Chapter 14: The Brain and Cranial Nerves (Anatomy & Physiology) – Practice Flashcards

14.1 Overview of the Brain

• the brain is the central processor and director of nervous system receives and filters

  • consciousness, thought, memory, learning, emotions, language, all reside in the brain

• is the only known organ to be aware of itself

  • Directional Terms

    • Rostral — toward forehead

    • Caudal — toward spinal cord

  

• Major Portions

  • Forebrain

  • Cerebellum

  • Brainstem

• Cerebrum (largest part; 83\% of volume)

  • Cerebral hemispheres (pair of half-globes)

  • Gyri (thick folds) on cerebrum surface

  • Sulci (shallow grooves)

  • Longitudinal cerebral fissure (separates hemispheres)

  • Corpus callosum (thick commissural bundle at fissure base)

• Cerebellum (second-largest) 10% of volume

  • Sits in posterior cranial fossa

  • has half of the brains neurons with densely packed nerves, enabling precise coordination and control of movement, as well as the integration of sensory information.

  • Separated from cerebrum by transverse cerebral fissure

  • Surface folds = folia

• Brainstem = remainder (midbrain, pons, medulla oblongata)

  • diencephalon inside the brain

    • includes the thalamus, epithalamus, hypothalamus

    • Midbrain: Located between the forebrain and hindbrain, it plays a crucial role in vision, hearing, and eye movement.

    • pons: The pons is a bridge connecting different parts of the nervous system, particularly the brain and spinal cord, and it is involved in regulating sleep, respiration, swallowing, and facial expressions.

    • medulla oblongata: The medulla oblongata is responsible for regulating vital functions such as heart rate, blood pressure, and respiration, making it essential for maintaining homeostasis.

    • Cranial Nerves: There are twelve pairs of cranial nerves that emerge directly from the brain and are primarily responsible for motor and sensory functions of the head and neck.

    • Midbrain: The midbrain, situated above the pons, plays a crucial role in processing visual and auditory information and helps in the coordination of motor functions.

• Central sulcus: The central sulcus is a prominent groove in the cerebral cortex that separates the frontal lobe from the parietal lobe and is critical in delineating areas responsible for motor and sensory functions.

• Lateral sulcus: The lateral sulcus, also known as the Sylvian fissure, is a significant groove that separates the temporal lobe from the frontal and parietal lobes, playing an essential role in language processing and auditory perception.

• precentral gyrus: (motor strip) is prominent fold of cortex located on the frontal lobe rostral to the central sulcus, and it is primarily involved in the planning and execution of voluntary movements.

• The postcentral gyrus, located in the parietal lobe, is positioned directly posterior to the central sulcus and is crucial for processing somatosensory information, including temperature, pain, and touch sensations. Is coronal to central sulcus

Gray vs. White Matter

• Gray Matter

  • Neuron cell bodies, dendrites, synapses

  • cortex (cerebrum & cerebellum)-Surface layers

  • nuclei-Deeper masses of gray matter, surrounded by white matter

• White Matter

  • composed of tracts-bundles of nerve fibers (axons)

  • Myelinated axon tracts

    • Lies deep to cortical gray; connects brain regions & spinal cord

Embryonic Development of CNS

• ectoderm: the outermost embryonic primary germ layer layer, crucial for developing the nervous system.

• Neurulation (first 3 weeks)

  • Neural plate → neural groove with neural folds → fusion forms hollow neural tube by day 26

  • Tube lumen becomes future ventricles & central canal

• Neural crest forms PNS, meninges (inner 2 layers), pigment cells, etc.

