Download Spinal cord

12
The Spinal Cord,
Spinal Nerves,
and Spinal
Reflexes
PowerPoint® Lecture Presentations prepared by
Alexander G. Cheroske
Mesa Community College at Red Mountain
© 2011 Pearson Education, Inc.
Section 1: Functional Organization of the Spinal
Cord
• Learning Outcomes
• 12.1 Discuss the anatomical features of the spinal
cord.
• 12.2 Describe the three meningeal layers that
surround the spinal cord.
• 12.3 Explain the roles of white matter and gray
matter in processing and relaying sensory
information and motor commands.
• 12.4 Describe the major components of a spinal
nerve.
© 2011 Pearson Education, Inc.
Section 1: Functional Organization of the Spinal
Cord
• Learning Outcomes
• 12.5 Describe the rami associated with spinal
nerves.
• 12.6 Relate the distribution pattern of spinal nerves
to the region they innervate and describe the
cervical plexus.
• 12.7 Relate the distribution pattern of the brachial
plexus with its function.
• 12.8 Relate the distribution patterns of the lumbar
plexus and sacral plexus with their functions.
© 2011 Pearson Education, Inc.
Section 1: Functional Organization of the Spinal
Cord
• The nervous system is highly organized allowing
efficient function
• Input pathways routing sensations
• Most often outside our awareness
• Only a fraction reaches our conscious awareness
• Processing centers prioritizing and distributing
information
• Motor centers directing responses to stimuli
© 2011 Pearson Education, Inc.
Section 1: Functional Organization of the Spinal
Cord
• Central Nervous System organization
• Brain, cranial nerves, cranial reflexes (Chapter 13)
• More complex
• Spinal cord, spinal nerves, spinal reflexes (this
chapter)
• Simpler
© 2011 Pearson Education, Inc.
A diagram of a functional perspective for studying the CNS
The Brain
Sensory
receptors
Sensory input
over cranial nerves
Reflex
centers
in brain
Motor output over
cranial nerves
Effectors
Muscles
The Spinal Cord
Glands
Sensory
receptors
Sensory input
over spiral nerves
Reflex
centers
in spinal
cord
Motor output over
spinal nerves
Adipose tissue
Figure 12 Section 1
© 2011 Pearson Education, Inc.
Module 12.1: Spinal cord functional anatomy
• Adult spinal cord dimensions
• Length: ~45 cm (18 in.)
• Width: ~14 mm (0.55 in.) maximum
• Superficial anatomy
• Cervical enlargement
• Supplies nerves to shoulder and upper limbs
• Lumbar enlargement
• Supplies nerves to pelvis and lower limbs
• Conus medullaris
• Tapered terminal end inferior to lumbar enlargement
© 2011 Pearson Education, Inc.
Module 12.1: Spinal cord functional anatomy
• Superficial anatomy (continued)
• Cauda equina (cauda, tail + equus, horse)
• Long, inferiorly extending dorsal and ventral roots +
filum terminale
• Resembles horse’s tail
• Filum terminale
• Slender thread of connective tissue attaching conus
medullaris to 2nd sacral vertebra
• Provides longitudinal support to spinal cord
© 2011 Pearson Education, Inc.
Module 12.1: Spinal cord functional anatomy
• Superficial anatomy (continued)
• 31 pairs of spinal nerves
• Arise from 31 segments of spinal cord
• Identified by adjacent vertebrae
• Cervical nerves
• From vertebrae immediately inferior
• Last vertebrae with this number system is C8
• Thoracic nerves
• From vertebrae immediately superior
© 2011 Pearson Education, Inc.
The 31 pairs of spinal nerves
Cervical spinal
nerves
Cervical
enlargement
Posterior median sulcus
Thoracic spinal
nerves
Lumbar enlargement
Conus medullaris
Lumbar
spinal
nerves
Interior tip of spinal cord
Cauda equina
Sacral spinal
nerves
Filum terminale
Coccygeal
nerve (Co1)
Figure 12.1
© 2011 Pearson Education, Inc.
1
Module 12.1: Spinal cord functional anatomy
• Spinal cord anatomy in cross section
• Posterior median sulcus
• Shallow longitudinal groove on posterior surface
• Dorsal root
• Contains axons of neurons whose cell bodies are in
dorsal root ganglion
• Dorsal root ganglion
• Contains sensory neuron cell bodies
• Each spinal segment contains one on each side
© 2011 Pearson Education, Inc.
Module 12.1: Spinal cord functional anatomy
• Spinal cord anatomy in cross section (continued)
• Spinal nerve
• Contains axons of both sensory and motor neurons
• Sensory enter CNS through dorsal root
• Motor exit CNS through ventral root
• Ventral root
• Contains axons of motor neurons extending into
periphery to control somatic and visceral effectors
• Anterior median fissure
• Deep groove along anterior (ventral) surface
© 2011 Pearson Education, Inc.
Module 12.1: Spinal cord functional anatomy
• Spinal cord anatomy in cross section (continued)
• White matter
• Superficial
• Contains large numbers of myelinated and
unmyelinated axons
• Gray matter
• Surrounds central canal
• Forms butterfly or H shape
• Dominated by cell bodies of neurons, neuroglia, and
unmyelinated axons
• Greater amount in spinal cord segments serving limbs
© 2011 Pearson Education, Inc.
Cross sections of three of the spinal cord’s 31 segments
Posterior median sulcus
Dorsal root
Dorsal root ganglion
White matter
Spinal nerve
Gray
matter
Segment C3
Ventral root
Anterior median fissure
White matter
Gray matter
Segment T3
Central
canal
Segment L1
Segment S2
© 2011 Pearson Education, Inc.
