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toward the forehead


toward the spinal cord

average weight of brain

Brain weighs about 1,600g (3.5 lb) in men, and 1,450g in women

three major portions of the brain



83% of brain volume; cerebral hemispheres, gyri and sulci, longitudinal fissure, corpus callosum


the portion of the brain that remains if the cerebrum and cerebellum are removed; diencephalon, midbrain, pons, and medulla oblongata


contains 50% of the neurons; second largest brain region, located in posterior cranial fossa

Longitudinal fissure

deep groove that separates cerebral hemispheres

Corpus callosum

thick nerve bundle at bottom of longitudinal fissure that connects hemispheres


shallow grooves


thick folds


what remains of the brain if the cerebrum and cerebellum are removed

Major components
-Medulla oblongata

Major components of the brainstem

-Medulla oblongata

Gray matter

—the location of neuron cell bodies, dendrites, and synapses
-Forms surface layer (cortex) over cerebrum and cerebellum
-Forms nuclei deep within brain

White matter

-bundles of axons
-Lies deep to cortical gray matter, opposite relationship in the spinal cord
-Pearly white color from myelin around nerve fibers
-Composed of tracts (bundles of axons) that connect one part of the brain to another or connect the cortex to the spinal cord


bundles of axons


outermost tissue layer of the embryo
-where the nervous system develops


early into the third week of development, dorsal streak appears along the length of embryo

what does the neruoectoderm thicken to form?

the neural plate

neural plate

-what the neuroectoderm thickens to form
-Destined to give rise to most neurons and all glial cells except microglia, which come from mesoderm

as thickening of the neural plate progresses...

Neural plate sinks and its edges thicken
Forming a neural groove with a raised neural fold on each side
Neural folds fuse along the midline
Beginning in the cervical region and progressing rostrally and caudally

where do the neural folds fuse?

along the midline. Beginning in the cervical region and progressing rostrally and caudally

by the fourth week of embryonic development, what is formed?

neural tube

neural tube

hollow channel formed by the fourth week

4th week of embryonic development

-Neural tube separates from overlying ectoderm
-Sinks deeper
-Grows lateral processes that later will form motor nerve fibers
-Lumen of neural tube becomes fluid-filled space
-Central canal in spinal cord
-Ventricles of the brain

Neural crest

formed from ectodermal cells that lay along the margins of the groove and separate from the rest forming a longitudinal column on each side
-Gives rise to the two inner meninges, most of the peripheral nervous system, and other structures of the skeletal, integumentary, and endocrine systems

what does the neural crest give rise to?

the two inner meninges, most of the peripheral nervous system, and other structures of the skeletal, integumentary, and endocrine systems

three anterior dilations (primary vessicles) that the neural tube exhibits by the fourth week...

Forebrain (prosencephalon)
Midbrain (mesencephalon)
Hindbrain (rhombencephalon)

what happens during the fifth week of embryonic development?

it subdivides into five secondary vessicles:

what two vessicles does the hindbrain divide into during the fifth week of embryonic development?


what two vessicles does the forebrain divide into during the fifth week of embryonic development?

Telencephalon—becomes cerebral hemispheres
Diencephalon—has optic vesicles that become retina of the eye

The midbrain during the fifth week of embryonic development

remains undivided.
called: mesencelphalon

The medulla oblongata

Cardiac center: Adjusts rate and force of heart

Vasomotor center: Adjusts blood vessel diameter

Respiratory centers: Control rate and depth of breathing

Reflex centers: For coughing, sneezing, gagging, swallowing, vomiting, salivation, sweating, movements of tongue and head

Cardiac center

Adjusts rate and force of heart

Vasomotor center

Adjusts blood vessel diameter

Respiratory centers

Control rate and depth of breathing

Reflex centers

For coughing, sneezing, gagging, swallowing, vomiting, salivation, sweating, movements of tongue and head

The midbrain

-Mesencephalon becomes one mature brain structure, the midbrain
-Short segment of brainstem that connects the hindbrain to the forebrain
-Contains cerebral aqueduct
-Contains continuations of the medial lemniscus and reticular formation
-Contains the motor nuclei of two cranial nerves that control eye movements: CN III (oculomotor) and CN IV (trochlear)

what two cranial nerves that control eye movement does the midbrain contain?

