Chapter 16
*Lecture Outline
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Chapter 16 Outline
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Gross Anatomy of the Spinal Cord
Spinal Cord Meninges
Sectional Anatomy of the Spinal Cord
Spinal Nerves
Reflexes
Development of the Spinal Cord
Spinal Cord—Introduction
The spinal cord provides a vital link between the
brain and the rest of the body. The spinal cord
and its attached spinal nerves serve two
important functions:
1. a pathway for sensory and motor impulses
2. responsible for reflexes, which are the
quickest reactions to a stimulus
Gross Anatomy
of the Spinal Cord
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Length: 42–45 cm, 16–18 inches
Roughly cylindrical, slightly flattened
posteriorly and anteriorly
Two longitudinal depressions on external
surface:
– Posterior median sulcus on posterior surface
– Anterior median fissure on anterior surface
Gross Anatomy
of the Spinal Cord
Parts of the spinal cord:
1. Cervical
2. Thoracic
3. Lumbar
4. Sacral
5. Coccygeal
Gross Anatomy of the Spinal Cord
Figure 16.1
Gross Anatomy of the Spinal Cord
Gross Anatomy
of the Spinal Cord
The diameter of the spinal cord changes along
its length because the amount of gray matter
and white matter and the function of the cord
vary in different regions.
• The cervical enlargement is located in the
inferior cervical part of the spinal cord and
innervates the upper limbs.
• The lumbosacral enlargement extends
through the lumbar and sacral parts of the
spinal cord and innervates the lower limbs.
Gross Anatomy
of the Spinal Cord
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The spinal cord is shorter than the
vertebral canal that houses it.
The tapering inferior end of the spinal
cord is called the conus medullaris
and is the official “end” of the spinal
cord proper (usually at the level of the
first lumbar vertebra).
Gross Anatomy
of the Spinal Cord
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Inferior to the conus medullaris,
groups of axons called the cauda
equina project from the spinal cord.
Within the cauda equina is the filum
terminale, which is a thin strand of pia
mater that helps anchor the conus
medullaris to the coccyx.
Gross Anatomy
of the Spinal Cord
The spinal cord is associated with 31 pairs of
spinal nerves that connect the CNS to muscles,
receptors and glands. Each side contains:
• 8 cervical nerves (C1–C8).
• 12 thoracic nerves (T1–T12).
• 5 lumbar nerves (L1–L5).
• 5 sacral nerves (S1–S5)
• 1 coccygeal nerve (Co1)
Spinal Cord Meninges
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The spinal cord is protected and
encapsulated by spinal cord
meninges, which are continuous with
the cranial meninges.
Some of the spaces between some of
the meninges have clinical
significance.
Spinal Meninges and
Structure of the Spinal Cord
Figure 16.2
Spinal Cord Meninges
Epidural space:
• lies between the dura mater and periosteum
covering the inner walls of the vertebra
• houses areolar connective tissue, blood
vessels, and adipose connective tissue
Dura mater:
• most external of the meninges
• fuses with the connective layers that
surround the spinal nerves
Spinal Cord Meninges
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Narrow subdural space separates
dura mater from arachnoid; a potential
space
Arachnoid mater is deep to the dura
mater and the subdural space
Subarachnoid space is a real space
filled with cerebral spinal fluid
Spinal Cord Meninges
Pia mater:
• innermost meningeal layer that adheres
directly to the spinal cord
• delicate layer composed of elastic and
collagen fibers and supports some of the
blood vessels supplying the spinal cord
• has paired, lateral triangular extensions
called denticulate ligaments, which
suspend and anchor the spinal cord laterally
to the dura mater
Sectional Anatomy
of the Spinal Cord
The spinal cord is partitioned into an inner
gray matter region and an outer white
matter region:
• Gray matter—dendrites and cell bodies
of neurons, unmyelinated axons, and
glial cells
• White matter—myelinated axons
Gray and White Components
of Spinal Cord
Figure 16.3
Gray and White Components
of Spinal Cord
Figure 16.3
Location and Distribution
of Gray Matter
Gray Matter:
• Centrally located in spinal cord
• Sectioned shape resembles butterfly
Subdivided into:
• Anterior Horns
• Lateral Horns
• Posterior Horns
• Gray Commissure
Location and Distribution
of Gray Matter
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Anterior horns house the cell bodies
of somatic motor neurons, which
innervate skeletal muscle
Lateral horns:
– found in the T1–L2 parts of the spinal cord
only
– contain cell bodies of autonomic motor
neurons, which innervate cardiac muscle,
smooth muscle, and glands
Location and Distribution
of Gray Matter
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The posterior horns contain axons of
sensory neurons and cell bodies of
interneurons.
