The brain is the most complex organ in the human body. It produces every thought, saves all memories, and executes every action each human body has ever done. The proper functioning of the brain is relied on in order to have a strong working mind and a healthy body. If the correct functioning of the brain falters due to damage, trauma, or other factors, consequences can arise that put obstacles in individuals lives that lie beyond their control. A type of issue many encounter due to neural damage are motor disorders. The motor cortex of the brain is responsible for correctly executing the intended movements an individual wishes to make such as writing with a pencil, opening up a door, smiling, and talking. If the motor cortex becomes damaged, there can be repercussions that adversely affect the everyday motor movements an individual normally does not struggle to execute. One specific motor skill many take for granted is speech production. Because proper speech production heavily relies on the motor cortex, it is often affected when certain areas of the motor cortex gets damaged. Hence, due to the complexity of the brain, depending on the location of damage or abnormal functioning within the motor system, there can be negative effects on one’s speech ability which can produce a variety of speech production disorders.
In order for the motor cortex to execute a motor movement properly, the desired motor movement must go through a series of steps within the motor cortex to ensure that it produces a smooth and precise motor movement when it is initiated. Within the frontal lobe which has the responsibility for both the planning and initiation of motor movements, distinct areas work together to produce a proper action called the prefrontal cortex, the premotor cortex, and the primary motor cortex (“The Forebrain and Initiating Movement” 2016). To begin, the prefrontal cortex’s responsibility involves the planning for complex behaviors (“The Forebrain and Initiating Movement” 2016). Although it does not indicate the particular movements that will be conducted, it merely distinguishes the goal of the proper movements needed to correctly carry out an action (“The Forebrain and Initiating Movement” 2016). In order to begin generating a plan to execute the desired action, the prefrontal cortex projects the signal to the premotor cortex, where the organization of the movement sequences begins (“The Forebrain and Initiating Movement” 2016). At this location, the premotor cortex generates a plan to properly carry out the desired action (Aronoff, slide 5). If the premotor cortex becomes damaged or experiences trauma, proper execution of motor sequences will be adversely affected causing the motor goal to not be completed. It is because of the premotor cortex that a person has the ability to organize multiple muscles for a single action (Aronoff, slide 10).
However, the premotor cortex cannot work alone in order to generate a proper motor plan to properly execute an action. Premotor areas rely on other brain circuits in order to smoothly arrange the motor behaviors. One of the circuits involves a number of major loops that refine motor actions called the basal ganglia. Signals passing through the motor cortex will be polished by the basal ganglia, and then the modified signal will project back to the motor cortex (Aronoff, slide 18). There are two major pathways in the basal ganglia that are responsible for initiating and inhibiting motor movements (Aronoff, slide 27-28). The direct pathway, despite having inhibitory connections, is an excitatory pathway in the basal ganglia meaning that it encourages a certain movement to happen (Aronoff, slide 27). On the contrary, the indirect pathway has an inhibitory effect on some motor movements to prevent them from occurring (Aronoff, slide 28). Therefore, these two pathways are working together to generate the movements a person wants while stopping the motions the person does not want (Aronoff, slide 29). However, voluntary movements are not initiated in this location (Knierim). It is the proper functioning of the excitation and the inhibition of the basal ganglia that will provide the fine movement for the desired motor action (Knierim). The regulation of the desired motor activity in this location contributes largely to the learned complex movements for speech production.
Once the motor plan gets refined by the basal ganglia, the signal gets sent to the primary motor cortex. At this location, the details of how the movement needs to be carried out begin to be specified which will lead to the generation of the motor action (“The Forebrain and Initiating Movement” 2016). Typically, it does not generally control individual muscles directly, but it seems to control individual motor sequences that require the activity of many muscle groups (Knierim). Additionally, although the responsibility of initiating the proper actions to carry out the desired movement is accredited to the primary motor cortex, other areas send information to this location to assist in carrying out the proper movement (Aronoff, slide 10). The motor system does not happen in isolation meaning that it needs information from other areas to help properly execute the motor movement (Aronoff, slide 10) For example, the primary motor cortex constantly receives input from the proprioceptive information from the somatosensory cortex in order to make any necessary motor adjustments (Aronoff, slide 10). For example, if an individual is walking, and something gets stuck under his or her foot, the primary motor cortex will utilize the information from other areas to adjust the motor movement in order to successfully carry out the same desired motor action.
