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Chapter 14
Lecture Outline
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1
14.1 Spinal Cord Gross
Anatomy
Describe the general
composition of the spinal cord.
2.
Identify the five anatomic
subdivisions of the spinal cord and
their associated spinal nerves.
3. Explain how the cauda equina
arises in development.
1.
Learning
Objectives:
2
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14.1 Spinal Cord Gross Anatomy
• Spinal cord
– Extends inferiorly from brain’s medulla through
vertebral canal
– Ends at L1 vertebra with conus medullaris
– Two widened regions with greater number of neurons
o Cervical enlargement: contains neurons innervating upper
limbs
o Lumbar enlargement: contains neurons innervating lower
limbs
3
14.1 Spinal Cord Gross Anatomy
• Spinal cord subdivided into five parts from top to bottom
– Cervical part (superiormost part)
o 8 pairs of cervical spinal nerves
– Thoracic part
o 12 pairs of thoracic spinal nerves
– Lumbar part
o 5 pairs of lumbar spinal nerves
– Sacral part
o 5 pairs of sacral spinal nerves
– Coccygeal part (inferior tip of spinal cord)
o 1 pair of coccygeal spinal nerves
4
Gross
Anatomy of
the Spinal
Cord and
Spinal Nerves
Figure 14.1a, c
5
14.1 Spinal Cord Gross Anatomy
• Spinal cord parts do not align with vertebrae names
– Vertebrae growth continues after spinal cord growth complete
– Rootlets from parts L2 and below extend inferiorly as cauda
equina
o Filum terminale: thin strand of pia attaching conus medullaris to coccyx
• Spinal nerves are named for attached spinal cord part
– Superiormost spinal nerve is C1 nerve
– Inferiormost spinal nerve is Co1 nerve
6
14.1 Spinal Cord Gross Anatomy
• Spinal cord size and shape vary along its length, but is roughly cylindrical
• Cross section reveals grooves on front (anterior median fissure) and
back (posterior median sulcus)
Figure 14.2 (partial)
What did you
learn?
7
•
•
Which parts of the spinal cord
are enlarged, and what
functions do these
enlargements serve?
What is the name of the
inferiormost cervical spinal
nerve?
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8
14.2 Protection and
Support of the Spinal Cord
Learning
Objectives:
Describe the locations and
function of the spinal cord
meninges.
2. Compare and contrast the three
spaces associated with the spinal
cord meninges.
1.
9
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14.2 Protection and Support of the Spinal Cord
• Spinal cord meninges
– Pia mater: delicate layer adhering to spinal cord
o Made of elastic and collagen fibers
o Denticulate ligaments: lateral extensions of pia; help suspend spinal cord
o Filum terminale: pia anchoring inferior end of spinal cord to coccyx
– Arachnoid mater: web-like layer, external to pia
o Arachnoid trabeculae: fibrous extensions of the membrane
o Subarachnoid space: area deep to arachnoid through which CSF flows
– Dura mater: tough, outermost layer
o One layer of dense irregular connective tissue that stabilizes spinal cord
o Subdural space is between dura and arachnoid
o Epidural space is between dura and vertebra
– Houses adipose, areolar connective tissue, blood vessels
10
Spinal Meninges and Structure of the Spinal Cord
Figure 14.3a
11
Spinal Meninges and Structure of the Spinal Cord Figure 14.3b
12
Clinical View: Lumbar Puncture
• Procedure for obtaining CSF for medical diagnosis
• Needle passes through
– Skin, back muscles, ligamentum flavum
– Epidural space, dura mater
– Arachnoid mater into subarachnoid space
• Adult spinal cord ends at L1
– Lumbar puncture below this, just above or below L4
– Spinous process of L4 at highest points of iliac crests
13
What did you
learn?
What is the name of the
fluid-filled space between
the two inner meninges?
Which meningeal layer is
thinnest and most
delicate?
•
•
14
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Distinguish the four anatomic
locations of gray matter in the spinal
cord.
2. Name the types of neurons and
functional groups (nuclei) found in
each gray matter region.
3.Identify the location of white matter
in the spinal cord.
4. Name the three anatomic divisions
of the white matter, and explain their
general composition.
1.
