Motoneurones

Motoneurones, also known as motor neurons, are nerve cells that form the core of the motor system. Their primary function is to transmit signals from the central nervous system (brain and spinal cord) to muscles, initiating and controlling movement. These specialized neurons play a crucial role in everything from voluntary actions like walking and grasping to involuntary reflexes.

There are two main types of motoneurones: upper motoneurones and lower motoneurones. Upper motoneurones originate in the brain, specifically in areas like the motor cortex, and descend through the spinal cord to synapse with lower motoneurones. Lower motoneurones are located in the brainstem and spinal cord. Their axons extend out of the central nervous system to directly innervate skeletal muscle fibers at structures called neuromuscular junctions.

When an upper motoneurone is activated, it sends an excitatory or inhibitory signal to a lower motoneurone. The lower motoneurone then integrates these signals and, if the net input is sufficient, fires an action potential. This electrical impulse travels down the axon of the lower motoneurone to the muscle. At the neuromuscular junction, the motoneurone releases neurotransmitters, typically acetylcholine, which bind to receptors on the muscle fiber membrane. This binding triggers a cascade of events leading to muscle contraction, resulting in movement. Damage or degeneration of motoneurones can lead to a range of debilitating conditions, including paralysis, muscle weakness, and atrophy, as seen in diseases like amyotrophic lateral sclerosis (ALS).