The Spinal Cord 2024-25 PDF

BI2432: Fundamental Neuroscience Spinal cord Dr Sean Wyatt ([email protected]) Each segment of the spinal cord gives rise to a pair of spinal nerves brain Spinal...

BI2432: Fundamental Neuroscience Spinal cord Dr Sean Wyatt ([email protected]) Each segment of the spinal cord gives rise to a pair of spinal nerves brain Spinal cord DRG sensory DRG C1-C8 neuron dorsal 30 pairs of spinal nerves root T1-T12 spinal nerves L1-L5 motor neuron spinal nerve S1-S5 ventral root  Vertebrae and spinal nerves C1 Each vertebra is associated with 7 Cervical vertebrae: C1 to C7 a pair of spinal nerves projecting C7 from the spinal cord T1 Exception C1 vertebra is associated with 2 pairs of spinal nerves 12 Thoracic vertebrae: T1 to T12 - C1 spinal nerves (contain motor axons only) arise from T12 spinal cord above C1 vertebra L1 - C2 spinal nerves (motor + sensory 5 Lumbar vertebrae: L1 to L5 axons) arise from spinal cord L5 below C1 vertebra 5 Sacral vertebrae: S1 to S5 (fused)) - C3 spinal nerves (motor + sensory Coccyx: 3-4 small fused vertebrae axons ) arise from spinal cord below C2 vertebra and so on ……………….  The spinal cord is protected spinous Spinous process dorsal process within the vertebral column epidural space containing small veins and fat dura vertebral arch meninges arachnoid spinal cord pia dorsal roots (sensory axons) The subarachnoid space between the arachnoid and pia contains csf and small blood vessels spinal nerve carrying sensory and motor neuron axons T1 to L2 spinal nerves also carry DRG postganglionic sympathetic neuron axons (sensory neurons) ventral roots (motor neuron axons) intervertebral disc + preganglionic sympathetic vertebral body neuron axons in T1-L2 ventral  Rootlets, Roots and Nerves dorsal DRG dorsal root (dorsal root rootlets dorsal ganglia) dorsal rootlets root dorsal roots dorsal root ganglia ventral root ventral spinal spinal nerves nerve rootlets Note: (a) how the rootlets form the roots and the roots form the nerves (b) the position of the DRG on the dorsal root (c) ventral roots are not associated with a ganglion  Organisation of a Vertebral Section Dorsal (posterior) ramus of spinal nerve supplies sensory, motor and sympathetic innervation to the back Ventral (anterior) ramus of spinal nerve supplies sensory, motor and Vertebral body sympathetic innervation to the antero- lateral parts of the trunk and the limbs  White and grey matter in the spinal cord dorsal Neuron types associated grey matter (neurons and glia) rootlets with the spinal cord Dorsal Root Ganglia sensory DRG neurons – centrally projecting axon terminals in the dorsal horn dorsal horn Motor neurons lateral ventral horn horn intermediate zone Preganglionic sympathetic neurons ventral intermedio-lateral horn horn (T1- L2) Interneurons (all grey matter regions) spinal nerve projection interneurons white matter (myelinated ventral rootlets axon fibre tracts) local circuit interneurons  The spinal cord is not uniform in cervical enlargement (origin of brachial plexus) shape and ends at vertebral level L1/L2 pedicles of vertebrae (connecting vertebral body to arch) The lumbosacral enlargement (thickening of the spinal cord) gives rise to: DRG - L1 to L5 lumbar spinal nerves - S1 to S5 sacral spinal nerves - the coccygeal nerve. lumbosacral enlargement Spinal cord segments in the lumbar enlargement (origin of lumbar are compressed compared to cervical and and sacral plexi) thoracic segments cauda equina After the spinal cord ends: L2 - S5 spinal nerve roots + the coccygeal nerve run caudally (downwards) in the cauda equina until they emerge at their appropriate vertebral level as spinal nerves  The cauda equina and lumbar cistern cervical enlargement The spinal cord ends at the conus medullaris (vertebral level L1/L2) The pia surrounding the spinal cord fuses and lumbosacral forms the filum teminale that anchors the enlargement spinal cord to the coccyx L2 to S5 spinal nerve roots + coccygeal T12 nerve run caudally in the cauda equina to conus L1 emerge at their appropriate vertebral level. medullaris conus L2 medullaris L3 Dura and underlying arachnoid continue to cauda S2/S3 vertebral levels where they merge with L4 lumbar cistern equina the filum terminale L5 filum Between L1/L2 and S2/S3 the csf S1 S2 Filum terminale S3 containing subarachnoid space is end of dura/ enlarged (lumbar cistern) arachnoid Clinical interventions associated with the spinal cord Epidural injection Lumbar puncture spinal cord Injection into epidural space Needle enters enlarged subarachnoid (outside of dura mater) space (lumbar cistern) to collect CSF The ratio of white to grey matter alters along the rostrocaudal axis rostral spinal cord The shape of grey matter C4 cervical Increasing proportion Cervical levels cervical enlargement of ascending and - skinny top butterfly descending axons C7 (white matter) Thoracic levels - underweight butterfly T2 thoracic Lumbar levels T12 - well proportioned butterfly lumbar enlargement L5 Sacral levels lumbar Increasing proportion - fat butterfly sacral S3 of grey matter caudal filum terminale S4 spinal cord Major ascending and descending axon tracts of the spinal cord Ascending tracts (sensory) Spinothalamic tract: pain, temperature, itch and simple