Chapter 14: The Brain and Cranial Nerves

6 Regions of the Brain

• Cerebrum• Cerebellum• Diencephalon• Mesencephalon• Pons• Medulla oblongata

Cerebrum

• Largest part of brain• Controls higher mental functions• Divided into left and right cerebral hemispheres • Surface layer of gray matter (neural cortex)– Also called cerebral cortex

• Folded surface increases surface area– Elevated ridges (gyri)– Shallow depressions (sulci)– Deep grooves (fissures)

Cerebellum

• Second largest part of brain• Coordinates repetitive body movements• 2 hemispheres• Covered with cerebellar cortex

Diencephalon

• Located under cerebrum and cerebellum• Links cerebrum with brain stem • 3 divisions:– Thalamus - relays and processes sensory

information– Hypothalamus - hormone production, emotion, &

autonomic function– Epithalamus (Pineal gland)

Pituitary Gland

• Major endocrine gland • Connected to hypothalamus• Via infundibulum (stalk)• Interfaces nervous and endocrine systems

The Brain Stem• Processes information between: – spinal cord and cerebrum or cerebellum

• Includes:– Mesencephalon (midbrain)• Processes sight, sound, and associated reflexes, &

maintains consciousness

– Pons• Connects cerebellum to brain stem & is involved in somatic

and visceral motor control

– Medulla oblongata• Regulates autonomic functions: HR, BP, and digestion

Primary Brain Vesicles

• The anterior end of the neural tube expands and constricts to form the three primary brain vesicles– Prosencephalon – the

forebrain– Mesencephalon – the

midbrain– Rhombencephalon –

hindbrain

Secondary Brain Vesicles

• In week 6 of embryonic development, secondary brain vesicles form– Telencephalon and

diencephalon arise from the forebrain

– Mesencephalon remains undivided

– Metencephalon and myelencephalon arise from the hindbrain

Adult Brain Structures

• Fates of the secondary brain vesicles:– Telencephalon – cerebrum:

cortex, white matter, and basal nuclei

– Diencephalon – thalamus, hypothalamus, and epithalamus

– Mesencephalon – brain stem: midbrain

– Metencephalon – brain stem: pons

– Myelencephalon – brain stem: medulla oblongata

Ventricles • Lateral ventricles

– Deep w/in cerebrum• Third ventricle

– Connected by interventricular foramen (Monro)

• Fourth ventricle– Connected by

cerebral aqueduct (Sylvius)

– Connects to subarachnoid space to return to bloodstream

Brain Protection and Support

• Physical protection:– bones of the cranium– cranial meninges– cerebrospinal fluid

• Biochemical isolation:– blood–brain barrier

The Cranial Meninges

• Has 3 layers:– dura mater– arachnoid mater– pia mater

• Is continuous with spinal meninges

• Protects the brain from cranial trauma

Characteristics of Cranial Meninges

• Dura mater:– inner fibrous layer (meningeal layer)– outer fibrous layer (endosteal layer) fused to

periosteum– venous sinuses between 2 layers

• Arachnoid mater:– contacts epithelial layer of dura mater• Subarachnoid space b/t arachnoid and pia mater

• Pia mater:– attached to brain surface by astrocytes

Dural Folds• Folded inner layer of

dura mater• Extend into cranial cavity

to stabilize and support brain

• Contain collecting veins (dural sinuses)

• Falx cerebri, tentorium cerebelli, and falx cerebelli

Cerebrospinal Fluid (CSF)

• Surrounds all exposed surfaces of CNS

• Interchanges with interstitial fluid of brain

• Functions of CSF1. Cushions delicate

neural structures2. Supports brain3. Transports nutrients,

chemical messengers, and waste products

Forming CSF

• Choroid plexus:– Specialized ependymal cells and capillaries:• secrete CSF into ventricles• remove waste products from CSF• adjust composition of CSF

