The reticular formation is the substance of the brainstem, between the main nuclei and tracts, and consists of aggregations of neurones without distinct anatomic boundaries, but often grouped because of their similar chemical composition and function.
The functions of the reticular formation have been examined using electrical stimuli, and this led to the vague concept of physiological 'centres', concerned with particular functions, such as cardiovascular or respiratory control. Modern studies have provided considerable detail as to the complex cellular circuitry underlying these functions.
The substance of the brainstem surrounding the main nuclei and tracts within the brainstem consists of a hotchpotch of nerve cells called the reticular formation; their axons appear to pass in many directions, intersecting with each other, rather than travelling together in large discrete bundles over long distances. Unlike the specific sensory pathways that transmit specific modalities - touch, pain, proprioception, vision or hearing - reticular formation neurones often have multimodal, non-specific, inputs.
Some reticular neurones are pacemakers, capable of generating intrinsic rhythms, such as those necessary for rhythmic breathing; these neurones contain ion channels that allow them to be rhythmically active. Others contain characteristic groups of putative neurotransmitters or receptors. Some cell groups send axons into the spinal cord (the descending reticular formation) while others send ther axons into the brain (the Ascending Reticular Activating System) or cerebellum; these control the activites of the spinal cord, and the sleep-waking cycle respectively.
Click on the image to sequence 3 pictures. Image 1: Overview of the functions of the Reticular formation Image 2: Ascending Cholinergic Pathways. Image 3: Ascending and Descending Noradrenergic Pathways.
Ascending Cholinergic Pathways.
Pathways that use acetylcholine within the brain include neurones in the basal forebrain concerned with arousal and the sleep/waking cycle including the Nucleus basalis, also called the Nucleus of Meynert. The nucleus basalis has projections that reach throughout the cerebral cortex.
A second group of cholinergic neurones project to the thalamus from the brainstem (the pedunculopontine nucleus in the rostral pons).
Acetylcholine is also present in a number of brainstem nuclei that project to the caudate nucleus.
Noradrenergic Pathways originte in the locus coeruleus (LC) and the lateral tegmental noradrenergic system. Locus coeruleus neurones send their axons to the hypothalamus and throughout the cortex, including the amygdala, and the cerebellum. They are concerned with the level of consciousness, stress and reward.
The axons of the brainstem noradrenergic nuclei descend into the spinal cord where they reach the dorsal horn, autonomic nuclei and the ventral horn. In addition, some project to the Nucleus tractus solitarius, involved in cardiovascular control.
The Reticular Formation : Structure and Function
The substance of the brainstem reticular formation consists of loose aggregations of neurones divided into nuclei or complexes based on their chemical properties.
Some neurones are pacemakers, capable of generating intrinsic rhythms, such as those necessary for rhythmic breathing; these neurones contain ion channels that allow them to be rhythmically active. Others contain characteristic groups of putative neurotransmitters or receptors.
Examples include the basal forebrain cholinergic neurones, the serotoninergic raphe nuclei, and the noradrenergic locus coeruleus; all of these give rise to separate and different pathways that project outside the reticular formation.
Other groups of neurones, such as the group known as the Bötzinger complex and the adjacent pre-Bötzinger complex, are the regions containing neurones that generate the intrinsic respiratory rhythm that controls breathing. To illustrate the principle of histochemical grouping, the neurochemical footprint of neurones in the pre-Bötzinger complex includes the expression of neurokinin-1 receptors, somatostatin, nitric oxide synthase and the vesicular glutmate transporter. This area was formerly included in a vaguely defined 'medullary respiratory centre', a functional concept without much detailed cellular structure.
Reticular formation neurones receive from a number of sensory and other systems, so their inputs are described as 'non-specific'.
Some groups of reticular neurones are concerned with regulating the activity of the forebrain, as in sleep, arousal and waking; these groups have been called the ascending reticular formation (ARAS), an ill-defined area of the brainstem, characterised by its ability to desynchronise the EEG when stimullated.
It is now believed to include the ascending cholinergic and noradrenergic systems arising from groups of histochemically defined groups of neurones - the pedunculopontine nucleus of Meynert and the Locus coeruleus.
Descending pathways form the Reticular Formation
Others areas of the brainstem are able to modulate the activity of spinal circuits, e.g., in the regulation of muscle tone, autonomic outflow or transmission of nociceptive information in the dorsal horn; these constitute the descending reticular formation.
The nucleus gigantocellularus reticularis and the midline medullary raphe nuclei contribute substantially to these descending pathways, by which the reticular formation can influence the activities of the spinal cord.
Role of the Brainstem in Cardiovascular and Respiratory control
Brainstem and the Cardiovascular System
The brainstem regulates the heart rate through its connections with the sympathetic and vagal efferents.
The tonic activity of both systems originates in the brainstem: at rest, vagal tone maintains a low heart rate by releasing acetylcholine in the SA node.
The tonic activity of the sympathetic system in maintaining arterial blood pressure is generated in the brainstem and depends on connections between the brainstem and the lateral horn of the spinal cord.
In addition to generating tonic activity, the brainstem participates in reflex changes in the cardiovascular system, initiated by the baroreceptors, chemoreceptors and atrial receptors.
These reflexes are responsible for adjustments to cardiovascular dynamics associated with postural change, altitude and pressures in the respiratory tract, e.g. the Valsalva manoeuvre.
More Details about the Organisation of Cardiovascular Control by the Brainstem
Brainstem and the Respiratory System
Breathing is essential for life. It involves pumping air in and out of the lungs using muscles of respiration (skeletal muscles), and in humans occurs 15-20 times a minute.
Inspiratory muscles such as the diaphragm and external intercostal muscles are responsible for drawing air into the lungs; if there are difficulties in breathing, other muscles can be recruited.
Expiration is a passive process at rest, but when ventilation is increased other accessory muscles become active, such as the internal intercostal muscles.
The sequence of events in each respiratory cycle is controlled by the brainstem.
Different groups of neurones are known to be active during the three phases of respiration: active inspiration, passive expiration and active expiration (when this occurs).
The basic rhythmicity of repiratory movements is controlled by areas of the medulla, formerly known as the 'medullary respiratory centre' and these movements could be modulated by another area of the pons, known as the 'pontine pneumotaxic centre'.
Modern studies have revealed much more detail about the roles of different cell groups in the control of respiration.
More about the Brainstem and the Control of Respiration.