CN III (oculomotor) and CN IV (trochlear)


-rooflike part of the midbrain posterior to cerebral aqueduct
-Exhibits four bulges, the corpora quadrigemina
-Upper pair, the superior colliculi, function in visual attention, tracking moving objects, and some reflexes
-Lower pair, the inferior colliculi, receives signals from the inner ear
-Relays them to other parts of the brain, especially the thalamus


-Dominated by the red nucleus
-Pink color due to high density of blood vessels
-Connections go to and from cerebellum
-Collaborates with cerebellum for fine motor control

Substantia nigra

-Dark gray to black nucleus pigmented with melanin
-Motor center that relays inhibitory signals to thalamus and basal nuclei preventing unwanted body movement
-Degeneration of neurons leads to tremors of Parkinson disease

Cerebral peduncles

-Bundle of nerve fibers that connect the cerebrum to the pons
-Carries corticospinal tracts


—three connective tissue membranes that envelop the brain
-Lies between the nervous tissue and bone
-As in spinal cord, they are the dura mater, arachnoid mater, and the pia mater
-Protect the brain and provide structural framework for its arteries and veins

Dura mater

-Most superficial of the 3 layers
-In cranial cavity; is actually composed of two layers
-An Outer periosteal—equivalent to periosteum of cranial bones
-An Inner meningeal—continues into vertebral canal and forms dural tissue that surrounds the spinal cord
-Cranial dura mater is pressed closely against cranial bones
-No epidural space
-Few blood vessels trapped between outer dura and cranial cavity
-Layers separated by dural sinuses—collect blood circulating through brain

Falx cerebri

separates the two cerebral hemispheres

Tentorium cerebelli

separates cerebrum from cerebellum

Falx cerebelli

separates the right and left halves of cerebellum

what does the dura mater do?

Folds inward to extend between hemispheres of the brain
-falx cerebri
-tentorium cerebelli
-falx cerebelli

Arachnoid mater

-Transparent membrane over brain surface
-Subarachnoid space separates it from pia mater below
-Subdural space separates it from dura mater above in some places

Pia mater

Very thin membrane that follows contours of brain, even dipping into sulci
Not usually visible without a microscope


four internal chambers within the brain
-2 lateral ventricles
-third ventricle
-fourth ventricle

Two lateral ventricles

: one in each cerebral hemisphere
Interventricular foramen—a tiny pore that connects to third ventricle

Interventricular foramen

a tiny pore that connects to third ventricle

Third ventricle

single narrow medial space beneath corpus callosum
Cerebral aqueduct runs through midbrain and connects third to fourth ventricle

Cerebral aqueduct

runs through midbrain and connects third to fourth ventricle

Fourth ventricle

-small triangular chamber between pons and cerebellum
-Connects to central canal, runs down through spinal cord

Choroid plexus

spongy mass of blood capillaries on the floor of each ventricle


-neuroglia that lines the ventricles and covers choroid plexus
-Produces cerebrospinal fluid

ventricles and CSF

-CSF continually flows through and around the CNS
-Driven by its own pressure, beating of ependymal cilia, and pulsations of the brain produced by each heartbeat
-CSF secreted in lateral ventricles flows through intervertebral foramina into third ventricle
-Then down the cerebral aqueduct into the fourth ventricle
-Third and fourth ventricles add more CSF along the way

what is CSF reabsorbed by?

arachnoid villi

arachnoid villi

Cauliflower-shaped extension of the arachnoid meninx
Protrudes through dura mater
Into superior sagittal sinus
CSF penetrates the walls of the villi and mixes with the blood in the sinus

Small amount of CSF fills the central canal of the spinal cord

All escapes through three pores
Median aperture and two lateral apertures
Leads into subarachnoid space of brain and spinal cord surface

Cerebrospinal fluid (CSF)

-clear, colorless liquid that fills the ventricles and canals of CNS
-Bathes its external surface

how much CSF does the brain produce/absorb per day?

