1. Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

2. General Organization - Nervous System
Highly organized, very efficient
Spinal Reflexes
Rapid, automatic nerve responses triggered by specific stimuli
Controlled by spinal cord alone; not the brain
Figure 13–1

3. Gross Anatomy of the Spinal Cord
Adult
About 18 inches (45 cm) long
1/2 inch (14 mm) wide
Ends between vertebrae L1 and L2
Bilateral Symmetry
Grooves divide the spinal cord into left and right
Posterior median sulcus: on posterior side
Anterior median fissure: deeper groove on anterior side
Figure 13-2

4. Enlargements of the Spinal Cord
Caused by:
amount of gray matter in segment
involvement with sensory and motor nerves of limbs
Cervical enlargement:
nerves of shoulders and upper limbs
Lumbar enlargement:
nerves of pelvis and lower limbs
The Distal End
Conus medullaris: thin, conical spinal cord below lumbar enlargement
Filum terminale: thin thread of fibrous tissue at end of conus medullaris, attaches to coccygeal ligament
Cauda equina: nerve roots extending below conus medullaris

5. 31 Spinal Cord Segments Roots
Based on vertebrae where spinal nerves originate
Positions of spinal segment and vertebrae change with age
Naming Spinal Nerves
Cervical nerves:are named for inferior vertebra
All other nerves: are named for superior vertebra
Roots
2 branches of spinal nerves:
ventral root: contains axons of motor neurons
dorsal root: contains axons of sensory neurons
Dorsal root ganglia:contain cell bodies of sensory neurons

6. The Spinal Nerve Mixed Nerves
Each side of spine: dorsal and ventral roots join and form a spinal nerve
Mixed Nerves
Carry both afferent (sensory) & efferent (motor) fibers
Spinal nerves are mixed nerves

7. Spinal Meninges The 3 Meningeal Layers
Specialized membranes isolate spinal cord from surroundings
Spinal meninges:
protect spinal cord
carry blood supply
continuous with cranial meninges
Meningitis:
viral or bacterial infection of meninges
The 3 Meningeal Layers
Dura mater: outer layer of spinal cord
Arachnoid mater: middle meningeal layer
Pia mater: inner meningeal layer
Figure 13–3

8. The Spinal Dura Mater The Epidural Space Are tough and fibrous
Cranially:
fuses with periosteum of occipital bone
is continuous with cranial dura mater
Caudally:
tapers to dense cord of collagen fibers
joins filum terminale in coccygeal ligament
The Epidural Space
Between spinal dura mater and walls of vertebral canal
Contains loose connective and adipose tissue
Anesthetic injection site

9. The Arachnoid Mater Inter-Layer Spaces Middle meningeal layer
Arachnoid membrane:
simple squamous epithelia
covers arachnoid mater
Inter-Layer Spaces
Subdural space:
between arachnoid mater and dura mater
Subarachnoid space:
between arachnoid mater and pia mater
contains collagen/elastin fiber network (arachnoid trabeculae)
filled with cerebrospinal fluid (CSF)

10. Cerebrospinal Fluid (CSF)
Is found in subarachnoid space
Carries dissolved gases, nutrients, and wastes
Spinal tap:
withdraws CSF
The Pia Mater
Is the innermost meningeal layer
Is a mesh of collagen and elastic fibers
Is bound to underlying neural tissue

11. Structures of the Spinal Cord
Paired denticulate ligaments:
extend from pia mater to dura mater
stabilize side-to-side movement
Blood vessels:
along surface of spinal pia mater
within subarachnoid space

12. Sectional Anatomy of the Spinal Cord
White Matter
Is superficial
Contains myelinated and unmyelinated axons
Gray Matter
Surrounds central canal of spinal cord
Contains neuron cell bodies, neuroglia, unmyelinated axons
Has projections (gray horns)
Figure 13–5a

13. Posterior gray horns:contain somatic and visceral sensory nuclei
Anterior gray horns: contain somatic motor nuclei
Lateral gray horns: are in thoracic and lumbar segments and contain visceral motor nuclei

14. Organization of White Matter
3 columns (funiculi) on each side of spinal cord:
posterior white columns
Lie between posterior gray horns and posterior median sulcus
anterior white columns
Lie between anterior gray horns and anterior median fissure
Area where axons cross from 1 side of spinal cord to the other
lateral white columns
Located on each side of spinal cord
Between anterior and posterior columns

