Spinal Tracts Flashcards
Describe the anterior spinocerebellar tract
Function: to convey information about whole limb movement and postural stability to the cerebellum which is of particular importance in the coordination and stability of upright gait
Route: The peripheral processes of the axons innervate the Golgi tendon organs of the ipsilateral lower limb and trunk. 1st order neurons of this system are located in the dorsal root ganglion and project to 2nd order neurons in the nucleus dorsalis in the lateral part of the base and neck of the dorsal horn. 2nd order neurons cross in the spinal cord and ascend in the contralateral white column to the pons where they then join the superior cerebellar peduncle (brachium conjunctivum) cross again to the other side and terminate in the vermis of the anterior lobe of the cerebellum.
The anterior spinocerebellar tract crosses first in the spinal cord and then again when it joins the superior cerebellar peduncle. This tract receives input from modulating descending tracts to the lower motor neurons and from reflex flexor arcs. The descending pathways adjust the output from the lower motor neurons independently from the corticospinal tract. The sum of these influences allows the tract to convey information about whole limb movement and postural stability to the cerebellum which is of particular importance in the coordination and stability of upright gait.
Location: 1st order: dorsal root ganglion, 2nd order: nucleus dorsalis, ascend in contralateral white column to the pons
Final Destination: Vermis of anterior lobe of cerebellum
Pathology:
Damage to the anterior spinocerebellar tract results in the loss of nonconscious proprioception and coordination in the lower limb.
Dorsal/ Posterior spinocerebellar tract
Function: Transmit info from muscle spindle and Golgi tendon organs (except head and neck) for non-conscious proprioception
Route: Peripheral 1st order neurones innerv muscle spindles and Golgi tenson organs > project to Clarke’s column (extends C8-L2/3, below L3 travel in fasciculus gracilis to L3 then synapses to Clarke’s column) > Axons ascend ipsilaterally in Clarke’s column (nucleus dorsalis of Clarke) to inferior cerebellar peduncle in medulla > terminate in ipsilateral cerebellar vermis of anterior lobe.
Location: Clarke’s column (C8-L2/3)
Final Destination: Ipsilateral cerebellar vermis of anterior lobe
Descending inputs from the corticospinal tracts modulate incoming proprioceptive information and integrate of all this within Clarke’s column streamlines information going to the cerebellum and filters out superfluous information. Now thought that a large part of the processing of proprioceptive signals related to motor planning and evaluation that was thought to occur in the cerebellum know occurs in Clarkes’ column. With this info the cerebellum is able to coordinate and integrate neural signals controlling movement of the individual lower limb muscles and posture.
Pathology:
Damage to the dorsal spinocerebellar tract results in the loss of nonconscious proprioception and coordination ipsilateral to the lesion.
Rostral Spinocerebellar tract
Function: Unconscious proprioception and coordination in ipsilateral upper limb
Route: The course of this tract is similar to the ventral spinocerebellar tract* except that its afferent axons are from the Golgi tendon organs located in the upper limb. The tract travels ipsilaterally and it enters the cerebellum via the inferior cerebellar peduncle (restiform body).
Damage to this tract results in the loss of nonconscious proprioception and coordination in the ipsilateral upper limb.
Location: 1st order: dorsal root ganglion, 2nd order: nucleus dorsalis, ascend in contralateral white column to the pons
Final Destination: Cerebellum via inferior cerebellar peduncle
* The peripheral processes of the axons innervate the Golgi tendon organs of the ipsilateral lower limb and trunk. 1st order neurons of this system are located in the dorsal root ganglion and project to 2nd order neurons in the nucleus dorsalis in the lateral part of the base and neck of the dorsal horn. 2nd order neurons cross in the spinal cord and ascend in the contralateral white column to the pons where they then join the superior cerebellar peduncle (brachium conjunctivum) cross again to the other side and terminate in the vermis of the anterior lobe of the cerebellum.
The anterior spinocerebellar tract crosses first in the spinal cord and then again when it joins the superior cerebellar peduncle. This tract receives input from modulating descending tracts to the lower motor neurons and from reflex flexor arcs. The descending pathways adjust the output from the lower motor neurons independently from the corticospinal tract. The sum of these influences allows the tract to convey information about whole limb movement and postural stability to the cerebellum which is of particular importance in the coordination and stability of upright gait.
