CoursesPhysiologyNervous systemMotor controlMotor neurons

Motor neurons

Learning objectives

After completing this study unit, you will be able to:

Define and classify motor neurons.
Describe the structure, location and functions of alpha and gamma motor neurons.
Define and describe the structure of a motor unit.
Describe the structure and function of a muscle spindle.

Introduction

Motor neurons are neurons originating in the brain, brainstem and spinal cord that supply effector organs such as muscles and glands. The somatic motor neurons supply skeletal (striated) muscles of the body. They can be classified into upper motor neurons (cell bodies in the cerebral cortex and brainstem) and lower motor neurons (cell bodies in the brainstem and spinal cord). Based on axonal diameter, there are three types of somatic motor neurons: alpha, beta and gamma motor neurons.

The larger alpha motor neurons have cell bodies topographically organized in the anterior horn of the spinal cord, with axons travelling through the anterior root of spinal nerves to reach the extrafusal contractile fibers of skeletal muscle. Each alpha motor neuron branches and supplies multiple muscle fibers forming a motor unit.

The cell bodies of the smaller gamma motor neurons are intermixed with those of the alpha motor neurons, and their axons supply the intrafusal muscle fibers of the muscle spindle.

The muscle spindle is a somatosensory receptor located between the extrafusal fibers of skeletal muscles. The intrafusal fibers of this spindle have both sensory (group Ia and II sensory nerve fibers) and motor (gamma motor neuron) innervation. They detect the length of muscle and rate of change of muscle length, providing the central nervous system with proprioceptive information. They also participate in an important reflex, known as the stretch reflex.

Explore concepts

Motor neurons

Explore the different types of motor neurons with these images!

Spinal cord organization

Take a look at how motor neurons are organized in the spinal cord with this gallery.

Motor units

A motor neuron innervates multiple muscles fibers forming a motor unit. Learn the parts of a motor unit with these images.

Muscle spindle

Learn the structure of one of the most complex somatosensory receptors in the body with this gallery!

Take a quiz

Flex your brain muscles as you test yourself with this motor neurons quiz!

Summary

Key facts about motor neurons
Definition	Neurons originating in the brain, brainstem and spinal cord that supply effector organs such as muscles and glands
Classification	Somatic motor neurons: supply skeletal muscle fibers Autonomic motor neurons: supply smooth muscles and glands
Somatic motor neurons	Based on anatomical location: Upper motor neurons: cell bodies in cerebral cortex and brainstem (E.g., corticonuclear fibers and corticospinal tracts) Lower motor neurons: cell bodies in the brainstem and spinal cord (E.g., cranial and spinal nerves) Based on axonal diameter: Alpha motor neurons: large cell bodies, heavily myelinated, supply extrafusal contractile skeletal muscle fibers Gamma motor neurons: smaller cell bodies, less heavily myelinated, supply intrafusal fibers of the muscle spindle
Spinal cord organization	Medial motor nuclei of spinal nerves: axial muscles Lateral motor nuclei of spinal nerves: limb muscles From medial to lateral: motor neurons of axial muscles > proximal limb muscles > distal limb muscles Motor neurons of flexor muscles are posterior to extensor muscles
Motor unit	Functional unit of skeletal muscle, composed of an alpha motor neuron with all the muscle fibers supplied by it; Innervation ratio: number of muscle fibers supplied by a single alpha motor neuron Size principle: motor units are recruited in ascending order of size
Muscle spindle structure	Encapsulated somatosensory receptor, composed of intrafusal fibers within a capsule: Nuclear bag fibers: Nuclear bag 1 (dynamic nuclear bag fibers) Nuclear bag 2 (static nuclear bag fibers) Nuclear chain fibers
Muscle spindle innervation	Sensory innervation: Group Ia sensory nerve fibers (anulospiral/primary endings) Group II sensory nerve fibers (flower spray/secondary endings) Motor innervation: Gamma motor neurons
Muscle spindle function	Proprioception Stretch reflex

Motor neurons

Introduction