1. 12
The Spinal Cord, Spinal Nerves, and Spinal Reflexes

2. Section 1: Functional Organization of the Spinal Cord
Central Nervous System organization
Brain, cranial nerves, cranial reflexes (Chapter 13)
More complex
Spinal cord, spinal nerves, spinal reflexes (this chapter)
Simpler

3. The Brain The Spinal Cord Effectors
A diagram of a functional perspective for studying the CNS
The Brain
Sensory input over cranial nerves
Motor output over cranial nerves
Effectors
Reflex centers in brain
Sensory receptors
Muscles
The Spinal Cord
Glands
Figure 12 Section 1 The Functional Organization of the Spinal Cord
Sensory input over spiral nerves
Motor output over spinal nerves
Reflex centers in spinal cord
Sensory receptors
Adipose tissue
Figure 12 Section 1 3

4. Module 12.1: Spinal cord functional anatomy
Adult spinal cord dimensions
Length: ~45 cm (18 in.)
Width: ~14 mm (0.55 in.) maximum
Superficial anatomy
Cervical enlargement
Supplies nerves to shoulder and upper limbs
Lumbar enlargement
Supplies nerves to pelvis and lower limbs
Conus medullaris
Tapered terminal end inferior to lumbar enlargement

5. Module 12.1: Spinal cord functional anatomy
Superficial anatomy (continued)
Cauda equina (cauda, tail + equus, horse)
Long, inferiorly extending dorsal and ventral roots + filum terminale
Resembles horse’s tail
Filum terminale
Slender thread of connective tissue attaching conus medullaris to 2nd sacral vertebra
Provides longitudinal support to spinal cord

6. Module 12.1: Spinal cord functional anatomy
Superficial anatomy (continued)
31 pairs of spinal nerves
Arise from 31 segments of spinal cord
Identified by adjacent vertebrae
Cervical nerves
From vertebrae immediately inferior
Last vertebrae with this number system is C8
Thoracic nerves
From vertebrae immediately superior

7. Figure 12.1.1 The spinal cord contains gray matter and white matter
The 31 pairs of spinal nerves
Cervical spinal nerves
Cervical enlargement
Posterior median sulcus
Thoracic spinal nerves
Lumbar enlargement
Conus medullaris
Lumbar spinal nerves
Interior tip of spinal cord
Figure The spinal cord contains gray matter and white matter
Cauda equina
Sacral spinal nerves
Filum terminale
Coccygeal nerve (Co1)
Figure 7

8. Module 12.1: Spinal cord functional anatomy
Spinal cord anatomy in cross section (continued)
White matter
Superficial
Contains large numbers of myelinated and unmyelinated axons
Gray matter
Surrounds central canal
Forms butterfly or H shape
Dominated by cell bodies of neurons, neuroglia, and unmyelinated axons
Greater amount in spinal cord segments serving limbs

9. Figure 12.1.2 The spinal cord contains gray matter and white matter
Cross sections of three of the spinal cord’s 31 segments
Posterior median sulcus
Dorsal root
Dorsal root ganglion
White matter
Spinal nerve
Gray matter
Segment C3
Ventral root
Anterior median fissure
White matter
Gray matter
Central canal
Segment T3
Figure The spinal cord contains gray matter and white matter
Segment L1
Segment S2
Figure 9

10. Module 12.2: Spinal meninges
Series of specialized membranes that provide physical stability and shock absorption for the spinal cord
Blood vessels branching within deliver oxygen and nutrients to spinal cord
Are continuous with cranial meninges and connective tissues surrounding spinal nerves
Three layers
1. Dura mater – outermost dense collagen fibers
narrow subdural space separating from arachnoid mater
2.Arachnoid mater – middle contains CSF ( lumbar puncture)
3.Pia mater- innermost Meshwork of elastic and collagen fibers

11. A section demonstrating the procedure called a lumbar puncture or spinal tap
Dura mater
Epidural space
Body of third lumbar vertebra
Interspinous ligament
Lumbar puncture needle with tip in subarachnoid space
Cauda equina in subarachnoid space
Figure

