Download Spinal reflexes

Chapter 13
The Spinal Cord,
Spinal Nerves, and
Spinal Reflexes
Lecture Presentation by
Lee Ann Frederick
University of Texas at Arlington
© 2015 Pearson Education, Inc.
An Introduction to the Spinal Cord, Spinal
Nerves, and Spinal Reflexes
•  Spinal Reflexes
•  Rapid, automatic nerve responses triggered by
specific stimuli
•  Controlled by spinal cord alone, not the brain
© 2015 Pearson Education, Inc.
Figure 13-1 An Overview of Chapters 13 and 14.
CHAPTER 14: The Brain
Sensory
receptors
Sensory input
over cranial nerves
Reflex
centers
in brain
Motor output over
cranial nerves
Effectors
Muscles
CHAPTER 13: The Spinal Cord
Glands
Sensory
receptors
© 2015 Pearson Education, Inc.
Sensory input
over spinal nerves
Reflex
centers
in spinal
cord
Motor output over
spinal nerves
Adipose tissue
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
© 2015 Pearson Education, Inc.
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
White matter
Central
canal
Cervical
enlargement
Gray
matter
Spinal Ventral
nerve
root
Anterior median fissure
C3
T1
T2
T3
T4
T5
T6
T7
KEY
Spinal cord regions
Thoracic
spinal
nerves
T8
T9
=
Cervical
T10
=
Thoracic
T11
=
Lumbar
=
Sacral
T12
L1
Posterior
median sulcus
T3
Lumbar
enlargement
Conus
medullaris
L2
Lumbar
spinal
nerves
L3
Inferior
tip of
spinal cord
L4
Cauda equina
L5
L1
Sacral spinal
nerves
S1
S2
S3
S4
S5
Coccygeal
nerve (Co1)
Filum terminale
(in coccygeal ligament)
a The superficial anatomy and orientation of the adult spinal cord. The
S2
b
numbers to the left identify the spinal nerves and indicate where the
nerve roots leave the vertebral canal. The adult spinal cord extends from
the brain only to the level of vertebrae L1–L2; the spinal segments found
at representative locations are indicated in the cross sections.
© 2015 Pearson Education, Inc.
Inferior views of cross sections
through representative segments
of the spinal cord, showing the
arrangement of gray matter and
white matter.
Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 1 of 4).
Posterior median sulcus
Dorsal root
Dorsal root
ganglion
Cervical spinal
nerves
C1
C2
C3
C4
C5
C6
C7
C8
Central
canal
Cervical
enlargement
Spinal Ventral
nerve
root
The superficial anatomy and orientation of the adult spinal
cord. The numbers to the left identify the spinal nerves and
indicate where the nerve roots leave the vertebral canal. The
adult spinal cord extends from the brain only to the level of
vertebrae L1– L2; the spinal segments found at representative
locations are indicated in the cross sections.
© 2015 Pearson Education, Inc.
White matter
Gray
matter
Anterior median fissure
C3
Inferior views of cross
sections through
representative segments
of the spinal cord, showing
the arrangement of gray
matter and white matter.
Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 2 of 4).
T1
T2
T3
T4
T5
T6
T7
Thoracic
spinal
nerves
T8
T9
Posterior
median sulcus
T10
T11
T12
T3
Lumbar
enlargement
Conus
medullaris
The superficial anatomy and orientation of the adult spinal
cord. The numbers to the left identify the spinal nerves and
indicate where the nerve roots leave the vertebral canal. The
adult spinal cord extends from the brain only to the level of
vertebrae L1– L2; the spinal segments found at representative
locations are indicated in the cross sections.
© 2015 Pearson Education, Inc.
Inferior views of cross sections
through representative segments
of the spinal cord, showing the
arrangement of gray matter and
white matter.
Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 3 of 4).
L1
Conus
medullaris
L2
Lumbar
spinal
nerves
L3
Inferior
tip of spinal
cord
L4
Cauda equina
L5
L1
The superficial anatomy and orientation of the adult spinal
cord. The numbers to the left identify the spinal nerves and
indicate where the nerve roots leave the vertebral canal. The
adult spinal cord extends from the brain only to the level of
vertebrae L1– L2; the spinal segments found at representative
locations are indicated in the cross sections.
© 2015 Pearson Education, Inc.
Inferior views of cross sections
through representative segments
of the spinal cord, showing the
arrangement of gray matter and
white matter.
Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 4 of 4).
