The γ motor neurons are smaller than α motor neurons; they have a soma diameter of about 35 µm. The γ motor neurons that project to a particular muscle are located in the same regions of the ventral horn as the α motor neurons that supply that muscle. γ Motor neurons do not supply extrafusal muscle fibers; instead, they synapse on specialized striated muscle fibers called intrafusal muscle fibers, which traverse receptors called muscle spindles that are embedded in skeletal muscles. The function of γ motor neurons is to regulate the sensitivity of these receptors (discussed later).
Topographic Organization of Motor Neurons in the Ventral Horn
The spatial distribution of motor neurons in the spinal cord is highly organized. (This is also true in the cranial nerve nuclei.) A given skeletal muscle is supplied by a group
Aclinicallyusefulwaytomonitortheactivityofmotorunitsiselectromyography. Anelectrodeisplacedwithinaskeletalmuscletorecordthesummedactionpotentialsoftheskeletalmusclefibersofamuscleunit.Ifnospontaneousactivityisnoted,thepatientisaskedtocontractthemusclevoluntarilytoincreasetheactivityofmotorunitsinthemuscle.Astheforceofvoluntarycontractionincreases,moremotorunitsarerecruited.Inadditiontotherecruitmentofmoremotorneurons,contractilestrengthincreaseswithincreasesintherateofdischargeoftheactiveα motorneurons.Electromyographyisusedforvariouspurposes.Forexample,theconductionvelocityofmotoraxonscanbeestimatedasthedifferenceinlatencyofmotorunitpotentialswhenaperipheralnerveisstimulatedattwositesseparatedbyaknowndistance.Anotheruseistoobservefibrillationpotentialsthatoccurwhenmusclefibersaredenervated.Fibrillationpotentialsarespontaneouslyoccurringactionpotentialsinsinglemusclefibers.Thesespontaneouspotentialscontrastwithmotorunitpotentials,whicharelargerandhavealongerdurationbecausetheyrepresenttheactionpotentialsinasetofmusclefibersthatbelongtoamotorunit.
of α motor neurons, called a motor nucleus, located in the ventral horn. Each such motor nucleus takes the form of a rostrocaudally running column that can span several spinal cord levels (see
Fig. 9.1A ). Motor neurons that supply the axial musculature collectively form a column of cells that extends the length of the spinal cord. In the cervical and lumbosacral enlargements, these cells are located in the most medial part of the ventral horn; at other levels, they essentially form the entire ventral horn. The motor neurons innervating the limb muscles are in the cervical and lumbosacral enlargements, where they form columns that are lateral to those for the axial muscles. Motor neurons to muscles of the distal part of the limb are located most laterally, whereas those that innervate more proximal muscles are located more medially (
Fig. 9.1B). Also, motor neurons to flexors are dorsal to those that innervate extensors. Note that the α and γ motor neurons to a given muscle are found intermixed within the same motor neuron column.
The interneurons that connect with the motor neurons in the enlargements are also similarly topographically organized. In general, interneurons that supply the limb muscles are located mainly in the lateral parts of the deep dorsal horn and the intermediate region between the dorsal and ventral horns. Those that supply the axial muscles, however, are located in the medial part of the ventral horn. All of these interneurons receive synaptic connections from primary afferent fibers and from the axons of pathways that descend from the brain, and thus they are part of both spinal reflex arcs and descending motor control pathways.