Figure 12.1
2
Module 12.1 Review
a. A typical spinal cord has how many pairs of
spinal nerves, and where does the spinal
cord end?
b. Describe the composition of the gray matter
of the spinal cord.
c. Describe the gross anatomical features of a
cross section of spinal cord.
© 2011 Pearson Education, Inc.
Module 12.2: Spinal meninges
•
Spinal meninges
•
Series of specialized membranes that provide physical
stability and shock absorption for the spinal cord
• Blood vessels branching within deliver oxygen and nutrients to
spinal cord
•
Are continuous with cranial meninges and connective tissues
surrounding spinal nerves
•
Three layers
1.Dura mater
2.Arachnoid mater
3.Pia mater
© 2011 Pearson Education, Inc.
Module 12.2: Spinal meninges
•
Three layers
1. Pia mater (pia, delicate + mater, mother)
•
Innermost meningeal layer
•
Meshwork of elastic and collagen fibers
•
Bound to underlying nervous tissue
© 2011 Pearson Education, Inc.
Module 12.2: Spinal meninges
•
Three layers (continued)
2. Arachnoid mater (arachine, spider)
•
Middle meningeal layer
•
Includes:
•
Simple squamous epithelium (arachnoid membrane)
•
Network of collagen and elastin fibers connecting
layer to pia mater (arachnoid trabeculae)
•
Subarachnoid space
•
Contains cerebrospinal fluid (CSF) that acts as
shock absorber and diffusion medium
•
Lumbar puncture (spinal tap) withdraws CSF
© 2011 Pearson Education, Inc.
A section demonstrating the procedure called a lumbar
puncture or spinal tap
Dura mater
Epidural space
Body of third
lumbar vertebra
Interspinous
ligament
Lumbar puncture
needle with tip in
subarachnoid space
Cauda equina in
subarachnoid
space
Figure 12.2
© 2011 Pearson Education, Inc.
5
Module 12.2: Spinal meninges
•
Three layers (continued)
3. Dura mater (dura, hard)
•
Outermost covering
•
Contains dense collagen fibers oriented longitudinally
•
Has narrow subdural space separating from
arachnoid mater
•
Epidural space
•
Between dura mater and vertebral canal
•
Contains areolar connective tissue, blood vessels,
and protective adipose tissue
© 2011 Pearson Education, Inc.
A posterior view of the dissected spinal cord showing the
basic relationships among the spinal meninges
Gray matter
Pia mater
White matter
Ventral
root
Spinal
nerve
Dorsal
root
Arachnoid mater
Dura mater
Figure 12.2
© 2011 Pearson Education, Inc.
1
A cross-sectional view showing the structures surrounding the spinal
cord and the spaces between the meningeal layers
Cerebrospinal
fluid (CSF)
Anterior
Ventral
root
Spinal meninges
Vertebral body
Spinal cord
Epidural space
Dorsal root ganglion
Pia mater
Arachnoid mater
Dura mater
Dorsal
root
Figure 12.2
© 2011 Pearson Education, Inc.
2
Module 12.2: Spinal meninges
•
Supporting ligaments
•
Prevent lateral movement of spinal cord
•
Denticulate ligaments
•
•
Extend from pia mater through arachnoid to dura
mater
Prevent superior–inferior movement of spinal
cord
•
Dural connections at foramen magnum
•
Coccygeal ligament
© 2011 Pearson Education, Inc.
An anterior view of the cervical spinal cord
showing the meninges, supporting ligaments,
and the roots of the spinal nerves
Spinal cord
Anterior median fissure
Pia mater
Denticulate ligaments
Dorsal root
Blood vessels within
the subarachnoid space
Ventral root, formed by
several “rootlets” from
one cervical segment
Arachnoid mater (reflected)
Dura mater (reflected)
Figure 12.2
© 2011 Pearson Education, Inc.
3
Module 12.2 Review
a. Identify and describe the three spinal
meninges.
b. Where is the cerebrospinal fluid that
surrounds the spinal cord located?
c.
Name the structures and spinal coverings
that would be penetrated during a lumbar
puncture procedure.
© 2011 Pearson Education, Inc.
Module 12.3: Gray and white matter
•
Gray matter
•
Structural organization
•
Horns (projections toward outer surface of spinal cord)
•
Posterior gray horn
•
•
•
Lateral gray horn
•
Only in thoracic and lumbar segments
•
Contains visceral motor nuclei
Anterior gray horn
•
•
Contains somatic & visceral sensory nuclei
Contains somatic motor nuclei
Gray commissures (contain axons that laterally cross
spinal cord)
© 2011 Pearson Education, Inc.
A cross section showing most of the anatomical landmarks of the spinal cord
Posterior
median sulcus
Posterior gray
commissure
Structural Organization
of Gray Matter
The projections of gray
matter toward the outer
surface of the spinal cord
are called horns.
Anterior view of
spinal cord
Posterior gray horn
Central canal
Dura mater
Lateral gray horn
Arachnoid
mater (broken)
Anterior gray horn
Pia mater
Dorsal root ganglion
Anterior
median fissure
Anterior gray
commissure
Ventral root
Figure 12.3
© 2011 Pearson Education, Inc.
1
Module 12.3: Gray and white matter
•
Gray matter (continued)
•
Functional organization
•
Nuclei (groups of cell bodies)
•
Sensory nuclei
•
•
Receive and relay sensory information from
peripheral receptors
Motor nuclei
•
© 2011 Pearson Education, Inc.
