Motor Speech Disorders and Brain Structures

Motor Speech Disorders

• Motor speech disorders affect motor planning, programming, coordination, and timing of speech movements.
• Key brain regions important for speech production include the primary motor cortex, basal ganglia, and cerebellum.
• Dysarthria is a disturbance in the mechanisms controlling speech musculature. While language comprehension might be sound, the motor production of speech might be affected.
• Apraxia is an inability to plan or program sensory and motor commands for speech. It's a higher-level processing issue, not a muscle problem.

Structures of the Brain and Motor Control

• The brain has 15-20 billion neurons.
• Speech musculature is innervated bilaterally (left and right hemispheres), unlike limbs, which are controlled contralaterally (opposite side).
• Neural circuitry interacts with neurotransmitters (e.g., dopamine) facilitating signals from nerve cells to target cells (muscles).

Cerebellum

• The cerebellum, also known as the "little brain," coordinates movement by monitoring input from the brain and spinal cord.
• Fine and complex motor activities like speech production are coordinated by circuits in the cerebellum.
• Damage to the cerebellum results in movement discoordination.
• Excessive alcohol consumption can temporarily impair cerebellum function.

Cranial and Spinal Nerves

• The peripheral nervous system (PNS) consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.
• Cranial nerves are vital for speech production.
• Spinal nerves contribute to speech breathing, while the brainstem regulates the basic breathing process.

Basal Ganglia

• Basal ganglia (control circuits) regulate motor function via the primary motor cortex.
• They regulate the motor planning and automatic execution of learned movement sequences.
• Upper Motor Neurons (UMN): Part of the central nervous system located in the cerebral cortex; affect the opposite side of the body (contralaterally). Speech mechanisms, however, are innervated bilaterally through cranial nerves.
• Lower Motor Neurons (LMN): Part of the peripheral nervous system, connecting the central nervous system to the muscle; innervate the same side (ipsilaterally). Damage to the basal ganglia or cerebellum does not directly impact the LMNs.

Motor Speech Disorder: Dysarthria

• Dysarthria is most common in adulthood and involves disturbances in the central and peripheral nervous systems that control speech muscle production.
• It's NOT a language disorder as language comprehension might be intact, but speech motor functions are disrupted.
• Different types include: flaccid (weak muscles), spastic (slowed and reduced force), ataxic (coordination difficulties), hypokinetic (reduced movements), hyperkinetic (involuntary movements), and mixed.
• Common etiologies include Bell's palsy, cerebral palsy, multiple sclerosis, Parkinson's disease, and Huntington's disease.

Motor Speech Disorder: Apraxia

• Apraxia is a motor speech disorder where the individual can't produce speech correctly, although they may be able to write or type the word.
• It's not due to muscle weakness or damage, but rather a higher-level problem with motor control and memory retrieval of motor programs for speech.
• Damage to the left hemisphere, particularly the frontal lobe, is often the cause.
• Speech production is characterized by groping for correct articulatory position, inconsistent errors, wide variations in errors, and increased difficulty with complex or unfamiliar words.
• Recovery is possible with some interventions but progressive neurological diseases usually require AAC devices.

Evaluation of Motor Speech Disorders

• Comprehensive assessment involves examining the oral mechanism, assessing respiratory, phonatory, and articulatory functions, as well as speech quality and prosody.
• Specific speech production tasks can help differentiate between various motor speech disorders (e.g., imitation of single words, sentence imitation, reading aloud, and spontaneous speech).

Treatment and Management of Motor Speech Disorders

• Treatments are dependent on the underlying cause and type of motor speech disorder.
• For dysarthria, interventions may include improving respiratory coordination, voice amplification, and intensive programs like LSVT.
• Speech therapy, supporting non-speech oral motor treatment methods, plus assistive technologies (e.g., AAC) may support recovery.
• For apraxia, interventions include integral stimulation, melodic intonation therapy (MIT), and contrastive stress procedures.

Tic Disorders/Tourette Syndrome

• Characterized by sudden, rapid, recurrent movements (motor tics) or vocalizations (vocal tics).
• It typically starts in childhood.
• Premonitory urges may precede tics.
• Tics can be influenced by emotional/mental states and other co-occurring conditions.
• Neurological, genetic, and toxic prenatal exposures influence the condition's emergence and severity.

Differences between Dysarthria and Apraxia

• Dysarthria involves speech-sound distortions, often consistent errors related to targeted phonemes.
• Apraxia involves different speech-sound substitutions, inconsistent errors, and potentially sounds often not relating to the desired phoneme/sound.