The neurology of speech

What is the Speech Motor System?

•Part of motor system

•Comprises all levels of nervous system

•Organises, controls, executes movements for speech

Major functional divisions of the speech motor system?

1.Final common pathway

2.Direct activation pathway

3.Indirect activation pathway

4.Control circuits

5.Motor programmer

Final common pathway

the 'final' peripheral system through which all movement is achieved

Which system is the final common pathway a part of?

lower motor neurons

Cranial nerves associated with production of speech

•V Trigeminal: jaw movement (opening and closing, lateral movement, innervates soft palate veli palatini -> nasal sound production)

•VII Facial: lip movement (majority of facial muscles, lips most important for speech)

•X Vagus: laryngeal and velar movement (abduction and aductioon of VF, movement of Velum, movement of palatoglossus - important for velar sounds)

•XII hypoglossal: tongue movement (all extrinsic apart from palatoglossus, and all intrinsic)

Cranial nerves associated with speech:

•Have their origin in the brainstem (pons, medulla)

•Are paired

•Are innervated bilaterally, EXCEPT VII (lower face) and XII

•VII (facial) (lower branch) and XII (hypoglossal) are CONTRALATERALLY innervated only

•Are part of the peripheral nervous system

•Are also known as lower motor neurons

Damage to final common pathway

Presentation

•flaccidity (or reduced tone, hypotonicity)

•fasciculations (twitching muscles of tongue)

•Associated with (flaccid) dysarthria

Common causes:

•Trauma to Facial nerves(e.g. surgery, facial injury)

•Acoustic neuroma (vestibular schwannoma, presses on nerves)

•Motor Neurone's Disease

•Myasthenia Gravis (

Direct activation pathway

•Upper motor neurons (found in CNS)

•Pyramidal tract

•Corticonuclear (corticobulbar)-> from brainstem to cortex (and corticospinal tracts) -> from peripheral nerves to spinal column.

•Part of descending tract

•Damage results in weakness and loss of skilled movement (with some increased tone (hypertonicity)- spasticity).

•Bilateral damage is generally needed to cause spasticity in speech

•Damage is associated with dysarthria (spastic dysarthria if bilateral or unilateral upper motor neuron dysarthria if unilateral)

Spasticity

A condition of increased muscular tone causing stiff and awkward movements

Indirect activation pathway

•Extra-pyramidal tract

•Also upper motor neurones

•Functions for speech are poorly understood!

•Mediates supporting muscular role (posture, tone, supplementary movements)

•Tend to get spasticity and hyperreflexia (reflexes are heightened and exaggerated)

•Damage causes dysarthria (spastic dysarthria if bilateral or unilateral upper motor neuron dysarthria if unilateral)

Basal ganglia

•Complex system

•Comprises:

-Caudate nucleus and putamen (striatum) sensory function

-Globus pallidus and putamen (lentiform nucleus) motor function

•Various loops with frontal lobe, thalamus and brainstem.

•Function depends on balance of various neurotransmitters (acetylcholine, dopamine and gamma-aminobutyric acid)

Basal ganglia

•Exact function of individual components BG not understood well

•As a group they help to regulate posture, muscle tone, static muscle contraction, velocity, amplitude and initiation of movement. -> also evidence to suggest that they may mediate some language functions.

Damage to basal ganglia

Presentation

•Hypokinesia (reduced movement)

•Hyperkinesia (excess and involuntary movement)

•Dysarthria (hypokinetic or hyperkinetic)

Typical causes:

•Parkinson's disease (PD)

•Huntington's Disease (HD)

•CVA (cerebro-vascular accident)

Cerebellum function

the "little brain" at the rear of the brainstem; functions include processing sensory input and coordinating fine movement output and balance.

•Imposes its control on posture and movement

•Interacts with the pyramidal and extrapyramidal systems to provide coordination of muscles and muscle groups for production of motor speech

Cerebellum

Some localisation seen in damage

•Posterior= loss of equilibrium

•Anterior = affect posture, gait and truncal tone

•Lateral = skilled, voluntary movements

•Bilateral lesions & generalised degeneration interfere with speech

Damage to cerebellum

)Presentation

•Nystagmus (involuntary eye movement)

•Intention tremor

•Dysmetria (lack of coordination of movement typified by the undershoot or overshoot of intended position with the hand, arm, leg, or eye)

•Dysdiadochokinesia (impaired ability to perform rapid, alternating movements)

•Hypotonia

•Muscle stretch reflexes

•(Ataxic) dysarthria (Ataxic dysarthria results from damage to the part of your brain called the cerebellum)

Typical causes

•Ataxia (disorders that affect co-ordination, balance and speech.)

•Cerebellar CVA

•MS

•Alcohol induced

Motor planning

•Motor planning occurs when phonologic representation (linguistic planning) has been created

•Involves developing a plan to organise and coordinate speech movements (of muscles) Pt knows exactly what they want to say but has difficulty organising movements.

•Localisation: Language dominant hemisphere(left): Sensory and motor cortices, premotor cortex (Broca's area) and supplementary cortex.

•Damage: Apraxia of speech (AOS)

Neurological location for AOS

•Left (dominant) hemisphere

•Fronto-parietal area/ perisylvian area (•Peri = about, around)

•Near Broca's area (for expressive speech)

•AOS therefore often accompanies EXPRESSIVE language problems

•influence of basal ganglia and cerebellum, reticular formation, thalamus, limbic system, R hemisphere

Speech disorders - an important distinction

dysarthria v apraxia of speech.

What is dysarthria?

the collective name for a group of neurologic speech disorders resulting from abnormalities in the strength, speed, range, steadiness, tone or accuracy of movements required for the control of the respiratory, phonatory, resonatory, articulatory and prosodic aspects of speech production. The responsible pathophysiologic disturbances are due to central or peripheral nervous system abnormalities and most often reflect weakness, spasticity, incoordination, involuntary movements, or excessive, reduced or variable muscle tone.

•Dysarthria is a NEUROMUSCULAR speech difficulty

•Due to damage to neurological system

•Affects muscular activity

What is apraxia of speech (AOS)?

"[AOS] is a neurologic speech disorder reflecting an impaired capacity to plan or program sensorimotor commands necessary for directing movements that result in phonetically and prosodically normal speech. It can occur in the absence of... the dysarthrias and in the absence of the disturbance in any component of language"

-acquired AOS is a problem with planning and executing the movements needed for speech. (muscles are not weak, no spasticity or flaccidity).

Differentiating between dysarthria and AOS

Dysarthria

•Results from weakness, incoordination, slowness or imprecision of the articulators

•Consistent errors

•Abnormal oral mechanism findings

•Can affect all components of speech

-fatigue effect

-volitional/spontaneous tasks the same

AOS

•Results from difficult with planning or programming speech movements

•Inconsistent errors (trial and error groping of articulators)

•Oral mechanism findings can be normal (but not typically)

•Typically affects articulation and prosody only

-no fatigue effect

-easier to produce spontaneous speech v volitional. (e.g., able to yawn but not able to open mouth on command.)

What causes dysarthria and AOS?

Dysarthrias and AOS are associated with

damage at particular levels of the nervous

system and with specific stages of the speech neuromotor processes.