Brain Anatomy and Structure

Overview of the Brain

• In humans, brain size correlates with body size, not intelligence
• Rostral means "toward the nose," while caudal means "toward the tail" or spinal cord
• The brain has three conceptual divisions: cerebrum, cerebellum, and brainstem
• The cerebrum, about 83% of the brain's volume, consists of two cerebral hemispheres with folds called gyri separated by grooves called sulci, and the deep longitudinal fissure separates the two hemispheres
• The corpus callosum connects the hemispheres at the bottom of this fissure
• The cerebellum is the second-largest part of the brain, located in the posterior cranial fossa inferior to the cerebrum, making up 10% of the brain’s volume while containing over 50% of the brain's neurons
• The brainstem comprises all brain parts except the cerebrum and cerebellum
• Major components of the brainstem include the diencephalon, midbrain, pons, and medulla oblongata

Gray and White Matter

• Spinal cord and brain are made up of gray and white matter
• White matter has a pearly white color due to myelin around nerve fibers
• Gray matter contains less myelin and has a duller white color
• The cortex, a surface layer over the cerebrum and cerebellum, is formed by gray matter
• Deeper masses called nuclei are surrounded by white matter
• In the spinal cord, the white matter lies deep to the cortical gray matter
• White matter in the brain comprises tracts/bundles of axons

Brain Development

• The mature brain anatomy develops from embryonic forebrain, midbrain, and hindbrain
• The nervous system develops from ectoderm
• Early in the third week of development, a neural plate forms along the embryo's dorsal midline, sinking and thickening at the edges to form a neural groove and folds
• The neural folds fuse along the midline, beginning in the cervical region, to create the neural tube
• By day 26, a hollow channel, the neural tube, is created
• The neural tube later separates from overlying ectoderm, sinks, and motor nerve fibers develop
• The lumen later becomes the central canal of the spinal cord and ventricles of the brain
• Ectodermal cells form neural crests on each side of the neural tube
• Neural crest cells form the arachnoid mater and pia mater, most of the PNS as well as skeletal, integumentary, and endocrine system structures
• The neural tube exhibits three primary vesicles by the fourth week: forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon)
• The neural tube subdivides into five secondary vesicles by the fifth week
• The forebrain becomes the telencephalon and diencephalon
• The telencephalon has lateral growths that become the cerebral hemispheres
• The diencephalon has optic vesicles that become the retinas
• The midbrain remains undivided retaining the name mesencephalon
• The metencephalon and the myelencephalon arise from the hindbrain

Meninges, Ventricles, CSF and Blood Supply

• Meninges, three connective tissue membranes, envelop the brain between nervous tissue and bone
• The three meninges are the dura mater, arachnoid mater, and pia mater
• The dura mater in the cranial cavity has two layers: an outer periosteal and an inner meningeal layer
• Only the meningeal layer forms the dural sheath by continuing into the vertebral canal
• In limited places, the dura mater presses against cranial bone (around the foramen magnum, sella turcica, crista galli, and sutures)
• The meningeal layer of the dura folds inward to separate the brain's major parts
• The falx cerebri is a wall between the cerebral hemispheres
• The tentorium cerebelli is like a roof over the posterior cranial fossa, separating the cerebellum from the cerebrum
• The falx cerebelli separates the right and left halves of the cerebellum
• The arachnoid is a transparent membrane over the brain surface
• The subarachnoid space separates it from the pia mater
• The pia mater is a thin, delicate membrane that follows brain contours and sulci

Ventricles and CSF

• The brain has four internal chambers (ventricles) filled with cerebrospinal fluid
• The largest and most rostral ventricles are the lateral ventricles, forming an arc in each cerebral hemisphere
• The lateral ventricles connect via the interventricular foramina to the third ventricle
• From the third ventricle, the cerebral aqueduct leads to the fourth ventricle
• The central canal extends into the spinal cord as the fourth ventricle narrows caudally
• Choroid plexus containing blood capillaries is in each ventricle
• Ependyma, lining the ventricles and canals, resembles cuboidal epithelium and produces cerebrospinal fluid

