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Diagrammatic section through the neural tube, with the floor plate region
enlarged.Nervous System
Questions Received:
Responses:
Imagine a group of people sitting together, perhaps listening to a lecture. The lecturer may be doing quite a good presentation, but at some point, someone yawns. Seeing this, a companion tries to stifle a yawn, and then several others yawn. The lecturer feels increasingly apprehensive! Yawning, like laughter, can be infectious. As to why this should be, there is little hard evidence, although yawning may be an arousal response that arose early in evolution and whose aim is to improve oxygenation of brain tissue and increase alertness, with a view to enhanced survival. In social groupings, the sight of one member yawning might act as a signal to cause other members to yawn - in some way the yawn is a warning that attention is slipping and the group may be at risk…
Research into the physiology and pathology of yawning appears to be quite limited. However, I have come across a couple of useful references. The first gives a general description of what happens when we yawn:
"Yawning is a response to a wide variety of stimuli, including sleepiness and an awakening from sleep, hunger, boredom, nervousness, and intracranial disease. It is infectious, occurs more frequently in the newborn, and is likely to be more vigorous (especially the accompanying stretch) in early morning than late at night."
"A yawn consists of an expansion of the chest, descent of the diaphragm and larynx, elevation of the ala nasi and soft palate, downward and backward movement of the tongue, abduction of the vocal cords, wide opening of the mouth, contraction of the tensor veli palatini muscle, closing of the eyes, lacrimation, pulling the head backwards, stretching of the upper arms sideward and the lower arms upward, and, less frequently, stretching of the legs. Vasoconstriction in the digits and cardiac acceleration have also been found to accompany the yawn. The whole act occupies 5 to 10 seconds, most of which time is spent in the inspiratory phase."
"Although yawning certainly is an effective way of fully expanding the lungs and increasing the venous return to the heart, its psychic implications are also of great interest. The author finds that merely writing this is a potent stimulus towards yawning, and perhaps the reader is similarly stimulated." (Fenn and Rahn 1964)
A good review of yawning is provided by Askenasy (1989), and the points that follow are derived from this source. Apparently, yawning movements have been observed in fetuses as early as the 15th week after conception, and continue to occur sporadically throughout the remainder of pregnancy. The newborn baby yawns a few minutes after birth. The frequency of yawning gradually diminishes as the baby matures.
Several causes of yawning are discussed in the review:
Psychological - this includes boredom, drowsiness, fatigue, separation, imitation, and autosuggestion
Hormonal - ACTH and MSH
Neurological - various brain pathologies including coma, encephalitis, hypoxia, and neoplasms
Psychiatric - for example schizophrenia, psychosis, depression and withdrawal
General Pathologies - ear disorders, gastric and biliary disorders, and motion sickness
Drug Overdose - for example naloxone, valproate, imipramine, clomipramine, seratonin, and pentobarbital
Drug Withdrawal - from heroin, morphine, lefetamine, methadone, and pentazocin.
References
Askenasy, J.J.M. (1989) Is yawning an arousal defense reflex? The Journal of Psychology, 123(6), 609-621.
Fenn, W.O., and Rahn, H. (1964) Handbook of physiology, Section 3: Respiration, Volume 1. Washington: American Physiological Society. (p 319)
Why do so many sensory and motor pathways in the central nervous system cross from one side to the other?
3rd November 1997; updated 23/11/98 and 23/4/99
The need for crossed pathways is linked with bilateral symmetry - sensory inputs from both sides of the body and motor outputs to muscles and glands on both sides require integration, and this necessitates both crossed and uncrossed sensory and motor pathways. Decussations and commissures - crossed pathways - are evolutionarily ancient and present in all the vertebrate nervous systems so far studied. Even animals that are relatively simple in organisation respond to a stimulus on one side of their body by contracting muscles on the other side in order to move away from potential danger.
A familiar example of the role of crossed pathways is seen in the visual system: information from the nasal half of each retina crosses to the opposite side at the optic chiasma and is combined with information from the lateral half of the retina on that side. In this way all the information coming from one side of the visual field is brought together in the visual cortex of the opposite side of the brain where stereoscopic depth can be perceived. For the system to work effectively, it is necessary then for the visual areas in the two hemispheres to exchange information, much of it passing via the corpus callosum.
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Diagrammatic section through the neural tube, with the floor plate region
enlarged.
| nt = neural tube |
| n = notochord |
The developmental mechanisms producing crossed and uncrossed pathways are the same in many species, from worms and flies to rats and humans (Dickson 1998). In the floor plate of the neural tube, the products of the netrin and roundabout genes influence the path taken by developing axons. The netrins have a relatively long-range effect, causing axons to turn towards the midline and grow across to the other side, while roundabout acts over a shorter range and causes axons to course along their own side of the midline without crossing. More recently, the gene slit has been identified as having a role in the formation of commissures, axon guidance, and the migration of muscle cells (Harris and Holt, 1999). The protein product of slit acts as a repellent to neurons bearing receptors specified by the roundabout gene, and stops them from crossing the midline.
References
Dickson, B. (1998) A Roundabout way of avoiding the midline. Nature, 391, 442-443.
Harris, W.A., and Holt, C.E. (1999) Slit, the midline repellent. Nature, 398, 462-463.
