Fundamentals of Anatomy & Physiology – Chapter 14: Brain & Cranial Nerves

14-1 Identify major brain regions, embryonic vesicles & adult ventricles; state location + function.

• 14-2 Describe protection/support mechanisms: cranial bones, meninges, CSF formation & flow, BBB.

• 14-3 Contrast medulla oblongata vs. spinal cord; list nuclei & autonomic roles.

• 14-4 Enumerate pons components; respiratory & relay functions.

• 14-5 List midbrain components; visual/auditory reflexes, motor modulation.

• 14-6 Detail cerebellar anatomy; postural & motor-learning roles.

• 14-7 Detail diencephalon parts; endocrine & sensory integration.

• 14-8 Map limbic system; emotional & memory circuitry.

• 14-9 Outline cerebrum subdivisions, cortical areas, white-matter tracts, EEG waveforms.

• 14-10 Provide examples of cranial (somatic & visceral) reflexes.

General Brain Facts

• Contains approximately 97\% of all nervous tissue in the body.

• Average mass is approximately 1.4\,\text{kg} (males typically have \approx+10\% larger brains than females, though brain size does not correlate with intelligence).

• Normal adult brain volume ranges from 750\text{ mL} to 2100\text{ mL}.

Gross Regions

• Cerebrum – The largest and most superior part of the brain, responsible for conscious thought, intellectual functions, and memory. It is divided into left and right hemispheres, each controlling the opposite side of the body. The outer layer, known as the "cerebral cortex," is composed of gray matter (neuron cell bodies) and is highly folded, featuring gyri (ridges) that increase surface area and sulci or fissures (grooves) that divide the brain into lobes and functional areas.

• Cerebellum – The second largest part of the brain, located posterior to the brainstem and inferior to the cerebrum. Its primary role is in the coordination and modulation of repetitive movements, balance, and motor learning. It consists of an outer cerebellar cortex (gray matter) and internal arbor vitae (white matter tracts).

• Diencephalon – Located deep within the cerebrum, superior to the brainstem. It includes three main structures:

  • Thalamus (paired structures) – Serves as a major sensory relay and processing center for all sensory information (except olfaction) en route to the cerebral cortex.

  • Hypothalamus – A vital center for autonomic nervous system control, endocrine regulation, and thermoregulation, linking the nervous and endocrine systems.

  • Epithalamus – Contains the pineal gland, which secretes melatonin, regulating circadian rhythms.

• Brainstem – Connects the spinal cord to the higher brain centers (cerebrum and cerebellum), acting as a crucial relay station for nerve signals. It is made up of three parts:

  • Midbrain (Mesencephalon) – Involved in processing visual and auditory reflexes, maintaining consciousness, and regulating alertness.

  • Pons – A bulge anterior to the cerebellum, acting as a "bridge" connecting the cerebellum to the brainstem and cerebrum. It contains nuclei involved in respiration and relays sensory and motor information.

  • Medulla Oblongata – Connects directly to the spinal cord; contains vital autonomic centers regulating cardiovascular (heart rate and blood pressure) and respiratory functions (breathing rhythm).

Embryology & Ventricles

• Primary vesicles (formed by week 3 of embryonic development):

  • Prosencephalon (forebrain).

  • Mesencephalon (midbrain).

  • Rhombencephalon (hindbrain).

• Secondary vesicles (formed by week 6):

  • Telencephalon (develops from prosencephalon) \rightarrow gives rise to the adult cerebrum.

  • Diencephalon (develops from prosencephalon) \rightarrow retains its name and forms the thalamus, hypothalamus, and epithalamus.

  • Mesencephalon (retains its name) \rightarrow forms the adult midbrain.

  • Metencephalon (develops from rhombencephalon) \rightarrow forms the pons and cerebellum.

  • Myelencephalon (develops from rhombencephalon) \rightarrow forms the medulla oblongata.

• Ventricular derivatives: These fluid-filled cavities within the brain are continuous with each other and the central canal of the spinal cord, containing cerebrospinal fluid (CSF).

  • Lateral ventricles – Paired C-shaped cavities located within the cerebral hemispheres (telencephalon), separated by the septum pellucidum.

  • Third ventricle – A narrow, midline cavity located within the diencephalon, which communicates with the lateral ventricles via the interventricular foramina (of Monro).

