reticular formation Flashcards
What does that reticular formation constitute? and what does it exclude?
-Substantial part of the dorsal part of the brainstem
-excludes:
–nuclei of CN
–long tracts that pass through the brainstem
–more conspicuous masses of grey matter
–medial lemniscus and nucleus ambiguous (located in territory of reticular formation)
what are the general functions of the reticular formation?
-sleep-arousal cycle
-perception of pain
-control of movement
-regulation of visceral activity
what are the aggregations of neurons that are recognised in the reticular formation?
- the precerebellar nuclei
- the raphe nuclei
- the central group of nuclei
- the cholinergic and catecholamine cell groups,
- the lateral parvocellular reticular area
- the parabrachial area
- the superficial medullary neurons.
What is the precerebellar reticular nuclei made up of?
-lateral reticular nucleus
-paramedian reticular nucleus
-pontine reticulotegmental nucleus
describe the precerebellar nuclei projections and function
-They all send projections into the cerebellum, and they are functionally quite separate from the rest of the reticular formation; hence they are generally described with the cerebellum.
-Function: coordination & muscle contraction
describe the location of raphe nuclei, and the main transmitter in this location
-groups of neurons either in or adjacent to the midline (raphe) of brainstem
-many of these neurons synthesize and secrete serotonin
describe the axons of serotonergic raphe neurons
thin, unmyelinated, greatly branched –> distributed to gray matter throughout CNS
what pathways that connect to the medullary raphe nuclei are important for suppression of conscious awareness of pain?
-connections of medullary raphe nuclei with periaqueductal gray matter and the spinal dorsal horn (and trigeminal sensory nuclei)
where do the pontine and mesencephalic raphe nuclei go to?
-cerebellum and all parts of cerebrum, incl. cerebral cortex, basal ganglia and limbic system
what does the more rostrally located raphe nuclei play a part in?
-role in sleep
what does the central group of the reticular formation include?
This group of nuclei includes:
-the medially located nuclei (paramedian pontine reticular formation (PPRF) and gigantocellular reticular nuclei) in the medulla and pons
-the laterally located cuneiform and subcuneiform nuclei in the midbrain (included in the central group because of their similar connections and functions).
what does the paramedian pontine reticular formation (PPRF) of the reticular formation include?
-includes neurons in the medial parts of the 2 pontine reticular nuclei
What do the gigantocellular reticular nucleus include in type of neurons?
some serotonin neurons
what are the central group afferents that are received ?
The central nuclei receive afferents from:
-all the general and special sensory systems
-from the reticular formation of the midbrain
-cholinergic reticular nuclei/cholinergic pedunculopontine nucleus,
-hypothalamus
-premotor area of the cerebral cortex/motor cortex
-cerebellar nuclei
-spinal cord
Neurons of the central reticular nuclei typically have axons with…
Long ascending and descending branches
describe descending axons from the central group
-The long descending axons constitute the reticulospinal tracts, located in the ventral and lateral funiculi of the spinal white matter. The reticulospinal tracts are important motor pathways.`
Describe the ascending axons from the central group
-Ascending axons from the central group of reticular nuclei travel in the central tegmental tract. The involvement of the ascending projections is in maintaining consciousness.
-The reticulothalamic projection also provides an interaction with the corpus striatum, which has motor and other functions.
where are the cholinergic neurons of the reticular formation located and what transmitter do they use?
-They have two groups of neurons in the reticular formation and are located in the rostral part of the reticular formation.
-Acetylcholine as their synaptic transmitter.
What are the 2 cholinergic neurons in the retiuclar formation?
-pedunculopontine nucleus –> larger. located in the rostral pons and caudal midbrain
-lateral dorsal tegmental nucleus –> smaller. Extends from the pontine periventricular grey matter into the periaqueductal grey matter
where do cholinergic neurons receives afferents from?
–nearby noradrenergic (locus coeruleus)
–serotonergic (raphe) nuclei,
–from histaminergic neurons in the hypothalamus
–inhibitory (gamma-aminobutyrate [GABA]) descending fibres from the pallidum, and
–from the preoptic area.
Cholinergic neurons have long, branching axons, which synapse to:
–neurons in the central group of pontine reticular nuclei
–the locus coeruleus
–substantia nigra
–subthalamic nucleus
–intralaminar thalamic nuclei, and
–basal cholinergic nuclei of the forebrain
What do electrophysiological studies typically implicate cholinergic reticular nuclei in?
-motor functions, locomotion, consciousness and arousal
what are catecholamine?
-noradrenaline
-adrenaline
-dopamine
where is the largest central noradrenergic neurons?
