Download Stretch reflex

Peripheral Nervous system
Chapter 13 Part D
Peripheral
Nervous
System
© Annie Leibovitz/Contact Press Images
© 2016 Pearson Education, Inc.
PowerPoint® Lecture Slides
prepared by
Karen Dunbar Kareiva
Ivy Tech Community College
2
13.6 Peripheral Motor Endings
• Motor endings: PNS elements that activate
effectors by releasing neurotransmitters
• These element innervate skeletal muscle,
visceral muscle, and glands
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3 Innervation of Skeletal Muscle
• Takes place at neuromuscular junction
• Neurotransmitter acetylcholine (ACh) is
released when nerve impulse reaches axon
terminal
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4 Innervation of Skeletal Muscle (cont.)
• ACh binds to receptors, resulting in:
– Movement of Na+ and K+ across membrane
– Depolarization of muscle cell
– An end plate potential, spreads to adjacent areas
of sarcolemma, which triggers opening of Na+
voltage-gated channels
– Results in an action potential, which leads to
muscle contraction
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5 Focus Figure 9.1 neuromuscular junction,
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Action
potential (AP)
Sarcolemma of
the muscle fiber
1 Action potential arrives at
axon terminal of motor neuron.
2 Voltage-gated Ca2+
channels open. Ca2+ enters the
axon terminal, moving down its
electrochemical gradient.
Ca2+
Ca2+
Axon terminal
of motor neuron
Fusing synaptic
vesicles
3 Ca2+ entry causes ACh (a
neurotransmitter) to be released
by exocytosis.
ACh
4 ACh diffuses across the
synaptic cleft and binds to its
receptors on the sarcolemma.
Na+ K+
ACh
Degraded ACh
Na+
Acetylcholinesterase
K+
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Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
5 ACh binding opens ion channels in
the receptors that allow simultaneous
passage of Na + into the muscle fiber and
K+ out of the muscle fiber. More Na+ ions
enter than K+ ions exit, which produces a
local change in the membrane potential
called the end plate potential.
6 ACh effects are terminated by
its breakdown in the synaptic
cleft by acetylcholinesterase and
diffusion away from the junction.
Synaptic vesicle
containing ACh
Synaptic
cleft
Postsynaptic membrane
ion channel opens;
ions pass.
Ion channel closes;
ions cannot pass.
6 Part 4 – Reflex Activity
13.6 Peripheral Motor Endings
• Inborn (intrinsic) reflex: rapid, involuntary,
predictable motor response to stimulus
– Examples: maintain posture, control visceral
activities
– Can be modified by learning and conscious effort
• Learned (acquired) reflexes result from practice
or repetition – cerebellum, premotor cortex
– Example: driving skills, basketball etc.
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7 Components of a Reflex Arc
• Components of a reflex arc (neural path)
1. Receptor: site of stimulus action
2. Sensory neuron: transmits afferent impulses
to CNS
3. Integration center: either monosynaptic or
polysynaptic region within CNS
4. Motor neuron: conducts efferent impulses
from integration center to effector organ
5. Effector: muscle fiber or gland cell that
responds to efferent impulses by contracting or
secreting
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8 Components of a Reflex Arc (cont.)
• Reflexes are classified functionally as:
– Somatic reflexes***
• Activate skeletal muscle
– Autonomic (visceral) reflexes
• Activate visceral effectors (smooth or cardiac muscle
or glands)
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9 Figure 13.15 The five basic components of all reflex arcs.
Stimulus
Unipolar nerve
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
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10 Table 11.1, pg.393
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11
13.9 Spinal Reflexes
• Spinal reflexes occur without direct
involvement of higher brain centers
– Brain is still advised of spinal reflex activity and
may have an effect on the reflex
• Testing of somatic reflexes important clinically
to assess condition of nervous system
– If exaggerated, distorted, or absent, may indicate
degeneration or pathology of specific nervous
system regions
– Most commonly assessed reflexes are stretch,
flexor, and superficial reflexes
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12 Stretch and Tendon Reflexes
• To smoothly coordinate skeletal muscle,
nervous system must receive proprioceptor
input regarding:
– Length of muscle
• Information sent from muscle spindles
– Amount of tension in muscle
• Information sent from tendon organs
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13 Figure 13.16 Anatomy of the muscle spindle and tendon organ.
