Ascending/descending tracts

• List most important ascending tracts

  • Dorsum collum-medial leminiscus pathway
  • spinothalamic (anterolateral system) pathway
  • Spinocerebellar pathway
• List the pathways related with touch and pressure

  • Dorsal column-medial lemniscal pathway
  • Anterior spinothalamic tract
  • Trigeminothalamic tract (from face)
  • Spinoreticular tract
• Anterior spinothalamic tract:

  • Receptors: merkels’s disks, free nerve endings
  • Sensory input from peripheral receptors → synapse with 1 neuron in DRG→ enters spinal cord via dorsal root→ fibers ascend/descend within lissaer’s tract→then synapses with 2neurons in dorsal horn(nucleus proprios)→fibers of 2neurons decussate through the ant white comissure→ ascend as Ant spinothalamic tract situated in ant funiculi of spinal cord→ synpase with 3neurons in ventroposterolateral nucleus of thalamus→ 3 neurons axons project into primary somatosensory cortex in postcentral, gyrus of parietal lobe
• List main pathways in CNS carrying pain and temperature sensation

  • Spinothalamic tract (lateral spinothalamic tract)
  • Trigeminothalamic tract ( from face)
  • Lissauer’s tract (modulates entry and initial processing of pain and temperature signals in spinal cord).
• Lateral spinothalamic tract

  • Receptors: nociceptors and thermoreceptors. Cell bodies of these neurons are in DRG. Axons enters spinal cord through dorsal root and may ascend/descend few segments in lissauer’s tracr before synapsing → Synapse with 2neurons in dorsal horn (substancia gelatinosa)→ axons decussates through the ant white comissure→fiber in lat white commissure ascend as lateral spinothalamic tract→reach thalamus and synapse in ventralposterolateral nucleus→project through internal capsule to 1 somatosensory cortex.
• Lesions to lateral spinothalamic tract

  • Loss of pain and temperature sensation on contralateral side of body bellow level of lesion.
  • Syringomyelia: cysts form within spinal cord often affecting central canal. This can disrupt crossing fibers of lateral spinothalamic tract leading to cape-like loss of pain and temperature sensation over shoulders and arms while sparing light touch and propioception.
• List the pyramidal tract

  • corticospinal tract
  • Corticobulbar tract
• Classify the descending tract

  • Functionally: voluntary (pyramidal) and involuntary (extrapyramidal) control
  • Somatotopically: Lateral motor system (lateral corticospinal + rubrospinal tracts) + Medial motor system (anterior corticospinal + vestibulospinal + reticulospinal + tectospinal tracts)
• 1.Basal nuclei (thalamus/Subthalamus:) prefrontal cortex→M1,S1, PM,SMA→basal nuclei →cerebral cortex: basal nuclei elaborate info about the mov →produce a new blueprint of mov that has been modified + corrected→ allows for right mov to be performed.

  2. Cerebellum: prefrontal cortex→M1,S1,PM and SMA→cerebral cortex: cerebellum received info about proprioception of body →creates another blueprint of the mov contained this info.

  Cerebral cortex has all info it needs to perform coordinate, organised and precise mov
• There are 2 check point to ensure mov is as precise and coordinated and possible

  • Basal ganglia + thalamus and subthalamus checkpoint
  • Cerebellum checkpoint

  
• Corticospinal tract

  • Giant pyramidal cell of betz (UMN) receive fully ready blueprint of mov from basal nuclei + cerebellum→ descend through ant 2/3 of posterior limb of internal capsule→descend through crus cerebri of midbrain→pons→at level of medulla pyramidal tract produces pyramids on both hemispheres on ventral surface of medulla→ at end of pyramids 90% of fibers decussates at pyramidal decussation forming Anterior and lateral corticospinal tracts.
• Lateral corticospinal tract

  • Made up of 80-85% of fibers that decussate at pyramids→ descend through lateral funiculus of spinal cord→ axon synapse on both alpha and gamma motor neurons (LMN)→ LMN leave spinal cord through ventral root and travel to specific skeletal muscle through spinal nerves→activation of muscle in order to perform movement.
  • Associated with distal limb musculature (fingers, arms) →perform very fine and precise movements.
• Anterior corticospinal tract

  • Made up of the 20% of fibers that do not decussate in pyramids→ descend in anterior funiculis of spinal cord
  • Some fibers decussate at level they terminate in spinal cord→go to contralateral ventral grey horn→synapse with alpha + gamma motor neurons associated with axial muscles (trunk and head) .
  • Some fibers remain uncrossed→ on ipsilateral side→ provided bilateral innervation ensuring coordination between the 2 sides of body.
• Lesions to corticospinal tract

