Somatosensory Systems


The Somatosensory System
From the moment we awaken in the morning until we fall asleep at night, our bodies are bombarded with information from the outside world. Most of us awake to the sound of an alarm clock that is detected by our auditory system. Our eyes focus on the clock to send information to the visual system and confirm that it is time to get up. We then climb out of bed in the morning into the warmth of a hot shower that is sensed by nerve endings of the somatosensory system. At the breakfast table the smell of toast and coffee comes wafting to our noses and olfactory system, and the flavor of a rich cup of coffee stimulate the taste buds of our gustatory system. Throughout the day we are continually exposed to information from the external environment that must reach the brain so that we can respond accordingly. The detection and transmission of this information are the functions of our sensory systems.


Figure Legend.  Summary of sense organs.


Information about our external and internal environments are detected by sensory receptor cells located within various sense organs.  Information about the extenal environment is transduced by extero receptors while information about our internal environment is transduced by interoreceptors. Sensory information from exteroreceptors is transmitted to the neocortex by different pathways depending on the type of sensation. Somatic sensation from the body and limbs, for example, is transmitted from receptor cells to the spinal cord, where it then ascends to the higher levels of the nervous system.  Auditory information detected by the ears, on the other hand, is transmitted from sensory receptor cells in the inner ear and transmitted to the brain stem, where it ascends to eventually reach the neocortex.  In general, a sensory receptor will respond only to a specific type of stimulus.  After detecting the stimulus, it transduces this information into electrical impulses that encode the stimulus intensity.  In this section we will consider the mechanisms by which sensory receptors on the surface of the body detect and transduce information about touch, pain, and temperature.






General Mechanisms of Sensory Transduction
 



 
Figure Legend.  Transduction of a stimulus to an action potential.
Sensory receptors are activated when they detect a specific stimulus. This specific stimulus is called an adequate stimulus and is unique to each sensory receptor. In the visual system, for example, the photoreceptors detect colors of light but are insensitive to frequencies of sound. Likewise, the auditory receptors in the ear respond only to sound and are insensitive to light.



 
Figure Legend.
  Transduction of pressure sensation by a Pacinian corpuscle receptor to action potentials. Upper panel – cross section of a Pacinian corpuscle and axon. Middle panel – axon membrane potential without pressure.  Lower panel – axon membranepotential with pressure.


An adequate stimulus will produce a change in the membrane potential of the sensory receptor cell.This change in the membrane potential is called a generator or receptor potential. In some sensory receptors, such as somatic sensory receptors, the generator potential is a depolarization of the membrane. In others, such as the photoreceptors of the eye, it is a hyperpolarization of the membrane. The generator potential, in turn, produces an action potential or a series of action potentials. These action potentials can be generated by the receptor cell itself or by a neuron connected to the receptor cell. They transmit information about the nature of the stimulus to the central nervous system.


The transduction mechanisms of all sensory systems, therefore, involve (1) an adequate stimulus, (2) a generator potential, and (3) the initiation of action potentials. In order to examine these mechanisms in greater detail, let us consider a somatic sensory receptor called a Pacinian corpuscle that responds to touch, or tactile, stimuli. This type of receptor is located beneath the skin and consists of a free nerve ending encapsulated by layers of connective tissue.  The nerve ending is wrapped with myelin along the length of the fiber. The fiber itself extends into the spinal cord, where it forms synapses with other nerve cells.



The adequate stimulus for the Pacinian corpuscle is pressure applied to the skin. This pressure causes the layers of connective tissue and free nerve endings to compress.  The compression of the free nerve ending causes an opening of Na+ channels embedded within the nerve membrane. The resulting influx of Na+ ions, in turn, depolarizes the membrane and produces a generator potential. If the generator potential is of sufficient amplitude, it will then depolarize the membrane in the region of the first node of Ranvier to threshold and initiate an action potential.
 

Figure Legend.  Axon membrane potential changes with pressure applied to the Pacinian corpuscle.




















 

Figure Legend.  Frequency coding of stimulus intensity.  Increasing pressure produces increases in the frequency of action potential generation.



The intensity of a stimulus is encoded by the frequency of action potentials

The number of action potentials generated per unit time is a function of the intensity of the pressure. Greater pressures produce a greater number of action potentials in a single fiber. This is known as the frequency code of stimulus intensity.  The increase in the number of action potentials generated with greater pressure is due to the continued opening of the Na+ channels and continued depolarization of the generator potential above the threshold value. As a consequence, after the repolarization of one action potential, another is generated.



