Notes on the Web
Nervous System:  Brain (CNS) and Cranial Nerves (PNS)

Bruce G. Stewart

Related Textbook Readings

• Marieb (2004) Chapter ___

Lecture Outlines for the Brain and the Cranial Nerves

I. Brain

A. Principle Parts and Miscellaneous Information

1. General Characteristics

a. 1000 billion neurons!

b. 1300 g (3lb)

c. four main parts

• brain stem – lower part of brain including the medulla oblongata, pons, and mesencephalon (midbrain)
• diencephalon- above the brain stem and consists of thalamus and hypothalamus
• cerebrum – spreads over diencephalon and makes up above 7/8 of total brain weight
• cerebellum – inferior to cerebrum and posterior to brain stem

2. Growth – rapidly during first few years of life but due to increase in neuron cell size, proliferation of neuroglia, development of fibers, myelination of fiber tracts

3. Embryonic origins and regions: study this in your text and know the general origin of the nervous system (neural tube formation, etc) and the secondary brain regions:

a. telencephalon - develops into the cerebrum and includes the lateral ventricles

b. diencephalon - develops into the epithalamus (including the pineal gland), the thalamus, the hypothalamus, and includes the third ventricle

c. mesencephalon - develops into the midbrain

d. metencephalon - develops into the pons (ventrally) and the cerebellum (dorsally) and includes part of the fourth ventricle

e. myelincephalon - develops into the medulla oblongata and includes the fourth ventricle

B. Protection and Coverings

1. cranial meninges – same as around spinal cord except that dura mater has two layers ( an outer endosteal layer and an inner meningeal layer which corresponds to the dura mater of spinal cord

a. composition & function

• 80-150 ml of watery liquid with proteins, glucose, urea, salts & some lymphoctes
• protection from shocks
• circulation of nutrients and removal of wastes

b. ventricles

• lateral ventricles – in cerebrum & connected to third ventricle by interventricular foramen
• third ventricle – vertical slit between and inferior to the right and left halves of the thalamus and between the lateral ventricles – connect to subarachnoid space of brain and spinal cord
• fourth ventricle – between inferior brain stem and cerebellum and has apetures that connect to subarachnoid space of brain and spinal cord

c. choroid plexuses- beds of capillaries and ependymal cells in ventricles that produce CSF

• blood-cerebrospinal fluid barrier: selective in materials filtered into CSF & thus helps protect the brain from harmful substances

C. Blood Supply

1. Highly vascularized by cerebral arterial circle (circle of Willis)

a. arteries penetrate inward but are surrounded by pia mater – space between vessel and pia mater is the perivascular space

b. brain is most metabolically demanding organ in the body

• 2% of body weight but 20%of O₂ use
• high glucose demand which can cause everything from confusion to convulsions if too low

2. Regulation of flow

a. levels of CO₂ and O₂ affect vasodilation

• chemical mechanism linking carbon dioxide concentration in blood to the hydrogen ion concentration: CO₂ + H₂O ---> H₂CO₃ --->HCO₃ ---> + H⁺
• hydrogen ion concentration is directly proportional to dilation; the higher the hydrogen ion concentration, the greater the diameter of blood vessels and thus the greater the flood of blood with oxygen to the cells

3. Blood-brain barrier (BBB) - a chemical protector of sensitive brain cells

a. allows rapid passage of glucose, oxygen, & some ions

b. allows slow passage of insulin, sucrose, chloride, urea, creatinine

c. blocks proteins and most antibiotics

d. lipid soluble substances can get through (e.g. alcohol, nicotine, and heroin )

e. capillaries of brain different than in other parts of the body

• brain capillaries are non-fenestrated which means they do not have spaces between the endothelia cells that form the capillary walls; this blocks or greatly slows passage of many proteins, antibiotics, and other large, charged, and/or non-lipid soluable compounds.

