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
Dendrites
Nucleolus
Axon
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
Types of Neurons
CB= Cell Body
Bipolar
Unipolar
Axons
Multipolar
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
Synapse
PRESYNAPTIC
NEURON
Vesicles
Synaptic
Cleft
POSTSYNAPTIC
NEURON
Lock & Key
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
Synapses
Myelin
Neuron A
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+)
Extracellular fluid
Sodium-Potassium
Pump
Sodium
Channel
Potassium (K+)
Intracellular fluid
Potassium
Channel
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+
A
C
Na+
CL-
A= resting state;
ionic imbalance
B
K+
B= depolarization;
Sodium
channels open;
potential positive
Na+ K+
D
K+ K+
C= Opening of
potassium
channels; potential
returns negative
D= return to rest as
sodium-potassium
pump works
A
B C
D
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
Premotor &
Supplementary
Cortex
Primary
Motor Cortex
Somatosensory
Cortex
Broca’s Area
-Stimulation of these areas can initiate, stop or distort vocalization.
-These behaviors occur in dominant & nondominant hemispheres.
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.
Projections to Cerebral Cortex
•Acts as a relay for
impulses in lower areas
of the brain.
Diencepahalon
Thalamus: What Does it Do?
•Integrates emotion into a
complex motor act.
Pons
Thalamus
Midbrain
Projections
to Cerebellar
Cortex
•Plays a major role in:
• coordinating outgoing information
from cortex,
• integrating incoming
sensory information
• adding emotionality
to speech
Nuclei in thalamus that project to parts of the
cerebral cortex
• Motor area receives its
projections from the
ventrolateral nucleus.
to & from Prenucleus
to & from
Sup. Parietal Lobule
to & from
Parietal
Lobe
Massa
Intermedia
• 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.
Dorsal
Median
Lateral
Dorsal
Ventral
Lateral
Ventral posterior
Lateral
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, CollegeHill 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.