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Where are interneurons in Autonomic Nervous System?

Where are interneurons in Autonomic Nervous System?


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According to the following, it implies that preganglionic neurons are interneurons.

Interneuron called preganglionic neuron whose cell body is situated in the "intermediolateral" column of spinal cord( also called 'lateral column').1

From this:

The interneuron (connector neuron) of somatic system has cell body in the dorsal horn and terminates in the ventral horn, while that of ANS has cell body in the intermediolateral horn and terminates in the autonomic ganglia… In the ANS, there are two efferent neuron chains between CNS and the effector organ: first efferent neuron (pre-ganglionic neuron) has its cell body in CNS while the second efferent neuron (post-ganglionic neuron) has its cell body out-side the CNS in the ganglion.

But this source implies quite opposite:

The spinal sympathetic interneurons that most directly influence the activity of sympathetic preganglionic neurons after spinal cord injury…

So which one is correct?


It sort of depends on your definition of interneuron. Many school-level textbooks suggest that any neuron that has neurons both pre and post synaptically are interneurons. Therefore both the preganglionic neuron and the spinal sympathetic interneurons could be considered interneurons. This means that all neurons in the brain are interneurons and vastly oversimplifies the CNS.

I would argue that it's probably more helpful to think of the preganglionic neurons as "projecting" neurons, as they come down the spinal cord. The post-ganglionic neuron is more like a motor neuron.

Edit: I answered a similar question in more detail here: Is the bipolar neuron of the retina considered a sensory neuron?


Where are interneurons in Autonomic Nervous System? - Biology

The central nervous system includes the brain and spinal cord. The brain and spinal cord are protected by bony structures, membranes, and fluid. The brain is held in the cranial cavity of the skull and it consists of the cerebrum, cerebellum, and the brain stem. The nerves involved are cranial nerves and spinal nerves.

Figure 1. The nervous system.

The nervous system has three main functions: sensory input, integration of data and motor output. Sensory input is when the body gathers information or data, by way of neurons, glia and synapses. The nervous system is composed of excitable nerve cells (neurons) and synapses that form between the neurons and connect them to centers throughout the body or to other neurons. These neurons operate on excitation or inhibition, and although nerve cells can vary in size and location, their communication with one another determines their function. These nerves conduct impulses from sensory receptors to the brain and spinal cord. The data is then processed by way of integration of data, which occurs only in the brain. After the brain has processed the information, impulses are then conducted from the brain and spinal cord to muscles and glands, which is called motor output. Glia cells are found within tissues and are not excitable but help with myelination, ionic regulation and extracellular fluid.

The nervous system is comprised of two major parts, or subdivisions, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord. The brain is the body’s “control center.” The CNS has various centers located within it that carry out the sensory, motor and integration of data. These centers can be subdivided to Lower Centers (including the spinal cord and brain stem) and Higher centers communicating with the brain via effectors.

The PNS is a vast network of spinal and cranial nerves that are linked to the brain and the spinal cord. It contains sensory receptors which help in processing changes in the internal and external environment. This information is sent to the CNS via afferent sensory nerves. The PNS is then subdivided into the autonomic nervous system and the somatic nervous system. The autonomic has involuntary control of internal organs, blood vessels, smooth and cardiac muscles. The somatic has voluntary control of skin, bones, joints, and skeletal muscle. The two systems function together, by way of nerves from the PNS entering and becoming part of the CNS, and vice versa.


The central nervous system (CNS) is made up of the brain and spinal cord.

The peripheral nervous system (PNS) includes the nerves that lead into and out of the central nervous system. The peripheral nervous system consists of the autonomic nervous system and the somatic nervous system.

The autonomic nervous system is not consciously controlled. The autonomic nervous system is made up of the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system sets off the “fight-or-flight” reaction. The parasympathetic nervous system has an effect opposite to that of the sympathetic nervous system.

The somatic nervous system is made up of sensory nerves that carry impulses from the body’s sense organs to the central nervous system. This system also consists of motor nerves that transmit commands from the central nervous system to the muscles.


