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Reticular Formation and the Neuromodulatory System


Study Guidelines

  • 1.

    Outline the subdivisions and functions of the reticular formation.

  • 2.

    Describe the location, type, and role of aminergic brainstem neurons of the neuromodulatory system.

  • 3.

    Define a chemoreceptor and its role in respiratory control.

  • 4.

    Summarise how the central nervous system monitors and controls respiration.

  • 5.

    Define a baroreceptor and its role in cardiovascular control.

  • 6.

    Summarise how the central nervous system monitors and controls blood pressure.

  • 7.

    Define nociception and pain, and outline the pathway of nociceptive transmission within the spinothalamic and spinobulbar pathways.

  • 8.

    Describe the role of the periaqueductal grey (PAG) substance and the magnus raphe nucleus in pain modulation.

  • 9.

    Define the gate control of nociceptive transmission and its clinical importance.

  • 10.

    Explain how a ‘flip-flop’ mechanism can explain the relationship between wake versus sleep and nonrapid eye movement (NREM) versus rapid eye movement (REM) sleep.

  • 11.

    Define the ascending reticular activating system (ARAS) and how our understanding of its origin has changed.

Reticular Formation

The term reticular formation refers only to the polysynaptic network in the brainstem, although the network continues rostrally into the thalamus and hypothalamus, and caudally into the propriospinal network of the spinal cord. The ground plan is shown in Fig. 24.1A . The medial reticular (tegmental) field contains large-celled neurons and the lateral reticular (tegmental) field small-celled neurons.

Fig. 24.1, (A) Reticular formation and subdivisions. (B) Aminergic and cholinergic cell groups of the neuromodulatory system.

The medial field is a predominantly efferent system . The axons are relatively long, and some ascend to synapse in the midbrain reticular formation or in the thalamus. Others have both ascending and descending branches contributing to a polysynaptic network. Projections from the premotor cortex, corticoreticular fibres , project here and give rise to the pontine and medullary reticulospinal tracts .

Neurons within the lateral field have long dendrites that branch at regular intervals. They have a predominantly transverse orientation, and their interstices are penetrated by long pathways running to the thalamus. The lateral network is mainly afferent in nature and receives fibres from all sensory pathways, including the special senses:

  • Olfactory fibres are received through the medial forebrain bundle, which passes alongside the hypothalamus.

  • Visual pathway fibres arise from the superior colliculus.

  • Auditory pathway fibres originate from the superior olivary nucleus and vestibular fibres from the medial vestibular nucleus.

  • Somatic sensory fibres are received from the spinoreticular tracts and from the spinal cord and principal (chief or main pontine) nucleus of the trigeminal nerve.

Axons from these neurons ramify extensively among the dendrites of the medial field and some synapse within the nuclei of cranial nerves and act as pattern generators.

Functional Anatomy

The range of functions served by different parts of the reticular formation is indicated in Table 24.1 .

Table 24.1
Components of the Reticular Formation and their Perceived Functions.
Function Component
Bladder control Pontine micturition centre
Cardiovascular control

  • Caudal ventrolateral medulla (CVLM)

  • Posterior ventrolateral medulla (PVLM)

  • Parabrachial nuclear complex

Locomotion

  • Mesencephalic locomotor region (pedunculopontine and cuneiform nucleus)

  • Pontomedullary reticular formation

Patterned cranial nerve activities (chewing, coughing, conjugate eye movements, sneezing, swallowing, vomiting, yawning) Premotor cranial nerve nuclei (central pattern generators)
Respiratory control

  • Dorsal respiratory group (DRG)

  • Ventral respiratory group (VRG)

  • Parabrachial nuclear complex

  • Retrotrapezoid nucleus

Salivary secretion, lacrimation Salivatory nuclei
Wake and sleep

  • Parabrachial nuclear complex

  • Parafacial zone

  • Pedunculopontine nucleus

  • Subcoeruleus region

Pattern Generators

Central pattern generator s (CPG) are integrated neuronal circuits that generate repetitive or stereotyped patterns of motor behaviour that can be independent of any sensory input or feedback. However, CPGs are subject to significant modulation that allows flexibility of response and integration with other systems. Patterned activities involving cranial nerves include:

  • Conjugate (parallel) movements of the eyes locally controlled by premotor nodal points ( gaze centres ) in the midbrain and pons linked to the nuclei of the ocular motor nerves (see Chapter 23 ).

  • Rhythmic chewing movements are controlled by the supratrigeminal premotor nucleus in the pons (see Chapter 21 ).

  • Swallowing, vomiting, coughing, yawning, and sneezing are controlled by separate premotor nodal points in the medulla and linked to specific cranial nerves and the respiratory centres.

  • Higher-level bladder controls are described in Basic Science Panel 24.1 .

  • Locomotor pattern generators are described in Basic Science Panel 24.2 .

