Psych/NS&B 255
Neural Basis of Movement
H. Sinnamon
Basic Principles of Motor Physiology
How the Nervous System Works to Control Movement
Physiology of Basic Spinal Circuits
I. The spindle control system provides a model for sensory-motor
integration
A. Key properties:
alpha motor neuron contracts extafusal fibers
gamma motor neuron contracts intrafusal fibers (spindles)
tension on spindle coded by primary and secondary afferents
tension on spindle is produced by stretch of extrafusals or contraction of spindle
activity in spindle afferents excites alpha motor neuron
function: gamma motor neurons adjust sensitivity of spindles over wide range
of muscle lengths.
B. Merton’s Servo theory
Vallbo’s critical experiment
alpha-gamma coactivation
II. Basic reflex and control circuits
1. Reflexes are fundamental circuits that relate sensory and motor events
A. they perform basic functions
B. they provide building blocks for complex behavior
C. they provide models for understanding motor control circuitry
2. The myotatic reflex (stretch reflex)
Key properties
Ia primary afferent directly excites
the homonymous motorneuron
the synergistic motorneuron
the inhibitory interneuron acting on the antagonistic motorneuron
(reciprocal inhibition)
general role: participates in the feedback control of length
Parts of the stretch reflex circuit are used by higher systems to coordinate
movements
3. Golgi Tendon organ connections (“Inverse Myotatic Reflex”)
Ib fibers directly excite:
inhibitory interneurons for same muscle
inhibitory interneruons for synergists
excitatory interneurons for antagonists
general role: participates in force control, details not known
speculation: Ib fibers activate the circuits that terminate the movement
when the force goal or an obstacle is reached
4. Recurrent inhibition
the Renshaw Cell
excited by motorneurons and supraspinal systems
inhibits same motorneuron and its neighbors
disinhibits antagonist motorneurons
general role: participates in force control
specific functions:
dampen activity
focus activity by inhibiting weakly excited motorneurons
adjust sensisitivity of motorneuron pools
5. Circuitry for flexion withdrawal reflex
6. Circuitry for rhythmic movements
The half center model