• Primary brain vesicles (week 4):

  • Prosencephalon (forebrain)

  • Mesencephalon (midbrain)

  • Rhombencephalon (hindbrain)

• Secondary vesicles (week 5):

  • Telencephalon & diencephalon (from forebrain)

  • Mesencephalon remains

  • Metencephalon & myelencephalon (from hindbrain)

14.2 Meninges, Ventricles, CSF & Blood Supply

Meninges

• 

  Three membranes (external → internal): dura mater, arachnoid mater, pia mater

• Cranial dura mater

  • Periosteal layer (outer) = cranial periosteum

  • Meningeal layer (inner) → continues into vertebral canal and forms spinal dural sheath around spinal cord

  • Dural sinuses- spaces located between periosteal and meningeal layers separate

    • collect venous blood

    • Superior sagittal sinus (midline)-just under calvaria along median line

    • Drains blood from the lateral aspects of the cerebral hemispheres into the confluence of sinuses.

    • Transverse sinus (horiz. posterior)-runs horizontally rom rear of head toward each ear

  • Dural folds:

    • falx cerebri: separates the two cerebral hemispheres

    • tentorium cerebelli: separates cerebrum from cerebellum

    • falx cerebelli: separates right and left halves of cerebellum

  • Subdural space: in between dura and arachnoid

  • subarachnoid space: space between the arachnoid and pia mater filled with cerebrospinal fluid
    

• Arachnoid mater

  • Transparent; subarachnoid space with CSF & vessels

• Pia mater

  • Thin, follows gyri & vessels or arteries as they penetrate into cerebrum

• Meningitis

  • Inflammation of meninges (usually pia & arachnoid) from viral/bacterial infection; peaks 3 mo – 2 yrs; diagnosed via lumbar puncture

  • Pia mater and arachnoid are most often affected

  • Can cause swelling of the brain, enlargement of the
    ventricles, and hemorrhage

  • May progress to coma, then death within hours of onset

  • Diagnosed by examining CSF obtained by lumbar
    puncture (spinal tap)

    
    

Ventricular System & CSF

• Brain has 4 CSF-filled spaces within the brain called ventricles, which include the two lateral ventricles, the third ventricle, and the fourth ventricle.

• ventricles (internal CSF reservoirs)-four internal, fluid-filled chambers of brain

  • two Lateral ventricles (pair) connected by third ventricle

  • Third ventricle: narrow medial space beneath corpus callosum (midline, under corpus callosum)

    • Connected to lateral ventricle by interventricular foramen

  • Fourth ventricle: small triangular chamber between pons and cerebellum (between pons & cerebellum)

  • Connections: interventricular foramina → cerebral aqueduct → central canal

    • interventricular foramen—pore that connects lateral ventricles to third ventricle

    • Cerebral aqueduct—tube running through midbrain that connects third ventricle to fourth ventricle

    • Central canal—tube that connects to fourth ventricle and runs through center of spinal cord

• CSF Production

• Cerebrospinal fluid (CSF) cushions, nourishes, and protects the CNS

  the space between meninges (subarachnoid space) filled with CSF

  • Production of CSF begins with filtration of blood plasma
    through capillaries of the brain

  • Choroid plexus: spongy mass of blood of capillaries in each ventricle that secretes cerebrospinal fluid

  • Ependymal cells—neuroglia that line ventricles and cover
    choroid plexus

  • ependymal cells modify the filtrate

    • Blood plasma filtered at choroid plexus; modified by ependymal cells

    • Composition: ↑Na^+, Cl^-; ↓K^+, Ca^{2+}, glucose, proteins

    • Compared to plasma, CSF has more sodium and chloride, less
      potassium, calcium, glucose, and very little protein

• Flow Path: CSF continuously flows & circulates through ventricles and meninges

  • Driven by its own pressure, beating of ependymal cilia,
    and pulsations of the brain produced by each heartbeat

  • secreted in Lateral ventricles → through interventricular foramina → into third ventricle (adds CSF) → down cerebral aqueduct → into fourth ventricle (3rd and 4th adds more CSF along the way) → escapes through median & lateral apertures into the → subarachnoid space and spinal cord surface → CSF is reabsorbed by arachnoid granulations → protrude through superior sagittal sinus→ CSF penetrates the walls of the villi and mixes with the blood in the sinus→ allowing for the maintenance of homeostasis within the central nervous system.

