Transcript Document

REFLEX SYSTEMS
Reflexes are an involuntary response to internal or external stimuli which are (generally)
protective 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
BRAIN ORGANIZATION
Cerebral Cortex
Thalamus
Red Nucleus
Basal ganglia
Medulla Pons Cerebellum
Afferent axon
(from Sensory Cell)
Spinal Cord
Efferent axon (to Muscle)
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 body’s various parts are with respect to the environment
There are 5 classes of receptors in muscles , tendons and joints:
(i)
Free endings ( pain, temperature, etc.)
(ii) Paciniform corpuscles (vibration, pressure)
(iii) Joint receptors (state of segments forming the joint, pain)
(iv) Golgi Tendon Organs (force)
(v) Muscle spindles (length, speed of stretch)
REFLEX SYSTEMS III
Elements which comprise the MYOTATIC REFLEX LOOP (engineer’s view)
- motorneuron efferent
Stretch
Muscle
Mechanical
Muscle
GIa
Dynamics
Coupling
Spindle
afferent
Synapse with
Anterior Horn
Cell
- motorneuron efferent
To analyze the ‘what’, ‘how’, and ‘when’ of this system, we would have to
(i) Model the muscle dynamics, spindle and anterior horn cell synapse
(ii) Model the encoding and decoding of spike trains in neurons
(iii) Recognize that the effects of other receptors and higher centers are neglected
REFLEX SYSTEMS II
Interaction of inhibition and excitation on a motorneuron
interneurones
From antagonist
muscle spindles
IPSP
From agonist muscle spindles
EPSP
Motorneuron
IPSP
Motorneuron
EPSP
Interneuron
AP
Axonal
AP
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
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
mv
TIME in msec
Triceps Surae EMG
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
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:
SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES II
Normal subject lying prone with a foot tied in an oscillating boot
Position
Input
Torque
Output
IEMG
Activity
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
Activity
RESULT: Increased fusimotor drive gives larger sensitivity to stretch
SUPRASPINAL EFFECTS ON PERIPHERAL REFLEXES V
Subject with paraplegia lying prone with foot tied in oscillating boot
Position
Input
Torque
Output
IEMG
Activity
RESULT: Inhibitory ’ downflow’ removed
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 animal’s
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???)
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)
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
MUSCLE SPINDLE SUMMARY
The muscle spindle reacts to a muscle’s 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.