14.3 Sectional Anatomy
of the Spinal Cord
Learning
Objectives:
15
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14.3a Distribution of Gray Matter
• Gray matter
– Made of neuron’s cell bodies, dendrites, and unmyelinated
axons; also glial cells
• Masses of grey matter project from center of spinal cord
– Anterior horns house cell bodies of somatic motor neurons
– Lateral horns house cell bodies of autonomic motor neurons
o Only present in parts T1–L2
– Posterior horns house axons of sensory neurons and cell
bodies of interneurons
16
Gray Matter and White Matter Organization
Spinal Cord
in the
Figure 14.4
17
14.3a Distribution of Gray Matter
• Gray commissure
– Horizontal band of gray matter surrounding central canal
– Contains unmyelinated axons connecting left and right gray matter
• Nuclei: groups of cell bodies
– Sensory nuclei in posterior horn contain interneurons
o Somatic sensory nuclei receive signals from skin, muscle, joints
o Visceral sensory nuclei receive signals from blood vessels, viscera
– Motor nuclei in anterior and lateral horns contain motor neurons
o Somatic motor nuclei (anterior) innervate skeletal muscle
o Autonomic motor nuclei (lateral) innervate smooth muscle, heart, glands
18
Neuron Pathways and Nuclei Locations
Figure 14.5
19
14.3b Distribution of White Matter
• White matter: myelinated axons to and from the brain
• Regions of white matter
– Posterior funiculus
o Sits between posterior gray horns and posterior median sulcus
o Contains sensory tracts (axon bundles called fasciculi)
– Lateral funiculus
o Sits on lateral sides of spinal cord
o Contains ascending (sensory) and descending (motor) tracts
– Anterior funiculus
o Sits between anterior gray horns and anterior median fissure
o Left and right anterior funiculi are interconnected by white commissure
o Contains ascending (sensory) and descending (motor) tracts
20
Clinical View: Treating Spinal Cord Injuries
• May leave individuals paralyzed and unable to perceive
sensations
• Prompt use of steroids after injury
– May preserve muscle function
• Early antibiotics
– Have reduced number of deaths due to pulmonary and
urinary infections
• Neural stem cells
– May be used in future to regenerate CNS axons
21
What did you
learn?
In which parts of the
spinal cord are lateral
horns found?
Which horn houses
somatic motor neurons?
What is the posterior
funiculus?
•
•
•
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22
14.4 Spinal Cord
Conduction Pathways
Learning
Objectives:
Name the components of a
conduction pathway and list the
features common to all pathways.
2. Compare and contrast sensory and
motor pathways.
3. Define a sensory pathway, and
describe its action.
4. List the neurons in the sensory
pathway chain, and describe their
roles.
1.
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23
Describe the three major
somatosensory pathways.
6.
Define a motor pathway, and
describe its actions.
7.
Distinguish between an upper
motor neuron and a lower motor
neuron, based on function and cell
body location.
8.Compare and contrast the direct and
indirect motor pathways.
5.
14.4 Spinal Cord Conduction
Pathways (continued )
Learning
Objectives:
24
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14.4a Overview of Conduction Pathways
• Spinal pathways are sensory or motor
– Sensory pathways ascend toward brain
– Motor pathways descend from brain
• Common pathway characteristics
– Cell locations: axons are in spinal cord tracts; cell bodies are in
ganglia, spinal cord gray horns, and brain gray matter
– Each pathway is made of a chain of two or more neurons
– Pathways are paired: there is a left and a right tract
– Most pathways decussate: axons cross midline so brain
processes information for contralateral side
o Uncrossed pathways work on the ipsilateral side of body
25
14.4b Sensory Pathways
• Sensory (ascending) pathways
–
–
–
–
Signals for proprioception, touch, temperature, pressure, pain
Somatosensory pathways carry signals from skin, muscles, joints
Viscerosensory pathways carry signals from viscera
Use a series of neurons to relay signal to brain
o Primary (1st order) neuron has peripheral ending, cell body in posterior
root ganglion, and axon leading to secondary neuron
o Secondary (2nd order) neuron is an interneuron; receives primary input
and extends to tertiary neuron or to cerebellum
o Tertiary (3rd order) neuron is an interneuron; receives secondary neuron
input and extends to somatosensory cortex of parietal lobe of cerebrum
26
14.4b Sensory Pathways
• Posterior funiculus–medial lemniscal pathway