Figure Legend.  Population code for stimulus intensity.  Increasing pressure recruits additional sensory receptors.


A second method of informing the brain about the intensity of the stimulus is a population code.With this method of coding, more sensory receptors of the Pacinian corpuscle type are activated as the pressure becomes greater. This occurs because the increased pressure affects a greater area beneath the skin.










The quality of a stimulus is encoded by the pathway of transmission

The type of information sent to the brain is also coded in the way that nerve fiber pathways leading to the brain are arranged physically. The skin, for example, contains sensory receptors for
 

Figure Legend.  Labeled-line coding of stimulus quality. Different types of stimuli are transmitted along different pathways.


temperature in addition to pressure. The information about the temperature of the skin reaches the brain by a different nerve fiber pathway than does information about pressure. In this way, information about the quality, or type, of stimulus is maintained within each pathway without becoming mixed with other types of stimuli. This aspect of the nervous system, the mechanisms of coding for the type of stimulus detected by a sensory receptor, is called the labeled-line code of stimulus quality.









Sensory Adaptation 

Often, when a stimulus is continuously applied, the brain at some point no longer consciously perceives it. This adjustment happens, for example, with background noise such as the ticking of a clock. After a period of time it is unnoticed. This phenomenon is called sensory adaptation. The adaptation to a sensory stimulus can be caused by mechanisms either within the brain or at the receptor site. Adaptation mechanisms that work at receptor sites are the most clearly understood and can be illustrated by the Pacinian corpuscle.


Figure Legend. Sensory adaptation to stimulus intensity.


When pressure applied to the Pacinian corpuscle is continuously maintained, the free nerve ending eventually reverts back to its original shape even though the layers of connective tissue remained deformed. Since the Na+ channels embedded within the nerve membrane are opened only when the nerve fiber is deformed, they close when the form of the nerve is again circular. Consequently, even though pressure on the Pacinian corpuscle is still maintained, it ceases to produce a generator potential when it resumes its normal shape and thus no longer transmits information about the pressure of the stimulus.



Mechanisms of adaptation are also found at the molecular level.  During sunny days, our eyes quickly adapt to the brightness of the outdoors by the "bleaching" of the photoreceptors. The underlying basis of this bleaching process is the removal of molecular receptors that capture light.


Types of Somatic Sensory Receptors 

The major types of somatic sensory receptors are (1) tactile receptors or mechanoreceptors activated by the mechanical stimulation of the body's surface, (2) thermal receptors activated by changes in temperature on the surface of the body, (3) pain receptors or nociceptors activated by noxious (harmful) stimuli, and (4) proprioceptive receptors activated by the movement of the limbs. Each type of sensation is detected by a variety of sensory receptors.


Tactile Receptors for Touch, Pressure, and Vibration Sense.

 

Figure Legend.  Types of somatic sensory receptors.

 

Tactile receptors are called mechanoreceptors. These receptors are responsible for detecting touch, pressure, and vibrations applied to the skin.  We have already examined the functional properties of one type of pressure-sensing mechanoreceptor, the Pacinian corpuscle. In addition to pressure, however, the Pacinian corpuscle is capable of detecting vibrations. The mechanisms that allow it to detect vibrations are extensions of its ability to rapidly adapt to applied pressures.

Recall that when pressure is applied to the Pacinian corpuscle for a length of time, it quickly stops generating action potentials. As we discussed earlier, this occurs when the Pacinian nerve ending reverts back to its original shape.  As the applied pressure is released, the layers of connective tissue surrounding the nerve ending spring back to their original ellipsoidal form and then become compressed again in the opposite direction because of the elastic nature of the connective tissue.  This recompression deforms the nerve ending and again causes Na+ channels to open and produce a generator potential and action potentials. In this way, both the application and release of pressure result in the discharge of action potentials. This oscillation of pressure is the basis of a vibratory stimulus. It should be clear from this example that the Pacinian corpuscles must adapt very quickly. This ability to quickly adapt to a stimulus classifies these corpuscles as rapidly adapting receptors.

Another rapidly adapting tactile mechanoreceptor is the hair receptor. The hair receptor is a hair follicle on the surface of the skin that has a nerve fiber wrapped around its base. The bending of the hair cell such as that caused by a slight breeze produces a mechanical displacement of the nerve fiber at the base of the follicle. This displacement, in turn, opens Na+ channels, resulting in a generator potential and the initiation of action potentials. This type of receptor is very sensitive to a gentle touch across the surface of the skin.

