Brain and Cranial Nerves

third week development

dorsal midline of embryo thickens to form neural plate

fourth week development

neural tube exhibits three primary vesicles at anterior end

1. Forebrain (prosencephalon)

2. Midbrain (mesencephalon)

3. Hindbrain (rhombencephalon)

fifth week development

neural tube subdivides into 5 secondary vesicles

1. Forebrain divides into 2 vesicles

   -telencephalon

   -diencephalon

2. Mid brain remains undivided

   -mesencephalon

3. Hindbrain divides into two vesicles

   -metencephalon

   -myelencephalon

4 major portions of the brain

1. Cerebrum: 83% brain volume

2. Cerebellum: second largest brain region

   • 50% neurons, 10% brain volume

   • located in posterior cranial fossa

   • separated from cerebrum by transverse cerebral fissure

3. Diencephalon

   • Thalamus, Epithalamus and hypothalamus

4. Brainstem

   • midbrain, pons, medulla oblongata

Rostral and Caudal

Rostral: toward the forehead

Caudal: toward spinal cord

Longitudial fissure

Gyri

Sulci

Longitudial fissure: deep grove that separates the cerebral hemisphere

Gyri: thick folds on brain surface

Sulci: shallow grooves between gyri

Corpus Callosum

thick nerve bundle at bottom of longitudinal fissure that connects hemispheres

White matter

called “tracts” in the CNS, “nerves” in the PNS

• deep to cortical gray matter in brain but superficial to gray matter of the spinal cord

• made up of bundles axons, white color is fr the myelin around axons

Gray matter

surface layer (cortex) over cerebrum and cerebellum that contains cell bodies, dendrites, and synapses

• darker color due to very little myelin present in tissue

• forms nuclei deep within brain

Meninges

3 membranes that lie between nervous tissue and bone surrounding brain and spinal cord

• protect brain and provide structural framework for arteries and veins

From outermost to innermost

• dura mater

• arachnoid mater

• pia mater

2 layers of Dura mater

1. outer periosteal layer

2. inner meningeal layer

separated by dural sinuses that collect blood circulating through brain

Dura mater presses closely against ____ bones. In the spinal cord the space between the bones of the vertebrae and the dura mater is the _____ space. No epidural space between the skull and dura mater but can develop with a traumatic bleed

cranial, epidural

Folds of the dura mater

1. Falx cerebri: separate two cerebral hemis

2. Tentorium cerebelli: separate cerebrum from cerebellum

3. Falx cerebelli: separate right and left halves of cerebellum

Arachnoid mater

transparent membrane over brain surface

• subarachnoid space separates it from pia mater below

Pia mater

very thin membrane, not usually visible without microscope

• follows all contours of brain

• follows arteries as they penetrate into cerebrum

Meningitis

inflammation of the meninges due to bacterial or viral invasion of CNS through nose and throat

• pia mater and arachnoid most often affected

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

Signs of meningitis

high fever, stiff neck, drowsiness, intense headache

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

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

Ventricles

four fluid-filled chambers within brain

1. Two later ventricles: one in each cerebral hemisphere, connected via the interventricular foramen

2. Third ventricle: narrow medial space beneath corpus callosum connected via cerebral aqueduct to the

3. Fourth ventricle: a small triangular chamber between pons and cerebellum

Cerebrospinal fluid (CSF)

clear, colorless liquid that fills ventricles and canals of CNS

• surrounds brain and spinal cord and bathes its external surface

production of CSF begins with _____ of blood plasma through capillaries of brain

filtration

Choroid plexus

spongey mass of blood capillaries on the floor of each ventricle produces CSF

Ependyma

type of neuroglia that lines ventricles and covers choroid plexus

• Ependymal cells modify the filtrate

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

How does CSF flow through CNS

1. CSF secreted by choroid plexus in each later ventricle

2. CSF flows through interventricular foramina into 3rd ventricle

3. Choroid plexus in 3rd ventricle adds more CSF

4. CSF flows down cerebral aqueduct to 4th ventricle

5. Choroid plexus in 4th ventricle adds more CSF

6. CSF flows out of two later apertures and one median aperture

7. CSF fills subarachnoid space and bathes external surfaces of brain and spinal cord

8. At arachnoid villi, CSF reabsorbed into venous blood of dural venous sinuses

Functions of CSF

• buoyancy

• allow brain to attain considerable size without being impaired by own weight

• if brain rested heavily on floor of cranium, pressure would kill nervous tissue

• protection

• protect brain from striking cranium

• shaken child syndrome

• chemical stability

Brain barrier system

regulates what substances can get from bloodstream into tissue fluid of the brain to prevent pathogens from entering brain

• selectively permeable

  • to water, glucose, lipid-soluable substances (oxygen, CO2, alcohol, caffeine, nicotine, anesthetics)