Issue motor commands to peripheral effectors
A diagrammatic view of the organization of the gray matter of the spinal cord
Site of a frontal
section that separates
the sensory (posterior,
or dorsal) nuclei from
the motor (anterior,
or ventral) nuclei
Posterior
gray horn
Dorsal
root
ganglion
Functional Organization
of Gray Matter
Gray commissures
Lateral
gray horn
Anterior
gray horn
Somatic
The cell bodies of neurons
in the gray matter of the
spinal cord are organized
into functional groups
called nuclei.
Visceral
Sensory nuclei
Visceral
Motor nuclei
Somatic
Ventral root
Figure 12.3
© 2011 Pearson Education, Inc.
2
Module 12.3: Gray and white matter
•
White matter
•
Columns (areas of white matter on spinal cord
sides)
•
Posterior white column
•
•
Lateral white column
•
•
Either side between anterior and posterior columns
Anterior white column
•
•
Between posterior gray horns and posterior median sulcus
Between anterior gray horns and median fissure
Anterior white commissure
•
Interconnects anterior white columns
© 2011 Pearson Education, Inc.
Module 12.3: Gray and white matter
•
White matter (continued)
•
Tracts (bundle of axons relatively uniform in
diameter, myelination, conduction speed, and
functional type)
•
Ascending tracts
•
•
Carry sensory information toward brain
Descending tracts
•
Convey motor commands to spinal cord
© 2011 Pearson Education, Inc.
Organization of Tracts
in the Posterior White
Column
The organization of the white matter into columns containing tracts
The posterior white
column contains
ascending tracts
providing sensations
from the trunk and limbs.
Leg
Hip
Trunk
Arm
Structural and
Functional
Organization
of White Matter
Posterior white column
Lateral white column
Anterior white column
Flexors/Extensors
Anterior white commissure
Trunk Shoulder Arm
Forearm Hand
In the cervical enlargement, which
contains neurons involved with
sensations and motor control of the
upper limbs, the motor nuclei of the
anterior gray horn are grouped by region,
with motor neurons controlling flexor
muscles medial to those controlling
extensor muscles.
Figure 12.3
© 2011 Pearson Education, Inc.
3
Module 12.3 Review
a. Differentiate between sensory nuclei and
motor nuclei.
b. A person with polio has lost the use of his leg
muscles. In which area of his spinal cord
would you expect the virus-infected motor
neurons to be?
c. A disease that damages myelin sheaths
would affect which portion of the spinal cord?
© 2011 Pearson Education, Inc.
Module 12.4: Spinal nerve structure and
distribution
•
Connective tissue layers of a spinal nerve
1. Epineurium
•
Outermost covering of nerve
•
Dense network of collagen fibers
2. Perineurium
•
Middle layer
•
Fibers extend inward from epineurium
•
•
Divide nerve into compartments that contain bundles of
axons (fascicles)
Branching blood vessels from epineurium continue
on to form capillaries in endoneurium
© 2011 Pearson Education, Inc.
Module 12.4: Spinal nerve structure and
distribution
•
Connective tissue layers of a spinal nerve
(continued)
3. Endoneurium
•
Innermost layer
•
Delicate connective tissues surrounding individual
axons
•
Capillaries here supply axons, Schwann cells, and
fibroblasts
© 2011 Pearson Education, Inc.
A sectional view of a spinal nerve showing its connective tissue layers
Connective Tissue
Layers of a
Spinal Nerve
Epineurium
Perineurium
Endoneurium
Artery and vein within
the perineurium
Fascicle
Schwann cell
Myelinated axon
Figure 12.4
© 2011 Pearson Education, Inc.
1
Module 12.4: Spinal nerve structure and
distribution
•
Spinal nerve branches
•
Called rami (singular ramus, a branch)
•
Some carry visceral motor fibers of autonomic
nervous system (ANS)
•
In thoracic and upper lumbar segments,
sympathetic division (“fight or flight”) motor fibers
© 2011 Pearson Education, Inc.
Module 12.4: Spinal nerve structure and
distribution
•
Specific rami
•
Dorsal ramus
•
•
Innervates muscles, joints, and skin of back
Ventral ramus
•
•
Innervates structures of lateral & anterior trunk
and limbs
Communicating rami
•
Present in thoracic and superior lumbar segments
•
Contain axons on sympathetic neurons
Animation: Peripheral Distal Spinal Nerves
© 2011 Pearson Education, Inc.
The branching of a spinal nerve to form rami
Dorsal root
Dorsal root
ganglion
Dorsal ramus
Ventral ramus
Ventral
root
Communicating rami
Autonomic nerve
Sympathetic
ganglion
Figure 12.4
© 2011 Pearson Education, Inc.
2
Module 12.4: Spinal nerve structure and
distribution
•
Dermatome
•
Specific bilateral region of skin surface monitored
by single pair of spinal nerves
•
C1 usually lacks sensory branch to skin
•
•
When present, helps monitor scalp with C2 and C3
Face is monitored by pair of cranial nerves
•
Boundaries between dermatomes overlap
•
Clinically important to determine damage or
infection of spinal nerve or dorsal root ganglion
•
Loss of sensation or signs on skin in dermatome
© 2011 Pearson Education, Inc.
Dermatomes, the specific bilateral regions of the skin
surface monitored by a single pair of spinal nerves
Anterior
Posterior
Figure 12.4
© 2011 Pearson Education, Inc.
3
Module 12.4: Spinal nerve structure and
distribution
•
Shingles
•
Viral infection of dorsal root ganglia
•
Caused by varicella-zoster virus
•
Same herpes virus as chickenpox
•
Produces painful rash and blisters on dermatome
served by infected nerves
•
Those who have had chickenpox are more at risk
•
Virus can remain dormant within anterior gray horns
•
Unknown trigger for reactivation
© 2011 Pearson Education, Inc.
Figure 12.4
© 2011 Pearson Education, Inc.
