How Do Sensory Receptors Convert Physical Stimuli to Action Potentials - Coursework Example

Running Head: Sensory Reception    Your name   Course name       Professors’ name Date Introduction Information from one animal to another or response from the environment that surrounds us is through their ability to decode information sent from the surrounding and this elicits a reaction. Sensory receptors within the nervous system make it possible for sensations to experienced and communicated from the brain. Stimulus can be acknowledged. In her research (Sadava ,2009) found out that sensory receptors react to stimuli by initiating and activating transduction and forming action potentials in cells within. For us to clearly understand, we must however discuss on the role of the sensory receptors which act as the first section in any sensory system. These receptors react to particular stimuli modalities which are determined by its adequate stimulus. Sensory receptors such as taste and smell are bound to chemicals which are triggered by the interaction with molecular structures on the molecule responsible. If the sensory modality is smell, then the odor molecule is activated. If the modality is sight then the photoreceptors of the eye will react to light. Pressure can also lead to stimulation of the photoreceptors and this is clearly exhibited when a punch in the eye results in a feeling of sensations of light. It is important to note that there are some sensory receptors which are very complex and are responsible for conversion of stimuli to electrical signals. In his book (Holmes ,1990), state that receptor potentials are activated by the sensory receptors and they vary in accordance to the concentration of the stimulus. The major categories of sensory receptors are chemoreceptor, mechanoreceptors, photoreceptors and thermo receptors. All these receptors elicit different perceptions or modalities. Action potentials are generated when changes in membrane potential are transmitted to sensory neurons and if the stimulus is adequate to achieve threshold as a boost in potential intensity directly translates to a greater frequency of action potentials in the neurons. In his discussion (Wynn,2002) summarizes that an action potentials can be described as an event that includes the rapid rise and fall of electrical membrane potential. This is normally after a stereotyped trajectory has been followed. In neurons, action potentials functions in the cell to cell communication and also plays various roles in other types of excitable cells. These cells could be muscle cells and endocrine cells where the role of action potentials could be to set off intracellular processes. For example, the release of insulin is as a result of provocation of the beta cell in the pancreas. Transmission to the Central Nervous System In his discussions (Goldstein, 2009) theorizes that humans and animals alike need to have constant information about their surrounding and this communication. Physical input is initially identified through sensory receptors with some integrated with organs. Certain stimuli can activate sensory receptors and signal generated afterwards is then amplified chemically within the receptor cells. The process by which energy is converted into a usable type in the central nervous system is known as transduction. In his studies (Kalat,2007), summarizes that this process varies in accordance to the nature of stimulus. However there are some features of mechanism that are common. An example is in photoreceptors and taste receptors, there are some proteins that constitute the integral membrane and they are homologous with receptor molecules. These membranes are typically joined to G-proteins. When activation occurs, the membranes triggers an external messenger system that amplifies the initial response leading to opening or closing of particular ion channels and consequent depolarization of receptor membrane. This whole process must maintain modality, time interval, intensity and location of the stimuli. Quality of the information received by the central nervous system is determined by the type of receptor that has been stimulated, routing of the responses along specific paths and the destination in the brain. Action potentials generated by all the receptors are identical to each other and the sensations that are very different are produced in the brain at cortical sites in the cerebrum. It is at this level that the inputs project. This phenomenon is known as Labeled Lines. Duration before the information is received by the central nervous system is programmed along receptor response time. In his studies (Holsclaw ,2005) concluded that there are two classes of receptors when discussing about duration, there are static receptors which carry on with responding throughout the interval of extended stimulus. The second class is dynamic receptors which respond to changing stimulus and stop when there is a period of inactivity. Such receptors act when important changes to the surrounding or environment have occurred. These receptors are thought to eliminate redundancy in the conduction continuous monitoring of some static condition. A good example of a receptor that monitors static events is the pressure receptors. Another example of dynamic receptors is the warmth receptors. It has been found out that many receptors have a dynamic component that is relative to the rate of change in addition to a static component that is relative to degree of preserved change. Sensing Intensity (Uttal ,1995) noted that when it comes to intensity, it can be programmed at some point in transduction whereby a change in membrane potential is caused by application of stimulus on a receptor. In his research, (Sherwood, 2008) found out that the extent of the receptor potential depends mostly on the amount