-Brain produces and absorbs 500 mL/day
-100 to 160 mL normally present at one time

-Production begins with the filtration of blood plasma through the capillaries of the brain
-Ependymal cells modify the filtrate, so CSF has more sodium and chloride than plasma, but less potassium, calcium, glucose, and very little protein

Functions of the CSF

Chemical stability

Functions of the CSF: Buoyancy

Allows brain to attain considerable size without being impaired by its own weight
If it rested heavily on floor of cranium, the pressure would impede impulse flow of neurons and kill the nervous tissue

Functions of the CSF: Protection

Protects the brain from striking the cranium when the head is jolted by decreasing the velocity of the brains movements

Functions of the CSF: Chemical Stability

Flow of CSF rinses away metabolic wastes from nervous tissue and homeostatically regulates its chemical environment

Blood Supply and the Brain Barrier System

-Brain is only 2% of the adult body weight, and receives 15% of the blood
750 mL/min.

-Neurons have a high demand for ATP, and therefore, oxygen and glucose, so a constant supply of blood is critical to the nervous system

-A 10-second interruption of blood flow may cause loss of consciousness
-A 1- to 2-minute interruption can cause significant impairment of neural function
-Going 4 minutes without blood causes irreversible brain damage

what can blood also be a source of?

Blood is also a source of antibodies, macrophages, bacterial toxins, and other harmful agents

Brain barrier system

strictly regulates what substances can get from the bloodstream into the tissue fluid of the brain

Two points of entry must be guarded
-Blood capillaries throughout the brain tissue
-Capillaries of the choroid plexus

Two points of entry must be guarded in the blood brain barrier system

Blood capillaries throughout the brain tissue
Capillaries of the choroid plexus

Blood-brain barrier

-protects blood capillaries throughout brain tissue
-Consists of tight junctions between endothelial cells that form the capillary walls
-Astrocytes reach out and contact capillaries with their perivascular feet
-Induce the endothelial cells to form tight junctions that completely seal off gaps between them
-Anything leaving the blood must pass through the cells, and not between them
-Endothelial cells can exclude harmful substances from passing to the brain tissue while allowing necessary ones to pass

Blood-CSF barrier

—protects the brain at the choroid plexus
-Forms tight junctions between the ependymal cells
-Tight junctions are absent from ependymal cells elsewhere
-Important to allow exchange between brain tissue and CSF

what is blood barrier system permeable to?

Blood barrier system is highly permeable to water, glucose, and lipid-soluble substances such as oxygen, carbon dioxide, alcohol, caffeine, nicotine, and anesthetics

Slightly permeable to sodium, potassium, chloride, and the waste products urea and creatinine

Crainal nerves

Brain must communicate with rest of body
-most of the input and output travels by way of the spinal cord
-12 pairs of cranial nerves arise from the base of the brain
-Exit the cranium through foramina
-Lead to muscles and sense organs located mainly in the head and neck

where do the 12 pairs of cranial nerves arise?

the base of the brain

where do the 12 pairs of cranial nerves exit?

through foramina

motor fibers of the cranial nerves

Most motor fibers of the cranial nerves begin in nuclei of brainstem and lead to glands and/or muscles

Sensory fibers of the cranial nerves

Sensory fibers begin in receptors located mainly in head and neck and lead mainly into the brainstem

cranial nerve impulses (sensory or motor)

Most cranial nerve impulses (sensory or motor) are generally ipsilateral
-i.e Lesion in left brainstem causes sensory or motor deficit on same side

-Exceptions: optic nerve where half the fibers decussate, and trochlear nerve where all efferent fibers lead to a muscle of the contralateral eye

Cranial nerve classification

Some cranial nerves are classified as motor, some sensory, others mixed

-Sensory (I, II, and VIII)

-Motor (III, IV, VI, XI, and XII)
-Stimulate muscle but also contain fibers of proprioception

-Mixed (V, VII, IX, X)
-Sensory functions may be quite unrelated to their motor function
-Facial nerve (VII) has sensory role in taste and motor role in facial expression

The Olfactory Nerve (I)

Sensory only
Sense of smell (olfaction)

The Optic Nerve (II)

Sensory only
Provides vision
Neurons project to the primary visual cortex in the occipital lobe

The Oculomotor Nerve (III)

Motor only
Controls muscles that turn the eyeball up, down, and medially, as well as controlling the muscle of the iris, lens, and upper eyelid
Damage causes drooping eyelid, dilated pupil, double vision, difficulty focusing, and inability to move eye in certain directions

The Trochlear Nerve (IV)

Motor only
Eye movement (superior oblique muscle)

The Trigeminal Nerve (V

Mixed motor and sensory
Largest of the cranial nerves

Most important sensory nerve of the face

Forks into three divisions:
-Ophthalmic division (V1): sensory
-Maxillary division (V2): sensory
-Mandibular division (V3): mixed

The Abducens Nerve (VI)

Motor only
Provides eye movement (lateral rectus m.)

