Brain and Cranial Nerves

Brain and Cranial Nerves

Overview of Brain Anatomy
Four major brain regions: Cerebrum Two hemispheres; five lobes per hemisphere Diencephalon Brainstem Cerebellum Outer surface is folded Gyri = ridges Sulci = depressions between ridges; fissures = deep sulci Anterior = rostral; Posterior = caudal

The Human Brain: Left Lateral View
Figure 13.1a (top)

The Human Brain: Inferior View
Figure 13.1b (top)

The Human Brain: Midsagittal View
Figure 13.1c (top)

Gray Matter and White Matter Distribution
Brain and spinal cord composed of gray and white tissue Gray matter made of neuron cell bodies, dendrites, and unmyelinated axons Cortex is superficial layer of gray matter Nucleus are regions of grey matter (clusters of cell bodies) either close to the surface or deep within the brain White matter consists of myelinated axons Organized in bundles called tracts

Coronal Section of Cerebrum and Diencephalon
Figure 13.4a

Cerebellum and Brainstem
Figure 13.4b

Cranial Meninges Cranial meninges Three connective tissue layers
Separate and support soft tissue of brain Enclose and protect blood vessels supplying the brain Help contain and circulate cerebrospinal fluid From deep to superficial Pia matter Arachnoid mater Dura mater

Figure 13.5

Cranial Meninges Pia mater Arachnoid mater
Innermost of the meninges; adheres to brain surface Thin layer of areolar connective tissue Arachnoid mater Lies external to pia mater Arachnoid trabeculae extend to pia mater through subarachnoid space Subarachnoid space contains cerebrospinal fluid Made of a web of collagen and elastic fibers Lies deep to dura mater Subdural space is a potential space that can fill with blood if a vein is ruptured

Cranial Meninges Dura mater Tough, outer membrane
Made of dense irregular connective tissue in 2 layers Meningeal layer (deeper layer of dura) Periosteal layer (more superficial layer of dura) Forms the periosteum on internal surface of cranial bones Layers are usually fused but in some areas they separate to form dural venous sinuses that drain blood from the brain The epidural space is a potential space between dura and skull Contains arteries and veins

©McGraw-Hill Education/Christine Eckel
Cranial Dural Septa Figure 13.6 ©McGraw-Hill Education/Christine Eckel

Meningitis and Encephalitis
Inflammation of the meninges, typically caused by viral or bacterial infections Symptoms—fever, headache, vomiting, stiff neck Pain from meninges sometimes referred to posterior neck May result in brain damage and death if untreated Bacterial meningitis with more severe symptoms Encephalitis Inflammation of the brain, most often from viral infections Symptoms – drowsiness, fever, headache, neck pain, and may result in death

Brain Ventricles Ventricles Cavities within the brain
Lined with ependymal cells Contain cerebrospinal fluid Connect with each other and with spinal cord’s central canal

Ventricles of the Brain
Figure 13.7 ©McGraw-Hill Education/Christine Eckel

Brain Ventricles Four ventricles within brain Two lateral ventricles
Large cavities in cerebrum Separated by medial partition, septum pellucidum Third ventricle Narrow space in middle of diencephalon Connected to each lateral ventricle by an interventricular foramen Fourth ventricle Sickle-shaped space between pons and cerebellum Connected to third ventricle by cerebral aqueduct Opens to subarachnoid space medially and laterally Narrows before merging with central canal of spinal cord

Cerebrospinal Fluid Cerebrospinal fluid (CSF)
Clear, colorless liquid surrounding CNS Circulates in ventricles and subarachnoid space Functions: Buoyancy – reduces brain’s apparent weight by 95% Protection – provides a liquid cushion Environmental stability – transport of nutrients / wastes and protects against fluctuations

Cerebrospinal Fluid CSF formation
Formed by choroid plexus in each ventricle Layer of ependymal cells and blood capillaries (within pia) Blood plasma is filtered through capillary and modified by ependymal cells Compared to plasma, CSF has more Na+, Cl–; less K+, Ca2+, glucose In addition, ependymal cell secretions and interstitial fluid from subarachnoid space help make up CSF CSF is continually formed and reabsorbed Excess CSF flows into arachnoid villi and drains into dural venous sinuses An arachnoid granulation is a collection of these villi

