Midbrain (“mesencephalon”)

Midbrain (“mesencephalon”)

• One inch in length• Extends from pons

to diencephalon• Cerebral aqueduct

connects 3rd ventricle above to 4th ventricle below

Structures of the Mesencephalon

• Tectum:• 2 pairs of sensory nuclei (corpora

quadrigemina):– superior colliculus (visual)– inferior colliculus (auditory)

Relationship of mesencephalon to pons and cerebellum


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

matter)


• Cerebral peduncles:– nerve fiber bundles on ventrolateral

surfaces– contain:

• descending fibers to cerebellum• motor command (pyramidal) fibers

Mesencephalon in Section

• Red nucleus-- rich blood supply & iron-containing pigment• Substantia nigra---helps controls subconscious muscle activity

– cortex & cerebellum coordinate muscular movements by sending information here from the cortex and cerebellum

• Cerebral peduncles---clusters of motor & sensory fibers

Figure 14–8a

The Mesencephalon

Midbrain(“mesencephalon” of the brainstem)

Table 14-4

Summary: The Mesencephalon

Reticular Formation (Don’t forget this diffuse

structure!)Motor function: helps regulate muscle

movementsSensory function: Reticular activating system

(RAS)Stimulation increased cortical activityInactivation sleep

Reticular Activating System• RAS filters out repetitive, weak, irrelevant

stimuli. • LSD removes the filtering effect sensory-

overload.

Head Trauma (eg., lightweight boxers) concussion (mild, transient loss of consciousness) coma (loss of consciousness, hours to lifetime).

Figure 14–7a

The Cerebellum

Cerebellum1.Adjusts postural muscles

2.Fine-tunes conscious and subconscious movements

Cerebellar Peduncles

• Three paired fiber tracts • Superior peduncles connect the cerebellum to

the midbrain• Middle peduncles connect the pons to the

cerebellum• Inferior peduncles connect the medulla to the

cerebellum and carry ascending and descending cerebellar tracts from the spinal cord.

• All fibers in the cerebellum are ipsilateral

Figure 14–7b

Structures of the Cerebellum

Arbor VitaeCerebellum

Arbor Vitae• Highly branched, internal white

matter of cerebellum • Cerebellar nuclei:

– embedded in arbor vitae – relay information to Purkinje cells

Purkinje Cells • Large, branched cells• Found in cerebellar cortex• Receive input from up to 200,000

synapses

Disorders of the Cerebellum

• Ataxia:– damage from trauma

or stroke – intoxication

(temporary disturbance)

– disturbs muscle coordination

Table 14-3

Summary: The Cerebellum

What are the main components of the

diencephalon and their functions?

Figure 14–5a

The Diencephalon

• Filters ascending sensory information for primary sensory cortex

• Relays information between basal nuclei and cerebral cortex

• Integrates sensory information and motor commands

The Diencephalon• Thalamus,

epithalamus, and hypothalamus

Figure 14–9

The Third Ventricle• Separates left thalamus and right

thalamus • Intermediate mass:

– projection of gray matter – extends into ventricle from each side

Thalamic Nuclei• Lateral geniculate nucleus: relays

visual information • Medial geniculate nucleus: relays

auditory information• Lateral group: involved in emotional

states and integration of sensory information

Table 14-5

Summary: Thalamic Nuclei

Hypothalamus• Located below the thalamus

it caps the brainstem and forms the inferolateral walls of the third ventricle

• Mammillary bodies– Small, paired nuclei;

Relay station for olfactory pathways– control reflex eating movements

• Infundibulum – stalk of the hypothalamus; connects to the pituitary gland

The DiencephalonHypothalamus

Diencephalon

Figure 14–10a

The Hypothalamus• Lies below thalamus

8 Functions of the Hypothalamus

1. Provides subconscious control of skeletal muscle

2. Controls autonomic function3. Coordinates activities of nervous

and endocrine systems

Functions of the Hypothalamus

4. Secretes hormones:– antidiuretic hormone (ADH) by

supraoptic nucleus– oxytocin (OT) by paraventricular

nucleus


5. Produces emotions and behavioral drives:

– the feeding center (hunger)– the thirst center (thirst)


6. Coordinates voluntary and autonomic functions

7. Regulates body temperature:– preoptic area of hypothalamus


8. Controls circadian rhythms (day–night cycles):

– suprachiasmatic nucleus

Table 14-6

Summary: The Hypothalamus

Epithalamus

• Above the thalamus• Pineal gland

– secretes melatonin:a hormone involved with sleep regulation, sleep-wake cycles, and mood

Pineal Gland

Questions?

