway, and this system provides control of posture. The corticospinal and the rubrospinal tract pathways belong to the lateral system which provides fine control of movement. [1]
This descending tract is consists of two parts: the medial (or pontine) and lateral (or medullary) reticulospinal tracts.
The reticulospinal tracts are involved in coordinating the activity of skeletal muscle (including tone and reflexes) by influencing the alpha and gamma motor neurons that innervate them. The tracts are involved in reciprocal inhibition of antagonist muscles: the tract ensure that contraction of specific flexors is accompanied by relaxation of their antagonist extensors - and vice versa. In collaboration with the lateral vestibulospinal tract, the tracts participate in maintaining balance and adjusting posture: this - as well as maintaining muscle tone - is a pre-requisite enabling voluntary movements to be performed properly. [3]
The neurons giving rise to the reticulospinal tracts receive (and integrate) afferents from the premotor cortex, supplementary motor area, basal nuclei, cerebellum, substantia nigra, and red nucleus, as well as from other areas of the reticular formation. [3]
The medial reticulospinal tract or pontine reticulospinal tract arises from the pontine reticular formation, which consists of the oral and caudal pontine reticular nuclei. The tract descends through the ipsilateral anterior funiculus of the spinal cord, projecting to all levels of the spinal cord. [3] It terminates in the laminae VII and VIII of the spinal cord.[ citation needed ]
It terminates by synapsing with interneurons and gamma motor neurons to exciting extensors and inhibiting flexors of the axial and proximal limb musculature (opposing the effects of the lateral reticulospinal tract). Many axons of this tract synapse with reflex propriospinal interneurons that in turn synapse with motor neurons of these muscles. A few axons also form inhibitory synapses with first-order sensory neurons innervating muscle spindles, thus inhibiting stretch reflex to enable smoother voluntary movements. [3]
The lateral reticulospinal tract or medullary reticulospinal tract arise from the medullary reticular formation (which consists of the gigantocellular and ventral reticular nuclei [3] - with the greatest contribution from the gigantocelular nucleus[ citation needed ]). [3] In the spinal cord, it descends mostly through the ipsilateral [3] anterior part of the lateral funiculus.[ citation needed ] It projects to all levels of the spinal cord. [3] The tract terminates mostly in laminae VII of the spinal cord, with some fibers terminating in laminae IX.[ citation needed ]
The tract terminates primarily by synapsing with the interneuron of the intermediate zone of spinal grey matter. It excites flexors and inhibits extensors of the axial and proximal limb musculature (opposing the effects of the medial reticulospinal tract). Some axons of the tract synapse with lower motor neurons of the distal limb musculature which then counteract the action of lower motor neurons innervating the axial extensors and promote those innervating limb flexors. [3]
The reticulospinal tracts provide a pathway by which the hypothalamus can control sympathetic thoracolumbar outflow and parasympathetic sacral outflow.[ citation needed ]
Two major descending systems carrying signals from the brainstem and cerebellum to the spinal cord can trigger automatic postural response for balance and orientation: vestibulospinal tracts from the vestibular nuclei and reticulospinal tracts from the pons and medulla. Lesions of these tracts result in profound ataxia and postural instability. [4]
Physical or vascular damage to the brainstem disconnecting the red nucleus (midbrain) and the vestibular nuclei (pons) may cause decerebrate rigidity, which has the neurological sign of increased muscle tone and hyperactive stretch reflexes. Responding to a startling or painful stimulus, both arms and legs extend and turn internally. The cause is the tonic activity of lateral vestibulospinal and reticulospinal tracts stimulating extensor motoneurons without the inhibitions from rubrospinal tract. [5]
Brainstem damage above the red nucleus level may cause decorticate rigidity. Responding to a startling or painful stimulus, the arms flex and the legs extend. The cause is the red nucleus, via the rubrospinal tract, counteracting the extensor motorneuron's excitation from the lateral vestibulospinal and reticulospinal tracts. Because the rubrospinal tract only extends to the cervical spinal cord, it mostly acts on the arms by exciting the flexor muscles and inhibiting the extensors, rather than the legs. [5]
Damage to the medulla below the vestibular nuclei may cause flaccid paralysis, hypotonia, loss of respiratory drive, and quadriplegia. There are no reflexes resembling early stages of spinal shock because of complete loss of activity in the motorneurons, as there is no longer any tonic activity arising from the lateral vestibulospinal and reticulospinal tracts. [5]
Articles related to anatomy include:
The brainstem is the stalk-like part of the brain that interconnects the cerebrum and diencephalon with the spinal cord. In the human brain, the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is continuous with the thalamus of the diencephalon through the tentorial notch.
