What is the mechanism of reflex arcs?

What is the mechanism of reflex arcs?

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When a reflex arc occurs the signal from the receptor passes straight to the motor neuron instead of being passed onto the brain.

This is a rather simplistic explanation, I was hoping to make it more detailed. How does the body know when a threshold, heat for example, has been crossed and what mechanism is able to activate the reflex?

Also where does this occur, in the receptor or the spine?

Withdrawal reflex

You asked specifically about the withdrawal reflex and the receptors that trigger this. The initiation of the reflex arc is determined at the level of the nociceptors (pain-transducing receptors) in the epidermis. For the most part, these are part of “free” (not encapsulated) nerve endings of sensory fibers. These fibers course within spinal nerves whose cell bodies are located in the dorsal root ganglia adjacent to the spinal cord. There are two types of nociceptive endings:

  • mechanical: with parent fibers Aδ
    • transduce severe mechanical deformation
  • polymodal: with parent C-fiber units
    • transduce heat, cold, irritant signals

This picture shows free nerve endings in the epidermis. The boxed portion of the big picture is expanded in the bottom right.

The withdrawal reflex initiated by these receptors is mediated by a polysnaptic (multiple neurons) arc. The sensory neuron synapses with interneurons in the spinal cord.1 Some of these use glutamatergic connections to activate appropriate muscle groups mediating withdrawal. Others inhibit the reciprocal muscle groups (reciprocal inhibition) to allow this movement to be unimpeded.

Here is a cartoon diagram showing the spinal components of the reflex arc in a cross-sectional level of the spinal cord. Notice the interneuron between the sensory and motor neurons (although it's not labeled as such here):

Stretch reflex

In order to provide a broader picture of the basic principles involved in spinal reflexes, it's helpful to also mention the stretch reflex. This is perhaps the quintessential reflex arc because it is monosynaptic, exemplified by the patellar tendon reflex. The stretch reflex is fast, with a latency (stimulus-response interval) of about 15-25 ms (contrast the polysynaptic withdrawal reflex with latency ~70-100 ms.)

The stretch is initiated at the Golgi tendon organs. These are found at the junction between muscle and tendon. The (Ib) nerve fiber splays out and intertwines with tendon fiber bundles. The nerve endings are activated by the tension of muscle contraction (either passive (as when the patellar tendon is tapped), or active (as during voluntary movement). The afferent sensory neurons directly excite homonymous motor neurons (i.e. motor neurons supplying the same muscles), and inhibit (via interposed “Ia internuncials”) the antagonist muscles.

I have limited the discussion here to the withdrawal reflex you asked about and another basic spinal reflex arc as an illustration of principle. There are many other, mostly more complex reflex arcs that are beyond the scope of this answer, but I highly recommend checking out a neuroanatomy or neurophysiology textbook such as the one I've listed below if you'd like more information.

1. Although the discussion here is limited to the anatomy of the reflex arc, there are additional synapses within the spinal cord that connect with ascending circuits, as described in the section about afferent neurons of spinal nerves in another answer. Some of these tracts end up at the cortex and are the basis for conscious perception. However, note that this process is multisynaptic and takes longer. The beauty of the spinal reflex arc is that it does not require information to travel all the way up to the brain and back down, which would slow down the response considerably. Practically, this means that you will pull your hand away from a hot stove before you have conscious sensory awareness of the burn.

Apart from linked article and images with sources below, all information is summarized from:
MJ Turlough FitzGerald, Gregory Gruener, Estomih Mtui. Clinical Neuroanatomy & Neuroscience © 2012, Elsevier Limited.

Image 1 from:

Image 2 from:

What is reflex mechanism?

Read complete answer here. Similarly one may ask, what is mechanism of reflex action?

Reflex action is a form of animal behaviour in which the stimulation of a sensory organ (receptors) results in the activity of some organ without the intervention of will. The sensory nerve fibres bring sensory impulses from the receptor organ to the central nervous system.

Also, what is reflex arc and how does it work? A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain.

Keeping this in view, what is reflex action and examples?

A few examples of reflex action are: When light acts as a stimulus, the pupil of the eye changes in size. Sudden jerky withdrawal of hand or leg when pricked by a pin. Coughing or sneezing, because of irritants in the nasal passages.

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. reflexive actionThe mechanism of reflexive action of the 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.

Nervous Control:

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.

Paralytic Secretion:

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.

Conditioned Reflex:

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.

Unconditioned Reflex:

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).

Spontaneous Secretion:

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.

1. Hyposalivation:

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.