• Produces about 500 ml of CSF/day– Room for ~ 150 ml

Circulating CSF• CSF produced from choroid plexus (in all ventricles)– Lateral Ventricle (thru interventricular foramen of

Monro)– Third Ventricle (thru cerebral aquaduct of Sylvius)– Fourth Ventricle (thru median & lateral apertures)– To central canal of spinal cord & subarachnoid space

around the brain, spinal cord, and cauda equina– Thru arachnoid villi/granulations (into dural venous

sinuses)– DVS’s merge into Internal Jugular Vein

Blood Supply to the Brain

• Supplies nutrients and oxygen to brain• Delivered by internal carotid arteries and

vertebral arteries• Removed from dural sinuses by internal

jugular veins

Cerebrovascular Disease

• Disorders interfere with blood circulation to brain

• Stroke or cerebrovascular accident (CVA):– shuts off blood to portion of brain– neurons die

• Transient ischemic attack (TIA): – temporary reduction in blood flow

Blood–Brain Barrier (BBB)

• Isolates CNS neural tissue from general circulation• Formed by network of tight junctions between

endothelial cells of CNS capillaries• Lipid–soluble compounds (O2, CO2), steroids, and

prostaglandins diffuse into interstitial fluid of brain and spinal cord

• Astrocytes control blood–brain barrier by releasing chemicals that control permeability of endothelium

4 Breaks in the BBB

1. Portions of hypothalamus:– secrete hypothalamic hormones

2. Posterior lobe of pituitary gland:– secrete hormones ADH and oxytocin

3. Pineal glands:– pineal secretions

4. Choroid plexus:– where special ependymal cells maintain blood–

CSF barrier

The Medulla Oblongata

• Allows brain and spinal cord to communicate

• Coordinates complex autonomic reflexes

• Controls visceral functions

Medulla Oblongata– Vital reflex centers (life sustaining

decisions)• Cardiac center – rate/force of contractions• Vasomotor center – smooth mm in b.v.’s (BP

control)• Respiratory center – rate/depth of breathing

(up into pons)– Non-vital reflex centers

• Vomiting, hiccough, swallowing, coughing, sneezing

– CN VIII through XII nuclei– Vestibular nuclei complex –

subconsciously maintain balance (equilibrium)

– Solitary nucleus - receives visceral sensory information

– Olivary nuclei (olives)…lateral to pyramids• Receive sensory info from proprioceptors

(position) in skeletal muscles & joints & act as a relay point to the cerebellum

The Pons• Links cerebellum with

mesencephalon, diencephalon, cerebrum, and spinal cord

• Nuclei of cranial nerves V, VI, VII, VIII

• Nuclei involved with respiration:– apneustic center and pneumotaxic

center – modify respiratory rhythmicity

center activity• Cerebellar peduncles – connect

cerebellum to brainstem (6 in all)• Superior cerebellar peduncles

– to midbrain• Middle cerebellar peduncles

– to pons (largest)• Inferior cerebellar peduncles

– to medulla

Functions of the Cerebellum

1. Adjusts postural muscles 2. Fine-tunes conscious and subconscious

movements• Ataxia:– damage from trauma or stroke – intoxication (temporary disturbance) – disturbs muscle coordination

Structures of the Cerebellum• Folia:

– surface of cerebellum– highly folded neural cortex

• Anterior and posterior lobes:– separated by primary fissure

• Cerebellar hemispheres:– separated at midline by vermis

• Flocculonodular lobe:– below fourth ventricle

• Arbor vitae: Highly branched, internal white matter of cerebellum

Structures of the Mesencephalon

• Tectum: (corpora quadrigemina):– superior colliculus (visual)– inferior colliculus (auditory)

• Tegmentum:– red nucleus (many blood vessels) – substantia nigra (pigmented gray

matter)