Two points of entry that must be guarded in brain barrier system

1. blood capillaries throughout brain tissue

2. capillaries of choroid plexus

at blood capillaries, the brain is protected by the ______ barrier

• consists of tight junctions between endothelial cells that form the capillary walls

blood-brain

During development, ______ reach out and contact capillaries with their perivascular feet which induces the endothelial cells to form tight junctions that completely seal off gaps between them

astrocytes

At choroid plexus, brain is protected by the blood ____ barrier

• forms tight junctions between ependymal cells

• tight junctions are absent from ependymal cells elsewhere

• important to allow exchange between brain tissue and CSF

CSF

the brain barrier system (BBS) can be an obstacle for delivering _______ such as antibiotics and cancer drugs

medications

Trauma and inflammation can damage ____ and allow pathogens to enter brain tissue

BBS

Circumventricular organs (CVOs)

places in the 3rd and 4th ventricles where the barrier is absent allows blood direct access to the brain and enables the brain to monitor and respond to fluctuations in blood glucose, pH, osmolarity, and other variables

Stroke (cerebral vascular accident— CVA)

interruption of blood supply causes death of brain tissue

Two kinds of stroke

1. Hemorrhagic stroke

   • rupture of cerebral or subarachnoid blood vessel

   • ischemic stroke

2. Obstruction of blood vessel

   • blood clot (thrombosis)

   • lipid deposit (atherosclerosis)

Medulla oblongata

comes from embryonic myelencephalon

• begins at foramen magnum of skull

• extends about 3 cm rostrally and ends at groove just below pons

• slightly wider than spinal cord

Features of the medulla oblongata

1. Pyramids:

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

2. Cranial nerves

   • four pairs begin or end in medulla— VII (in part), IX, X, XII

3. Olives

   • prominent bulges lateral to each pyramid

   • gracile and cunate fasciculi of spinal cord

     • continue as two pairs of ridges on posterior medulla

     • contain sensory fibers; synapse in gracile and cuneate nuclei

All ascending and descending fibers connecting brain and spinal cord pass through _____

medulla

Medial lemnicus

axons of gracile and cuneate nuclei decussate and form ascending (sensory) tract of thalamus

Corticospinal tracts

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

Inferior olivary nucleus

relay center for signals to cerebellum

Reticular formation

network of nuclei; extends through medulla, pons, midbrain and contains cardiac, vasomotor, and respiratory centers

Pons

anterior bulge in brainstem, rostral to medulla, develops from metencephalon

Sensory and Motor roles of cranial nerves

Sensory: hearing, equilibrium, taste, facial sensations

Motor: eye movement, facial expressions, chewing, swallowing, urination, and secretion of saliva and tears

Cerebellar peduncles

thick stalks on posterior pons that connect it (and the midbrain) to the cerebellum

• contains ascending sensory tracts and descending motor tracts

  • forms pathway in and out of cerebellum

Reticular formation

in pons contains additional nuclei concerned with sleep, respiration, posture

Midbrain:

Short segment of brainstem that connects hindbrain to forebrain contains cerebral aqueduct surrounded by _____ gray substance involved in controlling pain

contains continuations of _____ lemniscus and reticular formation

Short segment of brainstem that connects hindbrain to forebrain contains cerebral aqueduct surrounded by central gray substance involved in controlling pain

contains continuations of medial lemniscus and reticular formation

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

Tectum

roof-like part of midbrain posterior to cerebral aqueduct

Four prominent bulges

Superior colliculi: visual attention, tracking objects, and some reflexes

Inferior colliculi: relay signals from inner ear to thalamus and other parts of the brain

Cerebral peduncles

• what is its 3 parts?

two anterior midbrain stalks that anchor the cerebrum to the brainstem

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

Tegmentum

dominated by red nucleus

• connections go to and from cerebellum for motor control

Substantia nigra

black 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

bundle of nerve fibers that connect cerebrum to pons that carries corticospinal tracts

The Reticular Formation

loose web of gray matter that runs vertically through all levels of the brainstem and into upper spinal cord

• occupies space between white fiber tracts and brainstem nuclei

• has connections with many areas of cerebrum

  • more than 100 small neural networks without distinct boundaries

The Reticular Formation: 7 Functions of networks

1. Somatic motor control

2. Gaze Centers

3. Central pattern generators

4. Cardiovascular control

5. Pain modulation

6. Sleep and consciousness

7. Habituation

Somatic motor control

adjust muscle tension to maintain tone, balance, and posture, especially during body movements

• relay signals from eyes and ears to cerebellum

• integrate visual, auditory, balance and motion stimuli into motor coordination

Gaze centers

allow eyes to track and fixate on objects

Central pattern generators

neural pools that produce rhythmic signals to the muscles of breathing and swallowing

Cardiovascular control

cardiac and vasomotor centers of medulla oblongata

Pain modulation

some pain signals ascend through the reticular formation

• some descending analgesic pathways begin in the reticular formation

• end in the spinal cord where they block transmission of pain signals

Sleep and consciousness

plays a central role in consciousness, alertness, and sleep

• injury here can result in irreversible coma

Habituation

activity in cerebral cortex so that it ignores repetitive, inconsequential stimuli

Cerebellum

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

• contains more than half of all brain neurons

• contains many small granule cells and large Purkinje cell that have axons that synapse on deep nuclei

General structure of cerebellum

• right and left cerebellar hemispheres connected by vermis

• superficial cortex of gray matter with folds (folia), branching white matter (arbor vitae) and deep nuclei