2. Hypersalivation:

Also called sialorrhoea occurs during:

ii. Neoplasm of the mouth, tongue, carious tooth, oesophagus, stomach and pancreas,

Reflex action | Definition, Types and Mechanism and Important solved questions

A reflex action may be defined as a spontaneous, automatic and mechanical response to a stimulus acting on a specific receptor without the will of an animal.

Reflex action meaning

Reflex means Rapid, involuntary Motor Response to Stimulus

Reflex actions means something that you do without thinking, as a reaction to a situation.

What is an example of a reflex action ?

One example of reflex actions in man are knee-jerk reflex, movement of diaphragm during respiration, blinking of eyes, coughing, yawning, sneezing etc. In knee-jerk reflex, a gentle strike below the knee cap, while sitting with freely hanging legs, kicks the leg forward. Another very good example is afforded by the withdrawal of the leg of a decapitated frog*, also called a “spinal frog”, when touched with an acid or a live electric wire, Here the action of the frog does not at all involve its will as it is without a brain.

They are performed in the presence of the brain also, e.g., closing of the eyes if strong light is suddenly flashed on them or some object suddenly comes too near them, watering of mouth on seeing delicious food. These reflex actions, though performed without our will, are in our knowledge. There are many reflex actions which on without our knowledge, e.g., flow of bile from the gall- bladder into the duodenum when the food reaches there, peristalsis of the alimentary canal, beating of the heart, etc. These and other physiological actions are controlled by simple reflexes of autonomic nervous system.

Types of Actions

Animals show two types of actions: voluntary and involuntary. A voluntary action is performed by the animal with its will. In this action, the animal exercises its choice, so that the same stimulus may depending upon the situation. For example, seeing a snake in the way, one may run away, or call for help, or try to kill it to save oneself. An involuntary action, on the other hand, is performed by the animal without its will. It is very quick and the animal has no choice in it. Therefore, the same stimulus always gets the same response just as pressing a particular button of a machine brings into action a definite part of it. For example, the hand or foot is withdrawn every time it is suddenly pinched or pricked with a needle, or touched by a hot object. The involuntary actions are known as the reflex actions.

Mechanism of Reflex Action

A reflex action is brought about in the following way. When acid is applied to a toe of a decapitated frog, the stimulus is received by a receptor in the skin. Receptor is a general term for any type of sense organ. On receiving a stimulus, the receptor sets up a sensory impulse. The latter is carried to the spinal cord through the dorsal sensory root of a spinal nerve, i.e., sciatic nerve, in the above example. The spinal cord transforms the sensory impulse into a motor impulse. The latter is transmitted to the leg muscles. The muscles then contract and the leg is withdrawn to avoid the stimulus. The muscles are referred to as the effectors, where the impulse ends and response is given.

Reflex Arc

The path travelled by an impulse in a reflex action is called the reflex arc . A reflex arc starts from the sensory neuron which is affected by the stimulus, and ends with the last neuron that synapses with the affector including all the neurons in between.

Reflex arc is a type of emergency mechanism in your nervous system.

For Example – Suppose you touch a Cactus plant, by mistake. To keep your body safe, your arm has to be moved away from the point of contact with the cactus plant. That can be done when the muscles get the instructions for moving your hand away.

Now, your brain is already busy with controlling all the other metabolic pathways of your body. So, if the required signals for moving your hand has to come from your brain, it will take a long time and your finger will feel hurt or pain . To avoid that, the signals have to be generated from some other source, which is your spinal cord.

So what happens is, once the receptors in your finger tips get the ‘thorns in cactus plant ’ information, the afferent nerves will carry the signals to your spinal cord in the form of electric impulses. There, the information will be processed and the required electric impulses will be generated. Now, the efferent nerves will carry those impulses from your spinal cord to the effectors in your arm muscles. These impulses will then move your hand away from the cactus plant, by contracting and relaxing your muscles.

This whole process of carrying the impulses to your spinal cord, processing the impulses and carrying generated impulses to your muscles is called the Reflex Arc

What are the components of reflex arc ?

There are five components of reflex arc which are affecting inner organs and muscles.

(i) A specific receptor, the neurons of which receive a stimulus and set up a sensory impulse .

(ii) An afferent nerve, which brings the sensory impulse from the receptor to the central nervous system.

(iii) A portion of the central nervous system, brain or spinal cord, the neurons of which analyse and interpret the sensory impulse and set up an appropriate motor impulse. Brain and spinal cord are called modulators .

(iv) An efferent nerve, which carries the motor impulse from the central nervous system to the specific effectors (muscle fibres or gland cells.