Knowledge

q explain the transmission of a nerve impulse through a neuron, using the following terms:

– resting and action potential

– depolarization and repolarization

– sodium and potassium gates

q relate the structure of a myelinated nerve fibre to the speed of impulse conduction, with reference to myelin sheath, Schwann cell, node of Ranvier, and saltatory transmission q identify the major components of a synapse, including

– presynaptic and postsynaptic membranes

– calcium ions and contractile proteins

– excitatory and inhibitory neurotransmitters (e.g., norepinephrine, acetylcholine – ACh)

q explain the process by which impulses travel across a synapse

q describe how neurotransmitters are broken down in the synaptic cleft

q describe the structure of a reflex arc (receptor, sensory neuron, interneuron, motor neuron, and effector) and relate its structure to how it functions

C12 analyse the functional interrelationships of the divisions of the nervous system

q compare the locations and functions of the central and peripheral nervous systems

q identify and give functions for each of the following parts of the brain:

q explain how the hypothalamus and pituitary gland interact as the neuroendocrine control centre

q differentiate between the functions of the autonomic and somatic nervous systems

q describe the inter-related functions of the sympathetic and parasympathetic divisions of the autonomic nervous system, with reference to

– effect on body functions including heart rate, breathing rate, pupil size, digestion

– overall response (“fight or flight” or relaxed state)

q identify the source gland for adrenalin (adrenal medulla) and explain its role in the “fight or flight” response


Where are interneurons in Autonomic Nervous System? - Biology

The peripheral nervous system (PNS) is the connection between the central nervous system and the rest of the body. The CNS is like the power plant of the nervous system. It creates the signals that control the functions of the body. The PNS is like the wires that go to individual houses. Without those “wires,” the signals produced by the CNS could not control the body (and the CNS would not be able to receive sensory information from the body either).

The PNS can be broken down into the autonomic nervous system, which controls bodily functions without conscious control, and the sensory-somatic nervous system, which transmits sensory information from the skin, muscles, and sensory organs to the CNS and sends motor commands from the CNS to the skeletal muscles.

Autonomic Nervous System

Art Connection

Figure 1. In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a postganglionic neuron of the PNS. The postganglionic neuron, in turn, acts on a target organ. Autonomic responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one another. The sympathetic system activates the “fight or flight” response, while the parasympathetic system activates the “rest and digest” response.

The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control it can continuously monitor the conditions of these different systems and implement changes as needed. Signaling to the target tissue usually involves two synapses: a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ, as illustrated in Figure 1. There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.

Sympathetic Nervous System

The sympathetic nervous system is responsible for the “fight or flight” response that occurs when an animal encounters a dangerous situation. One way to remember this is to think of the surprise a person feels when encountering a snake (“snake” and “sympathetic” both begin with “s”). Examples of functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare an organism’s body for the physical strain required to escape a potentially dangerous situation or to fend off a predator.

Figure 2. The sympathetic and parasympathetic nervous systems often have opposing effects on target organs.

Most preganglionic neurons in the sympathetic nervous system originate in the spinal cord, as illustrated in Figure 2. The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic ganglia (the sympathetic ganglia form a chain that extends alongside the spinal cord). The acetylcholine activates the postganglionic neurons. Postganglionic neurons then release norepinephrine onto target organs. As anyone who has ever felt a rush before a big test, speech, or athletic event can attest, the effects of the sympathetic nervous system are quite pervasive. This is both because one preganglionic neuron synapses on multiple postganglionic neurons, amplifying the effect of the original synapse, and because the adrenal gland also releases norepinephrine (and the closely related hormone epinephrine) into the bloodstream. The physiological effects of this norepinephrine release include dilating the trachea and bronchi (making it easier to breathe), increasing heart rate, and moving blood from the skin to the heart, muscles, and brain making it easier to process information and run). The strength and speed of the sympathetic response helps an organism avoid danger, and scientists have found evidence that it may also increase LTP—allowing the animal to remember the dangerous situation and avoid it in the future.

Parasympathetic Nervous System

While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous system allows an animal to “rest and digest.” One way to remember this is to think that during a restful situation like a picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with “p”). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the bottom) spinal cord, as shown in Figure 2. The axons of the preganglionic neurons release acetylcholine on the postganglionic neurons, which are generally located very near the target organs. Most postganglionic neurons release acetylcholine onto target organs, although some release nitric oxide.

The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated (the common adrenaline dump you feel after a ‘fight-or-flight’ event). Effects of acetylcholine release on target organs include slowing of the heart rate, lowered blood pressure, and stimulation of digestion.

Sensory-Somatic Nervous System

The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Without its sensory-somatic nervous system, a person would be unable to process any information about their environment (what is seen, felt, heard, and so on) and could not control motor movements. Unlike the autonomic nervous system, which has two synapses between the CNS and the target organ, sensory and motor neurons have only one synapse—one ending of the neuron is at the organ and the other directly contacts a CNS neuron. Acetylcholine is the main neurotransmitter released at these synapses.

