Alpha and Gamma motoneurons

The parts of the skeletal muscle that actually contract are known as the extrafusal muscle fibers, and they are under direct control of the LMN or alpha motoneurons. Collectively, the LMN and muscle fibers that it innervates are known as a motor unit. The axon of an alpha motoneuron leaves the CNS inside a cranial or spinal nerve. It travels to the extrafusal muscle fibers, where it then branches out. Because of this branching, each axon in a nerve may innervate several muscle fibers. Also, each muscle fiber may be innervated by several different alpha motoneurons. This allows for flexibility in muscle functioning.

In addition to alpha motor neurons, motor nerves also house gamma motoneurons. These are smaller and slower conducting than the alpha motoneurons, and innervate muscle spindles (also known as intrafusal muscle fibers).


Sensory System – Somesthetic system

There are five main sensory systems of your nervous system – the somesthetic, auditory, gustatory, visual, and olfactory systems. The somesthetic and auditory systems are most important for speech purposes.

The somesthetic system includes the senses of touch, pressure, proprioception, heat, cold, and pain. Within this sensory system, as with all of the other sensory systems as well, there are different types (orders) of neurons along the pathway from PNS to CNS. In general, 1st order neurons have cell bodies which are in the ipsilateral ganglia of the PNS, and have central processes that extend to the ipsilateral nuclei of the brainstem or spinal cord. 2nd order neurons have their cell bodies in the ipsilateral nuclei of the brainstem or spinal cord, and their axons decussate and extend to the contralateral sensory nuclei of the thalamus. 3rd order neurons have their cell bodies in the contralateral sensory nuclei of the thalamus, and their axons extend to the ipsilateral primary sensory cortex via the internal capsule and corona radiata.

The somesthetic system uses a special kind of sensory receptor known as a muscle spindle. Muscle spindles serve as sensory receptors within the extrafusal muscle fibers. They provide information on the status of the normal stretch mechanisms in muscle. There are two types of intrafusal fibers: nuclear bag fibers and nuclear chain fibers. There is an aggregate of closely packed nuclei in the nuclear bag fibers. In the nuclear chain fibers, the nuclei are in single file.

Two types of sensory nerve fibers are associated with the muscle spindle: primary and secondary afferents. The primary afferents are called annulospiral endings, because they are wrapped around the center of the intrafusal fiber. The secondary afferents are called flower spray endings, and are slower conducting than the annulospiral endings.

Gamma motoneurons form a loop known as the gamma loop. This loop is important for maintaining muscle tone, which is a manifestation of the normal stretch reflex. There is always neural activity going on in nerves, even at rest. Thus, normal muscles are actually never really "relaxed". Muscle tone is achieved via normal stretch reflexes, which represent a muscle’s tendency to maintain its original length whenever it is stretched. Sustained muscle tone facilitates quick, unsustained skilled muscle movement, such as that used in speech.

When a gamma motoneuron fires, it causes the muscle spindle to contract. The shortening of the muscle spindle is detected by the annulospiral endings in the muscle spindle. This results in sensory feedback to the spinal cord or brainstem where the sensory nerve synapse with alpha motoneurons. The AMN, in turn, sends impulses back to extrafusal muscle fibers, stimulating them to contract until they are the same length as the muscle spindles. Once this equalization takes place, the sensory receptor no longer detects shortening, and the loop is inactivated.


Important cortical areas:


Primary motor cortex: Pre-central gyrus (Origin of gross motor movements)

Premotor area: Anterior to precentral gyrus in frontal lobe (Complex motor movements)

Broca’s Area: Inferior frontal lobe at juncture of lateral and central fissures. (Motor speech)

Supplemental Motor Area: Medial surface of frontal lobe.(Activation and Initiation of speech)

Primary somatosenstory cortex: Post-central gyrus (Perception of touch, pain, heat, pressure).

Primary Auditory Cortex: Also known as Heschl’s Gyrus, located in the poster two-thirds of lateral fissure on upper surface of temporal lobe. (perception of sounds)

Wernicke’s Area: Posterior section of superior temporal gyrus. (Auditory comprehension of language).

Supramarginal Gyrus: Fold of cortex that curves around the end of the lateral fissure. (Language processing)

Angular Gyrus: Immediately posterior to supramarginal gyrus. (written language)