16 Spinal Cord

STUDYAIMS

At the end of your study, you should be able to:

Draw a schematic transverse section through the spinal cord, meninges, and vertebrae

Understand the structure and function of the dorsal and ventral spinal nerve roots and rami

Understand the general topographyand synaptic transmitters of the autonomic nervous system

Know the levels of the principle dermatomes

Describe the vasculature supplyof the spinal cord and vertebral column

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GUIDE

Back and Spinal Cord: Spinal Cord

Spinal Cord

[Plate 157, Spinal Cord and Ventral Rami In Situ]

Originates at the inferior end of the medulla oblongata and terminates as conus medullaris Is approximately42 to 45 cm long from the foramen magnum to L2 (variable from T12-L3)

Is connected from the conus medullaris to the coccyxbythe filum terminale (a strand of connective tissue that connects exits from the dural sac and passes through the sacral hiatus)

Has two regional enlargements

Cervical-origin of the brachial plexus innervating the upper limb

Lumbosacral-origin of the lumbar and sacral plexuses innervating the lower limb Has the following features in cross section

Dorsal median sulcus and ventral median fissure that divide cord into symmetrical halves

Central canal carrying cerebrospinal fluid (CSF)

White matter surrounding an H-shaped core of greymatter (ventral and dorsal horns)

Structure of Spinal Nerves

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[Plate 158, Relation of Spinal Nerve Roots to Vertebrae]

page 89 page 90

31 pairs of spinal nerves

8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

C1-C7 exit superior to corresponding vertebrae

C8 exits inferior to the C7 vertebra

T1-Co exit inferior to corresponding vertebrae

Cauda equina: spinal nerve roots inferior to the conus medullaris, travelling obliquelyto exit vertebral canal Ventral roots

Carryefferent (motor) fibers with their cell bodies in the ventral horn of the cord

Maycontain presynaptic autonomic fibers Dorsal roots

Carryafferent (general and visceral sensory) fibers with their cell bodies in the dorsal root ganglion (DRG)

Maybe absent in C1 and Co

Ventral and dorsal roots: combine to form a (mixed) spinal nerve which exits through the intervertebral foramen and divides almost immediatelyinto (mixed) ventral and dorsal rami

Ventral rami

Anterior and lateral branches

Form plexuses and supplythe limbs and trunk Dorsal rami

Medial and lateral branches

Supplythe skin and true muscles of the back

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[Plate 162, Spinal Membranes and Nerve Roots]

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[Plate 163, Spinal Nerve Origin: Cross Sections]

Meninges

Dura mater: Tough fibroelastic membrane

Is continuous with the inner (meningeal) layer of the cranial dura

Attached to the margins of the foramen magnum and posterior longitudinal ligament

Separated bythe epidural space from vertebral periosteum

Extends as a sac from the margin of the foremen magnum to the level of S2

Pierced byspinal nerves

Anchored to the coccyxbythe external filum terminale

Forms dural root sleeves covering the spinal nerves before fusing with the epineurium Arachnoid mater: Delicate, avascular, fibroelastic membrane lining dural sac

Opposed (held to inner surface) to dura byCSF pressure

Is external to the subarachnoid space, between arachnoid and pia, containing CSF, traversed bystrands of connective tissue (arachnoid trabeculae)

Contains the lumbar cistern, an enlargement of subarachnoid space between L2 (end of spinal cord) and S2 (end of dural sac) Pia mater: Highlyvascular innermost layer covering roots of spinal nerves

Continues as the filum terminale

Suspends the spinal cord within the dural sac bylateral extensions between the anterior and posterior roots, called denticulate ligaments

Dermatomes: a well-defined strip of skin extending from the anterior to the posterior midline, supplied bya single spinal nerve

Cervical supplyneck and upper limb

Thoracic supplytrunk (T1 also supplies upper limb)

C5 abuts T1 on the superior anterior chest wall

Lumbar predominantlysupplyanterior lower limb

Sacral predominantlysupplyposterior lower limb

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Dermatome levels to know:

[Plate 159, Dermatomes]

page 90

page 91

Vasculature of Spinal Cord and Vertebral Column

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[Plate 167, Veins of the Vertebral Column: Vertebral Veins]

Anterior spinal artery

Is a single arteryrunning in ventromedian fissure

Arising from branches of the vertebral arteries (with contributions from the ascending cervical, deep cervical, intercostal, lumbar, and sacral arteries)

Supplies the anterior two thirds of the spinal cord and vertebral bodies Posterior spinal arteries

Are paired, longitudinal arteries arising from vertebral or posterior inferior cerebellar arteries

Supplyposterior one third of the spinal cord and vertebral bodies Radicular arteries

Are dorsal and ventral arteries arising from ascending cervical, deep cervical, intercostal, lumbar, and sacral arteries

Supplythe nerve roots (called segmental arteries if they reach the anterior or posterior spinal arteries)

Great anterior segmental artery(ofAdamkiewicz)

Occurs on the left side in 65% of individuals

Contributes to two thirds of the circulation to the inferior spinal cord

Veins: Usually3 anterior and 3 posterior longitudinal spinal veins with tributaries from the posterior medullaryand radicular veins. They drain into the valveless vertebral venous plexus.

Vertebral venous plexus is continuous with the cranial dural venous sinuses and contains no valves

Internal vertebral plexus (lying in the extradural space) drains the spinal cord External vertebral plexus connects with azygos vein, superior and inferior vena cavae

Autonomic Nervous System (ANS)

page 91 page 92

Sympathetic NS: catabolic system for fight or flight

T1-L2/3 (thoracolumbar) levels

Presynaptic (preganglionic) neurons have cell bodies located in the intermediolateral cell columns of the spinal cord (T1-L2 only) and utilize acetylcholine (Ach) as their neurotransmitter and synapse in paraor prevertebral ganglia

Postsynaptic (postganglionic) neurons have cell bodies in the paravertebral and prevertebral ganglia

Paravertebral ganglia linked to form right and left sympathetic chains (superior, middle and inferior cervical ganglia, T1-S5, ganglion impar)

Paravertebral ganglia attached to spinal nerves bywhite (T1-L2) and grey(C1-Co) rami communicantes

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Long postsynaptic neurons utilize norepinephrine as their neurotransmitter

Prevertebral ganglia (celiac, superior, and inferior mesenteric, aorticorenal) are in plexuses surrounding the origins of the main branches of the abdominal aorta

Splanchnic nerves are presynaptic fibers that pass through the paravertebral ganglia without synapsing to enter cardiac, pulmonary, esophageal, various abdominal and pelvic plexuses, where theysynapse

Sympathetic fibers innervate smooth muscle, modified cardiac muscle, glands, and medulla of suprarenal glands Parasympathetic NS: anabolic system for homeostasis

S2-S4 levels and cranial nerves III, VII, IX, X(craniosacral)

Long presynaptic neurons (Ach) with cell bodies in the mediolateral greymatter (S2-S4)

Short postsynaptic neurons arising near target organs (Ach)

Innervation of smooth muscle, modified cardiac muscle, and glands of thoracic, abdominal, and pelvic viscera Visceral afferent NS: provides sensoryinput from the body's internal environment

Provides visceral sensation

Can trigger both somatic and visceral reflexes

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