19. Reticular Formation

 

·          Oldest part of the brain phylogenetically

·          “reticulum” is a misnomer (more than just interneurons)

·          consists of a collection of nuclei running from caudal medulla à telencephalon

 

Location

Examples

Telencephalic

nucleus basalis of Meynert

- uses ACh

- substantia innominata

- connects with limbic system and cortex

Diencephalic

certain thalamic nuclei

Midbrain

periaqueductal gray

- made up of several distinct nuclei

- determines whether or not pain will be transmitted to higher brain centers

- vertical gaze center

Pontine

contains horizontal gaze center

caudal reticular formation

nucleus locus coeruleus

- pigmented neurons (use NE)

- projections to limbic system, cortex

Medullary

many: heart and lung function

nucleus raphe magnus

- pain transmission to brain

 

General Groupings

1.         Lateral nuclei (of midbrain, pontine, and medullary reticular formation)

- receive input (sensory, motor, limbic)

2.         Medial nuclei (of midbrain, pontine, and medullary reticular formation)

- outflow neurons (many: large motor)

3.         Raphe nuclei (extend from medulla to midbrain)

- use 5HT

a.         nucleus raphe magnus

- pain transmission to brain

 

Functions

1.         Motor functions

a.         Reticulo-spinal tract

- indirect spinal pathway (receives from all motor systems in the brain)

- maintenance of anti-gravity muscle tone and maintenance of posture

2.         Visceral functions

- receives input from hypothalamus and projects to spinal ANS

- compromise (as with tonsillar herniation) à death

a.         Respiration

- see clinical correlation: SIDS

b.         Circulation

c.         CO2 levels

3.         Pain modulation

- influenced by: opioid peptide binding or electrical stimulation produced analgesia (cross-tolerance)

- site of placebo influence (?): cortex communicates with RF

- RF acts on Rexed I,II to prevent pain transmission

sites involved

1.         periaqueductal gray

2.         nucleus raphe magnus

3.         Rexed I-V (especially II: substantia gelatinosa)

4.         Maintenance of Consciousness

- you are not conscious unless your reticular formation stimulates your cortex

- allows incoming stimulation to be acted on by cortex (ARAS: ascending reticular activating system)

- the ARAS acts as a filter (focuses on relevant incoming information) while asleep AND while awake (cocktail party)

- ARAS must activate cortex for memory storage to occur

- sleep is not a passive phenomenon, but an actively induced state

drug action

- hallucinogens: too much sensory input à brain tries to make sense out of it (LSD)

- amphetamines: low dose enhances ability of ARAS to focus (high dose à total inability to do this)

- barbiturates: suppress ARAS (information will not be perceived, acted upon or remembered)

consciousness disturbances

- occurs from ARAS compromise (physical, infection, etc.)

- can be measured on Glasgow Coma Scale or defined as follows

1. lethargic: diminished responsiveness, but can be aroused

2. obtunded/stuporous: unresponsive except to painful stimuli

3. comatose: unresponsive, unarousable

5.         Sleep/wake/dreaming cycles

- sleeping (REM and non-REM) and dreaming are actively induced states

involved RF areas

1.         caudal reticular formation (below mid-pons)

2.         raphe nuclei (other than nucleus raphe magnus)

3.         nucleus locus coeruleus: cortical activation as part of ARAS; activation of REM