Download Stretch Reflexes

Marieb Chapter 13 Part C
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Innervation of Skeletal Muscle
• Takes place at a neuromuscular junction
• Neurotransmitter ?
• NT binds to receptors, resulting in:
• Movement of Na+ and K+ across the
membrane
• Depolarization of the muscle cell
• An graded potential, which triggers an action
potential
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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
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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
Innervation of Visceral Muscle and Glands
• Autonomic motor endings and visceral
effectors are simpler than somatic junctions
• Branches form synapses via varicosities
• Acetylcholine and norepinephrine act
indirectly via second messengers
• Visceral motor responses are slower than
somatic responses
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Varicosities
Autonomic
nerve fibers
innervate
most smooth
muscle fibers.
Smooth
muscle
cell
Synaptic
vesicles
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Mitochondrion
Varicosities release
their neurotransmitters
into a wide synaptic
cleft (a diffuse junction).
Figure 9.27
Reflexes
• Inborn (intrinsic) reflex: a rapid, involuntary,
predictable motor response to a stimulus
• Learned (acquired) reflexes result from
practice or repetition,
• Example:
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Reflex Arc
•
Components of a reflex arc (neural path)
—site of stimulus action
1.
—transmits afferent impulses to the CNS
2.
3.
—either monosynaptic or polysynaptic
region within the CNS
4.
—conducts efferent impulses from the
integration center to an effector organ
5.
—muscle fiber or gland cell that responds
to the efferent impulses by contracting or secreting
What is this an example of?
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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
• Why do they exist?
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Stretch and Golgi Tendon Reflexes
• For skeletal muscle activity to be smoothly
coordinated, proprioceptor input is necessary
•
tell muscle length
•
tell muscle and tendon tension
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Secondary sensory
endings
Efferent (motor)
fiber to muscle spindle
Primary sensory
endings
Muscle spindle
Intrafusal muscle
fibers
Sensory fiber
Golgi tendon
organ
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Tendon
Figure 13.15
Muscle Spindles
•
Excited by stretch of muscle and muscle
spindle
•
Stretch causes an increased rate of
impulses in sensory fibers
•
Motor fibers then cause muscle contraction
•
What kind of feedback?
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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
• 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—Sensory
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
• Can you think of another example?
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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
2 The sensory neurons synapse directly with
motor neurons (red), which excite the stretched
by stretch, the associated sensory
muscle. Afferent fibers alsosynapse with
neurons (blue) transmit afferent impulses 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
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)
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
• Too much contraction?
• Contraction 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 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
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+ Excitatory synapse
– Inhibitory synapse
Let’s Combine The Two!
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
Also A Flexor-Crossed Extensor Reflex!
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Superficial Reflexes
• Elicited by gentle cutaneous stimulation
• Depend on central or spinal 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: same as in previous slide
• Response: dorsiflexion of big toe and fanning
of toes
• Present in infants due to incomplete
myelination
• In adults, indicates corticospinal or motor
cortex damage
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Plantar vs. Babinski Reflex
Normal or Abnormal?
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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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