Download Integrative Physiology I: Control of Body Movement

Chapter 13
Integrative Physiology I:
Control of Body Movement
(in the syllabus as Chapter 13: Reflex and Motor
Control)
For Friday, start with slide #35
Exam 3 will be on Monday November 21
Will cover chapters 11, 12, 13
May cover more, depends on how far we get
Neural Reflexes
(Table 13-1, p. 447)
All neural reflexes begin with a stimulus
Stimulus activates a receptor
Sends a message to the CNS
Efferent neurons bring a response to the stimulus
The efferent response goes out to an effector
An effector is either a muscle or a gland
Neural pathways can have negative feedback and or a
feed forward component
Negative feedback
– Signals from muscle and joint receptors
continuously inform the CNS of changing body
position
Feed forward
– Body anticipates a stimulus and begins the
response
– Example: bracing yourself in anticipation of a
collision
Classification of Neural Reflexes
1. Classified by the efferent division of the nervous
system that controls the response
2. Classified by the CNS location where the reflex is
integrated
3. Classified by whether the reflex is innate or learned
4. Classified by the number of neurons in the reflex
pathway
Classification of Neural Reflexes
1. Classified by the efferent division of the nervous
system that controls the response
– Somatic reflexes
• Involve somatic motor neurons and skeletal
muscles
– Autonomic reflexes
• Response is controlled by autonomic neurons
2. Classified by the CNS location where the reflex is
integrated
– Spinal Reflexes
• Integrated in the spinal cord
– Cranial Reflexes
• Integrated in the brain
3. Classified by whether the reflex is innate or learned
– Innate Reflex
• Genetically determined
• We are “born with them”
• Example: knee-jerk reflex
– Lower leg kicks when patellar tendon is
tapped
– Learned Reflex (Conditioned Reflex)
• Acquired through experience
• Example: Pavlov's dogs
– The dogs learned to salivate when a bell was
rung
4. Classified by the number of neurons in the reflex
pathway
Monosynaptic Pathway (fig. 13-1a)
– Only somatic motor reflexes are monosynaptic
– Monosynaptic reflexes have only 2 neurons with
one synapse between them
• One afferent (sensory) and one efferent neuron
– The 2 neurons synapse in the spinal cord
Figure 13-1a
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Polysynaptic Pathway (fig. 13-1b)
– 3 or more neurons and at least 2 synapses
– Can be quite complex with extensive branching in
the CNS
• The branching forms networks of multiple
interneurons within the CNS
Figure 13-1b
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– Divergence of pathways (fig. 8-25, p. 282) allows a
single stimulus to affect multiple targets
– Convergence of pathways integrates the input
from multiple sources to modulate the response
Figure 8-25a
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Figure 8-25b
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Autonomic (Visceral) Reflexes
All are polysynaptic
Many are characterized by tonic activity
– A continuous stream of action potentials that
creates ongoing activity in the effector
• Example: tonic control of blood vessels –a
continuously active autonomic reflex
Figure 13-2
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Autonomic (Visceral) Reflexes
Autonomic spinal reflexes
– Urination
– Defecation, etc.
Spinal reflexes can be modulated in the brain also
– Example; urination can be voluntarily initiated or
can be voluntarily inhibited (“bashful bladder
syndrome”)
Higher control of a spinal reflex is a learned response
– Example: toilet training
Autonomic (Visceral) Reflexes
Autonomic reflexes integrated in the brain
– Primarily in the
• Hypothalamus
• Thalamus
• Brain Stem
– These (above) contain centers which coordinate
body functions needed to maintain homeostasis
• Heart rate, blood pressure, breathing, eating,
water balance, maintenance of body
temperature
Autonomic (Visceral) Reflexes
The limbic system in the brain
– Site of “primitive drives” such as sex, fear, rage,
aggression, hunger, etc.
