Brain Structures and Mechanisms for Speech, Language, and Hearing

CNS versus PNS and overall chapter scope

CNS (central nervous system) includes the cerebral hemispheres, brainstem, and spinal cord; PNS (peripheral nervous system) includes nerves emanating from brainstem and spinal cord and ganglia located near but outside the CNS.
Neurons consist of a cell body (soma) and an axon; axons in the CNS form tracts, while axons in the PNS form nerves.
The chapter emphasizes gross anatomy for clinical relevance to speech, language, and hearing, with selective coverage of cells, meninges, ventricles, and blood supply; neurophysiology is interwoven with clinical implications for SLP and audiology.
Brain mechanisms refer to physiology at molecular, cellular, neurochemical, and system levels; brain structures refer to anatomy observable grossly or via imaging.
Somatotopic organization: body parts are represented in a structured way within motor and somatosensory cortices; representations can be overlapping for certain speech apparatus structures.

Anatomical planes and directional terms

Planes used to describe brain sections: coronal, sagittal (midsagittal = midline left-right halves), horizontal (axial/transverse).
Visibility and orientation depend on the plane; structures like the corpus callosum demonstrate variation across planes.
Deep-to-superficial and dorsal-ventral vs anterior-posterior distinctions apply, with caveats below brainstem where dorsal/posterior and ventral/anterior converge.
Deep vs superficial terminology often describes relative location from outside to inside the cortex.

White matter vs Gray matter; Nuclei vs Nerves; Ganglia vs Cells

Gray matter: clusters of neuron cell bodies (cortex is a major gray matter component of the hemispheres).
White matter: myelinated axons forming pathways connecting gray matter regions; appears whitish due to myelin.
Cortex = outer gray matter; subcortical gray matter includes nuclei (caudate, putamen, thalamus, etc.).
Subcortical nuclei are within the cerebral hemispheres (above brainstem) and include basal ganglia structures; cerebellar nuclei are within the cerebellum.
A nucleus is a cluster of neuron cell bodies; a tract (or fasciculus, lemniscus) is a bundle of axons.
Somatotopy: cortical and subcortical regions show body-part–specific clustering of neurons (e.g., primary motor cortex body map).

Somatotopic representation and its nuances

Primary motor cortex shows a somatotopic map with an upside-down body representation: lower body controlled by upper/medial cortex; face, tongue, larynx represented toward the bottom near the Sylvian fissure.
Somatotopic representations vary by function and region; overlap occurs for speech apparatus structures (pharynx, tongue, lips) near the central fissure/sylvian area, termed “somatotopy with overlap.”
Somatotopy is a framework, not an exact, uniform map across all tasks or modalities.

Neuronal circuitry: white matter tracts and connections

Neurons: cell body, axon, terminal button; myelin (in CNS) increases conduction speed; myelin wraps around axons with nodes of Ranvier—important for saltatory conduction.
Fiber tracts interconnect gray matter regions; tracts include association, commissural, and projection pathways.
Association tracts connect cortical areas within a hemisphere; major example: arcuate fasciculus (AF) linking Wernicke’s area (temporal) with Broca’s area (frontal) via parietal connections.
Projection tracts include corticobulbar (cortex to brainstem) and corticospinal (cortex to spinal cord); these are crucial for speech motor control.
Diffusion Tensor Imaging (DTI) allows visualization of major fiber tracts and maps connectivity (e.g., AF, inferior longitudinal fasciculus, uncinate fasciculus).
The arcuate fasciculus connects Wernicke’s area, angular/supramarginal gyrus, and Broca’s area; historically linked to the Wernicke–Geschwind model of language, though modern views recognize more complex networks.
Fiber tracts cross hemispheres via the corpus callosum; anterior and posterior commissures also provide interhemispheric connections.