Why do I continuously yawn when in conversation with someone? I am not tired or bored! It gets worse when they talk and I listen.
20th April 1999
Yawning is not associated only with tiredness (Aloe, 1994). It is a brain stem arousal reflex whose primary function is to improve oxygenation levels, particularly for the brain, so that performance levels can be maximised. Yawning is linked with a variety of situations, including social behaviour, and can be a sign of stress - you have probably seen a dog yawn (‘tension yawn’) when it is faced by a situation that it is unsure how to handle. Abnormal yawning is associated with a number of clinical problems, but in your case yawning becomes most noticeable when in conversation with someone, apparently not so much at other times. This tends to rule out abnormal yawning since you would be aware of copious yawning at other times. Thus it could be that your yawning is linked with a raised level of anxiety when in conversation under certain conditions, for example in a formal, social or work situations when you are keen to make a good impression. Incidentally, it has been reported that regular coffee drinkers who have been deprived of caffeine for a few hours also experience an increase in yawning activity, along with other withdrawal symptoms such as fatigue and sleepiness (Phillips-Bute and Lane, 1997). However, your converational yawning is not accompanied by tiredness, so this may not be relevant.
Yawning occurs in many species from reptiles to birds and mammals under different conditions (Argiolas and Melis, 1998). Although the physiology of yawning is not clearly understood, research has revealed that:
Yawning is under the control of a range of neurotransmitters and neuropeptides within the brain: dopamine, excitatory amino acids, acetylcholine, serotonin, nitric oxide, adrenocorticotropic hormone-related peptides, and oxytocin
Opioid peptides inhibit yawning
The paraventricular nucleus of the hypothalamus is involved in the control of yawning
When activated by dopamine, excitatory amino acids and oxytocin, neurons in the paraventricular nucleus facilitate yawning by releasing oxytocin at distant sites in the brain, ie: the hippocampus, pons and/or the medulla oblongata
The activation (and inhibition by opioids) of these oxytocinergic neurons is related to a concomitant increase (and decrease) of paraventricular nitric oxide synthase activity
Nitric oxide is also involved in yawning mediated by acetylcholine, serotonine, and adrenocorticotropic hormone (ACTH) and related peptides
Other neurotransmitters, i.e. gamma-aminobutyric acid (GABA) and noradrenaline, and neuropeptides, i.e. neurotensin and luteinizing hormone-releasing hormone (LH-RH), also influence yawning.
References
Aloe, F. (1994) Yawning. Arq Neuropsiquiatr, 52(2), 273-276 (Jun).
Argiolas, A., and Melis, M.R. (1998) The neuropharmacology of yawning. European Journal of Pharmacology, 343(1), 1-16 (Feb 5).
Phillips-Bute, B.G., Lane, J.D. (1997) Caffeine withdrawal symptoms following brief caffeine deprivation. Physiol Behav, 63(1), 35-39 (Dec 31).
What is righting reflex in experimental mice?
31st March 2000
Complex animals have several automatic righting reflexes that ensure that the head and body are correctly orientated in relation to each other and the environment. Most of these reflexes are integrated in the midbrain and hindbrain.
The main righting reflexes are:
Labyrinthine Righting Reflex - the stimulus is tilting of the head which stimulates the otolithic organs of the inner ear. The response is a compensatory contraction of the neck muscles to keep the head level. This reflex is co-ordinated in the midbrain.
Body on Head Righting Reflex - if the animal is laid on its side, the pressure on that side of the body initiates a reflex righting of the head, even if the labyrinths are not functioning. Again, this reflex is organised in the midbrain.
Body on Body Righting Reflex - if the animal is laid on its side but this time the head is held in the new position, the body alone may undergo righting movements. As before, this response requires midbrain integration.
Neck Righting Reflex - if the head is righted by either of the reflexes described in 1 and 2 above, and the body remains tilted in relation to the head, then some of the neck muscles become stretched as the others contract to move the head. This stretching initiates stretch reflexes in the affected muscles and they contract, righting the thorax in relation to the head. A wave of similar stretch reflexes passes along the body, righting the abdomen and hindquarters. These complex changes are co-ordinated by the hindbrain and spinal cord.
Optic Righting reflex - keeps the head correctly orientated by using visual information, even in the absence of labyrinthine or body stimulation. This reflex involves the visual cortex as well as the brainstem.
Righting reflexes are monitored in experimental mice for a variety of reasons. For example, changes in these reflexes are useful indicators in studies of anaesthesia (Ogli et al, 1994) and the progression of neurological problems in mutant or genetically modified mice (Bose et al, 1998; Crawley, 1999).
References
Bose, P., Fielding, R., Ameis, K.M., and Vacca-Galloway, L.L. (1998) A novel behavioral method to detect motoneuron disease in Wobbler mice aged three to seven days old. Brain Research, 813(2), 334-342 (Dec 7).
Crawley, J.N. (1999) Behavioral phenotyping of transgenic and knockout mice: experimental design and evaluation of general health, sensory functions, motor abilities, and specific behavioral tests. Brain Research, 835(1), 18-26 (Jul 17).
Ogli, K., Tsukamoto, I., Yokono, S., and Nogaya, J. (1994) Anaesthetic mechanism--from molecular to biological aspects. Annals of Academic Medicine Singapore, 23(4), 536-545 (Jul).