Sensory Pathways to the Brain

Sensory pathways to the brain 1. Dorsal column-medial lemniscal pathway (DCML) – convey tactile info 2. Spinothalamic tract – convey crude touch, pain and temp Dorsal column-medial lemniscal pathway • Transmits information from mechanoreceptors and proprioceptors…3 order of neurons: o First order neurons: transmit info from periphery and enter spinal cord through dorsal root → travel along the ipsilateral dorsal column and end in the dorsal column nuclei in the medulla oblongata → synapse with 2nd order neurons o Second order neurons: start from the medulla and cross over to the contralateral side of medulla in the medial lemniscus → ascend to thalamus o Third order neurons: transmit info from thalamus to the somatosensory cortex • Rapidly-conducting sensory axons (myelinated afferents) travel in the dorsal columns to the Gracile nucleus and the Cuneate nucleus in the medulla o Dorsal columns: ▪ Fasciculus gracilis: pathways from lower body ▪ Fasciculus cuneatus: pathways from upper body Spinothalamic tract • Consists of two adjacent pathways: anterior and lateral o Anterior: crude touch o Lateral: pain, temperature • Three orders of neurons: o o o First order: transmit info from periphery and enters spinal cord through dorsal root → ascend/descend 1-2 vertebrae → synapse with 2nd order neuron ipsilaterally (on same side) Second order: start from nucleus proprius or substantia gelationsa, cross over to contralateral side of spinal cord and ascend to thalamus Third order: transmit info from thalamus to somatosensory cortex Thalamus – relay centre of brain • All somatosensory pathways coming through spinal cord and brainstem converge on the Ventral posterior complex (VP) of the thalamus in an organised fashion • VPL (lateral): receives info from body • VPM (medial): relays info from face • Thalamus projects afferent axons to the primary (SI) and secondary (SII) somatosensory cortices Primary somatosensory cortex • Main input from thalamus to somatosensory cortex coming to layer 4 • Located in the postcentral gyrus and comprises 4 brodmann’s areas (1, 2, 3a, 3b) which form a somatotopic map of the body called homunculus o Cortical homunculus is a neurological map of the anatomical divisions of the body o Density of receptors in some body parts (lips, hands) is much greater → more sensitive Motor system • Part of the CNS involved with movement • Primary motor cortex (M1) lies along the precentral gyrus and generates the signals that control execution of movement (arranged somatotopically) • Secondary motor areas are involved in motor planning • Largest areas of M1 dedicated to hands and face → circuitry required for facial expression, speech and fine motor skills is very large and involves many neurons Pathway of motor system • Motor cortex projects info through UMNS that descend through corticospinal and corticobulbar tracts (main pathway for voluntary movement) o Axons of the corticobulbar tract terminate in the brainstem and innervate cranial nerves o Axons of the corticospinal tract terminate in the spinal cord Corticospinal tract • Most ‘Layer V pyramidal’ axons from the corticospinal tract descend through the internal capsule to the pyramidal decussation in the medulla, where they cross over to the contralateral side → fibres continue to descend through lateral column of spine, terminating at the appropriate spinal levels → synapse with LMNs • 10-20% of axons from corticospinal tract do not cross over and travel through the ventral columns of the spinal cord and terminate either ipsilateral or bilateral • Rapidly-conducting corticomotoneuronal axons travel in the lateral (90% - crossed) and anterior (10% - direct) corticospinal tracts Motor neurons • UMN: neurons that originate from motor cortex and innervate LMNs in ventral horn of spinal cord (1st order) • LMN: neurons that originate from the ventral horn of the spinal cord and leave via the ventral roots to innervate the muscle fibres (2nd order) o o Ventral roots fuse with dorsal roots to form the spinal nerves, then form peripheral nerves Muscles supplied by each spinal nerve are arranged segmentally as myotomes Myotomes • Group of muscles that a single spinal nerve root innervates (myotome = muscles + nerve) • Knowledge of the myotomes is clinically important for defining the level of spinal cord injury • Takes 40 ms for AP to travel from cortex, synapse in spinal cord, evoke AP in big toe Summary • Rapidly conducting sensory axons (afferents) project to the cortex in the dorsal columns of the spinal cord, synapsing first in the medulla (cuneate & gracile nuclei) and then to thalamus • Slowly conducting afferent project to cortex via lateral and anterior spinothalamic tracts, synapsing first in spinal cord and then in thalamus • Rapidly conducting motor axons (efferents) travel from primary motor cortex via the pyramidal tract (lateral corticospinal tract) and the extra-pyramidal tract (ventral corticospinal tract)