Chemoreceptors(Ke”mo-re-sep-torz)
Overview: Chemoreceptors respond to changes in the concentration of chemical substances.
Receptors associated with the senses of smell and taste are of this type. Chemoreceptors in internal organs detect changes in the blood concentrations of oxygen, hydrogen ions, glucose, and other chemicals. Chemoreceptors respond to some local chemical change. Usually refers to those which influence the respiratory and cardiovascular control centers in the brain stem: the medullary chemoreceptors, sensitive to pH changes in the cerebral extracellular fluid, and the arterial chemoreceptors which continually sense and respond to changes mainly in blood oxygen, carbon dioxide and pH, leading to appropriate reflex adjustments via afferent nerves to the brain stem control centres (e.g. increase in ventilation if arterial oxygen tension tends to fall and/or carbon dioxide to rise). See also breathing, hypoxia.
Chemoreceptors are specialized nerve cells which are designed to respond to chemical stimuli. The body contains both direct and distant chemoreceptors, all of which play important roles in bodily function and daily life. These cells are also sometimes known as chemosensors, because they behave like sensors which “sniff” for specific chemicals of interest. Like other neurons, these cells are usually designed with customized locks which only fit the keys of specific chemicals, which makes them sensitive only to certain types of chemicals or chemical families. An example of direct chemoreceptors are the cells located on the tongue. When people taste food, it is because these cells respond to the chemicals in the food, sending a signal to the brainto let the brain know about what's happening in the mouth. Specific regions of the mouth have areas which are targeted towards specific tastes, such as salty and sweet. This explains why foods can taste different as they are chewed and swallowed, and also why some foods have anaftertaste, as certain chemicals can take longer to stimulate the chemoreceptors.
Pain Receptors (Nociceptors) (no”se-sep-torz)
Overview: Pain receptors or nociceptors respond to tissue damage. Triggering factors include exposure to excess mechanical, electrical, thermal, or chemical energy.
Pain is an unpleasant feeling that is conveyed to the brain by sensory neurons. The discomfort signals injury to the body. However, pain is more than a sensation, or the physical awareness of pain; it also includes perception, the subjective interpretation of the discomfort. Perception gives information on the pain's location, intensity, and something about its nature. The various conscious and unconscious responses to both sensation and perception, including the emotional response, add further definition to the overall concept of pain. Pain arises from any number of situations. Injury is a major cause, but pain may also arise from an illness. It may accompany a psychological condition, such as depression, or may even occur in the absence of a recognizable trigger.
Acute pain often results from tissue damage, such as a skin burn or broken bone. Acute pain can also be associated with headaches or muscle cramps. This type of pain usually goes away as the injury heals or the cause of the pain (stimulus) is removed. As a group, these pain-sensing neurons are called nociceptors, and virtually every surface and organ of the body is wired with them. The central part of these cells is located in the spine, and they send threadlike projections to every part of the body. Nociceptors are classified according to the stimulus that prompts them to transmit a pain signal. Thermoreceptive nociceptors are stimulated by temperatures that are potentially tissue damaging. Mechanoreceptive nociceptors respond to a pressure stimulus that may cause injury. Polymodal nociceptors are the most sensitive and can respond to temperature and pressure. Polymodal nociceptors also respond to chemicals released by the cells in the area from which the pain originates.
Chronic and abnormal painChronic pain refers to pain that persists after an injury heals, cancer pain, pain related to a persistent or degenerative disease, and long-term pain from an unidentifiable cause. It is estimated that one in three people in the United States will experience chronic pain at some point in their lives. Of these people, approximately 50 million are either partially or completely disabled. One of the frustrating aspects of chronic pain is that the stimulus may be unknown. For example, the stimulus cannot be identified in as many as 85% of individuals suffering lower back pain. Scientists have long recognized a relationship between depression and chronic pain. In 2004, a survey of California adults diagnosed with major depressive disorder revealed that more than one-half of them also suffered from chronic pain.
Pain is the most common symptom of injury and disease, and descriptions can range in intensity from a mere ache to unbearable agony. Nociceptors have the ability to convey information to the brain that indicates the location, nature, and intensity of the pain. For example, stepping on a nail sends an information-packed message to the brain: the foot has experienced a puncture wound that hurts a lot. Pain perception also varies depending on the location of the pain. The kinds of stimuli that cause a pain response on the skin include pricking, cutting, crushing, burning, and freezing. These same stimuli would not generate much of a response in the intestine. Intestinal pain arises from stimuli such as swelling, inflammation, and distension.