Cuneocerebellar Tract
Function: Unconscious proprioception in the upper limb
Route: Afferent fibres entering the spinal cord above C8 ascend ipsilaterally in the fasciculus cuneatus and project to neurons in the accessory cuneate nucleus, giving rise to the cuneocerebellar tract which is functionally related to the upper limb conveying nonconscious proprioception.
As Clarkes’ column does not extend to the cervical levels, the fibres of the cuneocerebellar tract do not synapse in the spinal cord and travel with the fasciculus cuneatus to the medulla where they synapse in the accessory cuneate nucleus. From here fibres from this tract enter the cerebellum through the inferior cerebellar peduncle (posterior external arcuate fibres) These fibres terminate in the ipsilateral cerebellar cortex.
The processing in the accessory cuneate nucleus is analogous to the processing in Clarke’s column.
Location: Fasciculus cuneatus
Final Destination: Ipsilateral cerebellar cortex
Spinotectal tract
Function: Spinovisual reflexes and brings about movements of the eyes and head toward the source of stimulation – things in the corner of your eye
Route: Axons enter the spinal cord from the posterior root ganglion and travel to the gray matter where they synapse on 2nd order neurons. The 2nd order neurons cross the median plane and ascend as the spinotectal tract in the anterolateral white column close to the spinothalamic tract. After passing through the medulla oblongata and the pons they terminate by synapsing with neurons in the superior colliculus of the midbrain.
Location: Anterolateral white column close to the spinothlamic tract
Final Destination: Contralateral superior colliculus of the midbrain
Label the tracts in the diagram
Spino-olivary Tract
Function: Conveys info to cerebellum from cutaneous and proprioceptive organs, motor learning and muscle memory
Route: Axons contributing to this tract enter the spinal cord from the posterior root ganglion and terminate on 2nd order neurons. The 2nd order neurons cross the median plane and ascend as the spino-olivary tract in the white matter at the junction of the anterior and lateral columns. The axons end by synapsing on 3rd order neurons in the inferior olivary nucleus of the medulla oblongata. The axons of the 3rd order neurons cross the midline and enter the cerebellum through the inferior cerebellar peduncle.
Location: Junction of anterior and lateral columns
Final Destination: Ipsilateral inferior cerebellar peduncle
Dorsal Column/ Medial Lemniscal Pathway
Function: Tactile sensation (vibration, deep touch, 2 point discrimination), conscious proprioception
Route:
Cuneate: 1st order neurones dorsal root ganglia for Pacinian (tactile and vibration) and Meissner’s (touch) corpuscles of skin and proprioceptors ascend ipsilateral spinal cord > 2nd order neurones in ipsilateral nucleus cuneatus in the medulla travel as the internal arcuate fibres > Decussate and Ascend at medial lemniscus > Synapse with 3rd order neurons in contralateral ventral posterolateral nucleus of the thalamus > Terminate in lateral aspect sensorimotor cortex
Gracile: 1st order neurones dorsal root ganglia for Pacinian and Meissner’s corpuscles of skin and proprioceptors ascend ipsilateral spinal cord > 2nd order neurones in ipsilateral nucleus gracilis in the medulla travel as the internal arcuate fibres > Decussate and Ascend at medial lemniscus > Synapse with 3rd order neurons in contralateral ventral posterolateral nucleus of the thalamus > Terminate in medial aspect sensorimotor cortex
Location: Axons from lower thoracic, lumbar, sacral region (gracile T6-T12) located medially, Axons from higher levels (cuneate) more lateral
Final Destination: Gracile: terminates in medial aspect of sensorimotor cortex. Cuneate terms lateral aspect
Pathology:
The patient cannot perceive sensations such as touch or pressure and their movements are poorly co-ordinated and clumsy because of the loss of conscious proprioception of their position in space. Can be seen in demyelinating conditions such as multiple sclerosis
Lesion in cervical region can’t identify object in ipsilateral hand.
- Tabes Dorsalis*
- A neurological disorder seen in neurosyphilis - present with a characteristic loss of discriminative touch, vibration and conscious proprioception in the entire body excluding the head. The underlying pathology is the selective destruction of the posterior column-medial lemniscus pathway.*
Anterolateral / Spinothalamic
Direct Pathway
Function: Pain (location, intensity, quality), temp (lateral portion), Non-discrim touch (anterior portion)
Route: Axons from Ruffini’s corpuscles, nociceptors, thermoreceptors synapse with 1st order neurons in the dorsal root ganglia > Central axons enter spinal cord, branches travel up and down 1-2 spinal segments (tract of Lissauer) > Synapse with 2nd order neurons in nucleus proprius/ proper sensory nucleus* (located in Rexed laminae III and IV of dorsal horn) > Axons decussate via ventral white commissure and enter to contralateral side > Ascend in lateral funiculus > Synapse 3rd order neurons in ventral posterolateral nucleus of thalamus > Project to sensorimotor cortex
*cells from NP extend in to substantia gelatinosa where significant pain modulation occurs
Location: Somatotopically arranged -Fibres from lower body ascend dorsolaterally.
