The other senses, pain

• Sound

  • sound is waves of air compression
  • things that move, move the air around them
  • air gets compressed
  • molecules move, compression moves along
• Hearing

  • the ear has to translate changes in air pressure into nerve impulses
  • 3 parts:
  • outer: funnels the sound into ear
  • middle: registers and transmits the vibrations
  • inner: cochlea, generates nerve impulses

  
• Cochlea

  • identifies the amplitude and frequency of the sound
  • arranged in a spiral, getting narrower towards the end
  • 2 channels:
  • from oval window to end
  • from end to round window

  
• Neurons in the ear

  • basilar and vestibular membranes move when the pressure changes
  • this moves the hair cells
  • which have synapses onto cochlear neurons
  • which send messages to the brain
• Hearing frequency (pitch)

  • how does the cochlea identify frequency?
  • 3 leading theories:

  1. Place theory: different points along the cochlea are sensitive to diff frequencies

  • narrower areas react to higher frequencies
  • we must be phase-deaf

  1. Frequency theory: cochlear neurons fire at the frequency of the sound

  • human hearing range= 20-20,000 Hz
  • neurons can't fire that fast
  • partial: each cell could fire on some cycles

  1. combination

  • frequency theory for low frequency sounds (<100 Hz)
  • partial frequency theory for intermediate (<4000 Hz)
  • place theory for high frequency (>4000 Hz)
• Sound in the brain

  • from ear, the messages go to
  • cochlea nucleus (in brainstem)
  • thalamus (medial geniculate nucleus)
  • primary auditory cortex (A1) in the temporal lobe
  • like vision:
  • info crosses to contralateral sides
  • diff processing areas for diff features
  • some are very speacialized
  • Localized damage leads to various deficits
  • unlike vision:
  • basic hearing is sub-cortical
  • damage to A1 does not results in complete deafnes
• Localizing sound

  • often important to know where sound is coming from
  • we can use;
  • the difference in amplitude between the ears
  • the different in phase between the ears
  • the different in time between the ear
  • also many complex mechanisms in the cortex
  • cocktail party effect
  • attend to one ear only
  • ur name spoken in the other ear
  • name is heard but other things are not
• A neat trick


  Why are we not defined by our own speech?

  • the tympanic membrane reacts to vibrations
  • when we speak, we vibrate a lot of things
  • during activation of the larynx muscles, muscles in the middle ear also activates, inhibiting the movement of the middle ear bones
  • during speech, neurons in A1 are inhibited (probably by speech producing areas of the cortex)
  • if ur talking, ur literally not listening
• other senses

  • Mechanical
  • sensing physical changes (movements) of parts of the body
  • auditory; movement of the eardrum
  • vestibular; movement of head
  • touch; movement of the skin
  • chemical
  • sensing chemicals
  • smells; vaporized chemicals
  • taste; liquid/solid chemicals
• Vestibular sense

  • helps maintain balance and stabilize vision
  • we need to know the orientation of our own head (eyes) in 3 dimensions:
  • horizontal (move head side to side; say no)
  • vertical (look up, look down; say yes); posterior
  • roll (get water out of ear); anterior
  • each canal detects movement in one dimensions
  • similar mechanism to cochlea; moving fluid moves hair cells
  • helped by the utricle and saccule
  • also have hair cells
  • moved by otoliths

  
• Touch

  • many diff types of receptors in the skin
  • raw nerve endings
  • Merkel disks
  • Pacini corpuscles
  • different types of touch: soft sharp, deep, painful, itchy, hot, cold
  • pathways:
  • sensory nerves connect to spinal cord
  • pathway up through thalamus (ventral posterolateral nucleus)
  • then up to primary somatosensory cortex
• The chemical senses

  • taste and smell
  • much of the input combines in the brain
  • flavour= taste + smell
  • this is why our noses are over our mouths
  • unlike other senses
  • the receptors are exposed to the chemicals
  • receptors can be modified by chemicals
  • cells are damaged and need to be replaced throughout lifetime
• Taste buds

  • chemicals on the tongue are detected by taste receptors, inside tastebuds
  • some of them fire APs but its unknown
  • 5 types
  • salty, sweet, bitter, savoury (glutamate), sour
  • each type has diff receptors on membrane
  • uniformly arranged on tongue
• Taste pathway

  • receptors in the tongue activate sensory neurons
  • some cells respond to only one taste
  • some cells respond with diff patterns to all tastes
  • neurons enter medulla oblongata
  • to thalamus (ventral posterior medial nucleus)
  • to gustatory tract cortex (insula)
• Smell

  • unlike other senses;
  • hundreds of diff receptors, each for one odor
  • pathway
  • odor receptors in nose (neurons)
  • direct to limbic system, but also thalamus
  • to olfactory cortex
  • cortical cells are arranged topographically
  • neurons close to each other react to things that smell similar
• Sensory summary

  information from our senses:

  • enters through special receptors
  • translate the signal from a physical change in the environment into the neural signal
  • signal passes into brain
  • through the spinal cord if from the body
  • directly if from head
  • goes to thalamus
  • expect olfaction
  • dedicated nucleus
  • up to dedicated cortical area
  • further processing throughout the cortex

  
• Pain


  definitions

  • pain= perception of aching or unbearable sensation (feeling pain)
  • nociception= sensory process that provides signal possible causing pain
• Pain pathway

  • Nociceptors in the skin; raw nerve endings
  • several kinds of axons varying in thickness, myelination
  • some nociceptors are on internal organs, send signals to same part of spine
  • axons end in spinal cord
  • thalamus
  • then to cortex
  • somatosensory cortex
  • anterior cingulate gyrus; mediates emotional response to pain
  • other areas: hippocampus, amygdala, prefrontal cortex
• How pain hurts

  Several mechanisms modulate pain signals

  • bradykinin (small peptide):
  • released by cells in skin in response to damage
  • stimulates (depolarizes) some nociceptors (that react to heat)
  • contributes to some chronic pain (B1 receptors over-expressed)
  • prostaglandins (hormones):
  • sensitize nociceptors
  • cause inflammation, fever
  • substance P (neuropeptide):
  • released from nociceptor axons
  • intensifies perception of pain
  • causes vasodilation, histamine release
• Drugs

  • Substance P is inhibited
  • by opioids (morphine, herion)
  • by endorphins
  • which bind to presynaptic receptors and prevent it from being released into body
  • also prevent GABA from being released in the brain
  • which prevent GABA from preventing the release of dopamine, which increases dopamine levels, which decreases pain
• Other pains


  The pain pathway can be stimulated by other stimuli

  • emotional pain
  • sympathetic pain
  • these activate the same brain area, especially cingulate cortex
  • hurt feeling can be relieved with acetaminophen
• Reducing pain


  why does rubbing/pressing reduce pain?

  • gate control theory
  • pain tracts connect to nociceptors and touch cells
  • also connect inhibitory interneuron (excited by touch, inhibited by pain)
  • touch at the same time as pain can inhibit pain signals from processing to brain