Lecture 23 - Somatic Sensation

• Reflexes


  Rapid, reproducible and automatic responses to external stimuli, with no cortical involvement
• Voluntary movement


  Wide variety of different movements, requiring complex patterns of sensory and motor processing in cortex through motor pathways

  Requires complex patterns of sensory and motor processing in cortex through multiple pathways
• Spinal reflexes


  Use a single neural circuit that only goes as high as the spinal cord
• Reflexes do NOT require involvement of higher brain centres, whereas voluntary movements are initiated in the brain
• Stretch reflex


  1. Muscle stretch detected by muscle spindles

  2. Spindles send signal to spinal cord through sensory neurons.

  3. These neurons travel through bundle of nerves called dorsal root ganglion.

  4. Spinal cord sends signal back to muscle via. motor neurons.

  5. Muscle contracts rapidly

  Is monosynaptic!!
• Withdraw reflex


  1. Sensory neurons detect danger

  2. Sensory neurons pass through a bundle of nerve fibers called the dorsal root ganglion.

  3. Sensory neurons connect with interneurons in spinal cord

  4. Interneurons relay message to motor neurons

  5. Motor neurons instruct muscles to contract

  6. Hand pulls away

  Is polysynaptic!!
• Cerebellum


  • Coordinator
  • Located at back of brain, beneath cerebrum
  • Plans, executes, and learns motor programs
  • Integrates sensory information with planned actions
  • Modifies ongoing activity for smooth and accurate movements
  • Key for posture, balance, and body position
• Basal Ganglia


  • Located deep within cerebral hemispheres
  • Helps start, execute, and adjust movements
  • Also involved in habits and emotions
• You interact with your environment by picking up information and deciding what to do about it, this occurs in your body too!
• You have neurons in your body that pick up things like blood pressure, and muscle stretch that are specifically in tune with a particular stimulus
• There are some things we can't process because our sensory neurons can't pick them up, eg. some light waves
• Sensory transduction


  Conversion of a sensory stimulus into an action potential
• Sensory transduction (touch)


  1. Mechanoreceptor gets squished

  2. Membrane deformed, opening Na+ channels

  3. Na+ enters, depolarising membrane

  4. Action potentials sent to brain
• Information encoded by neural activity

  • Modality
  • Duration
  • Intensity
  • Location
• Thermoreceptor


  Responsible for sense of temperature
• Chemoreceptor


  Responsible for picking up chemicals
• Tactile receptor


  Mechanoreceptor responsible for touch, pressure, texture, vibration, and stretch
• Proprioceptor


  Mechanoreceptor responsible for sense of limb position relative to torso
• Baroreceptor


  Mechanoreceptor responsible for sense of pressure
• Nociceptor


  Responsible for sense of pain
• Thermoreceptor


  Nerve endings with temperature-gated ion channels, respond to different temperatures and changes in temperature, are phasic (fast-adapting)
• Transduction of temperature stimuli


  1. Temperature stimuli open temperature-gated Na+ channels

  2. Na+ enters, depolarising membrane

  3. If threshold reached, action potential fires
• Chemoreceptor


  Specialised receptor cells with chemically-gated ion channels, respond to different chemical concentrations
• Transduction of chemical stimuli


  1. Chemical stimuli bind to and open chemically-gated Na+ channels

  2. Na+ enters, depolarising membrane

  3. If threshold reached, action potential fires
• Mechanoreceptor


  Nerve endings with mechanically-gated ion channels, respond to physical forces that distort the plasma membrane
• Transduction of mechanical stimuli


  1. Mechanical stimuli deform membrane, opening mechanically-gated Na+ channels

  2. Na+ enters, depolarising membrane

  3. If threshold reached, action potential fires
• Tactile receptor


  Mechanoreceptor that responds to light touch, pressure, vibration, and/or stretch of skin, some are tonic (slow-adapting), some are phasic (fast-adapting)
• Proprioceptor


  Mechanoreceptor that responds to stretch/tension in receptors, informs about limb position in relation to torso, are tonic (slow-adapting)
• Nociceptor


  Nerve endings that respond to noxious (harmful, painful) stimuli, have temperature-gated, chemically-gated or mechanically-gated ion channels, are tonic (slow-adapting)
• Transduction of noxious stimuli


  Noxious stimuli cause ion channels to open, allowing Na+ to enter and depolarise the membrane
• Mechanoreceptors


  Detect touch, pressure, vibration, and/or stretch of skin
• Tonic (slow-adapting) mechanoreceptors


  • Constantly sending information, not easy to get them to stop sending information
• Phasic (fast-adapting) mechanoreceptors


  • Detect initial vibrations then stop firing
• Proprioceptors


  A type of mechanoreceptor
• Proprioceptors


  • Respond to stretch/tension in receptors (e.g. muscle spindles)
  • Inform about limb position in relation to the torso/trunk
  • Are tonic (slow-adapting)
  • Provide awareness of where your limbs are in relation to your torso
  • Required to move in a complex and coordinated way without the aid of your visual system
• Nociceptors


  Nerve endings that respond to noxious stimuli (harmful, painful) typically caused by tissue damage
• Nociceptors


  • Have either temperature-gated, chemically-gated or mechanically gated ion channels responding to extreme stimuli (e.g. excess heat or cold; chemical or membrane deformation)
  • Are tonic (slow-adapting)
• Transduction of noxious stimuli


  1. Noxious stimuli cause ion channels to open, allowing Na+ to enter and depolarise the membrane

  2. If threshold is reached in the nociceptor, an AP will fire and propagate to the brain
• The benefit of adaptation in phasic receptors means our brain is not 100% aware of every single stimulus, this stops us from becoming over stimulated - going mad
• Tonic Receptors (Slow-Adapting)


  • Provide a continuous response to a stimulus over time
  • Adapt slowly, meaning they continue to respond to a stimulus for a prolonged period without decreasing their firing rate
  • Are constantly active and provide ongoing information about the presence of a stimulus
  • Examples include nociceptors and proprioceptors