Somatic senses

• Somatic senses are a major component of the somatic nervous systems.
• Somatic sensations arise from stimulation of sensory receptors in the skin, mucus membranes, muscles, tendons and joints
• Somatic sensory receptors are distributed very unevenly:

  • some parts of the body contain only a few
  • some parts of the body are densely populated with receptors such as the fingertips, lips, tip of the tongue
• Tactile sensations include

  • touch
  • pressure and vibration
  • itch
  • tickle

  They arise by activation of the same type of receptor: encapsulated mechanoreceptors
• Touch: sensation resulting from stimulation of receptors in the skin or subcutaneous layer. They can be fast or slow adapting
• Pressure: sustained sensation felt over a large area. It occurs in deeper tissues than touch
• Vibration: result from rapidly repetitive sensory signs
• Itch: results from stimulation of free nerve endings, this stimulation comes from chemicals released in body
• Tickle: results from stimulation of free nerve endings, it requires stimulus from an outside self (effects of attempts to tickle oneself are blocked by signals to the cerebellum)
• Thermoreceptors are free nerve endings of 2 types

  • cold receptors are activated by temperatures from 10-40 degrees
  • warm receptors are activated by temperatures from 32-48 degrees
• Both types of thermoreceptors adapt rapidly at the start of a stimulus but continue to generate nerve impulses more slowly during a prolonged stimulus
• Temperatures below 10 or above 48 degrees stimulate nociceptors (pain) rather than thermoreceptors (detect danger)
• Nociceptors are free nerve endings. They respond to several types of stimuli:

  • excessive stimulation of sensory receptors (hot,cold)
  • excessive stretching of a structure (sprain)
  • presence of certain chemical substances
• Pain may persist even after pain producing stimulus is removed because nociceptors have very little adaptation (protective function). Nociceptors are found in every tissue
• Two types of pain

  • fast pain
  • slow pain
• Fast pain

  • occurs within 0.1 seconds of stimulus
  • known as acute pain
  • not felt in deeper tissues
  • very localized
• Slow pain

  • beings 1 second or after stimulus
  • gradually increases in intensity
  • can be felt in skin and deeper tissue
  • localized pain but over a larger area
• If slow pain stimulus arises from organ pain and can be felt in the area right above the organ or sometimes far from the organ, it is labelled as referred pain
• Proprioceptive sensations allow us to know where our head and limbs are located and how they are moving even if we are not looking at them
• Proprioceptors detect proprioceptive sensations. They are located in the:

  • muscles
  • tendons
  • joints
  • inner ear (head)
• Kinesthesia is the perception of body movements
• Signals sent from proprioceptors pass along to spinal nerves to spinal cord up to somatosensory area of cerebral cortex and cerebellum
• Proprioceptors have very little adaptation, the brain continually receives input on body position so it can constantly make adjustments to ensure coordination