Cerebellar peduncles

three pairs of stalks that connect brainstem and cerebellum (their fibers carry signals to and from cerebellum)

Inferior peduncles

connect to medulla oblongata

• most spinal input enters cerebellum through inferior peuncle

Middle peduncles

connect to pons

• most input from rest of brain enters through middle peduncle

Superior peduncles

connected to midbrain and carries cerebellar output

Recent functions of Cerebellum

• comparing texture of objects

• perceiving space

• recognizing objects from different views

• judging pitch of tones

• planning, scheduling, emotion control

Important functions of Cerebellum

• motor coordination

• aids in learning motor skills

• maintains muscles tone and posture

• smooths muscle contraction

• coordinates eye and body movements

• coordinates motions of different joints with each other

• lesions can cause ataxia— clumsy, awkward gait

two parts of forebrain

1. Diencephalon: encloses third ventricle

2. Telencephalon: develops chiefly into cerebrum

three parts of Diencephalon

1. Thalamus

2. Hypothalamus

3. Epithalamus

Thalamus

ovoid mass on each side of the brain with inter thalamic adhesion joining right and left thalami medially

• constitutes about four-fifths of the diencephalon

• composed of at least 23 nuclei within five major functional groups

what is the

• “gateway to the cerebral cortex”

• plays a key role in motor control

• involved in memory and emotion

thalamus

Hypothalamus

forms part of walls and floor of third ventricle

• attaches to pituitary through a stalk-like structure called the infundibulum

• major control center for autonomic nervous system and endocrine system

• plays essential role in homeostatic regulation of all body systems

Functions of hypothalamic nuclei

1. hormone secretion: metabolism, stress, labor contractions

2. autonomic effects: heart rate, blood pressure

3. thermoregulation: hypothalamic thermostat monitors body temp

4. food and water intake

5. sleep and circadian rhythms

6. memory

7. emotion and behavioral sexual responses

8. anger, aggression, fear, pleasure

Epithalamus

very small mass of tissue

• pineal gland: endocrine gland

• habenula: relay from limbic system to midbrain

• thin roof over third ventricle

Cerebrum

largest part of human brain

• sensory perception, memory, thought, judgment, voluntary motor actions

Two cerebral hemispheres divided by _____ fissure

longitudinal

Gyri and sulci increase amount of ____ in the cranial cavity, allowing for more information-processing capability

cortex

Frontal lobe

rostral to central sulcus

• voluntary motor functions, motivation, foresight, planning, memory, mood, emotion, social judgment, and aggression

Parietal lobe

between central sulcus and parieto-occipital sulcus

• integrates general senses, taste, and some visual information

Occipital lobe

caudal to parieto-occipital sulcus

• primary visual center of brain

Temporal lobe

lateral and horizontal; below lateral sulcus

• functions in hearing, smell, learning, memory, and some aspects of vision and emotion

Insula (hidden by other regions)

deep to lateral sulcus

• helps in understanding spoken language, taste and integrating information from visceral receptors

most of the volume of the cerebrum is ___ matter

white

three types of tracts in white matter

1. projection tracts

2. commissural tracts

3. association tracts

Projection tracts

extend vertically between higher and lower brain and spinal cord centers

• corticospinal tracts

Commissural tracts

cross from one cerebral hemisphere to the other allowing communication between two sides of cerebrum

• corpus callosum

• anterior and posterior commissures

Association tracts

connect different regions within same cerebral hemisphere

• long fibers connect different lobes

• short fibers connect gyri within a lobe

Neural integration is carried out in ___ matter of the cerebrum

gray

cerebral gray matter found in three places

1. cerebral cortex

2. basal nuclei

3. limbic system

cerebral cortex

covers surface of the hemispheres

• only 2-3 mm thick

• 40% brain mass

• 14-16 billion neurons

• 90% is neocortex- 6 layered tissue

cerebral cortex contains two principle type of neurons

1. stellate cells

2. pyramidal cells

stellate cells

spheroid somas with dendrite projecting in all directions

• receive sensory input, process information locally

pyramidal cells

tall and conical, with apex toward the brain surface

• thick dendrite with many branches with small, knobby dendritic spines

Limbic system

• components?

important center of emotion and learning

1. Cingulate: arches over corpus callosum in frontal and parietal lobes

2. Hippocampus: in medial temporal lobe (memory function)

3. Amygdala: immediately rostral to hippocampus (emotion functions)

LImbic system structures have centers for gratification and aversion

• what are they?

gratification: sensations of pleasure or reward

aversion: sensations of fear or sorrow

Basal nuclei

masses of cerebral gray matter buried deep in white matter, lateral to the thalamus

• receive input from substantia nigra of the midbrain and the motor areas of the cortex and sends signals back to both of these locations

• motor functions:

  • onset and cessation of intentional movements

  • repetitive hip and shoulder movements in walking

  • highly practiced, learned behaviors such as writing, typing, driving

at least three brain centers form the basal nuclei and are collectively called the corpus ______

• 3 parts of it

stratium

1. caudate nucleus

2. putamen

3. globus pallidus

Lentiform nucleus

putamen and globus pallidus together