  • Cerebral aqueduct (of Sylvius) – A narrow channel passing through the midbrain (mesencephalon), connecting the third ventricle to the fourth ventricle.

  • Fourth ventricle – Located between the pons/medulla anteriorly and the cerebellum posteriorly (derived from the hindbrain). It connects to the central canal of the spinal cord and the subarachnoid space via two lateral apertures (foramina of Luschka) and one median aperture (foramen of Magendie), allowing CSF to circulate over the brain and spinal cord surfaces.

Protection & Support

• Physical protection of the brain includes:

  • Skull bones – Provide a rigid bony enclosure.

  • Cranial meninges – Three connective tissue layers that surround and protect the brain and spinal cord, providing a framework for arteries, veins, and venous sinuses:

    • Dura mater (outermost, tough layer) – Consists of two fibrous layers: the periosteal layer (fused to the inner periosteum of the skull) and the meningeal layer (inner layer), which typically remain fused but separate to form dural folds and dural venous sinuses.

    • Arachnoid mater (middle layer) – A delicate, web-like membrane. It encloses the subarachnoid space filled with CSF.

    • Pia mater (innermost layer) – A thin, transparent membrane that adheres tightly to the surface of the brain, following every gyrus and sulcus, supplying blood vessels to the brain tissue.

  • Dural folds: Inward extensions of the dura mater that subdivide the cranial cavity, limiting brain movement within the skull:

    • Falx cerebri – Projects into the longitudinal fissure between the cerebral hemispheres, housing the superior and inferior sagittal sinuses.

    • Tentorium cerebelli – Separates the cerebrum from the cerebellum, housing the transverse sinus.

    • Falx cerebelli – Divides the two cerebellar hemispheres.

  • Cerebrospinal Fluid (CSF) – A clear, colorless fluid that surrounds the brain and spinal cord.

• CSF Characteristics and Circulation:

  • Formation: Formed by the choroid plexus, specialized capillaries within the ventricles, through active transport and filtration by ependymal cells. Approximately 500\,\text{mL/day} is produced. The total CSF volume is about 150\,\text{mL}, meaning it is entirely replaced roughly every 8 hours.

  • Circulation pathway: CSF flows from the lateral ventricles \rightarrow third ventricle \rightarrow cerebral aqueduct \rightarrow fourth ventricle. From the fourth ventricle, it exits via the median and lateral apertures into the central canal of the spinal cord and the subarachnoid space surrounding the brain and spinal cord. It then circulates over the brain and spinal cord surfaces and is reabsorbed back into the venous blood by specialized structures called arachnoid villi (or granulations) that project into the dural venous sinuses, primarily the superior sagittal sinus \rightarrow ultimately draining into the internal jugular veins.

  • Functions:

    • Buoyancy and support: The brain essentially floats in CSF, which reduces its effective weight by 97\% , preventing it from crushing itself under its own weight on the floor of the cranial cavity.

    • Cushioning: Provides a liquid cushion that protects the brain from physical trauma and sudden movements.

    • Nutrient and waste exchange: Carries nutrients (glucose, oxygen) to brain tissue and removes metabolic waste products.

• Blood Supply & Barriers

  • Arterial supply: The brain receives its blood supply from the internal carotid arteries (anteriorly) and the vertebral arteries (posteriorly), which merge to form the circle of Willis, ensuring continuous blood flow.

  • Venous drainage: Venous blood from the brain drains into the dural venous sinuses, which then empty into the internal jugular veins.

  • Cerebrovascular accident (CVA) (Stroke): Occurs when blood circulation to a brain area is blocked or bleeding occurs, leading to neuronal death within minutes due to lack of oxygen and nutrients. This can result from ischemia (blocked blood vessel) or hemorrhage (bleeding).

  • Blood-Brain Barrier (BBB): A highly selective physiological barrier that regulates the passage of substances from the blood into the brain tissue. It is formed by tight junctions between the endothelial cells of CNS capillaries, making them the least permeable capillaries in the body. Astrocytes play a crucial role in maintaining and regulating the BBB, signaling to the endothelial cells. Lipid-soluble molecules (e.g., oxygen, CO2, alcohol, anesthetics) can readily cross the BBB, while water-soluble or larger molecules (e.g., most drugs, proteins, ions) require specific transport mechanisms or are excluded.