• All CSF ultimately escapes through three pores that lead
  into subarachnoid space of brain and spinal cord surface

  • median aperture

  • two lateral apetures

• CSF is reabsorbed by arachnoid granulations

  • Cauliflower-shaped extensions of the arachnoid meninx

  • Protrude through dura mater into superior sagittal sinus

  • CSF penetrates the walls of the villi and mixes with the blood in the sinus

  • arachnoid villus reabsorbs CSF into venus blood returned to heart and reestablish pH of blood plasma

• Functions

  • Buoyancy allows brain to be large without being impaired by its size (brain mass ~1.5\,\text{kg} becomes effectively 50\,\text{g})

  • Protection from blows (though concussion still possible)

  • Chemical stability (waste removal, pH control)

Blood Supply & Brain Barrier System (BBS)

• Brain only contributes = 2\% body weight but receives 15\% of blood cardiac output (≈750\,\text{mL/min})

• Neurons have high demand for ATP (therefore oxygen and
  glucose) so constant supply of blood critical

  • 10\,\text{s} interruption = loss → unconsciousness; 1{-}2\,\text{min} → significant impairment of neural function; 4\,\text{min} → irreversible damage

• Barrier Components

• Brain barrier system (BBS)—regulates what substances can get from bloodstream into tissue fluid of the brain

  • Although blood is crucial, it can also contain harmful
    agents

  • Blood-brain barrier (BBB): a selective permeability barrier that prevents certain substances from passing into the brain while allowing essential nutrients to enter.

    • Highly permeable to water, glucose, and lipid-soluble substances
      such as oxygen, carbon dioxide, alcohol, caffeine, nicotine,
      anesthetics
      • Slightly permeable to sodium, potassium, chloride, waste products
      urea and creatinine

    • Two points of entry must be guarded:

      • Blood capillaries throughout the brain tissue; guarded by the blood-
        brain barrier

      • Capillaries of the choroid plexus; guarded by the blood-CSF barrier

  • Blood–brain barrier (BBB): protects the brain at the blood
    capillaries, consists of tight junctions in cerebral capillaries induced by astrocyte perivascular feet

    • Induce endothelial cells to form impenetrable tight junctions that prevent the passage of potentially harmful substances from the bloodstream into the brain, thereby maintaining the brain's chemical stability.

  • Blood–CSF barrier: protects the brain at the choroid plexus tight junctions between ependymal cells of choroid plexus

    • Selective permeability: passes H2O, glucose, O2, CO_2, lipids; limits ions, wastes, pathogens

• Circumventricular organs (CVOs)—BBB absent (parts of 3rd/4th ventricles) for homeostatic sensing but allow pathogen entry (e.g., HIV)

  • Enable the brain to monitor and respond to fluctuations in
    blood glucose, pH, osmolarity, and other variables
    • CVOs afford a route for invasion by the human immunodeficiency virus (HIV)

• Clinical notes

  • Stroke (CVA): sudden ischemia/hemorrhage; second-leading cause of death; \approx50\% die <1 yr

  • Hemorrhagic stroke—rupture of a cerebral or subarachnoid blood
    vessel
    • Ischemic stroke—obstruction of a blood vessel by blood clot or lipid deposit

The Brainstem:

•  brainstem connects brain to spinal cord, also connecting to cerebrum, thalamus, and cerebellum

• 3 regions of brainstem

  • Midbrain

  • pons

  • medulla oblongata (is continuous with the spinal cord)

• involved in unconscious reflexes and regulations of homeostasis

• helps regulate:

  • blood pressure

  • heart rate

  • breathing (pons)

  • swallowing

  • coughing/sneezing

• relays sensory impulses from spinal cord to thalamus

• 

14.3 Hindbrain & Midbrain

Medulla Oblongata (from myelencephalon)

• adult brain region that develops from embryonic myelencephalon

• Begins at foramen magnum of skull
  • Extends about 3 cm rostrally and ends at a groove just below pons
  • Slightly wider than spinal cord

• Landmarks: pyramids (corticospinal tracts), olives, gracile & cuneate fasciculi

  • Pyramids—ridges on anterior surface, resemble side-by-
    side baseball bats separated by anterior median fissure