Upper extremities & neck ascend ventromedially
Final Destination: Primary sensory cortex
Pathology:
Sectioning will result in total loss of pain, temp, and simple tactile sens on contralateral side. Loss experienced about 2 levels below due tract of Lisseur in 1st order neurones.
Damage to segment of spinal cord (eg syringomyelia) can result in bilateral loss of pain, temp, and simple tactile sens.
Expanding lesions in grey will affect cervical and thoracic firbres first bc ventromedial
Paleospinothalamic, Spinoreticular, Spinomesencephalic
Indirect Pathway
Paleospinothalamic tract
Route: Axons ascend bilaterally in ventrolateral quadrant of spinal cord and make several synapses in the reticular formation of brainstem. Project to intralaminar thalamic nuclei to finally project to cerebral cortex, esp limbic regions eg cingulate gyrus
Location: dorsal horn and intermediate grey matter
Final Destination: cerebral cortex, esp limbic regions eg cingulate gyrus
Spinoreticular tract
Route: Projections from spinal cord are both crossed and uncrossed and transmit sensory info to the reticular formation (influences consciousness) which activates the cerebral cortex thru 2o and 3o projections via midline and intralaminar thalamic nuclei. Thalamocortical projections are diffuse and affect wide areas of cerebral cortex.
Location: dorsal horn and intermediate grey matter
Final Destination: medullary and potine reticular formation
Spinomesencephalic tract
Route: Axons ascend ascend to the midbrain where they terminate in the periaqueductal grey activating it which inhibits pains sensation via descending fibres to the spinal cord. Projections to the superior colliculus will direct eyes to site of injury and important integrator and modulator of pain experiences. Also though to transmit sensory info to the amygdala via the parabrachial nuclei.
Location: dorsal horn and intermediate grey matter
Final Destination: cerebral cortex, esp limbic regions eg cingulate gyrus
Outline differences between direct and indirect spinothalamic pathways
Direct Pathway / Spinothalamic / Neospinothalamic
Function: Pain (location, intensity, quality), temp (lateral portion), Non-discrim touch (anterior portion)
Route: Axons from Ruffini’s corpuscles, nocicecptors, thermoreceptors synapse with 1st order neurons in the dorsal root ganglia > Central axons enter spinal cord, branches travel up and down 1-2 spinal segments (tract of Lissauer) > Synapse with 2nd order neurons in nucleus proprius/ proper sensory nucleus* (located in Rexed laminae III and IV of dorsal horn) > Axons decussate via ventral white commissure and enter to contralateral side > Ascend in lateral funiculus > Synapse 3rd order neurons in ventral posterolateral nucleus of thalamus > Project to sensorimotor cortex
*cells from NP extend in to substantia gelatinosa where significant pain modulation occurs
Location: Somatotopically arranged -Fibres from lower body ascend dorsolaterally.
Upper extremities & neck ascend ventromedially
Final Destination: Primary sensory cortex
Indirect Pathway / Paleospinothalamic, Spinoreticular, and Spinomesencephalic
Function: Involved in the autonomic, endocrine, motor and arousal components of pain, temperature and simple tactile sens (crude touch, pressure). Also, pain inhibiting mechanisms.
The axons of these neurons ascend the spinal cord bilaterally, show poor somatotopic organisation and make multiple synapses with the reticular formation, hypothalamus and limbic system. Can be described as the 2nd pain, the dull, throbbing poorly localised pain that includes the emotional response
Paleospinothalamic tract
Route: Axons ascend bilaterally in ventrolateral quadrant of spinal cord and make several synapses in the reticular formation of brainstem. Project to intralaminar thalamic nuclei to finally project to cerebral cortex, esp limbic regions eg cingulate gyrus
Location: dorsal horn and intermediate grey matter
Final Destination: cerebral cortex, esp limbic regions eg cingulate gyrus
Spinoreticular tract
Route: Projections from spinal cord are both crossed and uncrossed and transmit sensory info to the reticular formation (influences consciousness) which activates the cerebral cortex thru 2o and 3o projections via midline and intralaminar thalamic nuclei. Thalamocortical projections are diffuse and affect wide areas of cerebral cortex.
