The pons, (which is a Latin word meaning ‘bridge’), is a small structure in the lower brainstem, just above the medulla oblongata and below the midbrain. Behind it lies the cerebellum, with which it has many connections. Again we have another brain structure here whose size belies its importance.
The pons has been argued to be the seat of consciousness and there can be no doubt that it plays a large role in regulating our level of arousal. Also, injuries to the pons can produce coma! The main function of this structure is to pass information between the cerebellum and the cerebral cortex, in addition, it helps to send other messages to the brain, manage arousal feelings, and monitor respiration. Some scientists believe that the pons is an important part of dreaming, since it is responsible for Rapid Eye Movement (REM) sleep, which is an essential part of any sleep cycle.
There are a few sets of nuclei (brain cells) in the pons which are of vital importance to our performance as human beings.
The Raphe Nuclei. - These groups of cells in the middle of the pons are also sometimes classed as belonging to another brain structure called the ‘reticular formation.’ The reticular formation is a ‘net’ like structure which intertwines itself with many other structures in the lower brainstem and in higher structures too. However, back to the raphe nuclei of the pons, which have widespread connections to other areas of the brain and would seem to be a major producer of serotonin in the brain.
Studies have shown that the levels of activity of serotonin-containing cells in the raphe nuclei varies directly with the overall level of motor activity or arousal of the child. There are also changes in pattern associated with drowsiness and sleep (cells slow down firing while entering sleep, and stop firing during REM sleep).
The serotonin produced by the raphe nuclei is involved in a range of physiological and pathological functions in the human body, such as helping to regulate the release of serotonin that controls sleep patterns, mood, pain response, and motor functions. So when I am asking questions of parents in a consultation about their child’s development and those questions are centred on the child’s level of arousal, sleep pattern and mood, I am checking for injury here. Also when parents see me ‘pinpricking’ their children, I am checking their pain response at the level of the pons.
The way in which the raphe nuclei of the pons are considered to influence sleep by many is through their connections with the suprachiasmatic brain cells in the hypothalamus. This is where our ‘circadian clock’ is located, which regulates our sleep wake cycle. Children who have injuries which prevent these connections from operating appropriately, either sleep too much or conversely they might have terrible problems sleeping at all.
Locus coeruleus - These are a small collection of brain cells located in the pons and they are the major source of noradrenaline in the forebrain. Again it is arguable that the locus coeruleus belong to the reticular system, an area critical for arousal and wakefulness. Locus coeruleus neurons have extremely wide connections and they themselves are connected to by only a few brain stem nuclei and forebrain areas. The activity of these cells varies not only with arousal but also with specific cognitive processes, resulting in concerted release of noradrenaline in multiple target areas, with very complex effects. This key neuromodulatory system is currently thought to be critical for numerous functions including our response to stress. Therefore one might imagine how a child with a dysfunction of these neurons and their connections might experience high anxiety or even show symptoms of ADHD.
The locus coeruleus connects to the entire cortex, the thalamus, limbic structures such as the amygdala and the hippocampus, the globus pallidum and the cerebellum, as well as other brain cells which control the release of dopamine. In studies, activation of the locus coeruleus promotes high levels of vigilance and arousal.
The locus coeruleus also plays a major role in attention and behavioural flexibility. Indeed, when the activity of these cells is blocked, so that they cannot produce noradrenaline in the forebrain, we see problems in the ability to shift attention between different stimuli. Could injury here produce one of the phenomena which we see predominately in children with autism, but also in other neurodevelopmental disabilities, where the child becomes obsessed with certain objects, movements, sounds, tactile experiences, etc and focuses on that stimuli sometimes to the exclusion of all other stimuli?
On the other hand, enhancing noradrenergic function can facilitate shift of attention when the behavioral relevance of stimuli is varied experimentally, with resultant rapid behavioral adaptation. This would be typical of the child with AD(H)D, who could not focus on one particular task / object appropriately.
Cranial Nerves and the pons.
There are many cranial nerves which enter the brain at the level of the pons, they are as follows…
The trigeminal nerve. (Cranial nerve 5). - The trigeminal nerve is one of the main nerves of the face. There is one on each side. It comes through the skull from the brain in front of the ear. It is called trigeminal as it splits into three main branches. Each branch divides into many smaller nerves.
The nerves from the first branch go to your scalp, forehead and around your eye. The nerves from second branch go to the area around your cheek. The nerves from the third branch go to the area around your jaw.
The branches of the trigeminal nerve take sensations of touch and pain to the brain from your face, teeth and mouth. The trigeminal nerve also controls the muscles used in chewing, and the production of saliva and tears. So when we see a child who does not seem to feel sensation in his face, mouth, teeth, or has trouble chewing, produces too much or not enough saliva, or has watery or dry eyes, we have to consider that there may be injury to this nerve or its connections in the pons.
The abducens nucleus (Cranial nerve 6 ). – This nerve and it’s connections with the pons seem to control our ability to move our eyes. damage to the abducens nucleus causes loss of the ability to move the eye which is on the same side of the body as the injury, outward. It can also result in lateral gaze paralysis: loss of the ability to move either eye in the direction of the side with the lesion.
Facial nerve nucleus. (Cranial nerve 7). Lower down in the pons we find the facial nerve. The facial nerve, or cranial nerve (CN) VII, is the nerve of facial expression. So when parents see me gazing into the face of their child for prolonged periods, I am checking out even the most minute fluctuations in facial expression in order to ascertain if there is injury at this point.
Vestibulocochlear nuclei (vestibular nuclei and cochlear nuclei – Cranial nerve 8). - The vestibulocochlear nerve is a sensory nerve that conducts two special senses: hearing (audition) and balance (vestibular). It cannot be coincidental that these two senses travel into the brain together and the fact that so often I see children who have vestibular problems, who also have auditory processing disturbances. This is a common feature of autism and I postulate that the reason why so many children who have autism are so keen on vestibular stimulation is that it calms their auditory processing problems.
The eighth nerve enters the brain stem at the junction of the pons and medulla. The auditory component of the eighth nerve terminates in a sensory nucleus called the cochlear nucleus which is located at the junction of the pons and medulla. The vestibular part of the eight nerve ends in the vestibular nuclear complex located in the floor of the fourth ventricle.
The main connections from the vestibular cells are to the spinal cord (controlling head and body position), to the three, extraocular motor cells which play a part in controlling eye movements, to the thalamus where they are directed to the appropriate area of cortex for processing. They also go to the cerebellum which controls the coordination of postural adjustments.
The lateral vestibular tract starts in the lateral vestibular nucleus and descends the length of the spinal cord on the same side. This pathway helps us walk upright. The medial vestibular tract starts in the medial vestibular nucleus and extends bilaterally through mid-thoracic levels of the spinal cord in the MLF. This tract affects head movements and helps integrate head and eye movements.
Many interconnections are found between the vestibular nuclei and the cerebellum, which coordinate the postural adjustments. So you can see why vestibular stimulation of one form or another is a vital part of all Snowdrop rehabilitation programmes.
I think I have shown that the pons, although only a small structure in the lower brainstem, is of vital importance.