Chapter Summary
Study guidelines
- 1.
Outline the subdivisions of the reticular formation and describe the location and type of its aminergic brainstem neurons.
- 2.
List the functional anatomy or functions performed by the reticular formation.
- 3.
Explain how a ‘flip-flop’ mechanism can explain the relationship between the states of wake versus sleep and non-REM versus REM sleep.
- 4.
Define and describe the ‘components’ of the ascending arousal system (AAS).
- 5.
Describe the significance and role of the dorsal and ventral respiratory nucleus, magnus raphe nucleus, pedunculopontine nucleus, and pontine micturition control centre.
- 6.
Be able to recall how disturbance of function of aminergic projections from the reticular formation has been correlated with psychiatric states including major depression and schizophrenia.
The reticular formation is phylogenetically a very old neural network—it is a prominent feature of the reptilian brainstem. It originated as a slowly conducting, polysynaptic pathway intimately connected with olfactory and limbic regions. The progressive dominance of vision and hearing over olfaction led to localisation of sensory and motor functions within the tectum of the midbrain. Direct spinotectal and tectospinal tracts bypassed the reticular formation, which was largely relegated to automatic functions. In mammals the tectum in turn has been relegated to minor status with the emergence of very fast pathways linking the cerebral cortex with the peripheral sensory and motor apparatus.
In the human brain the reticular formation continues to be of importance in automatic and reflex activities and has retained its linkages to the limbic system.
Organisation
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 Figure 24.1A . In the midline the median reticular formation comprises a series of raphe nuclei ( pron. ‘raffay’ and derived from the Greek word for seam). The raphe nuclei are the major source of serotonergic projections throughout the neuraxis (see next section).
Next to this is the paramedian reticular formation . This part of the network contains magnocellular neurons throughout; in the lower pons and upper medulla some gigantocellular neurons also appear, before the network blends with the central reticular nucleus of the medulla oblongata.
Outermost is the lateral, parvocellular (small-celled) reticular formation . Parvocellular dendrites are long and 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. It receives fibres from all sensory pathways, including the special senses:
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Olfactory fibres are received through the medial forebrain bundle, which passes alongside the hypothalamus.
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Visual pathway fibres are received from the superior colliculus.
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Auditory pathway fibres are received from the superior olivary nucleus.
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Vestibular fibres are received from the medial vestibular nucleus.
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Somatic sensory fibres are received from the spinoreticular tracts and from the spinal and principal (chief or main pontine) nuclei of the trigeminal nerve.
Most parvocellular axons ramify extensively among the dendrites of the paramedian reticular formation. However, some synapse within the nuclei of cranial nerves and act as pattern generators (see later).
The paramedian reticular formation is a predominantly efferent system . The axons are relatively long. Some ascend to synapse in the midbrain reticular formation or in the thalamus. Others have both ascending and descending branches contributing to the polysynaptic network. The magnocellular component receives corticoreticular fibres from the premotor cortex and gives rise to the pontine and medullary reticulospinal tracts .
Aminergic neurons of the brainstem
Embedded in the reticular formation are sets of aminergic (or monoaminergic) neurons —neurons whose neurotransmitters are synthesised from an aromatic amino acid and that share several cellular properties ( Figure 24.1B ). They include one set producing the neurotransmitter serotonin , three sets producing catecholamines (dopamine, norepinephrine, and epinephrine) , and one set producing histamine ( Table 24.1 ).
- •
The serotonergic neurons have the largest territorial distribution of any set of central nervous system (CNS) neurons. In general terms, those of the midbrain project rostrally into the cerebral hemispheres; those of the pons ramify in the brainstem and cerebellum; and those of the medulla supply the spinal cord ( Figure 24.2 ). All parts of the CNS grey matter are permeated by serotonin-secreting axonal varicosities. Clinically, enhancement of serotonin activity is part of the treatment for a prevalent condition known as major depression ( Chapter 26 ).
Figure 24.2
Serotonergic projections from the brainstem midline (raphe).
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The dopaminergic neurons of the midbrain fall into two groups. At the junction of tegmentum and crus are those of the substantia nigra ( Chapter 33 ). Medial to these are those of the ventral tegmental nuclei ( Figure 24.3 ) that project mesocortical fibres to the frontal lobe and mesolimbic fibres to the nucleus accumbens in particular ( Chapter 34 ).
Figure 24.3
Dopaminergic projections from the midbrain.
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The noradrenergic (norepinephrine) neurons are only marginally less prodigious than the serotonergic ones. About 90% of the somas are pooled in the locus ceruleus (cerulean nucleus) , a ‘violet spot’ in the floor of the fourth ventricle at the upper end of the pons ( Figure 24.4 ). Neurons of the locus ceruleus project in all directions, as indicated in Figure 24.5 .