15. Nuclei Control and Location Nuclei: Sensory nuclei: Motor nuclei:
functional groups of cell bodies
Sensory nuclei:
dorsal (posterior)
connect to peripheral receptors
Motor nuclei:
ventral (anterior)
connect to peripheral effectors
Control and Location
Sensory or motor nucleus location within the gray matter determines which body part it controls

16. Tracts Tracts or fasciculi: Ascending tracts: Descending tracts:
in white columns
bundles of axons
relay same information in same direction
Ascending tracts:
carry information to brain
Descending tracts:
carry motor commands to spinal cord

17. 3 Connective Tissue Layers
Spinal Nerves
Every spinal cord segment:
is connected to a pair of spinal nerves
Every spinal nerve:
is surrounded by 3 connective tissue layers
that support structures and contain blood vessels
3 Connective Tissue Layers
Epineurium: outer layer, dense network of collagen fibers
Perineurium: middle layer, divides nerve into fascicles (axon bundles)
Endoneurium:inner layer, surrounds individual axons
Figure 13–6

18. Peripheral Distribution of Spinal Nerves
Motor
Spinal nerves:
form lateral to intervertebral foramen
where dorsal and ventral roots unite
then branch and form pathways to destination
Figure 13–7a

19. The first branch - from the spinal nerve:
carries visceral motor fibers
to sympathetic ganglion of autonomic nervous system
Sympathetic Nerves
Are postganglionic fibers
Innervate smooth muscles, glands, and organs

20. White Ramus Gray Ramus Rami Communicantes Preganglionic branch
Myelinated axons
Gray Ramus
Unmyelinated nerves
Return from sympathetic ganglion
Rejoin spinal nerve
Rami Communicantes
Also called communicating branches
Formed by gray ramus and white ramus together

21. Dorsal and Ventral Rami
Dorsal ramus:
contains somatic and visceral motor fibers
innervates the back
Ventral ramus:
larger branch
innervates ventrolateral structures and limbs

22. Peripheral Distribution of Spinal Nerves
Sensory fibers
Sensory Nerves
In addition to motor impulses: dorsal, ventral, and white rami also carry sensory information
Figure 13–7b

23. The 4 Major Plexuses of Ventral Rami
Nerve Plexuses
Complex, interwoven networks of nerve fibers
Formed from blended fibers of ventral rami of adjacent spinal nerves
Control skeletal muscles of the neck and limbs
The 4 Major Plexuses of Ventral Rami
Cervical plexus
Brachial plexus
Lumbar plexus
Sacral plexus

24. The Cervical Plexus Includes ventral rami of spinal nerves C1–C5
Innervates neck, thoracic cavity, diaphragmatic muscles
Major nerve: phrenic nerve (controls diaphragm)
Figure 13–10

25. The Brachial Plexus Includes ventral rami of spinal nerves C5–T1
Innervates pectoral girdle and upper limbs
Trunks and Cords of Brachial Plexus Nerves
that form brachial plexus originate from:
superior, middle, and inferior trunks
large bundles of axons from several spinal nerves
lateral, medial, and posterior cords
smaller branches that originate at trunks
Major Nerves of Brachial Plexus
-Musculocutaneous nerve (lateral cord)
Median nerve (lateral and medial cords)
Ulnar nerve (medial cord)
Axillary nerve (posterior cord)
Radial nerve (posterior cord)

26. Summary: Brachial Plexus
Table 13–2 (1 of 2)

27. The Lumbar and Sacral Plexuses
Innervate pelvic girdle and lower limbs
The Lumbar Plexus
Includes ventral rami of spinal nerves T12–L4
Major nerves:
genitofemoral nerve
lateral femoral cutaneous nerve
femoral nerve
Figure 13–12a, b

28. The Lumbar and Sacral Plexuses
Innervate pelvic girdle and lower limbs
The Sacral Plexus
Includes ventral rami of spinal nerves L4–S4
Major nerves:
pudendal nerve
sciatic nerve
Branches of sciatic nerve:
fibular nerve
tibial nerve
Figure 13–12a, b