Cerebrospinal Fluid (CSF)Cerebrospinal Fluid (CSF)• 80-150 ml (about ½ cup)• Clear liquid containing glucose, proteins, &

ions• Functions

– mechanical protection • floats brain & softens impact with bony walls

– chemical protection• optimal ionic concentrations for action potentials

– circulation• nutrients and waste products to and from blood

CSF CompositionCSF Composition

Differs from plasma: no (or very few) cellsprotein is lowerionic concentrations are differentpH affects brain blood flow &

respiratory rate.

Blood Brain BarrierBlood Brain Barrier

Ependymal cells around choroid plexus (which produce CSF) have tight junctions.

Capillaries of brain tissue have tight junctions between endothelial cells.

Astrocytes wrap the small vessels.

Many antibiotics and chemotherapy agents can not pass from blood to brain.

Blood Brain Barrier – details to knowBlood Brain Barrier – details to knowBreak-Down

radiation, infection, neoplasm (cancer), manitol (intentional disruption)

facilitated diffusion glucose, amino acids

simple diffusionSmall, neutrally charged molecules (i.e., lipid solublemolecules) pass easily.• Water, CO2, O2, • alcohol, caffeine, nicotine, heroin• “general” anesthetics

Brain Blood FlowBrain Blood Flow

Sources: 2 internal carotids (R and L)2 vertebral arteries and “Circle of Willis”(Study in more detail in lab.)

14% of cardiac output, and uses 20% of oxygen used by body

Brain Blood Brain Blood FlowFlow

Brain Blood FlowBrain Blood FlowFlow depends on CO2 more than O2 concentration • High CO2 increased blood flow• Low CO2 decreased blood flow

Hyperventilation blow off CO2 low blood flow dizzy spell

Other Factors: extreme blood pressuresintracranial pressureblood viscosity

Brain Blood Flow: Brain Blood Flow: Clinical Clinical IssuesIssues

• Low blood sugar (eg., too much insulin) starves neurons

• Mass (tumor, blood clot) decreased flow• Heart attack decreased flow and

confusion• 10 seconds without blood 10 seconds without blood pass out pass out• 4 minutes 4 minutes permanent brain damage permanent brain damage

• lysosomes release enzymeslysosomes release enzymes

Medical Example: Subdural Hematoma

An subdural hematoma is a blood collection Between the dura and arachnoid mater (external to the brain)Since the skull cannot expand, the brain shifts across the midline.

Clinical Cases:Bleeding in the Brain

• Epidural Bleed

• Subdural Hematoma

• Intraparenchymal Bleed


• Epidural Bleed


• Intraparenchymal Bleed


• Epidural Bleed


• Intraparenchymal Bleed(s)

Medical Examples

• Meningitis is inflammation of the meninges due to bacterial or viral infection.

• Encephalitis is inflammation of the brain

Parkinson’s Disease• Unknown cause.

• Basal ganglia involved.

• Tremor, rigidity.

• New therapy….

Basal Ganglia

• Connections to red nucleus, substantia nigra & subthalamus

• Input & output with cerebral cortex, thalamus & hypothalamus

• Control large automatic movements of skeletal muscles

Figure 14–17

The Electroencephalogram

Seizure• Is a temporary cerebral disorder• Changes the

electroencephalogram• Symptoms depend on regions

affected

4 Categories of Brain Waves1. Alpha waves:

– found in healthy, awake adults at rest with eyes closed

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

mentally stressed

4 Categories of Brain Waves3. 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

Synchronization• A pacemaker mechanism:

– synchronizes electrical activity between hemispheres

• Brain damage can cause desynchronization

CN V (trigeminal)Dermatomes

Herpes Virus

Next time:• Special Senses and Cranial Nerves

(not on pre-midterm)• Review of somatic sensory and

motor pathways• Integration areas of the brain• Limbic system• Learning and Memory

end

Midbrain (“mesencephalon”)

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