Sensory organs and structures in animals Channels for information to be delivered to

1. Sensory organs and structures in animals Channels for information to be delivered to the brain (“biological transducers”) What types of stimuli? (what forms of energy?) electrical mechanical chemical radiant Stimuli are transformed into nerve impulses interpreted by specialized areas of the brain (Muller, 1830s)

2. What are receptors? specialized neurons or epithelial cells often located in special sensory organs Exteroreceptors- detect external environment; located on or near body surface Interoreceptors detect stimuli within the body Proprioceptors- muscle tension; body position Energy is converted to membrane potentials

3. Sensory transduction (e.g., taste) G protein pathway

4. Taste involves chemoreceptors (for chemicals in solution) So does smell (for airborne chemicals)

6. A stimulus may be amplified Accessory structures in sensory organ Some receptors are more sensitive than others Touch receptors (along with hearing and equilibrium) utilize mechanoreceptors

7. Pain receptors (nocireceptors) harder to stimulate, thankfully Types of pain receptors proprioreceptors- body position and movement cutaneous receptors (aka generalized) touch, pressure, pain, heat, cold special senses- sight, hearing, taste, smell

8. Sensory adaptation Phasic receptors- slow down after initial stimulus adaptation- e.g., odor, touch, temperature fast-adapting Tonic receptors- fire at constant rate slow-adapting Receptors deliver specific stimulus; brain filters out a lot of the stimuli Produce action potential in response to stimulus

9. Cutaneous sensations Different neurons perceive different sensations Free nerve endings- light touch, temperature, pain Merkel’s discs-sustained touch and pressure (superficial) Ruffini’s corpuscles sustained pressure (deep) Meissner’s corpuscles- texture, slow vibration Pacinian corpuscles- deep pressure, fast vibration

11. Cold receptors are more numerous and closer to surface than heat receptors Cold receptors are inhibited by warming Sharp pain- conducted by myelinated axons dull pain by unmyelinated axons Capsaicin receptor- ion channel; produces sensation of pain in response to high heat

12. Neural pathways to postcentral gyrus Ascending fibers in dorsal columns of spinal cord Synapse in medulla oblongata Ascends to thalamus Sensory neurons project to postcentral gyrus

13. Receptive fields in skin large if receptors are few, small if receptors are dense measured by two-point touch threshold Lateral inhibition- sensory neurons most strongly affected by a stimulus inhibit others in nearby receptive fields

15. Hair cells are mechanoreceptors Often project into a fluid-filled compartment (as in inner ear)

16. Sound receptors systems in animals Arthropods- air pockets surrounded by tympanic membrane (most invertebrates have no “hearing”) Some moths can detect ultrasonic waves (to avoid bats that prey on them Vertebrate ear is much more complex! Human ear is example, of course

20. Evolution of middle ear Amphibians, reptiles and birds have a single ossicle (columella; stapes) Stapes originated in fish, as a jaw support Other ossicles also developed from early jaw structures

23. Equilibrium in invertebrates Statocysts function like utricle and saccule Statoliths “acquired” from environment

26. Lateral line Fishes and (aquatic) amphibians Analogous to inner ear

27. Photoreception (vision) Some invertebrates have light-sensitive cells scattered over their bodies locomotion, reproductive behaviors The dinoflagellate Nematodinium has a lens, light-gathering chamber and photoreceptive pigments- and is single-celled

28. Cannot form image

29. Invertebrates: single-lens or compound eyes Compound: insects, crustaceans, some annelids Cannot focus image well but can see movement Insects can see color, and some can see in ultraviolet range

32. Single-lens eyes Annelids, mollusks, spiders, vertebrates

35. Types of photoreceptors Rods- light intensity Cones- color vision, visual acuity Many more rods than cones In human, only cones are found on the fovea centralis

36. Rods can make more rhodopsin in the dark

39. Brain actually determines what we see