Inferior
tip of
spinal cord
Cauda equina
Sacral spinal
nerves
S1
S2
S3
S4
S5
Coccygeal
nerve (Co1)
Filum terminale
(in coccygeal ligament)
The superficial anatomy and orientation of the adult spinal
cord. The numbers to the left identify the spinal nerves and
indicate where the nerve roots leave the vertebral canal. The
adult spinal cord extends from the brain only to the level of
vertebrae L1– L2; the spinal segments found at representative
locations are indicated in the cross sections.
© 2015 Pearson Education, Inc.
S2
Inferior views of cross
sections through
representative segments of
the spinal cord, showing
the arrangement of gray
matter and white matter.
Figure 13-3a The Spinal Cord and Spinal Meninges.
White matter
Ventral rootlets
of spinal nerve
Gray matter
Dorsal root
ganglion
Ventral root
Spinal nerve
Dorsal root
Dorsal rootlets
of spinal nerve
Meninges
Pia mater
Arachnoid mater
Dura mater
a A posterior view of the
spinal cord, showing the
meningeal layers, superficial
landmarks, and distribution
of gray matter and white
matter
© 2015 Pearson Education, Inc.
Figure 13-3b The Spinal Cord and Spinal Meninges.
Meninges
ANTERIOR
Subarachnoid
space
Vertebral
body
Rami
communicantes
Dura mater
Arachnoid mater
Pia mater
Autonomic
(sympathetic)
ganglion
Ventral root
of spinal
nerve
Ventral
ramus
Dorsal
ramus
Spinal cord
Adipose tissue
in epidural space
Denticulate Dorsal root
ganglion
ligament
POSTERIOR
b A sectional view through the spinal cord and meninges,
showing the relationship of the meninges, spinal cord,
and spinal nerves
© 2015 Pearson Education, Inc.
13-2 Spinal Cord
•  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
1.  Dura mater
•  Outer layer of spinal cord
2.  Arachnoid mater
•  Middle meningeal layer
3.  Pia mater
•  Inner meningeal layer
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13-2 Spinal Cord
•  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
•  Joins filum terminale in coccygeal ligament
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13-2 Spinal Cord
•  The Dura Mater
•  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
•  Arachnoid membrane
•  Simple squamous epithelia
•  Covers arachnoid mater
© 2015 Pearson Education, Inc.
13-2 Spinal Cord
•  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)
© 2015 Pearson Education, Inc.
13-2 Spinal Cord
•  The Interlayer Spaces of Arachnoid Mater
•  Cerebrospinal Fluid (CSF)
•  Carries dissolved gases, nutrients, and wastes
•  Lumbar puncture or spinal tap withdraws CSF
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13-2 Spinal Cord
•  The Pia Mater
•  Is the innermost meningeal layer
•  Is a mesh of collagen and elastic fibers
•  Is bound to underlying neural tissue
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
Figure 13-4 The Spinal Cord and Associated Structures.
Spinal cord
Anterior median fissure
Pia mater
Denticulate
ligaments
Dorsal root
Ventral root, formed by
several “rootlets” from
one cervical segment
Arachnoid mater
(reflected)
Dura mater (reflected)
Spinal blood vessel
© 2015 Pearson Education, Inc.
13-3 Gray Matter and White Matter
•  Sectional Anatomy of the Spinal Cord
•  White matter
•  Is superficial
•  Contains myelinated and unmyelinated axons
•  Gray matter
•  Surrounds the central canal of spinal cord
•  Contains neuron cell bodies, neuroglia,
unmyelinated axons
•  Has projections (gray horns)
© 2015 Pearson Education, Inc.
13-3 Gray Matter and White Matter
•  Organization of Gray Matter
•  The gray horns
•  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; contain visceral motor nuclei
•  Gray commissures
•  Axons that cross from one side of cord to the other
before reaching gray matter
© 2015 Pearson Education, Inc.
13-3 Gray Matter and White Matter
•  Organization of Gray Matter
•  The cell bodies of neurons form functional groups
called nuclei
•  Sensory nuclei
•  Dorsal (posterior)
•  Connect to peripheral receptors
•  Motor nuclei
•  Ventral (anterior)
•  Connect to peripheral effectors
© 2015 Pearson Education, Inc.
13-3 Gray Matter and White Matter
•  Control and Location
•  Sensory or motor nucleus location within the gray
matter determines which body part it controls
© 2015 Pearson Education, Inc.
Figure 13-5a The Sectional Organization of the Spinal Cord (Part 1 of 2).
