Brain - Perception

TASTE AND SMELL 

This work was done as a part of Summer Program at Elio Academy of Biomedical Sciences.

Author: Riyaa Sri Ramanathan

 

 

Sense of taste is called gustation while the sense of smell is called olfaction. Just as sound is the perception of air pressure waves, sight is the perception of light, smell and taste are the perceptions of tiny molecules in the air and in the food. Both of these senses contribute to how food tastes, and are important for survival thereby enabling people to detect hazardous substances that might be inhaled or ingested. 

The cells processing taste and smell are exposed to the outside environment, leaving them vulnerable to damage. As a result of this, taste receptor cells and olfactory receptor neurons regularly regenerate. In fact, olfactory neurons are the only sensory neurons that are continually replaced throughout peoples’ lives.

Ability to taste food occurs as molecules are released during chewing or drinking. These molecules are detected by taste cells within taste buds located on the tongue, along the roof and back of the mouth. There are between 5000 and 10000 taste buds in the mouth that start to degenerate when humans turn around age 50. Each taste bud consists of 50 to 100 sensory cells that are receptive to at least five basic taste qualities: sweet, sour, salty, bitter, and umami (Japanese for “savory”). 

All the tastes are detected across the tongue and are not limited to specific regions. When taste receptor cells are stimulated, they send signals through three cranial nerves — the facial, glossopharyngeal, and vagus nerves — to taste regions in the brainstem known as Nucleus of solitary tract. The impulses are then routed through the thalamus to the gustatory cortex and insula, where specific taste perceptions are identified.

 

Odors enter the nose on air currents and bind onto specialized olfactory cells on the mucous membrane, found inside the nasal cavity. There are two olfactory bulbs found, one for each nostril. Axons of these sensory neurons enter the olfactory bulbs after passing through the tiny holes in the skull. From the bulb, the information travels to the olfactory cortex in the temporal lobe. Smell is the only sensory system that sends sensory information directly to the cerebral cortex without first passing through the thalamus. 

Humans have around 1,000 different types of olfactory cells, but can identify about 20 different types of smells. The tips of olfactory cells are equipped with several hair-like cilia that are sensitive to a number of different odor molecules. A specific smell will therefore stimulate a unique combination of olfactory cells, creating a distinct activity pattern. This “signature” pattern of activity is then transmitted to the olfactory bulb and on to the primary olfactory cortex located on the anterior surface of the temporal lobe. 

Olfactory information then passes to the nearby brain areas, where odor and taste information are combined, creating the perception of flavor. Recent research suggests that the size of the olfactory bulbs and the way neurons are organized change over time. One of the few brain areas that can generate new neurons throughout life are the olfactory bulbs in rodents, humans and primates.

 

TOUCH AND PAIN

 

The somatosensory system is responsible for all touch sensations that includes light touch, pressure, vibration, temperature, texture, itch, and pain. These sensations are perceived with various types of touch receptors whose nerve endings are located in different layers of the skin, the body’s main sense organ for touch. In particular, hairy skin areas have sensitive nerve cell endings which wrap around the bases of the hairs responding to slightest hair movements. Signals from touch receptors travel along sensory nerve fibers that connect to neurons in the spinal cord. From there, the signals move upward to the thalamus and on to the somatosensory cortex, where they are translated into a touch perception. 

Some touch information travels quickly along myelinated nerve fibers with thick axons (A-beta fibers), but other information is transmitted more slowly along thin, unmyelinated axons (C fibers). Somatosensory information from all parts of the body is spread onto the cortex in the form of a topographic map that curls around the brain. Very sensitive body areas like lips and fingertips stimulate much larger regions of the cortex than less sensitive parts of the body. The sensitivity of different body regions to tactile and painful stimuli depends largely on the number of receptors per unit area and the distance between them. 

In contrast to the lips and hands, which are the most sensitive to touch, touch receptors on the back are few and far apart, making the back much less sensitive. Neurologists measure this sensitivity using two-point discrimination, the minimum distance between two points on the skin that a person can identify as distinct stimuli rather than a single one. Acuity is greatest, and the two-point threshold is lowest in the most densely nerve packed areas of the body, like the fingers and lips. By contrast, two stimuli in the back can only be distinguished if they are several centimeters apart.