Humans have 12 cranial nerves, nerves that emerge from or enter the skull (cranium), as opposed to the spinal nerves, which emerge from the vertebral column. Each cranial nerve is accorded a name, which are detailed in Figure 3. Some cranial nerves transmit only sensory information. For example, the olfactory nerve transmits information about smells from the nose to the brainstem. Other cranial nerves transmit almost solely motor information. For example, the oculomotor nerve controls the opening and closing of the eyelid and some eye movements. Other cranial nerves contain a mix of sensory and motor fibers. For example, the glossopharyngeal nerve has a role in both taste (sensory) and swallowing (motor).

Figure 3. The human brain contains 12 cranial nerves that receive sensory input and control motor output for the head and neck.

  1. Olfactory: sensory for smell
  2. Optic: sensory, process visual information
  3. Oculomotor: motor, movement of eyes and smooth muscles controlling pupil and lens
  4. Trochlear: motor, eye movements
  5. Trigeminal: sensory of upper, and mid face and upper jaw motor for muscles of chewing
  6. Abducens: motor, eye movements
  7. Facial: motor for facial expression, tears and salivary glands sensory for taste
  8. Vestibulocochlear: sensory, hearing and equilibrium
  9. Glossopharyngeal: motor for mouth (swallowing) and for regulation of blood pressure sensory for tongue and pharynx and outer ear
  10. Vagus: motor for swallowing, speech, cardivascular and digestive regulation hunger and fullness sensory from visceral organs and taste. Main parasympathetic nerve
  11. Accessory: swallowing, and head, neck, shoulder movement
  12. Hypoglossal: tongue movements

Spinal nerves transmit sensory and motor information between the spinal cord and the rest of the body. Each of the 31 spinal nerves contains both sensory and motor axons. The sensory neuron cell bodies are grouped in structures called dorsal root ganglia and are shown in Figure 4. Each sensory neuron has one projection—with a sensory receptor ending in skin, muscle, or sensory organs—and another that synapses with a neuron in the dorsal spinal cord. Motor neurons have cell bodies in the ventral gray matter of the spinal cord that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the spinal cord but are sometimes directly stimulated by sensory neurons.

Figure 4. Spinal nerves contain both sensory and motor axons. The somas of sensory neurons are located in dorsal root ganglia. The somas of motor neurons are found in the ventral portion of the gray matter of the spinal cord.


Sensory-Somatic Nervous System

The sensory-somatic nervous system transmits sensory information from the body to the brain and motor movements from the brain to the body.

Learning Objectives

Explain the role of the cranial and spinal nerves in the sensory-somatic nervous system

Key Takeaways

Key Points

  • The sensory and motor neurons of the sensory-somatic system have only one synapse between the organ and a neuron of the CNS these synapses utilize acetylcholine to transmit signals across this synapse.
  • The twelve cranial nerves either enter or exit from the skull some transmit only sensory information, some transmit only motor information, and some transmit both.
  • There are 31 spinal nerves that convey both sensory and motor signals between the spinal cord and the rest of the body.

Key Terms

  • cranial nerve: any of the twelve paired nerves that originate from the brainstem instead of the spinal cord
  • spinal nerve: one of 31 pairs of nerves that carry motor, sensory, and autonomic signals between the spinal cord and the body
  • acetylcholine: a neurotransmitter in humans and other animals, which is an ester of acetic acid and choline

Sensory-Somatic Nervous System

The sensory-somatic nervous system is composed of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the central nervous system (CNS). Motor neurons transmit messages about desired movement from the CNS to the muscles, causing them to contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment (what it sees, feels, hears, etc. ) and could not control motor movements. Unlike the autonomic nervous system, which has two synapses between the CNS and the target organ, sensory and motor neurons have only one synapse: one ending of the neuron is at the organ and the other directly contacts a CNS neuron. Acetylcholine is the main neurotransmitter released at these synapses.

Cranial Nerves

Humans have 12 cranial nerves, nerves that emerge from or enter the skull (cranium), as opposed to the spinal nerves, which emerge from the vertebral column. Each cranial nerve has a name. Some cranial nerves transmit only sensory information. For example, the olfactory nerve transmits information about smells from the nose to the brainstem. Other cranial nerves transmit almost solely motor information. The oculomotor nerve controls the opening and closing of the eyelid and some eye movements. Other cranial nerves contain a mix of sensory and motor fibers. For example, the glossopharyngeal nerve has a role in both taste (sensory) and swallowing (motor).

Cranial nerves: The human brain contains 12 cranial nerves that receive sensory input and control motor output for the head and neck.

Spinal Nerves

Spinal nerves transmit sensory and motor information between the spinal cord and the rest of the body. Each of the 31 spinal nerves (in humans) contains both sensory and motor axons. The sensory neuron cell bodies are grouped in structures called dorsal root ganglia. Each sensory neuron has one projection with a sensory receptor ending in skin, muscle, or sensory organs, and another that synapses with a neuron in the dorsal spinal cord. Motor neurons have cell bodies in the ventral gray matter of the spinal cord that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the spinal cord, but are sometimes directly stimulated by sensory neurons.