-locus coeruleus nucleus pigmentosus –> lie at the pontomesencephalic junction
where do most efferent axons of the locus coeruleus go?
-travel rostrally in the central tegmental tract and the medial forebrain bundle
Where do the descending noradrenergic axons arise predominantly from?
lateral medullary catecholamine nuclei
What do noradrenaline from the locus coeruleus and related cell groups act as?
-modulator of synapses between other neurons
-has excitatory effects on spinal reflexes and alertness
-destructive lesions on LC does not cause unconsciousness
where is the parvocellular reticular area located?
-medulla and pons
-lateral to the central group and medial to the trigeminal nuclei
where do the neurons of the parvocellular reticular area send their axons and what is it involved with?
-to motor nuclei of hypoglossal, facial and trigeminal nerves
-involved in reflexes concerned with feeding
Where is the “excitatory centre” and what does stimulation of this region do?
-located within the medullary parvocellular reticular area
-stimulation can cause acceleration of heart and increased arterial blood pressure
where is the parabrachial area located?
-lateral part of the reticular formation of the caudal midbrain, close to superior cerebellar peduncle
-rostral to the parvocellular reticular area
describe the afferent fibres of the parabrachial area
-from the solitary nucleus and from cortex of the insula and adjoining parts of the parietal lobe
describe the parabrachial neurons projections rostrally
they project rostrally to:
-hypothalamus
-preoptic area
-intralaminar thalamic nuclei
-amygdala
with the exception of primates, the parabrachial nuclei also form part of the sensory pathway for…
Taste in many mammals
parabrachial area serves as a relay station in…
ascending pathways for visceral sensations
what is the pneumotaxic centre?
-contained in the parabrachial area
- region concerned with regulation of respiratory rhythm
-Dorsal pontine lesions can cause apneustic respiration, in which a pause of a few seconds takes place between full inspiration and the
beginning of expiration.
where is the superficial medullary reticular nuclei found?
in the central superficial reticular area in the medulla
where are the superficial medullary reticular neurons concerned with? and describe the afferents of this area
-cardiovascular and respiratory regulation
-afferents –> from spinal cord and solitary nucleus
what are the fibres of superficial medullary reticular nuclei activated by?
–the baroreceptors of the carotid and aortic sinuses
–the oxygen-sensitive chemoreceptors of the carotid and aortic bodies.
-Some of these medullary neurons respond directly to changes in the pH or carbon dioxide concentration
what are the efferent projections of the superficial medullary region
–the hypothalamus
–the preganglionic autonomic neurons in the medulla and spinal cord and
–the motor neurons that supply the muscles of respiration
where is the area postrema located? and what does it lack?
-this is a narrow strip of neural tissue in the caudal part of the floor of the fourth ventricle near the obex
-lacks blood-brain barrier
what does the area postrema have reciprocal connections with?
solitary nucleus
what area has been shown experimentally to contain chemoreceptors for emetics drugs such as apomorphine and digoxin?
the area postrema –> therefore, may function in physiology of vomiting
what does the perihypoglossal nuclei consist of?
Consists of three groups of neurons in the caudal medulla:
-the nucleus intercalatus,
-the nucleus of Roller (ventrolateral to the hypoglossal nucleus), and
-the nucleus prepositus hypoglossi
What is the largest of the perihypoglossal nuclei and what is it continuous with?
The nucleus prepositus hypoglossi, and it is continuous at its rostral end with the paramedian pontine reticular formation
where does the perihypoglossal nuclei sense efferent fibres to?
They send efferent fibres to the nuclei of cranial nerves III (3), IV (4), and VI (6), which they reach by passing into the medial longitudinal fasciculus.
perihypoglossal nuclei form part of the complex circuitry for…
-movements of the eyes –>lesions in nucleus prepositus hypoglossi impair the ability to keep the eyes fixed on a visual target
Where is the accessory oculomotor nuclei situated?
-junction of midbrain and diencephalon
what does the accessory oculomotor nuclei consist of?
-interstitial nucleus of Cajal,
-nucleus of Dark-schewitsch,
-the nucleus of the posterior commissure, and
-the rostral interstitial nucleus of the medial longitudinal fasciculus.
what is the accessory oculomotor nuclei concerned with?
-movement of eyes in vertical plane
where is the Dark-schewitsch located?
within the territory of the periaqueductal gray matter, but it is generally considered to be one of the accessory oculomotor nuclei
what does the periaqueductal grey matter surround?
The cerebral aqueduct of he midbrain
what does electrical stimulation of the periaqueductal matter cause?
analgesia –> mediated by the way of the descending projection of the nucleus raphes magus in the medulla
where is the interpeduncular nucleus located?
