The Speech Motor System

simple model of spoken language production

Conceptual Processing

Language Processing: Lemma selectiEdcodingon (Meaning) Phonological/ Grammatical (form)

Motor Planning Porgramming

Motor Execution (speech articulation)

SPEECH MOTOR SYSTEM

Final Common Pathway

Direct Activation Pathway

Indirect Activation Pathway

Control Circuits

Conceptual-Programming

Level

Sensory Feedback System

Lower Motor Neuron System (LMNS)

Cranial Nerves: once axons leave the brainstem becomes LMMNS

Spinal Nerves: once axons leave the spinal cord becomes LMMNS

FINAL COMMON PATHWAY

Motor commands originate in the cortex travel through the CNS or UMN system synapse with LMNs PNS carries messages to muscles

Control muscles contain

Alpha motor neurons

Gamma motor neurons

Every muscle is innervated by both alpha and gamma motor neurons

the motor neuron

is the only way in which the motor system can communicate with the muscles: all movements ultimately depend on the activity of lower motor neurons

Neurons are composed of:

cell body, axons, dendrites

Dendrites

extensions that receive information

Axon

extends from the cell body, carries electrical potential, sends a chemical message to adjacent neurons

Neurons are classified by the direction information is sent:

Sensory (or afferent) neurons: Motor (or efferent) neurons:

Interneurons:

Interneurons:

send information between sensory neurons and motor neurons. Most interneurons are located in the central nervous system

Motor (or efferent) neurons

send information AWAY from thecentral nervous system to muscles or glands

Sensory (or afferent) neurons

send information from sensory receptors (e.g., in skin, eyes, nose, tongue, ears) TOWARD the central nervous system

Flexion

contraction of a flexor muscle draws a limb IN

Extension

contraction of an extensor muscle moves the limb OUTWARD

A NOTE OR TWO ABOUT MUSCLE TYPES

Skeletal Muscle: Striated

Movements of our body are accomplished by contraction of the skeletal muscles

They are stimulated by a motor neuron

Skeletal muscle fibers have a striated (striped) appearance

ALPHA MOTOR NEURONS

Originate in cranial nerve nuclei in the brainstem and in spinal nerve nuclei in the spinal column• Axon is located inside cranial and spinal nerve

Innervate skeletal muscle and cause the muscle contractions that generate movement• Innervate the extrafusal fibers of skeletal muscles

Contract muscle fibers Alpha Motor Neurons do a LOT of work! Fun fact - a signal travels 268mph along an alpha motor neuron in the spinal cord, the fastest transmission in the human body!

GAMMA MOTOR NEURONS

Same origin, destination, and location as alpha motor neurons

Smaller size, fewer in number

Innervate the intrafusal fibers of skeletal muscles Contract muscle spindles (slight movement)

Help keep muscles ready

activation of the intrafusal fiber does not provide force to the muscle, but is necessary to keep the muscle spindle taut and sensitive to stretch over arrange of muscle lengths

Involved in muscle tone

FCP RECAP

Descending Upper Motor Neuron pathways deliver motor commands to motor nuclei of cranial and spinal nerves in the brainstem.

These messages are delivered to striated muscles via alpha and gamma motor neurons. Ascending pathways carry sensory information back to the cortex

Damage to LMN or FCP:

Damage to single alpha motor neuron may result in weakness or paresis

A muscle that loses input from all its LMNs may result in paralysis

Muscles may lose their bulk atrophy

Damage to the LMN system results in FLACCID DYSARTHRIA

Fasciculations

brief, localized twitches which can be seen

Direct Pathway - Pyramidal Tract/System

VOLUNTARY MOVEMENT

Indirect Pathway -Extrapyramidal Tract/System

INVOLUNTARY MOVEMENT

Control Circuits -

Extrapyramidal Tract/System

DIRECT PATHWAYS (UMN, PYRAMIDAL TRACT/SYSTEM)