29. Lumbar and Sacral Plexuses
Table 13-3 (1 of 2)

30. Reflexes Automatic responses coordinated within spinal cord
Through interconnected sensory, motor, and interneurons
Produce simple and complex reflexes

31. Functional Organization of Neurons
Sensory neurons:
about 10 million
deliver information to CNS
Motor neurons:
about 1/2 million
deliver commands to peripheral effectors
Interneurons:
about 20 billion
interpret, plan, and coordinate signals in and out

32. 5 Patterns of Neural Circuits in Neuronal Pools
Functional groups of interconnected neurons (interneurons)
Each with limited input sources and output destinations
May stimulate or depress parts of brain or spinal cord
5 Patterns of Neural Circuits in Neuronal Pools
Divergence:
spreads stimulation to many neurons or neuronal pools in CNS

33. Convergence: Serial processing:
brings input from many sources to single neuron
Serial processing:
moves information in single line
Figure 13–13b

34. Parallel processing: Reverberation:
moves same information along several paths simultaneously
Reverberation:
positive feedback mechanism
functions until inhibited
Figure 13–13d

35. Neural Reflexes The Reflex Arc
Rapid, automatic responses to specific stimuli
Basic building blocks of neural function
1 neural reflex produces 1 motor response
The Reflex Arc
The wiring of a single reflex
Beginning at receptor
Ending at peripheral effector
Generally opposes original stimulus (negative feedback)

36. 5 Steps in a Neural Reflex
Step 1: Arrival of stimulus, activation of receptor
physical or chemical changes
Step 2: Activation of sensory neuron
graded depolarization
Step 3: Information processing by postsynaptic cell
triggered by neurotransmitters
Step 4: Activation of motor neuron
action potential
Step 5: Response of peripheral effector
Figure 13–14

37. 4 Classifications of Reflexes
By early development
How reflex was developed:
innate reflexes:basic neural reflexes, formed before birth
acquired reflexes: rapid, automatic, learned motor patterns
By type of motor response-Nature of resulting motor response:
somatic reflexes:
involuntary control of nervous system
superficial reflexes of skin, mucous membranes
stretch reflexes (deep tendon reflexes) e.g., patellar reflex
visceral reflexes (autonomic reflexes):
control systems other than muscular syste

38. Complexity of neural circuit: monosynaptic reflex:
By complexity of neural circuit
Complexity of neural circuit:
monosynaptic reflex:
sensory neuron synapses directly onto motor neuron
polysynaptic reflex:
at least 1 interneuron between sensory neuron and motor neuron
By site of information processing
Site of information processing:
spinal reflexes:
occurs in spinal cord
cranial reflexes:
occurs in brain

39. Spinal Reflexes Range in increasing order of complexity:
monosynaptic reflexes
simplest of all reflexes
Just one synapse
The fastest of all reflexes
Example – stretch-knee-jerk reflex
polysynaptic reflexes
more common type of reflex
Most have a single interneuron between the sensory and motor neuron
Example – withdrawal reflexes

40. 1. Monosynaptic Reflexes – patellar reflex

41. 2. Polysynaptic Reflexes
More complicated than monosynaptic reflexes
Interneurons control more than 1 muscle group
Withdrawal or flexor reflex is an example
Move body part away from stimulus (pain or pressure): e.g., flexor reflex:pulls hand away from hot stove
Strength and extent of response:
depends on intensity and location of stimulus

42. The Tendon Reflex Prevents skeletal muscles from:
developing too much tension
tearing or breaking tendons
tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example.

43. Reflex Arcs
When a reflex arc consists of only two neurons (one sensory neuron and one motor neuron), it is defined as monosynaptic.
In the case of peripheral muscle reflexes (patellar reflex, achilles reflex), brief stimulation to the muscle spindle results in contraction of the agonist or effector muscle.
polysynaptic reflex pathways, one or more interneurons connect afferent (sensory) and efferent (motor) signals.
All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the spinal cord.

44. Crossed Extensor Reflexes
Occur simultaneously, coordinated with flexor reflex
e.g., flexor reflex causes leg to pull up:
crossed extensor reflex straightens other leg
to receive body weight
maintained by reverberating circuits

45. The Babinski Reflexes Normal in infants
May indicate CNS damage in adults
Figure 13–19