-midline, ventral to the periaqueductal grey matter and near the roof of the most rostral part of the interpeduncular fossa
What does the interpeduncular nuclei lie the pathway for?
-pathway through which the limbic system projects to the autonomic nuclei in the brainstem and spinal cord
where is the ventral tegmental area?
Lateral to the interpeduncular nucleus (in the medial part of the cerebral peduncle) –> dopamine secreting neurons
describe the reticular formation and pain
-The central group of reticular nuclei forms part of an ascending pathway (through spinal afferents and projections to the thalamus) for the poorly localized perception
of pain.
-A descending inhibitory pathway consists of the axons of serotonergic raphe neurons
that project to the dorsal horn and spinal trigeminal nucleus.
-The descending pathway inhibits the rostral transmission of action potentials that report pain.
-Electrical stimulation of the periaqueductal grey matter (which projects to the raphe nuclei in the medulla) results in loss of the ability to experience pain (Analgesia) from sites of injury or disease.
What is the raphespinal tract?
-a reticulospinal pathway best known for the involvement of its
serotonergic neurons in the modulation of pain sensation
-However, raphespinal projections may also modulate the activities of motor neurons, which are made more excitable by serotonin.
-Drugs that block the action of serotonin have been used clinically to alleviate the spasticity that
follows damage to the major descending motor pathways
Some regions of the reticular formation regulate visceral functions and breathing through connections with…
-the amygdala and the hypothalamus (rostrally)
-the nuclei of the autonomic outflow (caudally) and
-the respiratory motor neurons in the phrenic nucleus and thoracic
cord.
which nucleus coordinates the responses for maximal inspiration?
The gigantocellular nucleus
which nucleus coordinates the responses for maximal expiration
Parvocellular reticular area
What does stimulation in the medial and lateral part of the reticular formation of the medulla do?
medial – has a depressor effect on the circuitry system (slowing HR and lowering BP)
lateral – has opposite effect
REM sleep is episodes of?
sleep in which there are rapid eye movements (REM)
What happens during REM sleep?
-the muscles of the trunk and limbs are relaxed, and
-a substantial sensory stimulus is needed for arousal (individual is deeply asleep).
what is the muscular relaxation is mediated by for REM sleep?
-Mediated by neurons in the reticular formation that inhibit
motor neurons in the spinal cord. These inhibitory neurons use glycine as an inhibitory transmitter
is cerebral cortex active or inactive during REM sleep?
very active
describe the inhibition of transmission during REM sleep
-During REM sleep, there is an inhibition of transmission from the thalamus to the cerebral cortex in all the specific sensory pathways (e.g., somatic, auditory), hence, a
resistance to arousal occurs in REM sleep.
-The suppression of the transmission in specific sensory
pathways is believed to be mediated by rostrally projecting cholinergic neurons that stimulate the reticular nucleus of the thalamus.
-This nucleus contains GABA-ergic neurons that inhibit transmission from the other thalamic nuclei to the cerebral cortex.
what does EEG measure?
-The fluctuations in voltage recorded from a point on the scalp
-The recordings/signals represents the sum of the variations in the membrane potentials of the dendrites of neurons in the underlying cerebral cortex.
-Dendritic potentials are responses to activity of afferent axons, most of which come from neurons in the thalamus.
-Large potentials are recorded when groups of thalamic neurons fire synchronously, low-voltage activity indicates that each cortical neuron is responding differently to
its thalamic afferents.
describe the EEG waves of someone who’s fully alert, deep sleep and in REM sleep
-The EEG waves of a fully alert person are of low voltage and high frequency, indicating desynchronization of thalamocortical circuits.
-The EEG of a person in deep sleep shows much taller waves (synchronization) and longer waves (“slow-wave sleep”).
-In REM sleep, the EEG is desynchronized despite the fact that such sleep is deep in the sense of being resistant to sensory stimulation.
irreversible coma follows…
-bilateral destruction of the medial parts of the brain stem at or above the upper pontine levels
-Transmission in the more laterally located sensory pathways is not interrupted by medially located lesions that cause coma.
What is essential for maintaining a conscious state?
-intact rostral pontine reticular formation and central tegmental tract
-At the level of the midbrain and rostral pons, the central tegmental tract contains three populations of axons from the reticular formation that directly or indirectly stimulate the whole cerebral cortex:
-Noradrenergic neurons
-Cholinergic neurons of the pedunculopontine nucleus
-The central group of reticular nuclei
describe the noradrenergic neurons and, sleep and arousal
Noradrenergic neurons:
–provide an ascending projection that excites neurons throughout the cerebral cortex.
