A motor neuron (or motoneuron) is a nerve cell (neuron) whose cell body is located in the spinal cord and whose fiber (axon) projects outside the spinal cord to directly or indirectly control muscles.[1] Motor neurons are efferent nerves (also called effector neurons), that carry signals from the spinal cord to the muscles to produce (effect) movement.[2] Types of motor neurons are alpha motor neurons, beta motor neurons and gamma motor neurons.
Clinical neuroscientists refer to upper and lower motoneurons, with the cell type described above being a lower motoneuron. Upper motoneurons are cortico-spinal interneurons that arise from the motor cortex and descend to the spinal cord where they activate the lower motoneurons through synapses. Non-clinical neuroscientists tend to restrict the use of the term 'motoneuron' to the efferent neurons that actually innervate muscles (the lower motoneurons).
