Diencephalon, Brain Stem and Cranial Nerves

Day 5Pages: 242-246

Diencephalon• Located between the cerebral hemispheres and above

midbrain• Composed largely of gray matter• Surrounds 3rd ventricle• Includes:

– Thalamus– Hypothalamus– Optic tracts– Optic chiasma– Infundibulum– Posterior pituitary gland– Mammillary bodies– Pineal gland

Parts to Diencephalon

• Optic tracts/optic chiasma– Formed by optic nerve fibers crossing over each other

• Infundibulum– Behind optic chiasma, attachment for pituitary gland

• Posterior pituitary gland– Hangs from floor of hypothalamus

• Mammillary Bodies– Two rounded structures behind infundibulum

• Pineal gland– Cone shaped structure attached to upper portion of

diencephalon

Thalamus• Bulge into 3rd ventricle• Central relay station for all sensory impulses except smell (ascending

fibers)• Channels impulses to appropriate regions of cortex for interpretation• Can communicate with cerebral cortex by means of descending fibers

Hypothalamus• Located below thalamus and forms floor of 3rd ventricle• Maintains homeostasis and links endocrine to nervous

system• Regulates:

– Heart rate and arterial BP– Body Temp.– H2O and electrolyte balance– Control of hunger and body weight– Control movements and glandular secretions of stomach and

intestines– Production of neurosecretory substances and stimulation of

pituitary gland to secrete hormones– Sleep and wakefulness

Other functions of Diencephalon

• Limbic System– Comprised of parts of: cerebral cortex, frontal and

temporal lobes, hypothalamus, thalamus basal ganglia and other deep masses called nuclei

• Controls emotional experiences and expressions• Can modify the way a person acts– Fear, anger, pleasure, and sorrow

• Guides persons behavior towards a likely increase in survival.

Brain Stem

• Bundle of nervous tissue that connects cerebrum to spinal cord.

• Consists of three parts– Midbrain– Pons– Medulla Oblongata

Midbrain

• Located at the top between diencephalon and pons

• Contain corticospinal tracts which are the main motor pathways between cerebrum and lower parts of nervous system

• Contains several masses of gray matter that serve as reflex centers.

Pons• Rounded bulge on

underside of brain stem• Dorsal side relays

impulses to and from M.O and cerebrum.

• Ventral side transmits impulses to cerebrum and cerebellum.

• Also relays sensory impulses from PNS to higher brain centers

Medulla Oblongata

• End of brain stem• All ascending and descending nerve fibers pass through MO• Controls vital visceral activites– Cardiac center

• Alters heart rate– Vasomotor Center

• Constricting and dilating of blood vessels– Respiratory Center

• Regulate rate, rhythm, and depth of breathing

• Also responsible for coughing, sneezing, swallowing and vomiting.

Reticular Formation

• Found throughout the brain stem• Network of nerve fibers• Responsible for taking sensory impulses and

activating cerebral cortex into a state of wakefulness• Decreased activity in reticular formation is known as

sleep.• Comatose state:– Point at which the reticular formation is injured and

cannot be aroused even with strong stimulation.

Cerebellum

• Large mass of tissue located below occipital bone.

• Divided into two hemispheres

• Surrounded by cerebral cortex

• Communicates with CNS by three pairs of nerve tracts– Cerebellar peduncles

Cellebellar Peduncles

• Inferior – Brings sensory information concerning position of limbs,

joints, and other body parts• Middle– Transmits signals from the cerebral cortex to the

cerebrellum concerning desired positions of these parts. After interpretation/analysis, sends pulses on to 3rd pair

• Superior– Incorporated into motor impulses that get sent down

brainstem to move body in desired way.

Cerebellum

• Reflex center for integrating sensory information concerning position of body parts and coordination of skeletal muscle movements

• Maintains posture• Damage/Injury– Tremors– Inaccurate movements of voluntary muscles– Loss of muscle tone– Reeling walk– Loss of equilibrium

Review

• What are the major functions of the thalamus? The hypothalamus?

• How may the limbic system influence behavior?• List the structures of the brain stem.• What vital reflex centers are located in the brain

stem?• What is the function of the reticular formation?• Where is the cerebellum located?• What are the major functions of the cerebellum?