Chapter 16 *Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for...

Chapter 16

*Lecture Outline

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

*See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into PowerPoint

without notes.

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:– Posterior median sulcus on posterior surface– Anterior median fissure on anterior surface


Parts of the spinal cord:

1. Cervical

2. Thoracic

3. Lumbar

4. Sacral

5. Coccygeal


Figure 16.1


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 cervical enlargement is located in the inferior cervical part of the spinal cord and innervates the upper limbs.• The lumbosacral enlargement extends through the lumbar and sacral parts of the spinal cord and innervates the lower limbs.


• The spinal cord is shorter than the vertebral canal that houses it.

• The tapering inferior end of the spinal cord is called the conus medullaris and is the official “end” of the spinal cord proper (usually at the level of the first lumbar vertebra).


• Inferior to the conus medullaris, groups of axons called the cauda equina 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.


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 meninges, 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


Epidural space:• lies between the dura mater and periosteum

covering the inner walls of the vertebra• houses areolar connective tissue, blood

vessels, and adipose connective tissueDura mater:• most external of the meninges• fuses with the connective layers that

surround the spinal nerves


• Narrow subdural space separates dura mater from arachnoid; a potential space

• Arachnoid mater is deep to the dura mater and the subdural space

• Subarachnoid space is a real space filled with cerebral spinal fluid


Pia 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 Anatomyof 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

Location and Distributionof Gray Matter

Gray Matter:• Centrally located in spinal cord• Sectioned shape resembles butterflySubdivided into:• Anterior Horns• Lateral Horns• Posterior Horns• Gray Commissure


• Anterior horns house the cell bodies of somatic motor neurons, which innervate skeletal muscle

• Lateral 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


• The posterior horns contain axons of sensory neurons and cell bodies of interneurons.

• The gray commissure contains unmyelinated axons and serves as a communication route between the right and left side.

• The gray commissure houses a narrow central canal.


• Within the gray matter are functional groups of neuron cell bodies called nuclei:

– Sensory nuclei in the posterior horns contain interneuron cell bodies of:

– somatic sensory nuclei– visceral 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 Distributionof White Matter

The white matter of the spinal cord is external to the gray matter and is partitioned into three regions, each called a funiculus:• posterior funiculus• lateral funiculus• anterior funiculus

– interconnected by the white commissureThe 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 posterior ramus is the smaller of the two main branches and innervates the deep muscles of the back and the skin of the back.

• The anterior 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 dermatome 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 referred visceral pain, where a pain in a dermatome may arise from an organ nowhere near the dermatome.

Nerve Plexuses

A nerve plexus 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 intercostal nerves because they travel in the intercostal spaces between adjacent ribs.

• Spinal nerve T12 is called a subcostal 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 brachial plexus.• T2 innervates the intercostal muscles of the

second intercostal space and is sensory for the axilla and medial surface of the arm.

• T3–T6 innervate the intercostal muscles and are sensory for the anterior chest wall.

• T7–T12 innervate the intercostal muscles, the abdominal 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 phrenic 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 brachial 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:– superior trunk—nerves C5 and C6– middle trunk—nerve C7 – inferior 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. axillary nerve2. median nerve3. musculocutaneous nerve4. radial nerve5. ulnar nerve

Brachial Plexus

Figure 16.9

Brachial PlexusCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Superolateral arm

Cutaneous InnervationMotor InnervationAnteriorRami

Terminal Branch

Branches of the Brachial PlexusTable 16.3

Axillary NerveFormed from posterior cord, posterior division of thebrachial plexus

C5, C6 Deltoid (arm abductor)Teres minor (lateral rotator ofarm)

Posterior cord

Teres minor

Axillary nerve

Deltoid


Median Nerve

Formed from medial and lateral cords, anterior divisionof the brachial plexus

Lateral cord

Medial cord

Posterior cord

Pronator teres

Flexor carpi radialis

Palmaris longusFlexor digitorumsuperficialis

Flexor digitorumprofundus(lateral half)

Flexor pollicislongus

Lateral twolumbricals

Most anterior forearm muscles(pronators, flexors of wrist,digits)

Palmar aspects and dorsal tips oflateral 3-1/2 digits (thumb, indexfinger, middle finger, and 1/2 ofring finger)

Lateral two lumbricals (flex MPjoints and extend PIP and DIPjoints)

Flexor pollicis brevisAbductor pollicis brevisOpponens pollicis

Thenar (thumb) muscles (movethumb)

