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I know that the pathway of nerve impulses during a reflex action is called a reflex arc. However, I want to know whether the term reflex arc can be called either a reflex path or simple reflex?
A reflex arc is sometimes called the reflex pathway. While there is no involvement of the brain in the output/response to stimulus, it does receive signals via the spinal cord informing it of the action. In other words, the reflex action is registered in the brain as soon as it occurs.
Our body performs many actions that are not under conscious control. The nervous system of our body detects certain stimuli and performs a response action before we become aware of the stimulus. Reflex action is one example of such responses.
A reflex action is an involuntary response to a stimulus, mostly a pain stimulus, that takes place instantaneously. The action is performed by the components of the peripheral nervous system. Once the action is performed, information is said to the higher areas of the brain so that a person becomes aware of what has happened.
The pathway followed by a nerve impulse during a reflex action is called the reflex arc. In this article, we will discuss the components of the reflex arc, some examples of reflex actions, its importance, and associated clinical conditions.
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Reflex, in biology, an action consisting of comparatively simple segments of behaviour that usually occur as direct and immediate responses to particular stimuli uniquely correlated with them.
Many reflexes of placental mammals appear to be innate. They are hereditary and are a common feature of the species and often of the genus. Reflexes include not only such simple acts as chewing, swallowing, blinking, the knee jerk, and the scratch reflex, but also stepping, standing, and mating. Built up into complex patterns of many coordinated muscular actions, reflexes form the basis of much instinctive behaviour in animals.
Humans also exhibit a variety of innate reflexes, which are involved with the adjustment of the musculature for optimum performance of the distance receptors (i.e., eye and ear), with the orientation of parts of the body in spatial relation to the head, and with the management of the complicated acts involved in ingesting food. Among the innate reflexes involving just the eyes, for example, are: (1) paired shifting of the eyeballs, often combined with turning of the head, to perceive an object in the field of vision (2) contraction of the intraocular muscles to adjust the focus of the retina for the viewing of near or far objects (3) constriction of the pupil to reduce excessive illumination of the retina and (4) blinking due to intense light or touching of the cornea.
In its simplest form, a reflex is viewed as a function of an idealized mechanism called the reflex arc. The primary components of the reflex arc are the sensory-nerve cells (or receptors) that receive stimulation, in turn connecting to other nerve cells that activate muscle cells (or effectors), which perform the reflex action. In most cases, however, the basic physiological mechanism behind a reflex is more complicated than the reflex arc theory would suggest. Additional nerve cells capable of communicating with other parts of the body (beyond the receptor and effector) are present in reflex circuits. As a result of the integrative action of the nervous system in higher organisms, behaviour is more than the simple sum of their reflexes it is a unitary whole that exhibits coordination between many individual reflexes and is characterized by flexibility and adaptability to circumstances. Many automatic, unconditioned reflexes can thus be modified by or adapted to new stimuli, making possible the conditioning of reflex responses. The experiments of the Russian physiologist Ivan Petrovich Pavlov, for example, showed that if an animal salivates at the sight of food while another stimulus, such as the sound of a bell, occurs simultaneously, the sound alone can induce salivation after several trials. The animal’s behaviour is no longer limited by fixed, inherited reflex arcs but can be modified by experience and exposure to an unlimited number of stimuli.
Biology – Reflex and Voluntary Actions
Definition: A reflex action is an involuntary or automatic action in response to impulses initiated by a stimulus. A reflex is something that your body does natural. In other words, reflex action is a rapid or quick response to stimuli which are not consciously controlled by the brain. It is fast, inborn (instinctive) and stereo-typed. A given stimulus always produces the same response.
Characteristics of Reflex Action
- Reflex action is a response without prior thought or planning.
- It is not under the control of the control of the will or it is automatic.
- It is a quick response.
- It is entirely in stereotyped in nature.
- It involves minimum member of nerves cells.
- It terminates at the spinal cord.
Examples of Reflex Actions
- Blinking of the eyes
- Jerking of the kneel
- Secretion of glands
- Beating of the heart
Definition: Voluntary actions are responses to stimuli that are consciously controlled or co-ordinated by the brain. In other words, voluntary actions are actions taken deliberately, involving the exercise of the will or brain.
Characteristics of Voluntary Actions
- It involves prolonged response.
- Its response is controlled by the brain.
- Its response to a particular stimulus may vary.
