Download 5-2 PNS part 2

5-2 PNS part 2
Spinal Nerves
• 31 pairs of mixed nerves named according
to their point of issue from the spinal cord
– 8 cervical (C1–C8)
– 12 thoracic (T1–T12)
– 5 Lumbar (L1–L5)
– 5 Sacral (S1–S5)
– 1 Coccygeal (C0)
Cervical plexus
Brachial plexus
Cervical
enlargement
Intercostal
nerves
Cervical
nerves
C1 – C8
Thoracic
nerves
T1 – T12
Lumbar
enlargement
Lumbar plexus
Sacral plexus
Cauda equina
Lumbar
nerves
L1 – L5
Sacral nerves
S1 – S5
Coccygeal nerve Co1
Figure 13.6
Spinal Nerves: Roots
• Each spinal nerve connects to the spinal
cord via two roots
• Ventral roots
– Contain motor (efferent) fibers from the ventral
horn motor neurons
– Fibers innervate skeletal muscles)
Spinal Nerves: Roots
• Dorsal roots
– Contain sensory (afferent) fibers from sensory
neurons in the dorsal root ganglia
– Conduct impulses from peripheral receptors
• Dorsal and ventral roots unite to form
spinal nerves, which then emerge from the
vertebral column via the intervertebral
foramina
Gray matter
White matter
Ventral root
Dorsal root
Dorsal root
ganglion
Dorsal ramus
of spinal nerve
Ventral ramus
of spinal nerve
Spinal nerve
Dorsal and
ventral rootlets
of spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
Anterior view showing spinal cord, associated nerves, and vertebrae.
The dorsal and ventral roots arise medially as rootlets and join
laterally to form the spinal nerve.
Figure 13.7 (a)
Spinal Nerves: Rami
• Each spinal nerve branches into mixed
rami
– Dorsal ramus
– Larger ventral ramus
– Meningeal branch
– Rami communicantes (autonomic pathways)
join to the ventral rami in the thoracic region
Spinal Nerves: Rami
• All ventral rami except T2–T12 form
interlacing nerve networks called plexuses
(cervical, brachial, lumbar, and sacral)
• The back is innervated by dorsal rami via
several branches
• Ventral rami of T2–T12 as intercostal nerves
supply muscles of the ribs, anterolateral
thorax, and abdominal wall
Dorsal ramus
Ventral ramus
Spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
Intercostal nerve
Dorsal root
ganglion
Dorsal root
Ventral root
Branches of intercostal
nerve
• Lateral cutaneous
• Anterior cutaneous
Sternum
(b) Cross section of thorax showing the main roots and
branches of a spinal nerve.
Figure 13.7 (b)
Cervical Plexus
• Formed by ventral rami of C1–C4
• Innervates skin and muscles of the neck,
ear, back of head, and shoulders
• Phrenic nerve
– Major motor and sensory nerve of the
diaphragm (receives fibers from C3–C5)
Ventral rami
Segmental
branches
Hypoglossal
nerve (XII)
Lesser occipital
nerve
Greater auricular
nerve
Transverse
cervical nerve
Ansa cervicalis
Ventral
rami:
C1
C2
C3
C4
Accessory nerve (XI)
Phrenic nerve
C5
Supraclavicular
nerves
Figure 13.8
Table 13.3
Brachial Plexus
• Formed by ventral rami of C5–C8 and T1 (and
often C4 and T2)
• It gives rise to the nerves that innervate the
upper limb
• Major branches of this plexus:
–
–
–
–
Roots—five ventral rami (C5–T1)
Trunks—upper, middle, and lower
Divisions—anterior and posterior
Cords—lateral, medial, and posterior
Roots (ventral rami):
C4
C5
Dorsal scapular
Nerve to
subclavius
Suprascapular
Cords
C6
Posterior
divisions
C7
Lateral
C8
Posterior
T1
Upper
Middle
Trunks
Lower
Long thoracic
Medial pectoral
Lateral pectoral
Medial
Axillary
Musculocutaneous
Radial
Upper subscapular
Median
Ulnar
Medial cutaneous
nerves of the arm
and forearm
Lower subscapular
Thoracodorsal
(a) Roots (rami C5 – T1), trunks, divisions, and cords
Anterior
divisions
Posterior
divisions
Trunks
Roots
Figure 13.9 (a)
Anterior
divisions
Posterior
divisions
Major terminal
branches
(peripheral nerves)
Musculocutaneous
Median
Ulnar
Radial
Axillary
Trunks
Cords
Roots
