Biol 2401

Biol 2401 A&P Lecture Notes Brain & CN Dr. Weis

BRAIN AND CRANIAL NERVES

Brain development from the cranial end of the neural tube and will form three primary vesicles :

1. Forebrain vesicle ----> Prosencephalon

2. Midbrain vesicle ----> Mesencephalon

3. Hindbrain vesicle ----> Rhombencephalon

Further development and division into secondary vesicles by the embryo's 5th week of development :

Prosencephalon................Telencephalon (endbrain)
Diencephalon (second brain)
Mesencephalon.................Mesencephalon (midbrain)
Metencephalon (afterbrain)
Rhombencephalon...............Myelencephalon (spinalbrain)

then will finally develop into ADULT structures --->

Telencephalon ---->     Cerebrum

Diencephalon ---->     Epithalamus, Thalamus, Hypothalamus

Mesencephalon ---->     Midbrain (brainstem)

Metencephalon ---->     Pons (brainstem)

Cerebellum

Myelencephalon ---->    Medulla oblongata (brainstem)

Cerebrum : cerebral cortex

highest level of the CNS

for sensory perception, consciousness, memory

personality

Fxn... to store and process information

Two hemispheres separated dorsally by the DORSAL LONGITUDINAL

FISSURE. Each hemisphere has GYRI (outward folds) and SULCI (inward folds or depressions).
This pattern combination is called convoluted or GYRENCEPHALON and is species specific in arrangement and nomenclature.
In lower animals, the cerebral cortex is smooth and referred to as Lissencephalic.

Another fissure, the TRANSVERSE FISSURE, will separate the cerebrum from the cerebellum.

Gray and white matter of the brain is arranged as follows :

1. Inner gray around a central cavity

2. Middle white

3. Outer gray called the neural cortex :

Cerebral cortex in the cerebrum

Cerebellar cortex in the cerebellum

The corpus callosum is a thick fibrous band of neuron tissue that holds the two hemispheres together
and allows for symmetrical communication between right and left halves. (Commissural fibers).

The lumen within the cerebrum is paired and is called the LATERAL VENTRICLES and will make, store, and circulate cerebral spinal fluid (CSF).

If the CSF pressure increases, the ventricles dilate at the expense of the cortex and will create a condition known as HYDROCEPHALUS.

The cerebral cortex contains three functional areas :

1. Motor : control of voluntary motor

2. Sensory : conscious awareness of sensation

3. Association : integrate information

Overall, the cerebral cortex

instigates all conscious or voluntary action
integrates sensory inputs with motor outputs
controls behavior

Cerebral hemispheres can be further divided into lobes named by the cranial bones that overlie them. The lobes are as follows :

Frontal, Parietal, Temporal, Occipital and the INSULA which lies deep to the temporal, parietal, and frontal lobes.

Each hemisphere is primarily concerned with the sensory and motor functions of the opposite (contralateral) side of the body.

MOTOR AREAS

I. Frontal Lobe --> Primary motor cortex

@ the precentral gyrus (behind the central sulcus)

conscious voluntary motor control from the pyramidal cells

Representation of the motor control of the body's musculature by the cortex can be diagrammed
and is called the MOTOR HOMUNCULUS

Frontal lobe lesions will appear on the contralateral side

II. Premotor Cortex

anterior to precentral gyrus

controls learned motor skills of a repetitive or patterned nature

III. Broca's area

premotor area, anterior and inferior region

motor speech area in left hemisphere

IV. Frontal Eye field

voluntary eye movements

premotor area, anterior and superior region

SENSORY AREAS

I. Parietal Lobe : Primary Sensory Cortex

postcentral gyrus
receptors in skin (tactile) for pressure, pain, temperature receptors in skeletal muscles for proprioception (position sense)
Represented by the SENSORY HOMUNCULUS (see drawing)
Lesions seen on the contra lateral side the left parietal lobe is important for language comprehension and object recognition

II. Occipital Lobe : PRIMARY VISUAL CORTEX

medial aspect of occipital lobe

visual reception and interpretation

Lesions ---> Blindness

depending on the extent of the damage, it can range from unilateral to bilateral

III. Temporal Lobe : PRIMARY AUDITORY CORTEX

superior margin of temporal lobe

memory and auditory functions

Lesions ---> can be seen as

convulsive seizures, expressions of fear/rage, immobility

masticatory movements, salivation, dilated pupils, and abnormal sex changes

IV. Rhinencephalon : Primary Olfaction (smell brain)

ventral portion of the cerebrum, near the ventral portion of the temporal lobe
responsible for sense of smell and related behavioral responses
(smell --> salivate --> hunger --> find source)

ASSOCIATION AREAS :

analyze, recognize sensory

work with motor cortex to act on sensory inputs

All lobes have these areas with which to communicate

* somatosensory association area : within parietal lobe
* visual association area : within occipital lobe
* auditory association area : within temporal lobe

Other association areas :