• Cerebral peduncles:– Contain descending fibers to

cerebellum• motor command fibers

Figure 14–5a

The Diencephalon

• Integrates sensory information and motor commands

• Thalamus• Epithalamus– Secretes hormone

melatonin

• Hypothalamus

The Thalamus

• Filters ascending sensory information for primary sensory cortex

• Relays center to funnel information between basal nuclei and cerebral cortex

5 Groups of Thalamic Nuclei1. Anterior group: part of limbic system (emotions)2. Medial group: provides awareness of emotional

states3. Ventral group: relays sensory information4. Posterior group: 1. pulvinar nucleus (sensory)2. lateral geniculate nucleus (visual)3. medial geniculate nucleus (auditory)5. Lateral group: affects emotional states– integrates sensory info

Structures of the Hypothalamus• Mamillary bodies:

– process olfactory and other sensory information– control reflex eating movements

• Infundibulum:– a narrow stalk– connects hypothalamus to pituitary gland

8 Functions of the Hypothalamus

1. Provides subconscious control of skeletal muscle 2. Controls autonomic function3. Coordinates activities of nervous and endocrine

systems4. Secretes hormones:– antidiuretic hormone (ADH) & oxytocin (OT)

5. Produces emotions and behavioral drives:– the feeding center (hunger) & the thirst center

6. Coordinates voluntary and autonomic functions7. Regulates body temperature:8. Controls circadian rhythms (day–night cycles)

The Limbic System• Is a functional grouping

that:– establishes emotional

states– links conscious functions of

cerebral cortex with autonomic functions of brain stem

– facilitates memory storage and retrieval

Components of the Limbic System• Amygdala: deals with anger, danger, and fear responses– interfaces limbic system, cerebrum, and sensory systems

• Limbic lobe of cerebral hemisphere:– cingulate gyrus - plays a role in expressing emotions via

gestures, and resolves mental conflict– hippocampus - converts short-term memory to long-term

memory…memories charged w/ emotion are often retained• Fornix: tract of white matter– connects hippocampus with hypothalamus

• Anterior nucleus of the thalamus:– relays information from mamillary body to cingulate gyrus

• Reticular formation:– stimulation or inhibition affects emotions (rage, fear, pain,

sexual arousal, pleasure)

The Cerebrum• Is the largest part of the

brain• Controls all conscious

thoughts and intellectual functions

• Processes somatic sensory and motor information

• Gray matter:– in cerebral cortex and basal

nuclei• White matter:

– deep to cortex– around basal nuclei

Structures of the Cerebrum

• Gyri of neural cortex: increase surface area (number of cortical neurons)

• Longitudinal fissure: separates cerebral hemispheres

• Lobes: divisions of hemispheres• Central sulcus divides: anterior frontal lobe from

posterior parietal lobe• Lateral sulcus divides: frontal lobe from temporal

lobe• Parieto-occipital sulcus divides: parietal lobe from

occipital lobe

3 Functional Principles of the Cerebrum

1. Each cerebral hemisphere receives sensory information from, and sends motor commands to, the opposite side of body

2. The 2 hemispheres have different functions although their structures are alike

3. Correspondence between a specific function and a specific region of cerebral cortex is not precise

3 Types of Axons in the Cerebrum

1. Association fibers– Connections within 1

hemisphere

2. Commissural fibers– Bands of fibers connecting 2

hemispheres

3. Projection fibers– Link cerebral cortex with

diencephalon, brain stem, cerebellum, and spinal cord

Basal Nuclei

• Are masses of gray matter embedded in white matter of cerebrum

• Direct subconscious activities

• Are involved with:– the subconscious control

of skeletal muscle tone– the coordination of

learned movement patterns (walking, lifting)

• Caudate nucleus:• Lentiform nucleus:

– globus pallidus – putamen

Motor and Sensory Areas of the Cortex• Central sulcus separates motor and sensory areas• Precentral gyrus (Primary motor cortex) of frontal lobe:

– directs voluntary movements

• Postcentral gyrus (Primary sensory cortex) of parietal lobe:– receives somatic sensory information (touch, pressure, pain,

vibration, taste, and temperature)