- It involves many nerves cells.
Example of Voluntary Actions
- Walking etc.
Differences Between Reflex and Voluntary Actions
Action is initiated by muscle receptor.
Action is initiated in the brain.
It is neither automatic nor fast.
Nerve impulses do not reach the brain.
Nerve impulses always reach the brain.
The Reflex Arc
Reflex reactions in humans are controlled by the reflex arc. When the safety of an organism demands a very quick response, the signals may be passed directly from a sensory neuron, via a relay neurone, to a motor neurone for instant, unthinking action. This is a reflex action.
A reflex arc is a neural pathway that controls an action’s reflex, i.e. it is the nerve pathway which makes such a fast, automatic response possible. It does not matter how brainy you are – you will always pull your hand away from a flame without thinking about it. It is an in-built, or innate, behaviour, and we all behave in the same way.
A reflex arc consists of the following parts:
- Sensory cells : These cells receive the stimulus.
- Sensory or afferent neurone : This conducts or transmits nerve impulses from the sensory cells to the spinal cord or brain.
- Intermediate neurone : This conducts nerve impulses from the afferent to the efferent neurone.
- Motor or efferent neurone : This conducts or transmits impulses from the intermediate neurone to the effector.
- Effector (muscle or gland) : This takes action.
A Simple Reflex Arc
Conditioned reflex is a learned response or behaviour after birth. Once they are acquired, they can be performed without thinking about them. Most of our behaviours are conditioned reflexes. Examples are walking, driving, reading, writing, swimming etc. It takes a fairly long time to learn each of these skills. But once they have been mastered, they are performed very fast and efficiently without the individual thinking about them. They become habits.
Behaviour conditioning was first described by a Russian scientist, Ivan Pavlov in 1902 from his experiments on dogs. Naturally, a dog’s mouth becomes wet when it is given food due to reflex action. In one experiment, Pavlov rang a bell just before giving the dog its food. After repeating this several times the dog learnt to associate the bell with food. Hence, it salivated as soon as it heard the bell, i.e., before the food appeared. Pavlov called this response a conditioned reflex.
Role of Conditioned Reflex on Behaviour
- It helps individuals to acquire new skill through the learning of such habits.
- It helps to develop certain behaviour which are not originally shown in the individual.
- Unhealthy behaviour like smoking, drinking and drug taking may become bad habits when acquired through conditioned reflex and may be difficult to stop.
- The principle is used in the training of dogs for their special role in crime detection and for security.
Differences between Reflex Action and Conditioned Reflex
Absent stretch reflexes indicate a lesion in the reflex arc itself. Associated symptoms and signs usually make localization possible:
Peripheral neuropathy is today the most common cause of absent reflexes. The causes include diseases such as diabetes, alcoholism, amyloidosis, uremia vitamin deficiencies such as pellagra, beriberi, pernicious anemia remote cancer toxins including lead, arsenic, isoniazid, vincristine, diphenylhydantoin. Neuropathies can be predominantly sensory, motor, or mixed and therefore can affect any or all components of the reflex arc (see Adams and Asbury, 1970, for a good discussion). Muscle diseases do not produce a disturbance of the stretch reflex unless the muscle is rendered too weak to contract. This occasionally occurs in diseases such as polymyositis and muscular dystrophy.
Hyperactive stretch reflexes are seen when there is interruption of the cortical supply to the lower motor neuron, an "upper motor neuron lesion." The interruption can be anywhere above the segment of the reflex arc. Analysis of associated findings enables localization of the lesion.
The stretch reflexes can provide excellent clues to the level of lesions along the neuraxis. Table 72.1 lists the segmental innervation of the common stretch reflexes. For example, if the biceps and brachioradialis reflexes are normal, the triceps absent, and all lower reflexes (finger jerk, knee jerk, ankle jerk) hyperactive, the lesion would be located at the C6 level, the level of the triceps reflex. The reflex arcs above (biceps, brachioradialis, jaw jerk) are functioning normally, while the lower reflexes give evidence of absence of upper motor neuron innervation.
Segmental Innervation of Stretch Reflexes.
The laterality of reflexes is also helpful. For example, if all the reflexes on the left side of the body are hyperactive and those on the right side are normal, then a lesion is interrupting the corticospinal pathways to that side somewhere above the level of the highest reflex that is hyperactive.