of the stimulus applied. A change to the frequency of discharging of action potentials is as a result of the comparative change in the receptor potential. The frequency change is in a logarithmic manner in terms of the ratio of the extent of change to the strength of the stimulus. This phenomenon is dictated by the Weber-Fechner Rule. There is also the principle known as the frequency coding that dictates that the strength of the stimulus is related to the frequency of action potential which is related to the perceived concentration. Information can also be programmed in terms of the sensory organs that at many times are clustered together in overlapping fields. It is important to note that these same receptors respond to similar types of receptors with the difference only occurring in the degree of sensitivity. There are the most sensitive receptors are triggered by stimuli which can be categorized as low intensity and their numbers reacting to the strength of the stimuli. This type of programming is referred to population coding. In his studies (Greenberg, 2005) confirms that it is a well known fact that every receptor acts within a certain margin where it obtains input. Stimulus acting outside this margin is incapable of having an effect on the receptors. The area enclosed within this margin is often referred to as the receptive field. Receptors with neighboring receptive fields have a tendency of transporting their inputs along adjoining pathways to neighboring brain destinations. Receptive surfaces that are represented as topographical maps are projected to the sensory input. There are receptors that have a complex field in terms of reception an they obtain inputs from remote sources. This is the explanation as to why the brain expands as intricate information and signal processing that is linked with the distance receptors such as the nose receptors that is used for smell, eyes used for sight and ear receptors used for sound. Classification of Receptors In her book, (Latash, ,2008) discusses on receptors and how they are further classified on how information is collect and at the same time programmed. There are exteroreceptors that function by collecting information from the exterior environment of the body, interoreceptors whose main function is to check the inner condition in the body. There are also proprioceptors that detect movement and positions of parts that constitute the body as well as their orientation. There are also receptors that are categorized as Nociceptors that react to conditions that are harmful to bodily functions or directly to the body organs such as tissue. Example of such a condition is extreme heat. The resultant feeling of this category of receptors is usually pain. Many Nociceptors are chemoreceptive and react to the chemicals formed after injury to the body tissue. It is important to note that sensory receptors are connected with neurons of the first order. These sensory neurons have cell bodies that are on the outer surface of the central nervous system and are located in the ganglia within the spinal nerves. They can also be located in matching sensory ganglia within the cranial nerves. In his research (Spudich ,1993) concludes that different sensory receptors effect countless stimuli into an explosion of action potentials, which race across the many pathways in the central nervous system. This results in multifarious interaction and integrated patterns of brain neurological activity. Although these potentials are generated in the vicinity on patches of membranes that can be considered excitable, the resultant currents can affect potentials in adjacent stretches of membranes. This induces rapid propagation which are in contrast to inactive spread of potentials that are electric in nature. Action potentials are produced along the extensions of the membrane and spread without any decay. Voltage gated channels at biophysical levels cause action potential. When membrane depolarization accumulates as a result of neuron synaptic inputs it causes the membrane potential to reach a threshold triggering action potentials, in the process membrane potential abruptly shoots upwards then downwards up to below the resting level remaining there for sometime. In his discussions (Kolb ,2008) summarizes that action potential takes place in less than a thousandth of a second. Increased membrane potential results in opening of the sodium ion channels thus allowing the entry of sodium ions into the cell resulting to increased concentration of positively charged cations in the cell causing depolarization, where the cell’s potential is greater than the cell’s resting potential and consecutive exit of potassium ions out of the cell through the potassium ion channels. In his book (Starr, 2008) states that at the peak of action potential there is closure of the sodium ion channels while potassium ion channels are still open from the cell. Exit of potassium ions leads to hyper polarization of the cell. When small voltage increases from rest sodium current is lesser than potassium current thus voltage returns to the resting value. When increase in voltage passes the critical threshold than the normal resting value the sodium current dominates causing a positive feedback from sodium current stimulating more sodium channels thus triggering action potential. Chemoreception This is the process by which the body is able to distinguish definite molecules within its environment. The molecules are either in the air or in the water. The molecules help the body in detecting various objects within its surrounding and are vital in the location of food and evading of danger. In his studies, (Rowen, 2009) concludes that this process is divided into two types, gestation which is responsible for the ability to taste and olfaction which is responsible with