Divisions
Anterior
Lateral
Medial
Posterior
Anterior
Posterior
Posterior
Anterior
Posterior
Trunks
Upper
Roots
(ventral
rami)
C5
C6
Middle
C7
C8
Lower
T1
(d) Flowchart summarizing relationships within the
brachial plexus
Figure 13.9 (d)
Brachial Plexus: Nerves
• Axillary—innervates the deltoid, teres minor, and
skin and joint capsule of the shoulder
• Musculocutaneous—innervates the biceps
brachii and brachialis and skin of lateral forearm
• Median—innervates the skin, most flexors and
pronators in the forearm, and some intrinsic
muscles of the hand
• Ulnar—supplies the flexor carpi ulnaris, part of
the flexor digitorum profundus, most intrinsic
muscles of the hand, and skin of medial aspect of
hand
• Radial—innervates essentially all extensor
Axillary
nerve
Anterior
divisions
Posterior
divisions
Trunks
Roots
Humerus
Radial nerve
Musculocutaneous nerve
Ulna
Radius
Ulnar nerve
Median nerve
Radial nerve (superficial branch)
Dorsal branch of ulnar nerve
Superficial branch of ulnar nerve
Digital branch of ulnar nerve
Muscular branch
Median nerve
Digital branch
(c) The major nerves of the upper limb
Figure 13.9 (c)
Table 13.4
Lumbar Plexus
• Arises from L1–L4
• Innervates the thigh, abdominal wall, and
psoas muscle
• Femoral nerve—innervates quadriceps and
skin of anterior thigh and medial surface of
leg
• Obturator nerve—passes through obturator
foramen to innervate adductor muscles
Ventral rami
Iliohypogastric
Ilioinguinal
Genitofemoral
Lateral femoral
cutaneous
Obturator
Femoral
Lumbosacral
trunk
Ventral
rami:
Iliohypogastric
L1
Ilioinguinal
Femoral
Lateral femoral
L2
cutaneous
Obturator
L3
Anterior femoral
cutaneous
Saphenous
L4
L5
(a) Ventral rami and major branches
of the lumbar plexus
(b) Distribution of the major nerves from
the lumbar plexus to the lower limb
Figure 13.10
Table 13.5
Sacral Plexus
• Arises from L4–S4
• Serves the buttock, lower limb, pelvic structures,
and perineum
• Sciatic nerve
– Longest and thickest nerve of the body
– Innervates the hamstring muscles, adductor magnus,
and most muscles in the leg and foot
– Composed of two nerves: tibial and common fibular
Ventral rami
Ventral rami:
L4
Superior
gluteal
Lumbosacral
trunk
Inferior
gluteal
Common
fibular
Tibial
Posterior
femoral
cutaneous
Pudendal
Sciatic
L5
S1
S2
S3
S4
S5
Co1
Ventral rami and major branches
of the sacral plexus
Figure 13.11 (a)
Superior gluteal
Inferior gluteal
Pudendal
Sciatic
Posterior femoral
cutaneous
Common fibular
Tibial
Sural (cut)
Deep fibular
Superficial fibular
Plantar branches
(b) Distribution of the major nerves from
the sacral plexus to the lower limb
Figure 13.11 (b)
Table 13.6
Innervation of Skin
• Dermatome: the area of skin innervated by
the cutaneous branches of a single spinal
nerve
• All spinal nerves except C1 participate in
dermatomes
• Most dermatomes overlap, so destruction
of a single spinal nerve will not cause
complete numbness
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
C2
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T2
C5
C6
C6
C7
L1
C8
L2
T12
S2
S3
T2
C5
C6
L1
C8
L2
S1
L4
S2
S3
S4
S5
C6
C7
C6
C7
C8
C8
L2
S2
S1
L1
L3
L5
L4
T11
T12
L1
L3
L5
C7
C6
S1 S2
L3
C5
L2
L5
L4
L3
L5
L5
L4
S1
Anterior
view
S1
(b) Posterior
view
L4
L5
L4
L5
S1
Figure 13.12
Innervation of Joints
• Hilton’s law: Any nerve serving a muscle
that produces movement at a joint also
innervates the joint and the skin over the
joint
Motor Endings
• PNS elements that activate effectors by
releasing neurotransmitters
Review of Innervation of
Skeletal Muscle
• Takes place at a neuromusclular junction
• Acetylcholine (ACh) is the neurotransmitter
• ACh binds to receptors, resulting in:
– Movement of Na+ and K+ across the
membrane