Location: dorsal horn and intermediate grey matter
Final Destination: medullary and potine reticular formation
Spinomesencephalic tract
Route: Axons ascend ascend to the midbrain where they terminate in the periaqueductal grey activating it which inhibits pains sensation via descending fibres to the spinal cord. Projections to the superior colliculus will direct eyes to site of injury and important integrator and modulator of pain experiences. Also though to transmit sensory info to the amygdala via the parabrachial nuclei.
Location: dorsal horn and intermediate grey matter
Final Destination: cerebral cortex, esp limbic regions eg cingulate gyrus
Describe the difference between upper and lower motor neurons
Upper motor neurons are supraspinal neurons that arise from above the decussation of the pyramids and innervate lower motor neurons of the spinal cord and brainstem.
Upper motor neurons influence lower motor neurons.
Upper motor neurons that arise from the cerebral cortex and project to the spinal cord form the corticospinal (pyramidal) tracts.
Upper motor neurons that arise from the cerebral cortex and project to the motor nuclei (lower motor neurons) of the brainstem are the corticobulbar tracts (also pyramidal)
Lower motor neurons can be located in the motor nuclei of the brainstem or the anterior horn of the spinal cord, and send axons to innervate skeletal muscle through the anterior roots of the spinal nerves.
Motor neurones of cranial nerves are LMN
Describe the coticospinal tracts and their function
Corticospinal tract is involved in the control of fine movements and synergistic movement of limbs
Largest descending tract in humans comprising of a massive bundle of fibres containing about 1 million axons
Corticospinal tracts originate in the cerebral cortex and descend in the spinal cord
Fibres originate in both the prefrontal gyri of the frontal lobes, around one third from the primary motor cortex and one third from the premotor and supplementary motor cortex, the remaining
fibres originate in the post central gyri of the parietal lobes (somatic sensory region)
Sensory input to the cerebral control of movements permits ‘filtering out’ of proprioceptive and other sensory information generated by complex movements allowing the brain to increase the ‘signal-to-noise ratio’ and focus on more important or unexpected sensory feedback.
Descending fibres travel together with corticobulbar fibres (supplying CNs for face, tongue, and jaw) through the corona radiata and converge in the posterior limb of the internal capsule where they’re organised so that those concerned with the cervical areas are located more posteriorly than those concerned with the lower limb then enter the crus cerebri (anterior cerebral peduncle) of the midbrain
Entering the pons they are dispersed into bundles by the pontocerebellar fibres but converge again in the medulla
oblongatato form thepyramids
90% of the fibres decussate in the pyramidal decussation and descend contralaterally in the lateral white column of the spinal cord as the lateral corticospinal tract
Mainly terminate on the interneurons in the
intermediate regionsof the cord grey matter that control flexor and extensor muscles forsynergistic movement
Ultimate target neuron for LCST is in lateral portion of anterior horn which allows direct connecton from cortex to specific muscle for fine precise skilled movement
Descending tracts from the brainstem also act on LCST for gross and strength related movement
Some LCST fibres terminate on dorsal horn sensory relay neurons to affect sensory transmission
The 10% of fibres that do not decussate descend ipsilaterally in the anterior white column of the spinal cord as the anterior corticospinal tract. They decussate at the
segmental level at which they terminate (the majority will cross the midline in either the cervical or upper
thoracic regions). Supply the trunk and proximal musculature often bilateral.
Below L2 only LCST intervates LMNs
The anterior corticospinal tract is important for bilateral postural adjustments when voluntary movements are performed with the opposite extremity. For example when screwing in a light bulb there is contralateral input from the lateral corticospinal tract whilst bilateral input from the anterior corticospinal tract
activates truncal and proximal muscle groups to stabilize the body during the activity.
