 In this video, I will describe the descending somatic motor pathways and distinguish between flaccid and spastic paralysis. Here we have an illustration showing the corticospinal tract, which is the major descending pathway. The descending tracts that carry the motor commands down from the central nervous system to the peripheral nervous system. And then nerves will travel from the spinal cord out through the peripheral nervous system to skeletal muscles to stimulate contraction of the skeletal muscles. So this is the somatic, efferent fibers of the peripheral nervous system that excites skeletal muscles. And the corticospinal tract is formed from two neurons, an upper motor neuron in the brain and a lower motor neuron that has its cell body in the spinal cord. The upper motor neuron, also known as a Betz cell, is found in layer 5 of the cerebral cortex in the precentral gyrus of the frontal lobe, the region of the frontal lobe known as primary motor cortex. So the upper motor neuron's axon will extend down through the white matter of the cerebrum and through the cerebral peduncles of the midbrain and then through the pyramids of the medulla oblongata. Within the pyramids, these axons will cross the midline so that the right hemisphere of the brain will send commands to control the left half of the body and the left hemisphere of the brain will send commands to control the right half of the body. The upper motor neurons form synapses with lower motor neurons in the spinal cord within the anterior gray horn of the spinal cord, also known as the ventral horn of the spinal cord. And these axons travel down through the corticospinal tract of the spinal cord before forming that synapse in the anterior gray horn. Then the lower motor neuron has its cell body in the anterior gray horn and extends its axon out through the ventral root to a spinal nerve. The axon continues to travel through the spinal nerve down to the skeletal muscle where it will form a synapse with the skeletal muscle to stimulate contraction of that muscle, enabling motion of the body. So paralysis is the loss of the ability to move part of the body, and there's two major types of paralysis, either flaccid paralysis or spastic paralysis. Flaccid paralysis results from damage to the lower motor neurons, and flaccid paralysis results in inability to contract the skeletal muscles that are innervated by those lower motor neurons. So there's no voluntary control, but there's also no involuntary control of those muscles so they won't have muscle tone, they'll be limp all of the time, and the reflexes will also be lost as a result. In contrast, spastic paralysis results from damage to the upper motor neurons, whereas the lower motor neurons are still functioning, therefore muscle tone will still be present in the muscles that are innervated by this lower motor neuron. However, without the connection sending commands in from the upper motor neuron, there's no voluntary control over the muscles. But because the lower motor neuron is still functioning, there will be muscle tone, and there will still be some simple reflexes, and in fact those reflexes will be exaggerated or will be stronger responses than normal as a result of the fact that there is no information modulating the connection between the sensory neurons and the motor neurons within the spinal cord. The primary information coming down the corticospinal tract would influence the reflexes and modulate that activity. The loss of that upper motor neuron will lead to an even stronger reflex from a monosynaptic reflex where the integration center is found within the spinal cord. This illustration shows us the tracts of the spinal cord, so we can see the pyramidal tracts are synonymous with corticospinal tracts. You can see there are lateral and anterior corticospinal tracts, but the corticospinal tracts carry voluntary commands from primary motor cortex down the spinal cord. There are also extra pyramidal tracts. These extra pyramidal tracts carry information from nuclei in the brainstem that are involved in unconscious coordination of motor commands. Sensory feedback, such as proprioception, can be used in order to help fine tune our motor commands and help to maintain posture, and this unconscious coordination and fine tuning of motor commands involves motor commands traveling down the spinal cord through the extra pyramidal descending tracts.