(v) An effector, where impulse terminates and response is given as per instructions received from the modulator.

There may be connector (intermediate, relaying) neurons between the sensory and motor neurons.

A reflex pathway results in rapid responses to stimuli because it has a small number of synapses. In addition, the message need not make a lengthy trip to brain and back to give an appropriate response.

A nerve impulse can flow only in a single direction in a reflex arc (afferent to efferent neuron), because the nerve impulse can cross a synapse in one way only. Therefore, stimulating an effector or an efferent neuron cannot produce a reflex: response in a receptor.

Repeated stimulation of a receptor may temporarily suspend the reflex response because the synapses in the reflex arc are fatigued.

Why reflex action is important ?

Reflex action is very important. It has two advantages

(i) It enables the animal to respond immediately to the harmful stimuli so that no harm is caused to it.

(ii) It relieves the brain of too much work as the responses of routine nature take the form of reflex actions. If the animal were to exercise its will every time a wave of peristalsis started in the intestine, the brain would soon be exhausted.

Important Solved Questions for Exams

2. What is Reflex action ?
Ans. Reflex action is a fast, sudden, involuntary, unplanned, sequence of action that occur in response to a particular stimulus.

3. Which organ or part controls reflex action ?
Ans. The reflex actions controlled by spinal cord and brain are respectively called spinal reflex actions and cerebral reflex actions.

4. Which action is a reflex action ?
Ans. Involuntary actions, this actions is something that we cannot control. We do not tell our body to do it. Involuntary actions include breathing, blinking, our heart beating . In other words say this is the extremely quick, automatic, sudden action in response to something in the environment.

5. What happens during reflex action ?
Ans. Reflex action is a sudden response to changes in a enviroment without any thinking of feeling in control of a reaction . Example – I pulled my hand back from the flame suddenly.


A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. When a person accidentally touches a hot object, they automatically jerk their hand away without thinking. A reflex does not require any thought input.

The path taken by the nerve impulses in a reflex is called a reflex arc. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs.

Most reflex arcs involve only three neurons. The stimulus, such as a needle stick, stimulates the pain receptors of the skin, which initiate an impulse in a sensory neuron. This travels to the spinal cord where it passes, by means of a synapse, to a connecting neuron called the relay neuron situated in the spinal cord.

The relay neuron in turn makes a synapse with one or more motor neurons that transmit the impulse to the muscles of the limb causing them to contract and pull away from the sharp object. Reflexes do not require involvement of the brain, although in some cases the brain can prevent reflex action.

Reflex arc: The path taken by the nerve impulses in a reflex is called a reflex arc. This is shown here in response to a pin in the paw of an animal, but it is equally adaptable to any situation and animal (including humans).

Reflex Response

A reflex is a rapid, involuntary response to a stimulus.


A reflex arc is the pathway traveled by the nerve impulses during a reflex. Most reflexes are spinal reflexes with pathways that traverse only the spinal cord.

During a spinal reflex, information may be transmitted to the brain, but it is the spinal cord, and not the brain, that is responsible for the integration of sensory information and a response transmitted to motor neurons.

A reflex arc involves the following components:

The receptor is the part of the neuron (usually a dendrite) that detects a stimulus.

The sensory neuron transmits the impulse to the spinal cord.

The integration center involves one synapse (monosynaptic reflex arc) or two or more synapses (polysynaptic reflex arc) in the gray matter of the spinal cord.

A motor neuron transmits a nerve impulse from the spinal cord to a peripheral region.

An effector is a muscle or gland that receives the impulse form the motor neuron.

In somatic reflexes, the effector is skeletal muscle.

In autonomic (visceral) reflexes, the effector is smooth or cardiac muscle, or a gland.

An example of this that often used to check the status of a head injured person is:
The constriction of pupils in response to bright light is called the pupillary light reflex. If the light is shining directly into one eye, then the pupil in that eye will constrict (a direct response), but so will the pupil in the non-illuminated eye (a consensual response).

This reflex involves two cranial nerves: the optic nerve, which senses the light, and the oculomotor nerve, which constricts both pupils.

Explain the reflex arc

the reflex arc is a protective mechanism for organisms that protects them from immediate danger. Let us use the example of a hand pulling away from a boiling kettle. the Pain receptors in the skin sense the pain and send this as an electrical signal down the sensory neurone the sensory neurone takes the signal towards the spinal chord where it crosses a synapse (this is a gap between 2 neurones) and arrives at the relay neurone the relay neurone, which is located inside the spinal chord, passes the signal across a synapse to the motor neurone the motor neurone passes the signal to the muscle causing it to contract and pull the hand out of the danger of the kettle. the muscle is the effector. it Effects the change. effectors are either muscles or glands. when looking at a reflex arc always first try and find the receptor and the effect and then work out the path between the 2.

