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