Download The Nervous System Reflexes Spinal Reflexes Reflex Arc The

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Reflexes
• Rapid, involuntary, predictable motor response
to a stimulus
The Nervous System
Spinal Reflexes
Spinal Reflexes
Reflex Arc
• Components of a reflex arc
• 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
• Identical stimulus should always elicit the same
response stereotyped reflex
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to the
CNS
3. Synapses in gray matter—either monosynaptic or
polysynaptic region within the CNS
4. Motor neuron—conducts efferent impulses away
from cord
5. Effector—muscle fiber or gland cell that responds to
the efferent impulses by contracting or secreting
The Stretch Reflex
Stimulus
Skin
• Monosynaptic reflex
– 2 neurons (sensory and motor), 1 synapse
1 Receptor
Interneuron
• Muscle spindles
2 Sensory neuron
– Sensory receptors in belly of muscle
– Detects changes in length of muscle
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
• Muscle is stretched, reflex reverses the stretch
• Important for coordination, maintenance of
posture, keeps muscles from over stretching
Figure 13.14
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Secondary sensory
endings (type II fiber –
senses when muscle
is still)
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
3a
1
Muscle
spindle
Primary sensory
endings (type Ia
Fiber – senses
stretching)
Spinal cord
(L2–L4)
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.
3aThe motor neurons (red) send
+
–
α Efferent (motor)
fiber to extrafusal
muscle fibers
Extrafusal muscle
fiber
3b
3b
Patella
Hamstrings
(flexors)
Efferent (motor)
fiber to muscle spindle
Muscle spindle
Intrafusal muscle
fibers
Connective
tissue capsule
activating impulses to the quadriceps
causing it to contract, extending the
knee.
Excitatory synapse
Inhibitory synapse
Sensory fiber
Golgi tendon
Organ – senses
changes in
muscle tension
Figure 13.17 (2 of 2)
Reciprocal Inhibition
Tendon
Figure 13.15
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
• When a muscle is stretched, its antagonist is
flexing
• To avoid damage, it must be turned off in the
reflex
• Additional connection sends inhibitory signal to
the antagonist of the stretched muscle
– Polysynaptic
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
+
–
Patellar
ligament
Excitatory synapse
Inhibitory synapse
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.
3aThe motor neurons (red) send
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3bThe interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
Figure 13.17 (2 of 2)
Withdrawal Reflex
• Polysynaptic
• Polysegmental
– Association neurons
ascend and descend
spinal cord
– Synapse with motor
neurons in other spinal
nerves
Crossed-Extensor Reflex
• 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
• Quickly withdraws
(flexes) threatened limb
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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.
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
3