12. Module 12.2: Spinal meninges
Epidural space
Between dura mater and vertebral canal
Contains areolar connective tissue, blood vessels, and protective adipose tissue

13. A posterior view of the dissected spinal cord showing the basic relationships among the spinal meninges
Gray matter
Pia mater
White matter
Ventral root
Spinal nerve
Dorsal root
Arachnoid mater
Figure The spinal cord is surrounded by the meninges, which consist of the dura mater, arachnoid mater, and pia mater
Dura mater
Figure 13

14. A cross-sectional view showing the structures surrounding the spinal cord and the spaces between the meningeal layers
Cerebrospinal fluid (CSF)
Anterior
Ventral root
Vertebral body
Spinal meninges
Spinal cord
Figure The spinal cord is surrounded by the meninges, which consist of the dura mater, arachnoid mater, and pia mater
Epidural space
Dorsal root ganglion
Pia mater
Arachnoid mater
Dorsal root
Dura mater
Figure 14

15. An anterior view of the cervical spinal cord showing the meninges, supporting ligaments, and the roots of the spinal nerves
Spinal cord
Anterior median fissure
Pia mater
Denticulate ligaments
Dorsal root
Ventral root, formed by several “rootlets” from one cervical segment
Blood vessels within the subarachnoid space
Figure The spinal cord is surrounded by the meninges, which consist of the dura mater, arachnoid mater, and pia mater
Arachnoid mater (reflected)
Dura mater (reflected)
Figure 15

16. Anterior view of spinal cord
A cross section showing most of the anatomical landmarks of the spinal cord
Posterior
median sulcus
Posterior gray commissure
Structural Organization of Gray Matter
The projections of gray matter toward the outer surface of the spinal cord are called horns.
Anterior view of spinal cord
Posterior gray horn
Central canal
Dura mater
Lateral gray horn
Arachnoid mater (broken)
Anterior gray horn
Pia mater
Figure Gray matter is the region of integration, and white matter carries information
Dorsal root ganglion
Anterior
median fissure
Anterior gray commissure
Ventral root
Figure 16

17. Functional Organization of Gray Matter
A diagrammatic view of the organization of the gray matter of the spinal cord
Site of a frontal section that separates the sensory (posterior, or dorsal) nuclei from the motor (anterior, or ventral) nuclei
Functional Organization of Gray Matter
Gray commissures
The cell bodies of neurons in the gray matter of the spinal cord are organized into functional groups called nuclei.
Posterior gray horn
Somatic
Visceral
Sensory nuclei
Dorsal root ganglion
Lateral gray horn
Visceral
Motor nuclei
Anterior gray horn
Somatic
Figure Gray matter is the region of integration, and white matter carries information
Ventral root
Figure 17

18. Organization of Tracts in the Posterior White Column
The organization of the white matter into columns containing tracts
The posterior white column contains ascending tracts providing sensations from the trunk and limbs.
Leg
Hip
Trunk
Structural and Functional Organization of White Matter
Arm
Posterior white column
Lateral white column
Anterior white column
Flexors/Extensors
Figure Gray matter is the region of integration, and white matter carries information
Anterior white commissure
Trunk
Shoulder
Arm
Forearm
Hand
In the cervical enlargement, which contains neurons involved with sensations and motor control of the upper limbs, the motor nuclei of the anterior gray horn are grouped by region, with motor neurons controlling flexor muscles medial to those controlling extensor muscles.
Figure 18

19. Connective tissue layers of a spinal nerve
Epineurium
Outermost covering of nerve
Dense network of collagen fibers
Perineurium
Middle layer
Divide nerve into compartments that contain bundles of axons (fascicles)
Branching blood vessels from epineurium continue on to form capillaries in endoneurium
Endoneurium
Innermost layer
Delicate connective tissues surrounding individual axons
Capillaries here supply axons, Schwann cells, and fibroblasts

