1. Chapter 16 *Lecture Outline
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2. 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

3. 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

4. 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

5. Gross Anatomy of the Spinal Cord
Parts of the spinal cord:
1. Cervical
2. Thoracic
3. Lumbar
4. Sacral
5. Coccygeal

6. Gross Anatomy of the Spinal Cord
Figure 16.1

7. Gross Anatomy of the Spinal Cord

8. 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 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.

9. 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 conus medullaris and is the official “end” of the spinal cord proper (usually at the level of the first lumbar vertebra).

10. Gross Anatomy of the Spinal Cord
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.

11. 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)

12. 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.

13. Spinal Meninges and Structure of the Spinal Cord
Figure 16.2

14. Spinal Cord Meninges 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 tissue
Dura mater:
most external of the meninges
fuses with the connective layers that surround the spinal nerves

15. Spinal Cord Meninges
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

16. Spinal Cord Meninges 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

17. 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

18. Gray and White Components of Spinal Cord
Figure 16.3

19. Gray and White Components of Spinal Cord
Figure 16.3

20. Location and Distribution of Gray Matter
Centrally located in spinal cord
Sectioned shape resembles butterfly
Subdivided into:
Anterior Horns
Lateral Horns
Posterior Horns
Gray Commissure

21. Location and Distribution of Gray Matter
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

22. Location and Distribution of Gray Matter
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.

23. Location and Distribution of Gray Matter
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

24. Neuron Pathways and Nuclei Locations
Figure 16.4

25. 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 funiculus:
posterior funiculus
lateral funiculus
anterior funiculus
interconnected by the white commissure
The axons within each funiculus are organized into tracts.

26. Spinal Nerves 31 pairs Made up of motor and sensory axons
Contain connective tissue wrappings called endoneurium, perineurium, and epineurium

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. 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.

33. 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.

34. Spinal Nerve Branches
Figure 16.5

35. 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.

36. Dermatome Maps
Figure 16.6

37. 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.

38. 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.

39. 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.

40. 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.

41. Intercostal Nerves
Figure 16.7

42. 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

43. Cervical Plexus
Figure 16.8

44. Cervical Plexus

45. 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.

46. 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

47. 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

48. Brachial Plexus
Five major terminal branches emerge from the three cords:
1. axillary nerve
2. median nerve
3. musculocutaneous nerve
4. radial nerve
5. ulnar nerve

49. Brachial Plexus
Figure 16.9

50. Brachial Plexus
Figure 16.9

51. Brachial Plexus
Figure 16.9

52. Brachial Plexus Table 16.3 Branches of the Brachial Plexus
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Superolateral arm
Cutaneous Innervation
Motor Innervation
Anterior
Rami
Terminal Branch
Branches of the Brachial Plexus
Table 16.3
Axillary Nerve
Formed from posterior cord, posterior division of the
brachial plexus
C5, C6
Deltoid (arm abductor)
Teres minor (lateral rotator of
arm)
Posterior cord
Teres minor
Axillary nerve
Deltoid

53. Brachial Plexus Median Nerve C5–T1 Most anterior forearm muscles
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Median Nerve
C5–T1
Most anterior forearm muscles
(pronators, flexors of wrist,
digits)
Palmar aspects and dorsal tips of
lateral 3-1/2 digits (thumb, index
finger, middle finger, and 1/2 of
ring finger)
Formed from medial and lateral cords, anterior division of the brachial plexus
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
joints and extend PIP and DIP
joints)
Flexor carpi radialis
Palmaris longus
Flexor digitorum
superficialis
Flexor pollicis
longus
Flexor digitorum
profundus
(lateral half)
Pronator
quadratus
Thenar muscles
Lateral two
lumbricals

54. Brachial Plexus Table 16.3 Branches of the Brachial Plexus (continued)
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Table 16.3
Branches of the Brachial Plexus (continued)
Terminal Branch
Anterior
Rami
Motor Innervation
Cutaneous Innervation
Musculocutaneous Nerve
C5–C7
Anterior arm muscles (flex
humerus, flex elbow joint, supinate
forearm)
Lateral region of forearm
Formed from the lateral cord, anterior division of the brachial
plexus
Coracobrachialis
Biceps brachii
Brachialis
Lateral cord
Coracobrachialis
Musculocutaneous
nerve
Biceps brachii
Brachialis

55. Brachial Plexus
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Radial Nerve
C5–T1
Posterior arm muscles (extend
forearm)
Posterior region of arm
Posterior region of forearm
Dorsal aspect of lateral three digits
(except their distal tips)
Formed from the posterior cord, posterior division of the
brachial plexus
Triceps brachii
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
triceps brachii
Extensor pollicis longus
Long head of
triceps brachii
Extensor pollicis brevis
Abductor pollicis brevis
Extensor digiti minimi
Medial head of
triceps brachii
Extensor indicis
Brachioradialis
Brachioradialis (flexes forearm)
Anconeus
Extensor carpi
radialis
Supinator
Extensorcarpiulnaris
Abductor
pollicis longus
Extensor digiti minimi
Extensor digitorum
Extensor pollicis
longus and brevis
Extensor indicis

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

57. Brachial Plexus

58. 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.