(a) ©McGraw-Hill Education/Christine Eckel
Choroid Plexus Figure 13.8 (a) ©McGraw-Hill Education/Christine Eckel

Hydrocephalus Pathologic condition of excessive CSF
Often leads to brain distortion May result from obstruction in CSF restricting reabsorption May result from intrinsic problem with arachnoid villi In a young child, head enlarged with possible neurological damage May be treated surgically Implant shunts that drain CSF to other body regions

Cerebrum Cerebrum Origin of all complex intellectual functions
Two large hemispheres on superior aspect of brain Center of Intelligence and reasoning Thought, memory, and judgment Voluntary motor, visual, and auditory activities

Cerebral Hemispheres Cerebrum composed of left, right hemispheres
Longitudinal fissure: deep cleft separating hemispheres At a few locations white matter tracts connect the hemispheres Corpus callosum: largest tract providing connection between them Regions of cerebrum may exhibit multiple functions, some not easily assigned to one region Connections with the body are generally crossed Left hemisphere receives sensory signals from right side of body and sends motor signals to right side of the body Some higher-order functions exhibit lateralization; they are primarily controlled by one side of the brain Speech is frequently located in left cerebral hemisphere

Cerebral Hemispheres Figure 13.11

Lobes of the Cerebrum Five lobes in each hemisphere
Four are named for overlying cranial bones Frontal, parietal, temporal, occipital Insular lobe not visible at surface Frontal lobe: anterior part of cerebrum Posterior border is deep central sulcus Precentral gyrus controls voluntary movement Lateral sulcus separates inferior frontal lobe from temporal lobe Frontal lobe has varied functions Motor control, concentration, verbal communication, decision making, planning, personality

Lobes of the Cerebrum Parietal lobe (superoposterior part of cerebrum)
Anterior border is central sulcus Postcentral gyrus is ridge just posterior to central sulcus Posterior border is parieto-occipital sulcus Lateral border is lateral sulcus Serves general sensory functions E.g., evaluating shape and texture of objects Temporal lobe (internal to temporal bone) Located inferior to lateral sulcus Functions include hearing and smell

Lobes of the Cerebrum Occipital lobe (posterior part of cerebrum)
Functions in vision and visual memories Insula (deep to lateral sulcus) Small lobe that can be observed by pulling away temporal lobe Functions in memory and sense of taste

Functional Areas of the Cerebrum
Motor areas Housed within frontal lobes Primary motor cortex located in precentral gyrus Also called somatic motor area Controls skeletal muscle activity on opposite side of body Project contralaterally (opposite side) within brainstem or spinal cord

Motor areas (continued ) Motor speech area (Broca area) Located in inferolateral portion of left frontal lobe (in most people) Controls movements for vocalization Frontal eye field On superior surface of middle frontal gyrus Regulates eye movements needed for reading and binocular vision Premotor cortex (somatic motor association area) Located anterior to premotor cortex Coordinates learned, skilled activities

Sensory areas Primary somatosensory cortex Located in postcentral gyrus of parietal lobes Receives somatic sensory information from Proprioceptors, touch, pressure, pain, temperature receptors Areas of the body sending input can be mapped as a sensory homunculus Distorted proportions reflect the amount of sensory information collected from that region Large regions for lips, fingers, genital regions Somatosensory association area Immediately posterior to postcentral gyrus (in parietal lobe) Integrates touch information allowing us to identify objects by feel

Primary Somatosensory Cortex
Figure 13.13b

Sensory areas (continued ) Primary visual cortex Located within occipital lobe Visual association area Surrounds primary visual cortex Integrates color, form, memory to allow us to identify things we see (e.g., faces) Primary auditory cortex Located within temporal lobe Auditory association area Located in temporal lobe Interprets sounds; stores and retrieves memories of sounds

Sensory areas (continued) Primary olfactory cortex Located within temporal lobe Provides conscious awareness of smells Primary gustatory cortex Located within insula Involved in processing taste information

Functional brain regions Integrate information from multiple association areas Prefrontal cortex Located rostral to premotor cortex (in frontal lobe) Complex thought, judgment, personality, planning, deciding Still developing in adolescence Wernicke area Typically located in left hemisphere Involved in language comprehension