  • Weakness (paresis) partial loss of voluntary motor function either contralateral or ipsilateral.
  • Paralysis (plegia) complete loss of voluntary motor function. Contralateral or ipsilateral depending on location.
  • Hyperreflexia: exaggerated reflexes due to loss of inhibitory modulation from upper motor neurons.
  • Contralateral motor deficits: above medullary decussation
  • Ipsilateral deficits: below medullary decussation.
• Corticobulbar tract


  Originates from:

  1. Primary motor cortex
  2. Motor association cortex: Premotor cortex and supplementary motor area
  3. Primary somatosensory cortex

  They descend through corona radiate, internal capsule, brainstem and along way they gibe collaterals to the cranial nerve nuclei where they synapse on lower Motor neurons→ Activation of nucleus→activation of different group muscle. As rule majority of fibers decussate and end up innervating contralateral motor nuclei.
• Corticobulbar tract innervates what cranial nerve nuclei

  • Trigeminal nerve nuclei: supplies mastication muscle
  • Fascial nerve nuclei: supply mimic muscle of face
  • Nucleus ambigous: nuclei for glossopharyngeal, vagus and accessory nerve
  • Hypoglossal nerve nuclei: supplies tongue muscle
  • Nuclei of oculomotor, trocheal and abducens n indirectly when corticobulbar tract passes through paramedian pontine reticular formation
• Corticobulbar tract provides bilateral innervation to all cranial nerve nuclei except

  • lower part of facial nuclei: that supplies muscle of facial expression bellow eye
  • Lower part of Hypoglossal nuclei: supplies genioglossus m.
• The corticonuclear tract also indirectly affects the nuclei of oculomotor, trochlear and abducens nerves within the midbrain, through intermediate structures such as the medial longitudinal fasciculus and reticular formation.
• • The corticonuclear tract provides voluntary control over the muscles of the face, head and neck. In contrast to the corticospinal tract which controls the movement of the torso and limbs. 

  Specifically, the corticonuclear tract carries upper motor neuron input to the motor nuclei of the trigeminal, facial, glossopharyngeal and accessory cranial nerves. This then supplies voluntary control over a number of head and neck functions
• • Nuclei directed supplied by corticobulbar tract:
  • Trigeminal nerve (V): motor component of trigeminal nerves supplies the muscles of mastication. 
  • Facial nerve (VII): motor nucleus has both dorsal and ventral regions. Neurons in the dorsal region innervate the muscles of the upper face, while neurons in the ventral region innervate the muscles of the lower face.
  • Glossopharyngeal nerve (IX):  innervates the muscles of the pharynx and larynx. 
  • The accessory nerve (XI): supplies the sternocleidomastoid and trapezius muscles. 
• Indirect supply of corticobulbar tract to

  • The corticonuclear tract influences the activity of muscles supplied by the third, fourth and sixth cranial nerves, indirectly modulating their activity. medial longitudinal fasciculus and reticular formation
  • It does this by synapsing with intermediate structures from the brainstem such as the 
• The corticobulbar tract supplies:

  o Mastication muscle: through trigeminal nerve (V) o Mimic muscles of the face: through facial nerve (VII)

  o Soft palate muscles, uvula, pharynx and larynx: through nucleus ambiguus (CN IX, X, XI). Important for speech and deglutition o Tongue muscles: through CN XII
• Rubrospinal tract:

  Receives impulses from:

  1. cerebral cortex: (M1, SMA, premotor cortex) these are cortico-rubro fibers.
  2. Cerebellum: concerned with propioception: superior cerebellar peduncle

  Pathway: rubrospinal originates in red nuclei of midbrain tegmentum→ decussates to contralateral side in ventral tegmental decussation→descend in lateral white column→ ends in lower motor neurons that innervates flexors of body. (Limbs)

  • Rubrospinal + medullary reticulospinal →stimulates flexor muscles.
• Retoculospinal tract: part of corticoreticulospinal system.

  • Consists of: corticoreticular, pontine snd medullary reticulospinal fiber tracts.

  Corticoreticular fibers arise feom M1 + SMA. → These fibers synapse bilaterally with neurons of medial zone of pontine and medullary reticular formation→ Give rise to 2 descending tracts:

  1. Medial (pontine reticulospinal)
  2. Lateral (medullary (reticulospinal)
• Extrapyramidal system is involved in regulation and modulation of motor activity particularly those aspects of mov that are not under direct voluntary control

  • Operates largely without conscious thought to ensure smooth, coordinated movements, maintaining posture and appropriate muscle tone
  • Arm swinging while walk ensuring it synchronised with legs to maintain balance
  • You stand upright without having to think about it (system continuously adjusts muscle tone and position to keep you right and balanced
  • Suppressing random twitches or tremos while holding a cup of coffee.
• List pathways of extrapyramidal system

  1. Rubrispinal
  2. reticulospinal
  3. vestibulospinal
  4. tectospinal
• Vestibular nuclear complex is divided into

  • Superior
  • Lateral
  • Medial
  • Inferior portions

  All of these are found din lower part of pons except for inferior portion which extends into medulla

  • Receives info from: vestibular branch of vestibulochochlear n, purkinje fibers from cerebellum and red nucleus
  • Send info to: extensor muscles and eye muscles
• • Vestibulospinal tract

  Info from acceleration comes from inner ear through vestibulochoclear n + info from proprioception comes from fastigial nucleus through purkinje fibers→ both finers synapse on vestibular nuclear complex→which then send motor signals down spinal cord as vestibulospinal tract.