20. Artery and vein within the perineurium
A sectional view of a spinal nerve showing its connective tissue layers
Connective Tissue Layers of a Spinal Nerve
Epineurium
Perineurium
Endoneurium
Figure Spinal nerves have a relatively consistent anatomical structure and pattern of distribution
Artery and vein within the perineurium
Fascicle
Schwann cell
Myelinated axon
Figure 20

21. Module 12.4: Spinal nerve structure and distribution
Spinal nerve branches
Called rami (singular ramus, a branch)
Some carry visceral motor fibers of autonomic nervous system (ANS)
In thoracic and upper lumbar segments, sympathetic division (“fight or flight”) motor fibers

22. The branching of a spinal nerve to form rami
Dorsal root ganglion
Dorsal ramus
Dorsal root
Ventral ramus
Ventral root
Communicating rami
Figure Spinal nerves have a relatively consistent anatomical structure and pattern of distribution
Sympathetic ganglion
Autonomic nerve
Figure 22

23. Module 12.4: Spinal nerve structure and distribution
Dermatome
Specific bilateral region of skin surface monitored by single pair of spinal nerves
C1 usually lacks sensory branch to skin
When present, helps monitor scalp with C2 and C3
Face is monitored by pair of cranial nerves
Boundaries between dermatomes overlap
Clinically important to determine damage or infection of spinal nerve or dorsal root ganglion
Loss of sensation or signs on skin in dermatome

24. Dermatomes, the specific bilateral regions of the skin surface monitored by a single pair of spinal nerves
Anterior
Posterior
Figure Spinal nerves have a relatively consistent anatomical structure and pattern of distribution
Figure 24

25. Module 12.4: Spinal nerve structure and distribution
Shingles
Viral infection of dorsal root ganglia
Caused by varicella-zoster virus
Same herpes virus as chickenpox
Produces painful rash and blisters on dermatome served by infected nerves
Those who have had chickenpox are more at risk
Virus can remain dormant within anterior gray horns
Unknown trigger for reactivation

26. Figure Spinal nerves have a relatively consistent anatomical structure and pattern of distribution
Figure 26

27. The cervical, brachial, lumbar, and sacral plexuses (at left), and the major peripheral nerves of each (at right)
Lesser occipital nerve
Cervical plexus
Great auricular nerve
Transverse cervical nerve
Supraclavicular nerve
Brachial plexus
Phrenic nerve
Axillary nerve
Musculocutaneous nerve
Thoracic nerves
Radial nerve
Lumbar plexus
Ulnar nerve
Median nerve
Iliohypogastric nerve
Figure Spinal nerves form nerve plexuses that innervate the skin and skeletal muscles; the cervical plexus is the smallest of these nerve plexuses
Sacral plexus
Ilioinguinal nerve
Genitofemoral nerve
Femoral nerve
Obturator nerve
Superior gluteal nerve
Inferior gluteal nerve
Pudendal nerve
Saphenous nerve
Sciatic nerve
Figure 27

28. Module 12.6: Spinal nerve plexuses introduction and the cervical plexus
Ventral rami of spinal nerves C1–C5
Branches innervate
Muscles of neck and to control
Diaphragmatic muscles (phrenic nerve)
Extends into thoracic cavity
Skin of neck
Skin of superior part of chest

29. The cervical plexus, which consists of the ventral rami of spinal nerves C1–C5, and some of the muscles its branches innervate
Cranial Nerves
Accessory nerve (XI)
Hypoglossal nerve (XII)
Lesser occipital nerve
Great auricular nerve
Nerve Roots of Cervical Plexus C1 C2 C3
Geniohyoid muscle
Transverse cervical nerve C4
Figure Spinal nerves form nerve plexuses that innervate the skin and skeletal muscles; the cervical plexus is the smallest of these nerve plexuses C5
Thyrohyoid muscle
Ansa cervicalis
Omohyoid muscle
Supraclavicular nerves
Phrenic nerve
Sternohyoid muscle
Clavicle
Sternothyroid muscle
Figure 29