Two-Pore-Domain (K2P) Potassium Channels: Leak Conductance Regulators of Excitability☆

Sensing the Environment

Oxygen-sensing and respiration

Molecular oxygen sensors mediate rapid cardiovascular and metabolic responses to environmental hypoxia such as increase in respiration rate and vasoconstriction. Located in highly specialized cells, including carotid body cells, pulmonary neuroepithelial body cells, fetal adrenomedullary chromaffin cells and pulmonary artery smooth muscle cells, these specialized cells share the same general mechanism which transduces decreased oxygen tension into cellular responses. Thus, closing of oxygen-sensitive potassium channels depolarizes oxygen-sensitive cells leading to increased excitability via opening of voltage-gated Ca 2 + channels, elevation of intracellular Ca 2 + level and cell type specific cellular responses.

Among a variety of potassium channels suppressed by hypoxia in peripheral chemoreceptors, there is thus far good evidence that one member of the K2P family, K2P3.1, is involved in an oxygen-sensing pathway. Contrary to earlier reports, data from several laboratories suggests K2P2.1 channels are not oxygen-sensitive under physiological conditions. The carotid body located above the bifurcation of the common carotid artery is the major peripheral site for oxygen sensation. K2P3.1 channels are expressed in carotid body type one cells and a native oxygen-sensitive channel in the cells shares biophysical and pharmacological properties of cloned K2P3.1, most notably inhibition by hypoxia. This suggests that the response of these cells to hypoxia is at least partially due to inhibition of K2P3.1 leading to membrane depolarization and reflex increase in respiration. Hypoxia-induced closing of native K2P3.1-like channels is thought to be mediated by cytosolic second messengers rather than direct oxygen-sensing by the channel since channels in excised membrane patches are not subject to regulation by oxygen tension.

K2P3.1 channels are also expressed in central respiratory-related neurons, including airway motoneurons and putative chemoreceptor neurons of locus coeruleus. In these central neurons, hypoxia-induced inhibition of K2P3.1-like currents increases excitability to enhance respiratory motoneuron output. Thus, modulation of K2P3.1 currents appears to contribute at both the central and peripheral levels to the respiratory reflex.

Temperature sensation

The ability to detect changes in temperature is fundamental for the survival of warm-blooded animals. Mammals sense temperature through primary afferent sensory neurons of dorsal root and trigeminal ganglia. Central to thermal regulation is control of body heat homeostasis by the hypothalamus. The mammalian sensory system is capable of detecting and discriminating thermal stimuli ranging from noxious cold (< 8°C) to noxious heat (> 52°C). This broad spectrum suggests the existence of temperature sensors with distinct thermal sensitivities. One group of temperature sensors is the TRP cation channel family that has at least six temperature-sensitive members: four activated by heat (TRPV1-4, Q10

10–25) and two responsive to cold (TRPM8, TRPA1). In addition, an epithelial sodium channel (ENaC) and a potassium leak conductance have been implicated in regulation of body temperature.

K2P channels appear to be involved indirectly in temperature-sensing pathways. K2P2.1, K2P4.1 and K2P10.1 show changes in activity with temperature in the physiological range of 24°C to 42°C, the cold to moderate warmth domain. The channels are more active with increasing temperature in both experimental systems and rat cerebellar granule cells and dorsal root ganglion neurons. Moreover, K2P2.1 currents in rat ventricular cardiomyocytes show similar temperature response. In contrast to TRP channels where the temperature sensor is inherent to the channel (and coupled to the voltage-dependence of channel activation), K2P temperature responses appear to be via activity of temperature-sensitive secondary messenger pathways that act on the channels allowing for moderate sensitivity (Q10

These observations show that K2P2.1, K2P4.1 and K2P10.1 channels are active at physiological body temperature. Their role in temperature sensation is supported by expression of mRNA in dorsal root ganglion cells with TRPM8, TRV1, TRV3, and TRV4. Here, K2P channels might serve as negative feedback regulators, suppressing heat-induced excitation produced by TRP channels. This is supported by the observation that K2P2.1 knockout mice are hypersensitive to thermal pain. In turn, prostaglandin E2, a sensitizer of peripheral and central thermoreceptors, inhibits K2P2.1 opening via cAMP-dependent phosphorylation of the channel. However, reduced temperature would be expected to inhibit K2P channels in cold sensing neurons (promoting excitability) and inhibition of K2P2.1 or K2P4.1 did not mimic native cold responses in isolated rat trigeminal neurons and K2P2.1 knockout mice showed wild type cold responses.