– Signals about proprioception, touch, pressure, and vibration with a
three neuron chain
– Primary neuron relays signal from skin to brainstem
o Peripheral receptor has axon in spinal nerve, posterior root, spinal cord
o Within the cord, axon is in the posterior funiculus—either its fasciculus
cuneatus or its fasciculus gracilis
o In the medulla the axon contacts a secondary neuron
– Secondary neuron relays signal from medulla to thalamus
o Cell body in either nucleus cuneatus or nucleus gracilis of medulla
o Axon decussates and joins medial lemniscus
o In thalamus, the axon contacts a tertiary neuron
– Tertiary neuron relays signal to primary somatosensory cortex
(postcentral gyrus)
27
Posterior
Funiculus–
Medial
Lemniscal
Pathway
Figure 14.7
28
14.4b Sensory Pathways
• Anterolateral pathway
– Signals related to crude touch, pressure, pain, and temperature
with a three-neuron chain
– Primary neuron relays signal from skin to spinal cord
o Axon is in spinal nerve and posterior root
o Axon contacts secondary neuron in spinal cord’s posterior horn
– Secondary neuron relays signal from spinal cord to thalamus
o Axon decussates and ascends in contralateral white matter (either the
anterior spinothalamic tract or the lateral spinothalamic tract)
o Axon contacts tertiary neuron in thalamus
– Tertiary neuron relays signal from thalamus to cerebral cortex
o Axon contacts target neuron in appropriate part of primary
somatosensory cortex
29
Anterolateral
Pathway
Figure 14.8
30
14.4b Sensory Pathways
• Spinocerebellar pathway
– Signals about proprioception with a two-neuron chain
– Primary neuron relays signal from skin to spinal cord
o Axon is in spinal nerve and posterior root
o Axon contacts secondary neuron in spinal cord’s posterior horn
– Secondary neuron relays signal from spinal cord to cerebellum
o Some secondary neuron axons cross, while others remain ipsilateral
o Axon ascends in either the anterior spinocerebellar tract or posterior
spinocerebellar tract
o Axon contacts cell within the cerebellum
31
Spinocerebellar
Pathway
Figure 14.9
32
Sensory Pathways in the Spinal Cord
Figure 14.6
33
14.4c Motor Pathways
• Motor (descending)
pathways
– Control effectors such as
skeletal muscles
– Start in brain and include at
least two neurons
o Upper motor neuron in motor
cortex, cerebral nucleus or
brainstem nucleus; contacts
lower motor neuron
o Lower motor neuron in cranial
nerve nucleus or spinal cord
anterior horn; excites muscle
Figure 14.10
34
14.4c Motor Pathways
• Direct (pyramidal) pathway
– Begins with upper motor neurons in primary motor cortex
– Axons course through internal capsule, cerebral peduncles
– Axons end in brainstem (corticobulbar tracts) or spinal cord
(corticospinal tracts)
– Corticobulbar tracts
o Originate from facial region of primary motor cortex
o Axons extending to brainstem where they synapse with lower motor
neurons in cranial nerve nuclei
35
14.4c Motor Pathways
• Direct (pyramidal) pathway (continued )
– Corticospinal tracts
o Descend from primary motor cortex through brainstem into spinal cord
o Synapse on lower motor neurons in anterior horn
o Lateral corticospinal tracts
– Upper motor neuron axons decussate within medulla’s pyramids
– Axons form white tracts in lateral funiculi and contact lower motor neurons
– Lower motor neurons innervate limb muscles for skilled movements
o Anterior corticospinal tracts
– Upper motor neuron axons form white tracts in anterior funiculi
– Decussate at level of spinal cord segment and contact interneurons or lower
motor neurons
– Lower motor neurons innervate axial skeletal muscle
36
Corticospinal
Tracts
Figure 14.11
37
14.4c Motor Pathways
• Indirect pathway
– Upper motor neurons originate in brainstem nuclei and take
complicated route to spinal cord
– Lateral pathway
o Regulates precise movement and tone in flexor limb muscles
o Consists of rubrospinal tracts originating in midbrain
– Medial pathway
o Regulates muscle tone and movements of head, neck, proximal limb,
trunk
o Consists of reticulospinal, tectospinal, vestibulospinal tracts
38
14.4c Motor Pathways
• Indirect pathway (continued )
– Reticulospinal tracts from reticular formation
o Help control reflexes related to posture and balance
– Tectospinal tracts from superior and inferior colliculi
o Regulate reflexive orienting responses to visual and auditory
stimuli
– Vestibulospinal tracts from vestibular nuclei of
brainstem
o Help maintain balance during sitting, standing, walking
39
Differences Between Sensory and Motor Pathways
Figure 14.12
40
What did you
learn?
