Reticulospinal tract
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The reticulospinal tracts, also known as the 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 descending 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 delicate manipulations.[1] 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 divided into two parts, the medial (or pontine) and lateral (or medullary) reticulospinal tracts (MRST and LRST).
Medial reticulospinal tract[edit]
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The medial reticulospinal tract is responsible for exciting anti-gravity, extensor muscles. The fibers of this tract arise from the caudal pontine reticular nucleus and the oral pontine reticular nucleus and project to lamina VII and lamina VIII of the spinal cord.
Lateral reticulospinal tract[edit]
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The lateral reticulospinal tract is responsible for inhibiting excitatory axial extensor muscles of movement. It is also responsible for automatic breathing. The fibers of this tract arise from the medullary reticular formation, mostly from the gigantocellular nucleus, and descend the length of the spinal cord in the anterior part of the lateral column. The tract terminates in lamina VII mostly with some fibers terminating in lamina IX of the spinal cord.
The ascending sensory tract conveying information in the opposite direction is known as the spinoreticular tract.
Function[edit]
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- Integrates information from the motor systems to coordinate automatic movements of locomotion and posture
- Facilitates and inhibits voluntary movement; influences muscle tone
- Mediates autonomic functions
- Modulates pain impulses
- Influences blood flow to lateral geniculate nucleus of the thalamus.[3]
Clinical significance[edit]
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]
References[edit]
- ^ a b c Squire L (2013). Fundamental neuroscience (4th ed.). Amsterdam: Elsevier/Academic Press. pp. 631–632. ISBN 978-0123858702.
- ^ FitzGerald MT, Gruener G, Mtui E (2012). Clinical Neuroanatomy and Neuroscience. Philadelphia: Saunders Elsevier. p. 192. ISBN 978-0702037382.
- ^ Brownstone, Robert M.; Chopek, Jeremy W. (2018). "Reticulospinal Systems for Tuning Motor Commands". Frontiers in Neural Circuits. 12: 30. doi:10.3389/fncir.2018.00030. ISSN 1662-5110. PMC 5915564. PMID 29720934.
- ^ Pearson, Keir G; Gordon, James E (2013). "Chapter 41 / Posture". In Kandel, Eric R; Schwartz, James H; Jessell, Thomas M; Siegelbaum, Steven A; Hudspeth, AJ (eds.). Principles of Neural Science (5th ed.). United States: McGraw-Hill. The Brain Stem and Cerebellum Integrate Sensory Signals for Posture, p. 954. ISBN 978-0071390118.
- ^ a b c Michael-Titus et al (2010b), Box 9.5 Decorticate and decrebrate regidity, p. 172
