Sensory Pathways.docx

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Western University
Anatomy and Cell Biology
Anatomy and Cell Biology 3319
Kem Rogers

Sensory & Motor Pathways in Central Nervous System • Brain is connected to the body periphery by multi-neuron pathways that travel the white matter of CNS • Pathways composed of interconnected fibre tracts that relay information from parts of CNS to others • Pathways that carry information to more rostral regions of the CNS are called ascending pathways • Pathways that carry information to more caudal regions of the CNS are called descending pathways • Most pathways cross from 1 side of the CNS to the other or decussate at some point along the course • Most pathways consist of chain of 2 or 3 linked neurons that contribute to successive tracts • Most pathways are spatially arranged in a specific way, according to the body region they supply • Ex. One ascending tract, axons transmitting impulses from the superior part of the body lie lateral to the axons carrying impulses from inferior body parts • All pathways are bilaterally symmetrical, occurring on both right & left side of the brain or spinal cord • Segments of pathways that travel through spinal cord - called ascending & descending spinal tracts: o Ascending tracts: carry sensory information to the brain o Descending tracts: carry motor instructions to the effectors of the body • Spinal tract are named according to their origin & destination • Afferent neuronal signals from skin and deeper structures are segregated in the spinal cord; eg. spinothalamic tract vs. dorsal funiculi • Axons of pain and temperature cross midline in the spinal cord • Axons of discriminative touch and conscious proprioception ascend ipsilaterally to medulla; after synapsing in the medulla the axons cross midline • VPL in the thalamus is the primary site of termination for somatosensory information to the postcentral gyrus Ascending Pathways • Conduct general somatic sensory impulses superiorly through chains of two or three neurons to various regions of the brain • First-order neuron o First neuron in pathway o Sensory neuron which extends from sensory receptor into spinal cord o Synapses in the CNS with another neuron in the pathway • Second-order neuron o In some pathways, synapse with another neuron in the pathway: third-order neuron • 3 main ascending pathways o Spinocerebellar pathway o Dorsal column pathway o Spinothalamic pathway Spinocerebellar Pathway • Arises from second-order neurons in dorsal horn of spinal cord • Terminates on cerebellum • Carries information on proprioception from lower limbs & trunk to cerebellum – muscle spindles • Cerebellum uses information to coordinate body movements • Fibres either do not decussate or cross twice (undoing the decussation) – project ipsilaterally Dorsal Column-Medial Lemniscal Pathway • Carries information on fine touch, pressure & conscious aspects of proprioception • Discriminative senses – senses localized very precisely on the body surface (skin) • Axons of first-order neurons, sensory neurons, enter spinal cord & send axonal branch up one of the dorsal white column tracts – either medial fasiculus gracilis (lower spinal regions) or lateral fasiculus cuneatus (higher spinal regions) • Axons ascend in spinal tract to medulla oblongata • In medulla oblongata – axons synapse with second-order neurons in nucleus gracilis (lower body) or nucleus cuneatus (upper body) – sensory relay • Axons from these brain nuclei from a tract called medial lemniscus tract – decussates in medulla & ascends through pons & midbrain to thalamus  post-central gyrus (somatosensory cortex) o Ventro-posterolateral nucleus of thalamus (body) o Ventro-posteromedial nucleus of the thalamus (head & neck) • Third-order neurons, originating in thalamus send axons to primary somatosensory cortex on the post- central gyrus – sensory information is processed, resulting in awareness of precisely localized sensations Spinothalamic Pathway • Carries information on pain, temperature, deep pressure & nondiscriminative touch – stimuli we are aware of but cant localize precisely on body surface – smaller not myelinated axons slower o Nociceptors, thermoreceptors – receptors on skin • Axons of first-order sensory enter spinal cord – synapse on interneurons in dorsal gray horn • Axons of second-order neurons decussate in spinal cord (anterior white commissure) – enter lateral & ventral funiculi as the spinothalamic tract – ascend to thalamus: ventral posterolateral nucleus  postcentral gyrus & int
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