30. Module 12.7: Brachial plexus
Innervates pectoral girdle and upper limb
Contributions from ventral rami of nerves C4–T1
Clinical importance of cutaneous nerve
Damage or injury can be precisely localized by testing sensory function in hand
Animation: Brachial Plexus

31. The brachial plexus, which innervates the pectoral girdle and upper limbs with contributions from the ventral rami of spinal nerves C4–T1
Spinal Nerves Forming Brachial Plexus
Trunks of Brachial Plexus
Dorsal scapular nerve
C4 nerve
Suprascapular nerve
C5 nerve
Superior
C6 nerve
Middle
C7 nerve
Inferior
C8 nerve
T1 nerve
Musculocutaneous nerve
Median nerve
Ulnar nerve
Radial nerve
Lateral antebrachial cutaneous nerve
Superficial branch of radial nerve
Deep radial nerve
Figure The brachial plexus innervates the pectoral girdle and upper limbs
Ulnar nerve
Median nerve
Palmar digital nerves
Figure 31

32. The distribution of the cutaneous nerves of the wrist and hand
Anterior
Posterior
Radial nerve
Ulnar nerve
Median nerve
Figure The brachial plexus innervates the pectoral girdle and upper limbs
Figure 32

33. Module 12.8: Lumbar and sacral plexuses
Arise from lumbar and sacral segments of spinal cord
Innervate pelvic girdle and lower limbs
Lumbar plexus
Innervates mostly anterior and side surfaces
Sacral plexus
Innervates mostly posterior surfaces
Contains sciatic nerve (longest & largest nerve in body)
Animation: Lumbar Sacral Plexus

34. The origins of the spinal nerves of the sacral plexus
Spinal Nerves Forming the Sacral Plexus
Lumbosacral trunk
L4 nerve
L5 nerve
Nerves of the Sacral Plexus
S1 nerve
Superior gluteal
S2 nerve
Inferior gluteal
S3 nerve
S4 nerve
Figure The lumbar and sacral plexuses innervate the skin and skeletal muscles of the trunk and lower limbs
Sciatic
Posterior femoral cutaneous
Co1
Pudendal
Sacral plexus, anterior view
Figure 34

35. A posterior view of the lower limb showing the distribution of the nerves of the sacral plexus
Superior gluteal nerve
Inferior gluteal nerve
Pudendal nerve
Posterior femoral cutaneous nerve
Sciatic nerve
Tibial nerve
Common fibular nerve
Figure The lumbar and sacral plexuses innervate the skin and skeletal muscles of the trunk and lower limbs
Sural nerve
Figure 35

36. The dermatomes of the sensory nerves innervating the ankle and foot
Saphenous nerve
Sural nerve
Sural nerve
Saphenous nerve
Fibular nerve
Tibial nerve
Figure The lumbar and sacral plexuses innervate the skin and skeletal muscles of the trunk and lower limbs
Fibular nerve
Figure 36

37. An anterior view of the lower trunk and lower limb showing the distribution of the nerves of both the lumbar and sacral plexuses
Iliohypogastric nerve
Ilioinguinal nerve
Genitofemoral nerve
Lateral femoral cutaneous nerve
Femoral nerve
Obturator nerve
Superior gluteal nerve
Inferior gluteal nerve
Pudendal nerve
Posterior femoral cutaneous nerve (cut)
Sciatic nerve
Saphenous nerve
Figure The lumbar and sacral plexuses innervate the skin and skeletal muscles of the trunk and lower limbs
Common fibular nerve
Superficial fibular nerve
Deep fibular nerve
Figure 37

38. Module 12.9: Reflexes Reflexes
Are rapid, automatic responses to specific stimuli
Show little variability
Preserve homeostasis by making rapid adjustments in functions of organs or organ systems
In neural reflexes:
Sensory fibers carry information from peripheral receptors to integration center
Motor fibers carry motor commands to peripheral effectors
Reflex arc
“Wiring” of a single reflex from receptor to effector
Animation: Components of a Reflexive Arc

