5-2 PNS part 2

1
5-2 PNS part 2
2
Spinal Nerves

• 31 pairs of mixed nerves named according to their
  point of issue from the spinal cord
• 8 cervical (C1C8)
• 12 thoracic (T1T12)
• 5 Lumbar (L1L5)
• 5 Sacral (S1S5)
• 1 Coccygeal (C0)

3
Cervical plexus
Cervical nerves C1 C8
Brachial plexus
Cervical enlargement
Thoracic nerves T1 T12
Intercostal nerves
Lumbar enlargement
Lumbar nerves L1 L5
Lumbar plexus
Sacral plexus
Sacral nerves S1 S5
Cauda equina
Coccygeal nerve Co1
Figure 13.6
4
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)

5
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

6
Gray matter
White matter
Dorsal and ventral rootlets of spinal nerve
Ventral root
Dorsal root
Dorsal root ganglion
Dorsal ramus of spinal nerve
Ventral ramus of spinal nerve
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)
7
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

8
Spinal Nerves Rami

• All ventral rami except T2T12 form interlacing
  nerve networks called plexuses (cervical,
  brachial, lumbar, and sacral)
• The back is innervated by dorsal rami via several
  branches
• Ventral rami of T2T12 as intercostal nerves
  supply muscles of the ribs, anterolateral thorax,
  and abdominal wall

9
Dorsal ramus
Ventral ramus
Spinal nerve
Rami communicantes
Intercostal nerve
Dorsal root ganglion
Sympathetic trunk 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)
10
Cervical Plexus

• Formed by ventral rami of C1C4
• 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 C3C5)

11
Ventral rami
Segmental branches
Hypoglossal nerve (XII)
Ventral rami
Lesser occipital nerve
C1
Greater auricular nerve
C2
Transverse cervical nerve
C3
Ansa cervicalis
C4
Accessory nerve (XI)
C5
Phrenic nerve
Supraclavicular nerves
Figure 13.8
12
Table 13.3
13
Brachial Plexus

• Formed by ventral rami of C5C8 and T1 (and often
  C4 and T2)
• It gives rise to the nerves that innervate the
  upper limb
• Major branches of this plexus
• Rootsfive ventral rami (C5T1)
• Trunksupper, middle, and lower
• Divisionsanterior and posterior
• Cordslateral, medial, and posterior

14
Roots (ventral rami)
C4
Dorsal scapular
C5
Nerve to subclavius
C6
Suprascapular
Upper
Posterior divisions
C7
Middle
Trunks
C8
Lateral
Lower
Cords
T1
Posterior
Long thoracic
Medial pectoral
Medial
Lateral pectoral
Axillary
Upper subscapular
Musculo- cutaneous
Lower subscapular
Thoracodorsal
Radial
Medial cutaneous nerves of the arm and forearm
Median
Ulnar
(a) Roots (rami C5 T1), trunks, divisions, and
cords
Anterior divisions
Posterior divisions
Trunks
Roots
Figure 13.9 (a)
15
Anterior divisions
Posterior divisions
Trunks
Roots
Major terminal branches (peripheral nerves)
Roots (ventral rami)
Cords
Divisions
Trunks
Anterior
Musculocutaneous
C5
Upper
Lateral
Posterior
Median
C6
Medial
Anterior
Ulnar
Middle
C7
Posterior
Radial
C8
Anterior
Posterior
Lower
Axillary
T1
Posterior
(d) Flowchart summarizing relationships within
the brachial plexus
Figure 13.9 (d)
16
Brachial Plexus Nerves

• Axillaryinnervates the deltoid, teres minor, and
  skin and joint capsule of the shoulder
• Musculocutaneousinnervates the biceps brachii
  and brachialis and skin of lateral forearm
• Medianinnervates the skin, most flexors and
  pronators in the forearm, and some intrinsic
  muscles of the hand
• Ulnarsupplies the flexor carpi ulnaris, part of
  the flexor digitorum profundus, most intrinsic
  muscles of the hand, and skin of medial aspect of
  hand
• Radialinnervates essentially all extensor
  muscles, supinators, and posterior skin of limb

