File Name: ascending and descending tracts of spinal cord .zip
- Spinal cord
- Descending spinal tracts
- 14.5 Sensory and Motor Pathways
- The Descending Tracts of the Central Nervous System
The descending tracts are the pathways by which motor signals are sent from the brain to lower motor neurones. The lower motor neurones then directly innervate muscles to produce movement. There are no synapses within the descending pathways.
NCBI Bookshelf. The spinal cord is a vital aspect of the central nervous system housed in the spinal column. Its purpose is to send motor commands from the brain to the body and sensory information from the body to the brain, and coordinate reflexes. The spinal cord organizes segmentally with thirty-one pairs of spinal nerves emanating from it.
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Having focussed on the ascending sensory tracts of the spinal cord , this article will discuss the descending motor tracts. Any information in the tables presented in these articles is essential information for clinical medicine — we build on that information in the text.
Descending tracts carry motor information in efferent nerves from upper motor neurons of cortical structures like the cerebellum and cerebrum. The descending tracts transmit this information to lower motor neurons, allowing it to reach muscles. This will become particularly relevant during the discussion of pathologies that present in the motor pathways. There are many motor tracts in the spinal cord. Some of these are under conscious control and others under unconscious, reflexive or responsive control.
These motor tracts can be grouped functionally into pyramidal and extrapyramidal tracts. The pyramidal tracts are named as such due to their course through the pyramids of the medulla oblongata. The pyramidal tracts are responsible for the conscious, voluntary control of the body and face muscles.
The fourth cortical area the CST communicates with is the posterior parietal cortex for integration with and modulation of incoming sensory information. Neurons exiting the cerebral cortex in one of the three major regions above converge to form the white matter structure in the brain known as the internal capsule.
The internal capsule is located between the basal ganglia and thalamus; two highly vascularised structures in the deep brain.
After passing through the internal capsule, the fibres continue to pass down through the centre of the crus cerebri of the midbrain, before entering the pons and medulla. As the CST passes through the caudal medulla, it divides into the lateral and anterior corticospinal tracts:. These tracts then descend into the spinal cord, terminating in the ventral horn of the spinal cord where they synapse onto LMNs to supply the peripheral musculature. The anterior CST remains ipsilateral and descends only to the cervical and upper thoracic spinal cord, where they decussate at the level of the nerve root they supply.
Arising from the lateral aspect of the primary motor cortex the cephalic region of the motor homunculus , the CBTs receive mostly the same inputs as the CSTs. They follow a similar path but terminate in the brainstem at the motor nuclei rather than continuing down to the spinal cord.
In the brainstem, they synapse on the cranial nerve motor nuclei, which are LMN structures that supply the head and neck muscles. It is important to understand the clinical implication of damage to the CBTs. This is the case for all head and neck muscle cranial nerve nuclei except:.
The extrapyramidal tracts all originate in the brainstem and do not pass through the pyramids. These tracts all carry motor fibres to the spinal cord that allow for unconscious, reflexive or responsive movement of muscles to control balance, locomotion, posture and tone.
The rubrospinal tract begins in the red nucleus, where fibres immediately decussate and descend through the pons and medulla and into the spinal cord. It is thought that the rubrospinal tracts supply upper limb flexors as well as trunk flexors. Disinhibition of the rubrospinal tract leads to upper limb flexion. The tectospinal tract begins in the tectum, or roof of the midbrain, where the superior and inferior colliculi are located.
Collectively, the two superior and two inferior colliculi are referred to as corpora quadrigemina. Together, the colliculi send information about sights and sounds to the tectospinal tract, which decussates soon after leaving these structures, to supply muscles of the head and neck for reflexive localisation of these stimuli. These findings are shown in the table below:. You will notice that UMN lesions present with hypertonia and spastic paralysis, whereas LMN lesions are usually associated with hypotonia and flaccid paralysis.
This is because of the impaired ability for motor neurons to regulate descending signals, giving rise to disordered spinal reflexes. To understand more, we must integrate what we know about the central nervous system: as much as it is involved with activating pathways, it can also suppress pathway activity. That is, the corticospinal tract also helps in conscious inhibition of muscle lack of the signal.
If we sever the UMNs of the corticospinal tract, there is a loss of inhibitory tone of muscles. The effect of this is two-fold on a simple level; there is much more to it! This leads to the hypertonia and spastic paralysis we see in UMNs. If LMNs are damaged or lost, there is nothing to tell the muscles to contract, with resultant hypotonia and flaccid paralysis.
We have discussed that the upper half of the face receives a bilateral cortical supply, whereas the lower half of the face receives contralateral cortical supply only. The paralysis in an UMN facial lesion will classically be spastic. The paralysis of a LMN facial lesion is flaccid. Now that we understand the rubrospinal tract and the role it plays in adjusting flexor tone in the upper limb, we can discuss decorticate versus decerebrate posturing.
