Chapter 16 Outline

• Gross Anatomy of the • Spinal Cord • Sectional Anatomy of the Spinal Cord • Spinal • • 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 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 , groups of called the ______project from the spinal cord. • Within the is the , 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 (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 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 • ______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 . • 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 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 • • 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

• The left and right ______plexuses are networks of nerves that supply the upper limbs. • Each plexus is formed by the anterior rami of spinal nerves C5–T1. • Each brachial plexus innervates the pectoral girdle and the entire upper limb of one side. Brachial Plexus

• The anterior rami of C5–T1 form the roots of the brachial plexus • The roots unite to form the: – ______trunk—nerves C5 and C6 – ______trunk—nerve C7 – ______trunk—nerves C8 and T1 Brachial Plexus

• Portions of each trunk divide into an anterior division and a posterior division • The anterior and posterior divisions converge to form three cords: – posterior cord – medial cord – lateral cord Brachial Plexus

Five major terminal branches emerge from the three cords: 1. ______nerve 2 2. ______nerve 1 3. ______nerve 1 4. ______nerve 2 5. ______nerve 1 Brachial Plexus

Figure 16.9 Brachial Plexus

Figure 16.9 Brachial Plexus

Figure 16.9 Brachial Plexus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 16.3 Branches of the Brachial Plexus Terminal Branch Anterior Motor Innervation Cutaneous Innervation Rami Axillary Nerve C5, C6 Deltoid (arm abductor) Superolateral arm Formed from posterior cord, posterior division of the Teres minor (lateral rotator of brachial plexus arm)

Posterior cord

Axillary nerve Teres minor Deltoid Brachial Plexus

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Median Nerve C5–T1 Most anterior forearm muscles Palmar aspects and dorsal tips of (pronators, flexors of wrist, lateral 3-1/2 digits (thumb, index Formed from medial and lateral cords, anterior division digits) finger, middle finger, and 1/2 of of the brachial plexus ring finger) Flexor carpi radialis Lateral cord Flexor digitorum superficialis Posterior cord Pronator teres Medial cord Pronator quadratus Lateral 1/2 of flexor digitorum profundus Flexor pollicis longus Median nerve Thenar (thumb) muscles (move thumb) Flexor pollicis brevis Abductor pollicis brevis Opponens pollicis Pronator teres Lateral two lumbricals (flex MP Flexor carpi radialis joints and extend PIP and DIP Palmaris longus joints) Flexor digitorum superficialis Flexor pollicis longus Flexor digitorum Pronator profundus quadratus (lateral half) Thenar muscles Lateral two lumbricals Brachial Plexus

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Table 16.3 Branches of the Brachial Plexus (continued) Terminal Branch Anterior Motor Innervation Cutaneous Innervation Rami

Musculocutaneous Nerve C5–C7 Anterior arm muscles (flex Lateral region of forearm humerus, flex elbow joint, supinate Formed from the lateral cord, anterior division of the brachial forearm) plexus Coracobrachialis Biceps brachii Brachialis Lateral cord

Coracobrachialis Musculocutaneous Biceps brachii nerve

Brachialis Brachial Plexus

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Radial Nerve C5–T1 Posterior arm muscles (extend Posterior region of arm Formed from the posterior cord, posterior division of the forearm) Posterior region of forearm brachial plexus Triceps brachii Dorsal aspect of lateral three digits (except their distal tips) Anconeus Lateral cord Posterior forearm muscles (supinate forearm, extend wrist, digits, one muscle that abducts thumb)

Posterior cord Supinator Extensor carpi radialis muscles Medial cord Extensor digitorum Radial nerve Extensor carpi ulnaris Lateral head of Extensor pollicis longus Long head of triceps brachii Extensor pollicis brevis triceps brachii Abductor pollicis brevis Extensor digiti minimi Medial head of triceps brachii Extensor indicis Brachioradialis Brachioradialis (flexes forearm) Anconeus Extensor carpi Supinator radialis