The Facial Nerve (VII)

Mixed motor and sensory
Motor—major motor nerve of facial muscles: facial expressions; salivary glands and tear, nasal, and palatine glands
Sensory—taste on anterior two-thirds of tongue
Damage produces sagging facial muscles and disturbed sense of taste (no sweet and salty)

The Vestibulocochlear Nerve (VIII)

Sensory only
Nerve of hearing and equilibrium
Damage produces deafness, dizziness, nausea, loss of balance, and nystagmus (involuntary rhythmic oscillation of the eyes from side to side)

The Glossopharyngeal Nerve (IX)

Mixed motor and sensory
General sensation form the pharynx
Taste from posterior one-third of tongue
Damage results in loss of bitter and sour taste and impaired swallowing
Relays sensory input for Swallowing, salivation, gagging, control of BP and respiration

The Vagus Nerve (X)

Mixed motor and sensory
Most extensive distribution of any cranial nerve

Relays sensory information regarding cardiac, pulmonary, digestive, and urinary function to the brain

Motor - Swallowing, speech, regulation of viscera

Damage causes hoarseness or loss of voice, impaired swallowing, and fatal if both are cut

The Accessory Nerve (XI)

Motor only
Swallowing; head, neck, and shoulder movement
Damage causes impaired head, neck, shoulder movement; head turns toward injured side

The Hypoglossal Nerve (XII)

Motor only
Tongue movements for speech, food manipulation, and swallowing
If both are damaged: cannot protrude tongue
If one side is damaged: tongue deviates toward injured side; see ipsilateral atrophy

cerebral peduncles- 3 main components

-substantia nigra
-cerebral crus (crus cerebri)

the cerebellum

The largest part of the hindbrain and the second largest part of the brain as a whole

Consists of right and left cerebellar hemispheres connected by vermis

Has an outer cortex of gray matter with folds (folia) and four deep nuclei in each hemisphere

Cerebellar peduncles

three pairs of stalks that connect the cerebellum to the brainstem
Consist of thick bundles of nerve fibers that carry signals to and from the cerebellum

Inferior peduncles

connected to medulla oblongata
Most spinal input enters the cerebellum through inferior peduncle

Superior peduncles

-connected to the midbrain
-Carries cerebellar output

Middle peduncles

connected to the pons
Most input from the rest of the brain enters by way of middle peduncle

Forebrain consists of two parts

Encloses the third ventricle

Develops chiefly into the cerebrum


Encloses the third ventricle
Has three major subparts

Diencephalon: Thalamus

-ovoid mass on each side of the brain perched at the superior end of the brainstem beneath the cerebral hemispheres
-Constitutes about four-fifths of the diencephalon
-Two thalami are joined medially by a narrow intermediate mass
-Composed of at least 23 nuclei; to consider five major functional groups
-"Gateway to the cerebral cortex": nearly all input to the cerebrum passes by way of synapses in the thalamic nuclei, filters information on its way to cerebral cortex


—forms part of the walls and floor of the third ventricle
-Major control center of autonomic nervous system and endocrine system
-Plays essential role in homeostatic regulation of all body systems


—a stalk that attaches the pituitary gland to the hypothalamus

Functions of hypothalamic nuclei

Hormone secretion
Autonomic effects
Food and water intake
Rhythm of sleep and waking
Emotional behavior

Hormone secretion in hypothalamus

Controls anterior pituitary
Regulates growth, metabolism, reproduction, and stress responses
Controls anterior pituitary
Regulates growth, metabolism, reproduction, and stress responses

Autonomic effects in hypothalamus

Major integrating center for autonomic nervous system
Influences heart rate, blood pressure, gastrointestinal secretions, motility, etc.

