sensory information - gets to central nervous system through 4 steps 

• 4-step process
  • stimulation - activates sensory neuron
  • transduction - stimulus transformed into graded potentials
  • transmission - action potential lead to central nervous system
  • interpretation - brain analyzes/perceives senses from electrochemical messages
• 3 types of stimuli
  • mechanical forces - stimulate mechanoreceptors
  • chemicals - stimulate chemoreceptors
  • electromagnetic/thermal energy - stimulate photoreceptors
• free nerve endings - simplest sensory receptors, respond to mov’t of sensory neuron membrane, temperature change, chemicals in extracellular fluid
• exteroceptors - receptors receiving info from external environment
  • most developed in water for vertebrates
• interoceptors - receptors receiving info from within body
  • usually more simple than exteroceptors
• stimuli >> stimulus-gated ion channels open >> depolarization (receptor potential) >> info sent to brain

cutaneous receptors - skin receptors, respond to stimuli at border between external/internal 

• thermocreceptors - sensitive to changes in temperature
  • cold receptors - found right below epidermis
  • warm receptors - found deeper in dermis
• nociceptors - sensitive to pain
  • pain = stimulus causing damage to tissue
  • overstimulated sensory receptors can also produce pain
• mechanoreceptors - sensitive to forces applied to membrane
  • phasic receptors - intermittently activated, hair follicle receptors, Meissner’s corpuscles
  • tonic receptors - always activated, Ruffini corpuscles, touch dome endings (Merkel cells)
  • Pacinian corpuscles - monitor onset/removal of pressure

proprioceptors - muscle spindles giving info about position/mov’t of body parts 

• activated when muscle is stretched
• not found in bony fishes
• inhibits somatic motor neurons when muscle contracts too strongly

baroreceptors - monitor tension/stretch in blood vessel walls 

• measures blood pressure at carotid sinus (supplies blood to brain) and aortic arch (part of aorta very close to heart)
• low blood pressure >> less impulses from baroreceptors >> central nervous system stimulates sympathetic division to increase heart rate

chemoreceptors - chemicals/ligands lead to depolarization 

• used in smell/taste
• taste buds - collections of epithelial cells connected to neurons
  • most sensitive chemoreceptors in vertebrates
  • insects taste w/ their feet
  • papillae - raised areas in tongue/oral cavity where taste buds are found
  • sour/salty tastes act w/ ion channels
  • sweet/bitter tastes act w/ G proteins
• smell - receptors found in upper part of nasal passages
  • air particles must become extracellular fluid before activating the neurons
  • humans can tell apart many times more smells than tastes
• peripheral/central chemoreceptors - detect pH changes in blood and cerebrospinal fluid