The gray commissure contains
unmyelinated axons and serves as a
communication route between the
right and left side.
The gray commissure houses a
narrow central canal.
Location and Distribution
of Gray Matter
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Within the gray matter are functional groups
of neuron cell bodies called nuclei:
Sensory nuclei in the posterior horns
contain interneuron cell bodies of:
– somatic sensory nuclei
– visceral sensory nuclei
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–
Motor nuclei in the anterior horns contain
somatic motor nuclei
Autonomic motor nuclei are in the lateral
horns
Neuron Pathways
and Nuclei Locations
Figure 16.4
Location and Distribution
of White Matter
The white matter of the spinal cord is external to
the gray matter and is partitioned into three
regions, each called a funiculus:
• posterior funiculus
• lateral funiculus
• anterior funiculus
– interconnected by the white commissure
The axons within each funiculus are organized
into tracts.
Spinal Nerves
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31 pairs
Made up of motor and sensory axons
Contain connective tissue wrappings
called endoneurium, perineurium, and
epineurium
Spinal Nerves
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Multiple anterior rootlets arise from
the spinal cord and merge to form a
single anterior root.
Anterior roots contain motor axons
only.
The cell bodies of the motor axons
arise from cell bodies in the anterior
and lateral horns of the spinal cord.
Spinal Nerves
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Multiple posterior rootlets are
derived from a single posterior root.
Posterior roots contain sensory axons
only.
The cell bodies of the sensory axons
arise from cell bodies in the posterior
root ganglion, which is attached to
the posterior root.
Spinal Nerves
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Each anterior root and its
corresponding posterior root unite
within the intervertebral foramen to
become a spinal nerve.
A spinal nerve contains both motor
and sensory axons.
Spinal Nerves
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Spinal nerves are numbered according to
the location of the intervertebral canal.
In the cervical region the first seven pairs of
spinal nerves (C1–C7) exit the intervertebral
foramen above the vertebra of the same
number.
The eighth pair of cervical spinal nerves
(C8) exit above the first thoracic vertebra.
The remaining pairs of spinal nerves exit
below the vertebra of the same number.
Spinal Nerves
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Because the spinal cord is shorter
than the vertebral canal, the roots of
the lumbar and sacral spinal nerves
travel inferiorly to reach their
respective intervertebral foramen.
Spinal Nerve Distribution
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After leaving the intervertebral foramen, a
typical spinal nerve splits into branches
termed rami.
The posterior ramus is the smaller of the
two main branches and innervates the deep
muscles of the back and the skin of the
back.
The anterior ramus is the larger of the two
main branches and innervates the anterior
and lateral portions of the trunk and the
upper and lower limbs.
Spinal Nerve Distribution
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The anterior ramus splits into multiple
other branches.
Many of the anterior rami go on to
form nerve plexuses.
Additional rami, the rami
communicantes, extend between the
spinal nerve and the sympathetic trunk
ganglion.
Spinal Nerve Branches
Figure 16.5
Dermatomes
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A dermatome is a specific segment of
skin supplied by a single spinal nerve.
All spinal nerves except C1 innervate
a segment of skin.
The dermatome map follows a
segmental pattern along the body.