Individual nerve and root lesions can be identified by using information about the reflexes along with sensory and motor findings. Aids to the Investigation of Peripheral Nerve Injuries is a valuable pamphlet to carry in your bag to help in testing and analyzing muscles with respect to their innervation.
Vomiting (Emesis): Definition and Mechanism | Reflex | Human | Biology
In this article we will discuss about:- 1. Definition of Vomiting 2. Events takes Places During Vomiting 3. Mechanism.
Definition of Vomiting:
Vomiting is a reflex which serves to relieve the upper G.I. tract by forcible expulsion of gastric contents through the mouth. This may occur either because the contents are irritating or because the organs themselves or the nerves that supply them are more irritable than normal. Excessive distention and compression or irritation of the intestine, appendix, bile ducts and other abdominal viscera can also initiate this movement. This is a reflex movement (Fig. 9.49).
Events takes Places During Vomiting:
A number of events take place during vomiting, more or less, in the following order:
At the onset a feeling of nausea is experienced, followed by excess salivation.
2. Glottis becomes closed and the nasopharynx is also shut off by raising the soft palate:
The purpose is to prevent the entry of any vomited material into the trachea and nose.
3. The body of the stomach, the cardiac sphincter and the oesophagus relax:
The pylorus contracts and press­es its contents into the relaxed stomach. Weak contraction and antiperistalsis may take place in the stomach but they are not important, because vomiting normally takes place even if stomach is replaced by a blad­der. Pyloric sphincter remains closed.
4. Intra-abdominal pressure sharply rises:
This is caused by retching during which expiratory muscles, abdominal muscles and diaphragm contract. These contractions increase the intra-abdominal pressure so that the relaxed stomach is forcibly compressed. This increased intra-abdominal pressure is the chief motive force of vomiting. Gastric contents are pressed into oesphagus and then from the latter ejected out through the mouth. Antiperistalsis may take place in oesphagus helping ejection.
5. The process continues till the stomach is empty:
Towards the end diaphragm ascends (relaxes) and the expiratory muscles contract. Glottis being closed it raises the intrapulmonary pressure and compress the oesphagus. This helps to expel the last remnants of the vomitus from the oesophagus.
Mechanism of Vomiting:
Vomiting is a reflex process. Straightforward vomiting is governed by a vomiting centre which is situated in the dorsal part of the lateral reticular formation of the medulla lying ventral to the solitary tract and its nucleus.
It forms one of the components of the complex visceral centres which include salivation defaecation and vasomotor centre and vestibular nuclei. So there is consistent relationship of vomiting with salivation defae­cation, respiration and vasomotion. This centre can be directly stimulated (central vomiting) by certain drugs (apomorphine, etc.), certain toxins (such as those of uraemia), increased intracranial pressure (brain tumour, asphyxia, meningitis, etc.) and such others. It can be reflexly stimulated in various ways.
There are two pathways by which the vomiting centre is affected:
The nervous path lies in the various afferent pathways coming from various organs especially digestive tract. The most sensitive part is in the first part of duodenum. The afferent impulses may also arise in the throat (tickling—sensory nerves V and IX), stomach (irritation), intestine or other organs outside the gastro­intestinal tract like heart, kidney, uterus or semicircular canals.
Chemical substances (emetics) pass via body fluids and act on chemosensitive area called chemoreceptor trigger zone in the floor of IV th ventricle and cause vomiting. Destruction of this area results loss of response when emetics placed directly on the receptor site in chemoreceptor trigger zone.
The efferent impulses—both excitatory and inhibitory, are carried in the phrenic chiefly and vagus and the sympathetic.
The commonest cause of vomiting is gastric irritation and its purpose is to drive out the irritant from the stomach.
The central vomiting caused by stimulation of central nervous system (e.g., trauma and tumour in brain and irritation in meninges etc.) is projectile type having little feeling of nausea and absence of participation of voluntary muscles.
Persistent Type of Pernicious Vomiting of Pregnancy:
In this type the excitability of the centre is in­creased by metabolic disturbances (e.g., carbohy­drate starvation and dehydration with ketosis).
Loss of water, and both H + and Na + ions due to excessive vomiting might be harmful.
Emetics are substances that cause vomiting (such as drinking of sodium chloride and warm water).