– Can convert emotional stimuli into visceral,
emotionally-driven reflexes:
• “gut feelings”
• “butterflies in the stomach”
Autonomic (Visceral) Reflexes
Other emotion-linked visceral responses include:
– Urination
– Defecation
– Blushing
– Blanching (turning pale)
– Piloerection
• “I was so scared my hair stood on end”
• Arrector pili muscles in each hair follicle pull on
the hair shaft and make the hair stand up
Cross-section of skin
showing arrector pili
muscle (smooth muscle)
Human arm, showing hair
standing on end
due to cold or fright
Hyena, showing aggression
Chimp, showing aggression
Skeletal Muscle Reflexes
These are involved in almost everything we do
Proprioceptors sense changes in joint movements,
muscle tension, and muscle length
– Muscle spindles
– Golgi tendon organs
– Joint receptors
These receptors send the information to the CNS,
which responds with a signal to either
– Contract
– Or, inhibit contraction
Somatic motor neurons send only one signal to a
muscle: CONTRACT
In order to relax a muscle, sensory input activates
inhibitory interneurons in the CNS
The interneurons inhibit the activity of the somatic
motor neuron
Relaxation of the muscle then results from the
absence of excitatory input (from the somatic motor
neuron)
Skeletal Muscle Reflex Pathway
Proprioceptors are located in:
– Skeletal muscles
– Joint capsules
– ligaments
Proprioceptors monitor:
– Position of limbs in space
– Our movements
– Effort exerted in lifting objects
Skeletal Muscle Reflex Pathway
Sensory neurons carry the information from
proprioceptors into the CNS
CNS integrates this signal and acts on it
In a reflex, this is done subconsciously
Skeletal Muscle Reflex Pathway
Somatic motor neurons carry the output signal
The neurons which innervate skeletal muscle
contractile fibers are called alpha motor neurons
(fig. 13-3a)
The effectors for the alpha motor neurons are the
contractile skeletal muscle fibers, now called extrafusal
muscle fibers
Figure 13-3a
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Proprioceptors
Muscle Spindles (p. 451-453)
Golgi Tendon Organs (p. 453-454)
Joint Receptors (p. 451)
Proprioceptors
Joint Receptors (p. 451)
Found in the capsules and ligaments around joints
Stimulated by mechanical distortion
The distortion comes from changes in the relative
positioning of bones linked by flexible joints
Sensory information from joint receptors is integrated
primarily in the cerebellum
Muscle Spindles (p. 451-453)
– Located inside the skeletal muscle
– Sensory output activates muscle reflexes
These are stretch receptors
– Detect changes in muscle length
– Every skeletal muscle has lots of these (except for
one of the jaw muscles)
• Example: newborn human, 1 muscle in the
index finger has approximately 50 muscle
spindles
Muscle spindle structure:
– Small, elongate
– Scattered among and arranged parallel to the
contractile extrafusal muscle fibers
– Each spindle is made of:
• Intrafusal fibers wrapped in a connective tissue
capsule
Intrafusal fibers are modified muscle fibers:
– Ends are contractile
– Contractile ends are innervated by gamma
motor neurons
– Central region lacks myofibrils and is
wrapped by sensory neurons which are
stimulated by stretch
– These neurons project to the spinal cord and
synapse directly on alpha motor neurons
innervating the muscle in which the muscle
spindles are located
Figure 12-3c
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Figure 13-3
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Figure 13-3b
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Muscle Tone
When a muscle is at resting length, the central region
of each muscle spindle is stretched just enough to
activate the sensory fibers wrapped around it
As a result, sensory neurons from the spindles are
tonically active, sending a steady stream of APs to the
CNS
Because of this tonic activity, even a muscle at rest
maintains a certain level of tension, known as muscle
tone
Figure 13-4a
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Stretch Reflex
Muscle spindles are anchored in parallel to the
extrafusal muscle fibers
Any movement that increases length also stretches
the muscle spindles, causing their sensory fibers to fire
more rapidly
This creates a reflex contraction of the muscle, which
prevents damage from overstretching
Figure 13-4b
Copyright © 2010 Pearson Education, Inc.
When a resting muscle contracts and shortens, this
releases tension on the muscle spindle capsule
At the same time, the gamma motor neurons fire,
which causes the ends of the intrafusal fibers to
contract and shorten
The simultaneous firing is called Alpha-Gamma Coactivation
Contraction of the spindle end lengthens the central
region of the spindle and maintains stretch on the
sensory nerve endings. As a result, the spindle
remains active, even though the muscle contracts
Figure 13-5a
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Figure 13-5b
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Figure 13-5
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How muscle spindles work (fig. 13-6a-c)
Have an unsuspecting friend stand with eyes closed,
one arm extended (elbow at 90 degrees), and hand
with palm up
Place a small book or other object in the outstretched
hand
Arm muscles will contract to deal with the additional
weight
Figure 13-6a
Copyright © 2010 Pearson Education, Inc.