4
Module 12.4 Review
a. Identify the three layers of connective tissue
of a spinal nerve and identify the major
peripheral branches of a spinal nerve.
b. Describe a dermatome.
c. Explain the etiology (cause) of shingles.
© 2011 Pearson Education, Inc.
Module 12.5: Motor and sensory information in
spinal rami
•
Motor commands
•
Ventral root (axons of somatic & visceral motor
neurons)
•
Dorsal ramus (somatic & visceral motor fibers to
skin and skeletal muscles of back)
•
Ventral ramus (somatic & visceral motor fibers to
ventrolateral body surface, structures of body wall,
limbs)
© 2011 Pearson Education, Inc.
Module 12.5: Motor and sensory information in
spinal rami
•
Motor commands (continued)
Rami communicantes (“communicating branches”)
•
•
•
•
White ramus
•
Preganglionic fibers that are myelinated
•
Visceral motor fibers to sympathetic ganglion
Gray ramus
•
Postganglionic fibers that are unmyelinated
•
Innervate glands and smooth muscles of body wall or limbs
Sympathetic nerve
•
Preganglionic and postganglionic fibers to structures
of thoracic cavity
© 2011 Pearson Education, Inc.
Module 12.5: Motor and sensory information in
spinal rami
•
Sensory information
•
Sympathetic nerve (sensory information from
visceral organs)
•
Ventral ramus (sensory information from
ventrolateral body surface, body wall structures, and
limbs)
•
Dorsal ramus (sensory information from skin and
skeletal muscles of back)
•
Dorsal root (sensory information to spinal cord)
© 2011 Pearson Education, Inc.
Module 12.5 Review
a. Define gray ramus and white ramus.
b. Indicate whether the following fibers make up the white
rami or gray rami:
1) preganglionic fibers connecting a spinal nerve with a
sympathetic ganglion in the thoracic and lumbar region of the
spinal cord.
2) postganglionic fibers connecting a sympathetic ganglion in the
thoracic or lumbar region with the spinal nerve.
c. Which ramus innervates the skin and muscles of the back?
© 2011 Pearson Education, Inc.
Module 12.6: Spinal nerve plexuses introduction
and the cervical plexus
•
Nerve plexus (plexus, braid)
•
Complex interwoven network of nerves
•
Originally occurred during development as different
ventral rami innervating small skeletal muscles
•
Later, muscles fuse to form larger muscles with
compound origins
•
Ventral rami remain and blend fibers to form plexuses
© 2011 Pearson Education, Inc.
Module 12.6: Spinal nerve plexuses introduction
and the cervical plexus
•
Ventral rami plexuses
1. Cervical plexus
2. Brachial plexus
3. Lumbar plexus
4. Sacral plexus
Animation: Peripheral Nerves: Nerve Plexus
© 2011 Pearson Education, Inc.
The cervical, brachial, lumbar, and sacral plexuses (at left), and
the major peripheral nerves of each (at right)
Cervical
plexus
Brachial
plexus
Lesser occipital nerve
Great auricular nerve
Transverse cervical nerve
Supraclavicular nerve
Phrenic nerve
Axillary nerve
Musculocutaneous
nerve
Thoracic nerves
Radial nerve
Lumbar
plexus
Ulnar nerve
Median nerve
Iliohypogastric
nerve
Sacral
plexus
Ilioinguinal
nerve
Genitofemoral
nerve
Femoral nerve
Obturator nerve
Superior gluteal nerve
Inferior gluteal nerve
Pudendal nerve
Saphenous nerve
Sciatic nerve
Figure 12.6
© 2011 Pearson Education, Inc.
1
Module 12.6: Spinal nerve plexuses introduction
and the cervical plexus
•
Cervical plexus
•
Ventral rami of spinal nerves C1–C5
•
Branches innervate
•
Muscles of neck and to control
•
Diaphragmatic muscles (phrenic nerve)
•
Extends into thoracic cavity
•
Skin of neck
•
Skin of superior part of chest
© 2011 Pearson Education, Inc.
The cervical plexus, which consists of the ventral rami of spinal
nerves C1–C5, and some of the muscles its branches innervate
Cranial Nerves
Accessory
nerve (XI)
Hypoglossal
nerve (XII)
Lesser occipital
nerve
Great auricular nerve
Nerve Roots of
Cervical Plexus
C1
C2
C3
C4
C5
Geniohyoid muscle
Transverse cervical nerve
Thyrohyoid muscle
Ansa cervicalis
Omohyoid muscle
Supraclavicular
nerves
Clavicle
Phrenic nerve
Sternohyoid muscle
Sternothyroid muscle
Figure 12.6
© 2011 Pearson Education, Inc.
2
¯
Figure 12.6
© 2011 Pearson Education, Inc.
2
Module 12.6 Review
a. Define nerve plexus, and list the major nerve
plexuses.
b. An anesthetic blocks the function of the
dorsal rami of the cervical spinal nerves.
Which areas of the body will be affected?
c. Injury to which of the nerve plexuses would
interfere with the ability to breathe?
© 2011 Pearson Education, Inc.
Module 12.7: Brachial plexus
•
Brachial plexus
•
Innervates pectoral girdle and upper limb
•
Contributions from ventral rami of nerves C4–T1
•
Nerves originate from trunks and cords
• Trunks (large bundles of axons from several spinal nerves)
• Cords (smaller branches that originate in trunks)
•
Clinical importance of cutaneous nerve
• Damage or injury can be precisely localized by testing sensory
function in hand
Animation: Brachial Plexus
© 2011 Pearson Education, Inc.
