REFLEX SYSTEMS

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REFLEX SYSTEMS
Reflexes are an involuntary response to internal
or external stimuli which are (generally) protecti
ve of the body. A partial list of reflexes
follows i) Auditory - Loud noise causes eyes
to turn toward noise ii) Corneal - Light touch
of cornea causes eye blink iii) Cough - Food
or liquid in pharynx causes coughing to clear
airway iv) Gag - Stimulation of back of tongue
causes gag v) Pupillary - Light causes pupils
to constrict vi) Accommodation - Change of
focus point changes size of pupil vii) Muscle
- To be discussed in detail viii) Movement -
To be discussed in detail
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BRAIN ORGANIZATION
Cerebral Cortex
Thalamus
Red Nucleus
Basal ganglia
Medulla
Cerebellum
Pons
Afferent axon (from Sensory Cell)
Spinal Cord
Efferent axon (to Muscle)
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MUSCLE RECEPTORS

• The main job of muscle is for movement and/or
  force production.
• Muscle also must contain sensory cells to keep
  the brain (cerebellum) aware of
• where the bodys various parts are with respect
  to the environment
• There are 5 classes of receptors in muscles ,
  tendons and joints
• Free endings ( pain, temperature, etc.)
• Paciniform corpuscles (vibration, pressure)
• Joint receptors (state of segments forming the
  joint, pain)
• Golgi Tendon Organs (force)
• Muscle spindles (length, speed of stretch)

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REFLEX SYSTEMS III
Elements which comprise the MYOTATIC REFLEX LOOP
(engineers view)
- motorneuron efferent
Synapse with Anterior Horn Cell
Stretch
Muscle Dynamics
Muscle Spindle
GIa afferent
Mechanical Coupling
- motorneuron efferent

• To analyze the what, how, and when of this
  system, we would have to
• Model the muscle dynamics, spindle and anterior
  horn cell synapse
• Model the encoding and decoding of spike trains
  in neurons
• Recognize that the effects of other receptors and
  higher centers are neglected

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REFLEX SYSTEMS II
Interaction of inhibition and excitation on a
motorneuron
interneurones
From agonist muscle spindles
From antagonist muscle spindles
IPSP EPSP
Motorneuron IPSP Interneuron AP
Motorneuron EPSP Axonal AP
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OTHER SPINAL REFLEXES
Cutaneous reflexes - generally elicit complex
responses that are protective of the
organism e.g., 1) flexion withdrawal is a
total body response to remove painful
stimuli 2) crossed extensor reflex enhances
posture during 1) 3) extensor thrust caused by
light pressure on the plantar foot 4) scratch
reflex removes annoying stimuli by repetitive
movements Locomotory reflexes Central
Pattern Generators - from insects to higher
animals
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MODIFIABILITY OF REFLEXES
Often, reflexes are considered to produce a
repeatable pattern of neural signals. But all
reflexes have some degree of modifiability.
For example, consider gripping a fragile object
as opposed to a heavy object. Or consider
the Achilles tendon reflex Subject
Sitting Subject Standing Subject
Partial Standing
Stimulus Stimulus Stimulus
Triceps Surae EMG Triceps Surae EMG
Triceps Surae EMG
mv
TIME in msec
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EFFECTS OF HIGHER CENTERS ON SPINAL LEVEL
MECHANISMS
Reflex circuits provide higher centers of the
brain with a set of elementary patterns of
coordination from relatively simple combinations
(e.g., reciprocal inhibition at a single joint)
to more complex patterns (e.g., flexion
reflexes). Higher centers produce
voluntary movements by activating appropriate
reflex circuits. This allows higher
centers of the brain to control very complex
movement patterns with relatively simple
descending systems. Example of complexity of a
voluntary movement
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SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES
Downflow from the brain affects the excitability
of motorneurons involved in
peripheral
reflexes. This can be easily seen by changing
the excitability of the motorneuron pool during

elicitation of the tendon jerk response. NOTE
In general tonic signals from the brain are
inhibitory - i.e., try to keep the status
quo Other examples are seen when specific
lesion conditions are examined. Consider
the following experiments
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SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES II
Normal subject lying prone with a foot tied in an
oscillating boot
Position Input Torque Output IEMG Activit
y
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SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES IV
Subject with hemiplegia (due to stroke) lying
prone with affected foot tied in an
oscillating boot
Position Input Torque Output IEMG Activit
y
RESULT Increased fusimotor drive gives larger
sensitivity to stretch
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SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES V
Subject with paraplegia lying prone with foot
tied in oscillating boot
Position Input Torque Output IEMG Activit
y
RESULT Inhibitory downflow removed
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MUSCLE SPINDLE REDUX
Historically, Sherrington ( 1925) first
described the function of the muscle spindle and
its role in control of motor function. He
used the Bell-Magendie Law to deafferent an
animals limb. He found that the animal
could not use the limb after it was deafferented
so this became the accepted scientific
fact. About 50 years later (1970), it was found
that with (almost) total BILATERAL
DEAFFERENTATION, the limbs could be used quite
well. So how does the CNS use information from
muscle spindles? (i) controls motorneuron pool
excitability when a command signal comes
from the brain, if the motorneurons are close
to their threshold, then the command will more
likely be carried out (what would happen
without gamma motor system???)
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MUSCLE SPINDLE REDUX I
(ii) spindle acts as a feedback element through
the brain to keep track of the state of
muscles ( e.g., fatigue) (iii) spindles act as
learning elements that can adjust commands for
voluntary functional acts Historically,
(again) about 1955, the FOLLOW-UP SERVO THEORY
(Marsden, Merton, Morton) postulated that
movements occur by setting spindles through gamma
motorneurons and then the feedback loops
causing appropriate alpha motorneuron firing.
Physiological evidence for this came from
experiments on decerebrate cats in which turning
their head, caused leg muscle contraction which
disappeared if the dorsal roots were cut ( the
afferent signals remained unchanged)
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MUSCLE SPINDLE REDUX II
Alpha - Gamma Motorneuron Co-activation Command
signals are sent down from the brain to control
muscle action as well as to set the stiffness
(gain) of the reflex pathways. The stiffness
settings are learned, e.g., picking up heavy
looking but actually light objects causes large,
inappropriate movements
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MUSCLE SPINDLE SUMMARY
The muscle spindle reacts to a muscles overall
length, speed of stretch (and maybe
acceleration). Its response (i.e., the number of
afferent Action Potentials produced) is
controlled by a gamma efferent system which comes
from the CNS separate from the force - producing
alpha efferent system.