17
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)
18
Table 13.4
19
Lumbar Plexus

• Arises from L1L4
• Innervates the thigh, abdominal wall, and psoas
  muscle
• Femoral nerveinnervates quadriceps and skin of
  anterior thigh and medial surface of leg
• Obturator nervepasses through obturator foramen
  to innervate adductor muscles

20
Ventral rami
Ventral rami
Iliohypogastric
L1
Ilioinguinal
Femoral
Lateral femoral cutaneous
L2
Iliohypogastric
Ilioinguinal
Obturator
L3
Genitofemoral
Anterior femoral cutaneous
Lateral femoral cutaneous
Saphenous
L4
Obturator
L5
Femoral
Lumbosacral trunk
(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
21
Table 13.5
22
Sacral Plexus

• Arises from L4S4
• 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

23
Ventral rami
Ventral rami
L4
Superior gluteal
L5
Lumbosacral trunk
S1
Inferior gluteal
S2
Common fibular
Tibial
S3
Posterior femoral cutaneous
S4
Pudendal
S5
Sciatic
Co1
Ventral rami and major branches of the
sacral plexus
Figure 13.11 (a)
24
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)
25
Table 13.6
26
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

27
C2
C3
C2
C4
C3
C5
C6
C4
C7
C8
C5
T1
T2
C5
T1
T3
T2
T4
T3
T5
T4
T2
T2
T6
T5
T7
T6
T8
T9
T7
T10
T8
C6
C6
C5
C5
T11
T9
T12
C7
C7
T10
L1
C6
C6
S1
L2
C8
T11
C8
L3
L5
S2
L4
S3
T12
L1
L1
C6
S4
C6
S5
C7
S2
C7
C8
S3
C8
L2
L2
S1
S2
S1
S2
L1
L3
L3
L2
L5
L5
L4
L4
L3
L5
L5
L4
S1
S1
Anterior view
(b) Posterior view
L4
L4
S1
L5
L5
Figure 13.12
28
Innervation of Joints

• Hiltons law Any nerve serving a muscle that
  produces movement at a joint also innervates the
  joint and the skin over the joint

29
Motor Endings

• PNS elements that activate effectors by releasing
  neurotransmitters

30
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

31
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
Synaptic vesicle containing ACh
Ca2

3
Ca2 entry causes some synaptic vesicles to
release their contents (acetylcholine) by
exocytosis.
Mitochondrion
Axon terminal of motor neuron
Synaptic cleft
Fusing synaptic vesicles
Junctional folds of sarcolemma
4
Acetylcholine, a neurotransmitter, diffuses
across the synaptic cleft and binds to
receptors in the sarcolemma.
ACh
Sarcoplasm of muscle fiber
K
Na
Postsynaptic membrane ion channel opens ions
pass.
5
ACh binding opens ion channels that allow
simultaneous passage of Na into the muscle
fiber and K out of the muscle fiber.
Degraded ACh
ACh
Postsynaptic membrane ion channel closed ions
cannot pass.
Na
6
ACh effects are terminated by its
enzymatic breakdown in the synaptic cleft by
acetylcholinesterase.
K
Acetylcholinesterase
Figure 9.8
32
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

33
Varicosities
Autonomic nerve fibers innervate most
smooth muscle fibers.
Smooth muscle cell
Varicosities release their neurotransmitters into
a wide synaptic cleft (a diffuse junction).
Synaptic vesicles
Mitochondrion
Figure 9.27
34
Levels of Motor Control

• Segmental level
• Projection level
• Precommand level

35
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)
Motor output
Sensory input
Reflex activity
(a) Levels of motor control and their interactions
Figure 13.13a
36
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, oft-repeated
  motor activity

