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Spinoreticular tracts The spinoreticular tracts are the phylogenetically oldest somatosensory pathways. The reticular formation of the brainstem bilaterally distributed in terms of its ascending and descending connections. It is terminate at all levels of the brainstem and they is not somatotopically arranged. The spinoreticular system has two interrelated functions:1. to arouse the cerebral cortex, i.e. to induce or maintain the waking state2. to report to the limbic cortex about the nature of a stimulus. (emotional response)In summary, the phylogenetically old, ‘paleospinothalamic’ pathways through thereticular formation are concerned with the arousal and affective (emotional) aspectsof somatic sensory stimuli. In contrast, the direct, ‘neospinothalamic’ pathway isanalytical, encoding information about modality, intensity, and location.03/05/60 52
Spino-tectal tract 53The spinotectal tract runs alongside thespinothalamic pathway which it resembles in itsorigin and functional composition. It ends in thesuperior colliculus, where it joins crossed visualinputs involved in turning the eyes/head/trunktoward sources of sensory stimulation (visuospinalreflex).Spino-olivary tractThe spino-olivary tract sends tactile information tothe inferior olivary nucleus in the medulla oblongata.The inferior olivary nucleus has an important functionin motor learning through its action on thecontralateral cerebellar cortex. Spino-olivarydischarge can modify cerebellar activity in responseto environmental change, for example while climbinga surprisingly steep stairway. This feature iscalled motor adaptation. On the other hand, learningto perform routine motor programs automatically is afu03/n05c/60tion of the basal ganglia.
Spino-vestibular tractThe spinovestibular tractSpinovestibular fibers arise from all levels of the spinal cord and provideproprioceptive input primarily to the medial and lateral vestibular nuclei. Information concerning movement of the head through the visual world alsoreaches vestibular nuclei neurons through the accessory optic system. Vestibular nuclei neurons also receive input from the reticular formation,primarily from cells relaying information about proprioception but also fromserotonergic cells in raphe nuclei that exert modulatory or arousal effects. Finally, both the thalamus and several cortical regions provide directdescending connections to the vestibular nuclei.03/05/60 54
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Neuroanatomy Spinal Cord 56 Descending pathway Capt. Piyachat Chansela Department of Anatomy Academic year 201603/05/60
Outline & Objectives KNOW AND UNDERSTAND; The tracts descending the spinal cord and its particular set of motor neurons. CLINICAL PANELS; Upper motor neuron disease Lower motor neuron disease Spinal cord injury03/05/60 57
Two distinct pathways are involved in motor control: pyramidal and extrapyramidal Pyramidal : corticospinal, corticobulbar tract Extrapyramidal target lower motor neurons in the spinal cord that are involved in reflexes, locomotion, complex movements, and postural control. The extrapyramidal tracts include parts of the following: rubrospinal tract pontine reticulospinal tract medullary reticulospinal tract lateral vestibulospinal tract tectospinal tract03/05/60 58
The somatomotor cell columns Ventromedial (all segments) Erector spinae Dorsomedial (T1–L2) Intercostals, abdominals A E Ventrolateral (C5–C8, L2–S2) Arm/thigh B F D Dorsolateral (C6–C8, L3–S3) Forearm/leg C Retrodorsolateral (C8, T1, S1–S2) Hand/foot Central (C3–C5) Diaphragm Flexors Intermediolateral Central Anterolateral Posterolateral Retroposterolateral F E A D B C Posteromedial03/05/60 Extensors Anteromedial 59 http://clinicalgate.com/spinal-cord-2/
Corticospinal tract 60 AKA: Pyramidal tract The corticospinal tract is the greatvoluntary motor pathway. About 40%of its fibers take their origin from theprimary motor cortex in the precentralgyrus. Other sources include thesupplementary motor area on themedial side of the hemisphere, thepremotor cortex on the lateral side,the somatic sensory cortex, theparietal lobe, and the cingulate gyrus. 03/05/60
Leg Arm Head & Neck Corticospinal tract Corona radiataInternal capsule giant neurons (cells of Betz) LCST ACST Cerebral cortex (premotor cortex; area 4) 03/05/60 Corona radiata and posterior limb of the internal capsule Midbrain Pons 80% cross the midline in the pyramidal decussation Corticobulbar fiber Medulla oblongata Lateral corticospinal tract (LCST) C - spinal cord 10% Anterior corticospinal tract (ACST) *pyramidal fibers enter the LSCT on the same side (10% ) Uncross Anterior horn cells 61 LCST LS - spinal cord