  • Blood-CSF barrier: Formed by tight junctions between the ependymal cells of the choroid plexus, regulating the composition of CSF independently of the blood.

  • BBB "windows" (circumventricular organs): Specific areas of the brain where the BBB is absent or highly permeable, allowing direct monitoring of blood composition and hormone secretion. These include:

    • Hypothalamus: Involved in hormone release and regulation.

    • Posterior pituitary gland: Releases neurohormones directly into the bloodstream.

    • Pineal gland: Secretes melatonin directly into the blood.

    • Choroid plexus: Although it forms CSF, it has a distinct blood-CSF barrier via ependymal cells guarding the entry of substances from the blood into the CSF.

Medulla Oblongata (Myelencephalon)

• Reflex Centers: The medulla acts as a crucial autonomic reflex center:

  • Cardiovascular centers: Regulate heart rate (cardiac center) and blood pressure by controlling the force and rate of heart contraction and the diameter of blood vessels (vasomotor center).

  • Respiratory rhythmicity centers: Control the basic rhythm and rate of breathing, interacting with centers in the pons to set the pace.

  • Other nuclei control reflexes like vomiting, hiccuping, swallowing, coughing, and sneezing.

• Reticular formation: A network of nuclei and nerve fibers extending through the brainstem, involved in integrating various autonomic functions, arousal, and sleep.

• Cranial Nerve Nuclei: Contains nuclei for cranial nerves VIII (Vestibulocochlear), IX (Glossopharyngeal), X (Vagus), XI (Accessory), and XII (Hypoglossal).

• Relay Stations:

  • Gracile and cuneate nuclei: Receive somatic sensory information (fine touch, proprioception, vibration) from the spinal cord. Axons from these nuclei decussate (cross over) in the medulla, forming the medial lemniscus, which then relays signals to the thalamus.

  • Decussation of pyramids: The crossover point of the large corticospinal (pyramidal) tracts, explaining why one side of the brain controls the opposite side of the body.

  • Solitary nucleus: Receives visceral sensory information from the vagus, glossopharyngeal, and facial nerves, involved in taste and general visceral sensations.

  • Inferior olivary complex: Relays motor commands (especially regarding balance and coordination) from higher brain centers to the cerebellum.

Pons (Metencephalon)

• Cranial nerve nuclei: Contains nuclei for cranial nerves V (Trigeminal), VI (Abducens), VII (Facial), and parts of VIII (Vestibulocochlear).

• Respiratory modulation: Contains specific respiratory centers:

  • Apneustic center: Promotes inspiration by stimulating medullary inspiration centers.

  • Pneumotaxic center: Inhibits inspiration, leading to shorter, shallower breaths, thus adjusting the medullary rhythm.

• Ascending/descending tracts: Contains both sensory (ascending) and motor (descending) tracts that connect the cerebrum to the spinal cord and cerebellum.

• Transverse pontine fibers: Large bundles of fibers that cross the pons, connecting the cerebral cortex to the contralateral cerebellar hemisphere, crucial for coordinating voluntary movements.

Midbrain (Mesencephalon)

• Tectum (roof): The dorsal part of the midbrain, containing four prominent nuclei known collectively as the corpora quadrigemina:

  • Superior colliculi (two dorsal bumps) – Involved in visual reflexes, such as tracking moving objects and coordinating head and eye movements in response to visual stimuli.

  • Inferior colliculi (two ventral bumps) – Involved in auditory reflexes, such as the startle reflex to sudden loud noises and the relay of auditory information to the thalamus.

• Tegmentum: The ventral part of the midbrain, containing several important nuclei:

  • Red nucleus: Richly vascularized, involved in motor coordination, particularly of the upper limbs, contributing to limb flexion.

  • Substantia nigra: Contains neurons that produce dopamine and projecting to the basal nuclei. Its degeneration leads to a reduction in dopamine production, a hallmark of Parkinson’s disease, characterized by motor control deficits.

• Cerebral peduncles: Large bundles of nerve fibers on the ventral surface of the midbrain, containing descending corticospinal tracts (major motor pathways from the cerebrum to the spinal cord) and corticopontine fibers (from the cerebrum to the pons).

