Other Senses

Somatosensation

  • Types of Sensation, Intensity, Timing, and Location
  • Types: Temperature (thermoception), pressure (mechanoception), pain (nociception), and position (proprioception)
  • Timing: Non-adapting, slow-adapting, fast-adapting.
  • Location: Location-specific nerves to brain

somatosensation

  • Sensory Adaptation and Amplification
  • Adaptation is change over time of receptor to a constant stimulus – downregulation
    • As you push down with hand, receptors experience constant pressure. But after few seconds receptors no longer fire.
    • Important bc if cell is overexcited cell can die. Ex. If too much pain signal in pain receptor (capsaicin), cell can die.
  • Amplification is upregulation
    • Light hits photoreceptor in eye and can cause cell to fire. When cell fires AP, can be connected to 2 cells which also fire AP, and so on.
  • Somatosensory Homunculus
    • Your brain has a map of your body – the cortex.
    • This part of cortex is the sensory cortex – contains the homunculus.
    • Info from body all ends up in this somatosensory cortex.
    • If there was a brain tumor, to figure out what part it’s in neurosurgeons can touch diff. parts of cortex and stimulate them. If surgeon touches part of cortex patients can say they feel it. Do it to make sure they aren’t removing parts in sensation.
    • This creates topological map of body in the cortex.

sensory-homunculus

  • Proprioception and Kinaesthesia
  • How can you walk in a pitch black room? You rely on your sense of balance/position –proprioception.
    • Tiny little sensors located in our muscles that goes up to spinal cord and to the brain. It’s sensitive to stretching.
    • Sensors contract with muscles – so we’re able to tell how contracted or relaxed every muscle in our body is.
  • Kinaesthesia is talking about movement of the body. Proprioception was cognitive awareness of body in space. Kinaesthesia is more behavioural.
    • Kinaesthesia does not include sense of balance, while proprioception does.
  • Pain and Temperature
    • Pain = nociception, temp = thermoception
    • In order for us to sense temperature, we rely on the TrypV1 receptor.
      • Interestingly, this receptor is also sensitive to pain.
      • There are thousands of these in membranes. Heat causes a conformational change in the protein.
      • When cell is poked, thousands of cells are broken up, and releases different molecules that bind to TrypV1 receptor. Causes change in conformational change, which activates the cell and sends signal to brain.
    • 3 types of fibres – fast, medium, slow.
      • A-beta fibres – Fast ones are thick and covered in myelin (less resistance, high conductance)
      • A-delta fibres -– smaller diameter, less myelin.
      • C fibres – small diameter, unmyelinated (lingering sense of pain).
    • Pain also changes conformation of receptors – capsaicin binds the TrypV1 receptor in your tongue, and triggers the same response.

Taste and Smell

  • Olfaction – Structure and Function
    • When you have a cold, you aren’t able to taste things very well.
      • When you eat, molecules travel up back of throat and some go into back of your nose. So you’re using your sense of smell in conjunction with taste.
      • If your smell is knocked out, you can’t taste things as well.
    • Smell is also known as olfaction
      • Area in nostril called the olfactory epithelium. Separating the olfactory epithelium from the brain is the cribriform plate. Above the plate is an extension from the brain – olfactory bulb – a bundle of nerves that sends little projections through cribriform plate into the olfactory epithelium, which branch off.
        • At end of each connection are receptors, each sensitive to 1 type of molecule.
        • Molecule travels into nose, binds one of receptors on nerve endings.
      • Zoom in on olfactory bulb
        • Imagine there’s olfactory cell sending projection to olfactory bulb. There are thousands of types of epithelial cells, each with dif receptor. Say this one is sensitive to benzene rings.
        • When it binds to receptor, triggers events that cause cell to fire. AP will end up in olfactory bulb. All cells sensitive to benzene will fire to one olfactory bulb – called a glomerulus.
        • They then synapse on another cell known as a mitral/tufted cell that projects to the brain.
      • The molecule binds to the GPCR receptor, G-protein dissociates and causes a cascade of events inside the cell. Binds to ion channel, which opens and triggers an AP.

olfactory-bulb

  • Pheromones
  • Why do dogs pee on fire hydrant? There are molecules released in the urine, which can be sensed by other animals through the nose – pheromones.
    • They’re specialized olfactory cells.
    • Cause some sort of response in animal smelling them.
    • Pheromone is a chemical signal released by 1 member of the species and sensed by another species to trigger an innate response.
    • Really important in animals, particularly insects – linked to mating, fighting, and communication.
  • Specialized part of olfactory epithelium in animals – the accessory olfactory epithelium. It sends projections to the accessory olfactory bulb.
    • Within the accessory olfactory epithelium, you have the vomeronasal system.
    • In vomeronasal system, there are basal cells and apical cells. They have receptors at tips.
    • Triangle will come in and activate receptor on basal cell here. Basal cell sends axon through accessory olfactory bulb to glomerulus, which eventually goes to the
      • Amygdala is involved with emotion, aggression, mating etc.
    • Humans have vomeronasal organ, but no accessory olfactory bulb.

Gustation

  • We have 5 main tastes, localized on the tongue – bitter, salty, sweet, sour, and umami (ability to taste glutamate).
  • Taste buds are concentrated anteriorly on the tongue. Taste buds can be fungiform (anterior), foliate (side), and circumvallate (back).
    • In each taste bud are the 5 receptor cells that can detect each taste. Each taste can be detected anywhere on the tongue.
    • Mostly on anterior part of tongue.
  • Each receptor has an axon, which all remain separate to the brain. And they all synapse on dif parts of the gustatory cortex. Known as the labelled lines model.
    • Glucose hits tongue, activates sweet cell (because it has sweet sensitive receptors), triggers cascade of events so cell depolarizes, and travels down axon to the brain.
    • Glucose binds GPCR, conformational change, G-protein dissociates, opens ion channels, cause cell to depolarize and fire an AP
  • Sweet, umami, and bitter cells GPCR
  • Sour and salty rely on ion channels. They bind to receptor directly, ex. NaCl binds to receptor and causes ion channel to open, and + ions outside flow in. Cell depolarizes and fires an AP.
  • What happens if we put salty receptor inside a sweet cell? Receptors in membrane bind to glucose. But let’s insert a salty receptor. Since axon from cell leads to brain, if NaCl comes in, it activates the receptor, + ions go inside, sweet cell depolarizes and fires AP, and brain interprets it as a sweet signal.
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