Levels of Consciousness and Attention
PUPILLARY RESPONSES TO LIGHT
The parasympathetic reflex arc begins in the retina, traverses the base of the brain, runs through the midbrain, and returns to the pupil (see Chapters 8 and 9 Chapter 8 Chapter 9 ). Disorders altering pupillary constriction typically affect the midbrain or cranial nerve III. Compression of the superior colliculus (e.g., by a pineal region mass) interferes with input to the pretectal nuclei, resulting in pupils that are large (because the sympathetic system is not affected), unreactive to light, and sometimes displaying hippus. Lesions affecting the area of the Edinger‐Westphal nucleus and the origins of cranial nerve III are the most important because this area is adjacent to the superior pole of the midbrain reticular formation. Because the descending sympathetic efferent fibers also traverse this portion of the brain stem, dysfunction produces pupils that are midposition (4 to 6 mm in diameter), unreactive to light, and frequently slightly irregular. Such pupils are an ominous finding, usually indicating that coma is due to structural damage affecting the upper midbrain, and unless its etiology can be reversed quickly, the patient's coma is usually irreversible. Because the pupillary constrictor has a muscarinic, rather than a nicotinic, acetylcholine receptor, it is not affected by drugs given to block neuromuscular transmission. However, it is affected by systemic antimuscarinic drugs (e.g., atropine), so one must be cautious about interpreting the examination if such agents are being used.
Unilateral loss of pupillary constriction in the comatose patient may rarely indicate subarachnoid hemorrhage from an internal carotid aneurysm that compresses cranial nerve III at the origin of the posterior communicating artery (Video 32, Cranial Nerve III Palsy). Much more commonly, such a finding indicates the presence of a mass lesion that has shifted the diencephalon laterally. Although older studies suggested that this finding arose from compression of the third cranial nerve by the herniating temporal lobe, the unilaterally dilated pupil appears to develop before actual movement of the medial temporal structures over the tentorial edge. Ropper's work demonstrates that unilateral pupillary dilation results from traction on cranial nerve III produced when the diencephalon, being pushed away from an expanding lateral mass, pulls the midbrain with it. Because cranial nerve III is tethered anteriorly at the cavernous sinus, the nerve ipsilateral to the mass is subjected to stretching and the pupil dilates. Early in the course of this process, therapies that decrease the degree of shift (e.g., administration of mannitol) can reverse the pupillary dilation.
The sympathetic pathways begin in the hypothalamus, descend through the brain stem and spinal cord to the first thoracic level, and then exit the central nervous system to traverse the face and reach the pupil. Most sedative drugs produce bilateral small pupils by antagonizing sympathetic outflow at the hypothalamic level other agents, such as opiates, appear to have an additional effect of stimulating the parasympathetic system, resulting in very small (pinpoint) pupils. Lesions affecting the sympathetic system below the midbrain do not directly affect consciousness.
What is a reflex pathway? - Biology
A reflex action, also known as a reflex , is an involuntary and nearly instantaneous movement in response to a stimulus and does not require any thought input
When a person accidentally touches a hot object, they automatically jerk their hand away without thinking. The path taken by the nerve impulses from the stimulus to sensory neuron to motor neuron to reflex muscle movement in a reflex is called a reflex arc .
There are two types of reflex arcs: the autonomic reflex arc , affecting inner organs, and the somatic reflex arc , affecting muscles. When a reflex arc consists of only two neurons, one sensory neuron, and one motor neuron, it is defined as monosynaptic . Monosynaptic refers to the presence of a direct single synapse. No interneuron is present. By contrast, in polysynaptic reflex arcs , one or more interneurons connect afferent (sensory) and efferent (motor) signals. It causes the stimulation of sensory, association, and motor neurons.
The spinal cord provides the synapse for the reflex arc, even though the reflex arc is bypassing the brain, the brain is still aware of the occurrence.
The feedback loop is a situation when the response impacts or influences the stimulus it is of two main types namely, positive feedback and negative feedback the reflex arc is usually negative feedback.
- Positive feedback loops are unstable systems , in which a change in a given direction causes an additional change in the same direction. For example, uterine contractions lead to oxytocin release, causing more contractions.
- Negative feedback loops are stable systems, in which a change in a given direction causes a change in the opposite direction. For example, a drop in blood pressure causes an increase in the antidiuretic hormone, which increases blood pressure.