Figure 24.4
Part of a transverse section through the upper part of the pons, showing elements of the reticular formation.
Figure 24.5
Noradrenergic projections from the pons and medulla oblongata.
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Epinephrine-secreting neurons are relatively scarce and are confined to the rostral/caudal medulla oblongata. Some project rostrally to the hypothalamus, others project caudally to synapse upon preganglionic sympathetic neurons in the spinal cord.
| Transmitter | Location |
|---|---|
| Dopamine | Tegmentum of midbrain (substantia nigra, ventral tegmentum) |
| Epinephrine | Medulla |
| Histamine | Diencephalon |
| Norepinephrine | Midbrain, pons, medulla (locus ceruleus) |
| Serotonin | Raphe nuclei of midbrain, pons, medulla |
In the cerebral cortex the ionic and electrical effects of aminergic neuronal activity are quite variable. First, more than one kind of postsynaptic receptor exists for each of the amines. Second, some aminergic neurons also liberate a peptide substance capable of modulating the transmitter action—usually by prolonging it. Third, the larger cortical neurons receive many thousands of excitatory and inhibitory synapses from local circuit neurons, and they have numerous different receptors. Activation of a single kind of aminergic receptor may have a large or small effect depending on the existing excitatory state.
Although our understanding of the physiology and pharmacology of the aminergic neurons is far from complete, their relevance to a wide range of behavioural functions is unquestioned.
Functional anatomy
The range of functions served by different parts of the reticular formation is indicated in Table 24.2 .
| Element | Function |
|---|---|
| Aminergic neurons | Sleeping and waking, attention and mood, sensory modulation, blood pressure control |
| Ascending arousal system (AAS) | Arousal |
| Central reticular nucleus of medulla oblongata | Vital centres (circulation, respiration) |
| Lateral medullary nucleus | Conveys somatic and visceral information to the cerebellum |
| Magnocellular nuclei | Posture, locomotion |
| Medial parabrachial nucleus | Patterns of respiration during the waking state |
| Pontine locomotor centre | Pattern generation |
| Pontine micturition centre | Bladder control |
| Premotor cranial nerve nuclei | Patterned cranial nerve activities |
| Salivatory nuclei | Salivary secretion, lacrimation |
Pattern generators
Patterned activities involving cranial nerves include:
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Conjugate (in 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 ( Chapter 23 ).
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Rhythmic chewing movements controlled by the supratrigeminal premotor nucleus in the pons ( Chapter 21 ).
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Swallowing, vomiting, coughing, yawning, and sneezing, which are controlled by separate premotor nodal points in the medulla linked to the appropriate cranial nerves and to the respiratory centres.
Locomotor pattern generators are described in Box 24.1 . An overview of gait controls is shown in Figure 24.6 . Higher-level bladder controls are described in Box 24.2 .
From animal experiments, it has long been agreed that lower vertebrates and lower mammals possess locomotor pattern generators in the spinal cord, within the grey matter neurologically connected to each of the four limbs. These spinal generators comprise electrically oscillating circuits delivering rhythmically entrained signals to flexor and extensor muscle groups. Spinal generator activity is subject to supraspinal commands from a mesencephalic locomotor region (MLR) , which in turn obeys commands from motor areas of the cerebral cortex and is reciprocally connected to the corpus striatum.
The MLR contains the pedunculopontine nucleus, close to the superior cerebellar peduncle, where this passes along the upper corner of the fourth ventricle to enter the midbrain ( Figure 17.16 ). These nuclei send fibres down the central tegmental tract to the oral and caudal pontine nuclei serving extensor motor neurons and to medullary magnocellular neurons serving flexor motor neurons.
A major focus of spinal rehabilitation is on activation of spinal locomotor reflexes in patients who have experienced injury resulting in partial or complete spinal cord transection. It is now well established that even after complete transection at the cervical or thoracic level, a lumbosacral locomotor pattern can be activated by continuous electrical stimulation of the dura mater at lumbar segmental level. The stimulation strongly activates dorsal root fibres feeding into the generator in the base of the ventral grey horn. Surface electromyographic (EMG) recordings taken from flexor and extensor muscle groups reveal an oscillating pattern of flexor and extensor motor neuron activation, although the pattern is not identical to the normal one. A normal pattern requires the lesion to be incomplete, with preservation of some supraspinal projection from the pedunculopontine nucleus.
Generation of actual stepping movements is possible in complete lesions if the individual is supported over a moving treadmill belt while the dura is being stimulated, presumably because of the additional cutaneous and proprioceptive inputs to the generator. Muscle strength and stepping speed improve over a period of weeks but not enough to enable unassisted locomotion within a walking frame.
Current research aims at improving the opportunity for supraspinal motor fibres to ‘bridge the gap’ by clearing tissue debris from the gap and replacing that tissue with a medium having a matrix that will support regenerating axons both physically and chemically.
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