Although the primary motor cortex plays a role in correcting motor movements, the responsibility for correcting on-going motor movements is the cerebellum (Aronoff, slide 6). To illustrate, if someone is reaching out to grab an object, but his or her hand suddenly gets hit, the cerebellum is responsible for quickly correcting that on-going movement in order to continue to grab the object he or she were initially reaching for. Therefore, the cerebellum lies in the middle of the pathway from the motor cortex to the spinal cord and modifies the cortically generated motor plan (Aronoff, slide 5). It encompasses both afferent and efferent pathways meaning signals afferent from the motor cortex to the cerebellum with inputs for visual, auditory, vestibular, and propioceptive information, but signals from the cerebellum efferent to the brainstem, thalamus, and motor cortex which overall allows for it to mediate the motor activity (Aronoff, slide 20; Knierim). Because of the information the cerebellum receives and its ability to correct ongoing movement, it is vital in the timing and coordination of motor movements thus impacting speech production (Aronoff, slide 6). MAYBE ADD SOMETHING ABOUT THE PEDUNCLE PATHWAYS
After the primary motor cortex begins the initiation motor movement, the signal is then sent to the corticospinal tract and the corticobulbar tract (Aronoff, slide 23). The corticospinal tract contributes to motor movement by playing a role in controlling the voluntary movements of the skeletal muscles (Aronoff, slide 24). It projects signals to the spinal cord which will then be projected to the limbs (Aronoff, slide 23). If the signal projects from the primary motor cortex to the corticobulbar tract, then signals from the corticobulbar tract will then project to the cranial nerves which will then follow by sending signals to the muscles that are essential for speech production (Aronoff, slide 28). Out of the twelve cranial nerves, five cranial nerves play a very essential role in the production of speech (Aronoff, slide 27). The trigeminal nerve has the sensory nerves for the face, head, oral and orbital cavities, but it contains the motor nerves for the muscles of mastication such as the jaw and the velum (Aronoff, slide 29). The cranial nerve that is involved in the muscles required for facial expressions and the stapedial reflex also mediates the taste sensation in the anterior two-third of the tongue is the facial nerve (Aronoff slide 23). Therefore, the facial nerve is accountable for the lips which is one of the articulators for speech. Furthermore, the glossopharyngeal nerve is responsible for the tension, touch, temperature, and pain sensation for the pharynx, tonsils, eustachian tube, middle ear cavity, and the soft palate (Aronoff, slide 26). It also is responsible for the taste sensation of the posterior 1/3rd portion of the tongue. Overall, the glossopharyngeal nerve plays a big role in swallowing and talking (Aronoff, slide 26). To continue, the vagus nerve contributes to the muscles in the esophagus and the pharynx for swallowing and the muscles of the larynx for phonation (Aronoff, slide 28). Lastly, the hypoglossal nerve which has unilateral innervation is involved in the movement of the tongue, so if it got damaged, it could result in ipsilateral paralysis of the tongue causing that side to wrinkle and experience atrophy, tongue deviation to the side of the lesion, chewing difficulty, and bilateral damage that can lead to a profound difficulty in swallowing, eating, and speaking (Aronoff, slide 32-34). If any of the previous explained cranial nerves become damaged, many problems may arise in the production of an individual’s speech. The level of severity of speech production due to damage of the cranial nerves varies depending on the nerve that is damage as well as the severity of the damage. MAYBE ADD MORE SOURCES
Because each part of the motor system works together in order to properly execute the movements a person intends to make, for accurate speech protection, it is essential for the motor cortex to properly work. If there is damage to any part of the motor system, there will be various consequences that will cause differing issues which will result in interferences when accurately trying to produce speech. One specific speech production disorder than can result from damage to the motor system is acquired apraxia of speech. Apraxia can be defined as a motor speech planning disorder due to damage to the premotor cortex in the frontal lobe of the brain (Graff-Radford et al. 43). It can be a consequence resulting from damage brought on by varying things including neurodegenerative diseases or strokes (Graff-Radford et al. 43). Neurodegenerative diseases that cause apraxia of speech with another speech production disorder known as aphasia have demonstrated gray matter atrophy predominantly located in the superior premotor cortex extending to both to the precentral gyrus and the supplemental motor area (Graff-Radford et al. 46). On the contrary, if aphasia is not present, neurodegenerative damage causing apraxia of speech has illustrated atrophy of the premotor cortex with a greater incorporation of Broca’s area of the brain (Graff-Radford et al. 46). However, according to research, the most frequent cause of apraxia of speech results from strokes that cause damage to the premotor areas (Duffy et al. 88). Furthermore, the severity of apraxia of speech greatly correlates with how extensive the lesion or area of damage in the premotor cortices experiences (Trupe et al PG #). Chronic apraxia of speech has been strongly associated with larger infarcts (Trupe et al. PG #). ADD MORE TO FINISH OUT THE PARAGRAPH
Because apraxia of speech results from damage in the premotor area of the motor system, those with apraxia of speech struggle to generate a motor plan for proper movement of the articulators for speech production making speech a difficult task (Trupe et al. PG #). Therefore, those with apraxia of speech exemplify abnormal rhythm, stress, and pitch (Trupe et al. PG #). Additionally, they struggle with articulatory inconsistencies on repeated productions on the same words, and they have difficulty initiating the production of a word as well (Trupe et al. PG #). Unlike other speech production disorders, people with apraxia of speech are aware of their errors, therefore they may consistently make attempts to self-correct (Trupe et al. PG #) When trying to correct their failed attempts, the person may make different articulation errors from the original error in the process (Trupe et al. PG #). MAYBE MORE SOURCES
Because apraxia of speech becomes a burden for both the adults and the children who are diagnosed with it, therapy can be given by speech-language pathologists to help alleviate the symptoms. When providing treatment for speech motor disorders like apraxia of speech, the overall goal should be to provide a long-term improvement in speech (Edeal and Gildersleeve-Neumann 96).