What types of sensory signals
are processed by the
anterolateral pathway?
Where are lower motor
neurons located?
Which motor pathway
contains axons that cross in
the medullary and then help
control skilled limb
movements?
•
•
•
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41
Describe the components of a
typical spinal nerve.
2.
Compare and contrast the
anterior and posterior rami of a
spinal nerve.
3.
Define a dermatome, and
explain its clinical significance.
4.
Define a nerve plexus.
5. Identify the distribution of the
intercostal nerves
1.
14.5 Spinal Nerves
Learning
Objectives:
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42
14.5 Spinal Nerves
(continued )
Learning
Objectives:
6.
7.
8.
9.
List the nerves of the cervical
plexuses
Explain the action of the
phrenic nerve.
Explain the structure of the
brachial plexus, including the
three trunks, two divisions, and
three cords.
Describe the distribution of the
five major nerve branches that
arise from the three cords.
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43
14.5 Spinal Nerves
(continued )
10.
11.
Learning
Objectives:
12.
13.
Identify the spinal nerves that
make up the lumbar plexus.
Compare and contrast the
femoral and obturator nerve
composition and distribution.
List the spinal nerves that form
the sacral plexus.
Describe the composition of
the sciatic nerve and compare
its branches.
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44
14.5 Overview of Spinal Nerves
• Spinal nerve characteristics
– 31 pairs of spinal nerves (C1 to Co1)
– Each nerve formed from merger of anterior (ventral)
root and posterior (dorsal) root
o Anterior root is many axons of motor neurons whose
somas are in anterior and lateral horns
o Posterior root is many axons of sensory neurons whose
somas are in posterior root ganglion
45
14.5a Overview of Spinal Nerves
• Each nerve is named for part of spinal cord it comes from
and a number
– Cervical nerves exit intervertebral foramina superior to the
vertebra of the same number (e.g., C2 nerve exits between C2
and C1 vertebrae)
o Exception is nerve C8—it exits below C7 vertebra
– Below C8, nerves exit inferior to the vertebra of the same
number (e.g., T2 nerve exits between T2 and T3 vertebrae)
– Lumbar, sacral, and coccygeal spinal nerves have long roots that
extend inferiorly before exiting vertebrae
o These roots form the cauda equina
46
14.5a Overview of Spinal Nerves
• Distribution of spinal nerves
– After intervertebral foramen, spinal nerve splits
– Posterior ramus—small branch
o Innervates muscles and skin of back
– Anterior ramus—large branch
o Splits into multiple other branches
o At different levels, this ramus innervates anterior and lateral trunk, upper
limb, lower limb
o Participates in plexuses
– Rami communicantes—small branches of autonomic fibers
o Extend between spinal nerve and sympathetic trunk ganglion
– Ganglia interconnected in sympathetic trunk parallel to vertebral column
47
Spinal Nerve
Branches
Figure 14.13
48
14.5a Overview of Spinal Nerves
• Dermatomes
– Segment of skin supplied by single spinal nerve
o Some overlap in innervated regions
˗ E.g., T10 dermatome = horizontal ring of skin around umbilicus
– Can help localize damage to one or more spinal nerves
o E.g., loss of sensation on medial arm and forearm indicates C8 damage
– Involved in referred visceral pain
o E.g., appendicitis pain often referred to T10 dermatome
49
Dermatome
Maps
Figure 14.14
50
Clinical View: Shingles
•
•
•
•
•
•
•
Reactivation of chickenpox infection
Virus remaining latent in posterior root ganglia
Reactivated, travels through sensory axons to dermatome
Rash and blisters along the dermatome
Burning and tingling pain
Antiviral medication to reduce severity
Vaccine to prevent or reduce disease severity
51
14.5b Nerve Plexuses
• Nerve plexus
– Network of interweaving anterior rami of spinal
nerves
– Four main plexuses occur bilaterally: cervical,
brachial, lumbar, and sacral plexuses
o Most thoracic spinal nerves and nerves S5–Co1 do not
form plexuses
– Individual rami branch repeatedly
o Damage to one nerve or spinal segment does not deprive a