1. Prefrontal cortex (psychomotor)

anterior portion of frontal lobe

involved with intellect, cognition, personality,

abstract ideas, judgement, reasoning, planning

Lesions --> can range from :

apathy, stupidity, inactivity, agressiveness,

ill-tempored, and hyperactivity

2. General Interpretive area (Gnostic area)

left hemisphere

region input from ALL sensory association areas of

the temporal, parietal, & occipital lobes

storage site for complex memory patterns associated with sensation

3. Language areas

    a. Wernicke's : left hemisphere

    posterior portion of temporal lobe

for sounding out unfamilar words (medical terms !)

    b. Affective language : nonverbal, emotional

components of language (interpreting tone of voice, how words were said)

4. Visceral Association : INSULA

for perception of visceral sensations

(full stomach, distended intestines etc)

Cortical Function

1. Lateralization

cerebral hemisphere dominance

left hemisphere : language, math, logic
right hemisphere : creative, visual, emotional, visual-spatial skills

2. White Matter

myelinated fiber tracts (CNS axons)

classified according to direction

a. Commissural

between two hemispheres, connect rt/lf

e.g. Corpus callosum

Anterior Commissure

b. Association

fibers in one hemisphere

connect lobes within that hemisphere

c. Projection

connect lower brain or spinal cord with the

Cerebral hemispheres.

3. Subcortical Nuclei

Basal nuclei (gray) within white matter
Lentiform : putman and globus pallidus
Caudate nucleus

Receive inputs from cerebral cortex :

* relay with other nuclei

* influence motor control

* monitor movements

* allow ability to perform several activities at once.

II. DIENCEPHALON (second brain)

consists of three structures : Thalamus, Hypothalamus, Epithalmus

contains optic neurons for CN II, the optic nerve

the dorsal part forms the epithalamus
the superiolateral part forms the thalamus
the inferolateral part forms the hypothalamus
its lumen will form the third ventricle

A. Thalamus........relay center

1. major sensory integration

ALL sensory information (except olfaction) which reaches a conscious level MUST PASS through the Thalamus
and synapse on one of its many nuclei : anterior, medial, ventral, and posterior thalamic nuclei.

The posterior nuclei will be further divided into the pulvinar and the geniculates (medial/lateral)

2. Conduct impulses to and from the cerebral motor cortex

3. Involved in memory processing

4. Part of the Limbic system (emotion brain)

B. Hypothalamus

inferiolateral walls of the third ventricle

main visceral control center

maintains overall body homeostasis

part of the limbic system

functions ::

appetite/hunger centers
water balance (thirst) : osmotic receptors
sexual behavior
sleep-wake cycle
body temperature
body glucose levels
emotions (limbic system)
hormonal control for the pituitary
autonomic nervous function

C. Epithalamus

roof of the third ventricle which contains part of the choroid plexus

pineal body : secretion of melatonin

regulates sleep/wake cycles
regulates reproductive cycles

BRAINSTEM : Mesencephalon, Metencephalon, Myelencephalon

A. Mesencephalon (midbrain)

ascending and descending tracts
associated Cranial nerves III, IV
relay for optic reflexes and auditory pathway
body locomotion and equilibrium in regards to posture and righting
lumen : mesencephalic aquaduct
nuclei : corpora quadrigema, colliculi (superior, inferior), substantia nigra, red nucleus, reticular formation nuclei

B. Metencephalon (after brain)

ascending and descending tracts, lumen : 4th ventricle
associated with Cranial nerves V, VI, VII
dorsally develops into the Cerebellum (not considered to be part of the brain stem)
ventrally develops into the PONS ---> motor control and muscle tone
contain nerve fiber tracts that link the cerebellum
with all portions of the brainstem and the cerebellum with the spinal cord
involuntary control of respiration with regards to rhythm : pneumotaxic center and apneustic center

C. Myelencephalon............spinal brain --> MEDULLA OBLONGATA

connects the brain to the spinal cord, therefore contains ascending and descending tracts and becomes a relay center for information
associated with Cranial nerves ; VIII, IX, X, XI, XII
central canal lumen : 4the ventricle
with associated capillary blood vessels and ependymal cells to form a plexus --> Choroid Plexus that produces and regulates CSF.
controls respiration and cardiovascular function
reflex center for vomiting, swallowing, gagging, coughing
nuclei : olivary, cochlear, vestibular, nucleus gracilis, nucleus cunneatus

CEREBELLUM

develops from the dorsal portion of the metencephalon

grossly, the cerebellum is divided into lobes :

anterior, posterior, and flocculonodular

also is divided into right and left hemispheres by a midline band called the VERMIS (worm)

The gray matter folds are called folia

The white matter appears like a branching tree and is known as the ARBOR VITAE (tree of life)

There are paired fiber tracts called the cerebellar peduncles that connect the cerebellum with the brainstem