Dermatome Maps
Figure 16.6
Dermatomes
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The dermatome map can be important
because anesthesia (numbness) in one or
more of the segments could indicate
potential spinal nerve damage.
Dermatomes are also involved in referred
visceral pain, where a pain in a dermatome
may arise from an organ nowhere near the
dermatome.
Nerve Plexuses
A nerve plexus is a network of interweaving
anterior rami of spinal nerves.
• The anterior rami of most spinal nerves form
nerve plexuses on both sides of the body.
• The plexuses split into multiple named nerves
that innervate body structures.
• The principle plexuses are the: cervical
plexuses, brachial plexuses, lumbar
plexuses, and sacral plexuses.
Intercostal Nerves
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The anterior rami of spinal nerves T1–T11
are called intercostal nerves because they
travel in the intercostal spaces between
adjacent ribs.
Spinal nerve T12 is called a subcostal
nerve, because it arises below the ribs.
With the exception of spinal nerve T1, the
intercostal nerves do not form plexuses.
Intercostal Nerves
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T1 forms part of the brachial plexus.
T2 innervates the intercostal muscles of the
second intercostal space and is sensory for
the axilla and medial surface of the arm.
T3–T6 innervate the intercostal muscles
and are sensory for the anterior chest wall.
T7–T12 innervate the intercostal muscles,
the abdominal muscles, and the overlying
skin.
Intercostal Nerves
Figure 16.7
Cervical Plexus
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Formed by anterior rami of spinal nerves
C1–C4
Branches of the cervical plexus innervate
anterior neck muscles and the skin of the
neck and head and shoulders.
The phrenic nerve originated primarily from
C4 and some contributing axons of C3 and
C5.
– travels through the thoracic cavity to innervate the
diaphragm
Cervical Plexus
Figure 16.8
Cervical Plexus
Brachial Plexus
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The left and right brachial plexuses
are networks of nerves that supply the
upper limbs.
Each plexus is formed by the anterior
rami of spinal nerves C5–T1.
Each brachial plexus innervates the
pectoral girdle and the entire upper
limb of one side.
Brachial Plexus
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The anterior rami of C5–T1 form the
roots of the brachial plexus
The roots unite to form the:
– superior trunk—nerves C5 and C6
– middle trunk—nerve C7
– inferior trunk—nerves C8 and T1
Brachial Plexus
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Portions of each trunk divide into an
anterior division and a posterior division
The anterior and posterior divisions
converge to form three cords:
– posterior cord
– medial cord
– lateral cord
Brachial Plexus
Five major terminal branches emerge from the
three cords:
1. axillary nerve
2. median nerve
3. musculocutaneous nerve
4. radial nerve
5. ulnar nerve
Brachial Plexus
Figure 16.9
Brachial Plexus
Figure 16.9
Brachial Plexus
Figure 16.9
Brachial Plexus
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Table 16.3
Branches of the Brachial Plexus
Terminal Branch
Axillary Nerve
Formed from posterior cord, posterior division of the
brachial plexus
Posterior cord
Axillary nerve
Teres minor
Deltoid
Anterior
Rami
Motor Innervation
Cutaneous Innervation
C5, C6
Deltoid (arm abductor)
Teres minor (lateral rotator of
arm)
Superolateral arm
Brachial Plexus
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Median Nerve
C5–T1
Formed from medial and lateral cords, anterior division
