Introduction to Behavioral Neuroscience Lecture Notes (PDF)

Introduction to Behavioral Neuroscience PSYC 211 Lecture 12 of 24 – Touch, Taste, Smell, Pheromones (Rest of chapter 7 in the textbook) Professor Jonathan Britt Questions? Concerns? Please write to [email protected]  THE SOMATOSENSORY SYS...

Introduction to Behavioral Neuroscience PSYC 211 Lecture 12 of 24 – Touch, Taste, Smell, Pheromones (Rest of chapter 7 in the textbook) Professor Jonathan Britt Questions? Concerns? Please write to [email protected]  THE SOMATOSENSORY SYSTEM  The somatosensory system provides information about touch, pressure, temperature, and pain, both on the surface of the skin and inside the body.  There are three interacting somatosensory systems: - the exteroceptive system (cutaneous/skin senses) responds to external stimuli applied to the skin (e.g., touch and temperature) - the interoceptive system (organic senses) provides information about conditions within the body and is responsible for efficient regulation of its internal milieu (e.g., heart rate, breathing, hunger, bladder) - the proprioceptive (kinesthesia) system monitors information about the position of the body, posture, and movement (e.g., the tension of the muscles inside the body)  THE CUTANEOUS SENSES The cutaneous senses (skin) encode different types of external stimuli: - Pressure (touch) is caused by mechanical deformation of the skin - vibrations occur when we move our fingers across a rough surface - temperature is produced by objects that heat or cool the skin - pain can be caused by many different types of stimuli, but primarily tissue damage  ANATOMY OF THE SKIN The outermost layer of skin is called epidermis (“above dermis”). Cells here get oxygen from the air (not the blood). The middle layer is called dermis. The deepest layer is called the hypodermis (or subcutaneous, “below the skin”). Sensory neurons are scattered throughout these layers. Merkel’s disks respond to Glabrous skin is ‘hairless’ skin local skin indentations (e.g., palms of hands and feet) (simple touch) Ruffini corpuscles Free nerve endings are sensitive to primarily respond to stretch and the temperature and pain. kinesthetic sense of finger position and movement Meissner’s corpuscles are Pacinian corpuscles only found in glabrous skin. respond to skin They detect very light touch vibrations. and localized edge contours (brail-like stimuli).  PERCEPTION OF TEMPERATURE Temperature There are two categories of thermal receptors: those that respond to warmth and those that respond to coolness. Pain information is also conveyed by some of these cells. This information is poorly localized, and the axons that carry it to the CNS are unmyelinated or thinly myelinated. Some of the receptor proteins that are sensitive to temperature can also be activated by certain ligands (e.g., capsaicin molecules activate heat receptors and menthol molecules activate cold receptors). THERMAL GRILL ILLUSION  PAIN Sensations of pain and temperature are transduced by free nerve endings in the skin There are several types of pain receptor cells (usually referred to as nociceptors – “detectors of noxious stimuli”) One type is the high-threshold mechanoreceptors (pressure receptor cells), which are free nerve endings that respond to intense pressure, like striking, stretching, or pinching. Other types of free nerve endings respond to extreme heat (or the presence of chemicals such as capsaicin, the active ingredient in chili peppers). From the Body to Primary Somatosensory Cortex Axons from skin, muscles, and internal organs enter the CNS via spinal nerves. There are 2 main pathways: 1. Poorly localized information (e.g., crude touch, temperature, and pain) crosses over the midline in the spinal cord, just after the first synaptic connection. This information ascends to the thalamus through the spinothalamic tract. 2. Highly localized information (e.g., fine touch) ascends ipsilaterally through the dorsal column of the spinal cord. The first synapse in this pathway is in the medulla. From there the information crosses over to the contralateral side as it ascends to the thalamus. Both pathways get bundled together in the midbrain before synapsing in the thalamus. From there, information goes to primary somatosensory cortex in the parietal lobe. The Somatosensory Homunculus When different sites of primary somatosensory cortex are electrically stimulated, patients report somatosensory sensations in specific parts of their bodies. The relationship between cortical stimulations and body sensations is reflected in a somatotopic map of the body surface The somatotopic map is often referred to as the somatosensory homunculus (“little man”)  Tactile Agnosia Patients with tactile agnosia have trouble identifying objects by touch alone.  When touching an object, people might think this is that: - pine cone -> brush - ribbon -> rubber band - snail shell -> bottle cap  However, these patients can often draw objects that they are touching, without looking, and they can sometimes identify objects from their drawings.  PHANTOM LIMB Phantom limb is a form of pain sensation that occurs after a limb has been amputated. Amputees report that the missing limb still exists and that it often hurts. One idea is that phantom limb sensation is due to confusion in the somatosensory cortices (primary and association). The brain gets nonsense signals (in part from the cut axons, sensory and motor) and it has difficulty interpreting them. Most treatments (whether pharmacological, electrical, or behavioural) have not proven to be very effective. The mirror box has received lots of attention because it is cheap and easy, but its effectiveness is unclear.  GUSTATORY INFORMATION (TASTE) Six different categories of taste receptors have been identified. These receptors detect: 1. sweetness (molecules of sugar) the most common amino acid in animal detected with a single metabotropic receptor products (meat and cheese). MSG (monosodium glutamate) activates 2. umami (molecules of glutamate/glutamine) both salt and umami receptors. detected with a single metabotropic receptor 3. bitterness (a variety of molecules) detected with 50 different metabotropic receptors that bind different bitter molecules 4. saltiness (positive ions such as sodium) detected with an ion channel that is highly permeable to sodium 5. sourness (pH level; the concentration of free hydrogen ions) detected with an ion channel that is highly permeable to free protons cells that detect sourness are also responsible for the detection of astringency (e.g., the flavor of “salty licorice”) and carbonation (bubbles), but the details are murky 6. fat (fatty acids) detected with metabotropic receptors and fatty acid transporters  PERCEPTION OF GUSTATORY INFORMATION Transduction of taste is similar to chemical transmission that takes place at synapses When a tasted molecule binds to a taste receptor protein, it produces a change in membrane potential (either directly through an ion channel or through g protein signaling cascades). Different tastes relate to the activation of different types of taste receptor proteins. Taste buds contain 20 to 150 taste receptor cells, some for each type of taste (sugar, umami, bitter, salt, sour, or fat). Taste receptor cells do not have traditional action potentials. They release neurotransmitter in a graded fashion. Taste receptor cells are replaced about every ten days, because they are directly exposed to a rather hostile environment.  