Posterior white column
Posterior
gray horn
Dorsal root
ganglion
Lateral
white
column
Lateral
gray horn
Anterior
gray
horn
Anterior white column
The left half of this sectional view shows important
anatomical landmarks, including the three columns
of white matter. The right half indicates the
functional organization of the nuclei in the anterior,
lateral, and posterior gray horns.
© 2015 Pearson Education, Inc.
Figure 13-5a The Sectional Organization of the Spinal Cord (Part 2 of 2).
Posterior
median sulcus
Posterior gray
commissure
Somatic
Functional Organization
of Gray Matter
The cell bodies of neurons in the
gray matter of the spinal cord are
organized into functional groups
called nuclei.
Sensory nuclei
Visceral
Visceral
Motor nuclei
Somatic
Ventral root
Anterior gray commissure
Anterior white commissure
Anterior median fissure
The left half of this sectional view shows important anatomical landmarks,
including the three columns of white matter. The right half indicates the
functional organization of the nuclei in the anterior, lateral, and posterior
gray horns.
© 2015 Pearson Education, Inc.
13-4 Spinal Nerves and Plexuses
•  Anatomy of Spinal Nerves
•  Each spinal cord segment:
•  Is connected to a pair of spinal nerves
•  Each spinal nerve:
•  Is surrounded by three connective tissue layers
•  That support structures and contain blood vessels
© 2015 Pearson Education, Inc.
13-4 Spinal Nerves and Plexuses
•  Three Connective Tissue Layers of Spinal Nerves
1.  Epineurium
•  Outer layer
•  Dense network of collagen fibers
2.  Perineurium
•  Middle layer
•  Divides nerve into fascicles (axon bundles)
3.  Endoneurium
•  Inner layer
•  Surrounds individual axons
© 2015 Pearson Education, Inc.
Figure 13-6a A Peripheral Nerve.
Blood vessels
Connective Tissue
Layers
Epineurium covering
peripheral nerve
Perineurium (around
one fascicle)
Endoneurium
Schwann cell
Myelinated
axon
Fascicle
a A typical peripheral nerve and
its connective tissue wrappings
© 2015 Pearson Education, Inc.
Figure 13-6b A Peripheral Nerve.
Blood vessels
Connective Tissue
Layers
Perineurium (around
one fascicle)
Endoneurium
b A scanning electron micrograph showing the
various layers in great detail (SEM × 340)
© 2015 Pearson Education, Inc.
Figure 13-8 Peripheral Distribution of Spinal Nerves (Part 2 of 2).
To skeletal
muscles of
back
Postganglionic fibers to
smooth muscles,
glands, etc., of back
2
The spinal nerve forms just
lateral to the intervertebral
foramen, where the dorsal
and ventral roots unite.
3
Dorsal root
Dorsal root
ganglion
4
1
The ventral root of each spinal
nerve contains the axons of
somatic motor and visceral
motor neurons.
The dorsal ramus contains
somatic motor and visceral motor
fibers that innervate the skin and
skeletal muscles of the back.
The axons in the relatively
large ventral ramus supply
the ventrolateral body
surface, structures in the
body wall, and the limbs.
Visceral motor nuclei
To skeletal
muscles of body
wall, limbs
Somatic motor nuclei
Rami
communicantes
KEY
= Somatic motor
commands
= Visceral motor
commands
Postganglionic fibers to
smooth muscles,
glands, visceral organs
in thoracic cavity
Preganglionic fibers
to sympathetic
ganglia innervating
abdominopelvic
viscera
© 2015 Pearson Education, Inc.
Sympathetic
ganglion
7
Postganglionic fibers
to smooth muscles,
and glands of body
wall, limbs
5
The white ramus communicans is the first branch from
the spinal nerve and carries visceral motor fibers to a
nearby sympathetic ganglion. Because these preganglionic
axons are myelinated, this branch has a light color and is
therefore known as the white ramus. White rami are only
found between T1 and L2.
A sympathetic nerve
contains preganglionic
and postganglionic
fibers innervating
structures in the
thoracic cavity. 1
6
The gray ramus communicans contains preganglionic
fibers that innervate glands and smooth muscles in the
body wall or limbs. These fibers are unmyelinated and have
a dark gray color. Gray rami are associated with each
spinal nerve.
13-4 Spinal Nerves and Plexuses
•  Peripheral Distribution of Spinal Nerves
•  Sensory nerves
•  In addition to motor impulses:
•  Dorsal, ventral, and white rami also carry sensory
information
•  Dermatomes
•  Bilateral region of skin
•  Monitored by specific pair of spinal nerves
© 2015 Pearson Education, Inc.
