Block D

• Sensory systems


  Allow us to interact with the external environment
• Sensory transduction


  Sensory stimulus is picked up by the sensory organ (through specialised cells - sensory receptors), which are transmitted as electrical signals until it reaches the brain (neural impulses) where a response is coordinated
• General sensory pathway


  1st order neuron (afferent neuron) -> 2nd order neuron -> sensory cortex (electrical signals can propagate)
• Sensory systems


  • Work by adaptation, sensory coding, a receptive field and topographic organisation
• Adaptation


  Stimulus intensity and the perception of the stimuli both reach a peak. While the stimulus intensity remains constant, the perception of it gradually decreases over a period of time, until baseline. This is adaptation of the organism's surroundings - the stimuli is eventually ignored/disappeared (if not a threat to organism)
• Sensory coding


  An input (e.g., visual), leads to action potentials, activation of the sensory neuron, and a sensory input (no. of neurons vs. no. of action potentials)
• Receptive field


  A region of space in which the presence of a stimulus will alter the firing of the neuron (alter the neural pathway)
• Characteristic frequency


  Frequency which can emit sound-evoked responses at minimum intensity (2 domains - frequency and intensity)
• Topographic organisation


  The maps in the brain, which part of the brain controls which part of the various sensory systems
• Topographic maps


  • Visual system - retinotopic map
  • Auditory system - tonotopic map
  • Somatosensory system - somatotopic map
  • Chemical systems - to some extent (depends on area)
• Eye structure


  • Cornea
  • Pupil
  • Lens
  • Retina (laminar structure; bottom contains cone and rod cells (photoreceptor cells))
  • Optic nerve
• Photoreceptor cells


  • Rods - dim light
  • Cones - bright light (RBG)
• Microvilli


  In the photoreceptor cells provide a large surface area for capturing photons to enhance the ability of the cell to function as a light detector
• Phototransduction


  1. Receptors - rhodopsin and retinal

  2. Rhodopsins trigger phototransduction by receiving photons (GPCR transmembrane protein)

  3. Retinal observes photons and causes rhodopsin to change

  4. Photoreception triggers intracellular signalling to hyperpolarise photoreceptor cells

  5. After the initial photoreceptor in cells, neural signals propagate through retinal layers. Retinal ganglion cells (output cells) sends signals to 2nd order visual area - lateral geniculate nucleus (thalamus)
• Visual pathway


  1. Retina (photoreceptors to ganglion cells)

  2. Optic nerve

  3. Optic chiasm

  4. Lateral geniculate body (thalamus)

  5. Primary visual cortex (LGB sends signals here)
• The LHS of the scene forms in the right hemisphere. The RHS of the scene forms in the left hemisphere
• In the brain, a visual network is constructed. The cerebral cortex has many visual cortical areas
• Audition


  The ability to detect and interpret sound waves
• Hearing ranges


  • Frequency - 20Hz-20kHz (humans), 40Hz-60kHz (Dogs)
  • Intensity - calm room (20-30 db SPL), normal conversation (40-60 dB SPL) [non-linear]
• Sound


  • Frequency - no. of complete wavelengths that travel past a given point/s (Hz)
  • Wavelength - distance from 1 peak of a wave to the next peak
  • Amplitude (intensity) - magnitude of a wave
  • Short wavelength = high frequencies, high pitch/tone
• Mammalian ear


  • 3 main components - outer, middle and inner
  • Outer - external ear pinna, auditory canal
  • Middle - 3 bones/ossicles (malleus, incus and stapes)
  • Inner - cochlea (coiled chamber of bone) and vestibular system
• Perilymph


  Fluid found with the cochlea, vestibular and tympanic canals (separated by cochlear duct)
• Transmission of sound waves

  Sound waves travel through the outer ear and into the eardrum. This causes the tympanic membrane to vibrate, these vibrations are transferred to the oval window, and the cochlea vibrates. This vibration of the oval window and cochlea sends pressure waves through the perilymph in the vestibular and tympanic canal. This causes vibrations on the basilar membrane (sheath-like, elastic fibres tensed across the cochlear duct)
• Frequency


  • Low - pressure waves take the complete route (vestibular and tympanic)
  • High - pressure waves pass form the vestibular, through the cochlear duct, through the basilar membrane, and reaches the tympanic canal
• Auditory pathway