The brachial plexus, which innervates the pectoral girdle
and upper limbs with contributions from the ventral rami
of spinal nerves C4–T1
Trunks of
Brachial Plexus
Dorsal scapular nerve
Suprascapular nerve
Spinal Nerves
Forming Brachial
Plexus
C4 nerve
C5 nerve
C6 nerve
C7 nerve
C8 nerve
T1 nerve
Superior
Middle
Inferior
Musculocutaneous
nerve
Median nerve
Ulnar nerve
Radial nerve
Lateral antebrachial
cutaneous nerve
Superficial branch
of radial nerve
Deep radial nerve
Ulnar nerve
Median nerve
Palmar digital
nerves
Figure 12.7
© 2011 Pearson Education, Inc.
1
The trunks and cords from which the nerves that form the brachial plexus originate
Dorsal scapular
nerve
 Roots (ventral rami)
 Trunks
 Divisions
C5
SUPERIOR TRUNK
C6
Suprascapular nerve
 Cords
 Peripheral nerves
MIDDLE
TRUNK
Lateral cord
C7
Posterior cord
C8
Lateral pectoral nerve
Medial pectoral nerve
Subscapular nerves
T1
Axillary nerve
INFERIOR
TRUNK
Medial cord
Musculocutaneous
nerve
First
rib
Medial antebrachial
cutaneous nerve
Median nerve
Posterior brachial
cutaneous nerve
© 2011 Pearson Education, Inc.
Long thoracic
nerve
Ulnar
nerve
Radial nerve
Figure 12.7
2
The distribution of the cutaneous
nerves of the wrist and hand
Posterior
Anterior
Radial
nerve
Ulnar
nerve
Median
nerve
Figure 12.7
© 2011 Pearson Education, Inc.
3
Figure 12.7
© 2011 Pearson Education, Inc.
3
Module 12.7 Review
a. Define a nerve plexus trunk and cord.
b. Describe the brachial plexus.
c. Name the major nerves associated with the
brachial plexus.
© 2011 Pearson Education, Inc.
Module 12.8: Lumbar and sacral plexuses
•
Lumbar and sacral plexuses
•
Arise from lumbar and sacral segments of spinal
cord
•
Innervate pelvic girdle and lower limbs
•
Lumbar plexus
• Innervates mostly anterior and side surfaces
•
Sacral plexus
• Innervates mostly posterior surfaces
• Contains sciatic nerve (longest & largest nerve in body)
Animation: Lumbar Sacral Plexus
© 2011 Pearson Education, Inc.
The origins of the spinal
nerves of the sacral plexus
Spinal Nerves
Forming the
Sacral Plexus
Lumbosacral trunk
L4 nerve
L5 nerve
Nerves of the
Sacral Plexus
S1 nerve
Superior gluteal
S2 nerve
Inferior gluteal
S3 nerve
Sciatic
Posterior femoral
cutaneous
Pudendal
S4 nerve
Co1
Sacral plexus, anterior view
Figure 12.8
© 2011 Pearson Education, Inc.
1
A posterior view of the lower limb
showing the distribution of the nerves
of the sacral plexus
Superior gluteal nerve
Inferior gluteal nerve
Pudendal nerve
Posterior femoral
cutaneous nerve
Sciatic nerve
Tibial nerve
Common fibular
nerve
Sural nerve
Figure 12.8
© 2011 Pearson Education, Inc.
2
The dermatomes of the sensory nerves
innervating the ankle and foot
Saphenous
nerve
Sural nerve
Sural
nerve
Saphenous
nerve
Fibular
nerve
Tibial nerve
Fibular nerve
Figure 12.8
© 2011 Pearson Education, Inc.
3
An anterior view of the lower trunk
and lower limb showing the
distribution of the nerves of both
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
Figure 12.8
© 2011 Pearson Education, Inc.
4
Module 12.8 Review
a. Describe the lumbar plexus and sacral
plexus.
b. List the major nerves of the sacral plexus.
c. Compression of which nerve produces the
sensation that your lower limb has “fallen
asleep”?
© 2011 Pearson Education, Inc.
Section 2: Reflexes and Neural Circuits
• Learning Outcomes
• 12.9
Describe the steps in a neural reflex.
• 12.10 Describe the steps in the stretch reflex.
• 12.11 Explain withdrawal reflexes and crossed
extensor reflexes and the responses
produced by each.
• 12.12 CLINICAL MODULE Explain the value of
reflex testing and how higher centers control
and modify reflex responses.
© 2011 Pearson Education, Inc.
Section 2: Reflexes and Neural Circuits
•
Neuronal pools
•
Functional groups of interconnected neurons
•
Most cases are interneurons in CNS
•
May involve several regions of brain
•
May involve neurons in one specific location in brain or
spinal cord
•
Estimated number of pools ~100s to 1000s
•
Patterns of neuronal interactions suggest functional
classifications
•
•
Neural circuit (“wiring diagram”)
Simple circuits in PNS and spinal cord control reflexes
•
Preprogrammed responses to specific stimuli
© 2011 Pearson Education, Inc.
Section 2: Reflexes and Neural Circuits
•
Common neural circuit patterns
•
Divergence
•
Spread of information from one neuron to many
•
Permits broad distribution of a specific input
•
Example: sensory information coming to CNS
•
Parallel processing
•
Several neurons or neural pools process same
information simultaneously
•
Many responses can occur simultaneously
© 2011 Pearson Education, Inc.
The common types of neural circuits
Divergence
Divergence is the spread of information
from one neuron to several neurons, or
from one pool to multiple pools.
Divergence permits the broad distribution
of a specific input. Considerable
divergence occurs when sensory neurons
bring information into the CNS: The
information is distributed to neuronal
pools throughout the spinal cord and
brain.