Brain lobes and major cortical regions

Frontal lobe: motor planning and execution; primary motor cortex located along precentral gyrus; Broca’s area in the inferior frontal gyrus (premotor cortex) involved in speech planning; premotor cortex (PMI) and supplementary motor area (SMA) plan movement; prefrontal cortex supports executive functions and personality.
Parietal lobe: primary somatosensory cortex (postcentral gyrus); PPC (posterior parietal cortex) integrates sensory inputs for action planning; angular gyrus and supramarginal gyrus implicated in higher language functions (metaphor understanding, word meaning, phonology-to-meaning mapping).
Temporal lobe: primary auditory cortex (Heschl’s gyrus) in the superior temporal gyrus; planum temporale important for speech/language perception; Wernicke’s area located posterior to the Sylvian fissure and involved in language comprehension.
Occipital lobe: primary visual cortex (calcarine cortex); higher visual processing.
Insula: revealed when opercula are opened; implicated in speech, language, and swallowing; often left-lateralized for speech functions.
Limbic system: involved in emotion, motivation, memory; includes cingulate gyrus, parahippocampal gyrus, hippocampus, amygdala, and connections with insula and basal ganglia.
Lateralization and specialization: language is left-dominant in about 95% of right-handed individuals and about 85% of left-handed individuals; specialization exists within lateralized regions (e.g., Broca’s and Wernicke’s areas).
Planum temporale: often larger in the left hemisphere; leftward asymmetry linked to speech perception; asymmetry present even in preverbal infants; asymmetries are not necessary for language but are common.

Functional architecture: Broca’s area, premotor, SMA, and language networks

Broca’s area (inferior frontal gyrus) is central to planning and organization of motor behavior for speech; historically linked to expressive language.
Premotor cortex and SMA: PMA/SMA plan movements; primary motor cortex executes movements; distinction important for diagnosing whether speech motor disorders are planning-based (apraxia) or execution-based (dysarthria).
Premotor/SMA vs primary motor cortex: planning before execution; SMA is on medial surface; Broca’s area is part of PMA.
Lateralization: language tends to be left-dominant; specialization within left hemisphere for articulatory programming and motor planning of speech.

Temporal and auditory processing in language

Temporal lobe and auditory cortex: primary auditory cortex (Heschel’s gyrus) processes basic acoustic properties; surrounding secondary auditory cortices handle higher-level processing.
Planum temporale: important for speech and language perception; often larger on the left; asymmetry contributes to speech-processing specialization.
Wernicke’s area: historically linked to language comprehension; perisylvian language areas support comprehension and meaning extraction; imaging shows broad temporal involvement for language meaning and decision-making.
Auditory pathway: cochlea → auditory nerve → cochlear nuclei → brainstem nuclei → inferior colliculi → medial geniculate body (thalamus) → primary auditory cortex; rapid conduction enabling ABR (auditory brainstem response) measures (~5–6 ms latency to inferior colliculus).
Wernicke–Geschwind model summary (classic): acoustic analysis in auditory cortex → Wernicke’s area for meaning → arcuate fasciculus to Broca’s area for production; repetition tasks rely on intact connections among these regions.
Modern view: language networks are distributed and involve bilateral contributions; AF connections and other tracts (e.g., inferior longitudinal fasciculus, uncinate fasciculus) contribute to receptive and expressive processes; the planum temporale and broader perisylvian network show complex lateralization patterns.

