Chapter 16 Outline • Gross Anatomy of the Spinal Cord • Spinal Cord Meninges • Sectional Anatomy of the Spinal Cord • Spinal Nerves • Reflexes • Development of the Spinal Cord Spinal Cord—Introduction The spinal cord provides a vital link between the brain and the rest of the body. The spinal cord and its attached spinal nerves serve two important functions: 1. a pathway for sensory and motor impulses 2. responsible for reflexes, which are the quickest reactions to a stimulus Gross Anatomy of the Spinal Cord • Length: 42–45 cm, 16–18 inches • Roughly cylindrical, slightly flattened posteriorly and anteriorly • Two longitudinal depressions on external surface: – _______ median sulcus on posterior surface – _______ median fissure on anterior surface Gross Anatomy of the Spinal Cord Parts of the spinal cord: 1. _______ 2. _______ 3. _______ 4. _______ 5. Coccygeal Gross Anatomy of the Spinal Cord Figure 16.1 Gross Anatomy of the Spinal Cord Gross Anatomy of the Spinal Cord The diameter of the spinal cord changes along its length because the amount of gray matter and white matter and the function of the cord vary in different regions. • The _______ enlargement is located in the inferior cervical part of the spinal cord and innervates the upper limbs. • The _______ enlargement extends through the lumbar and sacral parts of the spinal cord and innervates the lower limbs. Gross Anatomy of the Spinal Cord • The spinal cord is shorter than the vertebral canal that houses it. • The tapering inferior end of the spinal cord is called the _______ _______ and is the official “end” of the spinal cord proper (usually at the level of the first lumbar vertebra). Gross Anatomy of the Spinal Cord • Inferior to the conus medullaris, groups of axons called the _______ _______ project from the spinal cord. • Within the cauda equina is the filum terminale, which is a thin strand of pia mater that helps anchor the conus medullaris to the coccyx. Gross Anatomy of the Spinal Cord The spinal cord is associated with 31 pairs of spinal nerves that connect the CNS to muscles, receptors and glands. Each side contains: • 8 cervical nerves (C1–C8). • 12 thoracic nerves (T1–T12). • 5 lumbar nerves (L1–L5). • 5 sacral nerves (S1–S5) • 1 coccygeal nerve (Co1) Spinal Cord Meninges • The spinal cord is protected and encapsulated by spinal cord _______, which are continuous with the cranial meninges. • Some of the spaces between some of the meninges have clinical significance. Spinal Meninges and Structure of the Spinal Cord Figure 16.2 Spinal Cord Meninges _______ space: • lies between the dura mater and periosteum covering the inner walls of the vertebra • houses areolar connective tissue, blood vessels, and adipose connective tissue _______ mater: • most external of the meninges • fuses with the connective layers that surround the spinal nerves Spinal Cord Meninges • Narrow subdural space separates dura mater from arachnoid; a potential space • _______ mater is deep to the dura mater and the subdural space • _______ space is a real space filled with cerebral spinal fluid Spinal Cord Meninges _______ mater: • innermost meningeal layer that adheres directly to the spinal cord • delicate layer composed of elastic and collagen fibers and supports some of the blood vessels supplying the spinal cord • has paired, lateral triangular extensions called denticulate ligaments, which suspend and anchor the spinal cord laterally to the dura mater Sectional Anatomy of the Spinal Cord The spinal cord is partitioned into an inner gray matter region and an outer white matter region: • Gray matter—dendrites and cell bodies of neurons, unmyelinated axons, and glial cells • White matter—myelinated axons Gray and White Components of Spinal Cord Figure 16.3 Gray and White Components of Spinal Cord Figure 16.3 Location and Distribution of Gray Matter Gray Matter: • Centrally located in spinal cord • Sectioned shape resembles butterfly Subdivided into: • _______ Horns • _______ Horns • _______ Horns • Gray Commissure Location and Distribution of Gray Matter • _______ horns house the cell bodies of somatic motor neurons, which innervate skeletal muscle • _______ horns: – found in the T1–L2 parts of the spinal cord only – contain cell bodies of autonomic motor neurons, which innervate cardiac muscle, smooth muscle, and glands Location and Distribution of Gray Matter • The _______ horns contain axons of sensory neurons and cell bodies of interneurons. • The _______ commissure contains unmyelinated axons and serves as a communication route between the right and left side. • The gray commissure houses a narrow _______ _______. Location and Distribution of Gray Matter • Within the gray matter are functional groups of neuron cell bodies called _______ : – Sensory nuclei in the