Describe the branches of the corticospinal tracts and their functions
Branches of the corticospinal tracts arising in the motor cortex are given off early and return to the cerebral cortex to inhibit activity in adjacent regions of the cortex, thereby sharpening the boundaries of the excitatory signal
Giant Betz cells - Largest neurons in CNS send axons down corticospinal tracts to anterior horns of spinal cord, originate in primary motor cortex
Caudate nucleus and putamen from there additional pathways extend into the brain stem and spinal cord mainly to influence and control body postural movements
A moderate number of fibres pass to the red nuclei and from these additional fibres pass down the cord through the rubrospinal tract
Reticular formation (substance) and vestibular nuclei of the brain stem. From there signals go to the spinal cord via the reticulospinal and vestibulospinal tracts others go to the cerebellum by way of the reticulocerebellar and vestibulocerebellar tracts
Large numbers of fibres synapse in the pontine nuclei which give rise to pontocerebellar fibres which carry signals to the cerebellar hemispheres
Collateral fibres terminate in the inferior olivary nuclei and from there the secondary olivocerebellar fibres transmit signals to multiple areas of the cerebellum
Branches keep the subcortical regions informed about the cortical motor activity allowing them to react when necessary and send their own signals to the alpha and gamma motor neurons Thus,
the basal nuclei, brain stem and cerebellum all receive strong motor signals from the corticospinal system
To emphasise the corticospinal tracts form pathways that confer speed and agility to voluntary movements and is thus used in performing rapid, skilled movements. Simple, basic voluntary movements are influenced by other descending tracts.
Describe the red nucleus and its relationship with the corticospinal tract
The red nucleus is located in the mesencephalon and functions in close association with the corticospinal tract.
Receives a large number of fibres from the corticorubral tract as well as branching fibres from the corticospinal tract.
These fibres synapse in the lower portion of the nucleus the, magnocellular portion, which contains large neurons that give rise to the rubrospinal tract
The magnocellular portion of the nucleus has a somatographic representation so stimulation of a single
point of the red nucleus causes contraction of a single muscle or group of muscles, however, this lacks the
fineness associated with the motor cortex, as humans have relatively small red nuclei
The corticorubrospinal pathway serves as an accessory route for transmission of discrete signals from the
motor cortex to the spinal cord. When corticospinal fibres are destroyed but the corticorubrospinal
pathway is intact discrete movements can still occur, except that fine control of the hands and fingers are considerably impaired.
Wrist movements are still intact which is not the case when the corticorubrospinal pathway is also blocked.
Describe the corticobulbar tracts and their function
Commands for voluntary movements mediated
by cranial nervesare conveyed to thebrainstem via the
corticobulbar tracts. These descending fibres serve as upper
motor neurons to the cranial nerve nuclei onto which they
synapse with.
Originates in the lateral aspect of the primary motor, premotor, supplementary motor and somatosensory cortex and functions similar to the corticospinal tract, only they innervate the
muscles of the face and head.
Descend through the corona radiata and converge in the genu of the internal capsule from where they descend through the basal pons and medullary pyramids together with the corticopontine and
corticospinal fibres.
They synapse directly or indirectly with
all motor cranial nerves, via interneurons in the reticular
formation
These motor neurons therefore are responsible
for the voluntary control of the muscles of facial expression,
eye movements, jaw opening and closing, and movements of
the tongue
In contrast to the corticospinal tract which
mostly crosses in the pyramidal decussation, most
corticobulbar fibres are directed towards the cranial nerve motor nuclei innervating these neurons bilaterally. This corresponds to how muscles on both sides of the face are
typically used simultaneously i.e. chewing and speaking.
As a result of this bilateral distribution, unilateral
damage to the internal capsule or corticobulbar tract ie UMN lesiondoes not cause substantial, lasting weakness of the muscles. The exception is the lower face (below the eyes), whose motor neurons receive a predom crossed corticobulbar input (contralateral innervation of the motor nucleus of CN VII and CN XII which supplies the genioglossus muscle)
Upper spares Upper
A subset of fibres that innervate the brainstem nuclei (III, IV and VI) and exert an influence on spinal cord neurons. These fibres terminate in the reticular formation, superior colliculus and red nucleus. These connections co-ordinate the cortical and brainstem motor systems.m
Describe the reticulospinal tracts and their function
Reticular formation in brainstem gives rise to the reticulospinal tracts of which there are two;
Medial (pontine) reticulospinal tract
Rises from medial part of the reticular formation within the pons and send axons through the spinal cord in the anterior white column
The pontine recticular nuclei have a high degree of excitability and receive strong signals from the vestibular nuclei and deep nuclei
of the cerebellum
Descends to all levels of the spinal cord where it synapses directly with alpha and gamma motor neurons or indirectly
through interneurons in anterior grey horn to control extensor muscles and vertebral column to support body against gravity
Ascending tracts (dorsal column, medial lemniscal pathway, spinothalamic, spinoreticular tracts) give off collaterals which stimulate the pontine nuclei
Lateral (medullary) reticulospinal tract
Arises from cells located in the reticular formation within medial two thirds of the medulla oblongata and sends axons through the spinal cord in the lateral white column
The actions of this pathway are opposite to medial reticulospinal tract ie stimulates the flexor muscles.
