Reticular formation

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24 Reticular formation

Organization

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 they 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 fibers from all of the sensory pathways, including the special senses:

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 fibers 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 neurons (Figure 24.1B). They include one set producing serotonin (5-hydroxytryptamine) and three sets producing catecholamines, as listed in Table 24.1.

The serotonergic neurons have the largest territorial distribution of any set of 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 gray 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 (Ch. 26).
The dopaminergic neurons of the midbrain fall into two groups. At the junction of tegmentum and crus are those of the substantia nigra, which will be considered in Chapter 33. Medial to these, dopaminergic neurons in the ventral tegmental nuclei (Figure 24.3) project mesocortical fibers to the frontal lobe and mesolimbic fibers to the nucleus accumbens in particular (Ch. 34).

Table 24.1 Aminergic neurons of the reticular formation

Transmitter Location
Serotonin Raphe nuclei of midbrain, pons, medulla
Dopamine Tegmentum of midbrain
Norepinephrine Midbrain, pons, medulla
Epinephrine 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 liberate a peptide substance also, 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 monoamines is far from complete, no-one disputes their relevance to a wide range of behavioral functions.

Functional Anatomy

The range of functions served by different parts of the reticular formation is indicated in Table 24.2.

Table 24.2 Elements of the reticular formation and their perceived functions

Element Function
Premotor cranial nerve nuclei Patterned cranial nerve activities
Pontine locomotor center Pattern generation
Magnocellular nuclei Posture, locomotion
Salivatory nuclei Salivary secretion, lacrimation
Pontine micturition center Bladder control
Medial parabrachial nucleus Respiratory rhythm
Central reticular nucleus of medulla oblongata Vital centers (circulation, respiration)
Lateral medullary nucleus Conveys somatic and visceral information to the cerebellum
Ascending reticular activating system (ARAS) Arousal
Aminergic neurons Sleeping and waking, attention and mood, sensory modulation, blood pressure control

Pattern generators

Patterned activities involving cranial nerves include:

Locomotor pattern generators are described in Box 24.1. An overview of gait controls is provided in Box 24.2. Higher-level bladder controls are described in Box 24.3.

Box 24.1 Locomotor pattern generators

From animal experiments, it has long been agreed that lower vertebrates and lower mammals possess locomotor pattern generators in the spinal cord, within the gray 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 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 fibers 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 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 posterior root fibers feeding into the generator in the base of the anterior gray horn. Surface 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.

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