Flexor carpi radialis

Flexor digitorum superficialis

Pronator teres

Pronator quadratus

Lateral 1/2 of flexor digitorumprofundusFlexor pollicis longus

C5–T1

Pronatorquadratus

Thenar muscles

Median nerve


Musculocutaneous Nerve

Terminal Branch

Branches of the Brachial Plexus (continued)Table 16.3Cutaneous InnervationMotor InnervationAnterior

Rami

Formed from the lateral cord, anterior division of the brachialplexus

C5–C7 Anterior arm muscles (flexhumerus, flex elbow joint, supinateforearm)

Lateral region of forearm

Coracobrachialis

Biceps brachii

Brachialis

Biceps brachii

Lateral cord

Coracobrachialis

Musculocutaneousnerve

Brachialis


Extensor indicis

Extensor pollicislongus and brevis

Extensor digitorum

Extensor digiti minimi

Extensorcarpiulnaris

Supinator

Anconeus

Medial head oftriceps brachii

Long head oftriceps brachii

Medial cord

Posterior cord

Lateral cord

Radial Nerve

Formed from the posterior cord, posterior division of thebrachial plexus

C5–T1

Radial nerve

Lateral head oftriceps brachii

Brachioradialis

Extensor carpiradialis

Abductorpollicis longus

Posterior arm muscles (extendforearm)

Triceps brachii

AnconeusPosterior forearm muscles (supinateforearm, extend wrist, digits,one muscle that abducts thumb)

Supinator

Extensor carpi radialis muscles

Extensor digitorum

Extensor carpi ulnaris

Extensor pollicis longus

Extensor pollicis brevis

Abductor pollicis brevis

Extensor digiti minimi

Extensor indicis

Brachioradialis (flexes forearm)

Posterior region of armPosterior region of forearmDorsal aspect of lateral three digits(except their distal tips)


Table 16.3Terminal Branch Cutaneous InnervationMotor InnervationAnterior

Rami

Branches of the Brachial Plexus (continued)

Ulnar Nerve

Formed from the medial cord, anterior division of thebrachial plexus

Lateral cord

Posterior cord

Medial cord

Ulnar nerve

Flexor carpi ulnaris

Flexor digitorumprofundus (medial half)

Adductorpollicis

Dorsal andpalmar interossei

Hypothenar muscles

Medial two lumbricals

Anterior forearm muscles(flexors of wrist and digits)

Medial half of flexor digitorumprofundusFlexor carpi ulnaris

Dorsal and palmar aspects ofmedial 1-1/2 digits (little finger,medial aspect of ring finger)

Intrinsic hand muscles

Hypothenar musclesPalmar interossei (adductfingers)Dorsal interossei (abductfingers)

Medial two lumbricals (flex MPjoints and extend PIP and DIPjoints)

C8–T1

Adductor pollicis (adductsthumb)

Brachial Plexus

Lumbar 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 femoral nerve.

• The main nerve of the anterior division is the obturator nerve.

Lumbar Plexus

Figure 16.10

Lumbar PlexusCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Femoral Nerve

Table 16.4

Main Branch AnteriorRami

Motor Innervation Cutaneous Innervation

Branches of the Lumbar Plexus

L2L3L4

Iliacus

Femoral nerve

Sartorius

Rectus femoris

Vastusintermedius

Vastus lateralisSartorius

Vastus medialis

Pectineus

Psoas major

L2–L4 Anterior thigh muscles

Pectineus1

Anterior thighInferomedial thigh

Most medial aspect of foot

Medial side of legQuadriceps femoris (kneeextensor)Iliopsoas (hip flexor)Sartorius (hip and knee flexor)

Lumbar PlexusL2–L4

L2

Obturator Nerve

Obturator externus

Adductor brevis

Adductor longus

Adductor magnus Gracilis

Adductor longus

Obturator nerveObturator externus (lateral rotatorof thigh)

Superomedial thighMedial thigh muscles (adductorsof thigh)

Pectineus1

Gracilis

AdductorsL3L4

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


Lumbar Plexus

Sacral 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 lumbosacral plexus.

• The anterior rami are organized into an anterior division and a posterior division.

Sacral Plexus

• The sciatic nerve is the largest and longest nerve in the sacral plexus and in the body.

• The sciatic nerve is composed of two divisions wrapped in a common sheath:

1. the tibial division2. the common fibular division.