Subcortical nuclei and cerebellum in speech motor control

Basal ganglia include caudate, putamen (together the striatum), globus pallidus (lentiform nucleus), subthalamic nucleus, and substantia nigra (brainstem but functionally linked to basal ganglia).
The caudate and putamen lie lateral to the internal capsule; globus pallidus medial to putamen; the subthalamic nucleus lies medial to the thalamus; substantia nigra lies in the midbrain.
Basal ganglia form cortico–striatal–cortical loops that refine motor commands and contribute to planning and execution of movements, including speech motor control.
The internal capsule carries converging fibers from the cortex to subcortical structures; corticobulbar fibers to brainstem and corticospinal fibers to spinal cord travel through the internal capsule and crus cerebri (cerebral peduncles).
Deep brain stimulation (DBS) can target the subthalamic nucleus or globus pallidus to alleviate Parkinsonian symptoms but may have side effects including mood and cognitive changes and potential speech effects.
Thalamus: relay station for sensory information to cortex and a node in sensorimotor loops; multiple thalamic nuclei process auditory, visual, tactile information and relay to cortical areas.
Cerebellum: connected to cortex via corticocerebellar loops and to spinal cord/brainstem via cerebellar peduncles; coordinates movement, balance, timing, and sequencing; may contribute to motor programming for skilled speech acts.

Brainstem and cranial nerves key for speech, swallowing, and hearing

Brainstem consists of midbrain (mesencephalon), pons, and medulla; houses cranial nerve nuclei and important motor/sensory pathways.
Surface features: ventral view shows cranial nerves V, VI, VII, VIII exiting pons/medulla; dorsal view highlights cranial nerve IV root at the midbrain; fourth ventricle boundaries are visible on ventral/dorsal surfaces.
Cranial nerves and nuclei (highlights for speech/language):
- CN V Trigeminal: mixed nerve; motor nucleus for jaw muscles (motor V) and sensory/trigeminal complex with mesencephalic, chief sensory, and spinal trigeminal nuclei; important for jaw movements and face sensation.
- CN VII Facial: mixed; motor nucleus controls facial expression; stapedius muscle (hearing protection); autonomic fibers to lacrimal and salivary glands; sensory components include sensation from ear and taste from anterior two-thirds of tongue.
- CN VIII Auditory-Vestibular: sensory; cochlear and vestibular components; essential for hearing and balance; nuclear and brainstem pathways form the auditory brainstem route to cortex; ABR uses these fast conduction times.
- CN IX Glossopharyngeal: mixed; motor nucleus ambiguus for stylopharyngeus; autonomic pathways to parotid gland via inferior salivatory nucleus; sensory from posterior tongue, pharynx, ear; taste from posterior tongue; autonomic sensory input from carotid bodies.
- CN X Vagus: mixed; motor to pharynx, larynx, velum; intrinsic laryngeal muscles; autonomic fibers to viscera; sensory from larynx, pharynx, ear canal; nucleus ambiguus and dorsal motor nucleus involved.
- CN XI Spinal Accessory: motor to sternocleidomastoid and trapezius; nucleus located in spinal cord (upper cervical) and ascends into skull with IX/X.
- CN XII Hypoglossal: motor to tongue muscles; contralateral innervation (left cortex → right hypoglossal nucleus, etc.).
Cortical innervation patterns for speech muscles: some nuclei are bilaterally innervated (V, VII upper face, nucleus ambiguus, XI), some contralaterally (XII); differences have clinical implications for unilateral lesions.
Clinical relevance: brainstem innervation patterns help localize lesions by observing facial expressions, tongue movements, and swallow/breathing coordination.

Spinal cord and spinal nerves

Spinal cord spans from the medulla to L1–L2 in adults; gray matter forms an H-shaped region with dorsal (sensory) horns and ventral (motor) horns; white matter contains ascending and descending tracts.
Dorsal root ganglia house first-order sensory neurons for many modalities; some muscle spindle fibers (e.g., stretch receptors) bypass dorsal root ganglia and synapse directly on dorsal horn neurons to speed reflexes.
Spinal nerves are mixed (sensory and motor) and exit at each level; levels of innervation generally map to body segments (cervical for arms/neck, thoracic for trunk, lumbar for legs); the diaphragm is innervated by C3–C5, illustrating exceptions to simple segment mapping.
Upper vs lower motor neuron lesions: UMN lesions cause increased muscle tone and hyperreflexia; LMN lesions cause atrophy, fasciculations, and reduced tone; both can yield weakness.

