1. BIOL 2401
Fundamentals of Anatomy and Physiology Mrs. Willie Grant

3. An Introduction to the Spinal Cord, Spinal Nerves, and Spinal Reflexes
Learning Outcomes
13-1 Describe the basic structural and organizational characteristics of the nervous system.
13-2 Discuss the structure and functions of the spinal cord, and describe the three meningeal layers that surround the central nervous system.
13-3 Explain the roles of white matter and gray matter in processing and relaying sensory information and motor commands.
13-4 Describe the major components of a spinal nerve, and relate the distribution pattern of spinal nerves to the regions they innervate.
13-5 Discuss the significance of neuronal pools, and describe the major patterns of interaction among neurons within and among these pools.
13-6 Describe the steps in a neural reflex, and classify the types of reflexes.
13-7 Distinguish among the types of motor responses produced by various reflexes, and explain how reflexes interact to produce complex behaviors.
13-8 Explain how higher centers control and modify reflex responses.

5. 13-2 Spinal Cord
Gross Anatomy of the Spinal Cord
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

6. 13-2 Spinal Cord
31 Spinal Cord Segments
Based on vertebrae where spinal nerves originate
Positions of spinal segment and vertebrae change with age
Cervical nerves (named for inferior vertebra)
All other nerves (named for superior vertebra)
Roots
Two 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

7. Figure 13-2 Gross Anatomy of the Adult Spinal Cord
Posterior median sulcus
Dorsal root
Dorsal root ganglion
White matter C1
Central canal
Gray matter C2
Cervical spinal nerves C3 C4 C5
Spinal nerve
Ventral root C6
Cervical enlargement C7
Anterior median fissure C8 C3 T1 T2 T3 T4 T5 T6 T7
Thoracic spinal nerves T8
Posterior median sulcus T9
T10 T3
T11
Lumbar enlargement
T12
Conus medullaris L1 L2
Inferior tip of spinal cord
Lumbar spinal nerves L3 L4
Cauda equina L5 L1 S1
Sacral spinal nerves S2 S3
1 What portion of the spinal cord connects with nerves of the upper limbs? S4 S5
Coccygeal nerve (Co1)
Filum terminale (in coccygeal ligament) S2 7

8. 13-2 Spinal Cord The Spinal Nerve On each side of spine
Dorsal and ventral roots join to form a spinal nerve
Mixed Nerves
Carry both afferent (sensory) and efferent (motor) fibers
The Spinal Meninges
Specialized membranes isolate spinal cord from surroundings
Functions of the spinal meninges include:
Protecting spinal cord
Carrying blood supply
Continuous with cranial meninges
Meningitis
Viral or bacterial infection of meninges

9. 13-2 Spinal Cord
The Three Meningeal Layers (Dura Mater, Arachnoid, Pia Mater)
The Dura Mater
Tough and fibrous
Cranially fuses with periosteum of occipital bone
Is continuous with cranial dura mater
Caudally tapers to dense cord of collagen fibers and 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

10. 13-2 Spinal Cord
The Arachnoid Mater (middle meningeal layer)
Simple squamous epithelia covering arachnoid mater
The Interlayer Spaces of Arachnoid Mater
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) Carries dissolved gases, nutrients, and wastes
Lumbar puncture or spinal tap withdraws CSF
The Pia Mater (the innermost meningeal layer)
Is a mesh of collagen and elastic fibers bound to underlying neural tissue

12. 13-2 Spinal Cord
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

13. 13-3 Gray Matter and White Matter
Sectional Anatomy of the Spinal Cord
White matter (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)
Organization of Gray Matter
The gray horns
Posterior gray horns contain somatic and visceralsensory nuclei
Anterior gray horns contain somatic motor nuclei
Lateral gray horns are in thoracic and lumbar segments; contain visceral motor nuclei
Gray commissures (axons that cross from one side of cord to other before reacing gray matter)
2 What is the difference between a horn and a column in the spinal cord?

