Spinocerebellar Tracts & Descending Tracts PDF

Fibres of the dorsal spinocerebellar tract come from the cells in the lamina 7 of cord segments t1-l2, known as either clarke’s column or the thalamic nucleus. The axons do not decussate they rise ipsilaterally and enter the cerebellum via the inferior cerebellar peduncle Meanwhile the ventral spi...

Fibres of the dorsal spinocerebellar tract come from the cells in the lamina 7 of cord segments t1-l2, known as either clarke’s column or the thalamic nucleus. The axons do not decussate they rise ipsilaterally and enter the cerebellum via the inferior cerebellar peduncle Meanwhile the ventral spinocerebellar tract originates from cells at the lumbosacral cord levels. Fibres decussate and ascend on the contralateral side and enter the cerebellum via the superior cerebellar peduncle. The cuneocerebellar fibres is the upper limb equivalent of the dorsal spinocerebellar tracts, and this is where the cervical enlargement is? It ascends ipsilaterally in the fasiculus cuneatus and terminates in the medulla just lateral to the principle cuneate nucleus in the external/ accessory cuneate nucleus It also joins the cerebellum via the inferior cerebellar peduncle The upper limb equivalent of the ventral spino cerebellar tracts is the rostral spinocerebellar tract. Axons arise from cells in the cervical enlargement and ascend ipsilaterally in the lateral funiculus, entering the cerebellum through the inferior peduncle Descending spinal tracts These are concerned with movement, muscle tone, reflexes, autonomic function, modulation of sensory transmission to higher centres Corticospinal tracts These are the major motor descending tracts Concerned with movements that are voluntary discreete skilled Often goes to distal portions of the limbs. Arise from cell bodies in the cerebral cortex, often the primary motor cortex. Betz cells give rise to the largest diameter corticospinal axons. They leave through the massive subcortical fibres of the corona radiata and the internal capsule to enter the crus cerebri of the midbrain They pass through the pons without decussating, forming the pyramids. They then decussate in the medulla (about 75-90%) They then enter the contralateral lateral corticospinal tract. This is deep to the dorsal spinocerebellar tracts 10-25% of pyramidal fibres remain ipsilateral and enter the ventral corticospinal tract lateral to the ventral median fissure. They synapse onto their lower motor neurones at the ventral horn of the level they branch off 55% of corticospinal neurones terminate at cervical levels 20% at thoracic 25% at lumbosacral Rubrospinal tracts These originate in the tegmentus in the midbrain Controls the tone of limb flexor muscles are excitatory to the motor neurones of these muscles axons leaving course ventromedially cross in the ventral tegmental decussation Descend to the spinal cord and lie ventrolateral and partly intermingled with the lateral corticospinal tract. Afferents (inward information) actually goes from the cortex and cerebellum TO the red nucleus. The tract therefore represents a non pyramidal route through which the cortex and cerebellum can influence spinal motor activity Tectospinal tracts Arise from the superior colliculus of the midbrain Axons pass around the aqueduct and come to front, also they decussate further back than the ruberospinal fibres, decussating instead at the dorsal tegmental region. Descending tectospinal fibres lie near the ventral median fissure and terminate predominantly in cervical segments Because is receives visual input (superior colliculus) the tectospinal tract is thought to mediate reflex movement in response to visual stimuli. Vestibulospinal tracts These arise from vestibular nowhere ucleui on the pons and medulla near the fourth ventricle. They receive input from the labyrinthine system by way of the vestibular nerve and also from the cerebellum Axons from the cells of the lateral vestibular nucleus descend ipsilaterally as the lateral vestibulospinal tract, located in the ventral funiculus mediate excitatory effects on extensor motor neurones serve to control extensor muscle tone in the anti gravity maintenance of posture The medial vestibular nucleus contributes descending fibres to the ipsilatteral medial longitudinal fasciculus It descends as far as cervical levels Contains the fibres that link the vestibular nuclei with the nuclei innervating the extraoccular muscles. Reticulospinal tracts The reticular formation of the pons and medulla gives rise to reticulospinal fibres. Axons from the pontine reticular formation descend ipsilaterally as the medial reticulospinal tract. Axon from the medulla descend bilaterally in the lateral reticulospinal tracts. Both of these are located in the ventral funiculus. These reticulospinal fibres influence voluntary movement, reflex activity and muscle tone by controlling the activity of both alpha and gamma motor neurones. Mediate pressor and depressor effects on the circulatory system and are involved in control of breathing