Motor and Sensory Pathways PDF

Motor & Sensory Pathways Images from Neuroanatomy: An Atlas of Structures, Sections, and Systems, 7th ed., by Duane E. Haines, © 2007 indicated by “HA”. 1 Pathways • consist of a series of neurons which communicate at synapses cell bodies synapses axons cell bodies synapses axons cell bodies...

Motor & Sensory Pathways Images from Neuroanatomy: An Atlas of Structures, Sections, and Systems, 7th ed., by Duane E. Haines, © 2007 indicated by “HA”. 1 Pathways • consist of a series of neurons which communicate at synapses cell bodies synapses axons cell bodies synapses axons cell bodies PATHWAY • number of neurons / pathway varies • neuronal cell bodies located in ganglia or nuclei • processing, modulation occurs within at synaptic relays • axons grouped to form nerves (in PNS) or tracts (in CNS) • in CNS, pathways are functionally segregated • in PNS, nerves are organized regionally, ie. no functional segregation • bilaterally paired, usually crossed • name often indicates origin & destination of tract 2 Somatic Motor Pathways Control Skeletal Muscle and Consist of a Two-Neuron Chain A B C CNS The upper motor neuron (UMN) D PNS A. cell body at higher centres (eg. cerebral hemispheres, brainstem) B. axon usually decussates, projecting to synapse onto: The lower motor neuron (LMN) C. cell body resides in the motor nucleus of a cranial nerve (face) OR in the ventral horn of the spinal cord (body) D. axon travels through either a cranial or a spinal nerve to synapse onto: Skeletal Muscle 3 CEREBRAL CORTEX Somatic Motor Pathways: LMNs • in the spinal cord, for control of skeletal muscle of the body: – cell bodies of LMNs are located in the ventral horn BRAIN STEM CERVICAL CORD – axons of LMNs reach their target muscle by travelling in the ventral root, spinal nerve, peripheral nerve to NMJ • in the brainstem, for control of skeletal muscle of the face: – cell bodies of LMNs are located in cranial nerve nuclei LUMBAR CORD – axons of LMNs reach their target muscle by travelling in a cranial nerve to the NMJ 4 The “Final Common Pathway”• Cerebral Cortex Vestibular Nuclei Reticular Formation Red Nucleus UMNs in several locations send signals to the LMN, however: • the LMN is the only neuron that contacts the muscle and therefore influences it BRAIN -STEM Lower Motor Neurons Muscle 5 • therefore, it is called the “final common pathway” (Charles Sherrington) • LMNs are typically large neurons with extensive dendritic trees that receive and integrate all the incoming signals BRAINSTEM LMNs Form Bilaterally Paired Motor Columns • brainstem: LMNs form a series of motor nuclei associated with certain cranial Ns • spinal cord: LMNs form the ventral horns CERVICAL CORD THORACIC CORD In the spinal cord: LMNs controlling axial musculature located at all spinal levels • innervate ipsilateral* axial muscles controlling posture & balance LMNs controlling distal limb muscles found in cervical and lumbosacral levels only • innervate ipsilateral distal limb muscles for skilled voluntary movement LUMBAR CORD • forms the lateral portion of the ventral horn, the “cervical (C5 – T1) and lumbosacral (L2 – S2) enlargements” Motor Columns of the Ventral Horn LOW CERVICAL THORACIC LUMBAR Axon-stained sections of the spinal cord 7 • Somatic Motor Pathways: UMNs A A B B LMNs are innervated by descending pathways consisting of upper motor neurons (UMNs) A. Lateral pathways: • UMNs descend in the lateral funiculus; include the lateral corticospinal tract (LCST) • innervate LMNs of the lateral dorsal horn to control distal limb muscles. • function in skilled voluntary movements B. Medial pathways: • UMNs descend in the ventral funiculus; inc. medial corticospinal tract (MCST) • innervate LMNs of the medial ventral horn to control axial and proximal limb muscles • function in the maintenance of posture and balance. 8 The Lateral Corticospinal Tract • Allows for skilled asymmetric limb mvt. 1° motor cortex • The corticospinal tract (CST) originates in 1° motor cortex of the precentral gyrus, which contains the cell bodies of UMNs. Brain stem • Axons of UMNs traverse the subcortical white matter of the cerebral hemispheres & brainstem. Cervical Cord • At the junction of the medulla and spinal cord, most fibres of the CST decussate & thus descend contralaterally in the lateral funiculus as the LCST. • (The remainder descend ipsilaterally as the MCST.) • Axons of the LCST synapse on LMNs in the lateral ventral horn, which innervate distal limb musculature for discrete, skilled movement. LCST LCST Lumbar Cord 9 Sensory Pathways • carry information from receptors to the CNS • interoceptors concern themselves with our internal environment – chemosensors, baroreceptors, stretch receptors, etc. • exteroceptors concern themselves with our external environment – “special senses” (sight, hearing, smell, vestibular) – somatic sensation (modalities: touch, pressure, vibration, conscious proprioception, nociception, temperature) • conscious sensation – a small fraction of sensory input reaches modality-specific 1° sensory cortex in the contralateral cerebral hemisphere – here, determine the location, nature of sensation 10 Basic Rules of Somatosensory Pathways 1. They consist of a three neuron chain, 1°, 2°, 3°. R PNS CNS Midline 2. The 1° neuron is pseudounipolar. Its cell body is located in a ganglion in the PNS. – for the body, the axon travels in a peripheral nerve, spinal nerve & dorsal root, and its cell body is located in a DRG. 3. The cell body of the 2° neuron is either a) in the dorsal horn of the spinal cord OR b) a brainstem nucleus (depending on the somatosensory modality); its axon projects to the contralateral thalamus. ∴ it’s the axon of this 2° neuron that decussates. 