Pain is both a sensory experience and an emotional experience. The sensory component signals tissue damage or the potential for damage, while the emotional component signals the unpleasant experience and distressing. Typically, pain is a warning signal that tells the brain that something is wrong with the body. Pain occurs when special sensory fibers, called nociceptors, respond to stimuli that can cause tissue damage. 

Normally, nociceptors respond only to strong or high-threshold stimuli. This response helps in detecting when something is truly dangerous. Different types of nociceptors are sensitive to different types of painful stimuli, such as thermal (heat or cold), mechanical (wounds), or chemical (toxins or venoms). Interestingly, these same receptors also respond to chemicals in spicy food, like the capsaicin in hot peppers, which might produce a burning pain, depending on persons’ sensitivity. Some types of nociceptors respond only to chemical stimuli that cause itch. A well-known example is histamine receptors that are activated when skin irritation, bug bites, and allergies trigger the release of histamine inside the body. But scientists have recently identified other itch-specific receptors as well. 

When tissue injury occurs, it triggers the release of various chemicals at the site of damaged areas, causing inflammation. The inflammation then triggers nerve impulses to cause continuous pain. This feeling of continuous pain helps to protect damaged parts of the body. Prostaglandins for example, enhance the sensitivity of receptors to tissue damage, causing the pain to feel more intense. 

These NSAIDs also contribute to a condition called Allodynia. Allodynia is a condition when soft touch on a sunburnt skin produces immense pain. In addition, long-lasting injury may lead to nervous system changes that enhance prolonged perceived pain, even in the absence of pain stimuli. The resulting state of hypersensitivity to pain, called neuropathic pain, is caused by a malfunctioning nervous system rather than by an injury. An example of this condition is diabetic neuropathy, in which nerves in the hands or feet are damaged by prolonged exposure to high blood sugar and send signals of numbness, tingling, burning, or aching pain.

Pain and itch messages travel to the spinal cord via small A-delta fibers and even smaller C fibers. The myelin sheath covering A-delta fibers helps nerve impulses to travel faster, and these fibers evoke the immediate, sharp, and easily identified pain that is produced. The unmyelinated C fibers transmit pain messages more slowly, and their nerve endings spread over a relatively large area producing a dull and diffuse ache or pain sensation whose origin is harder to identify. Pain and itch signals travel up the spinal cord through the brainstem and then to the thalamus. From there, the impulses are related to several areas of the cerebral cortex that monitor the state of the body and transform pain and itch messages into conscious experience. Depending on this relay, the brain responds to these messages.

Pain mainly depends on the strength of the stimulus, on a person’s emotional state, and the setting in which the injury occurs. When pain messages arrive in the cortex, the brain can process them in different ways. The cortex sends pain messages to a region of the brainstem called the periaqueductal gray matter. 

 Through its connections with other brainstem nuclei, the periaqueductal gray matter (PGM) activates descending pathways that modulate pain. These pathways also send messages to networks that release endorphins, which are opioids produced by the body that act like the analgesic morphine. Adrenaline produced in emotionally stressful situations also works as an analgesic, a drug used to relieve pain in emotionally stressful situations. The body’s release of these chemicals helps regulate and reduce pain by intercepting the pain signals ascending in the spinal cord and brainstem. 

Amount of pain a person’s feels depends on the efficacy and sensitivity of the brain circuits that exist in everyone. Because of this, people develop chronic pain that does not respond to irregular treatment. For people suffering from intense chronic pain, research shows that endorphins act as multiple types of opioid receptors in the brain and spinal cord. These endorphins, acting as opioid receptors, is an important implication for pain therapy. Opioid drugs are now given to patients in the spinal cord before, during and after the surgery to reduce pain. Scientists are studying ways to avoid the harmful effects of long — term opioid use by stimulating the spinal cord, electrically, to relieve pain. 

Variations in peoples’ perception of pain is leading to new avenues for therapy. Perceiving pain is influenced by emotional and sensory components and treatment to pain can be done through meditation, hypnosis, massages, cognitive behavioral therapy and controlled use of cannabis.

 

Author: Riyaa Sri Ramanathan

This work was done as a part of Extended Research Program at Elio Academy of Biomedical Sciences.