MCAT Official Prep (AAMC)
Online Flashcards Biology Question 9
Biology Question Pack, Vol. 1 Question 39
• Reflexes, or reflex actions, are involuntary, almost instantaneous movements in response to a specific stimulus.
• Reflex arcs that contain only two neurons, a sensory and a motor neuron, are considered monosynaptic.
• Monosynaptic refers to the presence of a direct single synapse no interneuron is present.
• Most reflex arcs are polysynaptic, meaning multiple interneurons (also called relay neurons) interface between the sensory and motor neurons in the reflex pathway.
• The spinal cord provides the synapse for the reflex arc, even though the reflex arc is bypassing the brain, the brain is still aware of the occurrence.
• The supraspinal circuit involves input from the brain or brainstem to process stimuli.
motor neuron : a neuron located in the central nervous system that projects its axon outside the CNS and directly or indirectly controls muscles
sensory neuron : a neuron responsible for converting various external stimuli that come from the environment into corresponding internal stimuli
reflex arc : a neural pathway that controls an action reflex
autonomic reflexes: are unconscious motor reflexes relayed from the organs and glands to the CNS
somatic reflexes: one of the two types of reflex arcs, and specifically involves the skeletal muscles. They are unlearned muscle reflexes that are mediated by the brainstem
interneurons: connect neurons within specific regions of the central nervous system
Mechanism of Salivary Secretion| Digestive System | Human | Biology
In this article we will discuss about:- 1. Nerve Supply of Salivary Glands 2. Mechanical Effects of Salivary Secretion 3. Observations 4. Rate of Flow and Composition 5. Adaptability 6. Disturbances.
Nerve Supply of Salivary Glands:
The salivary centre consists of superior and inferior salivary nuclei in the reticular formation of the medulla.
The salivary glands receive double nerve supply—both from the sympathetic and the parasympathetic. The parasympathetic fibres to the sub-maxillary (submandibular) and sublingual glands arise from the superior salivary nucleus (dorsal nucleus of the VII th cranial nerve) in the medulla as nervus intermedins and by-passing the geniculate ganglion descend downwards through the facial (VII th cranial) nerve and then through the chorda tympanic branch of the facial nerve.
The chorda tympanic nerve descends downwards and reaching the cavity of the mouth meets the lingual nerve. Then the secretory fibres leave the lingual nerve and end in the sub-maxillary (submandibular) ganglion (Langley’s ganglion in animals). From the sub-maxillary ganglion the postganglionic fibres arise and reach the sub-maxillary and sublingual glands and supply them with secretory and dilator fibres.
The parasympathetic or bulbar fibres to the parotid gland arise from the inferior salivary nucleus (dorsal nucleus of IX th nerve) in the medulla and descend downwards through the glossopharyngeal (IX th ) nerve and being separated as the tympanic branch pass through the tympan­ic plexus and then through the lesser superficial petrosal nerve end ultimately in the otic ganglion. From this the postganglionic fibres arise and reach the parotid gland through the auriculotemporal branch of the fifth nerve to supply it with secretory and dilator fibres.
The sympathetic fibres to all these glands is derived from first and second thoracic segments of the spinal cord and come out through the first three or four anterior thoracic nerve roots and end in the superior cervical ganglion.
The postganglionic fibres arise from this ganglion, pass along the walls of the arteries and supply all the salivary glands (Fig. 9.28). The sympathetic fibres are believed to end in the serous gland or in the serous part of the mixed gland and supply vasoconstrictor fibres to vessels of glands and myoepithelial cells of the duct.
On stimulation of the parasympathetic nerves in a cat the following effects are observed- (a) secretion of water (b) vasodilatation. Parasympathetic nerve fibres act through the medium of acetylcholine and so they are known as cholinergic fibres. Hilton and Lewis have found that after stimulation of the parasympathetic fibres an enzyme (kallikrein) is liberated in the tissue fluid which acts on the proteins and form a polypeptide known as bradykinin which produces vasodilatation.
On stimulation of the sympathetic nerves the following effects are observed:
(a) Secretion of viscous saliva with a higher solid content, and
Sympathetic nerve fibres act through the medium of adrenaline and adrenaline-like substance and so they are known as adrenergic fibres.
Atropine blocks the action of acetylcholine, and has been used in medicine to inhibit salivary secretion. A diet rich in carbohydrate increases the salivary amylase. Of the endocrine secretions, adrenocorticotrophic hormone lowers the sodium concentration of saliva.
