Transcript I. Stretch Reflex

Coordinated Purposeful Movements
Voluntary Motor Function:
* Posture Control (maintaining a position)
* Goal Directed Movements
* Rhythmic Movements
Reflexes:
Rapidly executed automatic and
stereotyped response to a given stimulus
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1. Decision making: Planning
Based on the task (will) and memory
2. Decision processing: Programming
Interpreting the descending
commands and processing them
3. Decision execution: Execution
- Executing the movement, and
- Informing the upper control level
Level 1
Decision
Making
Level 2
Decision
Processing
Level 3
Decision
Execution
Load
Sensory information
Functional basis of the voluntary movement control
Joint angles
and torques
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Receptors & Sensory Information
Mechanoreceptors: Cochlear hear cells,
Vestibular System
Kinesthesia: Proprioceptors, Exterocepts
(Somatosensory System)
Photoreceptors: Visual Information (direct)
1. Sensory Feedback
2. Defining the ambiance
Manipulating the sensory information!
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Brain components
Table 5.3 (1)
Page 144
Cerebral cortex
Cerebral cortex
Basal nuclei
(lateral to thalamus)
Basal nuclei
Thalamus
(medial)
Thalamus
Diencephalon
Hypothalamus
Hypothalamus
Cerebellum
Cerebellum
Midbrain
(Mesencephalon)
Brain stem
Brain stem
(midbrain, pons,
and medulla)
Pons
Medulla
oblongata
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Spinal cord
Hierarchical organization of the motor system
Cerebral Cortex
Motor Area
Thalamus
Basal
Ganglia
Other Sensory
Input
Brain
Stem
Cerebellum
Table 5.3 (1)Cerebral cortex
Page 144
Basal nuclei
(lateral to thalamus)
Spinal Cord
Thalamus
(medial)
Hypothalamus
Cerebellum
Brain stem
Sensory
Information
Sensory
Receptors
Muscle Contraction
and
Movement
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Hierarchical Organization:
Decentralized Control
Parallel Processing:
Production and Control
of Discrete types of
Movements, e.g.,
Reaching while
Controlling the Posture
Posture Control: Medial neuronal system of BS
Distal muscles of limbs: Lateral neuronal system of BS
Eye and Head movement Control : BS
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Cerebral Cortex Area:
Primary motor cortex (M1)
Premotor Cortex (PM)
Supplementary motor area (SMA)
Premotor area & SMA:
Coordinating and planning
sequences of movements
& receive information and
project it to the
primary motor cortex (to BS)
Somatotopic map!
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A Hierarchical Structure for motor control system
Execution
Limbic System
Need
Afferent pathway
Associative Cortex
Efferent pathway
Programming
Planning
?
Plan
Cerebellum
Motor Cortex
Basal
Ganglia
Spinal Cord
Sensory
System
Musculo-skeletal
System
Movement
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Pathways:
The centers and tracts that link the brain
with the rest of the body
Sensory pathways (Afferent):
Distribute information from peripheral
receptors to processing centers in brain
Motor pathways (Efferent):
Begin at CNS centers concerned with
motor control and end at the effectors they
control
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Usual sensory and motor pathways
(no reflex arc is considered)
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?
Limbic System
Limbic System:
* A functional grouping
* Includes nuclei and tracts
along the border between
the cerebrum, diencephalons
and mesencephalon
Need
Associative Cortex
Plan
Cerebellum
Motor Cortex
Spinal Cord
Musculo-skeletal
System
Sensory
System
Movement
Functions of the Limbic System:
* Establishing emotional states
* Linking the conscious, intellectual functions
of the cerebral cortex with the unconscious
and automatic function of the brain stem
* Facilitating memory storage and retrieval 11
Basal
Ganglia
Intra-spinal
(reflex loops)
Decision Making
Level 1
Decision Making
Level 2
Decision Processing
Proprioceptive and other sensory information
Supra-spinal
A proposed internal structure of the three levels
involved in performing a voluntary movement
Interneurons
-MN
-MN
Commands to muscles
Level 3
Agonist
Antagonist
Load
Load
Decision Execution
Joint angles
and torques
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Muscle fibers and Motor Neurons
Two types of muscle fibers:
1. Intrafusal muscle fibers
2. Extrafusal muscle fibers
(main body of the muscle)
Two types of motor neuron:
1. α-motor neurons
Innervate the extrafusal muscle fibers.