touch Dorsal column medial lemniscus pathway: discriminative touch and conscious proprioception Spinocerebellar pathway: non-conscious proprioception Descending tracts (motor) Pyramidal motor pathway: conscious movement – lateral and anterior corticospinal tracts Non-pyramidal motor pathways: largely non-conscious muscle control for balance/posture/reflexes For example:- - Reticulospinal tract: muscle tone, posture and simple stereotyped movements. - Vestibulospinal tract: coordinating balance and keeping the head balanced and eyes horizontal when moving  primary Spinothalamic Tract sensory (somatotopically mapped) cortex Pain, temperature, itch and simple touch = Spinothalamic tract Ventral posterior thalamic nuclei (somatotopically mapped) midbrain pons medulla DRG spinal cord projection interneuron laminae I or V decussation  primary Dorsal Column sensory cortex Pathway (somatotopically Discriminative (fine) touch Ventral posterior mapped) and conscious proprioception thalamic nuclei (somatotopically mapped) dorsal sacral gracile midbrain columns lumbar fasciculus thoracic cuneate cervical fasciculus Dorsal column pons nuclei (nucleus cuneatus (N.C.) (contralateral) & nucleus gracilis (N.G.) N.G. N.C. medial lemniscus (cell bodies of projection interneurons) medulla dorsal column fibre tract Low-threshold mechanoreceptive DRG neuron DRG spinal cord  primary motor cortex Pyramidal motor pathway (upper motor neurons) (corticospinal tracts) i.c. Upper motor neuron axons pass through the internal capsule (i.c.) on their way to the midbrain 90% of pyramidal motor axons cross the midline at midbrain the junction of the medulla and spinal cord - travel in the lateral corticospinal tract (l.c.t.) pons - make synaptic connections to lower motor neurons innervating limb muscles 10% of pyramidal neurons do not decussate here: - travel in the anterior/ventral corticospinal tract medulla - make connections to lower motor neurons innervating trunk/axial muscles Decussation (90%) - most decussate at level of lower motor neuron l.c.t. spinal cord a.c.t. Spinocerebellar pathway for non-conscious proprioception medulla lateral cuneate nucleus interneuron axon proprioceptive DRG neuron ipsilateral cerebellum dorsal (little decussation in columns this pathway) Spinocerebellar tracts Proprioceptive neurons innervate: muscle spindles – detect muscle nucleus of Clarke length (C8-L3 lamina VII) golgi tendon organs – detect muscle projection interneuron soma tension Motor neuron Reflex circuit = premotor interneuron  Spinal cord grey matter is divided into distinct regions (laminae) (i) Lamina I: contains the central terminals of nociceptive DRG sensory neurons and soma of nociception specific projection interneurons relaying pain information to the thalamus (spinothalamic tract) (limb muscles) Lamina II: central terminals of nociceptive DRG neurons + local inhibitory/excitatory interneurons that make (axial muscles) connections to sensory projection interneurons in I and V Skin and muscle DRG Lamina III-V: central terminals of mechanoreceptive DRG neurons and + associated local interneurons nociceptors Lamina V: soma of wide dynamic range sensory mechano- projection interneurons (relaying information on pain, non- receptors noxious temperature, simple touch, visceral sensation) prioprioceptors  Spinal cord grey matter is divided into distinct regions (laminae) (ii) Lamina V-VII: central terminals of proprioceptive and visceral sensory DRG neurons + associated interneurons Lamina VII: soma of sensory projection neurons of the spinocerebellar tract (non-conscious proprioception), (limb muscles) Lamina VII: soma of preganglionic sympathetic neurons (axial muscles) (T1-L2) and preganglionic parasympathetic neurons (S2- S4) Skin and muscle DRG Lamina VIII/IX: soma of lower motor neurons projecting to nociceptors skeletal muscle - distinct pools/nuclei innervate proximal (axial) mechano- receptors or distal (limb) muscles and either flexor or extensor muscles prioprioceptors  Somatic motor Cervical functions associated C1/2 movement of head and neck C3 diaphragm (breathing) with spinal nerves C4 diaphragm (breathing), shoulder shrug C5 deltoid (lifts arms sideways) + biceps C6 wrist extensors (lifts wrists back) Thoracic C7 triceps (straightens elbow) T1 hands and fingers C8 hands and fingers T2 -T5 chest muscles T6 -T8 chest + abdominal muscles T9 -T12 abdominal muscles Lumbar L1 hip muscles L2 hip muscles Sacral L3 knee extension muscles (straightens knee) S1 leg and toe muscles (points L4 knee flexion + ankle movement muscles foot) L5 ankle and toe muscles (lifts big toe S2 toe muscles + anal and and foot) bladder external sphincters S3 - S5 anal and bladder external sphincters Spinal cord lesions and somatic + autonomic motor function impairment functions affected functions affected breathing (C3-C4) + head/neck movement (C1-2) Regulation of heart rate (C4-C6) + shoulder movement (C4-5) wrist and elbow movement (C5-C7) hand and finger movement (C8-T1) sympathetic tone - including temperature regulation (T1-T12) trunk stability (T2-T12) ejaculation (T11-L2) and hip motion (L2) knee extension (L3) foot motion (L4-S1) and knee flexion (L4-5) penile erection (S2-S4) and and control of bladder/bowel activity (S2-S3)