  • An overview of gait controls is shown in Fig. 24.2 .

    Fig. 24.2, Overview of gait controls. Key nodes are indicated by an asterisk . (The assistance of Professor Tim O’Brien, Director, Gait Laboratory, Central Remedial Clinic, Dublin, is gratefully acknowledged.)

  • The salivatory nuclei belong to the parvocellular reticular formation of the pons and medulla and they contribute preganglionic parasympathetic fibres to the facial and glossopharyngeal nerves.

Respiratory Control

Groups of neurons located along either side of the upper medulla oblongata and organised into a dorsal respiratory nucleus ( DRN ) and ventral respiratory nucleus ( VRN ) regulate the respiratory cycle ( Fig. 24.3 ). Neurons of the DRN lie around and ventrolateral to the nucleus tractus solitarius and integrate sensory information (e.g. from cranial nerves IX and X via peripheral chemoreceptors) related to respiration, while other neurons project to related inspiratory motor neurons within the spinal cord.

Fig. 24.3, Respiratory control systems. All sections are viewed from below and behind. (A) is an enlargement taken from (B). (A) Inhibitory interaction between dorsal and ventral respiratory nuclei (DRN, VRN). Choroidal capillaries discharge cerebrospinal fluid (CSF) close to the medullary chemosensitive area (CSA) , whence neurons project to DRN. (B) The glossopharyngeal nerve (IX) contains chemoreceptive neurons reaching from the carotid body to the DRN. (C) Phrenic motor neurons are activated by the contralateral DRN. (D) Muscles of the abdominal wall are activated by the contralateral VRN to produce forced expiration. ICP , Inferior cerebellar peduncle.

The VRN lies ventral to the DRN. This column of neurons extends from the pons to the spinal cord and consists of three regions which receive sensory (afferent) information from the DRN. The rostral VRN ( Bötzinger complex ) portion is within the parafacial respiratory group of neurons and contains expiratory interneurons that project to other respiratory nuclei and the caudal VRN. In the intermediate VRN , the most rostral portion consists of inspiratory neurons (pre-Bötzinger complex) and possibly functions as the CPG of inspiration and for coordinating the other phases of the respiratory cycle. Within and near the intermediate VRN is the nucleus ambiguus (motor neurons of CN IX and X that when activated prevent collapse of upper airway during inspiration) and the nucleus paraambigualis that projects to inspiratory motor neurons in the spinal cord and inspiratory accessory muscles. The caudal VRN contains the nucleus retroambigualis (dorsal to the nucleus ambiguus; see Fig. 17.13 ) and is comprised of expiratory premotor neurons which project to spinal cord motor neurons that innervate accessory muscles of expiration (e.g. internal intercostals). As expiration is normally a passive process, these neurons are relatively inactive during normal breathing but become active during exercise.

The parabrachial nuclear complex surrounds the superior cerebellar peduncle and is in the dorsolateral pons. It participates in the regulation of cardiovascular and respiratory function. Stimulation of this nucleus by the amygdala in anxiety states results in characteristic hyperventilation (see Chapter 33 ).

Through numerous complementary and regulatory interactions between these brainstem structures and higher CNS centres, respiratory control is seamlessly integrated with speaking, swallowing, sleep–wake cycles, and emotions.

Medullary Chemosensitive Area

The choroid plexus of the fourth ventricle produces cerebrospinal fluid (CSF) that passes through the lateral aperture (of Luschka) of the fourth ventricle ( Fig. 24.3 ). At this location, cells of the lateral reticular formation at the medullary surface are exquisitely sensitive to the hydrogen (H + ) ion concentration in the neighbouring CSF. In effect, this medullary chemosensitive area ( CSA; retrotrapezoid nucleus ) samples the partial pressure of carbon dioxide (P CO2 ) level in the CSF, which is a direct reflection of the P CO2 in the blood supplying the brain (CSA also receives input from the peripheral chemoreceptors). Any increase in H + ions stimulates the dorsal respiratory nucleus through a direct synaptic linkage. (Several other nuclei within the medulla are also chemosensitive.)

Carotid Chemoreceptors

The carotid body is close to the bifurcation of the carotid artery ( Fig. 24.3 ) and receives a small branch from the external carotid artery that ramifies within the carotid body. Blood flow through the carotid body is so intense that the arteriovenous partial pressure of oxygen (P O2 ) changes by less than 1% during passage. The glomus cells chemoreceptors are neuroectodermal in origin, and release of their neurotransmitters triggers an action potential in the sensory endings of branches of the sinus nerve (branch of IX). The carotid chemoreceptors respond primarily to a fall in P O2 or rise in P CO2 and cause reflex adjustment of blood gas levels by altering the rate of breathing. Chemoreceptors in the aortic bodies (beneath the aortic arch) are relatively insignificant in humans but play a similar role and their afferents arise from the X cranial nerve.

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