39. Module 12.9: Reflexes Example: Simple withdrawal reflex
Arrival of stimulus and activation of receptor
Receptor is specialized cell or dendrites of sensory neuron
Sensitive to:
Physical or chemical changes in body
Or changes in external environment
Example: pain receptor in hand
Activation of sensory neuron
Stimulation of dendrites produces graded polarization leading to action potential
Action potential travels through dorsal root to spinal cord
Information processing
Sensory neuron releases excitatory neurotransmitters at postsynaptic membrane of interneuron

40. Module 12.9: Reflexes 4. Activation of motor neuron
Activation of interneuron leads to stimulation of motor neuron to carry action potential to periphery
Axonal collaterals may relay sensation to other centers in brain and spinal cord
Response of peripheral effector
Release of neurotransmitters by synaptic knobs leads to response by peripheral effector
Generally removes or opposes original stimulus
An example of negative feedback
Example: skeletal muscle contraction moving hand away from painful sensation

41. STEP 2 The Activation of a Sensory Neuron
The steps in a reflex arc: a simple withdrawal reflex
STEP The Arrival of a Stimulus and Activation of a Receptor
STEP 3 Information Processing
Dorsal root ganglion
To higher centers
REFLEX ARC
Receptor
Stimulus
STEP 4 The Activation of a Motor Neuron
Effector
STEP The Response of a Peripheral Effector
Figure Reflexes are rapid, automatic responses to stimuli
Sensory neuron (stimulated)
Excitatory interneuron
Motor neuron (stimulated)
Figure 41

42. Module 12.9: Reflexes Reflex categories Development Innate reflexes
Connections formed between neurons genetically or developmentally programmed
Generally appear in a predictable sequence
Example: simplest (withdrawal) to complex (suckling)
Acquired reflexes
Learned rather than preestablished
Enhanced by repetition

43. Module 12.9: Reflexes Reflex categories (continued) Nature of response
Somatic reflexes
Involuntary control of skeletal muscles
Example: withdrawal reflex
Rapid response that can later be supplemented voluntarily
Visceral reflexes (autonomic reflexes)
Control or adjust activities of smooth & cardiac muscle, glands, and adipose tissues

44. Module 12.9: Reflexes Reflex categories (continued)
Complexity of circuit
Polysynaptic reflexes
Involve at least one interneuron, one sensory neuron, and one motor neuron
Longer delay between stimulus and response due to increased number of synapses
Produce more complex reflexes
Monosynaptic reflexes
Simplest reflex arc involving one sensory and one motor neuron
Faster response time due to only one synapse

45. Module 12.9: Reflexes Reflex categories (continued) Processing site
Spinal reflexes
Occur in nuclei of spinal cord
Cranial reflexes
Occur in nuclei of brain

46. Module 12.10: Monosynaptic reflex
Stretch reflex
Best-known monosynaptic reflex
Provides automatic regulation of skeletal muscle length
Example: patellar reflex

47. The patellar reflex, a stretch reflex and the best-known monosynaptic reflex
STEP 2 Activation of a Sensory Neuron
STEP 1 Arrival of the Stimulus and Activation of a Receptor
STEP 3 Information Processing in the CNS
Stretch
Spinal cord
REFLEX ARC
Receptor (muscle spindle)
Contraction
Effector
STEP 4 Activation of a Motor Neuron
Figure The stretch reflex is a monosynaptic reflex involving muscle spindles
STEP 5 Response of a Peripheral Effector
Response
KEY
Sensory neuron (stimulated)
Motor neuron (stimulated)
Figure 47

48. Module 12.10: Monosynaptic reflex
Postural reflexes
Many stretch reflexes that help maintain upright posture
Coordinated activities of opposing muscles to keep body’s weight over feet
Example: leaning forward stretches calf muscle receptors which stimulate the muscles to increase tone
Returns body to upright position
Postural muscles generally have firm muscle tone and extremely sensitive stretch receptors
Allow for very fine, subconscious adjustments