the ability to taste. The gustatory receptors react to the dissolved molecules that get in touch with the receptors. Olfactory receptors react to molecules that are found in the air within our surrounding and the source may be located some distance away from the body. Receptors belonging to olfactory nerves are situated in the upper part in most animals, of the nasal cavity. These organs consist of hair like cells and are normally found at the end of a neuron. The Olfactory sense organ is uncomplicated when compared to other organs that are related to sight and sound. As much as they are very receptive to stimuli, they become exhausted after continued sensitivity. This is the reason that one may not notice the smell of something after being exposed for some time. When molecules are detected and absorbed by the mucous layer, they are transmitted to the cilia which are protrusions from the end of the nerve covered with mucous. It is at this stage that the chemical within the molecule is detected. The olfactory receptors only react when the chemicals from the molecules, dissolve in the mucus and further absorbed. The nerves that are responsible for the gustatory receptors are referred to as taste buds and can be found on the tongue and the top side of the mouth. There are four fundamental tastes sensations and they are sweet, sour, bitter, and salty. These sensations are the effect of stimulation from the taste buds which in turn leads to sensation within the stimulation of the olfactory receptor and organ related to it. This is the explanation handed out when one finds it difficult to taste when they are undergoing a bout of flu or cold. Taste buds later evolve over time to detect foods that have a high calorie content. Foods that have a high salt quantity and those that assist the body in maintaining water balance. The Gustatory receptors are important when a sour taste bud is activated; it could mean that the food ingested is sour or harmful if ingested in excess. Mechanoreception This is the ability of the body to detecting physical contact that may be applied on the surface of the skin. It could also be associated with movement within the body’s environment. This could be manifested through sound waves which could be detected in air or in water. In his research (Marieb ,2008) summarizes that mechanoreceptors vary in complexity as this is clearly in the skin’s connective tissue which is described as fairly simple to the intricate parts of the ear which are the inner and the middle part of the year. So one may ask themselves as to how the ear is able to convert sound energy to resultant nerve impulses. It has been found that the whole process of conversion starts at the tympanic membrane from where sound is first detected through vibrations and is transmitted to the ear drum followed by conduction to the middle ear through its three additional bones. From this point, vibrations are transmitted to the oval window which is then moved to the cochlea or the fluid found within. It has been researched and discovered that the cochlea is separated into 3 chambers whereby the two outer ones, are known as scala tympani and scala vestubuli respectively. All of these chambers are covered with a liquid containing high sodium concentrations. Having passed the oval window and cochlea chambers, sound vibrations are dissipated from end to end through the membrane of the window. Action potentials generate a number of impulses at specific times and this is relayed as information to the brain. Loudness is hence characterized by the change in amplitude of the sound vibrations; this is elicited in more hair movement and thus greater action potentials. The difference in wave frequencies is distinguished through pitch. High pitch is detected in the brain through stimulation of the basilar membrane which is considered as to be the adjoining the oval window. Low pitch is identified by stimulation of the neurons leading to the resultant transmission of signals to areas of the brain. The auditory nerves are found in a spiral ganglion and their main functions are to carry the action potential to the central nervous system. The mechanoreception mechanism varies from the categories of animals as this function varies. For example in fish, the system of mechanoreception is through a lateral line that enables detection of movement and these vibration are transmitted through the pores and stimulated resulting in the transmission of impulses through neurons. Fish have superior hearing than human beings as their system contains inner ear. Photoreception In discussing about the Structure of the eye (Gutman,2009) state that photoreception is the process of conversion of photons of light into neuronal signals through electrical translation. The human eye is very complex and the eyeball consists of the sclera which can be described as the organ that is connected through a white connective tissue to the choroid which is a thin pigmented layer. The cornea is contained within the sclera and it is in inside the cornea where light is allowed to penetrate through. The light passes the choroid which contains the iris that expands and collapsed in respect to the intensity of light that has passed through the cornea. The function of the iris is to control the amount of light entering the pupil. This is located centrally in the eye i.e. in the middle and is often seen as a hole in the eye. On the backside of the eye lies the retina which has cell that are responsible for the actual photoreception. The lens responsible for depth perception and gauging the distance is located in the middle of the cornea and the eyeball which is in the form of clear protein. The vitreous humor provides additional liquid to the lens. Operation: In his book (Field, 2003) discusses on the ability for any animal to see