NEURAL CONTROL of Skeletal Muscles

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Two major descending fiber tracts control skeletal muscle: pyramidal tracts & extrapyramidal tracts Chapter 8 Pyramidal tracts voluntary involuntary

Higher Motor Neuron Control of Skeletal Muscles Black = extrapyramidal- primitive, slower Red = pyramidal tracts PARKINSON’S EVOLVED

Somatic Motorneurons A motor unit consists of a somatic motor neuron and the muscle fibers it innervates Whenever a somatic motor neuron is activated, all of the muscle fibers it innervates are stimulated to contract

Muscle Spindle Apparatus sensory See next slide for descriptions

Alpha motoneurons: innervate extrafusal (contracting) muscle fibers Gamma motoneurons: innervate intrafusal (active stretch) muscle fibers Contraction of these fibers results in isometric contraction and enhances the stretch reflex. Provides enough tension during relaxation to maintain muscle tone. Both types are stimulated by upper motor neurons at the same time - coactivation

Knee-Jerk Reflex A monosynaptic reflex

Golgi Tendon Organ: this is a receptor that detects the amount of tension in the tendon. If tension is too great, the golgi tendon organ sends impulses that cause IPSPs and inhibit activity. This is a reflex.

Reciprocal Innervation Collateral branches simultaneously inhibit (IPSPs) an antagonistic muscle.

Crossed Extensor Reflex: A type of double reciprocal innervation seen when you step on a tack This is in the same level in the spinal cord.

REFLEXES TESTED IN NEUROLOGICAL EXAMS --BABINSKI - Fanning of the toes when you stroke the heel. Indicates immaturity of nervous system – normal in newborns --biceps reflex – tapping inner elbow, should flex upward --triceps reflex – opposite BABINSKI REFLEX Click here for a video

Learned “conditioned” somatic motor reflexes These are integrated at the level of the cerebral cortex Examples: voluntary control of urination/defacation, tying a shoelace, bicycling, swimming & skiing, driving a car

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