Chapter 14 – The Brain and Cranial Nerves

Major Brain Regions & Basic Functions

• Cerebrum- Largest region, comprising the majority of the brain's volume; divided into left and right hemispheres.

  • Responsible for higher mental functions: conscious thought, intellect, memory formation and retrieval, and voluntary motor control.

  • Its surface gray matter is the cerebral cortex, characterized by folds called gyri (ridges) separated by sulci (shallow grooves) and fissures (deep grooves). These folds increase the surface area.

• Cerebellum- The second largest brain region; located inferior to the cerebrum and posterior to the brainstem. It consists of two hemispheres and the cerebellar cortex.

  • Primarily functions in coordinating repetitive and complex somatic motor patterns, adjusting posture, and fine-tuning the output of other motor centers to ensure smooth, balanced movements.

• Diencephalon- Situated deep beneath the cerebrum and cerebellum, connecting them to the brainstem.

  • Thalamus: A crucial sensory relay and processing center, sending nearly all sensory information (except olfaction) to the cerebral cortex for interpretation.

  • Hypothalamus: Manages various functions including emotions, autonomic nervous system regulation, and hormone production. It forms a vital link between the nervous and endocrine systems via its connection to the pituitary gland through the infundibulum.

• Brainstem- Composed of the midbrain, pons, and medulla oblongata. It is directly continuous with the spinal cord.

  • Serves as a vital relay for information flowing between the spinal cord \leftrightarrow higher brain centers. It also houses essential autonomic and reflex nuclei controlling vital functions like breathing and heart rate.

Brain Protection & Support

• Blood Supply & Barriers- Blood is supplied to the brain primarily by the internal carotid and vertebral arteries. Venous drainage occurs via the internal jugular veins.

  • Cerebrovascular accident (stroke): Occurs when blood flow to a part of the brain is interrupted, leading to rapid neuron death (within minutes).

  • Blood–brain barrier (BBB): A protective barrier formed by tight junctions within CNS capillary endothelium, regulated by astrocytes, which limits the passage of substances from the blood. It permits diffusion of lipid-soluble agents (e.g., O2, CO2, steroids, alcohols).

  • Breaks in BBB: Specific regions lack a complete BBB for specific functions, including parts of the hypothalamus, posterior pituitary, pineal gland, and the choroid plexus (which forms a blood-CSF barrier).

Medulla Oblongata (MO)

• The most inferior part of the brainstem, connecting directly to the spinal cord, with a continuous central canal.

• Contains three primary groups of nuclei:

  1. Autonomic/Reflex Centers: Regulate vital involuntary functions.

     • Cardiovascular centers (cardiac and vasomotor) regulate heart rate and force of contraction, and control peripheral blood flow (blood pressure).

     • Respiratory rhythmicity centers set the basic pace of breathing, working in conjunction with the pons.

     • Reticular formation: Involved in autonomic integration and essential for consciousness.

  2. Cranial-nerve nuclei: Associated with cranial nerves VIII (part), IX, X, XI, and XII.

  3. Relay Stations: Process and transmit sensory information.

     • Gracile & cuneate nuclei: Receive somatic sensory input (fine touch, proprioception) from the spinal cord, with fibers decussating (crossing over) at the pyramids before ascending to the thalamus.

     • Solitary nucleus: Processes visceral sensory information (e.g., from the digestive and cardiovascular systems).

     • Inferior olivary complex: A motor relay to the cerebellum, crucial for motor learning.

Pons

• A prominent bulge anterior to the cerebellum, superior to the medulla oblongata, serving as a bridge linking the cerebellum \leftrightarrow brainstem.

• Key Contents:

  • Sensory and motor nuclei for cranial nerves V (Trigeminal), VI (Abducens), VII (Facial), and part of VIII (Vestibulocochlear).

  • Respiratory nuclei: Apneustic and pneumotaxic centers, which fine-tune and adjust the respiratory rhythm set by the medulla oblongata.

  • Relay nuclei and transverse pontine fibers, which connect opposite cerebellar hemispheres for coordination.

  • Ascending (sensory) and descending (motor) tracts that pass through the brainstem.