is through a set of definite protein molecules that form an optical pathway through which light is directed to precise surfaces that contain photoreceptors which are able to detect light in the form of photons. Light penetrates the eye through the cornea which bends light rays followed by further refraction which occurs just before the light enters the lens. Light rays are bent and refracted towards the lens where contraction and expansion of the cilia muscles occurs. This allows for focus of objects. When the muscles have relaxed, the lens is flattened and this allows the eye to focus on distant objects. In his research, (Bishop,1982), found out that the muscles are responsible for re-dimensioning the curve and thickness of eye lens. When it comes to viewing images that are closer, the lens contracts and this re-dimensions the lens to form a round shape. The eye can also achieve binocular convergence, through the ocular muscles that deviate light to both eyes in a way that figures received by both eyes are transmitted to the same point in the two retinas. Within the retina are categories of the receptor cells referred to as cones and rods. Rods are utilized in environments characterized by dim lights and cones are characterized in environments with bright lights. Both cells include pigments characterized by light absorbing molecules. These molecules are further linked to opsins which are proteins that direct and dictate the pigment that will be absorbed by each cell. In the rods, pigment is controlled by rhodopsin. When the bright light is shone on the eye, the rods become inactive as a result of the opsin and the retina detaching from each other. In comparison to cones, the rods are more reactive to sensitive light and this is the reason that they function better when the environment is composed of dim light. In the cones, there are several connections of neuronal cells which are nearer than the receptor cells and superior magnification of fragile stimuli is promoted by increased convergence. In an experiment, (Conn,1993) explained the phenomenon of why images viewed at night are better seen when not observed directly is because of the rods not being part of the fovea. The fovea is the part of the eye where images are best focused. At night we only observe objects in black and white as a result of the rod being stirred by the dim light. In the human body, there are 3 categories of cones namely: green, blue and orange. Particular Photo pigment molecules are contained with each cone and every molecule; there is a certain absorption rate that can be achieved by each molecule. Colors can only be detected by stimulation of two or three categories of cones. The fovea, which ensures that high quality vision is achieved, comprises of only cones and this enables the eye to observe objects in great detail. In times of light such as during the day, the fovea is able to take in as much light which hits the receptor cells or cones. A resultant charge across the membrane is formed; this should not be mistaken as a form of action potential. Images of objects are conveyed as action potentials after s series of synapsis between the neurons and the gangalion cells. The image of the object is expressed as an action potential to the central nervous system along the optic nerves. There is a point in the chiasma, central nervous system where the optic nerves of both eyes meet. it is also at this point that the blind spot is located. An interesting phenomenon is how animals observe objects in black and white and this can be explained by them having more rods that are located in the optical systems. In his studies, (Ansche, 2006) concluded that the eye contains pigmentation molecules that take in light in the form of photons. The molecules are composed of rhodopsin in addition to an absorbing retinal component. The process of isomerization of the cis-configuration leads to transconfiguration of the retinal. This process is important as it results in the division of the opsin from the retinal molecule which further transforms the conformation of the retinal. Proteins connected to the cell membranes are stimulated when light is absorbed; this results in the alteration of the membrane potential leading to the generation of an action potential. The action potential is transmitted to the central nervous system through the sensory neuron. Potential diffusion occurs across the membrane when a certain condition when the concentration of ions and cell membranes located outside, reach a predefined threshold. Thermoreception In his studies (Clarke, 2008), states that the process by which temperature within the body is synchronized with the surrounding through regulation of body’s responses to behavior and autonomic ability. Sensitivity to temperature is caused by thermoreceptors that are able to distinguish the ranges of temperature between hot and cold. There are also other animals that utilize thermoreceptors in the sensing of danger or in the sensing and detection of prey within its environment. Examples of these animals are snakes and some insects such as the mosquito that thrive on the blood of other animals. T has been shown that a rattle snake has the ability to sense the body temperature of mice 6that could be located around forty centimeters away from it. Conclusion We have so far discussed on the different categories of receptors and their roles in their generation of action potentials and also we have discussed about how they are able to do this. It is important to note that sensations occur after the receptors have been stimulated and through their respective mechanisms is when this information is carried to the central nervous system. This information is usually coded in a manner that allows the central nervous system to interpret to the physical stimuli. . Read More