 

Another group of tactile receptors, called slowly adapting tactile mechanoreceptors, continue to generate action potentials as long as the stimulus is applied. An example of a slowly adapting mechanoreceptor is the Ruffini corpuscle or bulbous corpuscle. This receptor is sensitive to stretching of the skin such as that produced during a massage.

Thermal Receptors for Detection of Heat and Cold. 

Sensory receptors that respond to temperature are called thermal receptors. There are two types of thermal receptors: one type responds to temperatures below 30°C, while the other type responds to temperatures above 30°C. These thermoreceptors are capable of adapting to the external environment. When exposed to cold weather, for example, the cold thermoreceptors do not become activated until temperatures lower than 30°C are reached, while warm temperatures become activated well before 30°C. 

Nociceptors for Detection Painful Touch and ExtremeTemperatures. 

Nociceptors are free nerve endings that detect painful and potentially damaging stimuli.  This information is sent to the spinal cord and brain where it is processed through a process called nociception so that the body can respond to minimize damage.  In general, there are several types of nociceptors: mechanical nociceptors activated by intense mechanical stimulation such as a knife cut on the arm or a slap to the head.  Heat nociceptors respond to temperatures above 45°C.  Chemical nociceptors respond to irritants.    

Proprioceptors for Detection of Position and Movement                   

Information processing about the relative position of the body's limbs is called proprioception and is detected by so-called proprioceptors. One type of proprioceptor detects the stationary position of the limbs in space with respect to the other parts of the body. Other proprioceptors transmit dynamic information about limb movement to convey the sense of movement, or kinesthesia. The brain needs this information to determine where the arms and legs are located in order to calculate how much further they need to go to complete a certain movement.

 The operation of these receptors can be tested by trying to bring the tips of your left and right index fingers together with your eyes closed. If your proprioceptors are working properly, your fingers should touch without your having to watch and visually guide them together.


The sense of stationary or static position is transmitted to the brain by mechanoreceptors located in joint capsules, cutaneous mechanoreceptors, and mechanoreceptors in muscles that are specialized to transduce the stretching of the muscle.  Extremes of joint angles are sensed by joint capsules, while intermediate angles are transduced by the muscle spindle receptor.  The functional properties of these receptors and muscle tension receptors will be discussed in detail later, in conjunction with the motor system

 

Figure Legend.  Action potential responses by static and dynamic proprioreceptors.



The static proprioceptors produce a continuous frequency of action potentials in response to different joint positions. If the joint is left in one particular position, the receptor will generate action potentials at one specific frequency.  This type of action potential response is called a tonic discharge. The dynamic proprioceptor generates action potentials only with a change in direction of movement. The burst of action potentials produced is very brief. This type of action potential response is called a phasic discharge.

The information from somatic sensory receptors is transmitted to the central nervous system by several different pathways organized according to three general principles: (1) Each type of somatic sensation has its own pathway; (2) most pathways cross over from one side of the brain to the other; (3) the nerve cells within each nucleus are topographically organized according to the location of their sensory receptors on the surface of the body. As we shall see, these three principles also apply to the organization of other sensory systems.




Anatomical Organization of Somatosensory  Nerve Cells


Figure Legend.  Dorsal root ganglion of the somatosensory system.

Each of the somatic sensory receptors previously described is associated with a peripheral nerve axon. The cell bodies of these sensory receptors are located within a cluster of cells immediately outside the spinal cord called the dorsal root ganglion.  The fibers with the largest diameters are those associated withtouch, pressure, and vibration sense. 



These are heavily myelinated fibers that range from 13 to 22 µm in diameter and have action potential conduction velocities of 70 to 120 meters per second. These fibers release the neurotransmitter glutamate at their terminal endings. The fibers with the smallest diameters are the lightly myelinated and unmyelinated fibers that convey pain and temperature information.  These fibers range from 1 to 5 µm in diameter and have action potential conduction velocities of 2 to 15 meters per second.These fibers release the neuropeptide substance P at their terminal endings.

Two major pathways transmit somatic sensory information from the surface of the body to the neocortex: the dorsal column pathway and the anterolateral pathway. These pathways sequentially transmit sensory information to nerve cell nuclei in the spinal cord, midbrain, thalamus, and neocortex. In order to understand how these pathways operate, we first need to consider the functional organization of the areas of the nervous system that are involved in the processing of somatic sensory information.