– Depolarization of the muscle cell
– An end plate potential, which triggers an action
potential
Myelinated axon
of motor neuron
Action
potential (AP)
Axon terminal of
neuromuscular
junction
Nucleus
1 Action potential arrives
at axon terminal of motor
neuron.
Sarcolemma of
the muscle fiber
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+
Ca2+
3 Ca2+ entry causes
some synaptic vesicles to
release their contents
(acetylcholine)
by exocytosis.
Axon terminal
of motor neuron
ACh
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
Na+
6 ACh effects are
terminated by its
enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase.
Mitochondrion
Synaptic cleft
Fusing synaptic
vesicles
4 Acetylcholine, a
neurotransmitter, diffuses
across the synaptic cleft
and binds to receptors in
the sarcolemma.
5 ACh binding opens ion
channels that allow
simultaneous passage of
Na+ into the muscle fiber
and K+ out of the muscle
fiber.
Synaptic vesicle
containing ACh
ACh
K+
Degraded ACh
Na+
Postsynaptic membrane
ion channel opens;
ions pass.
Postsynaptic membrane
ion channel closed;
ions cannot pass.
K+
Acetylcholinesterase
Figure 9.8
Review of Innervation of
Visceral Muscle and Glands
• Autonomic motor endings and visceral
effectors are simpler than somatic junctions
• Branches form synapses en passant via
varicosities
• Acetylcholine and norepinephrine act
indirectly via second messengers
• Visceral motor responses are slower than
somatic responses
Varicosities
Autonomic
nerve fibers
innervate
most smooth
muscle fibers.
Smooth
muscle
cell
Synaptic
vesicles
Mitochondrion
Varicosities release
their neurotransmitters
into a wide synaptic
cleft (a diffuse junction).
Figure 9.27
Levels of Motor Control
• Segmental level
• Projection level
• Precommand level
Precommand Level
(highest)
• Cerebellum and basal
nuclei
• Programs and instructions
(modified by feedback)
Internal
feedback
Feedback
Projection Level (middle)
• Motor cortex (pyramidal
system) and brain stem
nuclei (vestibular, red,
reticular formation, etc.)
• Convey instructions to
spinal cord motor neurons
and send a copy of that
information to higher levels
Segmental Level (lowest)
• Spinal cord
• Contains central pattern
generators (CPGs)
Sensory
input
Reflex activity
Motor
output
(a) Levels of motor control and their interactions
Figure 13.13a
Segmental Level
• The lowest level of the motor hierarchy
• Central pattern generators (CPGs):
segmental circuits that activate networks of
ventral horn neurons to stimulate specific
groups of muscles
• Controls locomotion and specific, oftrepeated motor activity
Projection Level
• Consists of:
– Upper motor neurons that direct the direct
(pyramidal) system to produce voluntary
skeletal muscle movements
– Brain stem motor areas that oversee the
indirect (extrapyramidal) system to control
reflex and CPG-controlled motor actions
• Projection motor pathways keep higher
command levels informed of what is
happening
Precommand Level
• Neurons in the cerebellum and basal nuclei
– Regulate motor activity
– Precisely start or stop movements
– Coordinate movements with posture
– Block unwanted movements
– Monitor muscle tone
– Perform unconscious planning and discharge
in advance of willed movements
Precommand Level
• Cerebellum
– Acts on motor pathways through projection
areas of the brain stem
– Acts on the motor cortex via the thalamus
• Basal nuclei
– Inhibit various motor centers under resting
conditions
Precommand Level
(highest)
• Cerebellum and basal
nuclei
• Programs and instructions
(modified by feedback)
Internal
feedback
Feedback
Projection Level (middle)
• Motor cortex (pyramidal
system) and brain stem
nuclei (vestibular, red,
reticular formation, etc.)