muscle or skin region of all innervation
52
14.5c Intercostal Nerves
• Intercostal nerves are located between ribs
– Anterior rami of spinal nerves T1–T11
o T12 is subcostal nerve, inferior to ribs
– T1 anterior ramus
o Part goes to brachial plexus; part goes to 1st intercostal space
– T2 anterior ramus
o Innervates intercostal muscles in second intercostal space
o Receives sensory information from axilla and medial arm surface
– T3–T6 anterior rami
o Innervate intercostal muscles
o Receive sensory signals from anterior and lateral chest wall
– T7–T12 anterior rami
o Innervate intercostal muscles and abdominal muscles
o Receive sensory signals from overlying skin
53
Intercostal
Nerves
Figure 14.15
54
14.5d Cervical Plexuses
• Cervical plexuses—anterior rami of C1–C4
– C5 contributes a few axons
– Branches innervate: anterior neck muscles, skin
of neck, portions of head and shoulders
– From rami of C3–C5 it gives rise to phrenic
nerve innervating diaphragm
55
Cervical Plexus
Figure 14.16
56
14.5e Brachial Plexuses
• Brachial plexuses—from anterior rami of C5–T1
– Network of fibers extending laterally from neck into axilla
– Composed of anterior rami, trunks, divisions, cords
• Rami unite to form trunks in posterior triangle of the neck
– Superior trunk from rami of nerves C5 and C6
– Middle trunk from ramus of nerve C7
– Inferior trunk from rami of C8 and T1
• Trunks divide into anterior and posterior divisions
– Axons innervate anterior and posterior parts of upper limbs
57
14.5a Brachial Plexuses
• Divisions converge to form cords near axillary artery
– Posterior cord
o Formed by posterior divisions of superior, middle, and inferior trunks
o Contains portions of C5–T1 nerves
– Medial cord
o Formed by anterior division of inferior trunk
o Contains portions of nerves C8–T1
– Lateral cord
o Formed from anterior divisions of superior and middle trunks
o Contains portions of nerves C5–C7
58
14.5a Brachial Plexuses
• Cords give rise to 5 major terminal branches
1.
2.
3.
4.
5.
Axillary nerve: to deltoid, teres major muscles; sensory input from
superolateral arm
Median nerve: to most anterior forearm muscles, thenar muscles,
lateral lumbricals; sensory input from palmar side and dorsal tips of
most fingers (not pinkie)
Musculocutaneous nerve: to anterior arm muscles (e.g., biceps
brachii); sensory input from lateral forearm
Radial nerve: to posterior arm muscles (e.g., triceps brachii), posterior
forearm muscles; sensory input from posterior arm and forearm and
dorsolateral hand
Ulnar nerve: to anterior forearm muscles, most intrinsic hand muscles;
sensory input from palmar and dorsal aspect of two medial fingers
59
Brachial Plexus
Figure 14.17a
60
Brachial
Plexus
Figure 14.17c
61
Clinical View: Brachial Plexus Injuries
• Minor injuries treated with rest
• Severe injuries may require nerve grafts or transfers
• Axillary nerve injury
– Can be compressed in axilla or damaged if neck of humerus broken
– Difficulty abducting the arm and anesthesia along superolateral skin
• Radial nerve injury
– By humeral shaft fractures or injuries to lateral elbow
– Causes paralysis of extensor muscles of forearm, wrist, fingers
– Causes anesthesia along posterior arm, forearm, part of hand
• Posterior cord (axillary and radial nerves) injury
– May be injured by improper use of crutches (crutch palsy)
62
Clinical View: Brachial Plexus Injuries
(continued )
• Median nerve injury
– May be compressed in carpal tunnel syndrome
– Causes paralysis of thenar muscles, lateral lumbricals; anesthesia in part of hand
• Ulnar nerve injury
– May be injured by fractures or dislocations of elbow
– Causes paralysis of most intrinsic hand muscles; sensory loss on medial hand
• Superior trunk (C5–C6) injury
– Injured by excessive separation of neck and shoulder
– Any brachial plexus branch that has these nerves affected
• Inferior trunk (C8–T1) injury
– Can be injured if arm excessively abducted
– Any brachial plexus branch from these nerves affected to some degree
63
14.5f Lumbar Plexuses
• Lumbar plexuses—from anterior rami of L1–L4
– Femoral nerve
o Main nerve in posterior division of plexus