Axons do not synapse with other cells until they reach their final destination (connect cortex to the Final Common Pathway)

Crucial to voluntary movement activity

Conscious, controlled, skilled, discrete, and rapid voluntary movements

Divided into corticobulbar (CN V, VII, IX, X, XI, XII, fibers with direct connections to the brainstem nuclei of cranial nerves) and corticospinal tract (spinal nerves, serve respiratory muscles)

Corticospinal Tract: Spinal Nerves

Starts in cerebral cortex and connects with LMNs for voluntary movement• Primary motor cortex and premotor cortex (in each hemisphere) are involved• Path this tract follows: cortex, corona radiata, internal capsule, cerebral peduncle, pons, medulla

Medulla (decussation)

Crossed: lateral corticospinal tract• Uncrossed: anterior corticospinal trac

Corticobulbar Tract: Cranial Nerves

Starts in cerebral cortex and connects with motor nuclei of cranial nerves Path this tract follows: cortex, corona radiata, internal capsule, cerebral peduncle, motor nucleus of brainstem, cranial nerves

UMN generally innervate LMN on the opposite side of the body

Speech cranial nerves are mostly bilaterally innervated

Primary Motor Cortex

Controls individual (or sequences of) movement that require activity of multiple muscle groups

Premotor Cortex

Selection of motor plans for voluntary movement

Sends axons to primary motor cortex and spinal cord

Neurons signal preparation for a movement

Neurons signal sensory information

Mirror neurons

activated when another person is observed performing a movement

Supplementary Motor Area (SMA)

Involved in programming complex sequences of movements and coordinating bilateral movements

Selects movements based on remember sequences of movements

Active during mental rehearsal

Damage to Direct Activation Pathway

Tracts occupy such a small area, even a small infarct can have a large effect

Each hemisphere's UMN pathway innervate LMN on the contralateral side of the body

UMN's of some cranial nerves have bilateral innervation• Lesions can produce weakness and loss or reduction of skilled movements

direct and indirect activation pathway (UMN) innervation of cranial nerves related to speech

CN V, VII upper face, IX, X, XI bilateral

VII lower face and XII contralateral

extrapyramidal tract (includes various short/interconnected pathways)

Corticoreticular tract

Corticorubral tract

Reticulospinal tract

Rubrospinal tract

Vestibulospinal tract

Corticopontine tract

Corticobulbar and Corticospinal tracts

indirect activation pathway (extrapyramidal pathway)

Components include basal ganglia, red nucleus,

substantia nigra, reticular formation, cerebellum

(stopping points)

All of these structures send information to the

LMN

This is a complex system that is not well

understood

It is hard to differentiate its activities from the

basal ganglia and cerebellar control circuits

Involved in automatic motor movements

Works with the autonomic nervous system to

help with posture and muscle tone

damage to indirect pathway

Lesions to the indirect activation pathway affect muscle tone and reflexes, spasticity, and hyperreflexia (effects differ based on muscle type)

Lesions are commonly referred to as UMN lesions

Clinical findings in UMN lesions can change over time

lesion is unilateral

impact can be mild (spasticity in speech, slow movement, or hyperadduction of the vocal folds during phonation)

UUMN dysarthria

lesion is bilateral

impact can be mild to severe (spasticity in speech, slow movement, or hyperadduction of the vocal folds during phonation)

Spastic dysarthria

Bilateral lesions can have hyperactive reflexes, pathologic reflexes, disinhibition of the physicalexpression of emotions

Distinctive Signs of Lesions LMN

Weakness of all movements (voluntary and automatic)

Diminished reflexes

Decreased muscle tone

Atrophy

Fasciculations

Distinctive Signs of Lesions UMN

Weakness and loss of skilled movement/dexterity Hyporeflexia Decreased muscle tone Babinski sign

Distinctive Signs of Lesions indirect activation pathway

Spasticity (and weakness) Clonus Hyperactive stretch reflexes Increased muscle tone

