Chapter 16 A, Sp 10

Human Anatomy, Second Edition McKinley & O'Loughlin Chapter 16A Lecture Outline: Spinal Cord and Spinal Nerves

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 16-

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 L 1 vertebra. 16-

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. 16-

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). 16-

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 (C 4 -T 1 ) Lumbar enlargement for neurons innervating lower limbs (T 9 -T 12 ) 16-

Spinal Tap (Lumbar Puncture) At level of L 3 -L 4 or L 4 -L 5 which is below the end of spinal cord at L 1 -L 2 in adult . Iliac crests are at L 4 and serve as a landmark Spinal cord ends in infants at L 3 -L 4 until 4-5 years. 16-

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 C 1 –C 8 ) 12 thoracic nerves (T 1 –T 12 ) 5 lumbar nerves (L 1 –L 5 ) 5 sacral nerves (S 1 –S 5 ) 1 coccygeal nerve (C o ). 16-

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 L 2 is an epidural injection site dura mater dense, irregular C.T. extends to second sacral vertebra subdural space interstitial fluid 16-

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 16-

Meningitis Inflammation of the meninges (may be bacterial or viral) Most common is the inflammation of the arachnoid and pia mater 16-

Sectional Anatomy of the Spinal Cord 16-

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 16-

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 16-

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. 16-

Location and Distribution of White Matter The amount of white matter increases as one gets closer to the brain 16-

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 C 1 . Not all in a line! 16-

Coverings Endoneurium around the axon. Perineurium around the fascicle. Epineurium around the entire nerve. 16-

Dermatomes A specific segment of skin supplied by a single spinal nerve. All spinal nerves except for C 1 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 16-

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 ) 16-

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) 16-

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 . 16-

Intercostal Nerves Anterior rami of spinal nerves T 1 –T 11 form the intercostal nerves, but no thoracic plexus . However, T 1 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. T 12 is called a subcostal nerve because it is not between the ribs. 16-

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