• four pairs of Cranial nerves begin or end in medulla: VIII (part), IX, X, XII originate/terminate here

  • Olives—prominent bulges lateral to each pyramid

  • Gracile and cuneate fasciculi of spinal cord continue as

    two pairs of ridges on posterior medulla

    • Contain sensory fibers; synapse in gracile and cuneate nuclei

• Major tracts: medial lemniscus (sensory decussation), corticospinal (motor)

  • Medial lemniscus—axons of gracile and cuneate nuclei decussate and form ascending (sensory) tract to thalamus

  • Corticospinal tracts—descending motor tracts in pyramids; carry signals down to skeletal muscles

• contains numerous Nuclei: cardiac center, vasomotor center, respiratory centers, inferior olivary

  • Inferior olivary nucleus—relay center for signals to cerebellum

  • Reticular formation—loose network of nuclei extending through
    entire brainstem; contains cardiac center, vasomotor center, and respiratory centers

Pons (from metencephalon)

• adult brain region that develops from embryonic metencephalon

• Anterior bulge 2.5\,\text{cm} long; posterior cerebellar peduncles

• Cerebellar peduncles—thick stalks on posterior pons that
  connect it (and the midbrain) to the cerebellum

• Cranial nerves V, VI, VII, VIII

  • Sensory: hearing, equilibrium, taste, facial sensation

  • Motor: eye & facial movement, chewing, tears, saliva, urination

• Reticular formation in pons contains additional nuclei: sleep, respiration, posture

Midbrain (mesencephalon)

• brain region that develops from embryonic mesencephalon

  • Short segment of brainstem that connects hindbrain to
    forebrain

• Contains cerebral aqueduct, periaqueductal gray substance involved in pain modulation

• Motor nuclei of two cranial nerves that control eye
  movements: CN III (oculomotor) and CN IV (trochlear)

• Tectum (roof): four bulges, 2 superior colliculi (visual reflexes), 2 inferior colliculi (auditory relay)

  • Two superior colliculi—visual attention, tracking moving objects,
    and some reflexes

  • Two inferior colliculi—relays signals from inner ear to thalamus
    and other parts of the brain

• Cerebral peduncles

  • are two anterior midbrain stalks that anchor the cerebrum to the brainstem

  • Each peduncle has three parts: tegmentum, substantia nigra, and cerebral crus

    • Tegmentum-within cerebral peduncle (dominated by red nucleus → motor relay to cerebellum)

    • Substantia nigra (dopaminergic inhibitory motor; degeneration → Parkinson)

      • Nucleus within peduncle; dark nucleus pigmented with melanin
        • Motor center that relays inhibitory signals to thalamus and basal
        nuclei suppressing unwanted body movement
        • Degeneration of neurons leads to tremors of Parkinson’s disease

    • Cerebral crus (descending corticospinal fibers)

      • Bundle of nerve fibers that connect cerebrum to pons

      • Carries corticospinal tracts

        
        

Reticular Formation

• network Web of gray matter through brainstem into spinal cord; >100 nuclei

• allows information from eyes and ears to be integrated by rest of brain

• sends sensory info to the thalamus, which then relays it to appropriate cortical areas for processing and interpretation.

• mediates overall consciousness
  

• Functions:

  • Somatic motor control (tone, posture, gaze centers, CPGs for breathing/swallow)

  • Cardiovascular control

  • Pain modulation (descending analgesic pathways)

  • Sleep & consciousness (injury → coma)

  • Habituation/ pa (filters repetitive stimuli)

Cerebellum (metencephalon)

• controls balance, posture, and coordination of muscles

• >50\% of neurons (granule cells)

• Anatomy: hemispheres connected by vermis; cortex (folia), arbor vitae, deep nuclei

• Peduncles: inferior (input from medulla/spinal cord), middle (input from cerebrum via pons), superior (output to midbrain/thalamus)

• Functions: motor coordination, locomotion, timekeeping, texture, spatial perception, eye movement, language, emotion, planning

14.4 Forebrain

Diencephalon

Connects cerebrum to the rest of the brain: lies between the brainstem and cerebrum

Consists of 3 major structures

• Thalamus: Acts as the main relay station for sensory information before it reaches the cerebrum.