Decorticate or decerebrate posture

LMN origin

brainstem and spinal cord

UMN origin for direct and indirect activation pathway origin

cerebral cortex

LNN destination

muscle

UMN origin for direct and indirect activation pathway destination

Cranial and spinal nerve nuclei

LMN function

Produce muscle actions for reflexes and muscle tone Carry out UMN commands for voluntary movement sand postural adjustments

Direct Activation Pathway Function

Direct voluntary, skilled movements

Indirect Activation Pathway Function

Control posture, tone, and movements supportive of voluntary movement

"CONTROL CIRCUITS"(EXTRAPYRAMIDALTRACT)

Helps to 'control' or program skilled movements

part of the extrapyramidal tract because they play a role in the regulation and modulation of that tract

Coordinate, integrate information streams

do not have direct contact with LMNs

control circuits

The cerebellum is only 10% of the brain's volume, yet it contains over 50% of the total number of neurons in the brain

Regulates and interprets sensory information from muscles, influences next cortical motor output

Modifies the motor commands of the descending pathways to make movements more adaptive and accurate

The cerebellar control circuit appears to help coordinate:

Timing among components of movement

Scale the magnitude of muscle action

Coordinate the sequence of agonist and antagonistic muscle activity

Control Circuits Function

Maintain balance and posture

Input from vestibular receptors and proprioceptors

Coordination of voluntary movements

Timing and force of different muscle groups

Motor learning Adapting and fine-tuning motor programs to make accurate movements

effects of damage to the cerebellum

-Difficulty with posture and balance especially during movement

-Possible development of tremors, discoordination of limbs

-Ataxia

-Ataxic Dysarthria

Difficulties with vision/nystagmus

Ataxia

loss of coordination of muscle movement

basal ganglia contain the

caudate nucleus and putamen, globus pallidus, substantia nigra and subthalamic nucleus

basal ganglia influences

Neurotransmitter balance

movement and regulates posture and tone

Neurotransmitter balance

dopamine is made in the substantia nigra

movement balance

it facilitates or inhibits movement

what does basal ganglia assists in

generating speech programs, especially thosethat maintain a stable musculoskeletal environment inwhich skilled movements can occur

basal ganglia deficits

Hypokinesia - reduced movement

Hyperkinesia - involuntary movement

Damage to the basal ganglia Impact on movement in 2 main ways:

Reduced mobility or hypokinesia (too little movement) Involuntary movement or hyperkinesia (too much movement)

Hypokinetic dysarthria

too little movement, rigidity in movement, masked expression

Hyperkinetic dysarthria -

too much movement, excessive activity in nerve fibers that can no longer be dampened or checked

Basic function to the FCP

Stimulates muscle contraction and movement

Basic function to the Direct Activation Pathway

Influences consciously controlled, skilled voluntary movement

Basic function to the Indirect Activation Pathway

Mediates subconscious or automatic muscle activities

Basic function to the Control Circuits

Integration or coordination of sensory information

Basic function to the Basal Ganglia

plan and program postural and supportive components of motor activity

Basic function to the Cerebellum

Integrates and coordinates execution of smooth, directed movements

Major Structures to the Cerebellum

Cerebellum, thalamus, cerebral cortex (and others)

Major Structures to the Basal Ganglia

Basal ganglia, substantia nigra, subthalamic nucleus, cerebral cortex

Major Structures to the Indirect Activation Pathway

Corticobulbar tracts, corticospinal tracts (and other tracts not discussed)

Major Structures to the Direct Activation Pathway

Corticobulbar tracts,

corticospinal tracts

Major Structures to the FCP

Cranial nerves, spinal nerves

Related Designations FCP

Lower Motor Neuron System

related designations of direct activation pathway

Upper motor neuron system, direct motor system, pyramidal tract

related designations of basal ganglia

extrapyramidal system

Related Designations indirect activation pathway

Upper motor neuron system, indirect motor system, extrapyramidal tract

related designations cerebellum

cerebellum