Pain receptors are found on free nerve endings located in many tissues throughout the body. This includes skin, muscles, joints, connective tissues, and internal organs. These receptors are activated in response to a painful stimulus, usually involving tissue damage. Once activated, they release chemicals called neurotransmitters that send information about the painful stimulus along nerves to the spinal cord and the brain. This entire process of pain transmission is callednociception, and the pain receptors found in tissues are called nociceptors.
Thermoreceptors(ther”mo-re-cep-torz)
Overview: are receptors that are sensitive to temperature change.
Thermoreceptors are specialized neurons designed to be sensitive to changes in temperature. These cells generally detect temperature variations within the normal range, while neurons known as nociceptors detect temperatures that could be dangerous to the body. Many organisms rely on these neurons for a variety of things, from alerting them to the fact that they are stepping into a cold river to assisting with the regulation of internal body temperature.In the skin, thermoreceptors provide the brain with information about environmental temperature. This can be important, as it will alert the body to unusually cool or warm temperatures that might require an action, such as putting on a coat or shedding a layer of clothing to become more comfortable. These neurons are also used to provide more general information about a body's environment, such as that one area of a room is cooler than other spots.
Inside the body, these cells are part of the body's complex and interconnected series of systems that are designed to keep the body in balance. Neurons in the hypothalamus are sensitive to changes in core temperature that could require a response from the body, and there are others inside some body organs, such as the bladder. When these cells fire, they alert the brain to an imbalance in internal temperature that needs to be addressed; for example, cells sensitive to heat in the eyes alert the tear glands to produce more fluid to keep the eyes lubricated.
Mechanoreceptors(mek”ah-no re-sep-torz)
Overview: Mechanoreceptors sense mechanical forces by detecting changes that deform the receptors.
There are three types: Proprioceptors (pro”pre-o-sep-torz) which sense changes in the tensions of muscles and tendons, Baroreceptors (bar”o-re-sep-torz) aka pressoreceptors which detect changes in blood pressure, and Stretch receptors in the lungs which sense the degree of inflation. Mechanoreceptors are structures in the body that enable people to experience physical sensations. They feed tactile information to the brain so the brain can process it, providing information about objects in the environment people interact with, as well as vibrations in the air and other sources of physical sensation. There are a number of types of mechanoreceptors, designed to sense different kinds of tactile information, and these structures function in different ways. In disorders involving sensory sensitivity, people may have problems with their mechanoreceptors or the nerves that carry information from these structures to the brain. Physical sensations can create a sense of pressure, distortion, vibration, or tension in the mechanoreceptors. These cells are usually designed to adapt, meaning as a sensation is experienced, the signals sent to the brain change. This prevents mechanoreceptors from repeatedly sending the same signal over and over, preventing people from being bombarded with information about constant sensations like clothing. Adaption speeds vary, depending on the type of receptor. The fastest adapter is the Pacinian corpuscle, a type of receptor designed to sense vibrations. These structures are highly sensitized. Meissner's corpuscles and hair follicle receptors, provide fine touch and are designed to sense texture and the movement of hairs respectively, are slower adapters. They adjust to changes taking place within seconds, rather than fractions of seconds as with the Pacinian corpuscles. Finally, the slowest adapters include Ruffini cells for detecting tension and Merkel's discs for sensing pressure.
Photoreceptors (fo”to-re-sep-torz)
Overview: Photoreceptors are found in the eyes and respond to light energy of sufficient intensity.
Photoreceptors are nerve cells which have been designed to be sensitive to light. These cells are located in the eye, allowing an organism to see, and the process through which they work is complex and quite fascinating. There are three types of photoreceptors: rods, cones, and photosensitive ganglion cells, and each plays a distinct role in vision. When a photoreceptor is exposed to light, photosensitive proteins in the neuron are stimulated, triggering a series of responses which convert the light into a signal which can be read by the brain. This process happens in a fraction of a second, allowing a photoreceptor to provide constant information to the brain about the visual environment. Certain photoreceptors are sensitized to particular spectra, and the brain uses information from these cells to distinguish colors. Rather than literally seeing color, in other words, the photoreceptor responds to specific spectra, and the brain averages responses to determine what the eye is seeing.
Photoreceptor cells which have been sensitized to specific spectra are known as cones, the cells which famously allow people to “see” color. These cells have a cone-like shape when viewed under magnification, explaining the name, and they require bright light to function effectively. Rods, on the other hand, work in very low light, but do not distinguish color well. Many organisms have a mixture of rods and cones which is designed to strike a balance between being able to see well in the dark, and being able to distinguish colors.