14. 13-3 Gray Matter and White Matter
Organization of Gray Matter
The cell bodies of neurons form functional groups called nuclei (masses of gray matter within the CNS)
Sensory nuclei (Dorsal—posterior)
Connect to peripheral receptors
Motor nuclei (ventral—anterior)
Connect to peripheral effectors
Sensory or motor nucleus location within the gray matter determines which body part it controls

15. 13-3 Gray Matter and White Matter
Organization of White Matter
Posterior white columns lie between posterior gray horns and posterior median sulcus
Anterior white columns lie between anterior gray horns and anterior median fissure
Anterior white commissure area where axons cross from one side of spinal cord to the other
Lateral white columns located on each side of spinal cord between anterior and posterior columns
Tracts or fasciculi (a bundle of axons in the white columns that relay the same information in the same direction).
Ascending tracts—carry information to brain
Descending tracts—carry motor commands to spinal cord

16. Figure 13-5 The Sectional Organization of the Spinal Cord

17. 13-3 Gray Matter and White Matter
Spinal Cord Summary
Spinal cord has a narrow central canal surrounded by gray matter
Containing sensory (dorsal) and motor nuclei (ventral)
Gray matter
Is covered by a thick layer of white matter
White matter
Consists of ascending and descending axons organized in columns
Contains axon bundles with specific functions
Spinal cord is so highly organized
It is possible to predict results of injuries to specific areas

18. 13-4 Spinal Nerves and Plexuses
Anatomy of Spinal Nerves
Every spinal cord segment is connected to a pair of spinal nerves
Every spinal nerve is surrounded by three connective tissue layers that support structures and contain blood vessels
Three Connective Tissue Layers of Spinal Nerves
Epineurium (outer layer)
Dense network of collagen fibers
Perineurium (middle layer)
Divides nerve into fascicles (axon bundles)
Endoneurium (inner layer)
Surrounds individual axons

19. Figure 13-6 A Peripheral Nerve
Blood vessels
Connective Tissue Layers
Epineurium covering spinal nerve
Perineurium (around one fascicle)
Endoneurium
Myelinated axon
Fascicle
Schwann cell 19

20. 13-4 Spinal Nerves and Plexuses
Peripheral Distribution of Spinal Nerves
Spinal nerves that form lateral to intervertebral foramen where the dorsal and ventral roots unite then branch to form pathways to destination
Motor nerves—The first branch
White ramus
Carries visceral motor fibers to sympathetic ganglion of autonomic nervous system
Gray ramus
Unmyelinated nerves
Return from sympathetic ganglion to rejoin spinal nerve
Dorsal and ventral rami
Dorsal ramus
Contains somatic and visceral motor fibers. Innervates the back.
Ventral ramus
Larger branch. Innervates ventrolateral structures and limbs
3 Why are all spinal nerves classified as mixed nerves?

21. Sensory input is
conveyed from
sensory receptors
to the posterior gray
horns of the spinal
cord.

22. Motor output is conveyed from the anterior and lateral gray horns of the spinal cord to effectors (muscles and glands).

23. Dermatome is the specific bilateral region
of the skin surface monitored by a single
pair of spinal nerves.
Peripheral Neuropathy
Regional loss of sensory or motor function
Due to trauma or compression
Examples: when your leg falls asleep.

24. Includes ventral rami of spinal nerves C1-C5
Includes ventral rami of spinal nerves C1-C5. Innervates neck, thoracic cavity, diaphragm. PHRENIC NERVE
Spinal Nerves C5-T1. Innervates pectoral girdle/upper limbs. RADIAL NERVE, ULNAR NERVE, MEDIAN NERVE.
Spinal Nerves T12-L4. FEMORAL NERVE
Spinal Nerves L4-S4. SCIATIC NERVE branches into FIBULAR NERVE AND TIBIAL NERVE
4 Why does complete severing of the spinal cord at level C2 cause respiratory arrest?
5 What three important nerves arise from the brachial plexus?

25. 13-5 Neuronal Pools
Functional Organization of Neurons
Sensory neurons—Deliver information to CNS—10 million
Motor neurons—Deliver commands to peripheral effectors—1/2 million
Interneurons—Interpret, plan, coordinate signals in and out—20 billion
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

26. Figure 13-14 Neural Circuits: The Organization of Neuronal Pools

27. 13-6 Reflexes
Reflexes—automatic responses coordinated within spinal cord
Through interconnected sensory neurons, motor neurons, and interneurons, producing simple and complex reflexes
Neural Reflexes—rapid, automatic responses to specific stimuli
Basic building blocks of neural function
One neural reflex produces one motor response
Reflex arc
The wiring of a single reflex
Beginning at receptor
Ending at peripheral effector
Generally opposes original stimulus (negative feedback)

