11: Sensory Systems Pt. 1

• Sensory organs: detect changes in the external and internal environment
• Sensory neurons: encode environmental changes as changes in frequency of action potentials
• Sensory information is sent to the CNS for interpretation
• Sensory systems allow animals to respond physiologically and/or behaviourally to internal/external environmental changes
• Transduction of Environmental Stimuli:

  1. Stimulus
  2. Receptor
  3. Change in frequency of action potentials in the afferent neuron
  4. CNS
  5. Physiological/behavioral response
• Sensory receptors are categorized by the type of stimulus
• Many types of sensory receptors transduce the change in the environment into electrical signals
• Mechanoreceptors: respond to distortion caused by pressure
• Photoreceptors: respond to particular wavelengths of light
• Chemoreceptors: detect specific molecules
• Thermoreceptors: detect changes in temperature
• Nociceptors: sense harmful stimuli such as tissue injury
• Electroreceptors: detect electrical fields
• Magnetoreceptors: detect magnetic fields
• The ability to sense a change in the environment depends on 3 processes:

  1. Transduction: the conversion of an external stimulus to action potentials in sensory neurons
  2. Transmission of the signal (action potentials)) to the CNS
  3. Interpretation of the signals by the CNS
• Sensory information is transduced by a specialized sensory neuron or by a specialized receptor cell that communicates with a sensory neuron
• In most sensory cells, specific stimuli result in ions flowing across a membrane and a change in membrane potential (ex. neurons)
• If ion flows cause the inside to become less negative than the resting potential, the membrane is depolarized
• If ion flows cause the inside to become more negative than the resting potential, the membrane is hyperpolarized
• The amount of depolarization or hyperpolarization of the sensory receptor is proportional to the intensity of the stimulus
• Change in a sensory receptor's membrane potential may result in a change in the firing rate of action potentials sent to the brain
• Receptor cells tend to be highly specific for a certain type of stimulus
• Each type of sensory neuron sends its signal to a specific portion of the brain

  • Different regions are specialized for interpreting different types of stimuli
• Mechanoreception: used for a variety of changes in the environment, but are all based on a similar mechanism

  • Ex. of changes:
  • Direct physical pressure on the skin
  • Hearing by detecting pressure changes in air
  • Stretching of muscles and blood vessels
  • Orientation of the head relative to gravity
• Direct physical pressure on plasma membrane, or distortion of membrane structures by bending, changes the conformation of ion channels (mechanoreception)

  • Causes the channels to open or close
  • Consequent change in ion flow leads to either depolarization of hyperpolarization
  • These changes modulate the frequency of action potentials in sensory neurons
  • These changes are transmitted to the brain for analysis/interpretation
• Sound: oscillation in pressure propagated in a medium (air, water)
• Hearing: the sensation produced by the wavelike changes in air pressure called sound
• Frequency of sound: the number of pressure waves that occur in 1 second
• We perceive differences in sound frequency as different pitches
• The ear transduces sound waves into action potentials that carry information to the brain
• The human ear has 3 sections:

  1. The outer ear
  2. The middle ear
  3. The inner ear

  Each is separated from the others by a membrane
• Outer Ear:

  • Collects pressure waves and funnels them into the ear canal, where they strike the tympanic membrane/eardrum
  • Tympanic membrane vibrates with the same frequency as the sound waves and passes the vibrations to 3 tiny bones that vibrate against one another in response
• Tympanic membrane = eardrum
• Stapes: made of 3 tiny bones in the middle ear

  • Vibrates against the oval window (the membrane that separates the middle ear from the inner ear)
• When the stapes vibrates against the oval window, the oval window oscillates and generates waves in the fluid inside a chamber called the cochlea
• The cochlea is where hair cells (mechanoreceptors) transduce vibrations into neuronal signals
• Because the tympanic membrane is about 15x larger than the oval window, the amount of vibration induced by sound waves is increased by a factor of 15 when it reaches the oval window
• The 3 inner ear bones act as levers that further amplify the vibrations from the tympanic membrane
• The overall effect is an amplification by a factor of 22x, meaning that soft sounds are amplified enough to stimulate the hair cells of the cochlea
• The cochlea is a coiled tube with a set of internal membranes that divide it into 3 chambers

  • Hair cells form rows in the middle chamber effectively sandwiched between membranes
  • They are embedded in a tissue that sits atop the basilar membrane
  • The sterocilia of the hair cells touch the surface of the tectorial membrane