Motor neurons (or motoneurons) are efferent neurons that are responsible for transmitting signals from the spinal cord to muscles, enabling muscle contraction. Motor neuron cell bodies are located in the central nervous system (CNS), while their axons project outside the CNS. Spinal motor neurons, whose development is mapped in LifeMap DiscoveryTM, are generally classified into two types: somatic motor neurons, which are motor neurons that innervate skeletal muscles, and visceral motor neurons, which innervate neuronal or glandular targets. Spinal motor neurons primarily release the acetylcholine neurotransmitter.
Motor neuron damage, caused by injury or disease, results in an array of severe muscle-related conditions, such as muscle atrophy, muscle fasciculation and spasticity, and in extreme cases, difficulties in speaking, swallowing and even breathing. Amyotrophic lateral sclerosis (ALS), also known as the Lou Gehrig's disease, is the most well known motor neuron disease.
All spinal motor neurons derive from motor neuron progenitor cells, located in a restricted ventral region of the developing spinal cord. Spinal motor neurons acquire distinct identities, which are associated with the position of their cell bodies, arranged in longitudinal columns within the spinal cord, together with their axonal projection patterns in the periphery. Graded sonic hedgehog (SHH), Wnt4/5 and fibroblast growth factor (FGF) signaling control the dorsoventral expression of homeodomain transcription factors that specify four distinct motor neurons columns:
The lateral motor column (LMC), the most prominent type of motor neuron column, contains a set of motor neurons, which are arranged in longitudinal arrays and project to distinct regions in the periphery. The LMC is generated only at limb levels, at the brachial and lumbar spinal cord levels, and sends axons into the limb mesenchyme.
The median motor column (MMC) contains a set of motor neurons, which are arranged in longitudinal arrays, and project to dorsal axial muscles and to body wall muscles.
The hypaxial motor column (HMC) contains a set of motor neurons, located at thoracic spinal levels that innervate intercostal and abdominal wall musculature.
The preganglionic column (PGC) contains a set of visceral motor neurons, located at thoracic levels that innervate sympathetic chain ganglia (SCG).