Medical brain notes 27

  SENSORY PATHWAY   The impulses involved in sensations follow very pre-cise pathways, which all have the following parts:   1.   Receptors —detect changes ( stimuli ) and generate   impulses. Receptors are usually very specific with respect to the kinds of changes they respond to. Those in the retina detect light rays, those in the nasal cavities detect vapors, and so on. Once a spe-cific stimulus has affected receptors, however, they all respond in the same way by generating electri-cal nerve impulses.   2.   Sensory neurons —transmit impulses from recep-tors to the central nervous system. These sensory neurons are found in both spinal nerves and cranial nerves, but each carries impulses from only one type of receptor.   3.   Sensory tracts —white matter in the spinal cord or   brain that transmits the impulses to a specific part of the brain.   4.   Sensory areas —most are in the cerebral cortex.   These areas feel and ...

  CHARACTERISTICS OF SENSATIONS   Certain characteristics of sensations will help you understand how the sensory areas work with informa-tion from the receptors.   1.   Projection —the sensation seems to come from the   area where the receptors were stimulated. If you touch this book, the sensation of touch seems to be in your hand but is actually being felt by your cere-bral cortex. That it is indeed the brain that feels sensations is demonstrated by patients who feel  phantom pain  after amputation of a limb. After   loss of a hand, for example, the person may still feel that the hand is really there. Why does this hap-pen? The receptors in the hand are no longer pres-ent, but the severed nerve endings continue to generate impulses. These impulses arrive in the parietal lobe area for the hand, and the brain does what it has always done and creates the projection, the feeling that the hand is still there. For most amputees, phantom pain diminishe...

  CUTANEOUS SENSES   The dermis of the skin and the subcutaneous tissue contain receptors for the sensations of touch, pressure, heat, cold, and pain. The receptors for pain, heat, and cold are  free nerve endings , which also respond to any intense stimulus. Intense pressure, for example, may be felt as pain. The receptors for touch and pres-sure are  encapsulated nerve endings , meaning that there is a cellular structure around the nerve ending (Fig. 9–1).   The  cutaneous senses  provide us with informa-tion about the external environment and also about the skin itself. Much of the information about the environment is not of great importance and is processed at a subconscious level (suppressed by the thalamus), though we can choose to be aware of it. For example, could you distinguish a cotton T-shirt from denim jeans by touch alone? Probably, but you might not realize that you can do that until you try it by, say, sorting laundry in the dark. Figur...

  MUSCLE SENSE   Muscle sense (also called proprioception or kines-thetic sense) was discussed and will be reviewed only briefly here. Stretch receptors (also called proprioceptors or muscle spindles) detect stretching of muscles and generate impulses, which enable the brain to create a mental picture to know where the muscles are and how they are positioned. Conscious muscle sense is felt by the parietal lobes. Unconscious muscle sense is used by the cerebellum to coordinate voluntary movements. We do not have to see our muscles to be sure that they are performing their intended actions. Muscle sense also contributes to our ability to distinguish the shape of objects.

  SENSE OF TASTE   The receptors for taste are found in  taste buds , most of which are in papillae on the tongue (Fig. 9–2). These  chemoreceptors  detect chemicals in solution in the mouth. The chemicals are foods and the solvent is saliva (if the mouth is very dry, taste is very  indistinct). There are five (perhaps more) general types of taste receptors: sweet, sour, salty, bitter, and savory.   Savory (also called umami or glutamate) is a taste like grilled meat. We experience many more dif-ferent tastes, however, because foods are often com-plex chemicals that stimulate different combinations of receptors, and the sense of smell also contributes to our perception of food. Figure 9–2.  Structures concerned with the senses of smell and taste, shown in a mid-sagittal section of the head. QUESTION:  If we sniff something pungent, why can we often taste it as well? (Follow the   inhaled air.)   Some taste preferences have been foun...

  SENSE OF SMELL   The receptors for smell ( olfaction ) are  chemorecep-tors  that detect vaporized chemicals that have been   sniffed into the upper nasal cavities (see Fig. 9–2). Just as there are specific taste receptors, there are also specific scent receptors, and research indicates that humans have several hundred different receptors. When stimulated by vapor molecules,  olfactory re-ceptors  generate impulses carried by the olfactory   nerves (1st cranial) through the ethmoid bone to the olfactory bulbs. The pathway for these impulses ends in the olfactory areas of the temporal lobes. Vapors may stimulate many combinations of receptors, and it has been estimated that the human brain is capable of distinguishing among 10,000 different scents.   Figure 9–2.  Structures concerned with the senses of smell and taste, shown in a mid-sagittal section of the head. QUESTION:  If we sniff something pungent, why can we often taste it a...

  HUNGER AND THIRST   Hunger and thirst may be called  visceral sensations , in that they are triggered by internal changes. Hunger is a sensation that seems to be far more complex than was first thought, but thirst seems to be somewhat simpler. The receptors for both senses are specialized cells in the hypothalamus. Receptors for hunger are believed to detect changes in blood nutrient levels, the blood levels of hormones from the stomach and small intestine, and a hormone released by adipose tissue; all of these chemical signals are collected by the hypo-thalamus. The receptors for thirst detect changes in the body water content, which is actually the water-to-salt proportion.   Naturally we do not feel these sensations in the hypothalamus: They are projected. Hunger is pro-jected to the stomach, which contracts. Thirst is pro-jected to the mouth and pharynx, and less saliva is produced.   If not satisfied by eating, the sensation of hunger gradually diminishes...

  THE EYE   The eye contains the receptors for vision and a refracting system that focuses light rays on the recep-tors in the retina. We will begin our discussion, how-ever, with the accessory structures of the eye, then later return to the eye itself and the physiology of vision.   EYELIDS AND THE LACRIMAL APPARATUS   The eyelids contain skeletal muscle that enables the eyelids to close and cover the front of the eyeball. Eyelashes along the border of each eyelid help keep dust out of the eyes. The eyelids are lined with a thin membrane called the  conjunctiva , which is also folded over the white of the eye and merges with the corneal epithelium. Inflammation of this membrane, called  conjunctivitis , may be caused by allergies or by certain bacteria or viruses, and makes the eyes red, itchy, and watery.   Tears  are produced by the  lacrimal glands ,   located at the upper, outer corner of the eyeball, within the orbit (Fig. 9–3). Se...