Meninges, ventricles, and CSF circulation

Meninges: dura mater (two layers: periosteal and meningeal), arachnoid mater, and pia mater;
- Falx cerebri: dural partition between cerebral hemispheres.
- Tentorium cerebelli: dural tent over brainstem separating it from cerebellum.
- Supratentorial vs infratentorial regions refer to positions relative to tentorium.
Arachnoid mater contains subarachnoid space with CSF; arachnoid villi (arachnoid granulations) drain CSF into venous sinuses.
Pia mater: closely adherent to brain surface.
Meningeal spaces can bleed (epidural, subdural hematomas) creating life-threatening pressure.
Ventricular system and CSF production: choroid plexus in lateral ventricles produces CSF; CSF flows Monro foramen → third ventricle → cerebral aqueduct → fourth ventricle; CSF exits to subarachnoid space via foramina of Magendie and Luschka; central canal continues down the spinal cord.
Blood-brain barrier: vascular barrier protecting brain chemistry; important for CNS pharmacology (e.g., L-DOPA crosses barrier while dopamine itself does not).

Circulation and blood supply to the brain

Anterior circulation: supplied by internal carotid arteries via MCA and ACA branches; MCA supplies lateral hemispheres (perisylvian language areas in dominant hemisphere); ACA supplies medial frontal/parietal areas and parts of corpus callosum.
Posterior circulation: supplied by vertebral arteries forming basilar artery, giving off AICA and SCA; basilar artery gives PCA, supplying occipital lobes and parts of the thalamus and corpus callosum.
Circle of Willis: arterial circle providing collateral flow through ACA, MCA, PCA connections via anterior/posterior communicating arteries; individual variation exists.
Dominant hemisphere MRI and stroke risk: left MCA supplies most lateral perisylvian language regions; left MCA stroke commonly affects speech/language production and comprehension; blockages can vary in location to affect production, comprehension, or both.
Blood-brain barrier relevance in pharmacology and disease treatment.

The cerebellum and basal ganglia in motor control and speech

Cerebellum: coordinates movement sequencing, timing, and balance; connects to cortex via cortico-cerebello-cortical loops; interacts with brainstem and spinal pathways to fine-tune speech articulation and respiration.
Basal ganglia: include caudate, putamen (striatum), globus pallidus, subthalamic nucleus, and substantia nigra; form cortico-striatal-thalamic loops for motor refinement, especially in sequencing and rhythmical aspects of movement, including speech motor control.
DBS in Parkinson’s disease can improve limb function but may cause mood, cognitive, sleep, and speech side effects; careful consideration of trade-offs.

The brain’s functional architecture for speech, language, and hearing

Wernicke–Geschwind model (classic): auditory analysis in primary auditory cortex → Wernicke’s area for comprehension → arcuate fasciculus to Broca’s area for production; repetition tasks rely on intact connections.
Arcuate fasciculus (AF): connects Wernicke’s area to Broca’s area through parietal regions; AF is central to language networks; modern views acknowledge broader network involvement beyond this simple relay.
Diffusion tensor imaging (DTI): enables noninvasive tracking of fiber tracts (AF, inferior longitudinal fasciculus, uncinate fasciculus);
Planum temporale: left-dominant in many individuals; leftward asymmetry associated with speech/language capabilities; asymmetry is not strictly necessary for language.
Insula: left-lateralized involvement in speech motor control and possibly speech perception; insular damage can affect speech and swallowing.