59. Lumbar Plexus
Figure 16.10

60. Lumbar Plexus
Figure 16.10

61. Lumbar Plexus
Figure 16.10

62. Lumbar Plexus Table 16.4 Branches of the Lumbar Plexus Main Branch
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Table 16.4
Branches of the Lumbar Plexus
Main Branch
Anterior
Rami
Motor Innervation
Cutaneous Innervation
Femoral Nerve
L2–L4
Anterior thigh muscles
Anterior thigh
Quadriceps femoris (knee
extensor)
Inferomedial thigh L2 L3
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
intermedius
Vastus medialis
Sartorius
Vastus lateralis

63. Lumbar Plexus Obturator Nerve L2–L4 Medial thigh muscles (adductors
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Obturator Nerve
L2–L4
Medial thigh muscles (adductors
of thigh)
Superomedial thigh L2
Adductors L3 L4
Gracilis
Pectineus1
Obturator nerve
Obturator externus (lateral rotator
of thigh)
Obturator externus
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

64. Lumbar Plexus

65. 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.

66. 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 division
2. the common fibular division.

67. Sacral Plexus The main branches of the sciatic nerve are the:
tibial nerve
common fibular nerve
deep fibular nerve
superficial fibular nerve

68. Sacral Plexus
Figure 16.11

69. Sacral Plexus
Figure 16.11

70. Sacral Plexus
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Gluteus maximus
(cut)
Inferior gluteal
nerve
Sacrotuberous
ligament
Sciatic nerve
Gluteus medius
Gluteus minimus
Superior gluteal
Posterior femoral
cutaneous nerve
Pudendal nerve
(c) Right gluteal region
Biceps femoris
Gracilis
Semimembranosus
Tibial nerve
Common fibular
Popliteal artery
Medial sural
Semitendinosus
Plantaris
Gastrocnemius,
lateral head
Lateral sural
(d) Right popliteal region
medial head
c,d: © The McGraw-Hill Companies, Inc./Photo and Dissection by Christine Eckel
Figure 16.11

71. Sacral Plexus Table 16.5 Branches of the Sacral Plexus Main Branch
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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
nerves)
(See tibial and common fibular
nerves)
(Composed of tibial and common fibular
divisions wrapped in a common sheath)
Tibial Nerve
L4–S3
Posterior thigh muscles (extend
thigh and flex leg)
Branches to the heel, and via its
medial and lateral plantar nerve
branches (which supply the sole
of the foot) L4 L5
Long head of biceps femoris S1
Semimembranosus S2 S3
Tibial division
of sciatic nerve
Semitendinosus
Part of adductor magnus
Biceps femoris
(long head)
Adductor
magnus
Posterior leg muscles (plantar
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
and lateral plantar nerve
branches)
Soleus
Tibialis posterior
Flexor digitorum
longus
Flexor hallucis
longus
Medial plantar
nerve
Lateral plantar
nerve

72. Sacral Plexus Common Fibular Nerve L4–S2 Short head of biceps femoris
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Common Fibular Nerve
L4–S2
Short head of biceps femoris
(knee flexor); see also deep
fibular and superficial fibular
nerves
(See deep fibular and superficial
fibular nerves)
(Divides into deep fibular and superficial fibular
branches) L4 L5 S1 S2 S3
Common fibular
division of
sciatic nerve
Biceps femoris
short head
Common fibular
nerve
Fibularis longus
Tibialis anterior
Fibularis brevis
Superficial fibular
nerve
Deep fibular
nerve
Extensor digitorum
longus
Extensor hallucis
longus
Fibularis tertius
Extensor digitorum
brevis
Extensor hallucis
brevis

73. Sacral Plexus Table 16.5 Branches of the Sacral Plexus (continued)
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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,
extend toes)
Dorsal interspace between first and
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
Superficial fibular
nerve
Extensor digitorum brevis
Deep fibular
nerve
Extensor digitorum
longus
Extensor hallucis
longus
Fibularis tertius
Extensor digitorum
brevis
Extensor hallucis
brevis

74. Sacral Plexus Superficial Fibular Nerve L5–S2
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Superficial Fibular Nerve
L5–S2
Lateral leg muscles (foot evertors
and plantar flexors)
Anteroinferior part of leg; most of
dorsum of foot
Fibularis longus
Fibularis brevis
Common fibular
nerve
Fibularis longus
Fibularis brevis
Superficial
fibular nerve

75. 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.

76. 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

77. Simple Reflex Arcs
Figure 16.12

78. 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

79. 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

80. Monosynaptic and Polysynaptic Reflexes
Figure 16.12

81. 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.

82. Examples of Spinal Reflexes
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.

83. Examples of Spinal Reflexes
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.

84. Stretch Reflexes
Figure 16.14

85. Golgi Tendon Reflex
Figure 16.15

86. Spinal Cord Development
Figure 16.16