Significance of Double Nerve Supply:
Each glandular cell is supplied by two sets of nerves. Probably one helps in the secretion of fluid and salts, and the other for the secretion of organic constituents.
Some holds that the differences in action of these two sets of nerves are not due to their specific effect on the glandular cells, but are due to their different actions on the blood vessels. The sympathetic carries vasoconstrictor fibres hence their stimulation will cause vasoconstriction in the gland and produce consequently less amount of saliva which becomes necessarily thick. Parasympathetic fibres will cause vasodilatation thus increasing the amount of saliva, which becomes necessarily thin.
Claude Bernard observed that after section of the chorda tympanic nerve in a dog or cat, a scanty secretion of thin turbid saliva is produced which increases until the seventh or eighth day, at which it reaches a peak level, and diminishes about the third week. He called it as paralytic secretion.
His presumption was that section of chorda tympani removed the restraining influence on secretion and as a result there was continuous secretion of saliva. Emmelin in 1952 explained that paralytic secretion was due to increased sensitivity of the gland to adrenaline section of the chorda tympanic nerve.
For studying the mechanism of salivation, it is nec­essary to adopt certain experimental procedures, by which pure saliva unmixed with food can be collected outside.
This has been done in two ways:
(1) A cannula is inserted into the parotid duct and through this all the saliva secreted by the gland is collected outside.
(2) The opening of the duct is re­sected out and is shifted upon the outer surface of the cheek. Saliva can be collected outside through the opening (Fig. 9.29).
With such preparations it is seen that when food is given to the dog salivation takes place, but when the corresponding nerves are cut salivation completely ceases. This proves that salivation is a purely reflex phenomenon. There is no direct chemical stimulus involved in it.
On further analysis, it is found that two types of reflexes are involved in salivation:
(1) Conditioned or acquired reflex, and
(2) Unconditioned or inherent or inborn reflex.
It is believed that one type of reflex does not exclude the other both are called into play together under ordinary condition.
The existence of this reflex is proved by the fact that even the sight or smell of food can stimulate salivation, although no food is actually given. Various conditioned stimuli can be established which can produce salivation. Pavlov used to sound a gong just before giving food to the animal. After continuing this procedure for some days, it was seen that only the gong sound was sufficient to cause salivation even when no food was given. The gong sound here acts as the conditioned stimulus.
For this reflex, food should actually be given to the dog.
The sensory stimulus for this reflex may arise from various sources as follows:
This is the chief place from which the normal unconditioned stimulus for salivation arises. The act of chewing, the sensation of taste, the irritation caused by the presence of food upon the mucous membrane of mouth—all these act as the sensory stimuli which reflexly produce salivation (Fig. 9.30).
Here the effector is the salivary gland, the afferent path is represented in the trunks of the chorda tympani, the pharyngeal branches of the vagus and glossopharyngeal nerves, and the lingual, buccal and the palatine branches of the trigeminal nerve, the efferent path is the secretory fibres of chorda tympanic nerve with another peripheral relay station and its centre is the medulla.
ii. Oesophago-Salivary Reflex:
The sensory stimulus may arise from the oesophagus. When the food passes down the oesophagus, salivation is stimulated to some extent. Pathological conditions of oesophagus, such as ulcer, cancer, or the presence of a foreign body in the oesophagus, stimulates salivation. If the distal end of cut oesophagus is stimulated, salivation occurs. If the vagi are divided, the reflex is abolished.
The purpose of this reflex seems:
(a) To provide enough saliva necessary to wash away the irritating substance, and
(b) Swallowing of saliva will set up peristalsis like movement of oesophagus which is likely to drive on the irritant. Oesophageal movement cannot be initiated by mechanical irritation but only when something is swallowed.
(iii) Gastro-Salivary Reflex:
The stimulus may arise from the stomach. Irritation of stomach stimulates saliva­tion. When food is introduced in the stomach of a sleeping dog (to avoid psychic effects), salivation takes place after about 20 minutes. This is also seen in many irritating conditions of stomach, for instance, gas­tritis, gastric cancer, etc. Increased salivation, before vomiting, is a typical example.
(iv) From other Viscera:
It is possible that stimulus for salivation may arise in other viscera also. For instance, in pregnancy increased salivation occurs. It is believed that the sensory stimulus arises from the distended uterus.
