1. Peripheral Nervous system
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2. 13.6 Peripheral Motor Endings2
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. 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. 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. 5 Focus Figure 9.1 neuromuscular junction,
Myelinated axon
of motor neuron
Action
potential (AP)
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
Action potential arrives at axon terminal of motor neuron. 1
Ca2+
Voltage-gated Ca2+
channels open. Ca2+ enters the axon terminal, moving down its electrochemical gradient. 2
Ca2+
Synaptic vesicle
containing ACh
Axon terminal
of motor neuron
Synaptic
cleft
Fusing synaptic
vesicles
Ca2+ entry causes ACh (a neurotransmitter) to be released by exocytosis. 3
ACh
Junctional
folds of
sarcolemma
ACh diffuses across the
synaptic cleft and binds to its receptors on the sarcolemma. 4
Sarcoplasm of
muscle fiber
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. 5
Na+ K+
Postsynaptic membrane
ion channel opens;
ions pass.
ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction. 6
ACh
Degraded ACh
Ion channel closes;
ions cannot pass.
Na+
Acetylcholin-
esterase K+
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6. 13.6 Peripheral Motor Endings
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. 7 Components of a Reflex Arc
Components of a reflex arc (neural path)
Receptor: site of stimulus action
Sensory neuron: transmits afferent impulses to CNS
Integration center: either monosynaptic or polysynaptic region within CNS
Motor neuron: conducts efferent impulses from integration center to effector organ
Effector: muscle fiber or gland cell that responds to efferent impulses by contracting or secreting
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8. 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. 9 Figure 13.15 The five basic components of all reflex arcs.
Stimulus
Unipolar nerve
Skin 1
Interneuron
Receptor 2
Sensory neuron 3
Integration center 4
Motor neuron 5
Effector
Spinal cord
(in cross section)
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10. 10 Table 11.1, pg.393
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11. 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. 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. 13 Figure 13.16 Anatomy of the muscle spindle and tendon organ.
 Efferent (motor)
fiber to muscle spindle
Flower spray endings
(secondary sensory
endings)
α Efferent (motor)
fiber to extrafusal
muscle fibers
Anulospiral
endings (primary
sensory endings)
Extrafusal
muscle fiber
Muscle spindle
Intrafusal
muscle fibers
Capsule (connective
tissue)
Sensory fiber
Tendon organ
Tendon
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14. 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. 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. 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. 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. 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. 19 Focus Figure 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
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. 2
When stretch activates muscle spindles, the
associated sensory neurons (blue) transmit afferent
impulses at higher frequency to the spinal cord. 1
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
(stretched)
Antagonist muscle 3a
Efferent impulses of alpha motor neurons
cause the stretched muscle to contract, which
resists or reverses the stretch.
Efferent impulses of alpha motor neurons to
antagonist muscles are reduced (reciprocal inhibition). 3b
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20. 20 Focus Figure 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)
Tapping the patellar ligament stretches the
quadriceps and excites its muscle spindles. 1
Hamstrings
(flexors)
Patellar ligament
Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons 2
The motor neurons (red) send activating
impulses to the quadriceps causing it to
contract, extending the knee. 3a
The interneurons (green) make inhibitory
synapses with ventral horn neurons (purple)
that prevent the antagonist muscles
(hamstrings) from resisting the contraction
of the quadriceps. 3b
Excitatory synapse
Inhibitory synapse
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