Special Sensory Cortexes

• Visual cortex: occipital lobe– information from sight receptors

• Auditory cortex: temporal lobe– information from sound receptors

• Olfactory cortex: frontal & medial temporal lobe– information from odor receptors

• Gustatory cortex: insula (deep to temporal lobe)– information from taste receptors

Association Areas

• Premotor cortex: – coordinates somatic motor responses (learned

movements)

• Somatic sensory association area:– interprets input to primary sensory cortex (e.g.,

recognizes and responds to touch)

• Visual association area:– interprets activity in visual cortex

• Auditory association area:– monitors auditory cortex

Integrative Areas• Wernicke’s area (General Interpretive Area )– Present in only 1 hemisphere– Receives information from all sensory association areas– Coordinates access to complex visual and auditory

memories• Speech center: coordinates all vocalization functions – is associated with general interpretive area• Prefrontal cortex of frontal lobe:– integrates information from sensory association areas– performs abstract intellectual activities (e.g., predicting

consequences of actions)

Figure 14–15c

Brodmann’s Areas

• Patterns of cellular organization in cerebral cortex

Hemispheric Lateralization• Functional differences between left

and right hemispheres• In most people (90%), left brain

(dominant hemisphere) controls:– reading, writing, and math– decision-making– speech and language

• Right cerebral hemisphere relates to:– senses (touch, smell, sight, taste,

feel)– recognition (faces, voice

inflections)• Unclear dominance may lead to

dyslexia

Monitoring Brain Activity• Brain activity is assessed by an electroencephalogram

(EEG): – electrodes are placed on the skull – patterns of electrical activity are printed out (brain waves)

4 Categories of Brain Waves

1. Alpha waves:– found in healthy, awake adults at rest with eyes closed

2. Beta waves:– higher frequency– found in adults concentrating or mentally stressed

3. Theta waves:– found in children– found in intensely frustrated adults– may indicate brain disorder in adults

4. Delta waves:– during sleep– found in awake adults with brain damage

Cranial Nerves

• 12 pairs connected to brain

• Name • Number • Function

Olfactory – CN I

• Sensory• Terminates in

olfactory bulbs• Discrimination of

smell

Optic – CN II

• Sensory• Vision• Terminates at

optic chiasm

Occulomotor – CN III

• Motor• Inferior oblique,

sup/inf/medial rectus• Test planes of gaze• PSNS Function:• Test constrictor mm

of iris (ciliary mm)– Pupillary reflex

Trochlear – CN IV

• Motor• Superior oblique

(down & out)• Test planes of gaze

Trigeminal – CN V

• Mixed • 3 divisions: ophthalmic,

maxillary, mandibular

– Test:– Motor- mandibular

division to mm of mastication

– Corneal reflex to elicit blinking

Abducens – CN VI

• Motor • Lateral rectus• Test planes of gaze

Facial – CN VII

• Mixed• Taste on anterior 2/3 of

tongue• Motor to mm of facial

expression (not chewing)– 5 motor branches• Temporal, zygomatic,

buccal, mandibular, cervical

• PSNS to lacrimal gland• Test making faces

Vestibulocochlear – CN VIII

• Sensory• Equilibrium &

hearing• Test hearing

Glossopharyngeal – CN IX

• Mixed• Motor fibers to pharynx• PSNS to parotid salivary

gland• Sensory conduct taste from

posterior 1/3 of tongue; carotid sinus pressure & chemoreceptors (Oxygen), general sensation of pharynx

• Test swallowing (gag reflex w/ CN X)

Vagus – CN X

• Mixed• Only nerve to descend

beyond neck (into thorax & abdomen)

• PSNS serving heart, lung, abd viscera to regulate heart rate, breathing, & digestive activity

• Test gag reflex (w/ CN IX)

Spinal accessory – CN XI

• Motor• Trapezius &

SCM• Test shrugging

shoulders

Hypoglossal CN XII

• Motor• Extrinsic & intrinsic

muscles of the tongue

• Test by sticking out tongue