Cerebellum

• Anatomy: The cerebellum's surface is highly folded into parallel ridges called folia, which increase its surface area. It is divided into an anterior lobe and a posterior lobe, separated by the primary fissure. The vermis is a narrow, median-lobe that connects the two cerebellar hemispheres. The flocculonodular lobe, located on the inferior surface, is involved in balance and eye movements. The internal white matter forms a tree-like pattern known as the arbor vitae, within which are embedded deep nuclei that process cerebellar outputs.

• Purkinje cells: Large, distinctive neurons in the cerebellar cortex with extensive, elaborate dendritic trees (each cell can receive up to 200{,}000 synapses). These cells are crucial for cerebellar function, acting as the sole output of the cerebellar cortex, sending inhibitory signals to the deep cerebellar nuclei.

• Peduncles: Three pairs of fiber tracts that connect the cerebellum to other parts of the brainstem:

  • Superior cerebellar peduncles: Connect the cerebellum to the midbrain, mostly carrying efferent (output) fibers from the cerebellum.

  • Middle cerebellar peduncles: Connect the cerebellum to the pons, primarily carrying afferent (input) fibers from the cerebral cortex via the pons.

  • Inferior cerebellar peduncles: Connect the cerebellum to the medulla oblongata and spinal cord, carrying both afferent (sensory input, e.g., proprioception) and efferent fibers to and from the cerebellum.

• Functions: Crucial for coordinating voluntary movements, maintaining posture and balance, motor error-correction (comparing intended to actual movements and adjusting), and motor learning (e.g., learning new motor skills like riding a bike). Damage to the cerebellum (e.g., from stroke, trauma, or alcohol intoxication) can lead to ataxia, characterized by a lack of coordination, unsteady gait, and dysarthria (slurred speech).

Diencephalon

• Epithalamus: The most dorsal part of the diencephalon, forming the roof of the third ventricle. It contains:

  • Pineal gland: An endocrine gland that secretes melatonin, a hormone that regulates sleep-wake cycles (circadian rhythms) and reproductive functions.

• Thalamus: Comprised of paired, egg-shaped masses of gray matter, serving as the main relay and processing center for sensory information ascending to the cerebral cortex. It also plays a role in motor activities, cortical arousal, learning, and memory. Specific nuclei include:

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

  • Medial nuclei: Involved in emotional awareness and integration of sensory information.

  • Ventral nuclei: Extensive relays for motor (ventral lateral, ventral anterior) and somatic sensory (ventral posterior) information to the cortex.

  • Dorsal nuclei: Including the pulvinar, involved in visual processing and attention.

  • Geniculate bodies: Specialized nuclei for specific sensory inputs:

    • Lateral geniculate nucleus (LGN): A key relay for visual information from the retina to the primary visual cortex.

    • Medial geniculate nucleus (MGN): A key relay for auditory information from the inner ear to the primary auditory cortex.

• Hypothalamus: A small but extremely vital region inferior to the thalamus, forming the floor of the third ventricle. It is the main visceral control center of the body, crucial for maintaining homeostasis.

  • Structures within the hypothalamus:

    • Mammillary bodies: Paired pealike nuclei on its posterior aspect, involved in eating reflexes (licking, chewing, swallowing) and memory formation (part of the limbic system).

    • Infundibulum: A stalk that connects the hypothalamus to the pituitary gland, serving as a conduit for neurohormones.

    • Tuber cinereum: A bulge between the mammillary bodies and the infundibulum, involved in connections to the pituitary for hormone release.

  • Eight major functions of the hypothalamus:

  1. Secretes hormones: Produces antidiuretic hormone (ADH) by the supraoptic nucleus and oxytocin by the paraventricular nucleus, which are then transported to and released from the posterior pituitary.

  2. Thermoregulation: The pre-optic area acts as the body's thermostat, initiating cooling (sweating, vasodilation) or warming (shivering, vasoconstriction) mechanisms.

  3. Autonomic control centers: Regulates the activity of the autonomic nervous system (e.g., heart rate, blood pressure, digestive tract motility, pupil size).

  4. Integrates voluntary & autonomic actions: For example, coordinating cardiovascular changes during exercise or stress responses.

  5. Links nervous–endocrine systems: Controls the release of hormones from the anterior pituitary by secreting releasing and inhibiting hormones.

  6. Regulates circadian rhythms: The suprachiasmatic nucleus (SCN) acts as the body's biological clock, influencing sleep-wake cycles in response to light-dark cues.