• Convey instructions to
spinal cord motor neurons
and send a copy of that
information to higher levels
Segmental Level (lowest)
• Spinal cord
• Contains central pattern
generators (CPGs)
Sensory
input
Reflex activity
Motor
output
(a) Levels of motor control and their interactions
Figure 13.13a
Precommand level
• Cerebellum
• Basal nuclei
Projection level
• Primary motor cortex
• Brain stem nuclei
Segmental level
• Spinal cord
(b) Structures involved
Figure 13.13b
Reflexes
• Inborn (intrinsic) reflex: a rapid, involuntary,
predictable motor response to a stimulus
• Learned (acquired) reflexes result from
practice or repetition,
– Example: driving skills
Reflex Arc
•
Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to the
CNS
3. Integration center—either monosynaptic or
polysynaptic region within the CNS
4. Motor neuron—conducts efferent impulses from the
integration center to an effector organ
5. Effector—muscle fiber or gland cell that responds to
the efferent impulses by contracting or secreting
Stimulus
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
Figure 13.14
Spinal Reflexes
• Spinal somatic reflexes
– Integration center is in the spinal cord
– Effectors are skeletal muscle
• Testing of somatic reflexes is important
clinically to assess the condition of the
nervous system
Stretch and Golgi Tendon
Reflexes
• For skeletal muscle activity to be smoothly
coordinated, proprioceptor input is
necessary
– Muscle spindles inform the nervous system of
the length of the muscle
– Golgi tendon organs inform the brain as to the
amount of tension in the muscle and tendons
Muscle Spindles
• Composed of 3–10 short intrafusal muscle
fibers in a connective tissue capsule
• Intrafusal fibers
– Noncontractile in their central regions (lack
myofilaments)
– Wrapped with two types of afferent endings:
primary sensory endings of type Ia fibers and
secondary sensory endings of type II fibers
Muscle Spindles
• Contractile end regions are innervated by
gamma () efferent fibers that maintain
spindle sensitivity
• Note: extrafusal fibers (contractile muscle
fibers) are innervated by alpha () efferent
fibers
Secondary sensory
endings (type II fiber)
Primary sensory
endings (type Ia
fiber)
Muscle spindle
Connective
tissue capsule
Efferent (motor)
fiber to muscle spindle
 Efferent (motor)
fiber to extrafusal
muscle fibers
Extrafusal muscle
fiber
Intrafusal muscle
fibers
Sensory fiber
Golgi tendon
organ
Tendon
Figure 13.15
Muscle Spindles
•
Excited in two ways:
1. External stretch of muscle and muscle
spindle
2. Internal stretch of muscle spindle:
•
•
Activating the  motor neurons stimulates the
ends to contract, thereby stretching the spindle
Stretch causes an increased rate of
impulses in Ia fibers
Muscle
spindle
Intrafusal
muscle fiber
Primary
sensory (la)
nerve fiber
Extrafusal
muscle fiber
Time
Time
(a) Unstretched
muscle. Action
potentials (APs)
are generated at
a constant rate in
the associated
sensory (la) fiber.
(b) Stretched
muscle. Stretching
activates the muscle
spindle, increasing
the rate of APs.