Mechanical Effects of Salivary Secretion:
As the food is chewed, the contractions of the muscles of mastication help to press out the saliva accumulated in the ducts and acini of the glands. Hence, mastication acts not as a real stimulus but through its mechanical effect.
Observations to Prove that Salivation is a Secretory Process:
Although in the saliva, there are certain products of excretion, such as the thiocyanates, urea, etc., yet, the following observations prove that salivation is mainly a secretory phenomenon:
i. Saliva is Extremely Useful:
Saliva is extremely useful, hence, cannot be an excretory product.
During salivation the glands are found to be actively working. It is only to manufacture a secretion that a gland needs to undergo work. Excretory processes do not involve much work.
The following facts prove that the glands are actively working:
a. During salivation the glands increase in size, become vascular and their temperature increase.
b. The hydrostatic pressure in the salivary duct, during active secretion, may be double the amount of blood pressure in the carotid artery. Had it been a process of filtration, fluid ought to have passed from saliva into blood. But actually the reverse process takes place. This shows that the glands are working against pressure.
c. The osmotic pressure of blood is higher than that of saliva so that fluid ought to have been drawn out of saliva and passed into the blood stream. But since salivation is just the reverse process, the glands must be working against osmotic pressure.
d. During salivation the amount of oxygen used and CO2 produced by the glands, increase, i.e., the R.Q. value of resting gland is 0.6 to 0.8 increased to 1.0. This increase did not occur in the absence of glucose. Thus source of energy for salivary metabolism is glucose and to some extent fructose. This also is a very important evidence of work.
e. Saliva contains certain substances, which is not present in blood, viz., and ptyalin. Obviously, such things must have been manufactured in the glands. This is a sure proof of secretory activity of the gland.
iii. Histological Changes:
During activity a number of histological changes are seen in the gland. One import­ant change is that, the zymogen and mucinogen granules which are present in the resting glandular cells reduce to a much smaller number during activity. They take up water from the cytoplasm in the process of secretion and are released from the cells due to differences in osmotic pressure.
iv. Electrical Changes:
Change of electric potential takes place in the gland during secretion. The outer surface of the gland becomes electrically positive to the hilus. All these evidences prove that salivation cannot be a process of excretion it is chiefly a secretory phe­nomenon.
Reflex Control of Rate of Flow and Composition of Saliva:
The receptor-centre-efferent system has got discriminating power so it can govern the salivary secretion, i.e., rate of flow and composition depend on the nature and intensity of the stimulus (e.g., food).
The continuous secretion of saliva without any known stimulus is termed as spontaneous secretion. Although its mechanism is not known but the acetylcholine may be the factor which is constantly secreted by the parasympa­thetic postganglionic nerve endings in small amount. Since atropine cannot check and cyanide or other metabolic poisons stop this type of secretion, so it is indicated that this is related and dependent to metabolic functions.
Adaptability of Salivary Reflex:
The saliva secreted from the gland varies in both quantity and quality with the physical and chemical nature of the substances stimulating the secretion. The salivary gland does not secrete as a unit but different sets 6f epithelial cells of the gland contribute different components of secretion and their local productivity depends upon the intensity of excitation coming from the salivary centre.
The afferent nerves are also different groups which carry impulse of specific nature and stimulate the different components of salivary centre which is a compound structure consisting of several parts and these in turn excite reflexly and selectively the different epithelial groups for appropriate types of secretion.
Disturbances of Salivary Secretion:
The salivary secretion may be under certain conditions:
i. When decrease or absent called hyposalivation.
ii. When increase called hypersalivation.
Emotional state, e.g., anxiety, fear, fever and obstruction of the duct due to calculi (sialolithiasis).
Aptyalism is rare but when occurs is due to congenital hypoplasia or absence of the gland.
Also called sialorrhoea occurs during:
ii. Neoplasm of the mouth, tongue, carious tooth, oesophagus, stomach and pancreas,
Nursing, Allied Health, and Interprofessional Team Interventions
The nurse has a crucial role in assisting the clinician before the test by preparing the patient. During the test, she assists the clinician by holding the light source and helps the clinician observe the changes in the opposite pupil. After the test, the nurse alerts the clinician, if any untoward changes in the condition of the patient. The nurse confirms that the patient is fully informed about his care plan and gives the patient all the necessary contact details.