Central White Matter Lies deep to gray cerebral cortex
Composed of myelinated axons grouped into tracts Association tracts Connect regions of cerebral cortex within same hemisphere Arcuate fibers: short tracts connecting neighboring gyri Longitudinal fasciculi: longer tracts connecting gyri in different lobes Commissural tracts Commissures connect regions in different hemispheres Include corpus callosum, anterior and posterior commissure

Cerebral White Matter Tracts: Sagittal View
Figure 13.14a

Cerebral Lateralization
Hemisphere specialization (cerebral lateralization) Two sides of cerebrum exhibit differences in higher-order functions Categorical hemisphere is usually the left hemisphere Specialized for language abilities, functions in categorization and analysis Contains Wernicke area and motor speech area Representational hemisphere is usually the right hemisphere Concerned with visuospatial relationships, imagination, comparison of senses The two hemispheres communicate through the corpus callosum and other commissures

Lateralization develops in early childhood Seen prior to 5–6 years of age Lateralization differs between sexes Women’s posterior corpus callosum is thicker (more connections) Males suffer more functional loss when one hemisphere damaged

Figure 13.15b

Clinical View: Epilepsy and Cerebral Lateralization
Neurological disorder Neurons transmitting action potentials too frequently and rapidly Usually controlled by medications, but may require surgical removal of part of brain In most severe cases, may require hemispherectomy: removal of side of brain responsible for seizure activity Remaining hemisphere able to take over some functions of missing hemisphere

Cerebral Nuclei Cerebral nuclei: gray matter deep in cerebrum
Help regulate motor output Diseases of these nuclei associated with involuntary movements Caudate nucleus Enlarged head and slender tail paralleling lateral ventricle Helps produce pattern and rhythm of walking movements Lentiform nucleus Rounded mass between insula and diencephalon Composed of putamen and globus pallidus Putamen: helps control movements at subconscious level Globus pallidus: influences thalamus to adjust muscle tone

Cerebral Nuclei Figure 13.16

Cerebral Nuclei Cerebral nuclei (continued ) Claustrum
Thin sliver of gray matter immediately internal to insula cortex Processes visual information on a subconscious level Amygdaloid body (amygdala) Expanded region at tail of caudate nucleus Functions in mood, emotions

Clinical View: Brain Disorders
Headache Due to dilated blood vessels in skull or muscle contraction Migraine headaches: severe, recurring, often unilateral Not true brain disorder, but may accompany them Cerebral palsy Group of neuromuscular disorders Result from damage to infant brain before, during, or right after birth Impairment of skeletal muscle, sometimes mental retardation

Clinical View: Brain Disorders
Huntington disease Hereditary disease affecting cerebral nuclei Rapid, jerky, involuntary movements Intellectual deterioration Fatal within 10 to 20 years after onset Parkinson disease Affects muscle movement and balance Stiff posture, slow voluntary movements, resting tremor Caused by decreased dopamine production in substantia nigra

Diencephalon Includes the epithalamus, thalamus, and hypothalamus
Figure 13.17

Epithalamus Epithalamus
Forms posterior part of roof of diencephalon, covers third ventricle Pineal gland Endocrine gland secreting melatonin Helps regulate day-night cycles, circadian rhythm Habenular nuclei Help relay signals from limbic system to midbrain Involved in visceral and emotional responses to odors

Thalamus Thalamus Oval masses of gray matter on lateral sides of third ventricle Interthalamic adhesion Midline mass of gray matter connecting left and right thalamus Composed of about a dozen thalamic nuclei Axons from a given nucleus project to a particular region of cortex Receives signals from all conscious senses except olfaction Relays some signals to appropriate part of cortex and filters out other signals distracting from subject of attention (e.g., background noise in crowded room)

Hypothalamus Hypothalamus Functions of the hypothalamus
Anteroinferior region of the diencephalon Infundibulum: stalk of pituitary that extends from hypothalamus Functions of the hypothalamus Control of autonomic nervous system Influences heart rate, blood pressure, digestive activities, respiration Control of endocrine system Secretes hormones that control activities in anterior pituitary gland Produces antidiuretic hormone and oxytocin Regulation of body temperature Neurons in preoptic area detect altered temperature Signal other hypothalamic nuclei to heat or cool the body

Hypothalamus Functions of the hypothalamus (continued ) Food intake
Ventromedial nucleus monitors nutrient levels, regulates hunger Water intake Anterior nucleus monitors concentration of dissolved substances in blood, regulates thirst Sleep-wake rhythms Suprachiasmatic nucleus directs pineal gland to secrete melatonin, regulates circadian rhythms Emotional behavior Part of limbic system; controls emotional responses (pleasure, fear, etc.)