Sour taste

The ability to identify food that is nutrient-rich and avoid toxic substances is essential for survival. The gustatory system acts as a final checkpoint control for food acceptance or rejection. Taste receptor cells located in taste buds respond to gustatory stimuli using a complex system of ion channels, exchangers, receptors and intracellular signaling cascades. Taste transduction typically utilizes two or more pathways in parallel. When stimulated, taste cells produce action potentials that result in the release of neurotransmitters onto an afferent nerve fiber that in turn relays the identity and intensity of the gustatory stimulus to the brain. The sense of a sour taste is mediated by protons that appear to influence ion channels and transporter activity from both the intra- and extracellular sides of the membrane. The degree of sourness is a function of proton concentration. The amplitude of a proton-sensitive potassium leak current in rat taste receptor cells shows strong correlation with the resting membrane potential, suggesting that this conductance may be the major determinant of excitability in these cells. Acid-sensitive K2P channels are intriguing candidates for sour taste recognition in taste receptor cells because they share the important functional features with the native leak channels in taste bud cells of being voltage-independent and potassium-selective. Moreover, K2P channel expression has been detected in taste buds in rat (K2P3.1, K2P5.1, and K2P16.1) and in mouse (K2P1.1, K2P2.1, K2P3.1, K2P 5.1, K2P6.1, and K2P10.1).

What is the mechanism of reflex arcs? - Biology

A reflex arc is the neural pathway that mediates a reflex action. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by activating spinal.
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reflex arc n. Physiology. The neural path of a reflex. . In a three neuron reflex arc list the direction and the stages of the nerve .
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reflex ( ) adj. Bent, turned, or thrown back reflected. Physiology. Being an . In a three neuron reflex arc list the direction and the stages of the nerve .
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The Reflex Arc. How a Stimulus Elicits a Response. A. Knee-Jerk . called a simple reflex arc. Follow the sensory neuron. from the spindle (receptor) to .
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A reflex arc refers to the neural pathway that a nerve impulse follows. . It is the simplest reflex arc and the integration center is the synapse itself. .
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Reflex Arc. Organisation of the. Nervous System . In a simple reflex arc, such as the knee jerk, a stimulus is detected by a .
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A reflex arc is the neural pathway that mediates a reflex action. . When a reflex arc consists of only two neurons (one sensory neuron and one motor .
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Definition of Reflex-arc in the Medical Dictionary. Reflex-arc explanation. . reflex arc. Etymology: L, reflectere, to bend back, arcus, bow . reflex arc .
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Reflex arc - Wikipedia, the free encyclopedia . Example: Somatic Reflex Arc. A somatic reflex arc is one in which there is the simplest possible arrangement .
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5. Reflex arc - Be able to label the kinds of neurons and their parts, in the . The correct order of a reflex arc is: sensory receptor --> sensory neuron .
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Reflexes | World of Anatomy and Physiology. Reflexes summary with 3 pages of encyclopedia . The combined function of these neurons creates a reflex arc. .
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Encyclopedia article about reflex arc. Information about reflex arc in the Columbia Encyclopedia, Computer Desktop Encyclopedia, computing dictionary. reflex arcs
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The pathway that a reflex follows (reflex arc) does not directly involve the brain. . Reflex Arc: A No-Brainer . The plantar reflex may help doctors .
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. reflex) Jump to: navigation, search. For other uses, see Reflex . involving humans, reflex actions are mediated via the reflex arc this is not .
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Psychology research, information, analysis and reviews from Psychology Journals, Newspapers, and magazines at Free Trial, Credit Card Required.
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Decreased reflexes should lead to suspicion that the reflex arc has been affected. . may also decrease reflexes by damaging the afferent limb of the reflex arc. .
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Chemical Regulation

Chemical regulation in respiration is controlled to some extent by the concentration of respiratory gases in the blood. The respiratory centre is very sensitive to C02 concentration. It Increases if tension is slight breathing becomes deep and fast permitting more CO2 to leave the blood. Similarly, O2 concentration in the blood affects the breathing rate but in opposite direction.

Regulation of the respiratory system must be coordinated with the homeostatic mechanism for temperature control, metabolic control, water and salt balance and circulatory system activities.

If you are interested in biology also read this notes – Cell Theory Easy Notes