These cells could be muscle cells and endocrine cells where the role of action potentials could be to set off intracellular processes. For example, the release of insulin is as a result of provocation of the beta cell in the pancreas. Transmission to the Central Nervous System In his discussions (Goldstein, 2009) theorizes that humans and animals alike need to have constant information about their surrounding and this communication. Physical input is initially identified through sensory receptors with some integrated with organs.

Certain stimuli can activate sensory receptors and signal generated afterwards is then amplified chemically within the receptor cells. The process by which energy is converted into a usable type in the central nervous system is known as transduction. In his studies (Kalat,2007), summarizes that this process varies in accordance to the nature of stimulus. However there are some features of mechanism that are common. An example is in photoreceptors and taste receptors, there are some proteins that constitute the integral membrane and they are homologous with receptor molecules.

These membranes are typically joined to G-proteins. When activation occurs, the membranes triggers an external messenger system that amplifies the initial response leading to opening or closing of particular ion channels and consequent depolarization of receptor membrane. This whole process must maintain modality, time interval, intensity and location of the stimuli. Quality of the information received by the central nervous system is determined by the type of receptor that has been stimulated, routing of the responses along specific paths and the destination in the brain.

Action potentials generated by all the receptors are identical to each other and the sensations that are very different are produced in the brain at cortical sites in the cerebrum. It is at this level that the inputs project. This phenomenon is known as Labeled Lines. Duration before the information is received by the central nervous system is programmed along receptor response time. In his studies (Holsclaw ,2005) concluded that there are two classes of receptors when discussing about duration, there are static receptors which carry on with responding throughout the interval of extended stimulus.

The second class is dynamic receptors which respond to changing stimulus and stop when there is a period of inactivity. Such receptors act when important changes to the surrounding or environment have occurred. These receptors are thought to eliminate redundancy in the conduction continuous monitoring of some static condition. A good example of a receptor that monitors static events is the pressure receptors. Another example of dynamic receptors is the warmth receptors. It has been found out that many receptors have a dynamic component that is relative to the rate of change in addition to a static component that is relative to degree of preserved change.

Sensing Intensity (Uttal ,1995) noted that when it comes to intensity, it can be programmed at some point in transduction whereby a change in membrane potential is caused by application of stimulus on a receptor. In his research, (Sherwood, 2008) found out that the extent of the receptor potential depends mostly on the amount of the stimulus applied. A change to the frequency of discharging of action potentials is as a result of the comparative change in the receptor potential. The frequency change is in a logarithmic manner in terms of the ratio of the extent of change to the strength of the stimulus.

This phenomenon is dictated by the Weber-Fechner Rule. There is also the principle known as the frequency coding that dictates that the strength of the stimulus is related to the frequency of action potential which is related to the perceived concentration. Information can also be programmed in terms of the sensory organs that at many times are clustered together in overlapping fields. It is important to note that these same receptors respond to similar types of receptors with the difference only occurring in the degree of sensitivity.

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