Figure 12 Section 2
© 2011 Pearson Education, Inc.
The common types of neural circuits
Parallel Processing
Parallel processing occurs when
several neurons or neuronal pools
process the same information
simultaneously. Many responses can
occur simultaneously.
Figure 12 Section 2
© 2011 Pearson Education, Inc.
Section 2: Reflexes and Neural Circuits
•
Common neural circuit patterns (continued)
•
Serial processing
•
Information relayed from one neuron or neuronal pool to
another (stepwise fashion)
•
Example: sensory relay from one brain part to another
•
Convergence
•
Several neurons synapse on single postsynaptic neuron
•
Several patterns of activity by presynaptic neurons can
have same postsynaptic effect
•
Example: breathing movements of diaphragm and ribs
can be controlled subconsciously or consciously
© 2011 Pearson Education, Inc.
The common types of neural circuits
Serial Processing
In serial processing, information is
relayed in a stepwise fashion, from one
neuron to another or from one neuronal
pool to the next. This pattern occurs as
sensory information is relayed from one
part of the brain to another.
Figure 12 Section 2
© 2011 Pearson Education, Inc.
The common types of neural circuits
Convergence
In convergence, several neurons synapse on a
single postsynaptic neuron. Several patterns of
activity in the presynaptic neurons can therefore
have the same effect on the postsynaptic
neuron. Through convergence, the same motor
neurons can be subject to both conscious and
subconscious control. For example, the
movements of your diaphragm and ribs are now
being controlled by your brain at the
subconscious level. But the same motor
neurons can also be controlled consciously, as
when you take a deep breath and hold it.
Figure 12 Section 2
© 2011 Pearson Education, Inc.
Section 2: Reflexes and Neural Circuits
•
Reverberation
•
Collateral axonal branches extend back
toward source of impulse to further stimulate
presynaptic neurons
•
•
Like positive feedback involving neurons
Once activated, reverberating circuits will
continue to function until something breaks
cycle
•
Synaptic fatigue
•
Inhibitory stimuli
© 2011 Pearson Education, Inc.
The common types of neural circuits
Reverberation
In reverberation, collateral branches of axons
somewhere along the circuit extend back toward
the source of an impulse and further stimulate
the presynaptic neurons. Reverberation is like a
positive feedback loop involving neurons: Once
a reverberating circuit has been activated, it will
continue to function until the synaptic fatigue or
inhibitory stimuli break the cycle.
Figure 12 Section 2
© 2011 Pearson Education, Inc.
Module 12.9: Reflexes
• Reflexes
• Are rapid, automatic responses to specific stimuli
• Show little variability
• Preserve homeostasis by making rapid adjustments in
functions of organs or organ systems
• In neural reflexes:
• Sensory fibers carry information from peripheral receptors to
integration center
• Motor fibers carry motor commands to peripheral effectors
• Reflex arc
• “Wiring” of a single reflex from receptor to effector
Animation: Components of a Reflexive Arc
© 2011 Pearson Education, Inc.
Module 12.9: Reflexes
• Example: Simple withdrawal reflex
1. Arrival of stimulus and activation of receptor
•
Receptor is specialized cell or dendrites of sensory
neuron
•
•
Sensitive to:
•
Physical or chemical changes in body
•
Or changes in external environment
Example: pain receptor in hand
2. Activation of sensory neuron
•
Stimulation of dendrites produces graded polarization
leading to action potential
•
Action potential travels through dorsal root to spinal cord
© 2011 Pearson Education, Inc.
Module 12.9: Reflexes
• Example: Simple withdrawal reflex (continued)
3. Information processing
•
Sensory neuron releases excitatory neurotransmitters
at postsynaptic membrane of interneuron
•
Neurotransmitter produces EPSP which is integrated with
other simultaneous stimuli
4. Activation of motor neuron
•
Activation of interneuron leads to stimulation of motor
neuron to carry action potential to periphery
•
Axonal collaterals may relay sensation to other centers in
brain and spinal cord
© 2011 Pearson Education, Inc.
Module 12.9: Reflexes
• Example: Simple withdrawal reflex (continued)
5. Response of peripheral effector
•
Release of neurotransmitters by synaptic knobs
leads to response by peripheral effector
•
Generally removes or opposes original stimulus
•
•
An example of negative feedback
Example: skeletal muscle contraction moving
hand away from painful sensation
© 2011 Pearson Education, Inc.
STEP 2
The Activation of a Sensory
Neuron
The steps in a reflex arc: a simple withdrawal reflex
STEP 1
The Arrival
of a Stimulus
and Activation of
a Receptor
STEP 3
Information
Processing
Dorsal root
ganglion
To higher
centers
REFLEX
ARC
Receptor
Stimulus
Effector
STEP 5
The Response
of a Peripheral
Effector
STEP 4
The Activation
of a Motor
Neuron
Sensory neuron
(stimulated)
Excitatory
interneuron
Motor neuron
(stimulated)
Figure 12.9
© 2011 Pearson Education, Inc.
1
Module 12.9: Reflexes
• Reflex classification based on:
1. Their development
2. Nature of resulting motor response
3. Complexity of neural circuit involved
4. Site of information processing
•
Categories are not mutually exclusive
© 2011 Pearson Education, Inc.
Module 12.9: Reflexes
•
Reflex categories
1. Development
•
Innate reflexes
•
Connections formed between neurons genetically or
developmentally programmed
•
Generally appear in a predictable sequence
•
•
Example: simplest (withdrawal) to complex (suckling)
Acquired reflexes
•
Learned rather than preestablished
•
Enhanced by repetition
© 2011 Pearson Education, Inc.
