Voluntary Motor System Lower motor neurons are located in the spinal cord anterior grey horn and in the brain stem Damage to lower motor neurons can lead to Flaccid Paralysis Lower motor neurons are controlled by upper motor neurons Damage to upper motor neurons can lead to Spastic Paralysis Cell bodies of upper motor neurons are located in the pre central gyrus of the frontal lobe Cortex Cerebellum and Basal Ganglia Nuclei on 4 30 Lower Motor Neurons Alpha motor neurons Large multipolar neurons in the anterior grey horn and brain stem Have thick myelin sheath Conduct action potentials to skeletal muscle very quickly Innervate the extrafusal muscle fibers which are the large muscle cells that produce voluntary movement by causing movement at synovial joints Axons are found in all spinal nerves and some cranial nerves those that have motor functions Gamma motor neurons Small motor neurons similar to alpha neurons in that they are multipolar but they are smaller Thinner myelin sheeth Somewhat slow impulse conduction velocity Axons of gamma motor neurons innervate intrafusal motor fibers which are contained in muscle spindles Axons are also found in all spinal nerves and those cranial nerves that have a somatic motor function At spinal nerve levels the axons of these motor neurons form the limbs of reflexes LMN Disease Flaccid Paralysis Status of voluntary muscle Paralysis If the nerve is severed there is a complete paralysis of that muscle Significant atrophy of involved muscles shortly after nerve injury Location of lesion CNS Cell body or axons in CNS PNS Rootlets of peripheral nerves Types of Injury Trauma CVA Poliomyelitis Brain stem injuries that may cause cranial nerve injury do occur but are most often fatal to the individual Upper Motor Neurons Largely control and regulate the pools of LMN s Pre central gyrus of frontal lobe called area 4 is the primary voluntary motor cortex that is extremely important for initiating voluntary movement