Figure 13.16a, b
Muscle Spindles
• Contracting the muscle reduces tension on
the muscle spindle
• Sensitivity would be lost unless the muscle
spindle is shortened by impulses in the 
motor neurons
• – coactivation maintains the tension and
sensitivity of the spindle during muscle
contraction
Time
Time
(c) Only motor
(d) - Coactivation.
neurons activated.
Both extrafusal and
Only the extrafusal
intrafusal muscle
muscle fibers contract.
fibers contract.
The muscle spindle
Muscle spindle
becomes slack and no
tension is mainAPs are fired. It is
tained and it can
unable to signal further
still signal changes
length changes.
in length.
Figure 13.16c, d
Stretch Reflexes
• Maintain muscle tone in large postural
muscles
• Cause muscle contraction in response to
increased muscle length (stretch)
Stretch Reflexes
• How a stretch reflex works:
– Stretch activates the muscle spindle
– IIa sensory neurons synapse directly with 
motor neurons in the spinal cord
–  motor neurons cause the stretched muscle
to contract
• All stretch reflexes are monosynaptic and
ipsilateral
Stretch Reflexes
• Reciprocal inhibition also occurs—IIa fibers
synapse with interneurons that inhibit the 
motor neurons of antagonistic muscles
• Example: In the patellar reflex, the
stretched muscle (quadriceps) contracts
and the antagonists (hamstrings) relax
Stretched muscle spindles initiate a stretch reflex,
causing contraction of the stretched muscle and
inhibition of its antagonist.
The events by which muscle stretch is damped
1 When muscle spindles are activated
2 The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal fibers
by stretch, the associated sensory
of the stretched muscle. Afferent fibers also
neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor
at higher frequency to the spinal cord.
neurons (purple) controlling antagonistic muscles.
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
3a Efferent impulses of alpha motor neurons
3b Efferent impulses of alpha motor
cause the stretched muscle to contract,
which resists or reverses the stretch.
neurons to antagonist muscles are
reduced (reciprocal inhibition).
Figure 13.17 (1 of 2)
Stretched muscle spindles initiate a stretch reflex,
causing contraction of the stretched muscle and
inhibition of its antagonist.
The events by which muscle stretch is damped
1 When muscle spindles are activated
by stretch, the associated sensory
neurons (blue) transmit afferent impulses
at higher frequency to the spinal cord.
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
Figure 13.17 (1 of 2), step1
Stretched muscle spindles initiate a stretch reflex,
causing contraction of the stretched muscle and
inhibition of its antagonist.
The events by which muscle stretch is damped
1 When muscle spindles are activated
2 The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal fibers
by stretch, the associated sensory
of the stretched muscle. Afferent fibers also
neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor
at higher frequency to the spinal cord.
neurons (purple) controlling antagonistic muscles.
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
Figure 13.17 (1 of 2), step 2
Stretched muscle spindles initiate a stretch reflex,
causing contraction of the stretched muscle and
inhibition of its antagonist.
The events by which muscle stretch is damped
1 When muscle spindles are activated
2 The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal fibers
by stretch, the associated sensory
of the stretched muscle. Afferent fibers also
neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor
at higher frequency to the spinal cord.
neurons (purple) controlling antagonistic muscles.
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
3a Efferent impulses of alpha motor neurons
cause the stretched muscle to contract,
which resists or reverses the stretch.
Figure 13.17 (1 of 2), step 3a
Stretched muscle spindles initiate a stretch reflex,
causing contraction of the stretched muscle and
inhibition of its antagonist.
The events by which muscle stretch is damped
1 When muscle spindles are activated
2 The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal fibers
by stretch, the associated sensory
of the stretched muscle. Afferent fibers also
neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor
at higher frequency to the spinal cord.
neurons (purple) controlling antagonistic muscles.
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
3a Efferent impulses of alpha motor neurons
3b Efferent impulses of alpha motor
cause the stretched muscle to contract,
which resists or reverses the stretch.
neurons to antagonist muscles are
reduced (reciprocal inhibition).