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Corticonuclear (corticobulbar) 63 During its descent through thebrainstem, the corticospinal tractgives off fibers which activate motorcranial nerve nuclei, notably thoseserving the muscles of the face, jaw,and tongue. These fibers arecalled corticonuclear(corticobulbar). 03/05/60
Rubrospinal tract Red nucleus Anterior (ventral) tegmental Rubrospinal tract decussation medial RSTadjacent to the lateral corticospinal tractMagnocellular and Parvocellularrubrospinal fibers rubroolivary fiberscontralateral ipsilateral lateral RST Terminate in laminae V, VI, and VII 64excitatory influence to motor neurons innervating proximal limb flexors 03/05/60
Reticulospinal tractsThe reticulospinal tracts originate in the reticular 65formation of the pons (medial RST) and medulla oblongata(lateral RST). They are partially crossed.The reticulospinal system is involved in two different kindsof motor behavior: locomotion and postural control.They are called pattern generators. 03/05/60
Tectospinal tract The tectospinal tract is a crossed pathway descending from the tectum ofthe midbrain to the medial part of the anterior gray horn at cervical and upperthoracic levels. This tract is an important motor pathway, being responsible fororienting the head/trunk toward sources of visual stimulation (superiorcolliculus) or auditory stimulation (inferior colliculus). Vestibulospinal tract The vestibulospinal tract is an important uncrossed pathway whereby thetone of appropriate antigravity muscles is automatically increased when thehead is tilted to one side. It descends in the anterior funiculus and its function isto keep the center of gravity between the feet. It originates in the vestibularnucleus in the medulla oblongata. Raphespinal tract The raphespinal tract originates in and beside the raphe nucleus situated inthe midline in the medulla oblongata. It descends on both sides within theposterolateral tract of Lissauer. Its function is to modulate sensory transmissionbetween first- and second-order neurons in the posterior gray horn –particularly with respect to pain.03/05/60 66
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Upper motor neurons Corticospinal and corticonuclear neurons Lower motor neurons Below brainstem nucleus and spinal cord03/05/60 68
CLINICAL PANELSLower Motor Neuron SignsLower motor neurons are those cells whose axons synapse directly on skeletalmuscle. When these neurons or their axons are damaged, the innervatedmuscles will show some combination of the following signs: (1) flaccid paralysis followed eventually by atrophy, (2) fasciculations (involuntary contractions of one or a group of motor units), (3) hypotonia (decreased muscle tone), and (4) hyporeflexia, areflexia, weakening or absence of muscle stretch reflexes.03/05/60 69
CLINICAL PANELSUpper Motor Neuron SignsThe term upper motor neuron is commonly used in reference to corticospinalor corticonuclear cell bodies and their axons.Damage to upper motor neurons results in muscles that (1) initially weak and flaccid (acute phase) (2) eventually become spastic, (latent phase) (3) exhibit increased muscle tone (hypertonia) (4) increase in muscle stretch reflexes (hyper-reflexia), (5) Clonus (6) Babinski sign positive03/05/60 70
Why are motor neurons 71hyperexcitable? Renshaw cells normally have a tonic breaking action on α and γ motor neurons at their own segmental level 03/05/60
Spinal cord injuryInjury at thoracic or lumbar segmental level results in paraplegia - (paralysis of lower limbs)Injury at cervical level causes tetraplegia (quadriplegia), - (paralysis of lower limbs and upper limb)Spinal shockThe following features are found below the segmental level of theinjury in the first few days following a complete cord transection: • Paralysis of movement. The limbs are flaccid and tendon reflexes are absent. • Anesthesia (loss of all forms of sensation). • Paralysis of the bladder and rectum. Return of spinal function 72 Several days or weeks later, reflex functions of the cord become progressively restored, and ‘upper motor neuron signs’ appear. : Excessive muscle tone (spastic), hyper reflexia, babinski sign03/05/60 positive, ankle clonus.
Brown-Séquard syndrome 73A hemisection of the spinal cord (as inthe Brown-Séquard syndrome) results in acombination of sensory and motor losses.Sensory deficits include (1) contralateral loss of nociceptive and thermal sensations over the body below the level of the lesion (ALS damage) and; (2) - ipsilateral loss of discriminative tactile, vibratory, and position sense over the body below the level of the lesion (posterior column damage). - ipsilateral loss of motor power (lateral corticospinal tract) of the leg or leg and arm, depending on the level of the hemisection. 03/05/60
Brown-Séquard syndrome03/05/60 74
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