D. Review of structure and function of areas of brain

1. Brain stem

a. medulla oblongata (=myelencephalon)

• above foramen magnum of cranium & is lowest part of brain (about 3 cm long)
• contains all ascending and descending tracts
  • pyramids (mainly white matter)-triangular structures on ventral side where largest motor tracts cross (decussation of pyramids)
    • tracts come mainly from lateral cerebral cortex
    • causes left brain control of right side of body
• contains sensory nuclei which receive sensory fibers from ascending tracts
  • conveys messages to thalamus then to cerebral cortex – also descussates
• contains reticular formation which deals with consciousness and waking from sleep
  • knockout blows often to jaw affect this region
• contains three vital reflex centers ( and some non-vital ones e.g. for vomiting, sneezing, etc.)
  • cardiac center- heartbeat and force of contraction
  • medullary rhythmicity area- basic breathing rate
  • vasomotor center – regulates diameter of blood vessels
• contains nuclei for all or parts of cranial nerves
  • vestibulocochlear (VIII) (part)- hearing and equilibrium (sensory) functions
  • glossopharyngeal (IX) – functions related to swallowing (motor), salivation (motor), taste (sensory), and detection of blood pressure and oxygen tension in the carotid arteries (sensory)
  • vagus (X) – innervates a wide range of thoracic and abdominal visceral organs and tissues (motor and sensory) and has important parasympathetic and other functions
  • accessory (XI) nerve (cranial part) – functions in voluntary movement of head and shoulder (motor) by innervating certain skeletal muscles
  • hypoglossal (XII) nerve – functions in tongue movements (motor) by innervating nearly all intrinsic and extrinsic muscles affecting the tongue
• other stuff

b. pons (part of metencephalon along with cerebellum)

• lies above medulla and anterior to cerebellum & is about 2.5 cm long
• has nuclei and white matter tracts
• connects spinal tract and medulla to other parts of the brain
• two main connections
  • transverse fibers – connect to cerebellar peduncles (like branchium pontis) which connect to cerebellum
  • longitidinal tracts – motor and sensory tracts from medulla to / form rest of brain
• cranial nerve nuclei
  • trigeminal (V) – chewing (transmits motor impulses to muscles of mastication) and transmits sensory impulses from head and face sensory receptors
  • abducens (VI) – transmits motor impulses that control lateral eyeball movements (abduction) by innervating the lateral rectus muscle
  • facial (VII) – taste (sensory), salivation and tearing (motor), and facial expression (motor) functions
  • vestibulocochlear (VIII) (part)- equilibrium (sensory)
• reticular function – help control respiration ( along with medulla)

c. midbrain (=mesencephalon)

• ventral portion – cerebral peduncles – which convey motor impulses from cerebral cortex to pons, medulla, and spinal cord
• dorsel portion – tectum with corpora quadrigemina
  • superior colliculi – reflex centers for eyes, and head and neck in response to visual and other stimuli
  • inferior colliculi- reflex centers for movements of head and trunk in response to auditory stimuli
• red nucleus – receives fibers from cerebellum
• nuclei for cranial nerves
  • oculomotor (III) – transmits motor impulses that control some eyeball movements (innervates four of six extrinsic eye muscles) and changes pupil size and lens shape
  • trochlear (IV) – conducts impulses to the superior oblique muscle that helps move the eyeballs (inferiorly and laterally)
• other stuff as always (e.g. medial lemniscus which is common to pons, medulla and midbrain and goes to thalamus)

2. Diencephalon  

a. thalamus – oval structure above midbrain (3 cm long and 4/5 of diencephalon)

• mainly two gray masses connected by intermediate mass that crosses third ventricl e
• various roles as relay stations for all sensory impulses except smell to cerebral cortex
• also has interpretive functions – for pain, temperature, light touch and pressure
• has anterior nucleus that affects some emotions and memories & a bunch more

b. hypothalamus – small lower part of diencephalon but with several major regions, nuclei and functions