Midbrain (Mesencephalon)

• The most superior part of the brainstem, connecting the diencephalon to the pons.

• Tectum (roof): Contains the corpora quadrigemina, composed of two pairs of colliculi:

  • Superior colliculi: Involved in visual reflexes (e.g., head/eye movements in response to visual stimuli).

  • Inferior colliculi: Involved in auditory reflexes (e.g., head movements in response to sudden sounds).

• Tegmentum: Contains important nuclei involved in motor control.

  • Red nucleus: Highly vascularized, involved in subconscious motor control (e.g., upper limb tone).

  • Substantia nigra: Produces dopamine; plays a crucial role in regulating basal nuclei activity. Its degeneration is a primary cause of Parkinson’s disease.

• Cerebral peduncles: Large ventrolateral bundles of nerve fibers containing descending corticospinal and corticopontine fibers, which transmit motor commands from the cerebrum.

• Also plays a key role in maintaining consciousness via the reticular activating system.

Cerebellum

• Features a highly folded surface gray matter known as the cerebellar cortex, appearing as leaf-like folds called folia. It is divided into anterior, posterior, and flocculonodular lobes by fissures, including the primary fissure.

• The midline vermis separates the two cerebellar hemispheres.

• Internal white matter, known as the arbor vitae ("tree of life" due to its branching appearance), contains embedded cerebellar nuclei that relay output to Purkinje cells.

• Purkinje layer: Characterized by large, highly branched Purkinje neurons, each receiving approximately 2 \times 10^5 synapses, indicative of extensive integration.

• Cerebellar peduncles: Three pairs (superior, middle, inferior) of thick nerve tracts that connect the cerebellum to the midbrain, pons, and medulla oblongata, respectively, allowing communication with other CNS parts.

• Functions: Primarily involved in adjusting postural muscles to maintain balance and equilibrium, and in programming and fine-tuning both conscious and subconscious movements, ensuring coordination and precision.

• Disorder: Ataxia (lack of voluntary motor coordination, appearing as a clumsy, unstable gait) can result from trauma, stroke, or alcohol intoxication.

Diencephalon

• Epithalamus: Forms the roof of the diencephalon; houses the pineal gland, which secretes melatonin and plays a crucial role in regulating circadian rhythms (sleep-wake cycles).

• Thalamus- A bilateral, egg-shaped structure serving as a major relay and filter station for sensory information ascending to the cerebral cortex. The two sides are separated by the third ventricle and often linked by an interthalamic adhesion (intermediate mass).

  • Contains multiple nuclei groups, each specialized for processing and relaying different types of information:

    • Anterior nuclei: Part of the limbic system, involved in emotions and memory.

    • Medial nuclei: Contribute to awareness of emotional states and integration of sensory information.

    • Ventral nuclei: General sensory relay to the primary somatosensory cortex.

    • Dorsal nuclei (e.g., pulvinar): Involved in sensory integration and associations.

    • Geniculate bodies: Lateral geniculate bodies relay visual information, and medial geniculate bodies relay auditory information; both also subtly modify emotional output.

• Hypothalamus- Located inferior to the thalamus; key landmarks include the paired mammillary bodies (involved in eating reflexes and memory), the infundibulum (connecting stalk to the pituitary gland), and the tuber cinereum (involved in hormone production).

  • Exerts control over numerous vital functions through its eight key functions:

  1. Secrete ADH (from supra-optic nucleus) & oxytocin (from paraventricular nucleus): Hormones released into circulation by the posterior pituitary.

  2. Regulate body temperature: Via the pre-optic area, initiating sweating or shivering.

  3. Control autonomic centers: Directly oversees centers in the medulla oblongata and pons, influencing heart rate, blood pressure, digestion, etc.

  4. Coordinate voluntary & autonomic activities: Integrates conscious actions with unconscious bodily responses.

  5. Integrate nervous & endocrine systems: The primary bridge between these two major control systems.

  6. Drive subconscious skeletal muscle contractions: Associated with rage, pleasure, pain, and sexual arousal.

  7. Produce emotions & drives: Generates feelings of hunger, thirst, satiety, and sexual urges.

  8. Regulate circadian rhythms: The suprachiasmatic nucleus acts as the body’s master clock.

Limbic System

• A functional system (not a distinct anatomical structure) spanning portions of the cerebrum and diencephalon; critically integrates emotion, memory formation, and motivation.