37
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

38
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

39
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

40
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)
Motor output
Sensory input
Reflex activity
(a) Levels of motor control and their interactions
Figure 13.13a
41
Precommand level
Cerebellum
Basal nuclei
Projection level
Primary motor cortex
Brain stem nuclei
Segmental level
Spinal cord
(b) Structures involved
Figure 13.13b
42
Reflexes

• Inborn (intrinsic) reflex a rapid, involuntary,
  predictable motor response to a stimulus
• Learned (acquired) reflexes result from practice
  or repetition,
• Example driving skills

43
Reflex Arc

• Components of a reflex arc (neural path)
• Receptorsite of stimulus action
• Sensory neurontransmits afferent impulses to the
  CNS
• Integration centereither monosynaptic or
  polysynaptic region within the CNS
• Motor neuronconducts efferent impulses from the
  integration center to an effector organ
• Effectormuscle fiber or gland cell that responds
  to the efferent impulses by contracting or
  secreting

44
Stimulus
Skin
Interneuron
1
Receptor
2
Sensory neuron
3
Integration center
4
Motor neuron
Effector
5
Spinal cord (in cross section)
Figure 13.14
45
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

46
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

47
Muscle Spindles

• Composed of 310 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

48
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

49
Secondary sensory endings (type II fiber)
Efferent (motor) fiber to muscle spindle
? Efferent (motor) fiber to extrafusal muscle
fibers
Primary sensory endings (type Ia fiber)
Extrafusal muscle fiber
Muscle spindle
Intrafusal muscle fibers
Connective tissue capsule
Sensory fiber
Golgi tendon organ
Tendon
Figure 13.15
50
Muscle Spindles

• Excited in two ways
• External stretch of muscle and muscle spindle
• 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

51
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
52
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

53
Time
Time
(d) - Coactivation. Both extrafusal and
intrafusal muscle fibers contract.
Muscle spindle tension is main- tained
and it can still signal changes in
length.
(c) Only motor neurons activated. Only
the extrafusal muscle fibers contract.
The muscle spindle becomes slack and no
APs are fired. It is unable to
signal further length changes.



Figure 13.16c, d
54
Stretch Reflexes

• Maintain muscle tone in large postural muscles
• Cause muscle contraction in response to increased
  muscle length (stretch)

55
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

56
Stretch Reflexes

• Reciprocal inhibition also occursIIa 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