• controls and integrates the ANS through neurons that communicate to sympathetic and parasympathetic nuclei in the brain stem and spinal cord; e.g. regulating heart rate, food movement, and contraction of bladder.
• integration of sensory impulses from the viscera
• principal intermediary between nervous system and the endocrine system
  • release of regulating hormones (factors) that act on anterior pituitary
  • also produces anitdiuretic hormone (affects urine volume) and oxytocin (causes uterine contractions during labor and milk ejection)
• “mind-over-body phenomenon – cerebral cortex interprets strong emotions and stimulates hypothalamus which stimulates both the ANS and anterior pituitary (can affect heart rate, etc.; psychosomatic disorders are thus "real" disorders)
• associated with fellings of rage and aggression
• controls normal body temp via ANS
• regulates food intake (feeding center foe hunger sensations) vs. satiety center (inhibits feeding center)
• thirst center – monitors extracellular fluid volume
• helps maintain waking state and sleep patterns
• exhibits circadian ryhmicity and acts as a “pacemaker”

3. Cerebrum

a. Cerebral cortex – outer layer or gray matter made of billions of cells in 6 layers of cell bodies

• gyri (=convolutions) are folds that result from faster growth compared to white matter in early development
• sulci – the shallow grooves between folds;
• fissures – are the deep grooves between gyri
  • longitudinal fissure – separates the cerebral hemispheres
    • corpus collosum – connects hemispheres
    • falx cerebri – extension of the cranial dura mater that forms the superior and inferior sagital sinuses
• cerebral lobes and important gyri - cerebrum is divided into various lobes and gyri by sulci
  • central sulcus – divides frontal lobe form parietal lobe
  • precentral gyrus – a major gyrus just anterior to central sulcus
    • possesses the primary motor cortex
  • postcentral gyrus – just posterior to central sulcus
    • an important general sensory area of the cerebral cortex
  • lateral cerebral sulcus (fissure) – separates frontal lobe from the temporal lobe
  • parietooccipital sulcus – separates the parietal lobe from the occipital lobe
  • transverse fissure – separates the cerebrum from the cerebellum 
  • insula – deep structure (sometimes considered to be a separate lobe) within the lateral cerebral fissure under the parietal, frontal and temporal lobes. Here is a quote from the Merck Manuals Online Medical Library on the insula:

    The insula integrates sensory and autonomic information from the viscera. It plays a role in certain language functions, as evidenced by aphasia in patients with some insular lesions. The insula processes aspects of pain and temperature sensation and possibly taste.[from Introduction: Function and Dysfunction of the Cerebral Lobes in the Merck Manuals Online Medical Library)]

    Some suggest that the insula plays important roles in addiction that have not been thoroughly researched to date. This is yet to be determined. It has also been suggested that the insula plays important roles in feelings of empathy in humans and other great apes.

b. cranial nerves originating from the cerebrum

• olfactory ( I ) - purely sensory nerve dealing with sense of smell
• optic (II) - sensory nerve dealing with vision

c. white matter – under the cortex & made of myelinated axons running in three main directions

• association fibers – connect and transmit nerve impulses between gyri in same hemisphere
• commissural fibers – transmit impulses from gyri in one hemisphere to corresponding gyri in the other
  • main ones – corpus callosum, anterior commissure and posterior commissure
• projection fibers – form ascending and descending tracts to other parts of brain and spinal cord
  • example – internal capsule

d. basal nuclei – paired masses of gray matter in each cerebral hemisphere – have many critical functions

• example – caudate nucleus of corpus striatum
  • deals with motor control of skeletal muscles and coordination of movement
  • damage causes paralysis on side of body opposite damage (e.g. affects of stroke )

e. limbic system – parts of the cerebrum and also the diencephalon that encircles the brain stem