Neurons, glia, and basic signaling in the CNS

Glial cells: astrocytes (support, extracellular environment regulation, synaptic anchoring), oligodendrocytes (myelination in CNS; Schwann cells in PNS), microglia (remove dead material), and ependymal cells (CSF production walls of ventricles).
Neurons: signaling cells with soma, dendrites, axon, and terminal buttons; axons carry signals to terminals; dendrites receive signals via neurotransmitter receptors.
Resting potential: resting membrane potential ~ $V_{rest} \,\approx\, -70\ \text{mV}$ ; membrane is more permeable to K+ than Na+ at rest, yielding a negative intracellular environment.
Action potential: rapid depolarization to threshold (~ $V_{threshold} \,\approx\, -45\ \text{mV}$ ), Na+ influx causes peak potentials (~ $+40\ \text{mV}$ ), followed by K+ efflux and repolarization; after-hyperpolarization occurs briefly below resting potential.
Saltatory conduction: myelinated axons conduct impulses quickly by jumping between nodes of Ranvier; conduction speed depends on myelination and axon diameter (larger diameters conduct faster); example speeds: $v<em>{ulnar} \ge 60\ \mathrm{m/s},\ v</em>{facial} \approx 50\ \mathrm{m/s},\ v_{trigeminal} \approx 55\ \mathrm{m/s}$ .
Synapse: presynaptic terminal (releases neurotransmitter), synaptic cleft, postsynaptic membrane with receptors; neurotransmitter binding alters postsynaptic membrane permeability and potential.
Neurotransmitters: table of common neurotransmitters (glutamate, GABA, dopamine, norepinephrine, epinephrine, serotonin, acetylcholine) with primary excitatory/inhibitory roles and associated behaviors; note that some transmitters have multiple actions depending on receptor type.
Neuromuscular junction: peripheral synapse between motor neuron terminal and muscle endplate; acetylcholine released, binds to acetylcholine receptors on muscle to induce depolarization and contraction.

Cortical innervation patterns and clinical relevance

Cortical innervation patterns for five paired brainstem motor nuclei (V, VII, IX/X/XI, XII) show bilateral innervation for some nuclei and contralateral for others; e.g., facial nucleus shows bilateral innervation for upper face and contralateral for lower face; hypoglossal nucleus is predominantly contralateral.
These patterns help interpret clinical signs in unilateral lesions: unilateral cortical damage to tongue motor area typically causes contralateral tongue weakness; unilateral facial weakness often involves lower face on the side opposite the lesion due to contralateral control of lower-face muscles.
The innervation patterns matter for diagnosing speech motor disorders and understanding how lesions affect speech production and swallowing.

The spinal cord, spinal nerves, and sensory pathways

Spinal cord anatomy: gray matter in H-shaped region; dorsal horns (sensory) and ventral horns (motor); white matter on the outside contains ascending and descending tracts.
Spinal nerves: dorsal (sensory) roots and ventral (motor) roots join to form mixed spinal nerves; dorsal root ganglia contain first-order sensory neuron cell bodies.
Sensory pathways: posterior column-medial lemniscus conveys fine touch, vibration, and proprioception; crosses in the medulla; anterolateral tracts convey pain, temperature, crude touch and cross near entry into the spinal cord.
Motor pathways: corticospinal tract crosses at the pyramidal decussation in the medulla (~ $80\%$ cross, leaving about $20\%$ in the same side as origin); corticobulbar and corticospinal tracts descend via corona radiata, internal capsule, cerebral peduncles, pons, and medulla to brainstem or spinal motor neurons.
Upper motor neuron lesions vs lower motor neuron lesions can affect speech musculature depending on the site of damage along the pathway.