When the alpha nerve fires the motor unit
generates tension and/or shortens.
2. γ-motor neurons
Innervate the intrafusal muscle fibers.
When the gamma nerve fires the intrafusal
muscle fiber generates tension
and/or shortens.
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Proprioceptors
Two types:
1. Muscle Spindle Apparatus
2. Golgi Tendon Organ
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Muscle Spindle
Function: Sensing
a) Changes in muscle length:
Spatial position
b) Rate of change in muscle
length: Stretch reflex
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Two kinds of sensory neurons (afferents)
I. Primary afferents (Ia):
fast or dynamic endings
– Rate of stretch
– Function: react to oppose stretch
II. Secondary afferents (II):
slow or static endings
– Final length of muscle
– Function: maintain muscle tone,
posture, positional
awareness
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The effect of changes in muscle
spindle length on muscle tone
Muscle Tone: A Resting tension
Some motor units are always active.
The contraction does not produce
movement, but do tense and firm the
muscle.
Remark: the identity of the stimulated
motor units changes constantly.
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Golgi Tendon Organs
 Monitors tension
developed in muscle and
prevents damage
during excessive force
generation
Ib fibers supply the receptors.
Tension
Firing pattern in the Ib fiber
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Alpha-Gamma Coordination
Voluntary Contraction
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Reflexes
Reflex motor behavior occurs automatically, without instructions of higher centers
Processing Site
A. Spinal Reflex :
Many segments interact to produce
a coordinated, highly variable motor
response
B. Cranial Reflex
Directed by nuclei in brain
e.g., the reflex movements in response
to a sudden loud noise
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Complexity of the Circuit
I. Monosynaptic Reflex (stretch reflex ):
• One sensory neuron
• One interneuron
• One motor neuron
{
Disynaptic reflex
Trisynaptic reflex
II. Polysynaptic Reflex (scratch reflex)
• One sensory neuron
• Multiple interneurons
• Multiple motor neurons
Different reflexes:
Stretch reflex, Withdrawal or Flexion reflex, Tonic reflex,
Golgi tendon reflex, Crossed extensor reflex, etc.
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I. Stretch Reflex
Patellar reflex (knee jerk)
 Quick stretch of muscle distorts
nuclear bag (muscle spindle)
 Afferent signal via primary
sensory nerve (Ia)
 Monosynapse in spinal cord
with α-motor neuron
 Efferent signal via α-motor
neuron
 Muscle contraction results to
oppose stretch
 Reciprocal innervation
Collateral Ia synapses inhibitory interneurons
Inhibitory interneuron sends
efferent signal to antagonist
→ relaxation of antagonist muscle
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II. Withdrawal or Flexion Reflex
(Disynaptic)
 Protective reflex
 Longer latency than
stretch reflex
 Complex nature
(coordination of several joints)
 Reciprocal inhibition
 Nonlinear
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III. Tonic Stretch Reflex (monosynaptic)
• Contributes to muscle tone
• Stabilizes muscle length when it is under constant load
Result: Skeletal muscle length regulation,
→ Posture regulation
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IV. Golgi Tendon Reflex
 Disynaptic reflex
Rectus femoris
(extensor)
 Afferent fiber: Ib
 Synapses are based on
reciprocal inhibition
 Complements the tonic
stretch reflex
 Skeletal muscle length
regulation
→ Postural Control
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IV. Golgi Tendon Reflex
Rectus femoris
(extensor)
Mechanism of postural control
during standing:
• RF starts to fatigue
• Patelar tendon force decreases
• Activity in Ib declines
• Normal inhibition of the MN
supplying RF will be removes
• Muscle will be stimulated
to contract more
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V. Crossed Extensor Reflex
 Contralateral reflex arc
(crossed extensor reflex)
occurs on the opposite side
of stimulus
 The two occur simultaneously
With 250 msec delay between
Flexion and extension
Function: maintaining
posture and balance
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