of the brachial plexus
Lateral cord
Posterior cord
Medial cord
Median nerve
Flexor pollicis
longus
Pronator
quadratus
Thenar muscles
Lateral two
lumbricals
Pronator teres
Flexor carpi radialis
Palmaris longus
Flexor digitorum
superficialis
Flexor digitorum
profundus
(lateral half)
Most anterior forearm muscles
(pronators, flexors of wrist,
digits)
Flexor carpi radialis
Flexor digitorum superficialis
Pronator teres
Pronator quadratus
Lateral 1/2 of flexor digitorum
profundus
Flexor pollicis longus
Thenar (thumb) muscles (move
thumb)
Flexor pollicis brevis
Abductor pollicis brevis
Opponens pollicis
Lateral two lumbricals (flex MP
joints and extend PIP and DIP
joints)
Palmar aspects and dorsal tips of
lateral 3-1/2 digits (thumb, index
finger, middle finger, and 1/2 of
ring finger)
Brachial Plexus
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Table 16.3
Branches of the Brachial Plexus (continued)
Terminal Branch
Anterior
Rami
Motor Innervation
Cutaneous Innervation
Musculocutaneous Nerve
C5–C7
Anterior arm muscles (flex
humerus, flex elbow joint, supinate
forearm)
Lateral region of forearm
Formed from the lateral cord, anterior division of the brachial
plexus
Coracobrachialis
Biceps brachii
Lateral cord
Coracobrachialis
Biceps brachii
Musculocutaneous
nerve
Brachialis
Brachialis
Brachial Plexus
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Radial Nerve
C5–T1
Formed from the posterior cord, posterior division of the
brachial plexus
Posterior arm muscles (extend
forearm)
Triceps brachii
Anconeus
Posterior forearm muscles (supinate
forearm, extend wrist, digits,
one muscle that abducts thumb)
Lateral cord
Supinator
Posterior cord
Extensor carpi radialis muscles
Medial cord
Radial nerve
Long head of
triceps brachii
Lateral head of
triceps brachii
Extensor digitorum
Extensor carpi ulnaris
Extensor pollicis longus
Extensor pollicis brevis
Abductor pollicis brevis
Medial head of
triceps brachii
Extensor digiti minimi
Extensor indicis
Brachioradialis
Anconeus
Supinator
Extensorcarpiulnaris
Extensor digiti minimi
Extensor digitorum
Extensor pollicis
longus and brevis
Extensor indicis
Extensor carpi
radialis
Abductor
pollicis longus
Brachioradialis (flexes forearm)
Posterior region of arm
Posterior region of forearm
Dorsal aspect of lateral three digits
(except their distal tips)
Brachial Plexus
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Table 16.3
Branches of the Brachial Plexus (continued)
Terminal Branch
Anterior
Motor Innervation
Rami
Ulnar Nerve
C8–T1
Formed from the medial cord, anterior division of the
brachial plexus
Lateral cord
Posterior cord
Medial cord
Ulnar nerve
Anterior forearm muscles
Dorsal and palmar aspects of
(flexors of wrist and digits)
medial 1-1/2 digits (little finger,
Medial half of flexor digitorum medial aspect of ring finger)
profundus
Flexor carpi ulnaris
Intrinsic hand muscles
Hypothenar muscles
Palmar interossei (adduct
fingers)
Dorsal interossei (abduct
fingers)
Adductor pollicis (adducts
thumb)
Medial two lumbricals (flex MP
joints and extend PIP and DIP
joints)
Flexor carpi ulnaris
Flexor digitorum
profundus (medial half)
Adductor
pollicis
Dorsal and
palmar interossei
Hypothenar muscles
Medial two lumbricals
Cutaneous Innervation
Brachial Plexus
Lumbar Plexus
• The left and right lumbar plexuses
are formed from the anterior rami of
spinal nerves L1–L4.
• The lumbar plexus is subdivided into
an anterior division and a posterior
division.
• The main nerve of the posterior
division is the femoral nerve.
• The main nerve of the anterior division
is the obturator nerve.
