1. Sensory Pathways
Functions of sensory pathways: sensory reception, transduction, transmission, and integration
For example, stimulation of a stretch receptor in a crayfish is the first step in a sensory pathway

2. Fig. 50-2
Weak receptor potential
Action potentials
Membrane potential (mV)
–50
Membrane potential (mV)
–70
Slight bend: weak stimulus
–70
Brain perceives slight bend.
Dendrites
Stretch receptor
Time (sec) 1 2 4
Axon 3
Brain
Muscle
Brain perceives large bend.
Action potentials
Large bend: strong stimulus
Strong receptor potential
Membrane potential (mV)
Figure 50.2 A simple sensory pathway: Response of a crayfish stretch receptor to bending
Membrane potential (mV)
–50
–70 1
Reception
–70
Time (sec) 2
Transduction 3
Transmission 4
Perception

3. Sensations and perceptions
Sensory Systems
Sensations and perceptions
Begin with sensory reception, the detection of stimuli (physical or chemical) by sensory receptors
Intergration of sensory information by brain is Perception
Exteroreceptors
Detect stimuli coming from the outside of the body.
Interoreceptors
Detect internal stimuli

4. Functions Performed by Sensory Receptors
All stimuli represent forms of energy
Sensation involves converting this energy
Into a change in the membrane potential of sensory receptors
Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor
This change in membrane potential is called a receptor potential which is then transmitted to other parts of the nervous system for processing and interpretation
Many sensory receptors are very sensitive: they are able to detect the smallest physical unit of stimulus

5. Therefore sensory receptors (sensory cells) are excitable ( capable of generating a charge across the membrane called the receptor potential): Why?
Sensory receptors could be modified neurons or special cells capable of generating a receptor potential and then releasing neurotransmitters that in turn stimulate the nervous system

6. Larger receptor potentials generate more rapid action potentials
Transmission
After energy has been transduced into a receptor potential, some sensory cells generate the transmission of action potentials to the CNS
Sensory cells without axons release neurotransmitters at synapses with sensory neurons
Larger receptor potentials generate more rapid action potentials

7. Integration of sensory information begins when information is received
Some receptor potentials are integrated through summation

8. Perceptions are the brain’s construction of stimuli
Stimuli from different sensory receptors travel as action potentials along different neural pathways
The brain distinguishes stimuli from different receptors by the area in the brain where the action potentials arrive

9. Amplification and Adaptation
Amplification is the strengthening of stimulus energy by cells in sensory pathways
Sensory adaptation is a decrease in responsiveness to continued stimulation

10. Types of Sensory Receptors
Based on the energy they transduce, sensory receptors fall into categories
Mechanoreceptors
Chemoreceptors
Electromagnetic receptors
Thermoreceptors
Pain receptor
Photoreceptors

11. Figure 45.1 Sensory Cell Membrane Receptor Proteins Respond to Stimuli

12. Figure 45.4 Olfactory Receptors Communicate Directly with the Brain

13. Figure 45.5 Taste Buds Are Clusters of Sensory Cells

14. Figure 45.6 The Skin Feels Many Sensations

15. Figure 45.7 Stretch Receptors Are Activated when Limbs Are Stretched

16. Figure 45.10 Structures of the Human Ear

17. Vibrating objects create percussion waves in the air
Hearing
Vibrating objects create percussion waves in the air
That cause the tympanic membrane to vibrate
The three bones of the middle ear
Transmit the vibrations to the oval window on the cochlea

18. These vibrations create pressure waves in the fluid in the cochlea
That travel through the vestibular canal and ultimately strike the round window
Cochlea
Stapes
Oval window
Apex
Axons of sensory neurons
Round window
Basilar membrane
Tympanic canal
Base
Vestibular canal
Perilymph
Figure 49.9

19. The pressure waves in the vestibular canal
Cause the basilar membrane to vibrate up and down causing its hair cells to bend
The bending of the hair cells depolarizes their membranes
Sending action potentials that travel via the auditory nerve to the brain

20. The cochlea can distinguish pitch
Because the basilar membrane is not uniform along its length
Cochlea (uncoiled)
Basilar membrane
Apex (wide and flexible)
Base (narrow and stiff)
500 Hz (low pitch)
1 kHz
2 kHz
4 kHz
8 kHz
16 kHz (high pitch)
Frequency producing maximum vibration
Figure 49.10

21. Each region of the basilar membrane vibrates most vigorously
At a particular frequency and leads to excitation of a specific auditory area of the cerebral cortex

22. Figure 45.8 The Lateral Line System Contains Mechanoreceptors

23. Figure 45.9 Organs in the Inner Ear of Mammals Provide the Sense of Equilibrium

24. Similar mechanisms underlie vision throughout the animal kingdom
Many types of light detectors
Have evolved in the animal kingdom and may be homologous

25. Vision in Invertebrates
Most invertebrates
Have some sort of light-detecting organ
One of the simplest is the eye cup of planarians
Which provides information about light intensity and direction but does not form images
Light
Light shining from the front is detected
Photoreceptor
Visual pigment
Ocellus
Nerve to brain
Screening pigment
Light shining from behind is blocked by the screening pigment

26. Figure 45.15 Ommatidia: The Functional Units of Insect Eyes

27. Figure 45.16 Eyes Like Cameras

28. The human retina contains two types of photoreceptors
Rods are sensitive to light but do not distinguish colors
Cones distinguish colors but are not as sensitive

29. Figure Rods and Cones

30. Figure 45.12 Rhodopsin: A Photosensitive Molecule

31. Figure 45.14 Light Absorption Closes Sodium Channels

32. Figure 45.13 A Rod Cell Responds to Light

33. Figure 45.19 Absorption Spectra of Cone Cells

34. Figure The Retina

35. Signals from rods and cones
Travel from bipolar cells to ganglion cells
The axons of ganglion cells are part of the optic nerve
That transmit information to the brain
Left
visual
field
Right
eye
Optic nerve
Optic chiasm
Lateral geniculate nucleus
Primary visual cortex
Figure 49.24