 

Figure Legend.  The dorsal column pathway and anterolateral pathway for transmission of sensation.

Spinal Cord Organization of Somatosensory Function



The spinal cord is shaped like a tube that runs from the base of the head to the small of the back. This longitudinal orientation is called the rostro-caudal axis (rostral meaning "toward the nose" and caudal meaning "toward the tail" in Latin). The spinal cord is also oriented so that one part of the tube is facing the back and the other is facing the stomach. This transverse orientation is called the dorso-ventral axis (dorsal meaning "toward the back" and ventral meaning "toward the belly" in Latin).

Four other terms also used quite often to describe the location of nuclei and fiber tracts in the brain are: medial meaning "close to the midline" of the body; lateral meaning "to the side" of the body; anterior meaning "toward the front" of the body; and posterior meaning "toward the back" of the body.






 

Figure Legend.  Anatomy of the spinal cord and vertebra.

A transverse section of the spinal cord shows that it has a central gray area and a peripheral region that is white. The gray matter is composed of nerve cells of the spinal cord, whereas the white matter is composed of axons. The myelination of axons is what makes the peripheral area appear white. The gray region has four protrusions shaped like butterfly wings that are symmetrical about the midline. The dorsal protrusions, called the dorsal horns, contain nerve cells involved in the processing of sensory information. The ventral protrusions are called the ventral horns and contain nerve cells that innervate skeletal muscles and are thus involved in motor function.




 

Figure Legend.  The dorsal column pathway and the anterolaral pathway.

Nerve fibers that carry sensory information in the spinal cord follow one of the two major pathways previously mentioned: the dorsal column pathway or the anterolateral pathway. Nerve fibers carrying information about touch and pressure traverse the dorsal column pathway, whereas those carrying information about pain and temperature traverse the anterolateral pathway. When pain and temperature fibers enter the spinal cord, they cross over to the midline; consequently, the anterolateral pathway transmits information from the opposite (contralateral) side of the body. In contrast, fibers of the dorsal column pathway do not cross over until they reach the medulla.





Somatosensory Organization of Spinal Cord Segments 


 

Figure Legend.  Spinal cord segment dermatomes.

The spinal cord is subdivided into 31 segments.  Each spinal cord segment receives sensory information from particular areas of the skin called dermatomes. These dermatomes contain receptors for all of the somatic sensations. The loss of sensory information by certain dermatomes can thus be used as a clue to the location of damage to the spinal cord.  In the uppermost region of the cord are 8 cervical segments (C1 to C8). These segments receive sensory information from the back of the head, neck, shoulders, part of the arms, and hands. Next are 12 thoracic segments (T1 to T12), which receive sensory inputs from parts of the arms and hands and trunk of the body. Following the thoracic segments are 5 lumbar segments (L1 to L5), which receive sensory inputs from the waist, thighs, upper and lower legs, and parts of the feet. The remaining segments of the spinal cord are five sacral segments (S1 to S5) and one sacro coccygeal segment. These receive sensory input from the back of the legs, buttocks, and anus.

 

Figure Legend.  The dorsal column nucleus and thalamus are relay stations for transmitting sensation.

Sensory fibers of the dorsal column system enter the spinal cord and ascend toward the brain in the dorsal portion of the spinal cord.  These fibers continue to the medulla oblongata, where they form synapses with neurons in the dorsal column nuclei. Fibers from the dorsal column nuclei, in turn, cross the midline and become part of the medial lemniscal pathway. These fibers reach the thalamus on the opposite side of the brain. Neurons in the thalamus, in turn, extend their fibers to the cortex.

















Effects of Lesions of the Spinal Cord on Somatosensory Function

 

Figure Legend.  Loss of function with spinal cord lesion.


In accidents that damage the spinal cord, the loss of cutaneous sensation appears in patterns that are characteristic of the level of the lesion. For example, an accident that severs half of the spinal cord on the right side of the body at the waist causes a loss of pain and temperature sensation on the left leg and a loss of touch and vibration sense on the right leg.

 

This pattern occurs because tactile information is transmitted in the spinal cord along the same side of the body that receives information about touch and pressure. Pain and temperature information, on the other hand, crosses the midline once it enters the spinal cord. Obviously, accidents that transect all or half of the spinal cord will also result in some form of paralysis. This loss of motor control after spinal cord lesions will be considered in later sections of the textbook.