Cerebrospinal Fluid (CSF)

• CSF is a clear, colorless liquid that fills the CNS and bathes its surface
• The brain can produce about 500 mL of CSF a day and constantly reabsorbs it
• Normally, 100 to 160 mL of CSF is present
• CSF production starts with blood plasma filtration via the brain’s capillaries
• Ependymal cells modify the filtrate
• More sodium and chloride are in the CSF but less potassium, calcium, glucose, and protein
• Circulation of CSF occurs via pressure, beating of cilia on ependymal cells, and rhythmic pulsations of heartbeat
• CSF secreted in the lateral ventricles flows to the third ventricle to the fourth ventricle and then down to the cerebral aqueduct through the interventricular foramina
• Third and fourth ventricles add more CSF
• A small amount of CSF fills the central canal of the spinal cord, but it all escapes through a median aperture and two lateral apertures in the walls of the fourth ventricle
• CSF is reabsorbed by arachnoid granulations protruding through the dura mater into the superior sagittal sinus
• The purposes of CSF: buoyancy, protection, and chemical stability

Brain’s Blood Supply

• The brain needs a critical blood supply, and the brain barrier system protects it from dangerous agents
• The brain is 2% of adult weight, but blood supply is 15%, and it consumes 20% of oxygen and glucose
• A 10-second interruption in blood flow can cause loss of consciousness
• 1 to 2 minutes of blood flow interruption can impair function
• 4 minutes without blood causes irreversible brain damage
• The brain barrier system regulates substance movement from the bloodstream into the brain's tissue fluid
• The blood–brain barrier (BBB)'s tight junctions between endothelial cells protect the brain
• Astrocytes induce formation of tight junctions during development
• Anything leaving the blood must pass through endothelial cells
• Blood–CSF barrier protects choroid plexuses
• Tight junctions are absent from ependymal cells elsewhere, which allows exchange between the brain and CSF
• The BBS is highly permeable to water, glucose, and lipid-soluble substances
• Examples: oxygen, CO2, alcohol, caffeine, nicotine, and anesthetics
• The BBS is slightly permeable to sodium, potassium, chloride, urea, and creatinine
• The BBS is an obstacle to medication delivery
• Examples: antibiotics and cancer drugs
• Trauma and inflammation damage the BBS allowing pathogens to enter brain tissue
• Circumventricular organs (CVOs) lack the barrier in the third and fourth ventricles, giving blood direct brain access
• HIV uses this route of invasion

Hindbrain and Midbrain

• The medulla oblongata of the adult hindbrain differentiates from the embryonic myelencephalon
• The medulla begins at the foramen magnum and extends about 3 cm rostrally to the groove between the medulla and pons
• The anterior surface contains pyramids with ridges wider at the rostral end, which taper caudally, separated by the anterior median fissure
• A bulge called the olive exists laterally to each pyramid
• The gracile and cuneate fasciculi of the spinal cord continue as two pairs of ridges on the medulla posteriorly
• Nerve fibers pass through the medulla connecting the brain to the spinal cord
• The ascending fibers include first-order sensory fibers of the two fasciculi
• These end in the gracile and cuneate nuclei which synapse with second-order fibers that form the medial lemniscus
• The second-order fibers rise to the thalamus, synapse with third-order fibers and continue to the cerebral cortex
• The biggest descending fibers are the corticospinal tracts filling the pyramids on the anterior surface
  • These carry motor signals to stimulate skeletal muscles
  • About 90% of these fibers cross over at the pyramidal decussation controlling muscles below the neck contralaterally
  • The tectospinal tract controls the neck muscles

Medulla Functions

• The medulla contains neural networks involved in sensory and motor functions
• Sensory functions include touch, pressure, temperature, taste, and pain.
• Motor functions such as chewing, salivation, swallowing, respiration, speech, cardiovascular control, head, neck, and shoulder movements
• Signals enter and leave through four pairs of cranial nerves
  • These cranial nerve pairs are vestibulocochlear (CN VIII), glossopharyngeal (CN IX), vagus (CN X), and hypoglossal (CN XII) nerves
• The inferior olivary nucleus is a relay center
• Reticular formation is a loose network of nuclei extending throughout the medulla, pons, and midbrain
  • It includes a cardiac center, a vasomotor center, two respiratory centers, and motor function nuclei in the medulla