How muscle spindles work (fig. 13-6a-c)
Suddenly, drop a heavier load (another book) onto the
hand
The added weight sends the arm downward,
stretching the biceps and activating its muscle
spindles
Sensory input into the spinal cord will activate the
alpha motor neurons, the biceps will contract, bringing
the arm back up to its original position
Figure 13-6b
Add extra weight,
arm drops
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Figure 13-6c
Muscle spindles
activated
Send message
to spinal cord,
arm comes back
up
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Golgi Tendon Organs (p. 453-454)
– Found at the junctions of tendons and muscle
fibers
– Respond primarily to the tension a muscle
develops during an isometric contraction
– The response is a relaxation reflex
• Isometric (static) contraction
– Creates force without moving a load
• Isotonic contraction
– Creates force and moves a load
Golgi Tendon Organs
Structure:
Free nerve endings that weave in between collagen
fibers inside a connective tissue capsule
When a muscle contracts, this pulls the collagen fibers
tight, pinching the sensory endings of the afferent
neurons, causing them to fire
Figure 13-3
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Figure 13-3c
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Golgi Tendon Organs
When the Golgi tendon organ is activated, afferent
input excites inhibitory interneurons in the spinal cord
These inhibit alpha motor neurons innervating the
muscle and muscle contraction decreases or ceases
This reflex slows muscle contraction as the force of
contraction increases
It also prevents excessive contraction which can injure
the muscle
Figure 13-6d
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Figure 13-6e
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Movement around most flexible joints is controlled by
groups of synergistic and antagonistic muscles acting
in a coordinated manner
Sensory neurons and efferent motor neurons are
linked by diverging and converging pathways of
interneurons within the spinal cord
Myotatic Unit:
– The collection of pathways controlling a single joint
Patellar tendon (knee jerk) reflex
This is a monosynaptic stretch reflex
Sit on the edge of a table, lower legs relaxed and
hanging off of the table
Tap the patellar tendon (just below the kneecap)
The tap stretches the quadriceps muscle
This activates the muscle spindles and sends
sensory information to the spinal cord
Patellar tendon (knee jerk) reflex
The sensory neurons synapse directly onto the motor
neurons that control contraction of the quadriceps
Excitation of the motor neurons causes the quadriceps
motor units to contract, causing the lower leg to swing
forward
Reciprocal Inhibition:
In order for this to work, the antagonistic flexor
muscles (hamstrings) must relax
Figure 13-7
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The single tap on the tendon causes both the
contraction of the quadriceps and the reciprocal
inhibition of the hamstrings
How this works:
The sensory fibers branch upon entering the spinal
cord.
Some of the branches activate motor neurons of the
quadriceps and other branches synapse on inhibitory
neurons
The inhibitory neurons suppress activity in the motor
neurons controlling the hamstrings (polysynaptic
reflex)
Flexion Reflexes
Polysynaptic, cause an arm or a leg to be pulled away
from a painful stimulus
These reflexes rely on divergent pathways in the
spinal cord also
Figure 13-8, p. 456
Person stepping on a tack
(next slide)
Figure 13-8, overview
Copyright © 2010 Pearson Education, Inc.
Movement (Table 13-2)
Can be classified as:
Reflex
Voluntary
Rhythmic
Reflex Movement
Least complex, integrated primarily in the spinal cord
Can be modulated by higher brain centers
Input that initiates reflexes also goes up to the brain
where it helps coordinate voluntary movements and
postural reflexes
Postural reflexes: help us to maintain body position,
integrated in brain stem, require continuous sensory
input from visual, vestibular, and muscle systems
Voluntary Movements
Most complex
Require integration at the cerebral cortex
Can be initiated at will, without external stimuli
Learned voluntary movements improve with practive,
to the point where they can become “involuntary”
“Muscle Memory” is the ability of the unconscious
brain to reproduce voluntary, learned movements and
positions
Rhythmic Movements
Examples: walking, running
Combinations of reflex and voluntary movements
initiated and terminated by the cerebral cortex
Once activated, these movements are kept going by
Central Pattern Generators (CPGs) which are
networks of CNS interneurons
Changes in rhythmic activity are initiated by the
cerebral cortex
Example: change from skipping to walking
Next:
Chapter 14
Heart: Cardiovascular Physiology