Upper motor neuron versus lower motor neuron Lower motor neurons are the anterior gray horn cells (alpha motor neuron) in the spinal cord and the motor neurons of the cranial nerve nuclei situated in the brain stem., which innervates the muscles directly. The lower motor neuron are under the influence of the upper motor neurons. Upper motor neurons are the neurons in the higher centers of the brain, which control the lower motor neurons.

Spinal Reflexes Somatic reflexes mediated by the spinal cord are called spinal reflexes These reflexes may occur without the involvement of higher brain centers The brain can facilitate or inhibit them

Every striated muscle has encapsulated muscle fibers scattered throughout the muscle called muscle spindles. The extrafusal muscle fibers are innervated by Alpha motor neuron The intrafusal muscle fibers are innervated by Gamma motor neurons Extrafusal and intrafusal fibers of Muscle

Anatomy of Muscle Spindle The contractile region of the intrafusal muscle fibers are limited to their ends as only these areas contain actin and myosin filaments These regions are innervated by gamma (  ) efferent fibers Primary sensory endings are Stimulated by both the rate and degree of stretch. Secondary sensory endings are stimulated only by degree of stretch

The Stretch Reflex Exciting a muscle spindle occurs in two ways – Applying a force that lengthens the entire muscle – Activating the  motor neurons that stimulate the distal ends of the intrafusal fibers to contact, thus stretching the mid- portion of the spindle (internal stretch)

Stretch reflex (monosynaptic)

Golgi tendon reflex When muscle tension increases moderately during muscle contraction, GTO receptors are activated and afferent impulses are transmitted to the spinal cord

The Crossed Extensor Reflex The reflex occurs when sensory receptors are excited by painful stimulus. There is a rapid lifting of the affected foot (ipsilateral withdrawal reflex ) while the contralateral response activates the extensor muscles of the opposite leg (contralateral extensor reflex) to support the weight shifted to it.

upperlower Muscle toneHypertoniaHypotonia Type of paralysisSpastic typeFlaccid type Deep tendon reflexIncreaseAbsent Wasting of muscle (atrophy) No wastingpresent Electrical activityNormalAbsent Muscles affectedGroups of musclesIndividual muscles FasciculationAbsentPresent Effects of upper motor versus lower motor neuron lesion