posterior horns contain interneuron cell bodies of: – _______ sensory nuclei – _______ sensory nuclei – Motor nuclei in the anterior horns contain somatic motor nuclei – Autonomic motor nuclei are in the lateral horns Neuron Pathways and Nuclei Locations Figure 16.4 Location and Distribution of White Matter The white matter of the spinal cord is external to the gray matter and is partitioned into three regions, each called a _______ : • posterior funiculus • lateral funiculus • anterior funiculus – interconnected by the white commissure The axons within each funiculus are organized into tracts. Spinal Nerves • 31 pairs • Made up of motor and sensory axons • Contain connective tissue wrappings called endoneurium, perineurium, and epineurium Spinal Nerves • Multiple anterior rootlets arise from the spinal cord and merge to form a single anterior root. • Anterior roots contain motor axons only. • The cell bodies of the motor axons arise from cell bodies in the anterior and lateral horns of the spinal cord. Spinal Nerves • Multiple posterior rootlets are derived from a single posterior root. • Posterior roots contain sensory axons only. • The cell bodies of the sensory axons arise from cell bodies in the posterior root ganglion, which is attached to the posterior root. Spinal Nerves • Each anterior root and its corresponding posterior root unite within the intervertebral foramen to become a spinal nerve. • A spinal nerve contains both motor and sensory axons. Spinal Nerves • Spinal nerves are numbered according to the location of the intervertebral canal. • In the cervical region the first seven pairs of spinal nerves (C1–C7) exit the intervertebral foramen above the vertebra of the same number. • The eighth pair of cervical spinal nerves (C8) exit above the first thoracic vertebra. • The remaining pairs of spinal nerves exit below the vertebra of the same number. Spinal Nerves • Because the spinal cord is shorter than the vertebral canal, the roots of the lumbar and sacral spinal nerves travel inferiorly to reach their respective intervertebral foramen. Spinal Nerve Distribution • After leaving the intervertebral foramen, a typical spinal nerve splits into branches termed rami. • The _______ ramus is the smaller of the two main branches and innervates the deep muscles of the back and the skin of the back. • The _______ ramus is the larger of the two main branches and innervates the anterior and lateral portions of the trunk and the upper and lower limbs. Spinal Nerve Distribution • The anterior ramus splits into multiple other branches. • Many of the anterior rami go on to form nerve plexuses. • Additional rami, the rami communicantes, extend between the spinal nerve and the sympathetic trunk ganglion. Spinal Nerve Branches Figure 16.5 Dermatomes • A _______ is a specific segment of skin supplied by a single spinal nerve. • All spinal nerves except C1 innervate a segment of skin. • The dermatome map follows a segmental pattern along the body. Dermatome Maps Figure 16.6 Dermatomes • The dermatome map can be important because anesthesia (numbness) in one or more of the segments could indicate potential spinal nerve damage. • Dermatomes are also involved in _______ _______ _______, where a pain in a dermatome may arise from an organ nowhere near the dermatome. Nerve Plexuses A _______ _______ is a network of interweaving anterior rami of spinal nerves. • The anterior rami of most spinal nerves form nerve plexuses on both sides of the body. • The plexuses split into multiple named nerves that innervate body structures. • The principle plexuses are the: cervical plexuses, brachial plexuses, lumbar plexuses, and sacral plexuses. Intercostal Nerves • The anterior rami of spinal nerves T1–T11 are called _______ nerves because they travel in the intercostal spaces between adjacent ribs. • Spinal nerve T12 is called a _______ nerve, because it arises below the ribs. • With the exception of spinal nerve T1, the intercostal nerves do not form plexuses. Intercostal Nerves • T1 forms part of the _______ plexus. • T2 innervates the intercostal muscles of the second intercostal space and is sensory for the _______ and _______ surface of the arm. • T3–T6 innervate the _______ muscles and are sensory for the anterior chest wall. • T7–T12 innervate the intercostal muscles, the _______ muscles, and the overlying skin. Intercostal Nerves Figure 16.7 Cervical Plexus • Formed by anterior rami of spinal nerves C1–C4 • Branches of the cervical plexus innervate anterior neck muscles and the skin of the neck and head and shoulders. • The _______ nerve originated primarily from C4 and some contributing axons of C3 and C5. – travels through the thoracic cavity to innervate the diaphragm Cervical Plexus Figure 16.8 Cervical Plexus Brachial Plexus