BBS 4032 Neuroscience of speech and language

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BBS 4032 Neuroscience of speech and language

• Lecture 4 neuroscience of speech production

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Motor cortex

• Precentral gyrus of the frontal lobe (BA4)
• Pyramidal cells are unique to this cortical area
• Homunculus representation of the human body
  mapped onto motor cortex control of the head,
  face, and muscles for speech is in the lower
  third of the motor strip

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Pyramidal system

• Descending pathway from motor cortex
• A two-neurone system UMN in the motor cortex,
  LMN in the anterior horn of the spinal cord
• Corticospinal and corticobulbar tracts
• Corticobulbar tract is most involved in the
  innervation of muscles of speech production
• Motor pathways are largely INHIBITORY

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Corticospinal Tract

• Corona radiata
• Internal capsule
• Cerebral peduncle of the midbrain
• Pontine nuclei
• Decussation at the medulla (pyramids)
• Uncrossed anterior corticospinal tract
• Crossed lateral corticospinal tract

Hole's Essentials of Human Anatomy and
Physiology, 7th Edition
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Corticobulbar tract

• Terminates in medulla, at nuclei of the cranial
  nerves
• Control of upper facial muscles is bilateral
• Control of lower facial muscles is contralateral

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Damage to Pyramidal tract

• Corticospinal lesions
• Above decussation ? paralysis on the
  contralateral side of the body
• Below decussation ? ipsilateral paralysis
• Right-sided hemiparesis in someone with a
  cortical lesion suggests left hemisphere damage
• Lesions of the motor strip or pyramidal tract
  alone may ? dysarthria disorders of motor speech
  resulting from neurological impairment
• Corticobulbar lesions
• Corticobulbar tracts have ipsilateral as well as
  contralateral fibers, so effects of (unilateral)
  damage tend to be less severe than effects of
  corticospinal lesions
• Corticobulbar tracts synapse with cranial nerve
  nuclei, so symptoms vary depending on which
  cranial nerve is affected

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Cranial nerves for speech

• Trigeminal mixed
• Facial motor / mixed
• Auditory/vestibular sensory
• Glossopharyngeal mixed
• Vagus mixed (mainly sensory)
• Spinal accessory motor
• Hypoglossal motor

Nuclei in pons
Nuclei in medulla
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Upper and lower motor neurons

• UMN all contained within the neuraxis
• Anterior and lateral corticospinal tracts also
  corticobulbar fibers from cortex to brainstem
• Primarily inhibitory
• Damage ? spastic hypertonicity, hyperreflexia,
  Babinskis sign, clonic movements
• LMN second order neurons
• Cranial and spinal nerves that send motor axons
  to periphery
• Final common pathway
• Damage ? flaccid hypotonicity, hyporeflexia,
  fasciculations (muscular dystrophy)

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Signs of UMN and LMN paralysis

• Upper motor neurone signs
• Damage to the corticospinal tract ? hypertonia
• Spastic paralysis
• Increased muscle tone, resistance to joint
  flexion, hyperreflexia (e.g. Babinski sign)
• Lower motor neurone signs
• Lesions to nerves, spinal cord or brainstem ?
  denervation, hypotonia
• Flaccid paralysis
• Atrophy, fasciculations, areflexia / hyporeflexia
• Causes damage to motor units
• E.g. Muscular dystrophy, myasthenia gravis

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Diseases of the motor neurones

• Amyotrophic Lateral Sclerosis (ALS), the most
  common form of MND, both upper and lower motor
  neurones degenerate, resulting in muscle
  weakness, fasciculations (twitching) and
  stiffness.
• Progressive Bulbar Palsy (PBP) (corticobulbar
  pathways) degenerates the nerves that control
  speech and swallowing
• Pseudobulbar palsy damage to both upper and
  lower motor neurones in the corticospinal tract
• Progressive Muscular Atrophy (PMA) affects mainly
  the lower motor neurones
• Primary Lateral Sclerosis (PLS) affects primarily
  the upper motor neurones
• there is much overlap, and other types of motor
  neurone may be affected

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Extrapyramidal system

• Basal ganglia caudate, globus pallidus, putamen
• Other structures reticular formation,
  subthalamus, substantia nigra, red nucleus

http//www.science.uwc.ac.za/physiology/neurology/
Directpw.jpg
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Basal ganglia
http//www.driesen.com/basal_ganglia.htm
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Extrapyramidal functions

• Maintains posture, unconscious control of tone
  and gait
• May influence facial expression
• Carrying out learned motor activities
• Disorders of the extrapyramidal system ?
  akinesia, dyskinesia

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Disorders of the Extrapyramidal system

• Movement disorders often seen as side effects of
  antipsychotic drugs
• Akathisia a pacing / inability to sit still
• Torticollis muscular spasms of neck
• Tardive dyskinesia involuntary, irregular
  muscle movements
• Parkinsons disease tremor, rigidity of limbs
  and trunk, bradykinesia, postural instability /
  impaired balance and coordination

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Cerebellar feedback
http//nawrot.psych.ndsu.nodak.edu/Courses/Psych46
5.S.02/Movement/Cerebellum.html
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Cerebellar damage

• Movement deficits, especially affecting timing
  and sequencing ?cerebellar ataxia
• Discoordination of voluntary movement
• Particular difficulties with initiation and
  termination of movements
• Intention tremors (dysmetria)
• Hypotonia of muscles
• Problems with motor learning
• Causes stroke, trauma, tumor
• Diseases Friedrichs ataxia, CJD

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Dysarthrias

• Spastic dysarthria UMN lesion
• Flaccid dysarthria LMN lesion
• Ataxic dysarthria cerebellar lesion
• Hypokinetic
• Hyperkinetic
• Mixed
• Darley, Aronson Brown (1969)
• Mayo Clinic classifications

Extrapyramidal system
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The dysarthrias
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Research the neural underpinnings of speech
production

• fMRI studies find that speech production involves
  the interaction of many brain areas
• SMA
• Motor cortex
• Cerebellum
• Intrasylvian cortex
• Subtraction methodologies suggest that
  intrasylvian cortex (left anterior insula) is the
  area crucially involved in articulation

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Anterior Insula

• All the areas involved in inner speech are also
  involved in real speech, apart from the
  anterior insula
• The role of the insula seems to be in the
  phonetic and temporal specification of
  articulatory gestures
• Lesions in this region ? Apraxia of speech
• Activation abnormalities affecting the anterior
  insula may ? stuttering