Transcript Muscle Spindles

PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
13
The Peripheral
Nervous
System and
Reflex Activity:
Part D
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Motor Endings
• PNS elements that activate effectors by
releasing neurotransmitters
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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
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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
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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
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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
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Projection Level
• Consists of:
• Upper motor neurons that direct the
(pyramidal) system to produce voluntary
skeletal muscle movements
• Brain stem motor areas that oversee the
indirect (extra pyramidal) system to control
reflex and CPG-controlled motor actions
• Projection motor pathways keep higher
command levels informed of what is
happening
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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
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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
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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
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Figure 13.13a
Reflexes
• Inborn (intrinsic) reflex: a rapid, involuntary,
predictable motor response to a stimulus
• Learned (acquired) reflexes result from
practice or repetition,
• Example: driving skills
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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
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Stimulus
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
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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
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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
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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
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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
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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
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Stretch Reflexes
• Maintain muscle tone in large postural
muscles
• Cause muscle contraction in response to
increased muscle length (stretch)
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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
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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
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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
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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
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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.
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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).
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Figure 13.17 (1 of 2), step 3b
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
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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
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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
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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.
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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
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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
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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
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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
Flexor and Crossed-Extensor Reflexes
• Flexor (withdrawal) reflex
• Initiated by a painful stimulus
• Causes automatic withdrawal of the
threatened body part
• Ipsilateral and polysynaptic
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Flexor and Crossed-Extensor Reflexes
• Crossed extensor reflex
• Occurs with flexor reflexes in weightbearing 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
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+ 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.
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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 cordlevel reflex arcs
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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
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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
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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
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