Pons and Cerebellum

• The pons and cerebellum are metencephalon developments
• The pons, about 2.5 cm long, appears as a broad anterior medulla bulge
• Posteriorly, thick cerebellar peduncles connect the pons and midbrain
• The pons contains continuations of the reticular formation, medial lemniscus, and tectospinal tract
• The anterior pons has white matter tracts including transverse fascicles and longitudinal fascicles that carry sensory and motor signals
• V to VIII cranial nerves begin/end in the pons, with sensory and motor functions
• Reticular formation contains nuclei involved in sleep, respiration, and posture

Midbrain

• The mesencephalon becomes the midbrain connecting the hindbrain and forebrain
• The midbrain contains the cerebral aqueduct, continuations of the medial lemniscus, and reticular formation
• Also present are oculomotor (CN III) and trochlear (CN IV) nerve motor nuclei to control eye movements
• The tectum exhibits four bulges in the roof-like part of the midbrain posterior to the cerebral aqueduct (the corpora quadrigemina)
• The upper pair (superior colliculi) controls vision, eye-related functions, visual tracking blinking and focusing
• The lower pair (inferior colliculi) receives signals from the inner ear and relays to the thalamus

Peduncles and Tegmentum

• The midbrain mainly consists of two cerebral peduncles anterior to the cerebral aqueduct anchoring the cerebrum to the brainstem
• Three main components exist in each peduncle: tegmentum, substantia nigra, and cerebral crus
• The tegmentum features the red nucleus, connects to the cerebellum for fine motor control
• The substantia nigra is a pigmented motor center that relays inhibitory signals to the thalamus and basal nuclei
• Degeneration of its neurons causes Parkinson Disease tremors
• The cerebral crus has fibers connecting the cerebrum to the pons and carrying the corticospinal nerve tracts

Central Gray Matter

• The central (periaqueductal) gray matter surrounds the cerebral aqueduct involved with the reticulospinal tracts controlling awareness of pain
• Reticular formation is a web of gray matter running through all brainstem levels
• The reticular formation occupies the space between the white fiber tracts and brainstem nuclei, and connects with the cerebrum
• More than 100 small neural networks carry out five functions in the reticular formation: somatic motor control, cardiovascular control, pain modulation, sleep and consciousness, and habituation

Reticular Formation

• Somatic motor control includes motor neurons connected to reticular formation nuclei adjusting muscle tone, balance, and posture during movement
• It relays signals from the eyes and ears to the cerebellum
• The cardiovascular center controls the heart and blood vessels
• It processes pain signals to the cerebral cortex as well as blocking pain signal transmission
• It’s vital for alertness/sleep states, and damage may cause irreversible coma
• Habituation occurs through repetitive stimuli

Cerebellum

• The cerebellum is the biggest hindbrain part connected by the vermis, a wormlike bridge
• Folias are parallel folds on each hemisphere are separated by shallow sulci
• The cerebellum has a gray matter surface cortex and a white matter deeper layer, which exhibits a pattern called arbor vitae
• Four masses of gray matter are in each hemisphere named deep nuclei embedded in the white matter responsible for all outputs
• All cerebellum input goes to the cortex
• 10% of the brain's mass, 60% of the cerebral cortex's surface area, and over half of all brain neurons
• Tiny granule cells are the most neuron abundant type
• Large Purkinje cells have compressed dendrites
• The cerebellum connects to the brain stem through three pairs of cerebellar peduncles
• Inferior peduncles connect to the medulla oblongata
• Middle peduncles connect to the pons
• Superior peduncles connect to the midbrain
• Spinal input enters via the inferior peduncles
• Input from the rest of the brain enters via the middle peduncles
• Output from the cerebellum is via the superior peduncles
• Cerebellar lesions also lead to sensory, linguistic, emotional, and nonmotor functioning deficits besides coordination and locomotor ability
• The cerebellum is active during tactile exploration/spatial perception, a timekeeping center as well as impairments in pitch/language output
• Difficulties planning and scheduling/impulse control, as well as children with ADHD experiencing abnormally small cerebella exhibit other cerebellar lesion affects

