Ch17 Spinal Cord Nerves PDF

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CHAPTER 17 The Spinal Cord and the Spinal Nerves LECTURE OUTLINE A. Spinal Cord Anatomy I. Surrounding and protecting the delicate nervous tissue of the spinal cord (and, in a similar way, the brain) are: 1. the vertebral column, including: i. vertebrae ii. vertebral ligaments 2. three spinal meninges (surrounding the brain and continuous with the spinal meninges are the cranial meninges), which include: 1. the outermost meninx, called the dura mater a. It is composed of dense, irregular connective tissue. b. It extends from the level of the foramen magnum to the second sacral vertebra. c. Surrounding it is the epidural space, which is filled with fat and connective tissue that provide additional protection for the spinal cord. ii. the middle meninx, the avascular arachnoid mater a. It consists of connective tissue with a spiderweb-like arrangement of collagen fibers and some elastic fibers. b. Surrounding it is the subdural space, which is filled with interstitial fluid. iii. the innermost meninx, the pia mater a. It is attached to the surface of the spinal cord (and brain). b. It is a layer of connective tissue that contains collagen fibers and some elastic fibers, as well as many blood vessels that provide nutrients and oxygen to the spinal cord (and brain). c. Surrounding it is the subarachnoid space, which is filled with cerebrospinal fluid, d. The arachnoid mater and the pia mater are collectively called the leptomeninges. iv. All three spinal meninges cover the spinal nerves up to the point of exit from the vertebral column through the intervertebral foramina. v. Membranous extensions of the pia mater called denticulate ligaments suspend the spinal cord in the middle of its dural sheath to provide protection against sudden displacement that could result in shock. 3. a cushion of cerebrospinal fluid (produced in the brain) II. External Anatomy of the Spinal Cord 1. It is roughly cylindrical and extends from the medulla oblongata to the superior border of the second lumbar vertebra in an adult. 2. It has two enlargements, which are sites where nerves supplying the limbs emerge:

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the cervical enlargement extends from the fourth cervical to the first thoracic vertebra ii. the lumbar enlargement extends from the ninth to the twelfth thoracic vertebra At its lower end, the spinal cord has a tapering, cone-shaped portion called the conus medullaris, which in an adult ends at the level of the intervertebral disc between the first and second lumbar vertebrae. An extension of the pia mater called the filum terminale extends inferiorly from the conus medullaris and fuses with the arachnoid mater and dura mater to attach the spinal cord to the coccyx. The roots of spinal nerves emerging from the lower part of the spinal cord travel inferiorly to form the cauda equina.

III. Internal Anatomy of the Spinal Cord 1. Two grooves divide the spinal cord into right and left sides: i. the anterior median fissure, a deep, wide groove on the ventral side ii. the posterior median sulcus, a shallow, narrow groove on the dorsal side 2. The spinal cord contains a core of gray matter, shaped like a butterfly when viewed in transverse section, that is surrounded by white matter. i. The gray matter consists primarily of cell bodies of neurons, neuroglia, unmyelinated axons, and the dendrites of interneurons and motor neurons. ii. The white matter consists of bundles of myelinated axons of motor neurons, interneurons, and sensory neurons. 3. The gray commissure is a region of gray matter that connects the two wings of the butterfly. 4. At the center of the gray commissure is the central canal, which extends throughout the entire length of the spinal cord; it is continuous with the fourth ventricle of the brain. 5. Anterior to the gray commissure is the anterior (ventral) white commissure, which connects the white matter of the left and right sides of the spinal cord. 6. The gray matter contains sensory nuclei and motor nuclei; the gray matter on each side of the spinal cord is subdivided into regions called horns. i. The anterior (ventral) gray horns contain cell bodies of somatic motor neurons and motor nuclei, which provide nerve impulses for the contraction of skeletal muscles. ii. The posterior (dorsal) gray horns contain somatic and autonomic sensory nuclei. iii. The lateral gray horns (which are present only in the thoracic, upper lumbar, and sacral segments of the spinal cord) contain cell bodies of autonomic motor nuclei that regulate the activities of involuntary effectors. 7. The white matter is subdivided by the anterior and posterior gray horns into regions called columns: i. anterior (ventral) white columns ii. posterior (dorsal) white columns iii. lateral white columns 8. Each column contains bundles of nerve axons called tracts, which are continuous with tracts in the brain:

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Sensory (ascending) tracts transmit nerve impulses upward to the brain. ii. Motor (descending) tracts transmit nerve impulses downward from the brain. The various spinal cord segments vary in size, shape, relative amounts of gray and white matter, and distribution and shape of gray matter; these features are summarized in Table 17.1.

B. Spinal Nerves I. Spinal nerves and their branches are components of the PNS. 1. Each of the 31 pairs of spinal nerves emerges from a spinal segment; the spinal nerves are named and numbered according to the region and level of the spinal cord from which they emerge: i. 8 pairs of cervical nerves (the first pair emerges between the atlas and the occipital bone), represented as C1–C8 ii. 12 pairs of thoracic nerves, represented as T1–T12 iii. 5 pairs of lumbar nerves, represented as L1–L5 iv. 5 pairs of sacral nerves, represented as S1–S5 v. 1 pair of coccygeal nerves, represented as Co1 II. Every spinal nerve (and cranial nerve) is surrounded and protected by connective tissue coverings. 1. Each nerve fiber (axon and its associated glial cells) is wrapped in a layer called the endoneurium. 2. Groups of axons with their endoneuria are arranged in bundles called fasciculi, and each fasciculus is wrapped in a layer called the perineurium. 3. Groups of fasciculi collectively form a nerve, which is covered by a layer called the epineurium. III. Each spinal nerve arises from the spinal cord as a series of small rootlets that converge to form larger roots; each spinal nerve is formed by the merger of two roots: i. The anterior or ventral root contains motor neuron axons, which transmit nerve impulses from the spinal cord to effector organs and cells. ii. The posterior or dorsal root contains sensory nerve fibers, which transmit nerve impulses from the periphery into the spinal cord; it has an enlargement called the posterior or dorsal root ganglion that contains the cell bodies of these sensory neurons. IV. The merger of a posterior root and an anterior root (which occurs at an intervertebral foramen) results in every spinal nerve being a mixed nerve called a spinal nerve trunk. V. Shortly after passing through its intervertebral foramen, a spinal nerve divides into several branches called rami. 1. The posterior (dorsal) ramus serves the deep muscles and skin of the posterior surface of the trunk. 2. The anterior (ventral) ramus serves the muscles and structures of the limbs and the skin of the lateral and anterior surfaces of the trunk. 3. The meningeal branch reenters the vertebral canal via the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges. 4. The rami communicantes are components of the autonomic nervous system. VI. The anterior rami of spinal nerves, except for thoracic nerves T2–T12, form networks on both the right and left sides of the body that are called plexuses; emerging from the

plexuses are nerves whose names often describe the general regions they serve or the routes that they follow. VII. The major pairs of plexuses (see Exhibits 17.A–17.D) are: 1. cervical plexuses 2. brachial plexuses 3. lumbar plexuses 4. sacral plexuses (two small coccygeal plexuses are also present) IX. Dermatomes (Omit) C. Spinal Cord Functions (Omit)...