Both types of posture involve lower limb extension. Decerebrate posturing refers to an adopted posture of upper limb extension:. Decorticate posturing refers to an adopted position of upper limb flexion:. Clinical Examination. Heart Murmurs. Eye Drops Overview. Ankle X-ray Interpretation. Shoulder X-ray Interpretation. How to Perform a Literature Search. Evaluation of Haematuria. A collection of surgery revision notes covering key surgical topics. A man with tight foreskin. Laryngeal Cancer.
Muscles of the Foot. Small Intestine. Visual Pathway and Visual Field Defects. A man with penile swelling. PSA Question Bank. Medical Student Finals Question Bank. ABG Quiz. Share Tweet. Last updated: February 7, Table of Contents. These functional groups contain several anatomical tracts, one for each side of the body: Pyramidal : conscious control of muscles from the cerebral cortex to the muscles of the body and face; and Extrapyramidal : unconscious , reflexive or responsive control of muscles from various brainstem structures to postural or anti-gravity muscles Pyramidal tracts The pyramidal tracts are named as such due to their course through the pyramids of the medulla oblongata.
As the CST passes through the caudal medulla, it divides into the lateral and anterior corticospinal tracts: Lateral CST : decussate in the pyramid of the medulla Anterior CST : stay ipsilateral These tracts then descend into the spinal cord, terminating in the ventral horn of the spinal cord where they synapse onto LMNs to supply the peripheral musculature.
There are two reticulospinal tracts: Medial reticulospinal tract: originates in the pons and contributes to voluntary movements and increases in muscle tone in response to alerting or activating stimuli that stimulate the reticular activating system; Lateral reticulospinal tract: originates in the medulla and contributes to inhibition of voluntary movements , and also reduces muscle tone.
Vestibulospinal tracts The vestibulospinal tracts do not decussate. There are two vestibulospinal tracts that control anti-gravity muscles via LMNs: Medial vestibulospinal tract: originates in the medial vestibular nucleus, to control ipsilateral postural and tone adjustments in response to the vestibular apparatus.
Lateral vestibulospinal tract: originates in the lateral, superior and inferior vestibular nuclei, to control ipsilateral postural and tone adjustments in response to the vestibular apparatus. Rubrospinal tract The rubrospinal tract decussates. Inhibition of the rubrospinal tract leads to upper limb extension. Tectospinal tract The tectospinal tract decussates. The superior colliculus is involved in reflexive responses to visual stimuli.
The inferior colliculus is involved in reflexive responses to auditory stimuli. Clinical relevance — decerebrate and decorticate posturing Now that we understand the rubrospinal tract and the role it plays in adjusting flexor tone in the upper limb, we can discuss decorticate versus decerebrate posturing. Decerebrate posturing refers to an adopted posture of upper limb extension: This occurs when a lesion below the red nucleus prevents the red nucleus from activating the upper limb flexors, resulting in upper limb extension.
This results in upper limb flexion in decorticate posturing for lesions aove the red nucleus. In decerebrate posturing, the loss of the rubrospinal tract causes the lateral reticulospinal tract to be overwhelmed by the other extrapyramidal tracts, resulting in upper limb extension.
Disruption of the lateral corticospinal tract allows the medial reticulospinal and medial and lateral vestibulospinal tracts of the lower limbs to overcome the input from the lateral reticulospinal tract.
This results in lower limb extension in both decorticate or decerebrate posturing. Join the community. See all results.
Descending spinal tracts
Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior dorsal horn, or even the anterior ventral horn, at the level of the spinal cord where they enter. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch may also turn into the posterior dorsal column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways.
14.5 Sensory and Motor Pathways
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The spinal cord is a long, thin, tubular structure made up of nervous tissue , which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column.
The Descending Tracts of the Central Nervous System
Tracts descending to the spinal cord are involved with voluntary motor function, muscle tone, reflexes and equilibrium , visceral innervation, and modulation of ascending sensory signals. The largest, the corticospinal tract, originates in broad regions of the cerebral cortex. Smaller descending tracts, which include the rubrospinal tract, the vestibulospinal tract, and the reticulospinal tract, originate in nuclei in the midbrain, pons , and medulla oblongata. Most of these brainstem nuclei themselves receive input from the cerebral cortex, the cerebellar cortex, deep nuclei of the cerebellum, or some combination of these. In addition, autonomic tracts, which descend from various nuclei in the brainstem to preganglionic sympathetic and parasympathetic neurons in the spinal cord, constitute a vital link between the centres that regulate visceral functions and the nerve cells that actually effect changes.
Nachum Dafny, Ph. Figure 3. The spinal cord is the most important structure between the body and the brain. The spinal cord extends from the foramen magnum where it is continuous with the medulla to the level of the first or second lumbar vertebrae. It is a vital link between the brain and the body, and from the body to the brain.
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