Forebrain

• The diencephalon and telencephalon make up the forebrain
• The thalamus, hypothalamus, and epithalamus are diencephalon derivatives enclosing the third ventricle
  • The thalamus on each brain side, an ovoid mass beneath the cerebral hemisphere at the brainstem's superior end, is about four-fifths of the diencephalon
  • It protrudes medially into the third ventricle and laterally into the lateral ventricles that join medially
• It is composed of 23 nuclei classified in five main groups
• Major groups: anterior, posterior, medial, lateral, and ventral
• The thalamus is the cerebral cortex “gateway” in that input passes via the thalamic nuclei
• It relays signals from the cerebellum for motor control, and provides feedback loops for the cerebral cortex and basal nuclei

Function of the Thalamus

• The thalamus participates in the limbic system's functions, such as memory and emotion
• The walls/floor of the third ventricle forming the hypothalamus extending near the mammillary bodies
• Each mammillary body contains three to four mammillary nuclei relaying limbic signals to the thalamus
• The pituitary gland attaches to the hypothalamus via a stalk (infundibulum)
• It's the autonomic/endocrine systems major center concerned visceral functions
• It monitors and controls the anterior pituitary regulating growth, metabolism, reproduction, and stress
• The hypothalamus stores water conservation hormones in the posterior pituitary, as well as involved with labor contractions and lactation

Autonomic Hypothalamus Effects

• Integrative center for the autonomic nervous system, such as influencing heart rate and blood pressure
• Body temperature regulated by the hypothalamic thermostat composed of mostly preoptic nucleus neurons
• It regulates sensations of hunger and satiety
  • Receptors exist for hormones increasing/decreasing hunger and energy expenditure
  • A blood osmolarity monitor stimulates behavioral/hormonal change when dehydrated
• It’s the caudal part of the reticular formation regulating sleep
  • The suprachiasmatic nucleus controls circadian rhythms and is near the optic chiasm
• Mammillary nuclei lie in the pathway of hippocampus signals for memory
• Nuclei lesions cause memory deficits
• It involves anger, aggression, fear, pleasure, sex
• The epithalamus has a pineal gland, a relay known as the habenula, and roof for the third ventricle
• It's the cerebrum's most conspicuous section
• The cerebrum has two hemispheres separated by the longitudinal fissure and connected by the corpus callosum
• Sulci, or folding into gyri, allows greater cortex amounts to fit into the brain cavities
• Frontal, parietal, occipital, temporal, and insula are the five distinct lobes

Brain Lobes

• The frontal lobe lies behind the frontal bone extending to the central sulcus used for voluntary motor/higher mental function
• The parietal lobe forms the brain’s upper part extending to the parieto-occipital sulcus
• Receives/interprets general senses and visual processing
• The occipital lobe in the rear is caudal to the parieto-occipital sulcus and is the visual center
• The lateral temporal lobe is deep to the temporal bone concerned with hearing, smell, and emotional aspects

Cerebral Tracts

• Glia and myelinated nerve fibers comprise the cerebrum's white matter volume into three kinds of cerebral tracts
• Projection tracts, carry corticospinal motor signals
• In general, projection tracts extend between higher & lower brain and spinal cord centers
• The projection tracts form a sheet/fanlike array
• The array has a dense sheet (internal capsule) and fanlike array (corona radiata)
• The cerebral commissural tracts cross from one hemisphere to the other
• Nearly all pass via the corpus callosum
• More rarely pass the smaller anterior and portions
• Within the same hemisphere association tracts connect separate regions
  • Short fibers (gyri), long fibers (lobes) are among the association tracts
  • Association tracts link perceptual and memory centers