Figure 13-8 Peripheral Distribution of Spinal Nerves (Part 1 of 2).
From interoceptors
of back
From exteroceptors,
proprioceptors of back
4
The dorsal root of each
spinal nerve carriers
sensory information to
the spinal cord.
3
The dorsal ramus carries sensory
information from the skin and
skeletal muscles of the back.
Somatic
sensory nuclei
2
The ventral ramus carries sensory
information from the ventrolateral
body surface, structures in the body,
wall, and the limbs.
Dorsal
root
ganglion
From exteroceptors,
proprioceptors of
body wall, limbs
From interoceptors
of body wall, limbs
Rami
communicantes
Ventral
root
KEY
= Somatic
sensations
= Visceral
sensations
© 2015 Pearson Education, Inc.
Visceral
sensory nuclei
1
The sympathetic nerve
carriers sensory information
from the visceral organs.
From interceptors
of visceral organs
Figure 13-7 Dermatomes.
NV
C2–C3
C2
C3
T2
C6
L1
L2
C8
T1
C7
L3
L4
KEY
Spinal cord regions
L5
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
S2
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L4 L3
L5
C4
C5
T2
C6
T1
C7
S4S3
L1
S5
C8
S1 L5
L2 S2
L3
= Cervical
= Thoracic
= Lumbar
S1
= Sacral
L4
ANTERIOR
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POSTERIOR
13-4 Spinal Nerves and Plexuses
•  Peripheral Neuropathy
•  Regional loss of sensory or motor function
•  Due to trauma or compression
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13-4 Spinal Nerves and Plexuses
•  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
© 2015 Pearson Education, Inc.
13-4 Spinal Nerves and Plexuses
•  The Four Major Plexuses of Ventral Rami
1. 
2. 
3. 
4. 
Cervical plexus
Brachial plexus
Lumbar plexus
Sacral plexus
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Figure 13-9 Peripheral Nerves and Nerve Plexuses (Part 1 of 2).
Cervical
plexus
Brachial
plexus
C1
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
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Lesser occipital nerve
Great auricular nerve
Transverse cervical nerve
Supraclavicular nerve
Phrenic nerve
Axillary nerve
Musculocutaneous
nerve
Thoracic nerves
Figure 13-9 Peripheral Nerves and Nerve Plexuses (Part 2 of 2).
T12
L1
Lumbar
plexus
Radial nerve
L2
Ulnar nerve
L3
Median nerve
L4
Sacral
plexus
L5
S1
S2
S3
S4
S5
Co1
Iliohypogastric
nerve
Ilioinguinal
nerve
Lateral femoral
cutaneous nerve
Genitofemoral
nerve
Femoral nerve
Obturator nerve
Superior
Inferior
Gluteal nerves
Pudendal nerve
Saphenous nerve
Sciatic nerve
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13-4 Spinal Nerves and Plexuses
•  The Cervical Plexus
•  Includes ventral rami of spinal nerves C1–C5
•  Innervates neck, thoracic cavity, diaphragmatic
muscles
•  Major nerve
•  Phrenic nerve (controls diaphragm)
© 2015 Pearson Education, Inc.
Figure 13-10 The Cervical Plexus (Part 1 of 2).
Cranial
Nerves
Accessory
nerve (N XII)
Hypoglossal
nerve (N XII)
Nerve Roots of
Cervical Plexus
C1
C2
C3
C4
C5
Clavicle
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Figure 13-10 The Cervical Plexus (Part 2 of 2).
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Figure 13-10 The Cervical Plexus (Part 2 of 2).
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13-4 Spinal Nerves and Plexuses
•  The Brachial Plexus
•  Includes ventral rami of spinal nerves C5–T1
•  Innervates pectoral girdle and upper limbs
•  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
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13-4 Spinal Nerves and Plexuses
•  The Brachial Plexus
•  Major nerves
•  Musculocutaneous nerve (lateral cord)
•  Median nerve (lateral and medial cords)
•  Ulnar nerve (medial cord)
•  Axillary nerve (posterior cord)
•  Radial nerve (posterior cord)
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Figure 13-11a The Brachial Plexus.
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Figure 13-11a The Brachial Plexus.
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Figure 13-11b The Brachial Plexus.
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Figure 13-11b The Brachial Plexus.
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Figure 13-11b The Brachial Plexus.
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Figure 13-11b The Brachial Plexus.