• Hypothalamus: Regulates vital functions such as temperature, hunger, and thirst, and maintains homeostasis.

• Epithalamus: Includes the pineal gland, which secretes melatonin and is involved in regulating the sleep-wake cycle.

Thalamus

• Pair of ovoid masses (~80\% of diencephalon) joined by interthalamic adhesion

• \ge23 nuclei (anterior, medial, lateral, ventral, posterior groups)

• Gateway to cortex: filters, relays sensory/motor information; role in memory & emotion via limbic links

  • sensory information: arrives at the thalamus and is relayed to the cerebral cortex

  • relays motor info between brain regions

  • involved in memory formation and emotional responses

Hypothalamus

the “unconscious bus driver” that helps regulate autonomic functions such as hunger, thirst, and body temperature, while also influencing emotional behavior and stress responses.

• also major integration center for ANS and endocrine system, playing a crucial role in maintaining homeostasis by coordinating the activities of both systems to respond to internal and external stimuli.

• Forms walls/floor of third ventricle; extends from optic chiasm to mammillary bodies; attached to pituitary via infundibulum

• Nuclei control:

  • Hormone secretion (anterior & posterior pituitary control)

  • Autonomic regulation (HR, BP, GI activity)

  • Thermoregulation (preoptic area)

  • Hunger & satiety (arcuate, ventromedial)

  • Water balance & thirst (osmoreceptors; supraoptic \rightarrow ADH)

  • Sleep/circadian (suprachiasmatic)

  • Memory relay (mammillary)

  • Emotion & sexual response

Epithalamus

• Regulation of circadian rhythms (pineal gland)

  • induces sleepiness, promotes repair functions during sleep

  • may assist in regulating onset of puberty
    

• Pineal gland (melatonin), habenula (limbic relay), thin roof of 3rd ventricle

Cerebrum- the largest part of the brain responsible for voluntary activities, sensory perception, and cognitive processes, including reasoning and problem-solving.

• conscious sensory awareness, control of movement, memory and learning, language and speech, emotional responses, intellectual processes

• Two hemispheres right and left; connected by corpus callosum, separated by longitudinal fissure

• surface shows numerous gyri and sulci↑ 3x surface area to fit in cranial cavity

• has sensory areas, motor area, and association areas

  • sensory area interpret nerve impulses as sensations

  • primary motor area control voluntary skeletal muscles

  • association area interrelate sensory and motor areas 75% of brain, responsible for cognition, memoryand higher-level functions such as reasoning and problem-solving, playing a crucial role in integrating and processing complex information.

• Lobes & Functions

  • Frontal: most rostral part voluntary motor, planning, mood, aggression

  • Parietal: most upper part, after central sulcus, general senses, taste, spatial perception

  • Occipital: lateral, vision

  • Temporal: hearing, smell, memory, emotion

  • Insula: only seen if temporal lobe is cut away language, taste, visceral integration

• White Matter Tracts

  • Projection (vertical cortex ↔ lower centers)

  • Commissural (corpus callosum, ant/post commissures)

  • Association (intrahemispheric; long & short)

• Cerebral Cortex

  • 2{-}3\,\text{mm} thick; \approx40\% brain mass

  • Cell types: stellate (local), pyramidal (output)

  • Neocortex = 90\%, 6 layers (I–VI)

• Limbic System (emotion & learning)

  • Cingulate gyrus, hippocampus, amygdala; interconnected loops; reward/aversion centers

• Basal Nuclei (motor control)

  • Caudate, putamen, globus pallidus

  • Corpus striatum (caudate + putamen + globus)

  • Lentiform nucleus (putamen + globus)

14.5 Integrative Functions

Brain Waves (EEG)