Spinal nerves: Spinal nerves contain both sensory and motor axons. The cell bodies of sensory neurons are located in dorsal root ganglia. The cell bodies of motor neurons are found in the ventral portion of the gray matter of the spinal cord.


The central nervous system (CNS) is made up of the brain and spinal cord.

The peripheral nervous system (PNS) includes the nerves that lead into and out of the central nervous system. The peripheral nervous system consists of the autonomic nervous system and the somatic nervous system.

The autonomic nervous system is not consciously controlled. The autonomic nervous system is made up of the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system sets off the “fight-or-flight” reaction. The parasympathetic nervous system has an effect opposite to that of the sympathetic nervous system.

The somatic nervous system is made up of sensory nerves that carry impulses from the body’s sense organs to the central nervous system. This system also consists of motor nerves that transmit commands from the central nervous system to the muscles.


The Peripheral Nervous System

The peripheral nervous system (PNS) is the connection between the central nervous system and the rest of the body. The PNS can be broken down into the autonomic nervous system, which controls bodily functions without conscious control, and the sensory-somatic nervous system, which transmits sensory information from the skin, muscles, and sensory organs to the CNS and sends motor commands from the CNS to the muscles.

Figure 16.6.6: In the autonomic nervous system, a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on a target organ. Activation of the sympathetic nervous system causes release of norepinephrine on the target organ. Activation of the parasympathetic nervous system causes release of acetylcholine on the target organ.

The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control it can continuously monitor the conditions of these different systems and implement changes as needed. Signaling to the target tissue usually involves two synapses: a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ (Figure 16.6.6). There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.

The sympathetic nervous system is responsible for the immediate responses an animal makes when it encounters a dangerous situation. One way to remember this is to think of the &ldquofight-or-flight&rdquo response a person feels when encountering a snake (&ldquosnake&rdquo and &ldquosympathetic&rdquo both begin with &ldquos&rdquo). Examples of functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare an organism&rsquos body for the physical strain required to escape a potentially dangerous situation or to fend off a predator.

Figure 16.6.7:The sympathetic and parasympathetic nervous systems often have opposing effects on target organs.

While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous system allows an animal to &ldquorest and digest.&rdquo One way to remember this is to think that during a restful situation like a picnic, the parasympathetic nervous system is in control (&ldquopicnic&rdquo and &ldquoparasympathetic&rdquo both start with &ldquop&rdquo). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the bottom) spinal cord (Figure 16.6.7). The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated including slowing of heart rate, lowered blood pressure, and stimulation of digestion.

The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment (what it sees, feels, hears, and so on) and could not control motor movements. Unlike the autonomic nervous system, which usually has two synapses between the CNS and the target organ, sensory and motor neurons usually have only one synapse&mdashone ending of the neuron is at the organ and the other directly contacts a CNS neuron.


Autonomic Nervous System

Figure (PageIndex<1>): In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a postganglionic neuron of the PNS. The postganglionic neuron, in turn, acts on a target organ. Autonomic responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one another. The sympathetic system activates the &ldquofight or flight&rdquo response, while the parasympathetic system activates the &ldquorest and digest&rdquo response.

Which of the following statements is false?

  1. The parasympathetic pathway is responsible for resting the body, while the sympathetic pathway is responsible for preparing for an emergency.
  2. Most preganglionic neurons in the sympathetic pathway originate in the spinal cord.
  3. Slowing of the heartbeat is a parasympathetic response.
  4. Parasympathetic neurons are responsible for releasing norepinephrine on the target organ, while sympathetic neurons are responsible for releasing acetylcholine.

The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control it can continuously monitor the conditions of these different systems and implement changes as needed. Signaling to the target tissue usually involves two synapses: a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ, as illustrated in Figure (PageIndex<2>). There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.


Nervous System Structure

The nervous system is an enormously complex system for coordinating an animal’s behavior and helping it navigate and react to the outside environment. In the least complex organisms, the nervous system can consist of only a few neurons and no central brain. At the other end of the spectrum, the human brain is capable of complex thought, symbology, and language.

Generally, the nervous system is structured so that inputs from the environment (vision, touch) are sent to the brain from the peripheral nervous system. Here, they are quickly processed and connected to nerves. Then, the brain sends signals to various other parts of the body. These can be somatic signals, which enact voluntary movements. Nerves which transport somatic signals are part of the somatic nervous system. Alternatively, they can be autonomous signals, which act on glands, smooth muscle, and other parts which are generally part of the subconscious. These nerves are part of the autonomic nervous system. The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems.