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13-4 Spinal Nerves and Plexuses
•  The Lumbar Plexus
•  Includes ventral rami of spinal nerves T12–L4
•  Major nerves
•  Genitofemoral nerve
•  Lateral femoral cutaneous nerve
•  Femoral nerve
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Figure 13-12a The Lumbar and Sacral Plexuses.
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Figure 13-12a The Lumbar and Sacral Plexuses.
© 2015 Pearson Education, Inc.
Figure 13-12b The Lumbar and Sacral Plexuses.
Nerve Roots of
Sacral Plexus
Lumbosacral trunk
Sacral Plexus
L4
Spinal Segments
Nerve and Distribution
Superior Gluteal
L4–S2
L4–S2
S1
Gluteus maximus muscle
S2
Sacrum
Skin over perineum and posterior thigh and leg
Semimembranosus, semitendinosus, and adductor magnus
muscles; branches into tibial and fibular nerves
Pudendal
S2–S4
S3
S4
Sciatic
L4–S3
L5
Inferior Gluteal
Posterior Femoral Cutaneous
S1–S3
L5
Gluteus minimus, gluteus medius, and tensor
fasciae latae muscles
The sacral
plexus is formed
by a branch from
L4 and ventral
rami of L5–S4.
S5
Co1
Muscles of the perineum; skin over external genitalia,
bulbospongiosus and ischiocavernosus muscles
Sacral plexus, anterior view
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Figure 13-12c The Lumbar and Sacral Plexuses.
Iliohypogastric nerve
Ilioinguinal nerve
Genitofemoral nerve
Lateral femoral
cutaneous nerve
Femoral nerve
Obturator nerve
Superior gluteal nerve
Inferior gluteal nerve
Pudendal nerve
Posterior femoral
cutaneous nerve (cut)
Sciatic nerve
Saphenous nerve
Common fibular
nerve
Superficial fibular
nerve
Deep fibular
nerve
Nerves of the lumbar
and sacral plexuses,
anterior view
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Figure 13-12d The Lumbar and Sacral Plexuses.
Superior gluteal
nerve
Inferior gluteal nerve
Pudendal nerve
Posterior femoral
cutaneous nerve
Sciatic nerve
Tibial nerve
Common fibular
nerve
Sural nerve
Nerves of the sacral
plexus, posterior view
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13-5 Neuronal Pools
•  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
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13-5 Neuronal Pools
•  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 13-13a Neural Circuits: The Organization of Neuronal Pools.
a Divergence
A mechanism for
spreading stimulation to
multiple neurons or
neuronal pools in the
CNS
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Figure 13-13b Neural Circuits: The Organization of Neuronal Pools.
b Convergence
A mechanism for providing
input to a single neuron from
multiple sources
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Figure 13-13c Neural Circuits: The Organization of Neuronal Pools.
c Serial
processing
A mechanism in
which neurons or
pools work
sequentially
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Figure 13-13d Neural Circuits: The Organization of Neuronal Pools.
d Parallel processing
A mechanism in which neurons
or pools process the same
information simultaneously
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Figure 13-13e Neural Circuits: The Organization of Neuronal Pools.
e Reverberation
A positive feedback
mechanism
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13-6 Reflexes
•  Reflexes
•  Automatic responses coordinated within spinal
cord
•  Through interconnected sensory neurons, motor
neurons, and interneurons
•  Produce simple and complex reflexes
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13-6 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 13-14 Spinal Reflexes (Part 1 of 4).
1
Arrival of
stimulus and
activation of
receptor
2
Dorsal
root
Activation of a
sensory
neuron
Sensation
relayed to the
brain by axon
collaterals
Spinal cord
Stimulus
5
Response by a
peripheral effector
Effector
Receptor
Information
processing
in the CNS
REFLEX
ARC
4
Ventral root
Activation of a
motor neuron
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3
KEY
Sensory neuron
(stimulated)
Excitatory
interneuron
Motor neuron
(stimulated)
13-6 Reflexes
•  Four Classifications of Reflexes
1. 
2. 
3. 
4. 