Corticoreticular fibres provide main stimulation to nuclei (giganto cellularis) within the reticular formation in the medulla
Ascending tracts give off collaterals which also stimulate the reticular formation
Fibres descend in the lateral white column giving off collaterals which stimulate alpha and gamma motor neurons in the anterior grey horn
Activation of this pathway inhibits voluntary movements and
cortically induced movementsandreduces muscle tone
by inhibiting muscle spindle activity through its effects
on gamma motor neurons.
Medullary (lateral) RST counterbalances the excitatory signals from the pontine reticular system, so under normal
conditions the body muscles are not normally tense.
When the brain stem of an animal is sectioned below the midlevel of the mesencephalon, but the pontine and medullary reticular formations as well as the vestibular system remain intact, the animal develops decerebrate rigidity.
This occurs only in the antigravity muscles of the neck, trunk and extensors of the legs. This is because there is no input from the cerebral cortex, the red nuclei or the basal nuclei to the medullary reticular nuclei. Due to this lack of input the medullary reticulospinal inhibitory system becomes non-functional and over activity of the excitatory pontine system occurs resulting in extensor rigidity.
Both sets of reticulospinal fibres enter the anterior grey columns of the spinal cord and may facilitate or inhibit the activity of the alpha and gamma motor neurons. By these means the reticulospinal tracts
influence voluntary movements and reflex activity.
The reticulospinal fibres are now thought to include
descending autonomic fibres thus providing a pathway by which the hypothalamus can control the sympathetic outflow and sacral parasympathetic outflow.
Tonically active neurons in the medulla project to the spinal cord and activate the skeletal muscles involved in breathing
RTS also mediates pressor and depressor effects on circulatory system
The reticular formation also contains neurons that use monoaminergic neurotransmitters (serotonin, noradrenaline and dopamine) which receive input from the limbic system and influence the emotional motor system.
Another set of reticulospinal tract fibres are involved in the modulation of pain in spinothalamic tract as they synapse with
enkephalinergic interneurons in the dorsal horn which then synapse on primary afferent pain fibres
Describe the vestibulospinal tracts and their function
Extensor muscles - anti-gravity muscles for maintaining posture and balance
Vestibulo-nuclear complex within upper part medulla
Receive stimuli from labyrinth inner ear (macula located in utricle and saccule) and another in the cristae amplullaris > vestibular branch of vestibulocochlear nerve AND ALSO fastigial nucleus of cerebellum
Lateral and medial parts of vestibulo-nuclear complex are main components of vestivbulo-spinal tract
Fibres enter spinal cord ipislaterally and supply alpha and gamma neurons in anterior grey horn
Medial VST
Arises from the ipsilateral and contralateral
medial vestibular nucleus,descendsbilaterallythrough the brainstem and travels in theanterior white columns (ventral funiculus)
Terminates in the ipsilateral ventral horn of the spinal cord to reach
cervical and upper thoracic levels of the spinal cord
Activate lower motor neurons associated with the spinal accessory
nerve
Causes rotation and lifting of the head as well as rotation of the shoulder blade around its axis. Important in producing an appropriate orientation in response to
forces that change posture and balance i.e. keeping the head stable while walking
Lateral VST - axial and appendicular extensors
Uncrossed and descends the
entire length of the spinal cord
Terminate predominantly on
interneurons that activate alpha and gamma (gamma is specifically for muscle spindles, alpha for muscle fibres) motor neurons innervating extensor (antigravity) muscles of
the trunk and ipsilateral lower limb
Red nucleus
- inhibits Vestibulonuclear complex to prevent excessive contraction of extensors (extensor hypertonus)
CN III, IV, VI via the medial longitudinal fasciculus for eye movements - vestibulo-ocular reflex
Drescribe the rubrospinal tract and their functions
Starts within red nucleus in midbrain
Primary motor, premoter cortex, supplementary motor areas in cortex tracts give stimuli (corticorubro fibres) and continues as corticospinal tract
Deep cerebellar nuclei (globose and emboliform nuclei) also send stimuli
Rubrospinal tract crosses in ventral tegmental decussation of midbrain
Enters lateral white column close to lateral CST
Terminates on alpha and gamma motor neurones in anterior grey horn which control flexor muscles mainly of upper limbs
Overall function: stimulating flexor muscles
Believed to more upper limb flexors and keeps lower limb flexors in check
Describe the tectospinal tract and its function
Arises from superior colliculus
Axons pass ventromedially around periaqueductal gray and cross in the dorsal tegmental decussation
Fibres descend in anterior white column near anterior median fissure and terminate mainly in cervical regions on neurons controlling neck movements
Superior colliculus receives visual input - tectospinal tract though to mediate reflexes and orientate head and neck in response to visual stimuli
What is the difference between decerebrate and decorticate?
