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Human Anatomy, Second Edition

McKinley & O'Loughlin

Chapter 16A Lecture Outline: Spinal Cord and Spinal

Nerves

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The Spinal Cord A vital link between the brain and the

rest of the body. Some functional independence from the

brain. The spinal cord and its attached spinal

nerves serve two important functions. pathway for sensory and motor

impulses reflexes

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Gross Anatomy of the Spinal Cord A typical adult spinal cord is

between 42 and 45 centimeters (16 to 18 inches) in length.

Extends inferiorly from the brain through the vertebral canal and ends at the level of the L1 vertebra.

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Parts of the Spinal Cord The cervical part is continuous with the medulla

oblongata The thoracic part The lumbar part The sacral part The coccygeal part is the inferior “tip” of the

spinal cord. Note: The parts do not line up with the vertebrae

of the same name because growth of the vertebrae continues longer than the spinal cord. Therefore, the adult spinal cord is shorter than the vertebral canal.

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Structure of the Spinal Cord The tapering end of the spinal cord is called

the conus medullaris. Nerve roots (groups of axons collectively

called the cauda equina) project inferiorly from the spinal cord.

Within the cauda equina is the filum terminale, a thin strand of pia mater that helps anchor the conus medullaris to the coccyx.

The filum terminale is part of the coccygeal ligament (extension of dura mater).

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Structure of the Spinal Cord In cross section, it is roughly a flattened

cylinder with two longitudinal depressions. the posterior (or dorsal) median sulcus the anterior (or ventral) median fissure

Enlargements of the spinal cord Cervical enlargement for neurons

innervating upper limbs (C4-T1) Lumbar enlargement for neurons

innervating lower limbs (T9-T12)

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Spinal Tap (Lumbar Puncture) At level of L3-L4 or L4-L5 which

is below the end of spinal cord at L1-L2 in adult. Iliac crests are at L4 and serve as a

landmark Spinal cord ends in infants at L3-L4

until 4-5 years.

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Structure of the Spinal Cord 31 pairs of spinal nerves connect the CNS

to muscles, receptors, and glands. Each side of the spinal cord contains

8 cervical nerves (called C1–C8) 12 thoracic nerves (T1–T12) 5 lumbar nerves (L1–L5) 5 sacral nerves (S1–S5) 1 coccygeal nerve (Co).

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Spinal Cord Meninges Continuous with the cranial meninges. Structures that encircle the spinal cord, listed from

outermost to innermost are: vertebra epidural space

fat, C.T., blood vessels below L2 is an epidural injection site

dura mater dense, irregular C.T. extends to second sacral vertebra

subdural space interstitial fluid

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Spinal Cord Meninges arachnoid

delicate collagenous fibers and elastic fibers, avascular

subarachnoid space CSF

pia mater transparent, collagen and elastic fibers, blood

vessels denticulate ligaments of pia mater

extend to the dura mater to hold cord laterally

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Meningitis Inflammation of the meninges

(may be bacterial or viral) Most common is the inflammation

of the arachnoid and pia mater

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Sectional Anatomy of the Spinal Cord

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Location and Distribution of Gray Matter Centrally located in the shape of a letter H or a

butterfly. The gray matter is dominated by

dendrites and cell bodies of neurons, glia cells and unmyelibnated axons.

The gray matter may be subdivided into the: anterior horns lateral horns posterior horns the gray commissure surrounds the

central canal

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Location and Distribution of Gray Matter Nuclei are groups of functional groups of

neuron cell bodies Sensory nuclei in the posterior horns

contain interneuron cell bodies Somatic sensory nuclei Visceral sensory nuclei

Motor nuclei in the anterior and lateral horns contain motor neuron cell bodies that send nerve impulses to muscles and glands

Somatic motor nuclei Autonomic motor nuclei

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Location and Distribution of White Matter The white matter of the spinal cord is

composed primarily of myelinated axons and is external to the gray matter.

Divided into three regions on each side. A posterior funiculus A lateral funiculus An anterior funiculus

The anterior funiculi are interconnected by the white commissure.

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Location and Distribution of White Matter The amount of white matter

increases as one gets closer to the brain

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Spinal Nerves 31 pairs of spinal nerves connect the central nervous

system to muscles, glands, and receptors Each spinal nerve is formed from the union of thousands of

motor and sensory axons. Motor axons originate from the anterior gray horn of the

spinal cord. Each anterior root and its corresponding posterior root

unite within the intervertebral foramen to become a spinal nerve.

Contain both motor axons and sensory axons, mixed. Each spinal nerve is associated with the vertebra of the

same number. Go through the intervertebral foramina except for C1.

Not all in a line!

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Coverings Endoneurium around the

axon. Perineurium around the

fascicle. Epineurium around the entire

nerve.

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Dermatomes A specific segment of skin

supplied by a single spinal nerve. All spinal nerves except for C1 innervate a

segment of skin, and so each of these nerves is associated with a dermatome.

The skin of the body may be divided into sensory segments that collectively make up a dermatome map.

Trunk regions overlap, limb regions less overlap, some areas served by one spinal nerve only

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Dermatomes and Myotomes Implications:

Diagnostically useful: stimulate a dermatome, if no sensation, then know spinal nerve(s) affected

Destruction of a single spinal nerve may not result in complete numbness because of overlap

For complete regional anaesthesia, at least 3 adjacent spinal nerves must be cut or blocked

Shingles is a reactivation of the chicken pox virus. The virus retreats to the dorsal root ganglion and later travels the sensory axons usually to the intercostal dermatomes. Blisters and skin discoloration are usually unilateral.

Destruction of a single spinal nerve can’t completely paralyze any limb muscle (myotomes)

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Spinal Nerve Distribution After the intervertebral foramen, the spinal nerve

branches into rami Dorsal ramus

Deep muscles and skin of posterior trunk Ventral ramus

Superficial muscles of back, muscles and structures of extremities and lateral and ventral trunk

Meningeal branch Reenters thru intervertebral foramen to vertebrae,

vertebral ligaments, blood vessels and meninges Rami communicantes

Part of ANS (cover later)

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Nerve Plexuses Anterior rami of most spinal nerves form nerve

plexuses, networks, on both the right and left sides of the body.

Nerve plexuses then split into multiple “named” nerves that innervate various body structures.

Nerves named by region or path Branches of nerves named by specific structures

innervated Nerves are mixed

Principal plexuses are the cervical plexuses, brachial plexuses, lumbar plexuses, and sacral plexuses.

Mainly serve the limbs.

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Intercostal Nerves Anterior rami of spinal nerves T1–T11 form the

intercostal nerves, but no thoracic plexus. However, T1 is part of the brachial plexus

Travel in the intercostal space sandwiched between two adjacent ribs.

Intercostal (thoracic) nerves serve intercostal and abdominal muscles and overlying skin mainly.

T12 is called a subcostal nerve because it is not between the ribs.

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