SPH405

Darley, et al, posited 6 major components or levels that are involved in motor (speech) organization. Each is related to certain anatomical structures and is characterized by specific functional signs. In later courses, your examination of Dysarthria types may be clarified by examining these neuro-motor levels

LOWER MOTOR NEURON (LMN).

This level is represented anatomically by the Anterior Horns of the Spinal Cord (with the Spinal nerves); the Motor Nuclei of the Brainstem (with the Cranial nerves). Remember that these are components of the Peripheral Nervous System. The Lower Motor Neuron is known by several names. "Final Common Pathway" is one of the most common, because all neurological impulses going to or from muscles are ultimately conveyed via the LMN.

It is a difficult but important concept for some to consider that several types of input feed into the Lower Motor Neuron. I like to imagine a soda fountain with carbonated water and maybe a couple of flavors pouring out into a cup. The cup is the LMN, the final common pathway to the muscle, through all the various types of motor impulses travel to the muscle fibers. If all is normal, these types of input assure that the muscle contraction will have the speed, strength, steadiness and accuracy intended by the individual.

You will also hear the terms "Alpha" and "Gamma" applied to the LMN. Alpha Motor Neurons are those that initiate direct or voluntary movement. They fire fast, and receive their input directly from the CNS. It is the Alpha LMN's that are properly called the "Final Common Pathways." Gamma Motor Neurons function in the maintenance of tonus for movement regulation. They receive unconscious input from afferent impulses arising in the muscle spindles in a feedback loop arrangement. They are slow firing.

The function of the LMN is reflexive. This means the neuron sends impulses to the muscle sufficient for the job at hand.

This job might be simply to support an extremity or it might be to perform a more active task.

"Direct" or Alpha LMN's perform quick, transient or phasic movements.

"Indirect" or Gamma LMN's perform sustained or postural movements.

If disease presents in the Lower Motor Neurons, the result is the loss of all motor innervation from the CNS.

Darley refers to the Lower Motor Neuron level as the BULBAR level. This might be a little confusing, since we know the term "Bulbar" as it refers to structures of the Brain Stem. This level is probably called "Bulbar" because the cranial nerves have their cell bodies in the Brain Stem. Cell bodies of other Lower Motor Neurons are, of course, found in the anterior (Ventral) Spinal Cord horns.

Disease of the Lower Motor Neurons produces FLACCIDITY. which is a complete loss of muscle tonus. Flaccid muscles become wasted and give no resistance to stretch. This loss occurs because there is no longer any connection between the muscle and the Central Nervous System.

The characteristics of flaccid speech are: hypernasality, imprecise consonants, breathiness, monopitch.

VESTIBULAR-RETICULAR level is next.

This level concerns the nuclear groups of the Brainstem, or the neuronal pools of the Brainstem; as well as certain tracts that project to the lower motor neurons.

These tracts include the Medial Lemniscus, Trapezoid Bodies, etc.

Darley avoids use of the term BULBAR to avoid confusion with the Lower Motor Neuron level, even though all structures of the Brainstem are, technically, Bulbar.

Disease at this level produces flaccidity mixed with other signs, such as disorders of the special senses. the special senses become involved since they have relay centers that course through the Brainstem.

Next up in the hierarchy is the EXTRAPYRAMIDAL level.

This level is represented anatomically by the BASAL GANGLIA, although there are other central nervous system components involved, including Brainstem structures and components of the Cerebellum.

Extrapyramidal structures are involved in the refinement of motor response.

1. The functions of the Basal Ganglia (a.k.a.: Basal Nuclei; Striate Bodies) functions are generally not conscious. They seem to function on their own. This automaticity is probably good, since we would never be able to keep up with our movement needs if we had to think about refining them. When there is disease at this level, there is very little the afflicted individual can do to correct the resulting movement disorder.

2. Disease at this level produces athetosis. Athetosis is characterized by writhing, continuous movements of the extremities. Hypokinetic and hyperkinetic muscle contractions occur in extrapyramidal disease, since it appears to be associated with the initiation and the termination of movement. the extrapyramidal level is also associated with "stereotyped" flexion or extension movements, such as "reaching" or "walking."

CEREBRAL CORTEX is the highest purely motor level.

Voluntary movement is associated with this level. The most important of these movements involve performance and learning of skillful acts.

Volitional movements involve inhibition or regulation of several reflexive patterns of movement.

Learning of skillful acts involve regulation of the patterned movements of extension for reaching or exploring and of withdrawal for defense and avoidance.

They also involve the coordination or integration of: inhibition of antigravity responses; replacement of the antigravity response to free an extremity or for sustained postural tone for infinite variations of movement patterns; performance of rapid, strong, accurate and phasic movements; synergy of antagonistic muscles; synchronization of multiple joints; programming and monitoring of the performance.

UPPER MOTOR NEURON is the term frequently associated with the motor component of the cerebral cortex.

Technically, there are two components of the upper motor system. the indirect system includes the premotor cortex, but also components of the extrapyramidal tracts. These tracts inhibit movement as described above.

The direct system is the one with which we are most familiar, consisting of the corticobulbar tracts. These upper motor neurons are associated with initiating the directed, volitional movements, but with inhibiting certain other movements.

Disease at this level produces SPASTICITY. Spasticity is most often characterized by hyperactive stretch reflex and increased muscle tone. The hyperactivity is thought to be related to the loss of cortical inhibition of the stretch reflex , and the individual has great difficulty using antagonistic muscle groups because of this disinhibition. It is noted that some sources have reported decreased muscle tone associated with disease at the cortical level.

The CEREBELLAR LEVEL is the 5th level.

The Cerebellum is generally attributed with the coordination and synergy of opposing groups of muscles. These muscles function as flexor-extender groups, and are called "Agonists or Antagonists."

The Cerebellar level may be considered as an "Collateral" or "Accessory" level and may not be properly placed in a hierarchy.

Disease at this level produces ATAXIA, or lack of coordination in synergistic movements.

At the highest level, the CONCEPTUAL PROGRAMMING LEVEL the planning of motor activity lies. In these movements, the individual performs purposeful, intentional movements, with some ideation or conceptualization involved before execution. At this level, the cognitive integration of sensory, and memory information is related to the ongoing movements of motor activity, including speech.

Dysfunction at this level interferes with planned movements and results in APRAXIA. Individuals with apraxia may perform unintentional movements with relative ease, but find it very challenging or impossible to perform planned, conceptualized movements.

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E-mail Bill Culbertson at bill.culbertson@nau.edu
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