Gray Matter and Neurons

• The cerebral gray matter carries out neural integration such as is the cortex, basal nuclei and limbic system
• The cerebral cortex, containing 14 to 16B neurons, is a layer of the hemispheres composing 40% brain mass, as well
• Stellate and pyramidal cells are neurons inside the cerebral cortex
• Input from sensory stimulus with local processing is from spheroidal stellate cells projecting short distances
• Output pyramidal cells are conical extending to the brain surface to pass to the white matter

Cortex

• 90% of cortex, from 600M years ago, the neocortex six-layered
  • Thickness/connections vary in different layers and regions
  • 3 major axons exit cortex: layers II, V, VI
  • The earliest paleocortex, archicortex and neocortex have six layers
    • Smell sensation exists in paleocortex areas of the insula
    • Human hippocampus features archicortex
    • Limbic system encircles learning, thalamus, corpus callosum
    • Prominent components: cingulate gyrus, hippocampus, amygdala
    • Components loop together and connect both hemispheres
• The limbic system regulates emotion from centers within the accumbens
• nuclei and amygdala nucleus

Basal Nuclei

• Basal nuclei are cerebral matter masses outside the central nervous system, such caudate, putamen and globus
• Together they compose striatum
• Globus pallidus and putamen form lentiform nuclei

Electrical Brain Activity

• Cortex is tied to sensations, and integration using cerebrum
• Brain's electrical activity features brainwaves
  • Four types: Alpha, beta, theta, delta, as shown on an EEG or electrical recording of the brain
  • Alpha is awakened area when at rest, 8 to 13Hz, especially the parieto-occipital regions
  • Waves absent when stimulated/deep sleep
  • Beta 14–30 frontal-parietal, dominant mental activity/stimulation
  • Theta 4–7, normal children-drowsy adults-also emotional brain disorders; delta less than 3, wake infants and deep sleep in others
• Abundance awake adults relates to brain damage
• SCN links eyes' rhythms/ external cues

Overview of Sleep

• Sleep is "temporary" stimuli
• Occurs with postural change/ paralysis and coma
• Sleep features stages that occur with the EEG patterns
• Stage one has drowsy
• Stage two light is the EEG wave decline, high amplitude, with interactions to the thalamus
• Stage three: occurs 20, after stage 1, vital sign decline, waves include theta, delta
• Stage four is delta, low vital sign
• Rapid eye movement (REM) or fast eye movement occurs as muscles paralyze

REM

• Rapid eye movement (REM) also resembles consciousness, increased vitals and sleep paralysis is increased
• Sleep features different types and phases
• The parasympathetic division stimulates both constricted pupils or sexual interaction
• Sleep includes complex interactions between cerebrum-thalamus-hypothalamus-reticular formation
• Retina and the suprachiasmatic nucleus rhythm stimulate the sleep day and night rhythm
• Later hypothalamus produces signal peptides for elevation and metabolic sleep (wakefulness)
• Orexin absent in narcolepsy
• Scientists cannot explain how dreaming and non-Rem cause
• Sleep includes energy, safe periods, and memory consolidations

Cognition

• Cognition is to gain/ understand and use memories distributed throughout the brain
• Cognition disorders arise from:
• Parietal lobe damage that leads to limb unawareness or side defects
• Long term sensory memories: vocabulary with planning and lesions cause severe amnesia
• Damage to temporal lobe relates to object recognition
• Damage to the frontal lobe relates to social and or personality disorders
• Higher understanding is prefrontal/ only primate based

Overview: Emotions and Sensations

• Emotions form by function and region and cannot work without both
• Emotional centers form with lesions but can cause misinterpretation
• Prefrontal leads, expression, intent, with emotions, like the hypothalamus; most are the amygdala for fear, also sensation and new functions
• The outputs of the amygdala:
• 1: Hypothalamus: lower movement and sensory signals
• 2: Influencing controls
• Aspects in personality as well: personality, learning, anger, pleasure
• Human connections: stimulus and interactions
• Most is based on what we get from the signals: a. MFB are involved in reward punishment sensations from medial bundle
• MFB stimuli even from SZ or pain medication still have a button causing tension relief
• Stimulation exists by senses to primary cortexes like taste, smell, touch
• Sensory input is first and areas respond
• Multimodal input from different responses like a meal to orbital areas and senses can be limited by a sense:
• Vision: occipital
• Hearing temporal
• The body touch is both cranial, trigeminal and spinal and the side of the body and thalamus/ cortex