  7. Subconscious skeletal muscle control: Influences muscle tone and activity, particularly in emotional responses.

  8. Regulates drives & emotions: Contains centers that regulate hunger, thirst, satiety, sex drive, and emotional behavior as part of the limbic system.

Limbic System

• The limbic system is a functional brain system, not a distinct anatomical structure, that includes parts of the cerebrum and diencephalon. It is often referred to as the "emotional brain" and plays a critical role in emotions, motivation, and memory.

• Key components:

  • Limbic lobe: Includes the cingulate gyrus (above corpus callosum, involved in expressing emotions through gestures and resolving mental conflict), parahippocampal gyrus, and dentate gyrus.

  • Hippocampus: Located in the medial temporal lobe, crucial for converting new information from short-term to long-term memory (memory consolidation and retrieval).

  • Amygdaloid body (amygdala): An almond-shaped nucleus anterior to the hippocampus, critically involved in processing fear, anger, pleasure, and other emotions, and linking them to memories. It also plays a role in facial recognition and social cognition.

  • Fornix: A C-shaped bundle of nerve fibers that connects the hippocampus to the hypothalamus (especially the mammillary bodies), forming a major output pathway of the hippocampus.

  • Anterior thalamic nuclei: Receive input from the mammillary bodies and project to the cingulate gyrus, participating in limbic circuits.

  • Reticular formation: Functional connections with the limbic system contribute to emotional states and arousal.

• Roles: The limbic system is the neural basis of emotions, affecting behavior, motivation (e.g., reward and punishment circuits), and crucial for memory formation, retrieval, and learning.

Cerebrum

Surface Landmarks

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

• Central sulcus: Divides the frontal lobe from the parietal lobe. The precentral gyrus (anterior to the sulcus) contains the primary motor cortex, while the postcentral gyrus (posterior to the sulcus) contains the primary somatosensory cortex.

• Lateral sulcus (Sylvian fissure): Separates the temporal lobe from the frontal and parietal lobes.

• Parieto-occipital sulcus: Separates the parietal lobe from the occipital lobe.

• Insula: A fifth lobe or deep cortical region, buried deep within the lateral sulcus, involved in taste, visceral sensation, and emotional processing.

• Lobes of the cerebrum: Each hemisphere is divided into five major lobes, named after the overlying skull bones (except insula):

  • Frontal lobe: Involved in voluntary motor control, decision-making, planning, problem-solving, personality, and executive functions.

  • Parietal lobe: Processes somatic sensory information (touch, pressure, temperature, pain, proprioception), spatial awareness, and navigation.

  • Temporal lobe: Involved in auditory processing, memory, and language comprehension.

  • Occipital lobe: Primarily responsible for visual processing.

  • Insular lobe: Involved in interoception, taste (gustatory cortex), visceral sensation, pain processing, and emotional responses.

White-Matter Tracts

• These fiber tracts allow for communication within the cerebrum and between the cerebrum and other parts of the CNS. They are myelinated axon bundles that appear white.

  • Association fibers: Connect different parts of the same cerebral hemisphere:

    • Arcuate fibers: Short fibers connecting gyri adjacent to each other.

    • Longitudinal fasciculi: Longer fibers connecting gyrus in one lobe to gyri in another lobe of the same hemisphere (e.g., superior longitudinal fasciculus).

  • Commissural fibers: Connect corresponding gray areas of the two hemispheres, allowing them to function as a coordinated whole:

    • Corpus callosum: The largest commissural fiber tract, connecting large areas of the two cerebral hemispheres.

    • Anterior commissure: Connects parts of the temporal lobes and olfactory bulbs of the two hemispheres.

  • Projection fibers: Connect the cerebrum with lower parts of the brain and spinal cord, forming efferent (motor) and afferent (sensory) pathways:

    • Internal capsule: A compact bundle of projection fibers that passes between the basal nuclei and the thalamus, linking the cerebral cortex to the brainstem and spinal cord. Most sensory information reaching the cortex and motor commands leaving it pass through this structure.

Basal Nuclei (Ganglia)

• A group of subcortical nuclei located deep within the cerebral white matter, crucial for motor control, eye movements, and some cognitive functions. They receive input from the cerebral cortex and send output to the thalamus, which then projects back to the cortex, forming a feedback loop.