Sacral Plexus

The main branches of the sciatic nerve are the:• tibial nerve• common fibular nerve• deep fibular nerve• superficial fibular nerve

Sacral Plexus

Figure 16.11

Sacral Plexus

Figure 16.11

Gluteus maximus(cut)

Inferior glutealnerve

Sacrotuberousligament

Sciatic nerve

Gluteus medius(cut)

Gluteus minimus

Superior glutealnerve

Posterior femoralcutaneous nerve

Gluteus maximus(cut)

Pudendal nerve

(c) Right gluteal region

Biceps femorisGracilis

Semimembranosus Tibial nerve

Common fibularnerve

Popliteal artery

Medial suralcutaneous nerve

Semitendinosus

Plantaris

Gastrocnemius,lateral headLateral suralcutaneous nerve

(d) Right popliteal region

Gastrocnemius,medial head

c,d: © The McGraw-Hill Companies, Inc./Photo and Dissection by Christine Eckel


Sacral PlexusTable 16.5

Sciatic Nerve

Main Branch

Branches of the Sacral PlexusAnterior Rami Motor Innervation Cutaneous Innervation

Tibial Nerve

L4–S3(Composed of tibial and common fibulardivisions wrapped in a common sheath)

(See tibial and common fibularnerves)

(See tibial and common fibularnerves)

L4L5S1S2S3

L4–S3 Posterior thigh muscles (extendthigh and flex leg)

Branches to the heel, and via itsmedial and lateral plantar nervebranches (which supply the soleof the foot)

Long head of biceps femoris

SemimembranosusSemitendinosus

Part of adductor magnus

Posterior leg muscles (plantarflexors of foot, flexors of knee)

Flexor digitorum longus

Flexor hallucis longusGastrocnemius

Soleus

Popliteus

Plantar foot muscles (via medialand lateral plantar nervebranches)

Biceps femoris(long head)

SemitendinosusSemimembranosus

Tibial divisionof sciatic nerve

Adductormagnus

Tibial nerve

Gastrocnemius

Popliteus

Soleus

Tibialis posterior

Flexor digitorumlongus

Flexor hallucislongus

Medial plantarnerve

Lateral plantarnerve

Tibialis posterior (inverts foot)


Sacral PlexusCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Short head of biceps femoris(knee flexor); see also deepfibular and superficial fibularnerves

(See deep fibular and superficialfibular nerves)

Common Fibular Nerve(Divides into deep fibular and superficial fibularbranches)

L4–S2

S3S2S1L5L4

Common fibulardivision ofsciatic nerve

Biceps femorisshort head

Common fibularnerveFibularis longus

Fibularis brevis

Superficial fibularnerveExtensor digitorumlongusFibularis tertius

Extensor digitorumbrevis

Tibialis anterior

Deep fibularnerveExtensor hallucislongus

Extensor hallucisbrevis

Sacral Plexus


Deep Fibular Nerve

Table 16.5 Branches of the Sacral Plexus (continued)

Main Branch Anterior Rami Motor Innervation Cutaneous Innervation

L4–S1 Anterior leg muscles (dorsiflex foot,extend toes)

Dorsal interspace between first andsecond toes

Tibialis anterior (inverts foot)

Extensor hallucis longus

Extensor digitorum longus

Fibularis tertius

Dorsum foot muscles (extend toes)

Extensor hallucis brevis

Extensor digitorum brevis

Common fibularnerve

Superficial fibularnerveExtensor digitorumlongus

Fibularis tertius

Extensor digitorumbrevis

Tibialis anterior

Deep fibularnerveExtensor hallucislongus

Extensor hallucisbrevis

Sacral PlexusCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Superficial Fibular Nerve L5–S2 Lateral leg muscles (foot evertorsand plantar flexors)

Fibularis longus

Fibularis brevis

Anteroinferior part of leg; most ofdorsum of foot

Common fibularnerveFibularis longus

Fibularis brevis

Superficialfibular nerve

Reflexes

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 reflex activity.

Reflexes

A reflex arc 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:• Ipsilateral—both the receptor and

effector organs are on the same side• Contralateral—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:• Monosynaptic—sensory axons

synapse directly on motor neurons, whose axons project to the effector

• Polysynaptic—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

• Withdrawal reflex—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.


• Stretch reflex—monosynaptic reflex arc

– Stretch in a muscle is monitored by a stretch receptor called the muscle spindle.

– When a stimulus results in the stretching of a muscle, the muscle reflexively contracts.


• Golgi tendon reflex:– 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 Golgi tendon organ.

– 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

Chapter 16 *Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for...

Documents

Transcript of Chapter 16 *Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for...