1 The Spinal Cord and the Spinal Nerves Chapter 18 CHAPTER SUMMARY This chapter provides a detailed description of the anatomy and functions of the spinal cord and spinal nerves. The protection provided by the vertebral column and the meninges is explained. The external anatomy and the internal anatomy of the spinal cord are portrayed in fine detail. The functions of the spinal cord are explained; the characteristics and roles of sensory tracts, motor tracts, and reflexes are explained. The 31 pairs of spinal nerves are identified and their protective coverings and their attachments to the spinal cord are described. The distribution of spinal nerves (i.e., branches, plexuses, and intercostal nerves) is portrayed. The dermatomes of the body are described. This chapter concludes with a list of key medical terms associated with the spinal cord and spinal nerves, a thorough study outline, an excellent self-quiz, critical thinking questions, and answers to questions that accompany chapter figures. STUDENT OBJECTIVES 1. 2. 3. 4. 5. 6. 7. Describe the protective coverings of the spinal cord. Describe the external and internal anatomy of the spinal cord. Describe the functions of the principal sensory and motor tracts of the spinal cord. Describe the functional components of a reflex arc and how reflexes contribute to homeostasis. Describe the components, connective tissue coverings, and branches of a spinal nerve. Define a plexus, and identify the distribution of nerves of the cervical, brachial, lumbar and sacral plexuses. Describe the clinical significance of dermatomes. LECTURE OUTLINE A. Spinal Cord Anatomy (p. 554) 1. Surrounding and protecting the delicate nervous tissue of the spinal cord (and, in a similar way, the brain) are: i. vertebral column, including: a. vertebrae b. vertebral ligaments ii. three spinal meninges (surrounding the brain and continuous with the spinal meninges are the cranial meninges), which include: a. outermost meninx is the dura mater - it is composed of dense, irregular connective tissue - it extends from the level of the foramen magnum to the second sacral vertebra where it is close-ended - surrounding it is the epidural space filled with fat and connective tissue which provide additional protection to the spinal cord b. middle meninx is the avascular arachnoid mater - it consists of connective tissue with a spider web-like arrangement of collagen fibers and some elastic fibers - surrounding it is the subdural space filled with interstitial fluid c. innermost meninx is the pia mater - it is attached to the surface of the spinal cord (and brain) - 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) - surrounding it is the subarachnoid space filled with cerebrospinal fluid All three spinal meninges cover the spinal nerves up to the point of exit from the vertebral column through the intervertebral foramina. Membranous extensions of the pia mater called denticulate ligaments suspend the spinal cord in the middle of its dural sheath to provide protection against shock and sudden displacement. 2 iii. cushion of cerebrospinal fluid (produced in the brain) 2. External Anatomy of the Spinal Cord: (p. 554) i. It is roughly cylindrical and extends from the medulla oblongata to the superior border of the second lumbar vertebra in an adult. ii. It has two enlargements which are sites where nerves supplying the limbs emerge: a. cervical enlargement extends from the fourth cervical to the first thoracic vertebra b. lumbar enlargement extends from the ninth to the twelfth thoracic vertebra iii. At its lower end, the spinal cord has a tapering, cone-shaped portion called the conus medullaris which ends at the level of the intervertebral disc between the first and second lumbar vertebrae in an adult. iv. An extension of the pia mater called the filum terminale extends inferiorly from the conus medullaris to attach the spinal cord to the coccyx. v. The roots of spinal nerves emerging from the lower part of the spinal cord travel inferiorly to form the cauda equina. vi. Each of the 31 pairs of spinal nerves emerge 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: a. 8 pairs of cervical nerves (the first pair emerge between the atlas and the occipital bone) represented as C1-C8 b. 12 pairs of thoracic nerves represented as T1-T12 c. 5 pairs of lumbar nerves represented as L1-L5 d. 5 pairs of sacral nerves represented as S1-S5 e. 1 pair of coccygeal nerves (represented as Co1) vii. Each spinal nerve is attached to a spinal segment by two bundles of axons called roots: a. 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 which contains the cell bodies of the sensory neurons b. anterior or ventral root contains motor neuron axons which transmit nerve impulses from the spinal cord to effector organs and cells 3. Internal Anatomy of the Spinal Cord: (p. 558) i. Two grooves divide the spinal cord into right and left sides: a. anterior median fissure is a deep, wide groove on the ventral side b. posterior median sulcus is a shallow, narrow groove on the dorsal side ii. The spinal cord contains a core of gray matter, shaped like a butterfly when viewed in cross section, that is surrounded by white matter: a. the gray matter consists primarily of cell bodies of neurons, neuroglia, unmyelinated axons, and dendrites of interneurons and motor neurons b. the white matter consists of bundles of myelinated and unmyelinated axons of motor neurons, interneurons, and sensory neurons iii. The gray commissure is a region of gray matter that connects the two wings of the butterfly. iv. 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. v. 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. vi. 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: a. anterior (ventral) gray horns contain cell bodies of somatic motor neurons and motor nuclei which provide nerve impulses for contraction of skeletal muscles b. posterior (dorsal) gray horns contain somatic and autonomic sensory nuclei c. 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 neurons that regulate activities of involuntary effectors 3 vii. The white matter is subdivided by the anterior and posterior gray horns into regions called columns: a. anterior (ventral) white columns b. posterior (dorsal) white columns c. lateral white columns viii. Each column contains bundles of nerve axons called tracts: a. ascending (sensory) tracts transmit nerve impulses upward to the brain b. descending (motor) tracts transmit nerve impulses downward from the brain Spinal cord tracts are continuous with tracts in the brain. ix. 