The anterior horn of the spinal cord is frequently mentioned in conversations about motor neuron diseases such as polio or amyotrophic lateral sclerosis (ALS). When the spinal cord develops, the posterior part becomes responsible for managing most aspects of sensation, and the anterior is more responsible for movement. When you move, the cells of your cerebral cortex send a message to cells in the spinal cord. These cells then relay the message out to the peripheral nervous system and muscles. The nerve cells that are responsible for relaying messages between the brain and the peripheral nervous system are called motor neurons. The nerves that send messages between the cerebral cortex and the spine are called upper motor neurons, and those that relay messages from the spine to the muscles are called lower motor neurons. These neurons communicate by synapses in the anterior horn of the spinal cord, as shown in the image.
Diseases that selectively attack these neurons are called motor neuron diseases. As the name suggests, motor neuron diseases reduce someone's ability to move. The best known example is amyotrophic lateral sclerosis, but other examples include polio, primary lateral sclerosis, and Kennedy's disease. Neurologists use their physical exam to determine where the disease is in the body. If the problem is with the upper motor neurons alone, then certain exam findings like rigidity and spasticity may develop, whereas if just the lower motor neurons are involved, other exam findings like atrophy and fasiculations are present. In some forms of motor neuron disease, such as ALS, both upper and lower motor neuron signs are present.