Upper motor neurons Upper motor neurons originate in the
motor cortex located in the
precentral gyrus. The cells that make up the
primary motor cortex are
Betz cells, which are giant
pyramidal cells. The axons of these cells descend from the cortex to form the
corticospinal tract.
Corticomotorneurons project from the primary cortex directly onto motor neurons in the ventral horn of the spinal cord. Their axons synapse on the spinal motor neurons of multiple muscles as well as on spinal
interneurons. Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas. •
Lateral corticospinal tract •
Rubrospinal tract •
Lateral reticulospinal tract •
Vestibulospinal tract •
Medial reticulospinal tract •
Tectospinal tract •
Anterior corticospinal tract Lower motor neurons Lower motor neurons are those that originate in the spinal cord and directly or indirectly innervate effector targets. The target of these neurons varies, but in the somatic nervous system the target will be some sort of muscle fiber. There are three primary categories of lower motor neurons, which can be further divided in sub-categories. According to their targets, motor neurons are classified into three broad categories: • Somatic motor neurons • Special visceral motor neurons • General visceral motor neurons
Somatic motor neurons Somatic motor neurons originate in the
central nervous system, project their
axons to
skeletal muscles (such as the muscles of the limbs, abdominal, and
intercostal muscles), which are involved in
locomotion. The three types of these neurons are the
alpha efferent neurons,
beta efferent neurons, and
gamma efferent neurons. They are called
efferent to indicate the flow of information from the
central nervous system (CNS) to the
periphery. •
Alpha motor neurons innervate
extrafusal muscle fibers, which are the main force-generating component of a muscle. Their cell bodies are in the
ventral horn of the spinal cord and they are sometimes called
ventral horn cells. A single motor neuron may synapse with 150 muscle fibers on average. The motor neuron and all of the muscle fibers to which it connects is a
motor unit. Motor units are split up into three categories: • Slow (S) motor units stimulate small muscle fibers, which contract very slowly and provide small amounts of energy but are very resistant to fatigue, so they are used to sustain muscular contraction, such as keeping the body upright. They gain their energy via oxidative means and hence require oxygen. They are also called red fibers. •
Gamma motor neurons innervate intrafusal muscle fibers found within the muscle spindle. They regulate the sensitivity of the spindle to muscle stretching. With activation of gamma neurons, intrafusal muscle fibers contract so that only a small stretch is required to activate spindle sensory neurons and the stretch reflex. There are two types of gamma motor neurons: Dynamic- These focus on Bag1 fibers and enhance dynamic sensitivity. Static- These focus on Bag2 fibers and enhance stretch sensitivity. ==See also==