Extensorcarpiulnaris Abductor pollicis longus Extensor digiti minimi

Extensor digitorum

Extensor pollicis longus and brevis Extensor indicis Brachial Plexus

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Table 16.3 Branches of the Brachial Plexus (continued) Anterior Terminal Branch Motor Innervation Cutaneous Innervation Rami Ulnar Nerve C8–T1 Anterior forearm muscles Dorsal and palmar aspects of Formed from the medial cord, anterior division of the (flexors of wrist and digits) medial 1-1/2 digits (little finger, brachial plexus Medial half of flexor digitorum medial aspect of ring finger) profundus Lateral cord Flexor carpi ulnaris Intrinsic hand muscles Posterior cord Hypothenar muscles Medial cord Palmar interossei (adduct fingers) Dorsal interossei (abduct fingers) Ulnar nerve Adductor pollicis (adducts thumb) Medial two lumbricals (flex MP joints and extend PIP and DIP joints)

Flexor carpi ulnaris

Flexor digitorum profundus (medial half)

Hypothenar muscles Adductor pollicis Medial two lumbricals

Dorsal and palmar interossei Brachial Plexus

• The left and right lumbar plexuses are formed from the anterior rami of spinal nerves L1–L4. • The lumbar plexus is subdivided into an anterior division and a posterior division. • The main nerve of the posterior division is the ______nerve. • The main nerve of the anterior division is the ______nerve. Lumbar Plexus

Figure 16.10 Lumbar Plexus

Figure 16.10 Lumbar Plexus

Figure 16.10 Lumbar Plexus

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 16.4 Branches of the Lumbar Plexus Main Branch Anterior Motor Innervation Cutaneous Innervation Rami L2–L4 Anterior thigh muscles Anterior thigh Quadriceps femoris (knee Inferomedial thigh L2 L3 extensor) Medial side of leg L4 Iliopsoas (hip flexor) Most medial aspect of foot Iliacus Sartorius (hip and knee flexor) Femoral nerve Psoas major Pectineus1

Sartorius Pectineus Rectus femoris Vastus Vastus medialis intermedius Sartorius Vastus lateralis Lumbar Plexus

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. L2–L4 Medial thigh muscles (adductors Superomedial thigh of thigh) L2 Adductors L3 L4 Gracilis Pectineus1 Obturator nerve Obturator externus (lateral rotator Obturator externus of thigh) Adductor longus

Adductor brevis Adductor longus Adductor magnus Gracilis

1 Pectineus may be innervated by the femoral nerve, obturator nerve, or branches from both nerves Lumbar Plexus

• The left and right sacral plexuses are formed from the anterior rami of spinal nerves L4–S4. • The lumbar and sacral plexuses are sometimes considered together as the . • The anterior rami are organized into an anterior division and a posterior division. Sacral Plexus

• The ______nerve is the largest and longest nerve in the sacral plexus and in the body. • The is composed of two divisions wrapped in a common sheath: 1. the tibial division 2. the common fibular division. Sacral Plexus

The main branches of the sciatic nerve are the: • ______nerve • ______nerve • ______nerve • ______nerve Sacral Plexus

Figure 16.11 Sacral Plexus

Figure 16.11 Sacral Plexus

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Gracilis Biceps femoris

Gluteus medius (cut) Gluteus minimus Semimembranosus Gluteus maximus (cut) Superior gluteal Semitendinosus nerve Common fibular nerve Inferior gluteal Popliteal artery nerve Plantaris Sacrotuberous ligament Sciatic nerve Medial sural Posterior femoral cutaneous nerve Gastrocnemius, cutaneous nerve lateral head Gastrocnemius, Lateral sural Gluteus maximus medial head cutaneous nerve (cut)

(c) Right gluteal region (d) Right popliteal region c,d: © The McGraw-Hill Companies, Inc./Photo and Dissection by Christine Eckel

Figure 16.11 Sacral Plexus

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 16.5 Branches of the Sacral Plexus Main Branch Anterior Rami Motor Innervation Cutaneous Innervation Sciatic Nerve L4–S3 (See tibial and common fibular (See tibial and common fibular (Composed of tibial and common fibular nerves) nerves) divisions wrapped in a common sheath) Tibial Nerve L4–S3 Posterior thigh muscles (extend Branches to the heel, and via its L4 thigh and flex leg) medial and L5 Long head of biceps femoris branches (which supply the sole S1 Semimembranosus of the foot) S2 S3 Tibial division Semitendinosus of sciatic nerve Part of adductor magnus Biceps femoris Adductor Posterior leg muscles (plantar (long head) magnus flexors of foot, flexors of knee) Flexor digitorum longus Semitendinosus Semimembranosus Flexor hallucis longus Gastrocnemius Tibial nerve Soleus Popliteus Gastrocnemius Tibialis posterior (inverts foot) Popliteus Plantar foot muscles (via medial Soleus and lateral plantar nerve Tibialis posterior branches) Flexor digitorum longus

