Neural Basis of Speech

2/29/00

Neuron

• Neuron = Nervous system cell

– Neuron cell body (contains nucleus)

– Nucleus (contains genetic material)

– Dendrites (projections; communication from 1 neuron to another)

– Axon (single long process which conducts nerve impulses to muscles, glands or other neurons)

– Rarely can be replaced

– Cannot regenerate

Neuron Structure

Nucleus

Nucleolus

Axon

Dendrites

Neuron• Three basic types:

– Sensory Neurons• Conduct nerve impulses from sensory receptor (eye or ear) to

the brain & spinal cord

• Travel from periphery to central site

• Direction of travel is afferent

– Motor Neurons• Carry neural instructions from the brain to muscles or glands

• Travel from central nervous system to the periphery

• Direction of travel is efferent

– Interneurons• Most numerous of all types

• Constitute neural tissue of brain & spinal cord

Neuron• Three primary structure types:

– Monopolar• Cell body located in a collateral section that connects to

transmitting zone of dendrite & axon

• Cell of somatic sense (sense of touch & pressure)

– Bipolar• Cell body along the main structure of the neuron with the

dendrite extending in one direction from body and the axon in the other direction

• Found in special senses (vision, audition, olfaction)

– Multipolar• Multiple dendrites project from cell body

• Neuron of the CNS & motor neuron innervating muscle

CB= Cell Body

Unipolar Bipolar Multipolar

Axons

Types of Neurons

Neural Connections

• Communication between neurons is achieved by the release of neurotransmitters– Synapse= Tiny gap between 2 neurons– Presynaptic Neuron= Transmits impulse– Postsynaptic Neuron= Receiving impulse

• Excitation (promoting neural activity)

• Inhibition (reducing neural activity)

– Neurotransmitter=Chemicals involved in neural communication

• Released from terminal boutons of one neuron into cleft of synaptic junction

• Contained in synaptic vesicles

PRESYNAPTICNEURON

POSTSYNAPTICNEURON

Vesicles

SynapticCleft

Lock & Key

Synapse

Neurotransmitters• 100 different kinds• Major:

– Glutamate– Aspartate– Gamma-aminobutyric acid (GABA)– Glycine

• Relatively simple & fast action• Central to basic life processes

• Slower Neurotransmitters:– Seratonin– Norepinephrine– Dopamine

Synaptic Connections

Neuron B Neuron C

Neuron A

Synapses

Myelin

Myelin & Glia

• Larger axon insulated with fatty coating- Myelin

– Increases speed of neural transmission

– Reduces interference with the neural message

– Multiple sclerosis- dymyelinating

• Neurons outnumbered by glial cells

– Hold neurons in place & provide nutrients

– Oligodendroglia (form myelin in CNS)

– Schwann (form myelin in the PNS)

Neural Impulse

• Neurons generate electrical impulse traveling the length of the nerve fibers

• Neural activity= electrical & chemical activity

• Neuron is like a battery– Stores electrical potential by accumulating positive

charge in one terminal & excessive negative at the other terminal

– An electrical potential across the membrane is created• Extracellular positive compared to intracellular

• Ions carry charges – positive: sodium (Na+ ) & potassium (K+)

– Negative: chlorine (CL=)

Neural Impulse

• Positive ions- concentrated outside the cell (sodium)

• Negative ions- concentrated inside the cell

• Resting membrane potential (-70 millivolts) created due to excessive positive outside cell– Maintained through sodium-potassium pump

• act to exchange sodium ions found inside the cell with potassium ions found outside the ell

• Neuron at rest= polarized

• Neural activity= depolarization

Sodium (Na+)

Potassium (K+)

Extracellular fluid

Intracellular fluid

Sodium Channel

Potassium Channel

Sodium-PotassiumPump

Neural Impulse

• Action potential occurs= Wave of depolarization

– Depolarization occurs when an action in another

neuron momentarily lowers the voltage of a region of a

membrane

• Causes voltage-controlled gates to open that regulate sodium

channels

• Sodium floods into the cell

• Polarity reverses from -70 to +30 mV

• Cell returns to rest (sodium-potassium pump)

Neural Impulse

• Depolarization effects tiny portion of

membrane at a time.

• Causes a wave down the entire membrane by

causing voltage gated channels to open

• Wave continues until the axon terminal

– Synapse with other neuron

– Transmitted to next neuron

Na+ Na+CL-

K+

K+ K+

A

A

B

B

C

C

D

D

K+Na+

A= resting state; ionic imbalance

B= depolarization;Sodium

channels open; potential positive

C= Opening of potassium

channels; potential returns negative

D= return to rest assodium-potassium

pump works

Neuroanatomy of the Vocal Mechanism

Neuroanatomy of the vocal Mechanism

• Volitional control of muscles of the larynx resides in the

brain.

• Connecting points in brain that have a role in control of

phonation: cortex, subcortical areas, midbrain & medulla.