• Key Components:

  • Caudate nucleus: A C-shaped structure with a head (anteriorly) and a long tail (posteriorly and inferiorly).

  • Lentiform nucleus: A lens-shaped structure composed of two parts:

    • Putamen: The outer larger part, involved in controlling learned, automatic movements.

    • Globus pallidus: The inner part, involved in regulating muscle tone and inhibiting unwanted movements.

  • Claustrum: A thin, irregular sheet of neurons located lateral to the lentiform nucleus, whose function is not fully understood but may be involved in integrating sensory information.

• Functions: Play a significant role in the initiation and termination of movements, regulation of muscle tone, and execution of patterned, subconscious movements (e.g., swinging arms while walking). Dysfunction (e.g., hyperactivity or degeneration of dopaminergic input from substantia nigra) can lead to motor disorders such as Parkinsonian signs (tremors, rigidity, difficulty initiating movement) or Huntington's disease (uncontrolled, jerky movements due to degeneration of neurons in the caudate and putamen).

Cortical Areas (see Table 14-3 for detailed mapping)

• The cerebral cortex is divided into functional areas based on their primary roles:

  • Primary motor cortex (located in the precentral gyrus of the frontal lobe) – Contains pyramidal cells (large neurons) that control voluntary movements of skeletal muscles. A specific region of the cortex controls specific body parts, forming a motor homunculus.

  • Premotor cortex (somatic motor association area, anterior to primary motor cortex) – Plans and coordinates skilled, learned motor sequences (e.g., typing, playing an instrument), sending instructions to the primary motor cortex.

  • Primary somatosensory cortex (located in the postcentral gyrus of the parietal lobe) – Receives general sensory information from the skin (touch, pressure, temperature, pain) and proprioceptors (muscle and joint position) from the opposite side of the body. Also forms a sensory homunculus.

  • Somatosensory association cortex (posterior to primary somatosensory cortex) – Interprets and integrates sensory inputs (e.g., temperature, pressure) arising from the primary somatosensory cortex, allowing recognition of objects by touch (stereognosis), and understanding of body position.

  • Special senses cortical areas:

    • Primary visual cortex (occipital lobe) – Receives and processes visual information.

    • Visual association area (surrounding primary visual cortex) – Interprets and gives meaning to visual stimuli (e.g., recognizing faces).

    • Primary auditory cortex (temporal lobe) – Processes sound information (pitch, rhythm, loudness).

    • Auditory association area (posterior to primary auditory cortex) – Interprets and gives meaning to sounds (e.g., understanding speech, music).

    • Olfactory cortex (medial temporal lobe, piriform cortex) – Processes smell information.

    • Gustatory cortex (insula) – Processes taste information.

  • Integrative centers: Complex, multimodal association areas that receive information from multiple sensory and motor areas, allowing for complex thought, judgment, and behavior. These include:

    • Wernicke’s area (located in the posterior temporal lobe, usually in the left hemisphere) – Critically involved in language comprehension (understanding spoken and written words). Damage here leads to fluent aphasia (words are produced but often nonsensical, and comprehension is impaired).

    • Broca’s area (located in the prefrontal cortex, usually in the left hemisphere) – Essential for speech production, controlling the muscles used for speaking. Damage here leads to non-fluent (expressive) aphasia (comprehension is intact, but speech is slow, distorted, and difficult to produce).

    • Frontal eye field (anterior to premotor cortex) – Controls voluntary eye movements.

    • Prefrontal cortex (anterior part of the frontal lobe) – The most complex and executive part of the brain, involved in abstract thought, reasoning, planning, judgment, personality, foresight, empathy, and predicting consequences of actions. Historically, psychosurgery (lobotomy) involved severing connections to this area to treat severe mental disorders, but it often led to undesirable personality changes.

• Hemispheric Lateralization: While both hemispheres work together, they specialize in different functions (division of labor):

  • Left hemisphere (dominant in 90\% of people): Typically specialized for language (speech, writing, comprehension), analytical thought, mathematics, and logic.

  • Right hemisphere: Typically specialized for spatial reasoning, facial recognition, artistic and musical appreciation, intuition, emotion, and perception of tones in language.

EEG Brain Waves

• Electroencephalogram (EEG) measures electrical activity in the brain, reflecting the summed electrical potentials of neurons. Different brain wave patterns are associated with different states of consciousness or activity.

