Taste, smell, and somatosensory systems

• Sense of taste responds to 5 different qualities; these correspond to 5 distinct taste receptors: bitter, sweet, salt, sour, and umami
• The tongue has 3 receptor types: temperature, tactile, and chemical receptors.
• Taste buds are concentrated on the tongue's edges, tips, and back. The middle of the tongue is relatively insensitive to taste. They are also found on the roof of the mouth and back of the throat. When food or other substances enter the mouth, molecules interact with saliva and are bound to taste receptors in the oral cavity and other locations
• The primary gustatory cortex is a brain structure responsible for the perception of taste. It consists of 2 substructures: the anterior insula and the frontal operculum.

• Olfaction is a chemoreception that forms the sense of smell. It has many purposes, such as the detection of hazards, pheromones, and food
• Olfaction integrates with other senses (e.g gustation) to form the sense of flavour
• Olfaction occurs when odorants bind to specific sites on olfactory receptors located in the nasal cavity. Glomeruli aggregate signals from these receptors and transmit them to the olfactory bulb, where the sensory input will start to interact with parts of the brain responsible for smell identification, memory, and emotion
• Mitral cells, located in the inner layer of the olfactory bulb, form synapses with the axons of the sensory neurons within glomeruli and send information about the odour to other parts of the olfactory system. Mitral cells leave the olfactory bulb in the olfactory tract
• The piriform cortex determines the chemical structure of the odourant molecules and also categorises odours. It projects to the thalamus, which projects to the orbitofrontal cortex, which projects to the entorhinal cortex, which projects to the amygdala, and finally projects to the hippocampus, storing odour information in long-term memory
• Pheromones are chemical signals found in natural body scents. They are thought to be partly responsible for sexual attraction

• The primary somatosensory cortex is located in the postcentral gyrus, in the parietal lobe. Tactile representation is orderly arranged from the toe (at the top of the cerebral hemisphere) to the mouth (bottom of the cerebral hemisphere)
• The primary motor cortex is located in the dorsal portion of the frontal lobe. It's the primary region of the motor system and works in association with other motor areas to plan and execute movements. Motor representation is ordered in the same inverted way the somatosensory cortex is arranged
• The amount of either cortex devoted to a body part isn't proportional to the size of the body surface, but instead to the relative density of cutaneous tactile/motor receptors on that body part
• Larger cortical areas indicate a body part that conducts complex movements
• Electrical stimulation of regions of the motor cortex will produce corresponding motor responses

• Our skin is the most sensitive of all body organs. Free nerve endings just below the surface of the skin (on palms of hands, fingertips, etc.) detect: painful stimuli, changes in temperature, and movement
• Skin responds to: vibration (when fingers run across a rough surface), pressure (someone squeezing your hand, causing mechanical deformation of the skin), heating/cooling (provides information about temperature), and stimuli that cause tissue damage - pain.
• Mechanoreceptors respond to pressure or distortion. The signal is sent to the somatosensory cortex
• Thermoreceptors detect warmth and cold via free nerve endings. They're the receptive portion of a sensory neuron, that code changes in temperature. The stimulus for a warm receptor is a temperature increase, which results in an increase in firing rate. Cold receptors increase firing rate during cooling and decrease during warming

The gate control theory of pain (Melzack & Wall, 1965) asserts that non-painful input closes the "gates" to painful input, which prevents pain sensation from travelling to the central nervous system. Therefore, stimulation by non-noxious input is able to suppress pain.

• A series of gates in the central nervous system are stimulated by signals sent from large nerve fibers (non-pain receptors) and small nerve fibers (pain receptors). For pain messages to be sent to the brain, small nerve fibers must be stimulated, opening the gates.
• Gates are closed at all other times and the messages of pain aren't received, e.g when a child falls over, rubbing their knees temporarily blocks the pain signal travelling from the injured knee to the brain