Summary of key terms and concepts

Central nervous system (CNS) vs Peripheral nervous system (PNS)
Planes: coronal, sagittal, horizontal; midsagittal plane; parasagittal
White matter vs gray matter
Nuclei vs ganglia; basal ganglia; thalamus; cerebellar nuclei
Somatotopic organization and somatotopy-with-overlap (speech muscles located toward ventral parts of motor cortex)
Association vs commissural vs projection tracts; arcuate fasciculus; other major tracts
Wernicke’s area; Broca’s area; premotor cortex; SMA; insula; planum temporale
Insula’s role in speech, language, and swallowing
Limbic system components and functions in emotion and memory
Diffusion Tensor Imaging (DTI) and tractography
Basal ganglia function and circuits; deep brain stimulation (DBS) implications
Brainstem organization and cranial nerves relevant to speech, swallowing, and hearing
Spinal cord organization and spinal nerves; reflexes
Meninges, CSF production and circulation; ventricles; arachnoid villi
Circle of Willis and cerebral circulatory patterns; MCA’s role in language
Blood-brain barrier and pharmacologic implications
Neurons and glia; resting potential, action potential, saltatory conduction; synapses; neurotransmitters; neuromuscular junction
Clinical implications: lateralization, aphasia, apraxia, dysarthria, dysphagia, and hearing disorders

Quantitative and formula-driven highlights

Resting potential: $V_{rest} \approx -70\ \text{mV}$
Action potential threshold: $V_{threshold} \approx -45\ \text{mV}$
Peak action potential: $V_{peak} \approx +40\ \text{mV}$
Conduction speeds (example fibers):
- $v_{ULNAR} \ge 60\ \mathrm{m/s}$
- $v_{FACIAL} \approx 50\ \mathrm{m/s}$
- $v_{TRIGEMINAL} \approx 55\ \mathrm{m/s}$
Language lateralization statistics:
- Left-hemisphere language dominance in about $95\%$ of right-handed and $85\%$ of left-handed individuals (approximate values given in sources).
Pyramidal decussation: about $80\%$ of corticospinal fibers cross at the pyramidal decussation; the remainder descend on the same side.
Two-thirds of the cortex may be hidden inside sulcal walls in the human brain (approximate estimate).

Important figures and tables (referenced concepts)

Figure 7–1: CNS vs PNS; major components of CNS and PNS
Figure 7–2/7–3: MR images showing corpus callosum in coronal, sagittal, and horizontal planes; direction terminology
Figure 7–4: Four views of the cerebral hemispheres; lobes colored; insula and limbic regions highlighted
Figure 7–5: Somatotopic representations along primary motor and sensory cortices
Figure 7–6/7–9: Planar/oblique views of auditory cortex and planum temporale; asymmetry left vs right
Figure 7–8: DTI reconstruction of arcuate fasciculus and other association tracts
Table 7–1: Classification of tracts (association, striatal, commissural, descending/ascending projection tracts)
Table 7–2: Cranial nerves, nuclei and basic functions
Table 7–3: Cortical innervation patterns of motor nuclei (bilateral vs contralateral)
Table 7–4: Glial cells and neurons types
Table 7–5: Neurotransmitters, primary functions, and behaviors
Figure 7–11/7–12/7–14: Internal capsule anatomy; corticobulbar and corticospinal tract paths
Figure 7–16/7–18/7–21: Brainstem surface anatomy and cranial nerve nuclei locations
Figure 7–31/7–32/7–35: Ventricular system and circle of Willis
Figure 7–37: Distribution of cerebral arterial supply with emphasis on MCA

Practical implications for speech-language pathology and audiology

Lesion localization helps interpret speech and language deficits; e.g., left MCA strokes frequently impact perisylvian language areas and internal capsule, affecting production and comprehension.
Somatotopy explains selective vulnerability: strokes affecting orofacial regions impact speech muscles (jaw, tongue, lips, larynx) more than distant body parts due to disproportionate representation.
Diffusion imaging (DTI) helps map language networks in individuals and guide surgical planning and prognosis.
Knowledge of cranial nerve nuclei and brainstem pathways informs assessment of dysphagia, dysarthria, phonation, resonance, and reflexes.
Insula and planum temporale involvement can influence speech motor control and perception; lateralization patterns may affect rehabilitation strategies.
Understanding CSF dynamics and meninges helps interpret clinical imaging findings for meningitis, hemorrhage, and hydrocephalus affecting communication abilities.