• Key Components:

  • Limbic lobe: Includes the cingulate gyrus, dentate gyrus, parahippocampal gyrus, and the hippocampus (vital for memory consolidation).

  • Amygdaloid body: Plays a central role in emotion (especially fear and aggression) and interfaces the limbic system \leftrightarrow sensory input.

  • Fornix: A C-shaped white-matter tract that connects the hippocampus \leftrightarrow the mammillary bodies of the hypothalamus, crucial for memory circuit.

  • Anterior thalamic nuclei: Relay nuclei for the limbic system, connecting the mammillary bodies with the cingulate gyrus.

  • Reticular formation: Contributes to general alertness, arousal, and sleep regulation.

Cerebrum – Gross Landmarks

• Longitudinal fissure: A deep groove that completely separates the left and right cerebral hemispheres.

• Central sulcus: A prominent groove that divides the frontal lobe from the parietal lobe. The pre-central gyrus (motor) lies anterior to it, and the post-central gyrus (sensory) lies posterior.

• Lateral sulcus (Sylvian fissure): Separates the frontal and parietal lobes superiorly from the temporal lobe inferiorly. Deep to this sulcus lies the insula (fifth lobe of the cerebrum).

• Parieto-occipital sulcus: Divides the parietal and occipital lobes, more prominent on the medial surface.

Cerebral White Matter Fibers

• Association fibers– Connect different parts within the same cerebral hemisphere.

  • Arcuate fibers: Short, connect gyri \leftrightarrow gyri within the same lobe.

  • Longitudinal fasciculi: Longer, connect lobes \leftrightarrow lobes within the same hemisphere.

• Commissural fibers– Connect corresponding gray areas of the two hemispheres.

  • Corpus callosum: The largest, most important commissure, forming a broad band of nerve fibers connecting the two hemispheres.

  • Anterior commissure: Smaller, connects parts of the temporal lobes and olfactory bulbs.

• Projection fibers– Connect the cerebral cortex \leftrightarrow lower brain centers (e.g., brainstem, spinal cord) or vice versa. These fibers converge and fan out from the internal capsule, a dense band of white matter.

Basal Nuclei

• A group of deep gray matter masses situated within the white matter of the cerebrum.

• Major nuclei include: caudate nucleus (with a distinctive head and tail), lentiform nucleus (subdivided into the putamen and globus pallidus), and the claustrum.

• Functions: Involved in subconscious muscle tone regulation, coordination of learned body movement patterns (e.g., swinging arms while walking), and inhibiting unwanted movements. Overactivity or imbalance in this system (often due to dopamine deficiency) is associated with Parkinson’s disease signs.

Functional Areas of Cortex

• Motor areas: Control voluntary movements.

  • Primary motor cortex: Located in the pre-central gyrus, containing large pyramidal cells that initiate voluntary movements.

  • Premotor cortex (somatic motor association area): Plans and coordinates learned, skilled movements.

• Sensory areas: Receive and interpret conscious sensory information.

  • Primary somatosensory cortex: Located in the post-central gyrus, receives general somatic sensory inputs (touch, pressure, pain, temperature, proprioception). Somatosensory association areas interpret these sensations.

• Special senses cortices: Dedicated areas for specific senses.

  • Visual cortex: Processes visual information.

  • Auditory cortex: Processes sound information.

  • Olfactory cortex: Processes smell information.

  • Gustatory cortex: Processes taste information.

• Integrative centers: Complex areas for processing and integrating information.

  • Wernicke’s area: Critical for language comprehension (understanding spoken and written language).

  • Broca’s area: Essential for speech production (forming words and speaking).

  • Prefrontal cortex: Located in the frontal lobe, involved in abstract intellect, prediction, planning, personality, and decision-making.

  • Frontal eye field: Controls voluntary eye movements.