Decerebrate posturing will be a symptom of a lesion at the level of the midbrain or rostral pons
Lesion includes the red nucleus and rubrospinal tract
All descending corticol motor pathways are interrupted
Excitatory and inhibitory components of the reticulospinal tract
receive from corticospinal or rubrospinal tract
No cortical modulation or input of reticulospinal tract
Extensor rigidity is the result of excessive input to spinal cord
(no medullary reticulospinal tract to inhibit) interneurons or circuit neurons via the pontine reticulospinal tract
Decorticate posturing will be a symptom of a lesion rostral to or
above the red nucleus
All descending input to the spinal cord and brainstem is interrupted
Both inputs from the rubrospinal and reticulospinal tract remain
intact
Lower extremities show increased extensor tone (reticulospinal tract)
Upper extremities show increased flexor tone (rubrospinal tract)
Brown-Séquard Syndrome
This involves a hemisection of the spinal cord.
The primary characteristic is the dissociation of function
between conscious proprioception and pain and temperature sensations.
Dorsal Columnn: Loss of conscious proprioception, vibration, and tactile discrimination occurs below the level of the lesion on the ipsilateral side
Lateral spinothalamic Tract: Loss of pain and temperature sensation occurs one or two segments below the level of the lesion on the contralateral side
Lateral corticospinal Tract: There is also an upper motor neuron paralysis (spastic, hypertonic, hyperreflexic, clonus) below the level of the lesion on the ipsilateral side. Lower motor neuron paralysis at the level of the lesion on the ipsilateral side.
This kind of incomplete transection may occur by fracture dislocation of vertebrae, tumour or missile wounds.
Amyotrophic lateral sclerosis (Lou Gehrig’s Disease)
ALS is a progressive degenerative disease in which the corticospinal tracts (upper motor neurons) and
ventral horn cells (lower motor neuron) degenerate.
Degeneration of the ventral horn cells in the cervical
spinal cordresults inweakness, wasting*,andultimately loss of control in muscles of the hand, trunk and lower limbs.
Involuntary twitching of muscle fascicles (fasciculations) occurs in these muscles.
Motor cranial nerve nuclei in the pons or medulla may be involved from the start (progressive bulbar palsy) or only terminally.
Bladder and bowel functions are also impaired due to the loss of descending autonomic fibres
Median survival is 3 years and death is from respiratory impairment and related complications.
*wasting not only due to disuse atrophy but also from loss of trophic (nourishing) factor produced by motor neurons and conveyed to muscles
Syringomyelia
This disease is due to a developmental or acquired abnormality and is characterised by an expansion of the
central canal of the spinal cord.
This expansion produces glial cell proliferation in this region, especially at the levels of the lower cervical and upper thoracic cord.
There is segmental loss of pain and thermal
sensationbecause of aninterruption of crossing fibres of the spinothalamic tracts in the same and
adjoining segments at the level of the lesion.
Tactile sensation is largely preserved, while pain and
thermal sensation is lostit’s an example ofdissociated sensory loss.
Tabes Dorsalis
Tabes dorsalis represents the late consequences of syphilitic infection of the nervous system also
referred to as tertiary syphilis or neurosyphilis.
In this syndrome the large diameter central processes of
the dorsal root ganglion neurons degenerate, especially in thelower thoracic and lumbosacral segments.
Fibres in the fasciculus gracilis degenerate and there is a loss of vibration sensation, two point discrimination and conscious proprioception.
The loss of proprioception results in ataxia as the sufferer is
deprived of sensory feedback signals that detect the position of the lower limbs at any given point.