By early development
By type of motor response
By complexity of neural circuit
By site of information processing
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13-6 Reflexes
•  Development of Reflexes
•  Innate reflexes
•  Basic neural reflexes
•  Formed before birth
•  Acquired reflexes
•  Rapid, automatic
•  Learned motor patterns
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13-6 Reflexes
•  Motor Response
•  Nature of resulting motor response
•  Somatic reflexes
•  Involuntary control of nervous system
•  Superficial reflexes of skin, mucous membranes
•  Stretch or deep tendon reflexes (e.g., patellar, or
“knee-jerk,” reflex)
•  Visceral reflexes (autonomic reflexes)
•  Control systems other than muscular system
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13-6 Reflexes
•  Complexity of Neural Circuit
•  Monosynaptic reflex
•  Sensory neuron synapses directly onto motor
neuron
•  Polysynaptic reflex
•  At least one interneuron between sensory neuron
and motor neuron
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13-6 Reflexes
•  Sites of Information Processing
•  Spinal reflexes
•  Occur in spinal cord
•  Cranial reflexes
•  Occur in brain
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Figure 13-15 The Classification of Reflexes.
Reflexes
can be classified by
development
response
complexity of circuit
processing site
Innate Reflexes
Somatic Reflexes
Monosynaptic
Spinal Reflexes
• Genetically
determined
Acquired Reflexes
• Learned
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• Control skeletal muscle contractions
• Include superficial and stretch reflexes
Visceral (Autonomic) Reflexes
• Control actions of smooth and cardiac
muscles, glands, and adipose tissue
• One synapse
Polysynaptic
• Multiple synapses
(two to several hundred)
• Processing in
the spinal cord
Cranial Reflexes
• Processing in
the brain
13-7 Spinal Reflexes
•  Spinal Reflexes
•  Range in increasing order of complexity
•  Monosynaptic reflexes
•  Polysynaptic reflexes
•  Intersegmental reflex arcs
•  Many segments interact
•  Produce highly variable motor response
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13-7 Spinal 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
•  Receptor is muscle spindle
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Figure 13-14 Spinal Reflexes (Part 2 of 4).
Stretch
Receptor
(muscle
spindle)
Spinal cord
REFLEX
ARC
Stimulus
Contraction
Effector
KEY
Response
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Sensory neuron
(stimulated)
Motor neuron
(stimulated)
13-7 Spinal Reflexes
•  Postural Reflexes
•  Stretch reflexes
•  Maintain normal upright posture
•  Stretched muscle responds by contracting
•  Automatically maintains balance
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13-7 Spinal Reflexes
•  Polysynaptic Reflexes
•  More complicated than monosynaptic reflexes
•  Interneurons control more than one muscle group
•  Produce either EPSPs or IPSPs
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13-7 Spinal Reflexes
•  The Tendon Reflex
•  Prevents skeletal muscles from:
•  Developing too much tension
•  Tearing or breaking tendons
•  Sensory receptors unlike muscle spindles or
proprioceptors
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13-7 Spinal Reflexes
•  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 13-14 Spinal Reflexes (Part 3 of 4).
Distribution within gray horns to
other segments of the spinal cord
Painful
stimulus
Flexors
stimulated
Extensors
inhibited
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13-7 Spinal Reflexes
•  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
•  Occur 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
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13-7 Spinal Reflexes
•  Crossed Extensor Reflexes
•  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 13-14 Spinal Reflexes (Part 4 of 4).
To motor neurons in other
segments of the spinal cord
Extensors
inhibited
Flexors
stimulated
Extensors stimulated
Flexors inhibited
KEY
Sensory neuron
(stimulated)
Motor neuron
(inhibited)
Excitatory
interneuron
Inhibitory
interneuron
Motor neuron
(stimulated)
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Painful
stimulus
13-7 Spinal Reflexes
•  Five General Characteristics of Polysynaptic
Reflexes
1. 
2. 
3. 
4. 
Involve pools of interneurons
Are intersegmental in distribution
Involve reciprocal inhibition
Have reverberating circuits
•  Which prolong reflexive motor response
5.  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
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13-8 The Brain Can Alter Spinal Reflexes
•  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
•  Creating EPSPs at reflex motor neurons
•  Facilitating postsynaptic neurons
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13-8 The Brain Can Alter Spinal Reflexes
•  Inhibition of Spinal Reflexes
•  Higher centers inhibit spinal reflexes by:
•  Stimulating inhibitory neurons
•  Creating IPSPs at reflex motor neurons
•  Suppressing postsynaptic neurons
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13-8 The Brain Can Alter Spinal Reflexes
•  The Babinski Reflexes
•  Normal in infants
•  May indicate CNS damage in adults
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Figure 13-17a The Babinski Reflexes.
a The plantar reflex
(negative Babinski
reflex), a curling of the
toes, is seen in healthy
adults.
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Figure 13-17b The Babinski Reflexes.
b 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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