Together, coordinated responses to almost any situation can be completed. Organisms without a brain typically coordinate actions in a similar manner, though their nerves are more evenly distributed throughout their bodies.


Reflex actions

Of the many kinds of neural activity, there is one simple kind in which a stimulus leads to an immediate action. This is reflex activity. The word reflex (from Latin reflexus, “reflection”) was introduced into biology by a 19th-century English neurologist, Marshall Hall, who fashioned the word because he thought of the muscles as reflecting a stimulus much as a wall reflects a ball thrown against it. By reflex, Hall meant the automatic response of a muscle or several muscles to a stimulus that excites an afferent nerve. The term is now used to describe an action that is an inborn central nervous system activity, not involving consciousness, in which a particular stimulus, by exciting an afferent nerve, produces a stereotyped, immediate response of muscle or gland.

The anatomical pathway of a reflex is called the reflex arc. It consists of an afferent (or sensory) nerve, usually one or more interneurons within the central nervous system, and an efferent (motor, secretory, or secreto-motor) nerve.

Most reflexes have several synapses in the reflex arc. The stretch reflex is exceptional in that, with no interneuron in the arc, it has only one synapse between the afferent nerve fibre and the motor neuron (see below Movement: The regulation of muscular contraction). The flexor reflex, which removes a limb from a noxious stimulus, has a minimum of two interneurons and three synapses.

Probably the best-known reflex is the pupillary light reflex. If a light is flashed near one eye, the pupils of both eyes contract. Light is the stimulus impulses reach the brain via the optic nerve and the response is conveyed to the pupillary musculature by autonomic nerves that supply the eye. Another reflex involving the eye is known as the lacrimal reflex. When something irritates the conjunctiva or cornea of the eye, the lacrimal reflex causes nerve impulses to pass along the fifth cranial nerve (trigeminal) and reach the midbrain. The efferent limb of this reflex arc is autonomic and mainly parasympathetic. These nerve fibres stimulate the lacrimal glands of the orbit, causing the outpouring of tears. Other reflexes of the midbrain and medulla oblongata are the cough and sneeze reflexes. The cough reflex is caused by an irritant in the trachea and the sneeze reflex by one in the nose. In both, the reflex response involves many muscles this includes a temporary lapse of respiration in order to expel the irritant.

The first reflexes develop in the womb. By seven and a half weeks after conception, the first reflex can be observed stimulation around the mouth of the fetus causes the lips to be turned toward the stimulus. By birth, sucking and swallowing reflexes are ready for use. Touching the baby’s lips induces sucking, and touching the back of its throat induces swallowing.

Although the word stereotyped is used in the above definition, this does not mean that the reflex response is invariable and unchangeable. When a stimulus is repeated regularly, two changes occur in the reflex response—sensitization and habituation. Sensitization is an increase in response in general, it occurs during the first 10 to 20 responses. Habituation is a decrease in response it continues until, eventually, the response is extinguished. When the stimulus is irregularly repeated, habituation does not occur or is minimal.

There are also long-term changes in reflexes, which may be seen in experimental spinal cord transections performed on kittens. Repeated stimulation of the skin below the level of the lesion, such as rubbing the same area for 20 minutes every day, causes a change in latency (the interval between the stimulus and the onset of response) of certain reflexes, with diminution and finally extinction of the response. Although this procedure takes several weeks, it shows that, with daily stimulation, one reflex response can be changed into another. Repeated activation of synapses increases their efficiency, causing a lasting change. When this repeated stimulation ceases, synaptic functions regress, and reflex responses return to their original form.

Reflex responses often are rapid neurons that transmit signals about posture, limb position, or touch, for example, can fire signals at speeds of 80–120 metres per second (about 180–270 miles per hour). However, while many reflex responses are said to be rapid and immediate, some reflexes, called recruiting reflexes, can hardly be evoked by a single stimulus. Instead, they require increasing stimulation to induce a response. The reflex contraction of the bladder, for example, requires an increasing amount of urine to stretch the muscle and to obtain muscular contraction.

Reflexes can be altered by impulses from higher levels of the central nervous system. For example, the cough reflex can be suppressed easily, and even the gag reflex (the movements of incipient vomiting resulting from mechanical stimulation of the wall of the pharynx) can be suppressed with training.

The so-called conditioned reflexes are not reflexes at all but complicated acts of learned behaviour. Salivation is one such conditioned reflex it occurs only when a person is conscious of the presence of food or when one imagines food.



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