–the cells of the locus coeruleus are most active in awake, attentive state
–they are less active in non-REM sleep and inactive in REM sleep
Describe the cholinergic neurons of the pedunculopontine nucleus and, sleep and arousal
-these project to the hypothalamus, basal cholinergic
nuclei of the forebrain, and intralaminar thalamic nuclei
-all these nuclei have extensive although sparse projections to all parts of the cerebral cortex.
-These neurons are active in the awake state and in REM sleep
-They are inactive in non-REM sleep
Describe the central group of reticular nuclei (especially the oral pontine reticular nucleus) and, sleep and arousal
-sends axons to the intralaminar nuclei of the thalamus and to the basal cholinergic nuclei of the forebrain.
-The central reticular neurons are a mixed population, differently active in all states of consciousness.
describe the intralaminar thalamic nuclei and, sleep and arousal
-The intralaminar thalamic nuclei provide an essential link in most of the ascending pathways concerned with both arousal and REM sleep.
-The intralaminar nuclei receive collateral branches from all the sensory tracts that go to other nuclei of the thalamus.
-Sensory stimuli that cause arousal from sleep may do so by way of these branches.
-Lesions that bilaterally damage the intralaminar nuclei cause
coma.
describe the tuberomammilary nucleus and sleep and arousal
-The tuberomammillary nucleus in the posterior part of the hypothalamus is composed of histamine-secreting neurons
-Their axons branch profusely in the thalamus and they also extend to many parts of the CNS, including the cerebral cortex.
-Pharmacological studies indicate that histamine of neuronal origin participates in arousal.
-The sedative side effects of traditional antihistaminic drugs (H1-receptor blockers) are probably caused by competitive inhibition of the action of histamine on cortical neurons.
what neurons stimulate neurons thorughout the cerebral cortex for sleep and arousal
-The basal cholinergic nuclei of the forebrain also stimulate neurons throughout the cerebral cortex
-Groups of neurons in the diencephalon and telencephalon
stimulate the cerebral cortex in a general way.
what are the neurons in the brainstem and hypothalamus that actively promote sleep?
- The serotonergic raphe neurons
- The lateral hypothalamus
- The suprachiasmatic nucleus of the hypothalamus
- The cholinergic neurons of the pedunculopontine and lateral dorsal tegmental nuclei
- GABA-ergic neurons
Describe deep (non-REM) sleep and the serotonergic raphe neurons
-Their axons go to all parts of the CNS.
-the raphe neurons are active in deep sleep, (caused partly by a wide-spread inhibitory action of serotonin in the thalamus and cerebral cortex).
-Serotonergic neurons are less active in REM sleep (likely due to the occasional release of the telencephalic neurons from serotonergic inhibition).
-A simultaneous reduction of inhibition of the caudal pontine reticular nucleus (the PPRF) may account for the accompanying movements of the eyes.
Describe deep (non-REM) sleep and the lateral hypothalamus
–some neurons produce a pair of peptides, orexin-A and orexin-B, which are active in the awake state.
–the orexins are also called hypocretins-1 and -2.
–the long axons of orexin neurons extend to most parts of the brain.
–One of their actions is stimulation of the cholinergic neurons of the pedunculopontine nucleus.
–Deficiency of orexin in humans is associated with narcolepsy (i.e., waking state is frequently interrupted by brief episodes of REM sleep).
Describe deep (non-REM) sleep and the suprachiasmatic nucleus of the hypothalamus
-this contains neurons whose patterns of firing follow a 24-hour cycle
-Thus it serves as an internal clock for the brain.
-Axons from the suprachiasmatic nucleus contact the orexin neurons of the lateral hypothalamus and the dorsomedial hypothalamic nucleus, which projects caudally to the locus coeruleus.
-These connections provide circuitry that may facilitate sleeping during the night rather than during the day.
Describe deep (non-REM) sleep and the cholinergic neurons of the pedunculopontine and lateral dorsal tegmental nuclei
-These nuclei are as active in REM sleep as they are in the awake state.
Describe deep (non-REM) sleep and the GABA-ergic neurons
-These are present in the preoptic area(immediately anterior to the hypothalamus)
-They contain a peptide, galanin.
-These inhibitory neurons are active in deep (non-REM) sleep.
-Their axons go to the tuberomammillary nucleus, the locus coeruleus, and the cholinergic reticular nuclei.
-Destructive lesions in the preoptic area cause insomnia, indicating that this region is essential for the occurrence of sleep