Brainstem Connects cerebrum, diencephalon, and cerebellum to spinal cord Contains ascending and descending tracts Contains autonomic nuclei, nuclei of cranial nerves, and reflex centers Consists of the midbrain, pons, and medulla oblongata

Brainstem: Anterior View
Figure 13.20a

Brainstem: Posterolateral View
Figure 13.20b

Midbrain Components of midbrain Cerebral peduncles
Motor tracts (pyramidal system) on anterolateral surface of midbrain Carry voluntary motor commands from primary motor cortex Superior cerebellar peduncles Connect cerebellum to midbrain Medial lemniscus Bands of ascending, myelinated axons running through brainstem Substantia nigra Cluster of cells with black appearance due to melanin Houses neurons producing dopamine Involved in movement, emotions, pleasure and pain response Its degeneration causes Parkinson disease

Midbrain Figure 13.21

Midbrain Components of midbrain (continued ) Tegmentum
Involved in postural motor control Contains red nuclei (pigmented) and reticular formation Cerebral aqueduct Connects third and fourth ventricles Surrounded by periaqueductal gray matter Houses nuclei of CN III (oculomotor) and IV (trochlear) Tectum Contains four mounds making a tectal plate Pair of superior colliculi control visual reflexes and tracking Pair of inferior colliculi control auditory reflexes

Pons Bulging region on anterior brainstem
Includes sensory and motor tracts connecting brain to spinal cord Middle cerebellar peduncles Transverse axons connecting pons to cerebellum Pontine respiratory center Helps regulate skeletal muscles of breathing Superior olivary nuclei Help with sound localization Cranial nerve nuclei (sensory and motor) Nuclei for CN V to CN VIII: trigeminal, abducens, facial, and vestibulocochlear nerves

Pons Figure 13.22

Medulla Oblongata Autonomic nuclei of medulla
Cardiac center regulates heart’s output Vasomotor center regulates blood vessel diameter Strong influence on blood pressure (vessel constriction increases pressure) Medullary respiratory center controls breathing rate Contains dorsal and ventral respiratory groups Communicates with pontine respiratory center Other nuclei for varied functions Coughing, sneezing, vomiting, salivating, swallowing

Structural Components of the Cerebellum
Cerebellum: 2nd largest brain area (after cerebrum) Cerebellar cortex: convoluted surface with folia (folds) Left and right cerebellar hemispheres Each hemisphere has an anterior and posterior lobe separated by primary fissure Vermis Narrow band of cortex between left and right lobes Receives sensory signals regarding balance Three regions of cerebellum: Cerebellar cortex: outer gray matter Arbor vitae: internal region of white matter Deep cerebellar nuclei of gray matter

Cerebellum Figure 13.24

Functions of the Cerebellum
Cerebellum coordinates and “fine-tunes” movements Ensures muscle activity follows correct pattern Stores memories of previously learned movements Regulates activity along voluntary and involuntary motor paths Adjusts movements initiated by cerebrum, ensuring smoothness Helps maintain equilibrium and posture Receives proprioceptive information from muscles and joints Continuously receives motor plans and sensory feedback May generate error-correcting signals to be sent to premotor and primary motor cortex

Cerebellar Pathways Figure 13.25

Limbic System Limbic system: the emotional brain
Composed of multiple cerebral and diencephalic structures that process and experience emotions Limbic system components Cingulate gyrus In sagittal plane, above corpus callosum Parahippocampal gyrus (associated with hippocampus) Cortical tissue in temporal lobe Hippocampus Superior to parahippocampal gyrus (temporal lobe) Helps form long-term memories

Limbic System Limbic system components (continued ) Amygdaloid body
Involved in many aspects of emotion and emotional memory, especially fear Olfactory bulbs, olfactory tracts, olfactory cortex Process odors that can provoke emotions Fornix Thin white tract connecting hippocampus with other limbic structures Nuclei that interconnect to other parts of limbic system E.g., anterior thalamic nuclei, habenular nuclei, septal nuclei, mammillary bodies