The Thalamus in Somatosensory Function 

The thalamus is one of the brain areas that receives most types of sensory information. It receives not only somatic information, but also visual, auditory, and taste information. The region of the thalamus that receives somatic sensory information is the ventral posterior lateral (VPL) nucleus. Neurons in the VPL of the thalamus then transmit sensory information to the neocortex.

The Somatic Sensory Cortex

 

Figure Legend.  The somatic sensory region of the neocortex.

The neocortex is the part of the brain that evolved most recently (neo meaning new). It is divided into discrete areas that receive somatic, visual, auditory, and gustatory sensations as well as areas for the control of movement and other functions.  The area of the neocortex that receives somatic sensations is called the somatic sensory cortex.









 

Figure Legend.  The map of the body in the somatic sensory cortex.

 

A cross-section of the somatic sensory cortex shows that it is organized like a map of the body.  Sensory information from the legs is sent to the medial portion of the cortex. Adjacent but more lateral regions of the cortex receive sensory information from the torso. The most lateral regions of the cortex receive sensory information from the arms, hands, and face.

This type of topographical organization of sensory information from various regions of the body on to the sensory cortex is called a somatotopic organization. The map that is created in the somatosensory cortex is a distortion of the true proportions of the body. The regions of the body that have the greatest density of sensory receptors are represented by a greater proportion of the cortex. The areas in the cortex representing the thumb and fingers of the hand, for instance, occupy substantially more space than that represented by the torso.

Receptive Fields of Neurons in the Somatic Sensory System 

 

Figure Legend.  The receptive field for dorsal root ganglion cells.


Stimuli detected by a sensory receptor are transmitted to various groups of nerve cells. The most effective location of a stimulus, however, differs for each cell within a cluster. For instance, pressure applied to an area of the skin directly above a Pacinian corpuscle will activate the associated dorsal root ganglion cell.

If pressure is applied to another location on the surface of the skin, another dorsal root ganglion cell is activated; however, the first dorsal root ganglion cell is not affected by pressure applied to this second location on the skin.  Thus, only a small area on the receptive surface of the skin can activate a nerve cell in the dorsal root ganglion.  This small area of skin represents the receptive field for that cell. The receptive field can be described as a circular area on the skin where pressure is applied to excite the cell. We will denote the ability to excite the cell with a "+," and the receptive field area of the skin by a circle as shown in the adjacent figure.


Nerve cells in the dorsal column nuclei, the next cluster of cells along the dorsal column pathway, have different receptive fields than those in the dorsal root ganglia. Dorsal column nerve cells generate a low rate of spontaneous action potentials. This is their tonic baseline activity. When pressure is applied to an area of the skin, a dorsal column nerve cell (cell #1 in figure below) is excited to produce a higher frequency of action potential discharge than its baseline rate of activity. However, when the pressure is applied to a different location, the frequency of the action potentials is decreased to less than the baseline rate.

 

Figure Legend.  The receptive field for dorsal column nerve cells.




This inhibition, "-", of the dorsal column nerve cell, is a result of activating a second Pacinian corpuscle, which in turn activates a different nerve cell (cell #2) within the dorsal column. The activation of this second dorsal column nerve cell leads to the activation of an inhibitory interneuron. This inhibitory neuron, in turn, prevents the excitation of the first dorsal column neuron. There are also other areas of the skin that, when pressed, will inhibit the response of dorsal column nerve cell #1. When the excitatory and inhibitory areas on the skin are mapped for nerve cell #1, an annulus or doughnut-shaped receptive field is revealed with an excitatory center and an inhibitory ring as shown in panel d in the figure above.

 

This annular receptive field is also found for nerve cells in the thalamus and certain areas of the somatosensory cortex. The cells in the somatosensory cortex that exhibit the annular receptive field are all located in area 3 of the primary somatosensory cortex. This is a narrow strip of cortical tissue that is also called Brodmann area 3 (named for the neuroanatomical studies by K. Brodmann in 1909). In Brodmann areas 1 and 2, the receptive fields are no longer annular. The stimulus that most effectively activates nerve cells in areas 1 and 2 is pressure applied to the skin in the form of a rectangle rather than a ring. The effectiveness of a rectangular stimulus is the result of nerve cells in area 3 that have overlapping receptive fields.

 

Figure Legend.  Changes in receptive field properties from the dorsal root ganglion to the cortex.





 

Figure Legend.  Convergence of receptive field properties.