28. 13-6 Reflexes
Five 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

29. Figure 13-15 Events in a Neural Reflex
Dorsal root
Arrival of stimulus and activation of receptor
Activation of a sensory neuron
Sensation relayed to the brain by axon collaterals
Information processing in the CNS
REFLEX ARC
Receptor
Stimulus
Response by effector
Effector
Ventral root
KEY
Sensory neuron (stimulated)
Activation of a motor neuron
Excitatory interneuron
Motor neuron (stimulated) 29

31. 13-7 Spinal Reflexes Spinal Reflexes
Monosynaptic reflexes (a stretch reflex) and Polysynaptic reflexes
Stretch reflex
Have least delay between sensory input and motor output
For example, stretch reflex (such as patellar reflex)
Completed in 20–40 msec. The receptor is muscle spindle

32. Polysynaptic Reflexes
Produce more complicated responses because the interneurons can control motor neurons that activate several muscle groups simultaneously.
A withdrawal reflex moves affected parts of the body away from a stimulus.
An example of a withdrawal reflex is the flexor reflex.

33. 13-7 Spinal Reflexes
The Tendon Reflex
Prevents skeletal muscles from:
Developing too much tension and tearing or breaking tendons
Sensory receptors unlike muscle spindles or proprioceptors

34. Ipsilateral reflex arcs occurs on same side of body as stimulus
13-7 Spinal Reflexes
Reflex Arcs
Ipsilateral reflex arcs occurs on same side of body as stimulus
Stretch, tendon, and withdrawal reflexes
Crossed extensor reflexes
Involve a contralateral reflex arc (occur on side opposite stimulus)
Occur simultaneously, coordinated with flexor reflex
For example, flexor reflex causes leg to pull up
Crossed extensor reflex straightens other leg
To receive body weight
Maintained by reverberating circuits

36. Stretch reflexes that help maintain normal upright posture.
Spinal Reflexes
Polysynaptic Reflexes
More complicated than monosynaptic reflexes
Interneurons control more than one muscle group
Withdrawal Reflex is an example of a Polysynaptic Reflex
Postural reflexes
Stretch reflexes that help maintain normal upright posture.
Stretched muscle responds by contracting
Automatically maintain balance

37. 13-7 Spinal Reflexes
Muscle Spindles
The receptors in stretch reflexes
Bundles of small, specialized intrafusal muscle fibers
Innervated by sensory and motor neurons
Surrounded by extrafusal muscle fibers
Which maintain tone and contract muscle

38. Figure 13-18 A Muscle Spindle
Gamma efferent from CNS
Extrafusal fiber
To CNS
Sensory region (central, enters CNS at dorsal root, synapses with gamma motor neurons
Intrafusal fiber
Muscle spindle
Gabba efferent from CNS (synapses back
Into intrafusalfibers) 38

39. 13-8 The Brain Can Alter Spinal Reflexes
Integration and Control of Spinal Reflexes
Reflex behaviors are automatic
But processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord
Voluntary Movements and Reflex Motor Patterns
Higher centers of brain incorporate lower, reflexive motor patterns
Automatic reflexes
Can be activated by brain as needed
Use few nerve impulses to control complex motor functions
Walking, running, jumping

40. 13-8 The Brain Can Alter Spinal Reflexes
Reinforcement of Spinal Reflexes
Higher centers reinforce spinal reflexes
By stimulating excitatory neurons in brain stem or spinal cord
Facilitating postsynaptic neurons
Inhibition of Spinal Reflexes
Higher centers inhibit spinal reflexes by:
Stimulating inhibitory neurons
Suppressing postsynaptic neurons

41. Figure 13-21a The Babinski Reflexes
Normal in infants
The plantar reflex
(negative Babinski relfex),
A curling of the toes, is seen in healthy adults. 41

42. Figure 13-21b The Babinski Reflexes
The Babinski sign (positive Babinski reflex) occurs in the absence of descending inhibition. It is normal in infants, but pathological in adults. 42

43. Clinical Case—Prom Night
Do you think Joe will regain motor or sensory activity in his lower extremities? Why or why not?
Do you think Joe’s Babinski relfexes are normal (toes plantar flexed) or abnormal (toes go up,
Dorsiflexed)? Why?