• Next slides will briefly review phonation neuroanatomy

& neurophysiology.

Cortical Mechanisms of Phonatory Control

• The cerebral cortex is responsible for:– conceptualization, planning, and execution of speech ,

including phonation.

• Three major areas of the cortex responsible for vocalization: – a) Precentral & postcentral gyrus, – b) Anterior (Broca’s) area, – c) Supplementary motor area.

Cortical Areas Involved in Speech Movement Control

-Stimulation of these areas can initiate, stop or distort vocalization.

-These behaviors occur in dominant & nondominant hemispheres.

Premotor & SupplementaryCortex

Broca’s Area

Primary Motor Cortex

Somatosensory Cortex

Speech and Phonation are complex motor acts

• Involves simultaneous activation and control of many muscles.

• Control of these motor acts occurs primarily in the cortex.

• Control of individual muscles occurs lower in the brain.

– No evidence that cortical stimulation produces a response in a single solitary muscle.

• Higher brain function = idealization of the event, integration of sensory information, feedback control, and coordination of various muscles.

Subcortical Mechanisms

• Motor cortex has connections to the Thalamus ( egg shaped in the middle of the cerebral cortex),

– A major portion of the diencephalon or interbrain.– Contain nuclei for language & speech– Relay station from cortical to subcortical brain– Thalamus has major pathways to the motor cortex &

Broca’s area.

• Parts of the diencephalon: a) hypothalamus, b) metathalumus, c) epithalumus, d) subthalumus, & e) third ventricle.

Thalamus: What Does it Do?

•Acts as a relay for impulses in lower areas of the brain.

•Integrates emotion into a complex motor act.

•Plays a major role in:

• coordinating out-going information from cortex,

• integrating incoming sensory information

• adding emotionality to speech

Thalamus

Projections to Cerebral Cortex

Midbrain

Projectionsto CerebellarCortex

Die

nce

pah

alon

Pons

Nuclei in thalamus that project to parts of the cerebral cortex

• Motor area receives its projections from the ventrolateral nucleus.

• 1971- ventrolateral nucleus shown to be responsible for initiation of speech movements & control of loudness, pitch, rate & articulation.

• Broca’s area- receives connections from dorsomedian nuclei.

VentralLateral Ventral posterior

Lateral

DorsalMedian

to & from Prenucleus

to & fromSup. Parietal Lobule

to & fromParietal Lobe

LateralDorsal

MassaIntermedia

Midbrain Structures

• Midbrain (mesencephalon) lies beneath the thalamus.

• Cerebral peduncles lie on anterior surface of the midbrain and connect the cerebrum with the brainstem and spinal cord.

• Posterior side has four colliculi: Superior (visual function), inferior (audition).

• Within midbrain lies the cerebral aqueduct of Sylvius, surrounded by periaqueductal gray.

Periaqueductal Gray: What does it do?

• Stimulation of dorsal and ventrolateral areas of periaqueductal gray = activity in some laryngeal muscles.

• 1985- Larson reported some cells in ventrolateral area stimulate muscle activity, whereas some suppress activity.

• Periaqueductal gray is an intermediate area between recognition of a stimulus and the production of a motor act.

Brainstem

• Bilateral structures in brainstem implicated in the neural control of phonation:

• Nucleus ambiguus

• Nucleus tractus solitarii

• Nucleus parabrachialis

• How do we know these structures are involved in phonation?

Yoshida, Mitsumasu, Hirano Study

• Traced connections among brainstem structures.

• Injected tracer chemical into one nucleus ambiguus.

• Found evidence of tracer throughout the contralateral nuclei, nuclei tractus solitarri bilaterally, in nucleus parabrachialis and bilaterally in the lateral and ventrolateral parts of the periaqueductal gray area, with a predominance ipsilaterally.

• Conclusion: Many interconnections bilaterally among the nucleus ambiguous, nucleus tractus solitarri, and motor roots of vagus.

Cerebellum

• Structure lying posterior to the midbrain area.

• Implicated in the control of movement.• Three main portions: a) vermis, b) pars

intermedia, c) hemispheres• Consists of many traverse folia- increases

surface area.• Fissura prima- fissure separating anterior &

posterior lobes.

References:• Colton, R.H. & Casper, J.K.,(1990), Understanding Voice

Problems: A physiological perspective for diagnosis and treatment,, Williams & Wilkins.

• Bhatnager, S.C. & Andy, O.J., (1995), Neuroscience for the study of communicative disorders, Williams & Wilkins.

• Kuehn, D.P., Lemme, M.L. & Baumgartner, J.M., (1989), Neural basis of speech, hearing, and language, College- Hill Press.

• Lieberman, M., (1991), Neuroanatomy made easy and understandable, Aspen Publishers.

• Netsell, R., (1985), Speech and language evaluation in neurology-adult disorders, Grune & Stratton.

• Poritsky, R., (1992), Neuroanatomy: a functional atlas of parts & pathways, Mosby-Year Book.