• Alpha 8{-}13\,\text{Hz} — awake, eyes closed

• Beta 14{-}30\,\text{Hz} — mental activity

• Theta 4{-}7\,\text{Hz} — drowsy/sleeping adults

• Delta <3.5\,\text{Hz} — deep sleep

Sleep

• Circadian rhythm (~24 h); stages identifiable on EEG

  • Stage 1: alpha→mixed

  • Stage 2: light; sleep spindles

  • Stage 3: theta + delta; vitals drop

  • Stage 4: deep; vitals lowest

  • REM: backtrack to Stage 2; paradoxical (EEG awake-like), dreaming, atonia, penile/clitoral erection

• Regulation: SCN (master clock) via orexins, DMN, pineal melatonin; reticular formation; sleep deprivation fatal in animals

Cognition

• Distributed association areas (~75\% cortex)

• Lesions: parietal → contralateral neglect; temporal → agnosias; frontal → personality change

Memory

• Hippocampus: consolidation; teaches cortex long-term memories

• Cerebellum: motor skill learning

• Amygdala: emotional memory

• Amnesia: anterograde (no new), retrograde (no old)

Emotion

• Prefrontal cortex: judgement & expression

• Amygdala: fear, food, sex, attention; outputs to hypothalamus (autonomic) & prefrontal cortex (behavior)

Sensation

• Primary sensory cortices receive; association areas interpret; multimodal areas integrate

• Special Senses:

  • Vision: occipital lobe

  • Hearing: superior temporal

  • Equilibrium: cerebellum, brainstem, parietal roof of lat. sulcus

  • Taste: postcentral gyrus inferior

  • Smell: medial temporal

• General Senses: postcentral gyrus (primary somatosensory); sensory homunculus & somatotopy

Motor Control

• Premotor (association) → primary motor cortex (precentral gyrus) → corticospinal tracts (upper & lower motor neurons)

• Motor homunculus proportional to motor units, not size

• Basal nuclei: movement initiation, patterned behaviors; lesions → dyskinesias

• Cerebellum: coordination, posture, learning; lesions → ataxia

Language

• Wernicke area: comprehension, formulation of phrases

• Broca area: motor program for speech/signing → primary motor cortex

• Right hemisphere equivalents handle emotional prosody

• Aphasia types: nonfluent (Broca), fluent (Wernicke), etc.

Cerebral Lateralization

• Left (categorical): language, analytical, sequential

• Right (representational): spatial, holistic, music, pattern

• Correlates: 96\% of right-handers left-categorical; lateralization ↑ with age; males > females

14.6 Cranial Nerves

• General Features

  • 12 pairs so 24 total; emerge from brain base via foramina; mostly ipsilateral (exceptions: optic partial decussation; trochlear complete)

  • Classified: sensory (I, II, VIII), motor (III, IV, VI, XI, XII), mixed (V, VII, IX, X)

  • most motor fibers of cranial nerves begin in nuclei of the brainstem and lead to the glands and muscles.

  • sensory fibers: begin in receptors located mainly receptors in the head and neck and lead mainly to the brainstem

  • sensory fibers for propriception begin in the muscles innervated by motor fibers of the cranial nerves but often travel to the brain in a different nerve from the one that supplies the motor innervation

  • Most nerves carry fibers between brainstem and ipsilateral receptors and effectors

  • Sensory nerves CN I and II: carry signals only from outlying sense organs to the brain

  • Motor nerves CN III, IV, VI, XI, XII carry signals only from the brainstem to outlying muscles and glands

  • Mixed nerves V, VII, VIII, IX, X carry signals both ways

    • have sensory functions quite unrelated to their motor functions like facial nerve CN VII, has a sensory role in tatse and a motor role in facial expression

      
      Plexus nerves, such as the brachial plexus, are networks of interwoven nerves from multiple spinal roots that provide motor and sensory innervation to the limbs.

• 

  Disorders

  • Trigeminal neuralgia (tic douloureux): stabbing facial pain (CN V)

  • Bell palsy: facial paralysis (CN VII)

Imaging Techniques

• PET: inject radiolabeled glucose; active areas “light up”

• fMRI: measures blood oxygen–dependent signal; maps functional activity