  • Alpha waves (\approx 8-13\,\text{Hz}): Characterize a relaxed state, often with eyes closed but awake. They are relatively regular and synchronous.

  • Beta waves (\approx 14-30\,\text{Hz}): High-frequency, low-amplitude waves associated with active concentration, mental alertness, problem-solving, and REM sleep. They are less synchronous.

  • Theta waves (\approx 4-7\,\text{Hz}): Common in children and during light sleep in adults. In awake adults, their presence can indicate emotional stress, frustration, or brain disorders.

  • Delta waves (<4\,\text{Hz}): High-amplitude, low-frequency waves characteristic of deep sleep, infancy, and certain pathological brain conditions (e.g., brain damage or tumors) when observed in awake adults.

• Synchronization: Brain wave patterns are often synchronized across large areas of the cortex, influenced by pacemaker neurons in the thalamus. Seizures are characterized by episodic, abnormal, and highly synchronized electrical discharges from a group of neurons, leading to temporary brain dysfunction.

Cranial Nerves (CN I–XII)

• There are 12 pairs of cranial nerves that arise from the brain and brainstem, serving primarily the head and neck region (except the Vagus nerve). They can be classified as sensory (afferent), special sensory (e.g., vision, hearing, smell, taste), motor (efferent), or mixed (containing both sensory and motor fibers). Many also carry autonomic (parasympathetic) fibers.

I Olfactory (Sensory/Special Sensory)

• Function: Special sense of smell (olfaction).

• Pathway: Olfactory receptors in the nasal cavity \rightarrow axons pass through the cribriform plate of the ethmoid bone \rightarrow synapse in the olfactory bulbs \rightarrow olfactory tracts \rightarrow primary olfactory cortex (pyriform cortex).

II Optic (Special Sensory)

• Function: Special sense of vision.

• Pathway: Retinal ganglion cells of the eye \rightarrow optic nerve \rightarrow optic chiasm (where partial decussation of medial fibers occurs) \rightarrow optic tracts \rightarrow lateral geniculate nucleus (LGN) of the thalamus (relay) \rightarrow primary visual cortex in the occipital lobe.

III Oculomotor (Motor)

• Function: Primarily motor to extrinsic eye muscles and intrinsic eye muscles.

• Origin: Midbrain.

• Target muscles & role: Controls the superior rectus (SR), inferior rectus (IR), medial rectus (MR), inferior oblique (IO) muscles (for eye movements like elevation, depression, adduction) and the levator palpebrae superioris muscle (elevates the eyelid). Also carries visceral (parasympathetic) fibers to the ciliary ganglion, innervating the constrictor pupillae (pupil constriction) and ciliary muscles (lens accommodation for near vision).

IV Trochlear (Motor)

• Function: Primarily motor to one extrinsic eye muscle.

• Origin: Midbrain.

• Target muscle & role: Innervates the superior oblique muscle, which produces downward and inward (intorsion) eye rotation. Exits the skull through the superior orbital fissure.

V Trigeminal (Largest, Mixed)

• Function: Mixed sensory and motor functions for the face.

• Divisions: Divides into three major branches that exit the skull separately, each with sensory innervation:

  • V1 Ophthalmic (sensory): Sensation from the forehead, upper eyelid, nose, and eyeball.

  • V2 Maxillary (sensory): Sensation from the lower eyelid, upper jaw, cheeks, upper teeth, and nasal cavity.

  • V3 Mandibular (mixed): Sensation from the lower jaw, lower teeth, anterior \frac{2}{3} of the tongue. Contains motor fibers for the muscles of mastication (chewing: masseter, temporalis, pterygoids). The sensory cell bodies are located in the semilunar (trigeminal) ganglion. Clinical condition: Tic douloureux (trigeminal neuralgia) is a severe, debilitating facial pain condition due to irritation of this nerve.

VI Abducens (Motor)

• Function: Primarily motor to one extrinsic eye muscle.

• Origin: Pons.

• Target muscle & role: Innervates the lateral rectus muscle, which abducts the eye (moves it laterally).

VII Facial (Mixed)

• Function: Complex mixed nerve involved in taste, facial expression, and glandular secretion.

• Sensory: Carries taste sensation from the anterior \frac{2}{3} of the tongue.