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 18.1. B. Spinal Cord Functions (p. 560) 1. The spinal cord has two major functions in maintaining homeostasis: i. The tracts in the white matter of the spinal cord transmit nerve impulses between the brain and the periphery. ii. The gray matter of the spinal cord receives and integrates the incoming and outgoing information. 2. Sensory and Motor Tracts: i. A tract is often named according to its position in the white matter, where it begins and ends, and the direction of nerve impulse transmission; e.g., the anterior spinothalamic tract is located in the anterior white column, and it begins in the spinal cord and ends in the thalamus - since it transmits nerve impulses upward, it is an ascending (sensory) tract. The tracts are summarized in Table 21.3 and 21.4. ii. Sensory information from receptors is transmitted up the spinal cord to the brain via two main ascending routes on each side of the spinal cord: a. spinothalamic tracts transmit nerve impulses for sensing: - pain - temperature - deep pressure - itching and tickling - crude, poorly localized touch b. posterior columns transmit nerve impulses for sensing: - proprioception - discriminative touch - two-point discrimination - light pressure - vibrations iii. Motor output from the brain to skeletal muscles travels down the spinal cord via two types of descending pathways on each side of the spinal cord: a. direct (pyramidal) pathways (the lateral corticospinal, anterior corticospinal, and corticobulbar tracts) transmit nerve impulses destined to cause precise, voluntary movements of skeletal muscles b. indirect (extrapyramidal) pathways (the rubrospinal, tectospinal, and vestibulospinal tracts) transmit nerve impulses that: - govern automatic movements - help coordinate body movements with visual stimuli - maintain skeletal muscle tone and posture - play a major role in equilibrium by regulating muscle tone in response to movements of the head 3. Reflexes and Reflex Arcs: (p. 562) i. The gray matter in the spinal cord promotes homeostasis by serving as the integrating center for spinal reflexes (the brain stem is the integrating center for cranial reflexes). ii. Reflexes are fast, predictable, automatic responses to changes in the environment that help maintain homeostasis: a. somatic reflexes involve contraction of skeletal muscles b. autonomic (visceral) reflexes involve responses of smooth muscle, cardiac muscle, and glands 4 iii. The pathway followed by nerve impulses that produce a reflex response is called a reflex arc (reflex circuit), which includes five functional components: a. sensory receptor (dendrite or associated sensory structure) responds to a specific stimulus and may trigger one or more nerve impulses b. sensory neuron transmits nerve impulses from the receptor to the gray matter of the spinal cord or brain stem c. integrating center is a region of gray matter within the CNS: - it may have a single synapse between the sensory neuron and a motor neuron to form a monosynaptic reflex arc - more commonly, it has one or more interneurons in the pathway to the motor neuron resulting in a polysynaptic reflex arc d. motor neuron transmits nerve impulses from the integrating center to a part of the body that will respond e. effector is a muscle or gland that will respond to the motor nerve impulse; its action is called a reflex: - in a somatic reflex, the effector is skeletal muscle - in an autonomic (visceral) reflex, the effector is smooth muscle, cardiac muscle, or a gland C. Spinal Nerves (p. 564) 1. A spinal nerve is formed by merger of a posterior root (containing sensory axons) and an anterior root (containing motor axons) at an intervertebral foramen; therefore, every spinal nerve is a mixed nerve. 2. Every spinal nerve (and cranial nerve) is surrounded and protected by connective tissue coverings: i. each axon is wrapped in a layer called the endoneurium ii. groups of axons with their endoneuria are arranged in bundles called fascicles, and each fascicle is wrapped in a layer called the perineurium iii. groups of fascicles collectively form a nerve which is covered by a layer called the epineurium The dura mater of the spinal meninges fuses with the epineurium as the nerve exits through the intervertebral foramen. 3. Shortly after passing through its intervertebral foramen, a spinal nerve divides into several branches called rami: i. posterior (dorsal) ramus serves the deep muscles and skin of the posterior surface of the trunk ii. anterior (ventral) ramus serves the muscles and structures of the limbs and the skin of the lateral and anterior surfaces of the trunk iii. meningeal branch reenters the vertebral canal via the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and the meninges iv. rami communicantes are components of the autonomic nervous system 4. The anterior rami of spinal nerves, except for thoracic nerves T2-T12, form networks on both right and left sides of the body that are called plexuses; emerging from the plexuses are nerves that bear names which often describe the general regions they serve or the routes that they follow. 5. The major pairs of plexuses (see Exhibits 18.1–18.4) are: i. cervical plexuses ii. brachial plexuses iii. lumbar plexuses iv. sacral plexuses (two small coccygeal plexuses are also present) 6. The anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses and are called intercostal or thoracic nerves; these nerves travel directly to the intercostal regions and the nearby muscles and skin regions that they innervate. 7. Dermatomes: (p. 577) i. The skin over the entire body is supplied by somatic sensory neurons that carry nerve impulses into the spinal cord and brain stem. ii. The underlying skeletal muscles receive signals from somatic motor neurons that carry impulses out of the spinal cord. iii. Each spinal nerve supplies a specific, predictable segment of the body; the area of the skin that provides sensory input to one pair of spinal nerves or to cranial nerve V (for the face and scalp) is called a dermatome. 5 iv. The nerve supply in adjacent dermatomes overlaps slightly or, in some cases, more extensively. D. Key Medical Terms Associated with the Nervous Tissue (p. 579) 1. Students should familiarize themselves with the glossary of key medical terms.