1. Superior cerebellar peduncles : connect cerebellum with midbrain

2. Middle cerebellar peduncles : connect cerebellum with the pons

3. Inferior cerebellar peduncles : connect cerebellum with medulla

Fxn :

determine where parts of body are : location, movement
coordination of force, direction, extent of muscle
contraction to insure smooth coordinated movements
Rostral portion --> concerned with muscle tone & body posture
Ventrolateral portion --> equilibrium and balance
Caudal portion --> fine control of muscle coordination

Cerebellar lesions/disease ::

cause an inability to regulate rate, range, force of movement

can be exhibited as delayed onset, exaggerated movement, excessive force, or intention tremors

ventrolateral cerebellar lesions can result in loss of equilibrium, broad based stance, abnormal posture, and nystagmus

Brain Systems : Limbic System, Reticular Formation

I. Limbic System

complex group of fiber tracts and gray matter that will form the emotional-visceral brain

Main structures :

Hippocampus and other parts of the rhinencephalon
Amygdala
Hypothalamus
Anterior thalamic nuclei
Gyrus (cingulate)

Integrates and responds to a wide variety of environmental stimuli

II. Reticular Formation

Brainstem reticular neurons with wide spread connections

Fxn : maintain alert, conscious state : Reticular Activating

System (RAS)

filter out repetitive, familiar, or weak signals

motor for coarse movement : reticulospinal tract

ANS centers for cardiac, respiratory, vasomotor, and visceral

In Summary :

The neuron systems pathway from the lowest to the highest consists of the spinal cord, cranial nerves, brainstem,
cerebellum, diencephalon, and cerebrum

Neurologic lesions can be manifested in one of three ways :

1. Irrative lesions

cause hyperactivity, hypersensitivity due to irritation of neurons, axons, or discharging regions (foci) of cells

2. Deficits

hypofunction, anesthesia, abscence of function due to destruction or neuron death

3. Release Phenomena

a return to poorly controlled activity in lower centers due to complete or partial destruction of higher centers

BRAIN PROTECTION AND SUPPORT

1. Skull, boney cranium

2. Meninges : Dura Mater, Arachnoid, Pia mater

Dura Mater

consists of two layers

outer is fused with the cranium (periosteum)

Between the two layers are large veins that form the Dural SINUSES.
These vessels will collect blood from the brain and direct it back to the heart by way of the internal jugular veins.

Inner layer (meningeal layer) separates and protects the hemispheres of the cerebrum and cerebellum
by creating dural folds called the falx cerebri and falx cerebelli

Another dural fold, tentorum cerebelli will separate the cerebrum from the cerebellum along the transverse fissure.

Arachnoid

smooth with some projections called the arachnoid villi that allow for CSF absorption into the venous blood
of the sinuses from the subarachnoid space

Pia Mater

closely adhered to the cortex

contains the superficial circulatory system

3. Circulation :

Arterial blood supply from : internal carotid arteries and

vertebral arteries

Blood brain barrier (BBB) :

is a selective barrier that isolates neural tissue from the general circulation

formed by :

1. continuous endothelial lining of the capillary walls. Tight junctions are formed

2. Thick basal lamina (part of the basement membrane

3. Astrocyte end processes (feet)

Selective barrier for : glucose, some AA and electrolytes

Nonpermeable for : wastes (urea, creatinine), proteins, K+

The BBB is ineffective for fat soluble molecules : Fats, Fatty acids, Oxygen (O2), and Carbon Dioxide

In some areas the BBB is absent : vomiting center in the brainstem, hypothalamus

4. CSF

formed by the choroid plexus in the ventricles

capillaries + ependymal cells (with ion pumps) create a fluid similar to blood plasma but with less
protein, more Na+, Cl-, Mg++, and H+, fewer K+, and Ca++

Will circulate through ventricles and go to :
central canal of the spinal cord
subarachnoid space through openings (aperatures) in the wall of the fourth ventricle

Circulation is aided by the long microvilli of the ependymal cells.

FXN :

cushions and supports
exchange of nutrients and wastes
pH (hydrogen ion concentration) in CSF is important in regulating cerebral blood flow and respiration
evaluation of this fluid can indicated disease process or injury.

PROBLEMS :

1. Trauma : head injuries --> concussion, contusion

ruptured vessels --> hemorrhage --> pressure

Edema

2. Degenerative :

a. CVA (cerebral vascular accident) a.k.a Stroke = decreased blood supply (ischemia) due to blockage

b. TIA (transient ischemic attack) is a transient, temporary and reversible ischemia

c. Alzheimer's : genetically based progressive brain degeneration in the cortex and hippocampus
Caused by deposits on cell surface and the breakdown of microtubules essential for intracellular transport in neurons

CRANIAL NERVES :

Extension of the tube in the upper 1/3 portion.

You are responsible for knowing all 12 pair :

name
number
type of nerve
foramen
function
a test that can be performed to detect proper (or improper) function.

Your lab book exercises can help you in your studies.

You WILL USE cranial nerves in your patient assessment !