  • Lateral portion fibers: control axial and appendicular extensor muscles
  • Medial portion fibers controls head and neck extensor muscles
• • Medial vestibulospinal: originates in medial vestibular nuclei→descends bilaterally through medial longitudinal fasciculus and targets cervical and upper thoracic levels of spinal cord→influences motor neurons controlling head and neck mov.
  • Lateral vestibulospinal: originates in lateral vestibular nuclei→ descend ipisilaterally through lateral funiculus and extends throughout entire length of spinal cord.
  • Motor neurons travel to extensor muscles and cause them to contract
  • o α-motor neurons→ To extrafusal fibers (cause contraction)
  • o γ-motor neurons→ To spindle fibers (maintaind muscle tone)
• Corresponding adjustment of eye position and stabilisation of gaze during head movements, is made by connections of vestibular nuclei with extraocular motor nuclei (abducens, oculomotor and trochlear nuclei).

  Fibers connecting these nuclei form the medial longitudinal fasciculus. → helps coordinate eye mov with head and body mov.
• • head rotates to right →activates receptors on right vestibular apparatus→ fire impulses along vestibular n→ vestibular nuclei→ left reticular formation→ activation of left abducens + right oculomotor nucleus→ contraction of left lat rectus + right med rectus
  • This rotation also affects receptors in left apparatus→these are inhibitory because fluid is moving in opposite way here→ acts on right abducens + left oculomotor→relaxation of right lat rectus + left medial rectus
  • All these coordinated actions causes eye to drift to left
• when head rotates to 1 side→semicircular canal detects motion→ transmitted to vestibular nuclei→ send signal through med. longitudinal fasciculus to appropriate eye muscles→ muscle on opposite side of head mov are activate to mov eye in opposite direction to head mov.
• Rubrospinal tract receives impulses from

  • Cerebral cortex( M1,SMA, premortor cortex): corticorubro fibers
  • Cerebellum: deep cerebellar nuclei sends fibers to red nucleus

  From red nucleus in midbrain→crosses contra-laterally in ventral tegmental decussation and descend in lat white column→ terminates in lower motor neurons that innervates flexors of body → activation of flexor inhibition of extensors

  
• • Vestibular spinal tract + Pontine reticulospinal tract → Stimulate extensor muscles

  Rubrospinal tract and medullary reticulospinal tract → Stimulate flexor muscles
• Reticulospinal tract originates from reticular formation, a network of neurons located in pons and medulla of brainstem and projects to spinal cord, influencing motor neurons that controls muscles of trunk and limbs

  • Divided into 2 main pathways: medial (pontine) reticulospinal and lateral (medullary) reticulospinal tracts.
• Pontine reticulospinal tract receives input from

  • impulses from spinothalamic tract (DCML pathway, spinothalamic and spinoreticular tract) as they ascend they give collaterals to reticular formation in pons which send fibers that stimulates extensor muscles
  • From the nuclei in pons→descend ipsilaterally thought anterior white column of spinal cord→ supply alpha + gamma motor neurons that supply extensors

  
• Medullary reticulospinal tract receives: cortico-reticular fibers + ascend tracts fibers to reticular formation.

  • Pathway originates from medullary reticular formation→ descend bilaterally though anterior white column stimulating alpha and gamma motor neurons in anterior gray horn that controls extensors of body.

  
• Tectospinal tract originates from neurons in superior colliculus of midbrain.→ Crosses periaqueduct gray in dorsal tegmental decussation and descend in anterior white column of spinal cord→ reaches motor neurons of oculomotor, trochlear and abducens and motor neurons that controls muscles of neck

  • function in coordination of eye and neck movement towards source of visual + auditory stimulu.
• Flash of light is detected by retina→ visual info is sent to superior colliculus→ process info and determines necessary mov to orient head towards light stimulus→ tectospinal tract sends motor commands from superior colliculus down to cervical spinal cord→ cervical motor neurons receives stimulus and activates muscles of neck→ head is turned reflexively towards source of light allowing focus + response to visual stimulus
• Motor cortex decides to initiate a movement

  1. Basal ganglia direct pathway facilitates this movement by decreasing inhibition to thalamus

  2. Increased motor cortex activity

  3. Indirect pathway ensures unnecessary movements are suppressed providing smooth/focused movement

  4. Hyperdirect pathways stand ready to rapidly inhibit movement if an obstacle appears