 Efferent (motor)
fiber to muscle spindle
Flower spray endings
(secondary sensory
endings)
Anulospiral
endings (primary
sensory endings)
Muscle spindle
Capsule (connective
tissue)
 Efferent (motor)
fiber to extrafusal
muscle fibers
Extrafusal
muscle fiber
Intrafusal
muscle fibers
Sensory fiber
Tendon organ
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Tendon
14 Stretch and Tendon Reflexes (cont.)
• Functional anatomy of muscle spindles
(cont.)
• Muscle spindles are stretched (and excited) in
two ways
– External stretch: external force lengthens entire
muscle
– Internal stretch:  motor neurons stimulate
spindle ends to contract, thereby stretching
spindle
• Stretching results in increased rate of impulses
to spinal cord
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15 Stretch and Tendon Reflexes (cont.)
• Stretch reflex
– Brain sets muscle’s length via stretch reflex
– Example: knee-jerk reflex is a stretch reflex that
keeps knees from buckling when you stand
upright
– Stretch reflexes maintain muscle tone in large
postural muscles and adjust it reflexively
• Causes muscle contraction on side of spine in
response to increased muscle length (stretch) on
other side of spine
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16 Stretch and Tendon Reflexes (cont.)
• Stretch reflex (cont.)
– How stretch reflex works:
• Stretch activates muscle spindle
• Sensory neurons synapse directly with  motor
neurons in spinal cord
•  motor neurons cause extrafusal muscles of
stretched muscle to contract
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17 Stretch and Tendon Reflexes (cont.)
• Stretch reflex (cont.)
– Reciprocal inhibition also occurs—afferent
fibers synapse with interneurons that inhibit 
motor neurons of antagonistic muscles
• Example: In patellar reflex, stretched muscle
(quadriceps) contracts, and antagonists (hamstrings)
relax
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18 Clinical – Homeostatic Imbalance 13.9
• Stretch reflexes can be hypoactive or absent if
peripheral nerve damage or ventral horn injury
has occurred
– Reflexes are absent in people with chronic
diabetes mellitus or neurosyphilis and during
coma
• Stretch reflexes can be hyperactive if lesions of
corticospinal tract reduce inhibitory effect of
brain on spinal cord
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19 Focus Figure 13.1-1 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 stretch activates muscle spindles, the
associated sensory neurons (blue) transmit afferent
impulses at higher frequency to the spinal cord.
Sensory
neuron
2 The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal fibers of
the stretched muscle. Sensory fibers also synapse with
interneurons (green) that inhibit motor neurons (purple)
controlling antagonistic muscles.
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
(stretched)
Antagonist muscle
3a Efferent impulses of alpha motor neurons
3b Efferent impulses of alpha motor neurons to
cause the stretched muscle to contract, which
resists or reverses the stretch.
antagonist muscles are reduced (reciprocal inhibition).
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20 Focus Figure 13.1-2 Stretched muscle spindles initiate a stretch
reflex, causing contraction of the stretched muscle and inhibition of its
antagonist.
The patellar (knee-jerk) reflex—an example of a stretch reflex
2
Quadriceps
(extensors)
3a
3b
3b
1
Patella
Muscle
spindle
(stretched)
Spinal cord
(L2–L4)
1 Tapping the patellar ligament stretches the
quadriceps and excites its muscle spindles.
Hamstrings
(flexors)
Patellar ligament
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons
3a The motor neurons (red) send activating
impulses to the quadriceps causing it to
contract, extending the knee.
3b The interneurons (green) make inhibitory
Excitatory synapse
Inhibitory synapse
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synapses with ventral horn neurons (purple)
that prevent the antagonist muscles
(hamstrings) from resisting the contraction
of the quadriceps.