Because damages to different areas of the motor cortex results in various consequences, damage to an area besides to premotor cortex will offer different symptoms than those seen in acquired apraxia of speech. Within the brain, there are multiple areas that are said to play a role in the motor programming of movement such as the supplementary motor area, frontal system, basal ganglia, and the cerebellum (Spencer and Rogers PG #). In particular, the cerebellar circuits are thought to play a crucial role in the programming of movement (Spencer and Rogers PG #). Therefore, if this portion of the motor cortex becomes damaged, it can lead to a different speech production disorder called Ataxic Dysarthria. This disorder is characterized as a sensorimotor speech disorder occurring from lesions to the cerebellum that affects its afferent and efferent pathways (Spencer and Slocomb PG #). The features that come along with ataxic dysarthria reflect the lacking of the precision of timing and coordination of motor movements mediated through the cerebellum (Spencer and Slocomb PG #). Because the cerebellar circuit likely mediates speech production similar to the way it controls motor movements, in general, the information about the intended speech goal is sent from the cortex to the cerebellum (Spencer and Slocomb PG #). This will then trigger movement planning of the articulators based on learning, experience, and preliminary sensory information (Spencer and Slocomb PG #). Therefore, the difficulty of the motor programming that has an impact on speech in ataxic dysarthria can be credited to the disruption of the processing of the feedforward processing by cerebellum (Spencer and Slocomb PG #). To elaborate, when an infant is beginning to learn to produce speech sounds, the feedforward commands for a syllable are adjusted on each speech production attempt (Spencer and Rogers PG #). In the beginning, auditory feedback is extremely relied on to produce the proper sound since the feedforward commands are inaccurate (Spencer and Rogers PG #). These corrective commands that the auditory feedback system gives forth are then stored in the cerebellar feedback command to be utilized for the next speech attempt (Spencer and Rogers PG #). The following attempts to produce that sound result in a better feedforward command and less auditory error (Spencer and Rogers PG #). This process will continue until the feedforward command is capable of producing the sound without any auditory error, so the need of the auditory feedback subsystem to contribute to the production of the speech sound is removed unless the speech becomes disturbed in any way (Spencer and Rogers PG #). Therefore, if the feedforward motor signals that are relied on to accurately produce the sound get disrupted due to damage in the cerebellum or its afferent and efferent pathways, then it can produce the motor programming abnormalities seen in ataxic dysarthria. MAYBE EXPLAIN WHAT THE FEEDFORWARD SIGNALS
Due to the damage in the cerebellum or its afferent and efferent pathways, individuals with ataxic dysarthria exhibit many speech abnormalities (Yorkston and Beukelman PG #). First, articulatory inaccuracy co-occurring with irregular articulatory breakdowns are often present (Yorkston and Beukelman PG #). Additionally, prosodic abnormalities such as excess and equal stress, monoloudness, slow rate, monopitch and prolonged syllables are exhibited (Yorkston and Beukelman PG #). Physiological characteristics of ataxic dysarthria illustrate slow articulatory movement and an overall reduced articulatory mobility (Yorkston and Beukelman PG #). Lastly, the acoustic aspect of ataxic dysarthria indicates timing control issues in prolongation of segments and an inclination to equalize segment durations (Yorkston and Beukelman PG #). ADD MORE INFO PLEASE