Lower Motor System Prof. K. Sivapalan

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. 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

Voluntary Movement. 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

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 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

Spinal influences to AHC Stretch reflex through muscle spindle. Inverse stretch reflex through Golgi tendon organ. Withdrawal reflex through pain receptors. Other polysynaptic reflexes. 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

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] 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

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. 1/17/2019 Lower Motor system

Lower Motor Neuron Lesion Flacidity Paralysis Absent reflexes Muscle wasting Faciculation Fibrilation 1/17/2019 Lower Motor system