Lumbar Plexus
Figure 16.10
Lumbar Plexus
Figure 16.10
Lumbar Plexus
Figure 16.10
Lumbar Plexus
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Table 16.4
Branches of the Lumbar Plexus
Main Branch
Anterior
Rami
Femoral Nerve
L2
L3
L4
Anterior thigh muscles
Quadriceps femoris (knee
extensor)
Iliopsoas (hip flexor)
Sartorius (hip and knee flexor)
Femoral nerve
Psoas major
Pectineus1
Sartorius
Pectineus
Iliacus
L2–L4
Motor Innervation
Rectus femoris
Vastus
intermedius
Vastus lateralis
Vastus medialis
Sartorius
Cutaneous Innervation
Anterior thigh
Inferomedial thigh
Medial side of leg
Most medial aspect of foot
Lumbar Plexus
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Obturator Nerve
L2–L4
L2
L3
L4
Medial thigh muscles (adductors
of thigh)
Adductors
Gracilis
Pectineus1
Obturator nerve
Obturator externus
Obturator externus (lateral rotator
of thigh)
Adductor longus
Adductor brevis
Adductor longus
Adductor magnus
1
Gracilis
Pectineus may be innervated by the femoral nerve, obturator nerve, or branches from both nerves
Superomedial thigh
Lumbar Plexus
Sacral Plexus
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The left and right sacral plexuses are
formed from the anterior rami of spinal
nerves L4–S4.
The lumbar and sacral plexuses are
sometimes considered together as the
lumbosacral plexus.
The anterior rami are organized into
an anterior division and a posterior
division.
Sacral Plexus
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The sciatic nerve is the largest
and longest nerve in the sacral plexus
and in the body.
The sciatic nerve is composed of two
divisions wrapped in a common
sheath:
1. the tibial division
2. the common fibular division.
Sacral Plexus
The main branches of the sciatic nerve
are the:
• tibial nerve
• common fibular nerve
• deep fibular nerve
• superficial fibular nerve
Sacral Plexus
Figure 16.11
Sacral Plexus
Figure 16.11
Sacral Plexus
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Gracilis
Biceps femoris
Semimembranosus
Tibial nerve
Semitendinosus
Common fibular
nerve
Gluteus medius
(cut)
Gluteus minimus
Gluteus maximus
(cut)
Superior gluteal
nerve
Inferior gluteal
nerve
Sacrotuberous
ligament
Sciatic nerve
Popliteal artery
Plantaris
Posterior femoral
cutaneous nerve
Pudendal nerve
Gluteus maximus
(cut)
(c) Right gluteal region
Medial sural
cutaneous nerve
Gastrocnemius,
lateral head
Lateral sural
cutaneous nerve
Gastrocnemius,
medial head
(d) Right popliteal region
c,d: © The McGraw-Hill Companies, Inc./Photo and Dissection by Christine Eckel
Figure 16.11
Sacral Plexus
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Table 16.5
Branches of the Sacral Plexus
Main Branch
Anterior Rami
Motor Innervation
Cutaneous Innervation
Sciatic Nerve
(Composed of tibial and common fibular
divisions wrapped in a common sheath)
L4–S3
(See tibial and common fibular
nerves)
(See tibial and common fibular
nerves)
Tibial Nerve
L4–S3
Posterior thigh muscles (extend
thigh and flex leg)
Long head of biceps femoris
Semimembranosus
Semitendinosus
Part of adductor magnus
Posterior leg muscles (plantar
flexors of foot, flexors of knee)
Flexor digitorum longus
Flexor hallucis longus
Gastrocnemius
Soleus
Popliteus
Branches to the heel, and via its
medial and lateral plantar nerve
branches (which supply the sole
of the foot)
L4
L5
S1
S2
S3
Biceps femoris
(long head)
Tibial division
of sciatic nerve
Adductor
magnus
Semitendinosus
Semimembranosus
Tibial nerve
Gastrocnemius
Popliteus
Soleus
Tibialis posterior
Flexor digitorum
longus
Flexor hallucis
longus
Medial plantar
nerve
Lateral plantar
nerve
Tibialis posterior (inverts foot)
Plantar foot muscles (via medial
and lateral plantar nerve
branches)
Sacral Plexus
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Common Fibular Nerve
(Divides into deep fibular and superficial fibular