Module 12.9: Reflexes
•
Reflex categories (continued)
2. Nature of response
•
Somatic reflexes
•
Involuntary control of skeletal muscles
•
•
•
Example: withdrawal reflex
Rapid response that can later be supplemented
voluntarily
Visceral reflexes (autonomic reflexes)
•
Control or adjust activities of smooth & cardiac
muscle, glands, and adipose tissues
© 2011 Pearson Education, Inc.
Module 12.9: Reflexes
•
Reflex categories (continued)
3. Complexity of circuit
•
•
Polysynaptic reflexes
•
Involve at least one interneuron, one sensory neuron,
and one motor neuron
•
Longer delay between stimulus and response due to
increased number of synapses
•
Produce more complex reflexes
Monosynaptic reflexes
•
Simplest reflex arc involving one sensory and one
motor neuron
•
Faster response time due to only one synapse
© 2011 Pearson Education, Inc.
Module 12.9: Reflexes
•
Reflex categories (continued)
4. Processing site
•
Spinal reflexes
•
Occur in nuclei of spinal cord
•
Two types
1. Single segmental (within one spinal segment)
2. Intersegmental (multiple segments)
•
Cranial reflexes
•
Occur in nuclei of brain
© 2011 Pearson Education, Inc.
Module 12.9 Review
a. Define reflex and list the components of a
reflex arc.
b. What are common characteristics of reflexes?
c. Describe the various classifications of
reflexes.
© 2011 Pearson Education, Inc.
Module 12.10: Monosynaptic reflex
•
Stretch reflex
•
Best-known monosynaptic reflex
•
Provides automatic regulation of skeletal
muscle length
•
Example: patellar reflex
© 2011 Pearson Education, Inc.
Module 12.10: Monosynaptic reflex
•
Steps of the patellar reflex
1. Arrival of stimulus and activation of receptor
•
Patellar tendon tapped by physician
•
Quadriceps tendon receptors stretch
2. Activation of sensory neuron
•
Receptors stimulate sensory neuron that extends into
spinal cord
•
Sensory neuron synapses with motor neuron
3. Information processing in CNS
•
Information processing at motor neuron cell body
© 2011 Pearson Education, Inc.
Module 12.10: Monosynaptic reflex
•
Steps of the patellar reflex (continued)
4. Activation of motor neuron
•
Motor neuron activated and action potential is
generated and propagated
5. Response of peripheral receptor
•
Stimulation of skeletal muscle fibers leads to
contraction of knee extensors
© 2011 Pearson Education, Inc.
The patellar reflex, a stretch reflex
and the best-known
monosynaptic reflex
STEP 2
Activation of a
Sensory Neuron
STEP 1
Arrival of the
Stimulus and
Activation of
a Receptor
STEP 3
Information
Processing in
the CNS
Stretch
Spinal cord
REFLEX
ARC
Receptor
(muscle spindle)
Contraction
Response
Effector
STEP 4
Activation of a Motor Neuron
STEP 5
Response of a
Peripheral Effector
KEY
Sensory neuron
(stimulated)
Motor neuron
(stimulated)
Figure 12.10
© 2011 Pearson Education, Inc.
1
Module 12.10: Monosynaptic reflex
•
Muscle spindles
•
Are sensory receptors of stretch reflex
•
Consist of a bundle of specialized muscle fibers
(intrafusal muscle fibers)
•
Are surrounded by larger skeletal muscle fibers
responsible for muscle tone and contraction of entire
muscle
•
Sensory neuron surrounds muscle spindle and is
always sending impulses to CNS through dorsal root
•
Are innervated by gamma motor neurons that alter
tension in spindle and control sensitivity of receptor
© 2011 Pearson Education, Inc.
The structure of a muscle spindle
Skeletal muscle
fibers
Axon of gamma motor
neuron in CNS
Intrafusal fibers
Route of impulses to the CNS
Muscle
spindle
Dendrites of the
sensory neuron
Axon of gamma motor
neuron in CNS
Figure 12.10
© 2011 Pearson Education, Inc.
2
The effects of distortion of intrafusal fibers
on skeletal muscle tone
Sensory
Region
Action Potential
Frequency in
Sensory Neuron
Effect on
Skeletal
Muscle
Normal
muscle
tone
persists
Resting
length
Muscle
tone
increases
Stretched
Muscle
tone
decreases
Compressed
Figure 12.10
© 2011 Pearson Education, Inc.
3
Module 12.10: Monosynaptic reflex
•
Postural reflexes
•
Many stretch reflexes that help maintain
upright posture
•
Coordinated activities of opposing muscles to
keep body’s weight over feet
•
Example: leaning forward stretches calf muscle
receptors which stimulate the muscles to
increase tone
•
•
Returns body to upright position
Postural muscles generally have firm muscle
tone and extremely sensitive stretch receptors
•
Allow for very fine, subconscious adjustments
© 2011 Pearson Education, Inc.
Module 12.10 Review
a. Define stretch reflex.
b. In the patellar reflex, identify the response
observed and the effectors involved.
c. ln the patellar reflex, in what way does
stimulation of the muscle spindle by gamma
motor neurons affect the speed of the reflex?
© 2011 Pearson Education, Inc.
Module 12.11: Polysynaptic reflexes
•
Polysynaptic reflexes
•
Responsible for automatic actions involved in
complex movements
•
•
Examples: walking and running
May involve sensory and motor responses on
the same side of body or opposite sides
•
Same side: ipsilateral reflexes
•
•
Examples: stretch reflex, withdrawal reflex
Opposite sides: contralateral reflexes
•
Example: crossed extensor reflex
© 2011 Pearson Education, Inc.
