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


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Title: Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

Chapter 13 The Spinal Cord, Spinal Nerves, and
Spinal Reflexes
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 131
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
  • Posterior median sulcus on posterior side
  • Anterior median fissure deeper groove on
    anterior side

Figure 13-2
Enlargements of the Spinal Cord
  • Caused by
  • amount of gray matter in segment
  • involvement with sensory and motor nerves of
  • 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
  • Cauda equina nerve roots extending below conus

31 Spinal Cord Segments
  • Based on vertebrae where spinal nerves originate
  • Positions of spinal segment and vertebrae change
    with age
  • Naming Spinal Nerves
  • Cervical nervesare 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 gangliacontain cell bodies of
    sensory neurons

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

Spinal Meninges
  • Specialized membranes isolate spinal cord from
  • 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 133
The Spinal Dura Mater
  • 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
  • Contains loose connective and adipose tissue
  • Anesthetic injection site

The Arachnoid Mater
  • 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)

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

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

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 135a
  • Posterior gray hornscontain 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

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

  • 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

  • Tracts or fasciculi
  • 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

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)
  • Endoneuriuminner layer, surrounds individual

Figure 136
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 137a
  • The first branch - from the spinal nerve
  • carries visceral motor fibers
  • to sympathetic ganglion of autonomic nervous
  • Sympathetic Nerves
  • Are postganglionic fibers
  • Innervate smooth muscles, glands, and organs

White Ramus
  • 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

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

Peripheral Distribution of Spinal Nerves
  • Sensory fibers
  • Sensory Nerves
  • In addition to motor impulses dorsal, ventral,
    and white rami also carry sensory information

Figure 137b
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
  1. Cervical plexus
  2. Brachial plexus
  3. Lumbar plexus
  4. Sacral plexus

The Cervical Plexus
  • Includes ventral rami of spinal nerves C1C5
  • Innervates neck, thoracic cavity, diaphragmatic
  • Major nerve phrenic nerve (controls diaphragm)

Figure 1310
The Brachial Plexus
  • Includes ventral rami of spinal nerves C5T1
  • 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)

Summary Brachial Plexus
Table 132 (1 of 2)
The Lumbar and Sacral Plexuses
  • Innervate pelvic girdle and lower limbs
  • The Lumbar Plexus
  • Includes ventral rami of spinal nerves T12L4
  • Major nerves
  • genitofemoral nerve
  • lateral femoral cutaneous nerve
  • femoral nerve

Figure 1312a, b
The Lumbar and Sacral Plexuses
  • Innervate pelvic girdle and lower limbs
  • The Sacral Plexus
  • Includes ventral rami of spinal nerves L4S4
  • Major nerves
  • pudendal nerve
  • sciatic nerve
  • Branches of sciatic nerve
  • fibular nerve
  • tibial nerve

Figure 1312a, b
Lumbar and Sacral Plexuses
Table 13-3 (1 of 2)
  • Automatic responses coordinated within spinal
  • Through interconnected sensory, motor, and
  • Produce simple and complex reflexes

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

Neuronal Pools
  • Functional groups of interconnected neurons
  • Each with limited input sources and output
  • May stimulate or depress parts of brain or spinal

5 Patterns of Neural Circuits in Neuronal Pools
  • Divergence
  • spreads stimulation to many neurons or neuronal
    pools in CNS

  • Convergence
  • brings input from many sources to single neuron
  • Serial processing
  • moves information in single line

Figure 1313b
  • Parallel processing
  • moves same information along several paths
  • Reverberation
  • positive feedback mechanism
  • functions until inhibited

Figure 1313d
Neural Reflexes
  • 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

5 Steps in a Neural Reflex
  • Step 1 Arrival of stimulus, activation of
  • physical or chemical changes
  • Step 2 Activation of sensory neuron
  • graded depolarization
  • Step 3 Information processing by postsynaptic
  • triggered by neurotransmitters
  • Step 4 Activation of motor neuron
  • action potential
  • Step 5 Response of peripheral effector
  • triggered by neurotransmitters

Figure 1314
4 Classifications of Reflexes
  • By early development
  • How reflex was developed
  • innate reflexesbasic 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

  • By complexity of neural circuit
  • Complexity of neural circuit
  • monosynaptic reflex
  • sensory neuron synapses directly onto motor
  • 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

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

1. Monosynaptic Reflexes patellar reflex
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 reflexpulls hand away
    from hot stove
  • Strength and extent of response
  • depends on intensity and location of stimulus

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.

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.

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

The Babinski Reflexes
  • Normal in infants
  • May indicate CNS damage in adults

Figure 1319