Thermoregulation in hypothalamus

Hypothalamic thermostat monitors body temperature
Activates heat-loss center when temp is too high
Activates heat-promoting center when temp is too low

Food and water intake in hypothalamus

Hunger and satiety centers monitor blood glucose and amino acid levels
Produce sensations of hunger and satiety
Thirst center monitors osmolarity of the blood

Rhythm of sleep and waking in hypothalamus

Controls 24-hour (circadian) rhythm of activity

Memory in hypothalamus

Mammillary nuclei receive signals from hippocampus

Emotional behavior in hypothalamus

Anger, aggression, fear, pleasure, and contentment


—very small mass of tissue composed of:
Pineal gland: endocrine gland


—largest and most conspicuous part of the human brain
Seat of sensory perception, memory, thought, judgment, and voluntary motor actions

Two cerebral hemispheres divided by longitudinal fissure

Connected by white fibrous tract, the corpus callosum
Gyri and sulci: increase amount of cortex in the cranial cavity
Gyri increases surface area for information-processing capability
Some sulci divide each hemisphere into five lobes named for the cranial bones overlying them

what are the two cerebral hemispheres divided by?

longitudinal fissure

5 parts of the cerebrum

frontal lobe
parietal lobe
occipital lobe
temporal lobe

Frontal lobe

Voluntary motor functions
Motivation, foresight, planning, memory, mood, emotion, social judgment, and aggression

Parietal lobe

Receives and integrates general sensory information, taste processing

Occipital lobe

Primary visual center of brain

Temporal lobe

Areas for hearing, smell, learning, memory

Insula (hidden by other regions)

Understanding spoken language, taste and sensory information from visceral receptors

The Cerebral White Matter

Most of the volume of cerebrum is white matter
Glia and myelinated nerve fibers transmitting signals from one region of the cerebrum to another and between cerebrum and lower brain centers

three types of tracts in cerebral white matter

projection tracts
commissural tracts
association tracts

Projection tracts

Extends vertically between higher and lower brain and spinal cord centers
Carries information between cerebrum and rest of the body

Commissural tracts

Cross from one cerebral hemisphere through bridges called commissures
Most pass through corpus callosum
Enables the two sides of the cerebrum to communicate with each other

Association tracts

Connect different regions within the same cerebral hemisphere
-Long association fibers—connect different lobes of a hemisphere to each other
-Short association fibers—connect different gyri within a single lobe

Long association fibers

connect different lobes of a hemisphere to each other

Short association fibers

connect different gyri within a single lobe

Cerebral gray matter found in three places

Cerebral cortex
Basal nuclei
Limbic system

Neural integration

is carried out in the gray matter of the cerebrum

Cerebral cortex

-layer covering the surface of the hemispheres
-Only 2 to 3 mm thick
-Cortex constitutes about 40% of brain mass
-Contains 14 to 16 billion neurons

two principal types of neurons in cerebral cortex

stellate cells
pyramidal cells

Stellate cells

Have spheroid somas with dendrites projecting in all directions
Receive sensory input and process information on a local level

Pyramidal cells

Tall, and conical, with apex toward the brain surface
A thick dendrite with many branches with small, knobby dendritic spines
Include the output neurons of the cerebrum
Only neurons that leave the cortex and connect with other parts of the CNS.

Basal nuclei

masses of cerebral gray matter buried deep in the white matter, lateral to the thalamus
Receives input from the substantia nigra of the midbrain and the motor areas of the cortex
Send signals back to both these locations
Involved in motor control

three brain centers form basal nuclei

Caudate nucleus
Globus pallidus

Lentiform nucleus

putamen and globus pallidus collectively

Corpus striatum

putamen and caudate nucleus collectively

Parkinson's disease

stiff, "frozen" limbs & rigid muscles
hypokinesia = slowness of movement
arises from dopamine depletion

Huntington's disease (chorea)

excessive motion, & flailing limbs
hyperkinesia = excess movement
arises from degeneration of caudate nucleus

Primary sensory cortex

sites where sensory input is first received and one becomes conscious of the stimulus

Association areas

nearby to sensory areas that process and interpret that sensory information

Sensory homunculus

-diagram of the primary somesthetic cortex which resembles an upside-down sensory map of the contralateral side of the body
-Shows receptors in the lower limbs projecting to the superior and medial parts of the gyrus
-Shows receptors from the face projecting to the inferior and lateral parts


point-to-point correspondence between an area of the body and an area of the CNS

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