• functions in the emotional aspects of behavior related to survival
• damage to this region results in loss of memory of recent events and inability to commit things to memory
• stimulation of certain areas result in extreme pleasure, pain, rage, etc.
• controls mainly involuntary aspects of behavior (sometimes called the “emotional brain”)

f. damage to cerebrum

• concussion – no visible bruising of brain but may result in post-traumatic amnesia
• contusion – visible brusing due to trauma and blood leaking from microscopic vessels
  • pia mater stripped from over injured area allowing blood to enter subarachnoid space
  • extended loss of consciousness from minutes to hours
• laceration – tearing the brain such as from gunshot or skull fracture
  • results in cerebral hematoma, edema, etc.
• damage to brain from injury also results from free radicals (charged oxygen moleculeswhich cause damage to DNA and protein enzymes)

g. functional areas of cerebral cortex

• sensory – interpret sensory impulses
• motor – control muscular movement
• association areas – concerned with emotional and intellectual processes
• study in diagrams in book & note how specific theses areas are
  • example – Broca’s brain (motor speech area) located superior to the lateral cerebral sulcus
    • controls larynx, larynx and mouth as they deal with speech and coordinates breathing to regulate proper flow of air past vocal cords

4. Cerebellum (part of metencephalon along with pons).

a. second largest area of brain (alomost 1/8^th mass) – found in inferior and posterior part of cranial cavity

• separated from cerebrum by transverse cerebelli ( an extension of the cranial dura mater)

b. general structure of cerebellum

• vermis - constricted central area
• hemispheres - lateral areas
  • lobes of hemispheres
    • anterior lobe and posterior lobes deal with subconscious movements of skeletal muscle
    • flocculonodular lobe deals with equilibrium
• cortex - surface of gray matter with “folia” (analogous to the gyri of the cerebrum)
• arbor vitae - white matter tracts
• cerebellar nuclei - communicate to brain centers and spinal cord
• cerebellar peduncles
  • inferior cerebellar peduncles – connect to medulla and spinal cord with afferent and efferent tracts
  • middle cerebellar peduncles – connect to pons and have only afferent fibers that bring impulses to the cerebellum
  • superior cerebellar peduncles – connect to midbrain have efferent fibers (send impulses out of cerebellum)

c. general function of cerebellum

• primarily this is a motor area which coordinates subconscious movements of skeletal muscles
  • receives sensory information from proprioceptors, receptors for equilibrium, and visual receptors from eyes
  • also receives “duplicate” motor messages from cerebrum and basal ganglia to help with interpretation of body movement and position
• helps with posture and equilibrium
• helps predict future position of body and allows the body to react appropriately
• clinical application: damage to the cerebellum can produce
  • symptoms involving skeletal muscles on same side of body as damage (due to double crossing of fibers)
  • ataxia – lack of muscle coordination
    • ataxia is also produced by drinking alcohol and is the based for the sobriety test of blindfolding a person and requiring them to stretching the arms out and then attempting to touch fingers to the nose
  • abnormal gait
  • severe dizzines

Laboratory Studies

Approximately four hours of laboratory time will be dedicated to dissection of the sheep brain, which serves as an excellent model for the human brain. Detailed instructions will be provided in lab; however, the following represents those structure for which you are responsible to dissect and subsequently to be tested in a dissection quiz and laboratory practical exam.

Reminder about Textbook Study

As with other topics, your textbook has excellent presentations of the materials on the brain and cranial nerves of the nervous system. While you should focus on the specific material in the Notes on the Web, you should always use your textbook as a resource for illustrations and for understanding content that your notes cover.

As with all materials throughout the semester, you will have opportunities to ask questions or ask that any relevant material from your assignments be discussed in class.

Related Internet Resources

• http://www.uofs.edu/sheep/ - Link to a very useful Internet sheep brain dissection guide; it has both labeled and unlabeled photograph viewing options so that you can self-test
• http://www.gwc.maricopa.edu/class/bio201/cn/cranial.htm - This links to a useful cranial nerve study and review site