Download PNS: Reflexes

Peripheral Nervous System & Reflex
Activity
Part D: Motor Control & Reflexes
Prepared by Janice Meeking & W. Rose.
Figures from Marieb & Hoehn 8th , 9th eds.
Portions copyright Pearson Education
Levels of Motor Control
• Segmental level
• Projection level
• Precommand level
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
Copyright © 2010 Pearson Education, Inc.
Figure 13.13a
Precommand level
• Cerebellum
• Basal nuclei
Projection level
• Primary motor cortex
• Brain stem nuclei
Segmental level
• Spinal cord
(b) Structures involved
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Figure 13.13b
Reflexes
• Inborn (intrinsic) reflex: rapid, involuntary,
predictable motor response to a stimulus
• Learned (acquired) reflex: requires practice
and/or repetition
• Driving
• Sports
Stimulus
Components of a reflex arc
(neural path)
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
Mediated by spinal cord
• Regulated by the brain
• Work (but abnormally) even in spinal cord injury
patients
• Effectors are skeletal muscle
• Examples: Stretch, Golgi tendon, flexor, crossed
extensor, cutaneous
Spinal reflex testing an important part of a
clinical neurological exam
Stretch and Golgi Tendon Reflexes
• Help coordinate muscle activity
• Require proprioceptive input
• Muscle spindles provide muscle length
information
• Golgi tendon organs provide muscle and
tendon force information
Stretch Reflex
• Maintains muscle tone in large postural muscles
• Muscle lengthening causes contraction of stretched
muscle, relaxation of antagonist
• Stretch activates muscle spindle
• IIa sensory neurons make excitatory synapses onto 
motor neurons in spinal cord
•  motor neurons cause stretched muscle to contract
• Stretch reflex is monosynaptic and ipsilateral
Sensors for the Stretch Reflex: Muscle Spindles
• 3–10 short modified (intrafusal) muscle fibers in a
connective tissue capsule
• Noncontractile in central region (no myofilaments)
• Wrapped with two types of afferent endings: primary sensory
endings of type Ia fibers and secondary sensory endings of type
II fibers
• Contractile end regions innervated by gamma () efferent fibers
that maintain spindle sensitivity
• Note: extrafusal fibers (regular muscle fibers) are
innervated by alpha () efferent fibers
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
Sensors for the Stretch Reflex: Muscle Spindles
• Excited by stretch, which could be caused by:
1. External stretch of muscle and muscle spindle
2. Internal stretch of muscle spindle due to activation of 
motor neurons, stimulating ends to contract, thereby
stretching spindle
• Stretch causes an increased rate of impulses
in Ia fibers
• – coactivation maintains tension and
sensitivity of spindle during muscle
contraction
Muscle
spindle
Intrafusal
muscle fiber
Primary
sensory (la)
nerve fiber
Extrafusal
muscle fiber
Time
Time
Time
(a) Unstretched
muscle. Action
potentials (APs)
are generated at
a constant rate in
the associated
sensory (la) fiber.
(b) Stretched
muscle. Stretching
activates the muscle
spindle, increasing
the rate of APs.
(c) Only motor
(d) - Coactivation.
neurons activated.
Both extrafusal and
Only the extrafusal
intrafusal muscle
muscle fibers contract.
fibers contract.
The muscle spindle
Muscle spindle
becomes slack and no
tension is mainAPs are fired. It is
tained and it can
unable to signal further
still signal changes
length changes.
in length.
Copyright © 2010 Pearson Education, Inc.
Time
Figure 13.16a, b
Stretch Reflex Example: Patellar (knee-jerk) 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.
Copyright © 2010 Pearson Education, Inc.
Figure 13.17 (2 of 2)
Golgi Tendon Reflex
• Only kicks in when force is large. May act to
prevent muscle tearing due to excessive force.
• Some evidence for a role in normal muscle
coordination too.
• When tendon stretches, this reflex causes
muscle to relax & antagonist to contract
• Opposite to stretch reflex response to
lengthening
• Polysynaptic
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: ipsilateral, polysynaptic
• Painful stimulus causes automatic withdrawal of the
threatened body part
• Crossed extensor reflex: contralateral; polysynaptic
• Occurs with flexor reflex in weight-bearing limbs to
maintain balance
• Contralateral extension while ipsi side flexes
+ 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.
Copyright © 2010 Pearson Education, Inc.
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 (cutaneous) reflexes
• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and cord-level
reflex arcs
• Plantar
• Abdominal
Plantar reflex
• Stimulus: stroke lateral aspect of sole of foot
• Normal response: downward flexion of toes
• Tests for function of corticospinal tracts
• Babinski’s sign: abnormal response
– Hallux dorsiflexes, smaller toes fan laterally
– Normally in infants <1 y.o. due to incomplete myelination
– In adults, indicates corticospinal or motor cortex damage
Reflex Testing
Normal Babinski:
http://library.med.utah.edu/neurologicexam/html/motor_normal.html#10
Normal Babinski (infant): http://video.google.com/videoplay?docid=3102473882446365023&pr=goog-sl
Positive Babinski (adult):
http://www.youtube.com/watch?v=bWKTrUjxkqs
Movies from the Neurologic Exam and PediNeurologic Exam websites by Paul D. Larsen, M.D., University of Nebraska
Medical Center and Suzanne S. Stensaas, Ph.D., University of Utah School of Medicine. Additional materials for Neurologic
Exam are drawn from resources provided by Alejandro Stern, Stern Foundation, Buenos Aires, Argentina; Kathleen Digre,
M.D., University of Utah; and Daniel Jacobson, M.D., Marshfield Clinic, Wisconsin.
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