49. Module 12.11: Polysynaptic reflexes
Responsible for automatic actions involved in complex movements
Examples: walking and running
May involve sensory and motor responses on the same side of body or opposite sides
Same side: ipsilateral reflexes
Examples: stretch reflex, withdrawal reflex
Opposite sides: contralateral reflexes
Example: crossed extensor reflex

50. Module 12.11: Polysynaptic reflexes
Withdrawal reflexes
Move affected body parts away from stimulus
Strongest are triggered by painful stimuli but other stimuli can initiate
Show tremendous versatility because sensory neurons activate many pools of interneurons
Intensity and location of stimulus affect:
Distribution of effects
Strength and character of motor responses
Example: flexor reflex, crossed extensor reflexes

51. Module 12.11: Polysynaptic reflexes
Withdrawal reflex example: flexor reflex
Grabbing an unexpectedly hot pan causes pain receptors in hand to be stimulated
Sensory neurons activate interneurons in spinal cord
Interneurons
Activate motor neurons in anterior gray horn to contract flexor muscles
Activated inhibitory interneurons keep extensors relaxed
= Reciprocal inhibition

52. Distribution within gray horns to other segments of the spinal cord
The flexor reflex, a representative withdrawal reflex
Distribution within gray horns to other segments of the spinal cord
Painful stimulus
Flexors stimulated
Sensory neuron (stimulated)
Figure Withdrawal reflexes and crossed extensor reflexes are polysynaptic reflexes
Muscles undergoing reciprocal inhibition
Excitatory interneuron
Extensors inhibited
Motor neuron (stimulated)
Motor neuron (inhibited)
Inhibitory interneuron
Figure 52

53. Module 12.11: Polysynaptic reflexes
Crossed extensor reflexes
Example: stepping on a tack
Flexor reflex pulls injured foot away
Flexor muscles stimulated
Extensor muscles inhibited
Crossed extensor reflex straightens uninjured leg and supports shifting weight
Activated by collaterals of excitatory and inhibitory interneurons
Extensor muscles stimulated
Flexor muscles inhibited

54. To motor neurons in other segments of the spinal cord
The crossed extensor reflex, which involves a contralateral reflex arc
To motor neurons in other segments of the spinal cord
Extensors inhibited
Extensors stimulated
Flexors stimulated
Flexors inhibited
Figure Withdrawal reflexes and crossed extensor reflexes are polysynaptic reflexes
Sensory neuron (stimulated)
Excitatory interneuron
Motor neuron (stimulated)
Motor neuron (inhibited)
Painful stimulus
Inhibitory interneuron
Figure 54

55. Brain influences on spinal reflexes
CLINICAL MODULE 12.12: Brain influences on spinal reflexes and diagnostics using reflexes
Brain influences on spinal reflexes
Can facilitate or inhibit motor neurons or interneurons involved
Facilitation = reinforcement
Example: voluntary movement to pull apart clasped hands can reinforce stretch reflexes and increase response (example: bigger kick after patellar tap)

56. Reflexes used in diagnostic testing
CLINICAL MODULE 12.12: Brain influences on spinal reflexes and diagnostics using reflexes
Reflexes used in diagnostic testing
Specific examples
Babinski reflex
Stroking lateral side of sole of foot
Positive response: toes fan due to lack of inhibitory control of reflex response from descending motor pathways
Normal in infants
Can indicate damaged higher centers or descending tracts in adults
Negative response: toes curl due to development and normal reflex response
= Plantar reflex

57. Reflexes used in diagnostic testing (continued)
CLINICAL MODULE 12.12: Brain influences on spinal reflexes and diagnostics using reflexes
Reflexes used in diagnostic testing (continued)
Specific examples (continued)
Abdominal reflex
Depends on descending facilitation
Light stroking of skin of anterior abdomen produces reflexive twitch of abdominal muscles
Absence of response may indicate damage to descending tracts