Lower Motor Neuron Lesion Lecture PDF

Lower Motor Neuron Lesion DR. DOHA AL-AFIFI Objectives At the end of this lecture the student will be able to: Describe lower motor neuron lesion. Describe the signs of lower motor neuron lesion. Differentiate between UMNL and LMNL. Over view Over view The pyramidal system, which arises...

Lower Motor Neuron Lesion DR. DOHA AL-AFIFI Objectives At the end of this lecture the student will be able to: Describe lower motor neuron lesion. Describe the signs of lower motor neuron lesion. Differentiate between UMNL and LMNL. Over view Over view The pyramidal system, which arises from the premotor and motor areas, then carries the cortical commands to the lower motor neurons (LMNs) located in the brainstem and spinal cord. In turn, the LMNs carry the commands to the contractile units of the voluntary muscles, and the movement occurs. During the execution of the movement, muscle receptors that record stretch send information back to the LMNs and to the cerebellum to fine-tune the coordination of the movement as it continues. The fine-tuning occurs via connections of the cerebellum with the motor cortex and the brainstem motor centers, both of which influence the LMNs. It should be remembered that all participate in commanded movements and all must be intact for normal voluntary movements to occur. For voluntary movement to occur: CNS influences the activity of skeletal muscle through two sets of neuron. Upper Motor Neuron (UMN). Lower Motor Neuron(LMN). Types of motor neuron Upper Motor Neurons (UMN): Neurons arise in the cerebral cortex brainstem, and their axons travel in descending tracts to synapse with lower motor neurons and/or interneurons in the brainstem or spinal cord. Lower motor neurons (LMN) : Neurons have their cell bodies in the spinal cord or brainstem and synapse with skeletal muscle fibers. UMN send impulses to LMN and thus exert control over LMN of spinal nerves and cranial nerves. To classify LMN lesion based on the basis of anatomical station affected:  1- AHC: they are special type of nerve cell situated in the anterior horn cell of the H- shaped grey matter of the spinal cord. They received motor impulse from the pyramidal tract. Their axon exit from the spinal cord at anterior roots. The motor nuclei of cranial nerves are similar in function to AHC, as they form the cell bodies of the LMN for the cranial nerves. 2- Peripheral motor nerve :carry motor impulse from the AHC to the voluntary muscle. LOWER MOTOR NEURONS Lower motor neurons are the neurons that convey signals to extrafusal and intrafusal skeletal muscle fibers. There are two types of lower motor neurons based on the type of muscle fibers they innervate: Alpha motor neuron Gamma motor neuron. Both types have cell bodies in the ventral horn of the spinal cord. Their axons leave the spinal cord via the ventral root, travel through the spinal nerve, and then travel through the peripheral nerve to reach skeletal muscle. Alpha Motor Neurons Alpha motor neurons have large cell bodies and large, myelinated axons. The axons of alpha motor neurons project to extrafusal skeletal muscle, branching into numerous terminals as they approach muscle. Normally an alpha motor neuron releases enough ACh that all of the muscle fibers it innervates contract. Extrafusal muscle fiber the most numerous type of muscle fiber and the one that involved in muscle contraction. Gamma motor neuron Gamma motor neurons have medium-sized myelinated axons. Axons of gamma motor neurons project to intrafusal fibers in the muscle 'spindle Lower motor neuron N.B In addition to alpha motor neurons, skeletal muscles are also supplied by gamma motor neurons. The axons of the gamma motor neurons innervate the intrafusal fibers of the muscle spindles, which are sensory organs that are stimulated by lengthening or stretching the muscle. The intrafusal fibers are located at the poles of the muscle spindles. When activated by the gamma motor neurons, the intrafusal fibers increase the tension on the muscle spindle receptors. The gamma motor neurons play an important role in muscle tone.  LMNs are also called alpha motor neurons. Whether in the spinal cord or brainstem, alpha motor neurons and their axons are the only connections between the central nervous system (CNS) and skeletal muscle contraction units, the extrafusal muscle fibers. Signs of LMNL lesion If LMN cell bodies and/or axons are destroyed, the affected muscles can undergo: 1- Muscle Atrophy: Muscle atrophy is the loss of muscle bulk. Disuse atrophy results from lack of muscle use, Neurogenic atrophy is caused by damage to the nervous system. 2- Involuntary Muscle Contractions: Fasciculations (quick twitches of muscle fibers of a single motor unit that are visible on the surface of the skin) e.g: the eyelid twitches that sometimes accompany anxiety. Fibrillations : It is a spontaneous contraction of a single muscle fiber. It is hardly visible, except in the tongue 3- Hypotonia: is abnormally low resistance to passive stretch Flaccidity is lack of resistance to passive stretch. Lower motor neuron syndrome Lower motor neuron syndrome Case study