NOTES: CH 50 - Sensory & Motor Mechanisms!

1. NOTES: CH 50 - Sensory & Motor Mechanisms!

3. Sensory Mechanisms • Sensations: action potentials that reach the brain via sensory neurons • Perception: interpretation of a sensation *(this occurs in the brain!) “If a tree falls in the woods….?”

4. Sensory receptors are: 1)exteroreceptors: detect stimuli outside the body (heat, light, pressure, chemicals) 2) interoreceptors: detect stimuli within the body (blood pressure, body position)

5. The transmission of signals to the nervous system begins with: • SENSORY TRANSDUCTION: -conversion of stimulus energy  a change in membrane potential

6. Sensory transduction is followed by: • AMPLIFICATION:strengthening of stimulus • TRANSMISSION: conduction of impulses to the CNS • INTEGRATION: processing of information (usually in the brain)

8. 5 types of sensory receptors: • MECHANORECEPTORS: stimulated by physical deformation (pressure, touch, stretch, motion) 2)PAIN RECEPTORS: stimulated by excess heat, pressure, specific chemicals (like those released by damaged tissues/cells)

10. Sensory receptors: (continued) 3) THERMORECEPTORS: respond to heat or cold; regulate body temp. 4) CHEMORECEPTORS: detect and transmit info. about solute concentration; involved w/taste & smell; osmoreceptors that regulate kidneys &rate of urine production 5) ELECTROMAGNETIC RECEPTORS: include photoreceptors (detect light); used in some animals for migration (use magnetic field of earth)

11. CHEMO-RECEPTORS in an insect!

12. CHEMORECEPTORS:

13. ELECTRO-MAGNETIC RECEPTORS:

14. ELECTROMAGNETIC RECEPTORS:

15. PHOTORECEPTORS: • PHOTORECEPTORS:  cells that contain pigment molecules that absorb light;  transduce the light stimulus into an electrical signal (by changing the membrane potential of the cell)

16. Vertebrate EYE: see fig. 50.17

17. Vertebrate EYE, continued

18. Vertebrate EYE, continued

21. Two types of photoreceptors on retina: 1) Rod cells: more sensitive to light (so they help us to see in dim light); do not distinguish colors; allow us to see at night; more numerous in nocturnal animals (approx. 125 million; make up 70% of all sensory receptors in the body!) 2) Cone cells: do not function at night; can distinguish colors in daylight; approx. 6 million

24. *Rods and cones contain visual pigments which consist of light-absorbing pigment molecules (retinal…derived from vitamin A) and a membrane protein (opsin). Opsin Retinal

26. RHODOPSIN: ● the visual pigment of ROD CELLS ● has 2 alternating forms depending on the conditions: DARK or LIGHT

27.  IN THE DARK… ● rhodopsin is INACTIVE. ● rod cells are highly permeable to sodium and are therefore in a DEPOLARIZED state ● rod cells in this INACTIVE (dark) state are releasing neurotransmitter molecules that INHIBIT the firing of postsynaptic cells in the retina. ● so in the dark, no message is sent from the rod cells to the visual centers of the brain

28.  IN THE LIGHT… ● rhodopsin absorbs light, and breaks apart, as its retinal component changes shape; opsin is now ACTIVE; ● this triggers a chain of metabolic events (signal-transduction pathway!) that makes the rod cell membrane less permeable to sodium and therefore hyperpolarizes the rod cell membrane; ● the rod cell synaptic terminals stop releasing neurotransmitter (which was inhibiting the postsynaptic cell);

29.  IN THE LIGHT… ● thus, a decrease in chemical signal to the cells with which the photoreceptors synapse serves as the message that rods have been stimulated by light…these postsynaptic neurons, now freed from inhibition, can develop action potentials which are transmitted to the brain for processing

30. Photoisomerization of rhodopsin:

33. ● over time, very bright light keeps the rhodopsin “bleached” (most of the rhodopsin decomposes into retinal and opsin) and rod cells eventually become unresponsive  cone cells take over; ● in the dark, enzymes convert the retinal back to its original form (rhodopsin) and the rod cells can once again respond to faint light (e.g. walking from a bright environment into a dark room or movie theatre…)

35. CONES and COLOR VISION • there are 3 subclasses of cone cells, each with a different opsin protein • each photopsin is best at absorbing a specific wavelength (color) of light • the 3 subclasses are: • red • green • blue

36. Integration of visual information: • begins in the retina • rod and cone cells synapse with: BIPOLAR CELLS, which synapse with: GANGLION CELLS • HORIZONTAL CELLS and AMACRINE CELLS also involved

39. Integration of visual info (cont.): Vertical pathway: info. passes directly from receptor (i.e. the rod or cone cell) to bipolar to ganglion cells Lateral pathway: info. passes from rod/cone to horizontal/amacrine cells and spreads out over several bipolar or ganglion cells *(nearby cells are stimulated; distant receptor & bipolar cells are inhibited  “lateral inhibition”…sharpens the edges of objects)

41. • Finally, the info. is transmitted along the optic nervesformed by axons of ganglion cells; • optic nerves from the 2 eyes meet at the OPTIC CHIASM;

42. • they pass through the thalamus; • they continue back to the primary visual cortex in the occipital lobe of the cerebrum (integration occurs here and in other cortex areas)

45. HEARING AND EQULIBRIUM: • vibrating objects create percussion waves in air; • these waves cause the tympanic membrane (EARDRUM) to vibrate w/same freq.;

47. • the 3 bones of the middle ear (malleus, incus, stapes) amplify the sound and transmit the mechanical movements to the oval window (membrane covering cochlea in the inner ear); • vibrations of oval window produce pressure waves in the fluid w/i the cochlea; • mechanoreceptors in the cochlea convert the energy of the vibrating fluid into action potentials (which travel on the auditory pathway to the cerebral cortex)