Limbic System Figure 13.26

Development of Higher-Order Brain Functions
CNS development Processing abilities become complex with maturation Most CNS axons still myelinating during first 2 years of life Brain growth is rapid in early childhood: 95% complete at age 5 Beyond early childhood, processing is more complex and more refined New connections made, while others are pruned Some CNS axons (especially prefrontal cortex) still undergoing myelination in adolescence Axons of PNS continue to myelinate past puberty

Sleep Sleep: natural, temporary absence of consciousness
Less cortical activity, vital brainstem functions maintained Sleep stages characterized by EEG frequency and eye movements Non-REM sleep: slower frequency brain waves Takes up about 75% of total sleep time Important for growth, rest, energy conservation, and strength renewal Divided up into substages with different EEG frequencies (delta, theta, etc.) REM (rapid eye movement) sleep: brain is active, eyes move Takes up about 25% of total sleep time Memorable dreaming Considered important for consolidation of memories

Memory Types of memory Sensory memory Short-term memory (STM)
Associations based on sensory input (e.g., smell of café) that last for seconds Short-term memory (STM) Limited capacity (about seven bits of information) Brief duration (seconds to hours) Long-term memory (LTM) Can be encoded from short-term memory if information repeated May exist indefinitely, but can be lost if not retrieved occasionally

Memory Brain areas involved in memory
Encoding (memory consolidation) requires amygdala and hippocampus LTM is housed primarily in appropriate association cortex area Motor memories stored in premotor cortex (and cerebellum) Memories of sounds stored in auditory association cortex

Model of Information Processing
Figure 13.30

Clinical View Alzheimer Disease: The “Long Goodbye”
Leading cause of dementia in developed world Slow, progressive loss of higher intellectual function Usually starts after age 65 Changes in mood and behavior Eventual loss of memory and personality Underlying cause unknown Significance of beta amyloid plaques and tau tangles is debated No cure, some medications to help slow course Seems identifiable with positron emission tomography (PET) Loss of sense of smell may be an early sign of the disease

Emotion Brain regions involved in emotion
Emotions are interpreted by limbic system, but expression controlled by prefrontal cortex Amygdaloid body and hippocampus are important structures If damaged or artificially stimulated, emotions are deadened or exaggerated and learning is impaired

Language Language involves reading, understanding, speaking, and writing words Wernicke area interprets language Motor speech (Broca) area initiates speech motor program Primary motor cortex signals motor neurons to produce speech Figure 13.31a

Language Categorical hemisphere (including Wernicke area) analyzes literal meaning of speech Representational hemisphere analyzes emotional content of speech Lesion to right hemisphere opposite Wernicke area can cause aprosodia—dull, emotionless speech Apraxia of speech: motor disorder Person is aware of what they want to say but cannot speak properly Aphasia: difficulty understanding or producing speech May not produce comprehensible speech; may not realize it Often due to head injury or stroke

Cranial Nerves 12 pairs of cranial nerves
Part of PNS, originating from brain Numbered with Roman numerals according to their position Begin with most anteriorly located nerve Name of nerve often related to its function

CN I Olfactory Nerve Sensory nerve for olfaction (smell)

CN II Optic Nerve Sensory nerve for vision

CN III Oculomotor Nerve
Motor nerve that controls muscles that move eye, lift eyelid, change pupil diameter, change lens shape

CN IV Trochlear Nerve Motor nerve that controls superior oblique eye muscle

CN V Trigeminal Nerve Mixed nerve that receives somatic sensation from face; controls muscles involved in chewing

CN VI Abducens Nerve Motor nerve that controls lateral rectus muscle that abducts eye

CN VII Facial Nerve Mixed nerve that controls muscles of facial expression and conducts taste sensations from tongue

CN VIII Vestibulocochlear Nerve
Sensory nerve involved in hearing and equilibrium

CN IX Glossopharyngeal Nerve
Mixed nerve that receives taste and touch from tongue; motor control of a pharynx muscle

CN X Vagus Nerve Mixed nerve that controls muscles in pharynx and larynx; conducts sensation from many viscera; major source of parasympathetic output

CN XI Accessory Nerve Motor nerve that controls muscles of neck, pharynx

CN XII Hypoglossal Nerve
Motor nerve that controls tongue muscles

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