Transcript The Spinal Nerve

BIOL 2401
Fundamentals of Anatomy and Physiology
Mrs. Willie Grant
[email protected]
210-486-2870
© 2012 Pearson Education, Inc.
13
The Spinal Cord,
Spinal Nerves,
and Spinal Reflexes
PowerPoint® Lecture Presentations prepared by
Jason LaPres
Lone Star College—North Harris
© 2012 Pearson Education, Inc.
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.
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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
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Figure 13-2 Gross Anatomy of the Adult Spinal Cord
Posterior median sulcus
Dorsal root
Dorsal root
ganglion
Cervical spinal
nerves
C1
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
White matter
Gray
matter
Central
canal
Cervical
enlargement
Spinal Ventral
nerve
root
Anterior median fissure
C3
T7
Thoracic
spinal
nerves
T8
T9
Posterior
median sulcus
T10
T11
T3
Lumbar
enlargement
T12
L1
Conus
medullaris
L2
Lumbar
spinal
nerves
L3
L4
Inferior
tip of
spinal cord
Cauda equina
L5
L1
Sacral spinal
nerves
1 What portion of
the spinal cord
connects with
nerves of the upper
limbs?
S1
S2
S3
S4
S5
Coccygeal
nerve (Co1)
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Filum terminale
(in coccygeal ligament)
S2
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
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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
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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
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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
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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
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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?
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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
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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
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Figure 13-5 The Sectional Organization of the Spinal Cord
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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
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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
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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
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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?
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Sensory input is
conveyed from
sensory receptors
to the posterior gray
horns of the spinal
cord.
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Motor output is
conveyed from the
anterior and lateral
gray horns of the
spinal cord to
effectors (muscles
and glands).
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Figure 13-8 Dermatomes
C2C3
NV
C2C3
Bilateral regions of skin
monitored by specific pairs
of nerves.
C2
C3
T2
C6
L1
L2
Peripheral Neuropathy
C8
Regional loss of sensory or
C7
T1
L3
L4
motor function
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L4 L3
L5
C4
C5
T2
C6
T1
C7
SS
S2
4 3
L1
S5
C8
S1 L5
L2 S2
Due to trauma or
L5
L3
compression
Examples: when your leg
falls asleep.
L4
ANTERIOR
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S1
POSTERIOR
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?
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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
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Figure 13-14 Neural Circuits: The Organization of Neuronal Pools
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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)
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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
Triggered by neurotransmitters
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Figure 13-15 Events in a Neural Reflex
Activation of a
sensory neuron
Arrival of stimulus and
activation of receptor
Dorsal
root
Sensation
relayed to the
brain by axon
collaterals
Information processing
in the CNS
REFLEX
ARC
Receptor
Stimulus
Response by effector
Effector
Ventral
root
Activation of a
motor neuron
KEY
Sensory neuron
(stimulated)
Excitatory
interneuron
Motor neuron
(stimulated)
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Figure 13-16 The Classification of Reflexes
Four classes: 1 Development, 2 Response, 3 Complexity of Circuit, 4 Processing Site
Figure 13-16 The Classification of Reflexes
Reflexes
can be classified by
1
2
3
development
response
Innate Reflexes
Somatic Reflexes
• Genetically
determined
• Control skeletal muscle
contractions
• Include superficial and stretch
reflexes
Acquired Reflexes
Visceral (Autonomic) Reflexes
• Learned
• Control actions of smooth and
cardiac muscles, glands, and
adipose tissue
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4
complexity of circuit
processing site
Monosynaptic
Spinal Reflexes
• One synapse
Polysynaptic
• Multiple synapse
(two to several
hundred)
• Processing in
the spinal cord
Cranial Reflexes
• Processing in
the brain
13-7 Spinal Reflexes
Spinal Reflexes
Monosynaptic reflexes (a stretch reflex) and Polysynaptic reflexes
Monosynaptic reflexes—A 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
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Figure 13-17 A Stretch Reflex
Receptor
(muscle
spindle)
Spinal cord
Stretch
REFLEX
ARC
Stimulus
Effector
Contraction
KEY
Sensory neuron
(stimulated)
Motor neuron
(stimulated)
Response
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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
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Figure 13-18 A Muscle Spindle
Extrafusal
fiber
Gamma
efferent
from CNS
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)
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Spinal Reflexes
Polysynaptic Reflexes
More complicated than monosynaptic reflexes
Interneurons control more than one muscle group
Intersegmental reflex arcs
Many segments interact to produce highly variable motor
response
Postural reflexes
Stretch reflexes that help maintain normal upright posture.
Stretched muscle responds by contracting
Automatically maintain balance
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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
Withdrawal Reflexes
Move body part away from stimulus (pain or pressure)
For example, flexor reflex
Pulls hand away from hot stove
Strength and extent of response
Depend on intensity and location of stimulus
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Figure 13-19 A Flexor Reflex
Distribution within gray horns to other segments of the spinal cord
Painful
stimulus
Flexors
stimulated
Extensors
inhibited
KEY
Sensory neuron
(stimulated)
Motor neuron
(inhibited)
Excitatory
interneuron
Inhibitory
interneuron
Motor neuron
(stimulated)
Reciprocal Inhibition
For flexor reflex to work
The stretch reflex of antagonistic (extensor) muscle must be inhibited
(reciprocal inhibition) by interneurons in spinal cord
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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
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Figure 13-20 The Crossed Extensor Reflex
To motor neurons in other segments of the spinal cord
Extensors
inhibited
Flexors
stimulated
Extensors
stimulated
Flexors
inhibited
KEY
Painful
stimulus
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Sensory neuron
(stimulated)
Motor neuron
(inhibited)
Excitatory
interneuron
Inhibitory
interneuron
Motor neuron
(stimulated)
13-7 Spinal Reflexes
Five General Characteristics of Polysynaptic Reflexes
Involve pools of interneurons
Are intersegmental in distribution
Involve reciprocal inhibition
Have reverberating circuits
Which prolong reflexive motor response
Several reflexes cooperate
To produce coordinated, controlled response
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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
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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
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Figure 13-21a The Babinski Reflexes
The Babinski Reflexes
Normal in infants
The plantar reflex
(negative Babinski relfex),
A curling of the toes, is
seen in healthy adults.
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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.
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