Muscle Contraction

• Intent to contract begins with from the (premotor) area of ​​the frontal lobes as then translated to a precentral, primary motor, in the frontal
• Signals from from the neurons descend to contraction
• As a homunculus, size of a body = number of muscles or units
• Pyramidal precentral neurons are upper motor, the tract to fibers is about 1M lower from sides of the spinal cord

Basal Nuclei & Cerebellum

• The basal nuclei start and stop most actions such hip , shoe tying with feedback from cerebrum
• All signals are to the nuclei except for primary and then thalamus
• Lesions lead to dyskinesias with rigid movements
• Cerebellum then aids all the signals and motor helps
  • The cerebellum then receives cerebrum signals to joints the two compare and correct any error

Language

• Language: ability to understand/speak/ write/ read Area of the brain for Language:
• Wernicke posterior, speech and writing
• Angular gyrus parietal, reading and parietal writing by translating and sending a motor program to a Broca location (prefrontal cortex).
• When PET scan rises prepare to
• To motor the body

Aphasia & Lateralization

• All these are connected and cortex opposite, with language problems is called Aprosdy
• Wernicke can lead to not recognizing what other people say the result is aphasia: language loss (lesions)
• Problems include understanding and then speaking and the cause and different damages

Hemispheric Lateralization

• Each cerebral hemisphere appears nearly symmetrical but presents functional asymmetry
• One hemisphere (usually the left) specializes in analytic / sequential functions (categorical hemisphere)
• The other (usually the right) specializes in holistic functions such as visual / spatial skills (representational hemisphere)
• Lateralization related to handedness: dominant left hemisphere 96% right-handers; 70% left-handers
• Children's hemispheres can take over for each other if damaged
• The effect of damage more functional on males

Cranial Nerves

• the brain communicates with 12 pairs of them and spinal cords
• The twelve begin with rostral pairing, they have a descriptive title
• Motor has the fiber with brains to gland and muscles
• Sensory has fiber with mainly neck located/ mainly brain
• Fibers for cranial muscles may travel a different direction
• Effects go to stem, stem and receptors exception- optic for 2/ trochlear (all contra-lateral)
• Some do stimulate, carry and assist hearing and taste, motor in faces: VII or sensory

Olfactory Nerve (CN I)

• Nerve with mucosa/ bulb
• Damage has smell impairment can be tested
• Vision is CNII: passes: optic foramen from the retina to thalamus/ midbrain
• Blindness is tested by sight/ testing
• Oculomotor/ III: goes orbital fissure
• Eye can move and control/ iris/ etc from the brain middle to fibers can cause double vision, size and tracking, etc.

Trochlear Nerve (CN IV)

• Primarily a midbrain motor to vision/ downward medially /orbital
• May experience vertical errors tested and tilted
• Trigeminal has the large largest important with three is orbital from the sinuses also tested and face
• The maxillary the pons, the molar and with damage from temperature

Abducens Nerve(CN VI)

• Abducens is V!: controls the eye medially orbit side not able look lateral
• The seven - VII has is mouth taste is a pain, can test ammonia or face by smiling

Vestibulocochlear Nerve CN VIII

• Inner ear problems mean hearing and balance loss also check inner ear
• Glossopharyngeal the IX does numerous sections, from pharynx is damage taste bad, also cough
• Vagal the the ten, to most long sections can result in heart or damage as vocal, you can look at this cough
• Cervical cord to C6 to shoulder and neck function will be hard to test for shrugging function
• Hypoglossal 12 tongue moves
• Tongue is damaged and swallowing