o Innervates anterior thigh muscles and sartorius
o Sensory input from anterior and inferomedial thigh and medial leg
– Obturator nerve
o Main nerve in anterior division of plexus
o Innervates medial thigh muscles
o Sensory input from superomedial skin of thigh
– Smaller nerves of lumbar plexus
o Innervate abdominal wall, portions of external genitalia
64
Lumbar
Plexus
Figure 14.18a
65
Lumbar
Plexus
Figure 14.18c
66
14.5g Sacral Plexuses
• Sacral plexuses—anterior rami of L4–S4
– Anterior division nerves tend to innervate flexor
muscles
– Posterior division nerves tend to innervate extensor
muscles
– Sciatic nerve
o Largest and longest nerve in body
o Formed from portions of anterior and posterior sacral plexus
o Composed of tibial division and common fibular division
– The two divisions split into two separate nerves just above popliteal
fossa
67
14.5g Sacral Plexuses
– Tibial nerve (from anterior division of sciatic)
o Innervates hamstrings and hamstring part of adductor
magnus muscle
o Splits into lateral and medial plantar nerves that innervate
plantar muscles of foot
o Receives sensory signals from skin on sole of foot
– Common fibular nerve (from posterior division of
sciatic)
o Innervates short head of biceps femoris muscle
o Splits into deep fibular nerve and superficial fibular nerve
68
14.5g Sacral Plexuses
– Deep fibular nerve
o Innervates anterior leg muscles and muscles on dorsum of
foot
o Sensory input from skin between dorsum of first and
second toes
– Superficial fibular nerve
o Innervates lateral compartment muscles of leg
o Sensory input from dorsal foot and anteroinferior leg
69
Sacral
Plexus
Figure 14.19a
70
Sacral
Plexus
Figure 14.19c
71
Clinical View: Sacral Plexus Injuries
• Superior or inferior gluteal nerves
– Can be injured by poorly placed gluteal injection
• Sciatica: injury to sciatic nerve
– Characterized by extreme pain down posterior thigh and leg
– May be caused by herniated intervertebral disc
• Common fibular nerve
– Prone to injury due to fracture of neck or compression from cast
– May cause paralysis of anterior and lateral leg muscles
– Person unable to dorsiflex and evert the foot
72
What did you
learn?
What is a dermatome and how
can it be useful in clinical
evaluations?
What functions are served by the
dorsal rami of spinal nerves?
Which terminal branch of the
brachial plexus innervates the
biceps brachii?
Anterior rami from which spinal
nerves participate in the lumbar
plexus?
•
•
•
•
73
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1.
14.6 Reflexes
Learning
Objectives:
Describe the properties of a reflex.
2. Explain how reflexes are classified.
3. List the structures involved in a reflex
arc and the steps in its action.
4. Explain the five ways a reflex may be
classified.
Name and describe four common
spinal reflexes.
6. Explain the indications of a hypoactive
reflex versus those of a hyperactive
reflex.
5.
74
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14.6a Characteristics of Reflexes
• Reflexes: involuntary responses
–
–
–
–
A stimulus is required to initiate a reflex
Response is rapid; involves a chain of only a few neurons
The response is preprogrammed; always the same
The response is involuntary; no intent or awareness of the reflex
before it happens
• A reflex is a survival mechanism
– We respond to a potentially detrimental stimulus immediately
and awareness comes later
75
14.6b Components of a Reflex Arc
Reflex arc: neural pathway responsible for generating the response
Figure 14.20
76
14.6c Classifying Spinal Reflexes
• Reflexes vary and can be classified in different ways
– Spinal or cranial: Is the spinal cord or brain the reflex
integration center?
– Somatic or visceral: Is the effector a skeletal muscle or is it
cardiac muscle, smooth muscle, a gland?
– Monosynaptic or polysynaptic: Do sensory neurons synapse
directly with motor neurons or are there interneurons in the
reflex arc?
– Ipsilateral or contralateral: Are receptor and effector on the
same side of the body or on opposite sides?
– Innate or acquired: Are you born with the reflex or do you
develop it after birth?