Flexor hallucis longus

Medial plantar Lateral plantar nerve nerve Sacral Plexus

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Common Fibular Nerve L4–S2 Short head of biceps femoris (See deep fibular and superficial (Divides into deep fibular and superficial fibular (knee flexor); see also deep fibular nerves) branches) L4 fibular and superficial fibular L5 nerves S1 S2 S3

Common fibular division of sciatic nerve

Biceps femoris short head

Common fibular nerve Fibularis longus Tibialis anterior Fibularis brevis Superficial fibular Deep fibular nerve nerve Extensor digitorum Extensor hallucis longus longus Fibularis tertius Extensor digitorum brevis Extensor hallucis brevis Sacral Plexus

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 16.5 Branches of the Sacral Plexus (continued) Main Branch Anterior Rami Motor Innervation Cutaneous Innervation

Deep Fibular Nerve L4–S1 Anterior leg muscles (dorsiflex foot, Dorsal interspace between first and extend toes) second toes Tibialis anterior (inverts foot) Extensor hallucis longus Extensor digitorum longus Common fibular nerve Fibularis tertius Dorsum foot muscles (extend toes) Tibialis anterior Extensor hallucis brevis Extensor digitorum brevis Superficial fibular Deep fibular nerve nerve Extensor digitorum Extensor hallucis longus longus Fibularis tertius Extensor digitorum brevis Extensor hallucis brevis Sacral Plexus

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Superficial Fibular Nerve L5–S2 Lateral leg muscles (foot evertors Anteroinferior part of leg; most of and plantar flexors) dorsum of foot Fibularis longus Fibularis brevis

Common fibular nerve Fibularis longus

Fibularis brevis

Superficial fibular nerve Reflexes

______are rapid, automatic, involuntary reactions of muscles or glands to a stimulus. • A stimulus is required to initiate a response to sensory input. • A rapid response requires that few neurons be involved and synaptic delay be minimal. • An automatic response occurs the same way every time. • An involuntary response requires no intent or pre-awareness of the activity. Reflexes

A is the neural wiring of a single reflex. • Always begins at a receptor in the PNS • Communicates with the CNS • Ends at a peripheral effector Simple Reflex Arcs

Figure 16.12 Reflexes

A reflex arc may be: • ______—both the receptor and effector organs are on the same side • ______—the sensory impulses from a receptor organ cross over through the spinal cord to activate effector organs on the opposite limb Reflexes

Reflexes may be: • ______—sensory axons synapse directly on motor neurons, whose axons project to the effector • ______—more complex pathways that exhibit a number of synapses involving interneurons within the reflex arc Monosynaptic and Polysynaptic Reflexes

Figure 16.12 Examples of Spinal Reflexes

• —polysynaptic reflex arc – Painful stimulus causes transmission of sensory information to the spinal cord. – Interneurons receive the sensory information and stimulate the motor neurons to direct flexor muscles to contract in response. – Simultaneously, antagonistic extensor muscles are inhibited so that the traumatized body part may be quickly withdrawn from the harmful stimulation. Examples of Spinal Reflexes

• —monosynaptic reflex arc – Stretch in a muscle is monitored by a stretch receptor called the . – When a stimulus results in the stretching of a muscle, the muscle reflexively contracts. Examples of Spinal Reflexes

• Golgi : – Golgi tendon organs are nerve endings located within tendons near a muscle–tendon junction. – As a muscle contracts, force is exerted on its tendon, resulting in increased tension in the tendon and activation of the . – Nerve impulses signal interneurons in the spinal cord, which in turn inhibit the actions of the motor neurons. Stretch Reflexes

Figure 16.14 Golgi Tendon Reflex

Figure 16.15 Spinal Cord Development

Figure 16.16