4. The cell body of the 3° neuron is located in the VP nucleus of the thalamus. Its axon projects to 1° SS cortex. 11 Functional Localization in the Spinal White Matter: Somatosensory Pathways DORSAL COLUMNS Fasciculus gracilis: touch, vibration & cons. proprioception ~ T6↓ Fasciculus cuneatus: touch, vibration & cons. proprioception ~ T6↑ Spinothalamic tract pressure, pain & temperature 12 The Spinothalamic Tract: Pressure, Pain & Temperature from the Body THALAMI • 1° neurons: • cell bodies lie in dorsal root ganglia in the PNS • project to the CNS via dorsal roots to synapse onto: SL XS THRU LUMBAR CORD: 1° SS CTX BRAIN STEM CERVICAL CORD STT • • • • 2° neurons: STT cell bodies lie in the dorsal horn of the spinal cord axons decussate ascend in the spinothalamic tract (STT) of the ventrolateral funiculus, becoming the: • spinal lemniscus (SL) in the brainstem, synapsing onto: • 3° neurons: • cell bodies lie in the VP nucleus of the thalamus & project to: • 1° somatosensory cortex in the postcentral gyrus LUMBAR CORD 13 Somatotopy within the Spinothalamic Tract As you ascend the spinal cord, more and more axons are added to the STT XS THRU CERVICAL CORD: STT C Homework Assignment 1: How might each lesion present over time? Assume a slowgrowing spaceoccupying lesion. Cervical intramedullary lesion T L S/Co Cervical extramedullary lesion 14 Basic Rules of Somatosensory Pathways 1. They consist of a three-neuron chain, 1°, 2°, 3°. R PNS CNS Midline 2. The 1° neuron is pseudounipolar. Its cell body is located in a ganglion in the PNS. – for the body, the axon travels in a peripheral nerve, spinal nerve & dorsal root, and its cell body is located in a DRG. 3. The cell body of the 2° neuron is either a) in the dorsal horn of the spinal cord OR b) a brainstem nucleus (depending on the somatosensory modality); its axon projects to the contralateral thalamus. ∴ it’s the axon of this 2° neuron that decussates. 4. The cell body of the 3° neuron is located in the VP nucleus of the thalamus. Its axon projects to 1° SS cortex. 15 Functional Localization in the Spinal White Matter: Somatosensory Pathways DORSAL COLUMNS Fasciculus gracilis: touch, vibration & cons. proprioception ~ T6↓ Fasciculus cuneatus: touch, vibration & cons. proprioception ~ T6↑ Spinothalamic tract pressure, pain & temperature 16 Fasciculus Gracilis and Cuneatus Dorsal root ganglion Dorsal column C7 dorsal rootlets Sulci: Dorsal median Dorsal intermediate Dorsal lateral C7 dorsal root Fasciculus gracilis Fasciculus cuneatus HA2.2 Dorsal view of cervical cord 17 Dorsal Columns / Medial Lemniscus System: Touch, Conscious Proprioception, Vibration Sense from the Body 1° neurons: • cell bodies lie in dorsal root ganglia and project to the CNS via the dorsal roots • for the lower body, they ascend in the ipsilateral fasciculus gracilis (FG) to synapse on: XS THRU CERVICAL CORD: THALAMI NG FG 2° neurons: • cell bodies in the nucleus gracilis (NG) of the medulla • axons decussate, ascend contralaterally as the medial lemniscus (ML) through the brainstem, synapsing onto: 3° neurons: • cell bodies in the VP thalamus and project to • 1° somatosensory cortex in the postcentral gyrus. 1° SS CTX BRAIN STEM ML CERVICAL CORD FG LUMBAR CORD Dorsal Columns / Medial Lemniscus System: Touch, Conscious Proprioception, Vibration Sense from the Body 1° neurons: • cell bodies lie in dorsal root ganglia and project to the CNS via the dorsal roots • for the upper body, they ascend in the ipsilateral fasciculus cuneatus (FC) to synapse on: DRG XS THRU CERVICAL CORD: THALAMI FG FC 2° neurons: • cell bodies in the nucleus cuneatus (NC) of the medulla • axons decussate, ascend contralaterally in the medial lemniscus (ML) through the brainstem, synapsing onto: 3° neurons: • cell bodies in the VP thalamus and project to • 1° somatosensory cortex in the postcentral gyrus. 1° SS CTX NG NC FC BRAIN STEM ML CERVICAL CORD FG LUMBAR CORD Fasciculus Gracilis and Cuneatus Dorsal root ganglion Dorsal column C7 dorsal rootlets Sulci: Dorsal median Dorsal intermediate Dorsal lateral C7 dorsal root Fasciculus gracilis Fasciculus cuneatus HA2.2 Dorsal view of cervical cord 20 1° mCTX Brain stem Cervical Cord Thalami SL STT 1° SS CTX Brain stem Cervical cord Thalami NG NC FC 1° SS CTX Brain stem ML Cervical cord FG Lumbar Cord Lumbar cord Lumbar cord 21 1° SS CTX Describe the effects of the lesion BRAIN STEM CERVICAL CORD LESION LUMBAR CORD 22 Describe the effects of the lesion XS THRU CERVICAL CORD: STT XS THRU THORACIC CORD: LESION STT XS THRU LUMBAR CORD: STT