Module 12.11: Polysynaptic reflexes
•
Polysynaptic reflexes (continued)
•
All share basic characteristics:
•
Involve pools of interneurons
•
Are intersegmental in distribution
•
Involve reciprocal inhibition
•
Have reverberating circuits that prolong
response
•
Several reflexes may cooperate to produce
coordinated, controlled response
© 2011 Pearson Education, Inc.
Figure 12.11
© 2011 Pearson Education, Inc.
3
Module 12.11: Polysynaptic reflexes
•
Withdrawal reflexes
•
Move affected body parts away from stimulus
•
Strongest are triggered by painful stimuli but other
stimuli can initiate
•
Show tremendous versatility because sensory
neurons activate many pools of interneurons
•
•
Intensity and location of stimulus affect:
•
Distribution of effects
•
Strength and character of motor responses
Example: flexor reflex, crossed extensor
reflexes
© 2011 Pearson Education, Inc.
Module 12.11: Polysynaptic reflexes
•
Withdrawal reflex example: flexor reflex
•
Grabbing an unexpectedly hot pan causes
pain receptors in hand to be stimulated
•
Sensory neurons activate interneurons in
spinal cord
•
Interneurons
•
Activate motor neurons in anterior gray horn to
contract flexor muscles
•
Activated inhibitory interneurons keep
extensors relaxed
•
= Reciprocal inhibition
© 2011 Pearson Education, Inc.
Module 12.11: Polysynaptic reflexes
•
Withdrawal reflex example: flexor reflex
(continued)
•
Mild discomfort might cause brief contraction
of hand and wrist muscles
•
More powerful stimuli may produce
coordinated muscle contractions of hand,
wrist, forearm, and arm
•
Severe pain may also involve muscles of
shoulder, trunk
•
Contractions may persist due to reverberating
circuits
© 2011 Pearson Education, Inc.
The flexor reflex, a representative withdrawal reflex
Distribution within gray
horns to other segments
of the spinal cord
Painful
stimulus
Flexors
stimulated
Sensory neuron
(stimulated)
Muscles undergoing
reciprocal inhibition
Extensors
inhibited
Excitatory
interneuron
Motor neuron
(stimulated)
Motor neuron
(inhibited)
Inhibitory
interneuron
Figure 12.11
© 2011 Pearson Education, Inc.
1
Module 12.11: Polysynaptic reflexes
•
Crossed extensor reflexes
•
Example: stepping on a tack
•
•
Flexor reflex pulls injured foot away
•
Flexor muscles stimulated
•
Extensor muscles inhibited
Crossed extensor reflex straightens uninjured
leg and supports shifting weight
•
Activated by collaterals of excitatory and inhibitory
interneurons
•
Extensor muscles stimulated
•
Flexor muscles inhibited
© 2011 Pearson Education, Inc.
The crossed extensor reflex, which involves
a contralateral reflex arc
To motor neurons
in other segments
of the spinal cord
Extensors
inhibited
Flexors
stimulated
Extensors
stimulated
Flexors
inhibited
Sensory neuron
(stimulated)
Excitatory
interneuron
Motor neuron
(stimulated)
Painful
stimulus
Motor neuron
(inhibited)
Inhibitory
interneuron
Figure 12.11
© 2011 Pearson Education, Inc.
2
Module 12.11 Review
a. Identify the basic characteristics of
polysynaptic reflexes.
b. Describe the flexor reflex.
c. During a withdrawal reflex of the foot, what
happens to the limb on the side opposite the
stimulus? What is this response called?
© 2011 Pearson Education, Inc.
CLINICAL MODULE 12.12: Brain influences on spinal
reflexes and diagnostics using reflexes
•
Brain influences on spinal reflexes
•
Can facilitate or inhibit motor neurons or
interneurons involved
•
•
Facilitation = reinforcement
Example: voluntary movement to pull apart clasped
hands can reinforce stretch reflexes and increase
response (example: bigger kick after patellar tap)
© 2011 Pearson Education, Inc.
CLINICAL MODULE 12.12: Brain influences on spinal
reflexes and diagnostics using reflexes
•
Reflexes used in diagnostic testing
•
Specific examples
•
Babinski reflex
•
Stroking lateral side of sole of foot
•
Positive response: toes fan due to lack of inhibitory control
of reflex response from descending motor pathways
•
•
Normal in infants
•
Can indicate damaged higher centers or descending
tracts in adults
Negative response: toes curl due to development and
normal reflex response
•
© 2011 Pearson Education, Inc.
= Plantar reflex
CLINICAL MODULE 12.12: Brain influences on spinal
reflexes and diagnostics using reflexes
•
Reflexes used in diagnostic testing (continued)
•
Specific examples (continued)
•
Abdominal reflex
•
Depends on descending facilitation
•
Light stroking of skin of anterior abdomen produces
reflexive twitch of abdominal muscles
•
Absence of response may indicate damage to descending
tracts
© 2011 Pearson Education, Inc.
Biceps reflex
Figure 12.12
© 2011 Pearson Education, Inc.
1
Triceps reflex
Figure 12.12
© 2011 Pearson Education, Inc.
1
Ankle-jerk reflex
Figure 12.12
© 2011 Pearson Education, Inc.
1
Babinski sign
Figure 12.12
© 2011 Pearson Education, Inc.
2
Plantar reflex
Figure 12.12
© 2011 Pearson Education, Inc.
3
Abdominal reflex
Figure 12.12
© 2011 Pearson Education, Inc.
4
CLINICAL MODULE 12.12 Review
a. Define reinforcement as it pertains to spinal
reflexes.
b. What purpose does reflex testing serve?
c. After injuring her back, 22-year-old Tina
exhibits a positive Babinski reflex. What does
this imply about her injury?
© 2011 Pearson Education, Inc.