Figure 13.17 (1 of 2), step 3b
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3b The interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
Figure 13.17 (2 of 2)
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
Quadriceps
(extensors)
1
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
+
–
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
Excitatory synapse
Inhibitory synapse
Figure 13.17 (2 of 2), step 1
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
+
–
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
Excitatory synapse
Inhibitory synapse
Figure 13.17 (2 of 2), step 2
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
Figure 13.17 (2 of 2), step 3a
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3b The interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
Figure 13.17 (2 of 2), step 3b
Golgi Tendon Reflexes
• Polysynaptic reflexes
• Help to prevent damage due to excessive
stretch
• Important for smooth onset and termination
of muscle contraction
Golgi Tendon Reflexes
• Produce muscle relaxation (lengthening) in
response to tension
– Contraction or passive stretch activates Golgi tendon
organs
– Afferent impulses are transmitted to spinal cord
– Contracting muscle relaxes and the antagonist
contracts (reciprocal activation)
– Information transmitted simultaneously to the
cerebellum is used to adjust muscle tension
1 Quadriceps strongly
2 Afferent fibers synapse
contracts. Golgi tendon
organs are activated.
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
+ Excitatory synapse
– Inhibitory synapse
3a Efferent impulses
3b Efferent
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension.
impulses to
antagonist
muscle cause
it to contract.
Figure 13.18
1 Quadriceps strongly
contracts. Golgi tendon
organs are activated.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
+ Excitatory synapse
– Inhibitory synapse
Figure 13.18, step 1
1 Quadriceps strongly
2 Afferent fibers synapse
contracts. Golgi tendon
organs are activated.
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
+ Excitatory synapse
– Inhibitory synapse
Figure 13.18, step 2
1 Quadriceps strongly
2 Afferent fibers synapse
contracts. Golgi tendon
organs are activated.
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
3a Efferent impulses
+ Excitatory synapse
– Inhibitory synapse
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension.
Figure 13.18, step 3a
1 Quadriceps strongly
2 Afferent fibers synapse
contracts. Golgi tendon
organs are activated.
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
+ Excitatory synapse
– Inhibitory synapse
3a Efferent impulses
3b Efferent
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension.
impulses to
antagonist
muscle cause
it to contract.
Figure 13.18, step 3b
Flexor and Crossed-Extensor
Reflexes
• Flexor (withdrawal) reflex
– Initiated by a painful stimulus
– Causes automatic withdrawal of the
threatened body part
– Ipsilateral and polysynaptic
Flexor and Crossed-Extensor
Reflexes
• Crossed extensor reflex
– Occurs with flexor reflexes in weight-bearing
limbs to maintain balance
– Consists of an ipsilateral flexor reflex and a
contralateral extensor reflex
• The stimulated side is withdrawn (flexed)
• The contralateral side is extended
+ Excitatory synapse
– Inhibitory synapse
Interneurons
Efferent
fibers
Afferent
fiber
Efferent
fibers
Extensor
inhibited
Flexor
stimulated
Site of stimulus: a noxious
stimulus causes a flexor
reflex on the same side,
withdrawing that limb.
Arm
movements
Flexor
inhibited
Extensor
stimulated
Site of reciprocal
activation: At the
same time, the
extensor muscles
on the opposite
side are activated.
Figure 13.19
Superficial Reflexes
• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and
cord-level reflex arcs
Superficial Reflexes
• Plantar reflex
– Stimulus: stroking lateral aspect of the sole of
the foot
– Response: downward flexion of the toes
– Tests for function of corticospinal tracts
Superficial Reflexes
• Babinski’s sign
– Stimulus: as above
– Response: dorsiflexion of hallux and fanning of
toes
– Present in infants due to incomplete
myelination
– In adults, indicates corticospinal or motor
cortex damage
Superficial Reflexes
• Abdominal reflexes
– Cause contraction of abdominal muscles and
movement of the umbilicus in response to
stroking of the skin
– Vary in intensity from one person to another
– Absent when corticospinal tract lesions are
present