57
Stretched muscle spindles initiate a stretch
reflex,causing contraction of the stretched
muscle andinhibition of its antagonist.
The events by which muscle stretch is damped
The sensory neurons synapse directly with
alphamotor neurons (red), which excite
extrafusal fibersof the stretched muscle.
Afferent fibers alsosynapse with interneurons
(green) that inhibit motorneurons (purple)
controlling antagonistic muscles.
2
When muscle spindles are activatedby
stretch, the associated sensoryneurons (blue)
transmit afferent impulsesat higher frequency to
the spinal cord.
1
Sensoryneuron
Cell body ofsensory neuron
Initial stimulus(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
3a
3b
Efferent impulses of alpha motor
neuronscause the stretched muscle to
contract,which resists or reverses the stretch.
Efferent impulses of alpha motorneurons
to antagonist muscles arereduced (reciprocal
inhibition).
Figure 13.17 (1 of 2)
58
Stretched muscle spindles initiate a stretch
reflex,causing contraction of the stretched
muscle andinhibition of its antagonist.
The events by which muscle stretch is damped
When muscle spindles are activatedby
stretch, the associated sensoryneurons (blue)
transmit afferent impulsesat higher frequency to
the spinal cord.
1
Sensoryneuron
Cell body ofsensory neuron
Initial stimulus(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
Figure 13.17 (1 of 2), step1
59
Stretched muscle spindles initiate a stretch
reflex,causing contraction of the stretched
muscle andinhibition of its antagonist.
The events by which muscle stretch is damped
The sensory neurons synapse directly with
alphamotor neurons (red), which excite
extrafusal fibersof the stretched muscle.
Afferent fibers alsosynapse with interneurons
(green) that inhibit motorneurons (purple)
controlling antagonistic muscles.
2
When muscle spindles are activatedby
stretch, the associated sensoryneurons (blue)
transmit afferent impulsesat higher frequency to
the spinal cord.
1
Sensoryneuron
Cell body ofsensory neuron
Initial stimulus(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
Figure 13.17 (1 of 2), step 2
60
Stretched muscle spindles initiate a stretch
reflex,causing contraction of the stretched
muscle andinhibition of its antagonist.
The events by which muscle stretch is damped
The sensory neurons synapse directly with
alphamotor neurons (red), which excite
extrafusal fibersof the stretched muscle.
Afferent fibers alsosynapse with interneurons
(green) that inhibit motorneurons (purple)
controlling antagonistic muscles.
2
When muscle spindles are activatedby
stretch, the associated sensoryneurons (blue)
transmit afferent impulsesat higher frequency to
the spinal cord.
1
Sensoryneuron
Cell body ofsensory neuron
Initial stimulus(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
3a
Efferent impulses of alpha motor
neuronscause the stretched muscle to
contract,which resists or reverses the stretch.
Figure 13.17 (1 of 2), step 3a
61
Stretched muscle spindles initiate a stretch
reflex,causing contraction of the stretched
muscle andinhibition of its antagonist.
The events by which muscle stretch is damped
The sensory neurons synapse directly with
alphamotor neurons (red), which excite
extrafusal fibersof the stretched muscle.
Afferent fibers alsosynapse with interneurons
(green) that inhibit motorneurons (purple)
controlling antagonistic muscles.
2
When muscle spindles are activatedby
stretch, the associated sensoryneurons (blue)
transmit afferent impulsesat higher frequency to
the spinal cord.
1
Sensoryneuron
Cell body ofsensory neuron
Initial stimulus(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
3a
3b
Efferent impulses of alpha motor
neuronscause the stretched muscle to
contract,which resists or reverses the stretch.
Efferent impulses of alpha motorneurons
to antagonist muscles arereduced (reciprocal
inhibition).
Figure 13.17 (1 of 2), step 3b
62
The patellar (knee-jerk) reflexa specific
example of a stretch reflex
2
Quadriceps(extensors)
3a
3b
3b
1
Patella
Musclespindle
Spinal cord(L2L4)
Tapping the patellar ligament
excitesmuscle spindles in the quadriceps.
1
Hamstrings(flexors)
Patellarligament
2
Afferent impulses (blue) travel to
thespinal cord, where synapses occur withmotor
neurons and interneurons.
3a
The motor neurons (red) sendactivating
impulses to the quadricepscausing it to
contract, extending theknee.
Excitatory synapseInhibitory synapse

3b
The interneurons (green) makeinhibitory
synapses with ventral horn neurons (purple) that
prevent theantagonist muscles (hamstrings)
fromresisting the contraction of thequadriceps.
Figure 13.17 (2 of 2)
63
The patellar (knee-jerk) reflexa specific
example of a stretch reflex
Quadriceps(extensors)
1
Patella
Musclespindle
Spinal cord(L2L4)
Tapping the patellar ligament
excitesmuscle spindles in the quadriceps.
1
Hamstrings(flexors)
Patellarligament
Excitatory synapseInhibitory synapse

Figure 13.17 (2 of 2), step 1
64
The patellar (knee-jerk) reflexa specific
example of a stretch reflex
2
Quadriceps(extensors)
1
Patella
Musclespindle
Spinal cord(L2L4)
Tapping the patellar ligament
excitesmuscle spindles in the quadriceps.
1
Hamstrings(flexors)
Patellarligament
2
Afferent impulses (blue) travel to
thespinal cord, where synapses occur withmotor
neurons and interneurons.
Excitatory synapseInhibitory synapse