In anatomy, the extrapyramidal system is a part of the motor system network causing involuntary actions. 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 tracts may directly innervate motor neurons of the spinal cord or brainstem, whereas the extrapyramidal system centers on the modulation and regulation of anterior (ventral) horn cells.
The pyramidal tracts include both the corticobulbar tract and the corticospinal tract. These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (corticobulbar) or spinal cord (corticospinal) and are involved in the control of motor functions of the body.
The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cortex of the postcentral gyrus.
The medial longitudinal fasciculus (MLF) is a prominent bundle of nerve fibres which pass within the ventral/anterior portion of periaqueductal gray of the mesencephalon (midbrain). It contains the interstitial nucleus of Cajal, responsible for oculomotor control, head posture, and vertical eye movement.
The reticular formation is a set of interconnected nuclei that are located in the brainstem, hypothalamus, and other regions. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formation make up a complex set of networks in the core of the brainstem that extend from the upper part of the midbrain to the lower part of the medulla oblongata. The reticular formation includes ascending pathways to the cortex in the ascending reticular activating system (ARAS) and descending pathways to the spinal cord via the reticulospinal tracts.
The rubrospinal tract is a part of the nervous system. It is a part of the lateral indirect extrapyramidal tract.
The fastigial nucleus is located in each hemisphere of the cerebellum. It is one of the four deep cerebellar nuclei.
The flocculonodular lobe (vestibulocerebellum) is a small lobe of the cerebellum consisting of the unpaired midline nodule and the two flocculi - one flocculus on either side of the nodule. The lobe is involved in maintaining posture and balance as well as coordinating head-eye movements.
The vestibulospinal tract is a neural 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.
Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.
The gigantocellular reticular nucleus is the (efferent/motor) medial zone of the reticular formation of the caudal pons and rostral medulla oblongata. It consists of a substantial quality of giant neurons, but also contains small and medium sized neurons.
The spinoreticular tract is a partially decussating (crossed-over) four-neuron sensory pathway of the central nervous system. The tract transmits slow nociceptive/pain information from the spinal cord to reticular formation which in turn relays the information to the thalamus via reticulothalamic fibers as well as to other parts of the brain. Most (85%) second-order axons arising from sensory C first-order fibers ascend in the spinoreticular tract - it is consequently responsible for transmiting "slow", dull, poorly-localised pain. By projecting to the reticular activating system (RAS), the tract also mediates arousal/alertness in response to noxious stimuli. The tract is phylogenetically older than the spinothalamic ("neospinothalamic") tract.
The medial vestibulospinal tract is one of the descending spinal tracts of the ventromedial funiculus of the spinal cord. It is found only in the cervical spine and above.
The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal cord is hollow and contains a structure called the central canal, which contains cerebrospinal fluid. The spinal cord is also covered by meninges and enclosed by the neural arches. Together, the brain and spinal cord make up the central nervous system.
Blocq's disease was first considered by Paul Blocq (1860–1896), who described this phenomenon as the loss of memory of specialized movements causing the inability to maintain an upright posture, despite normal function of the legs in the bed. The patient is able to stand up, but as soon as the feet are on the ground, the patient cannot hold himself upright nor walk; however when lying down, the subject conserved the integrity of muscular force and the precision of movements of the lower limbs. The motivation of this study came when a fellow student Georges Marinesco (1864) and Paul published a case of parkinsonian tremor (1893) due to a tumor located in the substantia nigra.
A spinal interneuron, found in the spinal cord, relays signals between (afferent) sensory neurons, and (efferent) motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. Most interneurons are found in the grey column, a region of grey matter in the spinal cord.
The raphespinal tract is an unmyelinated descending serotonergic tract involved in pain modulation. It is a descending pain-inhibiting pathway; it is a component of the reticulospinal tract.