The output fibers of these cells converge upon a nerve cell on area 1. Consequently, the simultaneous activation of all three input cells is needed in order to excite the one cell in area 1. This excitation occurs by the process of spatial summation.


Although sensory information of a particular type of stimulus is transmitted from area 3 to 1, and area 1 to 2, in any one Brodmann area of the somatosensory cortex, information from one type of somatic sensation tends to predominate. The nerve cells in Brodmann area 1, for example, respond to rapidly adapting cutaneous receptors; those in area 2 respond to deep pressure; those in area 3a respond to stretch receptors in skeletal muscle; and those in area 3b respond to rapidly and slowly adapting cutaneous receptors. In this way the somatic sensory cortex has several somatotopic maps of the body.



 

Figure Legend.  The vertical columns of neurons in the somatic sensory cortex are functional units of sensation.


The most effective shapes of stimuli needed to activate nerve cells in area 2 can be quite complex. In essence, the physical features of an object in the environment are first broken down into its fundamental components by the peripheral sensory receptors. This information is then assembled to reconstruct the holistic features of the object that are capable of activating nerve cells in higher centers of the brain. In our discussion of the other sensory system, we will see that a similar synthesis and reconstruction of the external environment takes place for vision and hearing.

 

The somatic sensory cortex is also organized so that all of the cells that respond to one type of sensation are located together within vertical columns. Each nerve cell within a vertical column of the somatic sensory cortex, for example, is activated by Pacinian corpuscles in one fingertip.  The location of the receptive fields for cells within a vertical column is also similar. Thus, the vertical columns in neocortex represent basic units of sensation-specific and location-specific function.










The Anterolateral Pain and Temperature Pathway 


Figure Legend.  Neurons in the substantia gelatinosa and the thalamus are relay stations for the transmission of pain sensation.


As do the nerve fibers that transmit tactile sensation, the fibers that send pain and temperature information enter the dorsal portion of the spinal cord.  After entering the spinal cord, however, the majority of them form synaptic connections with nerve cells located in the dorsal horn. This group of nerve cells is collectively called the substantia gelatinosa. These cells, in turn, cross the midline and ascend toward the brain as a bundle of fibers along the anterior and lateral portion of the spinal cord, forming the anterolateral pathway. These fibers have inputs to the reticular formation of the brain stem and the thalamus. The reticular formation serves as an alerting system and activates other areas of the brain.

Inputs to the thalamus terminate in the ventrobasal (VB) nuclei. This information in turn is sent to the primary somatic sensory cortex in a somatotopic fashion, and to the frontal cortex. The pain information that reaches the primary sensory cortex is associated with acute pain, whereas the information that reaches the frontal cortex is associated with chronic pain.



 

Figure Legend.  Modulation of pain sensation.  The perception of pain can be altered by descending pathways and peripheral pathways. 


The perception of pain can be altered by other sensory inputs from the periphery or controlled by descending inputs from the brain stem and other higher brain centers. This gating of pain information is carried out by an inhibitory interneuron located in the dorsal horn of the spinal cord.  As previously discussed, the pain fibers from the periphery release the peptide called substance P as the neurotransmitter. In the absence of substance P release, pain cannot be detected by the brain. The interneuron acts to inhibit the release of substance P by a mechanism called pre-synaptic inhibition. The axon terminal of the interneuron forms a synapse upon the terminal of pain fibers. The terminals of the interneurons release the peptide called enkephalin, which in turn inhibits the release of substance P from the terminals of the pain fibers.

Thus, the interneuron is the key element in altering the sensation of pain. It can be activated by sensory fibers associated with touch, pressure, and vibration, or by fibers from higher brain centers. The influence of other tactile inputs is seen in the reduction of pain caused by rubbing the skin surrounding a region of injury; this causes the pain to subside more quickly. The influence of higher brain centers on pain perception is seen in cases in which individuals involved in emergency situations do not realize that they have been injured until much later. The mechanisms that prevent the perception of pain under these highly stressful conditions are part of an autonomic "fight or flight" response that will be discussed in a later section.

 

Another aspect of pain sensation is the perception that pain actually caused by the distress of internal organs is coming from the surface of the body. This is often seen in cases of heart attacks in which the area of the chest and left arm become painful. This type of referred pain will be considered in the section on the autonomic nervous system.

 

 

External Links to Related Topics

 

Somatic Sensory System Tutorial – video

                                                                                                                    
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