Figure 13.17 (2 of 2), step 2
65
The patellar (knee-jerk) reflexa specific
example of a stretch reflex
2
Quadriceps(extensors)
3a
1
Patella
Musclespindle
Spinal cord(L2L4)
Tapping the patellar ligament
excitesmuscle spindles in the quadriceps.
1
Hamstrings(flexors)
Patellarligament
2
Afferent impulses (blue) travel to
thespinal cord, where synapses occur withmotor
neurons and interneurons.
3a
The motor neurons (red) sendactivating
impulses to the quadricepscausing it to
contract, extending theknee.
Excitatory synapseInhibitory synapse

Figure 13.17 (2 of 2), step 3a
66
The patellar (knee-jerk) reflexa specific
example of a stretch reflex
2
Quadriceps(extensors)
3a
3b
3b
1
Patella
Musclespindle
Spinal cord(L2L4)
Tapping the patellar ligament
excitesmuscle spindles in the quadriceps.
1
Hamstrings(flexors)
Patellarligament
2
Afferent impulses (blue) travel to
thespinal cord, where synapses occur withmotor
neurons and interneurons.
3a
The motor neurons (red) sendactivating
impulses to the quadricepscausing it to
contract, extending theknee.
Excitatory synapseInhibitory synapse

3b
The interneurons (green) makeinhibitory
synapses with ventral horn neurons (purple) that
prevent theantagonist muscles (hamstrings)
fromresisting the contraction of thequadriceps.
Figure 13.17 (2 of 2), step 3b
67
Golgi Tendon Reflexes

• Polysynaptic reflexes
• Help to prevent damage due to excessive stretch
• Important for smooth onset and termination of
  muscle contraction

68
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

69
1
2
Quadriceps strongly contracts. Golgi tendon
organs are activated.
Afferent fibers synapse with interneurons in
the spinal cord.
Interneurons
Quadriceps (extensors)
Spinal cord
Golgi tendon organ
Hamstrings (flexors)
3b
3a
Efferent impulses to antagonist muscle cause
it to contract.
Efferent impulses to muscle with stretched
tendon are damped. Muscle relaxes, reducing
tension.

Excitatory synapse

Inhibitory synapse
Figure 13.18
70
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
71
1
2
Quadriceps strongly contracts. Golgi tendon
organs are activated.
Afferent fibers synapse 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
72
1
2
Quadriceps strongly contracts. Golgi tendon
organs are activated.
Afferent fibers synapse with interneurons in
the spinal cord.
Interneurons
Quadriceps (extensors)
Spinal cord
Golgi tendon organ
Hamstrings (flexors)
3a
Efferent impulses to muscle with stretched
tendon are damped. Muscle relaxes, reducing
tension.

Excitatory synapse

Inhibitory synapse
Figure 13.18, step 3a
73
1
2
Quadriceps strongly contracts. Golgi tendon
organs are activated.
Afferent fibers synapse with interneurons in
the spinal cord.
Interneurons
Quadriceps (extensors)
Spinal cord
Golgi tendon organ
Hamstrings (flexors)
3b
3a
Efferent impulses to antagonist muscle cause
it to contract.
Efferent impulses to muscle with stretched
tendon are damped. Muscle relaxes, reducing
tension.

Excitatory synapse

Inhibitory synapse
Figure 13.18, step 3b
74
Flexor and Crossed-Extensor Reflexes

• Flexor (withdrawal) reflex
• Initiated by a painful stimulus
• Causes automatic withdrawal of the threatened
  body part
• Ipsilateral and polysynaptic

75
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

76
Excitatory synapse
Inhibitory synapse
Interneurons
Efferent fibers
Afferent fiber
Efferent fibers
Extensor inhibited
Flexor inhibited
Arm movements
Flexor stimulated
Extensor stimulated
Site of reciprocal activation At the same time,
the extensor muscles on the opposite side are
activated.
Site of stimulus a noxious stimulus causes a
flexor reflex on the same side, withdrawing that
limb.
Figure 13.19
77
Superficial Reflexes

• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and cord-level
  reflex arcs

78
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

79
Superficial Reflexes

• Babinskis 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

80
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