77
Monosynaptic and Polysynaptic Reflexes
Figure 14.21
78
14.6d Spinal Reflexes
• Stretch reflex
– Reflexive contraction of a muscle after it is stretched
– Stretch (or tendon tap) is detected by a muscle spindle receptor
o
Spindle contains intrafusal muscle fibers innervated by gamma motor
neurons and wrapped by sensory neurons
–
o
o
o
Fibers not within spindle are extrafusal muscle fibers innervated by large
alpha motor neurons
When stretched, spindle’s sensory axon fires impulses that are conducted
to the spinal cord
In spinal cord, the sensory axon excites alpha motor neurons of the same
muscle, causing contraction (monosynaptic)
Simultaneously, the sensory axon excites interneurons that inhibit motor
neurons of antagonist muscle (polysynaptic reciprocal inhibition)
– Stretch reflex classified as: spinal, somatic, ipsilateral, innate
79
Biceps Stretch Reflex
Figure 14.22
80
14.6d Spinal Reflexes
• Golgi tendon reflex
– Prevents muscles from contracting excessively
– Golgi tendon organs detect excessive tension
o
o
They are proprioceptors with sensory ending at muscle tendon junction
Their sensory axons excite interneurons in the spinal cord
– Some excited interneurons inhibit motor neurons of same muscle
o
Muscle relaxes (polysynaptic reflex), preventing it from damage
– Some excited interneurons excite motor neurons of antagonist
muscle (reciprocal activation)
o
Antagonist muscle contracts (polysynaptic reflex)
81
Golgi Tendon Reflex
Figure 14.23
82
14.6d Spinal Reflexes
• Withdrawal reflex
– Pulls a body part away from a painful stimulus
– Stimulus excites nociceptor sensory neuron that transmits
signal to spinal cord and excites interneurons
– Interneurons excite motor neurons of flexors so flexor
muscles (e.g., hamstrings) contract and limb is withdrawn
– Simultaneously, other interneurons reciprocally inhibit motor
neurons of extensors so that extensor muscles (e.g.,
quadriceps) relax and withdrawal happens quickly
83
14.6d Spinal Reflexes
• Crossed-extensor reflex
– Occurs in conjunction with withdrawal reflex
– Some interneurons excited by the nociceptor sensory neuron
cross midline and excite extensor motor neurons on other side
o E.g. as left leg is withdrawn, right leg’s quadriceps is excited
– Allows the opposite side limb to support body weight while the
hurt limb withdraws
84
Withdrawal and
CrossedExtensor
Reflexes
Figure 14.24
85
14.6e Reflex Testing in a Clinical Setting
• Reflexes are useful for diagnoses
– Can test function of specific muscles, nerves, spinal segments
– Hypoactive reflex: diminished or absent
o May indicate damage to spinal cord, or muscle disease, or damage to
neuromuscular junction
– Hyperactive reflex: abnormally strong response
o May indicate damage to brain or spinal cord, especially if accompanied
by clonus (rhythmic oscillating movements with reflex testing)
86
What did you
learn?
•
•
•
Is the flexion withdrawal
reflex ipsilateral or
contralateral?
What is the only known
monosynaptic reflex?
What is the function of the
Golgi tendon organ reflex?
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87
14.7 Development of
the Spinal Cord
Learning
Objectives:
1. Describe how the neural tube forms
the gray matter structures in the spinal
cord.
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88
14.7 Development of the Spinal Cord
• Spinal cord develops from caudal neural tube
– Neural canal of tube becomes central canal of cord
– By sixth week of embryonic development sulcus limitans forms
o Horizontal groove in lateral walls of canal
o Represents dividing point between basal and alar plates
– Basal plates: structures anterior to sulcus limitans
o Develop into anterior and lateral horns, and anterior gray commissure
– Alar plates: structures posterior to sulcus limitans
o Develop into posterior horns and posterior gray commissure
89
14.7 Spinal Cord Development
Figure 14.25
90
14.7 Development of the Spinal Cord
• Development of spinal cord length
– Embryo’s spinal cord is same length as vertebral canal
– In fetal period, vertebral growth is faster than spinal cord’s,
so newborn’s spinal cord ends at L3 vertebra
– By adulthood, spinal cord ends at L1 vertebra
– The difference in growth explains why names of spinal cord
parts, nerves, and vertebrae are confusing
o The coccygeal nerves attach to the spinal cord under a lumbar
vertebra
91
What did you
learn?
•
•
Which embryonic plates give
rise to the anterior horns of the
spinal cord?
During fetal development,
which grows faster: the spinal
cord or the vertebral column?
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92