• Motor: Controls muscles of facial expression (e.g., orbicularis oculi, zygomaticus), allowing for smiles, frowns, etc.

• Parasympathetic: Provides secretomotor innervation to the lacrimal glands (tear production), submandibular glands, and sublingual glands (saliva production) via the pterygopalatine and submandibular ganglia.

• Branches: Divides into five main motor branches in the face: Temporal, Zygomatic, Buccal, Marginal Mandibular, and Cervical. Clinical condition: Bell’s palsy, a temporary facial paralysis resulting from inflammation or damage to the facial nerve.

VIII Vestibulocochlear (Special Sensory)

• Function: Special senses of balance and hearing.

• Divisions: Consists of two main parts:

  • Vestibular division: Carries information about balance and head position from the vestibule and semicircular canals of the inner ear. Receptors \rightarrow vestibular ganglia \rightarrow pons/medulla.

  • Cochlear division: Carries auditory information from the cochlea of the inner ear. Receptors \rightarrow spiral ganglion \rightarrow pons/medulla.

IX Glossopharyngeal (Mixed)

• Function: Mixed nerve involved in taste, swallowing, and sensation from the pharynx.

• Sensory: Taste from the posterior \frac{1}{3} of the tongue. Also carries sensory information (chemo- and baroreceptor reflexes) from the carotid sinus (monitors blood pressure and O2/CO2 levels).

• Motor: Innervates the stylopharyngeus muscle, involved in swallowing.

• Parasympathetic: Innervates the parotid gland (saliva production) via the otic ganglion.

X Vagus (Mixed, Extensive)

• Function: The most extensive cranial nerve, with wide-ranging sensory, motor, and parasympathetic functions, primarily for visceral organs.

• Sensory: Receives sensory input from the external ear canal, diaphragm, pharynx, larynx, and most thoracic and abdominal visceral organs (e.g., heart, lungs, digestive tract).

• Motor: Controls skeletal muscles of the pharynx, larynx, and palate (involved in speaking and swallowing).

• Parasympathetic (major role): Provides the vast majority of parasympathetic innervation to the heart (slowing heart rate), lungs (bronchoconstriction, mucous secretion), and most of the digestive tract (stimulating motility and secretion) down to the large intestine. Sensory neuron cell bodies are in the superior and inferior vagal ganglia. Notable branch: the recurrent laryngeal nerve, critical for vocal cord movement. Involved in major autonomic reflexes like the gag reflex, baroreflex, and chemoreflex.

XI Accessory (Motor)

• Function: Primarily motor to specific neck and shoulder muscles.

• Roots: Has two origins:

  • Cranial root: Originates from the medulla and joins the vagus nerve (CN X) to innervate muscles of the palate and larynx (contributes to swallowing and voice production).

  • Spinal root: Originates from the cervical spinal cord (C1–C5) \rightarrow ascends into the cranial cavity \rightarrow exits through the jugular foramen \rightarrow innervates the sternocleidomastoid and trapezius muscles (responsible for head turning, shrugging shoulders).

XII Hypoglossal (Motor)

• Function: Primarily motor to the tongue muscles.

• Origin: Medulla oblongata.

• Target muscles & role: Innervates intrinsic and extrinsic tongue musculature (e.g., genioglossus, styloglossus, hyoglossus), allowing for tongue movements involved in speech, swallowing, and manipulation of food.

Representative Cranial Reflexes

• Corneal blink reflex: A somatic reflex where touching the cornea (sensory via CN V Trigeminal) causes involuntary blinking of the eye (motor via CN VII Facial).

• Pupillary light reflex: A visceral reflex where light shined into one eye (sensory via CN II Optic) causes both pupils to constrict (motor via CN III Oculomotor, specifically its parasympathetic fibers).

• Gag reflex: A mixed reflex, where touching the back of the pharynx (sensory via CN IX Glossopharyngeal) elicits contraction of pharyngeal muscles (motor via CN X Vagus), preventing aspiration.

• Vestibulo-ocular reflex (VOR): A complex reflex involving CN VIII (Vestibulocochlear) sensory input about head movement, which then coordinates eye movements via CN III (Oculomotor), CN IV (Trochlear), and CN VI (Abducens) to stabilize gaze and keep the eyes fixed on a target even when the head moves.