PERCEPTION OF GUSTATORY INFORMATION To study the taste system, researchers often manipulate the DNA of mice. For example, to identify the sugar taste receptor, researchers remove specific genes from their genome and then test if mice can discriminate between regular water and sugar water. Researchers have even created mice where the sugar receptor gene was replaced with a bitter receptor gene. These genetically engineered mice cannot taste sugar, but they love the bitter molecule that now activates the cells in their sweet taste receptor cells. These studies demonstrate that much of taste processing is innate (hard-wired from birth to be either pleasurable or aversive). Sugar and umami taste receptor cells are instinctively rewarding/reinforcing. Direct stimulation of them (or their downstream structures in the cerebral cortex) is inherently reinforcing. Bitter taste receptor cells are instinctively aversive. Animals can grow to appreciate some bitter taste cell activity, but it is an acquired taste. Interestingly, the entire cat family (including leopards, lions, tigers, cheetahs, jaguars, etc) cannot taste sugar. They have evolved to only enjoy the savory, salty taste of meat.  PRIMARY GUSTATORY CORTEX IS IN THE INSULA LOBE OF THE CEREBRAL CORTEX  OLFACTION (SMELL) The olfactory system is specialized for identifying specific molecules called odorants. Odorant molecules are volatile substances that have a molecular weight in range of approximately 15 to 300. Most of them are lipid soluble and of organic origin, however many substances that meet these criteria have no odor. The receptor proteins that transduce odorants into a change in membrane potential are metabotropic g protein-coupled receptors. Humans express ~400 different types of odorant receptors. Each one is sensitive to a specific molecule.  OLFACTION Olfactory epithelium is the tissue of the nasal sinus that sits underneath the skull (the cribriform plate) and contains olfactory receptors cells. Each olfactory cell expresses only one type of olfactory receptor protein. Olfactory receptor cells synapse in glomeruli in the olfactory bulb, which in turn sends axons into the brain. Each glomerulus processes information from just one type of olfactory receptor cell (expressing a particular type of olfactory receptor protein). Thus, each glomerulus processes a distinct odor. Although humans only have ~400 different olfactory receptor proteins/cell types, we can recognize up to ten thousand different odorants through combinatorial processing. Unlike taste, odors are largely not hard wired to be innately good or bad. Whether we like or dislike an odor is related to learned associations. Olfactory information does not relay in the thalamus. It goes directly to primary olfactory cortex in the temporal lobe and the amygdala.  PHEROMONES Although most odors are not innately perceived as good or bad in young animals, pheromones are different. Pheromones are molecules released by one animal to signal something to another member of the same species. Behavioural responses to pheromones are largely innate (hard-wired from birth). Pheromones strongly influence the behaviour of many organisms, from prokaryotic cells to complex multicellular animals, but their existence in humans is controversial. In many animals, especially insects, pheromones are used to attract or repel other members of the same species signal attractiveness and sexual receptivity mark a path to follow (as seen in ants) signal danger  PHEROMONE SIGNALING In mammals, the initial transduction and processing of pheromones occurs in the vomeronasal organ and ‘accessory olfactory bulb’, which are next to but distinct from the regular olfactory epithelium and ‘main olfactory bulb’, which process regular odors. Pheromones are detected by metabotropic vomeronasal receptors. These receptors are only distantly related to the olfactory receptors that detect normal odors, highlighting their different role. Humans, apes, and birds do not have functional vomeronasal organs. They only have regular olfactory epithelium that detects normal odors (but it is certainly possible that some pheromone-like signaling occurs in this structure in humans).  RODENT PHEROMONE EFFECTS Many mammals release pheromones in their urine. These molecules are usually not airborne. They must be actively sniffed or tasted to be detected. Rodents often sniff each other’s genitals and each others’ urine, and pheromone detection strongly influences their sexual behaviour. Female to male pheromone signaling is especially powerful. If the vomeronasal system is functional in a male mouse, they will only attempt to mate with female mice that are in heat. If the male vomeronasal system is damaged, they try to mate indiscriminately with any mouse, male or female. Male to female pheromone effects are more subtle. Females prefer males that have healthy testosterone levels (vs castrated males), presumably because of testosterone-induced male sex pheromone signaling. (Other male to female pheromone effects are described on the next slide.)  RODENT MALE TO FEMALE PHEROMONE EFFECTS Lee-Boot Effect – When female mice are housed together (without any male urine present), their estrous cycles slow down and eventually stop. Whitten Effect – Pheromones in the urine of male mice can trigger synchronous estrus cycles in groups of female mice. Vandenbergh Effect – Earlier onset of puberty seen in female animals that are housed with males. Bruce Effect – The tendency for female rodents to terminate their pregnancies following exposure to the scent of an unfamiliar male. The urine of castrated males does not produce these effects. In general, rodent male to female pheromone effects are subtle and hard to reproduce. And human pheromone effects are notoriously hard to replicate. Although several studies have found that females living in close proximity tend to have synchronized menstrual cycles, more recent work suggests that human menstrual synchrony does not exist.