Sensory

• Sensory system


  Part of the nervous system consisting of sensory receptors that receive stimuli from the internal and external environment, neural pathways that conduct this information to the brain and parts of the brain that processes this information
• Humans can perceive various types of sensations, and with this information, our motor movement is determined
• We become aware of the world by way of sensation
• Sensations can also be protective to the body, by registering environmental cold or warm, and painful needle prick, for example
• General sensations


  • Touch
  • Pain
  • Temperature
  • Proprioception
  • Pressure
• Special senses


  • Vision
  • Hearing
  • Taste
  • Smell
• Fine touch


  Receptors stimulated by light touch
• Deep touch


  Receptors stimulated by deep pressure
• Pressure


  Sense of pressure from long-standing sitting or pressure over any body part
• Vibration


  High-frequency vibrations perceived by the body to enable walking and fine movements
• Temperature


  Sensed by thermoreceptors to register hot and cold temperatures for protection
• Proprioception


  Sense of the position of different joints and muscles, perceived via receptors
• Pain


  Noxious sensation that is vitally important to remove the initiating stimulus
• All sensations begin with skin receptors and get conveyed through spinal neurons to the brain
• Receptors


  Structures that convert mechanical signals into electrical signals
• Types of receptors


  • Mechanoreceptors
  • Photoreceptors
  • Chemoreceptors
  • Thermoreceptors
  • Nociceptors
• Mechanoreceptors


  Activated by changes in pressure
• Photoreceptors


  Activated by light, including rods and cones in the retina
• Chemoreceptors


  Activated by chemicals, serving for olfaction and taste
• Thermoreceptors


  Located in the skin, including cold and warm receptors
• Nociceptors


  Activated by extreme pressure, temperature, or noxious chemicals, located in the skin
• Touch receptors


  • Merkel cells (slowly adapting type 1)
  • Ruffini corpuscles (slowly adapting type 2)
  • Pacinian corpuscles
  • Meissner corpuscles
• Merkel's disc


  Structurally simplest touch receptor, present in non-hairy skin, sensitive to stress/strain and useful for Braille reading
• Ruffini nerve endings


  Less dense touch receptors in hairy skin, sensitive to skin stretching
• Pacinian corpuscle


  Most sensitive touch receptor, a large onion-like structure that functions as a mechanical filter
• Meissner corpuscle


  Sensitive to dynamic changes in the skin, relatively insensitive to static changes
• Adaptation


  Degree to which receptors respond to sustained stimuli, ranging from very rapidly adapting to slowly adapting
• Thermal receptors


  Slowly adapting receptors that detect changes in skin temperature, including cold and warm receptors
• Vibration


  Perceived by Pacinian corpuscles and Meissner corpuscles, useful for balance and proprioception
• Pain


  Noxious stimuli activate free nerve endings in the dermis and epidermis
• Proprioception


  Sensed by Golgi tendon organs and muscle spindles, detecting joint position and movement
• Nerve fiber types


  • Group I (A-alpha)
  • Group II (A-beta)
  • Group III (A-delta)
  • Group IV (C)
• Sensory nerves emerge from the dorsal root ganglion and convey sensations from the peripheral nervous system to the central nervous system
• Sensory pathways


  1. Spinothalamic pathway (pain, temperature, touch, pressure)

  2. Dorsal column pathway (touch, vibration, proprioception)
• Spinothalamic pathway


  Anterior tract carries crude touch/pressure, lateral tract carries pain/temperature
• Dorsal column pathway


  Ascends ipsilaterally to medulla, then crosses midline to thalamus
• Sensory cortex


  Topographical representation of sensations in the parietal lobe, with areas 3, 1, and 2 processing different modalities
• Sensory system examination is necessary for patients with neuropathy or spinal injury
• Knowledge of the sensory system is essential to differentiate multiple types of sensations and understand diseases related to sensory derangement