1. Lower Motor System
Prof. K. Sivapalan

2. Motor output.
Motor output for external work may be reflex or voluntary action. This involves skeletal muscle innervated by somatic nerves.
Some reflexes are subject to voluntary adjustment and control by learning.
Internal motor functions are involuntary. They involve smooth and cardiac muscles and autonomic nervous system.
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Lower Motor system

3. Voluntary movement Brain plans a movement.
Selects appropriate muscles.
Sends impulses to muscles to generate movements with appropriate force.
Adjusts the movement by comparing the plan and performance.
Movements are learnt by doing- repetition improves performance.
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Lower Motor system

4. Voluntary Movement.
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Lower Motor system

5. Final motor output.
Muscle fibers contract in response to impulse in the motor nerve.
Each axon branches at the end and innervates several muscle fibers.
Motor unit is the muscle fibers innervated by one axon.
Several motor units constitute a muscle.
One or more muscles and joints can be involved in one movement.
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Lower Motor system

6. Lower motor neuron
Motor nerve from anterior horn cell is the final common pathway.
It is named α motor neuron because the axon is Iα fiber.
It is also known as lower motor neuron because it is influenced by several upper neurons
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Lower Motor system

7. Stimuli to Anterior Horn Cell.
Fibers from several sources converge on the α-motor neuron.
They include sensory and descending fibers directly or through inter neurons.
Some are excitatory and others are inhibitory.
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Lower Motor system

8. Spinal influences to AHC
Stretch reflex through muscle spindle.
Inverse stretch reflex through Golgi tendon organ.
Withdrawal reflex through pain receptors.
Other polysynaptic reflexes.
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Lower Motor system

9. Stretch and reverse stretch reflexes
Muscle spindles are stimulated by stretch of the skeletal muscle.
Afferents excite the AHC.
Output from AHC causes contraction of the muscle.
Colateral from spindle afferent inhibits antagonist muscles- reciprocal innervation.
Golgi tendon organ is stimulated when the force of stretch is very high.
It inhibits AHC through inter neuron.
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Lower Motor system

10. Structure of Muscle Spindle.
Muscle spindles are muscles with sensory area.
They are found in a capsule. Therefore they are called intrafusal fibers.
There are two types of spindles in each capsule- nuclear bag fibers (2) and nuclear chain fibers (4 or more).
Ends of the nuclear chain fibers are attached to the nuclear bag fibers.
Nuclear portion is sensory but both ends of the spindles are contractile.
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Lower Motor system

11. Innervation of the Spindle.
Rapidly conducting Ia fibers wrap round the nuclear portions.
Secondary [flower-spray] endings innervate nuclear chain fibers.
Aγ motor fibers innervate the spindles.
Gama motor fibers constitute 30 % in the ventral route.
Ventral horns have Gama motor neurons in addition to AHC.
β efferents- poorly understood.
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Lower Motor system

12. Function of the Spindle.
Nuclear bag fiber- dynamic response.
Nuclear chain fiber- static response.
No adaptation.
It maintains the tone of the muscle [resistance to stretch]
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Lower Motor system

13. Effects of γ Efferent.
Annulospiral endings discharge at slow rate at rest.
When muscle is stretched, elongation increases discharge.
When muscles contracts by α motor stimulation, spindle is relaxed and no discharge.
Increased γ efferent discharge stretches the nuclear portion and generates impulses.
Stretch of contracted muscle generates more impulses in afferents.
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Lower Motor system

14. Inverse stretch reflex.
As the stretch on the muscle increases, force of reflex contraction also increases.
Beyond a point, muscle relaxes due to inverse stretch reflex.
Golgi tendon organ is stimulated by strong force.
It is a protective response.
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Lower Motor system

15. Withdrawal reflex. Painful stimulus excites the flexor muscles.
At the same time it inhibits extensor muscles of the same limb.
If the stimulus is very powerful, it inhibits flexors and stimulates extensors in the opposite limb.
It is known as reciprocal inhibition.
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Lower Motor system

16. Lower Motor Neuron Lesion
Flacidity
Paralysis
Absent reflexes
Muscle wasting
Faciculation
Fibrilation
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Lower Motor system