branches)
L4
L5
S1
S2
S3
Common fibular
division of
sciatic nerve
Biceps femoris
short head
Common fibular
nerve
Fibularis longus
Fibularis brevis
Superficial fibular
nerve
Extensor digitorum
longus
Fibularis tertius
Extensor digitorum
brevis
Tibialis anterior
Deep fibular
nerve
Extensor hallucis
longus
Extensor hallucis
brevis
L4–S2
Short head of biceps femoris
(knee flexor); see also deep
fibular and superficial fibular
nerves
(See deep fibular and superficial
fibular nerves)
Sacral Plexus
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Table 16.5
Branches of the Sacral Plexus (continued)
Main Branch
Anterior Rami
Motor Innervation
Deep Fibular Nerve
L4–S1
Anterior leg muscles (dorsiflex foot, Dorsal interspace between first and
second toes
extend toes)
Tibialis anterior (inverts foot)
Extensor hallucis longus
Extensor digitorum longus
Common fibular
nerve
Fibularis tertius
Dorsum foot muscles (extend toes)
Tibialis anterior
Superficial fibular
nerve
Extensor digitorum
longus
Deep fibular
nerve
Extensor hallucis
longus
Fibularis tertius
Extensor digitorum
brevis
Extensor hallucis
brevis
Extensor hallucis brevis
Extensor digitorum brevis
Cutaneous Innervation
Sacral Plexus
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Superficial Fibular Nerve
L5–S2
Lateral leg muscles (foot evertors
and plantar flexors)
Fibularis longus
Fibularis brevis
Common fibular
nerve
Fibularis longus
Fibularis brevis
Superficial
fibular nerve
Anteroinferior part of leg; most of
dorsum of foot
Reflexes
Reflexes are rapid, automatic, involuntary
reactions of muscles or glands to a stimulus.
• A stimulus is required to initiate a response to
sensory input.
• A rapid response requires that few neurons
be involved and synaptic delay be minimal.
• An automatic response occurs the same way
every time.
• An involuntary response requires no intent or
pre-awareness of the reflex activity.
Reflexes
A reflex arc is the neural wiring of a
single reflex.
• Always begins at a receptor in the PNS
• Communicates with the CNS
• Ends at a peripheral effector
Simple Reflex Arcs
Figure 16.12
Reflexes
A reflex arc may be:
• Ipsilateral—both the receptor and
effector organs are on the same side
• Contralateral—the sensory impulses
from a receptor organ cross over
through the spinal cord to activate
effector organs on the opposite limb
Reflexes
Reflexes may be:
• Monosynaptic—sensory axons
synapse directly on motor neurons,
whose axons project to the effector
• Polysynaptic—more complex
pathways that exhibit a number of
synapses involving interneurons within
the reflex arc
Monosynaptic and
Polysynaptic Reflexes
Figure 16.12
Examples of Spinal Reflexes
•
Withdrawal reflex—polysynaptic
reflex arc
– Painful stimulus causes transmission of
sensory information to the spinal cord.
– Interneurons receive the sensory information
and stimulate the motor neurons to direct
flexor muscles to contract in response.
– Simultaneously, antagonistic extensor
muscles are inhibited so that the traumatized
body part may be quickly withdrawn from the
harmful stimulation.
Examples of Spinal Reflexes
•
Stretch reflex—monosynaptic reflex
arc
– Stretch in a muscle is monitored by a
stretch receptor called the muscle spindle.
– When a stimulus results in the stretching of
a muscle, the muscle reflexively contracts.
Examples of Spinal Reflexes
•
Golgi tendon reflex:
– Golgi tendon organs are nerve endings
located within tendons near a muscle–tendon
junction.
– As a muscle contracts, force is exerted on its
tendon, resulting in increased tension in the
tendon and activation of the Golgi tendon
organ.
– Nerve impulses signal interneurons in the
spinal cord, which in turn inhibit the actions of
the motor neurons.
Stretch Reflexes
Figure 16.14
Golgi Tendon Reflex
Figure 16.15
Spinal Cord Development
Figure 16.16