  1. Cochlea

  2. Cochlear nucleus

  3. Superior olivary complex

  4. Inferior colliculus

  5. Medial geniculate body

  6. Auditory cortex

  7. Auditory transduction leads to mechanical vibrations, leading to electrical signals (auditory signals)
• Somatosensory receptors in the skin

  • Mechanoreceptors (touch)
  • Thermoreceptors (temperature)
  • Nociceptors (pain)
• Dorsal column-medial lemniscal pathway (DCML)

  1. Deals with the conscious appreciation of fine touch, 2-point discrimination, conscious proprioception, and vibration sensations from the entire body (except head)

  2. Dorsal root ganglia (1st order neuron)

  3. Dorsal column nuclei

  4. Ventral posterior nucleus (thalamus)

  5. Primary somatosensory cortex
• Spinothalamic tract


  1. Sensory tract that carries nociceptive, temperature, crude touch and pressure from our skin. Responsible for quick withdrawal reaction to a painful stimulus

  2. Dorsal root ganglia

  3. Spinal cord

  4. Ventral posterior nucleus of thalamus

  5. Primary somatosensory cortex
• Differences between DCML and Spinothalamic tract

  DCML carries vibration sensation, proprioception, and 2 point discrimination. Spinothalamic tract carries pain, thermal stimuli (lateral), pressure, crude touch (ventral)
• Receptive fields for touch

  Depend on the organisation of the group of cells that pass information to the somatosensory neuron
• The somatotopic map is heavily deformed, with a heavy representation of the face and hands
• Pain perception/nociceptor system


  • A-delta fibres - myelinated, sends signals quickly
  • C-fibres - unmyelinated, sends signals slowly
  • This results in a bi-phasic pain perception
• Chemical senses - smell

  1. Olfactory receptor cells -> olfactory nerves -> olfactory bulb -> olfactory tract -> olfactory striae -> olfactory cortex -> output targets of olfactory cortex (piriform cortex)

  2. After olfactory information is transmitted from the olfactory receptor neurons to mitral and tufted cells in the glomeruli, the axonal projections of the mitral and tufted cells form bundles that pass through the olfactory bulb and run dorsally, merging to form the olfactory tract

  3. The olfactory pathway is unique as mitral cells send their output to multiple brain regions. The thalamus doesn't directly contribute to olfactory processing
• Topographic maps in the olfactory system

  • Depend on the brain region. In the piriform cortex, there's unbiased organisation, while in the amygdala there's biassed organisation
  • Anterior olfactory nucleus (AON) - topographic organisation
• The existence of a 'tongue map' has been discredited, there is psychophysical evidence that demonstrates a small significant difference in the taste sensitivity across the tongue, soft palate, and pharynx (sites where taste buds have been documented)
• Taste buds


  • Contains 50~150 taste receptor cells. These cells contain receptors that extend upward inside the taste pore - these extensions are microvilli which come into contact with the chemicals in the food/drink consumed

  
• Gustatory pathway


  1. Taste bud -> nucleus of the solitary tract -> ventroposterior medial nucleus (Thalamus) -> insular cortex

  2. Taste transduction is initiated when taste stimuli interact with receptor sites on the exposed apical microvilli of receptor cells. This increaction leads to an increase in intracellular Ca2+ and transmitter release from the taste cell

  
• Gustotopic map


  • Topographic separation of sweet and bitter into distinct and non-overlapping gustatory fields in primary taste cortex reveals the existence of a functional 'gustotopic map' - different from other chemosensory modality
  • Gustatory cortex is located in the anterior insula in the temporal lobe and frontal opercular region

  
• Mammals


  • Over 5500 species, around 125 families, 27~29 orders
  • Found underground, on land, in air, in water, in deserts, in rainforests, on ice caps etc.
  • Can be defined by how their young develop, their size and diet
• Common features of mammals

  • 4-chambered heart (2 atria and 2 ventricles)
  • Warm-blooded (endotherms) - high and constant body temperature, independent of environment
  • Muscular diaphragm is used during breathing
  • Lower jaw consists of 1 bone
  • 3 bones in middle ear - malleus, incus, and stapes (hammer, anvil and stirrup)
  • Hair at some point in development
  • Milk-producing glands in the female of the species