Ascending Tracts - Anatomy Notes PDF

‫المملكة العربية السعودية‬ ‫وزارة التعليم‬ ‫جامعة نجران‬ ‫العمادة‬/‫الكلية‬/‫اسم الوكالة‬ Ascending Tracts 2023 - 1444 ‫الع...

‫المملكة العربية السعودية‬ ‫وزارة التعليم‬ ‫جامعة نجران‬ ‫العمادة‬/‫الكلية‬/‫اسم الوكالة‬ Ascending Tracts 2023 - 1444 ‫العام‬ Dr. Aymen Nasreldin Dr. Asmaa Tolba Anatomy Department Anatomy Department Najran University Najran University  White matter of the spinal cord White in colour Surrounds the grey matter Mixture of: 1. Nerve fibers (Myelinated) 2. Neuroglial cells. 3. Blood vessels.  Divisions in white matter Anterior (Ventral) white column (funiculus) Lateral white column (funiculus) Posterior (Dorsal) white column (funiculus) Anterior white commissure.  Tracts Collection of nerve fibres with same Origin, Course and Termination Ascending (sensory) Tracts (Pathways) The information may be divided into two main groups: Exteroceptive information, which originates from outside the body, such as pain, temperature, and touch, and Proprioceptive information, which originates from inside the body, for example, from muscles and joints  Ascending (sensory) Tracts (Pathways) Three-neuron pathways: Primary sensory neurons (1st order neurons): From external receptors Travel through dorsal roots of spinal cord Secondary sensory neurons (2nd order or interneurons): Gives rise to an axon that decussates (crosses to the opposite side) and ascends to a higher level of the central nervous system Make up tracts in spinal cord and brainstem Tertiary sensory neurons (3rd order or thalamic neurons): From thalamus which gives rise to a projection fibers pass to primary sensory cortex Travel through internal capsule A) Anterolateral Spinothalamic Tract: Carries information of pain, temperature (thermal sensation), crude (light) touch and pressure. 1st order neuron enters spinal cord via dorsal root and synapses on 2nd order neuron then decussates and ascends to brain; synapse on 3rd order neuron (in thalamus); terminates in somatosensory cortex  Anterior spinothalamic tract Carries impulses concerned with Light (Crude) Touch and Pressure Axons of 1st order neurons entering the spinal cord from the posterior root ganglion proceed to the tip of the posterior gray column and divide into ascending and descending branches and travel for a distance of one or two segments of the spinal cord as posterolateral tract of Lissauer It is believed that these fibers of the first-order neuron terminate by synapsing with cells in the substantia gelatinosa group in the posterior gray column Axons of 2nd order, decussate obliquely within one segment of their origin, by passing through the ventral white commissure and terminate on 3rd order neurons in ventral posterolateral nucleus of the thalamus Thalamic neurons project to the somatosensory cortex through the posterior limb of internal capsule Lateral spinothalamic tract Carries impulses concerned with pain and temperature (thermal sensations). Axons of 1st order neurons entering the spinal cord from the posterior root ganglion proceed to the tip of the posterior gray column and divide into ascending and descending branches and travel for a distance of one or two segments of the spinal cord as posterolateral tract of Lissauer Fibers of the first-order neuron terminate by synapsing with cells in the posterior gray horn, including cells in the substantia gelatinosa. Axons of 2nd order, decussate obliquely within one segment of their origin, by passing through the ventral white commissure and terminate on 3rd order neurons in ventral posterolateral nucleus of the thalamus Thalamic neurons project to the somatosensory cortex through the posterior limb of internal capsule B) Posterior (Dorsal) Column (Fasciculus Gracilis and Fasciculus Cuneatus) and medial lemniscus Contains two tracts: Fasciculus gracilis (FG) Fasciculus cuneatus (FC) Carries information of Conscious proprioception (Muscle Joint Sense), Discriminative Touch and Vibratory Sense  Fasciculus Gracilis contains fibres received at sacral, lumbar and lower thoracic levels, Fasciculus Cuneatus contains fibres received at upper thoracic and cervical levels  Fibers ascend without interruption to terminate in 2nd order neurons in nucleus gracilis and nucleus cuneatus (in medulla oblongata) The axons of the 2nd order neurons decussate in the medulla as internal arcuate fibers and ascend through the brain stem as medial lemniscus, which terminates in the 3rd order neurons in ventral posterolateral nucleus of the thalamus Fibres project to the somatosensory cortex (thalamocortical fibers)  C) Spinocerebellar Tract: The spinocerebellar system consists of a sequence of only two neurons Two tracts: Posterior (Ipsilateral) Anterior (contralateral) Carry information of proprioception derived from muscle spindles, Golgi tendon organs (the sense organ that tells how much tension the muscle is exerting) and tactile receptors (Merkel's disks, Meissner's corpuscles, Ruffini endings, Pacinian corpuscle and Krause end bulbs) Terminates in the cerebellum Controls posture and coordination of movements  Posterior spinocerebellar tract Present only above level L3 The cell bodies of 2nd order neuron lie in Clark’s column (dorsal or posterior thoracic nucleus) Axons of 2nd order neuron terminate ipsilaterally (uncrossed) in the cerebellar cortex by entering through the inferior cerebellar peduncle  Anterior spinocerebellar tract The cell bodies of 2nd order neuron lie in base of the dorsal horn of the lumbosacral segments Axons of 2nd order neuron cross to opposite side, ascend to the midbrain, and enter the superior cerebellar peduncle The fibbers cross the midline again within the cerebellum before terminating in the cerebellar cortex Both spinocerebellar tracts convey sensory information to the same side of the cerebellum D) Spinotectal Tract From the posterior root ganglion, the axons enter the spinal cord and proceed to the grey matter to form synapses with unidentified second- order neurones. The second-order neurones' axons climb as the spinotectal tract in the anterolateral column after crossing the median plane They terminate by forming synapses with neurones in the superior colliculus of the midbrain This pathway provides afferent information for spinovisual reflexes and brings about movements of the eyes and head toward the source of the stimulation  E) Spinoreticular Tract From the posterior root ganglion, the axons enter the spinal cord and proceed to the grey matter to form synapses with unknown second-order neurones. These second-order neurones' axons ascend the the lateral white column's as spinoreticular tract The majority of the fibres are uncrossed and come to an end by making synapses with reticular formation neurones in the midbrain, medulla oblongata, and pons. The reticular formation receives an afferent fibres from the spinoreticular tract, which is crucial in determining consciousness levels. F) Spino-olivary Tract The axons enter the spinal cord from the posterior root ganglion and terminate on unknown second-order neurons in the posterior gray column. The second-order neurones ascend after crossing the midline at the junction of the anterior and lateral white columns as spino-olivary tract In the inferior olivary nuclei of the medulla oblongata, the axons terminate by forming synapses with third-order neurones. The axons of the third-order neurones enter the cerebellum via the inferior cerebellar peducle after crossing the midline. The spino-olivary tract conveys information to the cerebellum from cutaneous and proprioceptive organs. References 1. Clinical neuroanatomy / Richard S. Snell. — 7th ed. Lippincott Williams & Wilkins- Philadelphia 2. Netter Basic Science John T. Hansen, Netter's Clinical Anatomy with Online Access, 3e- Saunders (2014). 3. Last’s anatomy Regional and applied,12th edition. Chummy S. Sinnatamby FRCS, Churchill Livingstone (2011). 4. Gray's Anatomy. The Anatomical Basis of Clinical Practice. 40th Edition. Susan Standring, Ph.D. Churchill Livingstone Elsevier. 2008