Lower Motor System

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. 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. Lower Motor system

4. Voluntary Movement. 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. 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 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. 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. 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. 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. 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. • βefferents- poorly understood. 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] 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. 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. 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. Lower Motor system

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