• Hemispheric lateralization: The functional specialization of the two cerebral hemispheres, though they work together via the corpus callosum.

  • Left hemisphere: Typically dominant for language, mathematics, and logical reasoning.

  • Right hemisphere: Typically dominant for spatial perception, sensory analysis (e.g., recognizing faces and voices, appreciating music), social skills, and emotional context.

Electroencephalogram (EEG) & Brain Waves

• An EEG measures electrical activity of the brain, reflecting patterns of neuronal firing. These patterns are categorized into distinct brain waves:

  • Alpha waves: Characteristic of a resting brain in a healthy adult with eyes closed (idling state).

  • Beta waves: Indicate active concentration, mental engagement, or stress (actively thinking).

  • Theta waves: Common in children; in frustrated adults, they may signal brain pathology or emotional distress.

  • Delta waves: Prominent during deep sleep in all ages; in awake adults, their presence almost always indicates significant brain damage.

• Synchronization: Brain waves often show synchronized patterns, often due to an inter-hemispheric pacemaker, which helps coordinate activities. Desynchronization: Abnormal, uncontrolled electrical activity can lead to seizures, which are temporary cortical disorders.

Cranial Nerves Overview

• There are 12 pairs of cranial nerves that emerge directly from the brain (mostly the brainstem), serving primarily the head and neck, though some extend to the thoracic and abdominal cavities.

• They can be purely sensory, special sensory, motor, or mixed (containing both sensory and motor fibers). Many also carry autonomic (parasympathetic) fibers.

I Olfactory

• Type: Special sensory.

• Function: Sense of smell (olfaction).

• Pathway: Receptors in the nasal cavity send axons through the cribriform foramina of the ethmoid bone to synapse in the olfactory bulbs.

II Optic

• Type: Special sensory.

• Function: Vision.

• Pathway: Axons from the retina of the eye pass through the optic canals, cross partly at the optic chiasm, and continue as optic tracts to the lateral geniculate nucleus (LGN) of the thalamus.

III Oculomotor

• Type: Motor.

• Origin: Midbrain.

• Pathway: Exits the cranium via the superior orbital fissure.

  • Somatic motor: Innervates superior, inferior, and medial rectus muscles, inferior oblique muscle (eye movements), and the levator palpebrae superioris (lifts eyelid).

  • Visceral motor (parasympathetic): Controls intrinsic eye muscles (e.g., pupillary constriction, lens accommodation) via the ciliary ganglion.

IV Trochlear

• Type: Motor.

• Function: Innervates the superior oblique muscle of the eye (downward and outward rotation).

• Characteristics: The smallest cranial nerve, unique in that it is the only cranial nerve to exit the posterior aspect of the midbrain and decussate (cross) before innervation.

V Trigeminal (largest)

• Type: Mixed (sensory and motor).

• Divisions: It has three major branches extending from the semilunar (trigeminal) ganglion (which houses sensory neuron cell bodies).

  • V1 ophthalmic (sensory): Provides sensation from the orbit, forehead, and nasal areas. Exits via superior orbital fissure.

  • V2 maxillary (sensory): Provides sensation from the upper jaw, palate, and middle face. Exits via foramen rotundum.

  • V3 mandibular (mixed): Provides sensation from the lower jaw, anterior two-thirds of the tongue, and side of the head. Contains motor fibers for muscles of mastication. Exits via foramen ovale.

• Disorder: Tic douloureux (trigeminal neuralgia) is a severe, chronic pain condition affecting the face, typically in the distribution of one or more trigeminal nerve branches.

VI Abducens

• Type: Motor.

• Function: Innervates the lateral rectus muscle of the eye, responsible for abducting (moving laterally) the eyeball.

• Origin: Pons.

VII Facial

• Type: Mixed.

• Functions:

  • Special sensory: Taste sensation from the anterior two-thirds of the tongue.

  • Somatic motor: Controls muscles of facial expression.

  • Visceral motor (parasympathetic): Controls glandular secretions from the lacrimal (tear), submandibular, and sublingual salivary glands.

• Branches: Supplies facial muscles through major branches: temporal, zygomatic, buccal, marginal mandibular, and cervical.

