There are two different types of motoneurons in the ventral horn of the spinal c
ID: 34553 • Letter: T
Question
There are two different types of motoneurons in the ventral horn of the spinal cord: alpha and gamma motoneurons. These motoneurons innervate distinctly different muscle fibers and there are subclasses of both types of motoneurons.
a) Compare the muscle fibers innervated by the two types of motoneurons and their subclasses.
b) To execute a movement it is thought that we co-activate both alpha and gamma motoneurons. How does
activating each of these motoneurons lead to contraction of the extrafusal muscle?
Explanation / Answer
Somatic motor neurons, which originate in the central nervous system, project their axons to skeletal muscle (such as the muscles of the limbs, abdominal, and intercostal muscles), which are involved in locomotion .
Special visceral motor neurons, also called branchial motor neurons, which directly innervate branchial muscles (that motorize the gills in fish and the face and neck in land vertebrates).
General visceral motor neurons (visceral motor neurons for short) which indirectly innervate cardiac muscle and smooth muscles of the viscera ( the muscles of the arteries): they synapse onto neurons located in ganglia of the autonomic nervous system (sympathetic and parasympathetic), located in the peripheral nervous system (PNS), which themselves directly innervate visceral muscles
b)
In addition to voluntary skeletal muscle contraction, alpha motor neurons also contribute to muscle tone, the continuous force generated by noncontracting muscle to oppose stretching. When a muscle is stretched, sensory neurons within the muscle spindle detect the degree of stretch and send a signal to the CNS. The CNS activates alpha motoneurons in the spinal cord, which cause extrafusal muscle fibers to contract and thereby resist further stretching. This process is also called thestretch reflex.