• Ganglia: Geniculate ganglion (sensory cell bodies); pterygopalatine and submandibular ganglia (parasympathetic synapses).

• Disorder: Bell’s palsy is a temporary paralysis or weakness of muscles on one side of the face due to inflammation or damage to the facial nerve.

VIII Vestibulocochlear

• Type: Special sensory.

• Divisions: Two main components:

  • Vestibular branch: Responsible for balance and equilibrium, transmitting information from the semicircular canals and vestibule of the inner ear.

  • Cochlear branch: Responsible for hearing, transmitting auditory information from the cochlea of the inner ear.

• Pathway: Receptors in the inner ear send signals via the internal acoustic meatus.

IX Glossopharyngeal

• Type: Mixed.

• Functions:

  • Special sensory: Taste sensation from the posterior one-third of the tongue.

  • Visceral sensory: Receives input from the carotid sinus (baroreceptors for blood pressure) and carotid bodies (chemoreceptors for blood gases).

  • Somatic motor: Innervates pharyngeal muscles for swallowing.

  • Visceral motor (parasympathetic): Controls secretion from the parotid salivary gland.

• Ganglia: Contains superior & inferior ganglia (sensory) and the otic ganglion (visceral motor synapse).

X Vagus ("wandering")

• Type: Mixed.

• Characteristics: The longest and most widely distributed cranial nerve, extending extensively into the thoracic and abdominal cavities.

• Functions:

  • Sensory: Receives sensory input from the external ear, diaphragm, and most visceral organs in the thorax and abdomen.

  • Motor: Provides motor innervation to muscles of the palate and pharynx (swallowing, speech) and is the primary parasympathetic innervation to visceral organs (regulating heart rate, digestion, respiration).

• Ganglia: Contains superior & inferior vagal ganglia.

XI Accessory

• Type: Motor.

• Roots:

  • Cranial root: Joins the vagus nerve and contributes to the innervation of muscles of the palate, pharynx, and larynx.

  • Spinal root: Originates from the cervical spinal cord, ascends into the cranium, then exits again to innervate the sternocleidomastoid and trapezius muscles (head and shoulder movements).

XII Hypoglossal

• Type: Motor.

• Function: Exclusively innervates the tongue musculature, controlling tongue movements for speech and swallowing.

• Pathway: Exits the cranium via the hypoglossal canal.

Cranial Reflexes

• Automatic, involuntary responses mediated by mono- or polysynaptic circuits involving cranial nerves and brainstem nuclei.

• They are clinically important as diagnostic tools for assessing the integrity of specific cranial nerves and associated brain regions.

• Examples: Corneal blink reflex (involves CN V for sensation and CN VII for motor response leading to eyelid closure), pupillary light reflex (involves CN II for light detection and CN III for pupillary constriction), and the gag reflex (involves CN IX and CN X for sensory and motor components in the pharynx).

Ethical / Clinical Connections

• Prefrontal lobotomy: A historical psychosurgical procedure that involved severing connections in the prefrontal cortex, which dramatically illustrated the role of this brain region in personality, behavior, and intellect.

• BBB breaks: While typically protective, breaks or regions of natural permeability in the blood-brain barrier can be leveraged for therapeutic drug delivery (e.g., for brain tumors) but also pose a risk for toxin entry.

• Cerebrovascular diseases (stroke): Emphasize the brain's critical and constant need for adequate blood perfusion, as even brief interruptions can lead to irreversible neuronal damage and severe neurological deficits.

• Parkinson’s disease: A neurodegenerative disorder strongly tied to the degeneration of dopamine-producing neurons in the substantia nigra, disrupting the basal nuclei circuitry and leading to motor symptoms like tremors and rigidity.

• Bell’s palsy & trigeminal neuralgia: These conditions highlight the vulnerability of peripheral cranial nerves to inflammation, compression, or damage, leading to specific sensory or motor deficits.

• Ataxia: Often associated with cerebellar dysfunction, such as that caused by alcohol intoxication, underscoring the cerebellum's sensitivity to substances and its vital role in motor coordination.