1. The Peripheral Nervous System and Reflex Activity: Part A13
The Peripheral Nervous System and Reflex Activity: Part A

2. Peripheral Nervous System (PNS)
All neural structures OUTSIDE the brain and those starting in and ending outside of the spinal cord
Sensory nerves contain
Neurons receiving info in senses, skin, organs, muscles
Motor nerves contain
neurons going to muscles and glands

3. Structure of a nerve Axon Myelin sheath Endoneurium Perineurium
Epineurium
Fascicle
Blood
vessels
(b)
Figure 13.3b

4. How is a nerve different than a neuron?
A nerve is a group of axons
Most nerves have both Sensory and Motor neurons
Cranial nerves carry information to and from the head and shoulders and brain
Spinal nerves carry info to and from the body and the spinal cord.

5. Sensory “Receptors”
Are just modified dendrites of sensory neurons that respond to things other than just chemical neurotransmitters
They respond to changes in their environment (stimuli) rather than NT.

6. Perception: Modified dendrites respond to many things
Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch
Thermoreceptors—sensitive to changes in temperature
Photoreceptors—respond to light energy (e.g., retina)
Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry)
Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

7. Receptors can be classified by Location
1. Exteroceptors
-Respond to stimuli arising outside the body on skin and in eyes/ears/mouth/nose
2. Interoceptors (visceroceptors)
-Respond to stimuli from organs and blood vessels (low pressure=empty, high pressure= full)
3. Proprioceptors
-Respond to stretch in skeletal muscles, tendons, joints
-they Inform the brain of where your arms and legs are.
-How do you know where your hand is with your eyes closed?
Proprioceptors!

8. Receptors can be classified by how they are made
1. Unencapsulated dendrites
-free dendrites
-temperature, pain, chemicals
2. Encapsulated dendrites
-have specialized coverings
- to distinguish deep touch, light touch

9. Table 13.1

10. How do we perceive the environment?
Levels of neural integration in sensory systems:
Receptor level—the sensory receptors (dendrites or special receptors) (1st order)
Circuit level—ascending pathways through spinal cord/brainstem (2nd order)
Perceptual level—neuronal circuits in the cerebral cortex (3rd order)

11. Perceptual level (processing in cortical sensory centers)3
Perceptual level (processing in
cortical sensory centers)
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Cerebellum
Pons
Medulla 2
Circuit level
(processing in
ascending pathways)
Spinal
cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle 1
Receptor level
(sensory reception
and transmission
to CNS)
Joint
kinesthetic
receptor
Figure 13.2

12. Adaptation of Sensory Receptors
Have you ever gotten “used to” an itchy sweater, tight necktie or bra?
Adaptation is a change in sensitivity in the presence of a constant stimulus
Some receptors become less responsive over time and may turn off.

13. Adaptation of Sensory Receptors
Phasic receptors signal adapt quickly
only signal the beginning or end of a stimulus
receptors for pressure, touch, and smell
Tonic receptors adapt slowly or not at all
Nociceptors (you want to be aware of pain)
Proprioceptors (you want to know where your limbs are)

14. How does the brain know what stimuli are coming in?
Different senses GO to different places in the brain
Eye nerves go to back of brain (vision)
Touch nerves go to side of brain (feeling)
So different parts of the brain expect different information
The more intense the stimulus
the more action potentials are received
The bigger the area it is received from

15. Processing a the perceptual level
When information gets to the brain it needs to decode it:
By the frequency of APs sent by one or more neurons (how loud you shout!)
By the number of neurons sending information from a narrow or broad area of the body (how many people are shouting)

16. Perceptual level (processing in cortical sensory centers)3
Perceptual level (processing in
cortical sensory centers)
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Cerebellum
Pons
Medulla 2
Circuit level
(processing in
ascending pathways)
Spinal
cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle 1
Receptor level
(sensory reception
and transmission
to CNS)
Joint
kinesthetic
receptor
Figure 13.2

17. Pain is an important sense
Warns of actual or impending tissue damage
Stimuli include extreme pressure and temperature, and chemicals
Glutamate and substance P are NT that relay pain info
Some pain impulses are blocked by inhibitory NT called endorphins

18. Visceral Pain
Organs have pain, but there are very few sensory receptors in organs. Stimulation of visceral organ receptors is:
Felt as vague aching, gnawing, burning
Activated by tissue stretching, ischemia, chemicals, muscle spasms
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19. Referred Pain Referred pain
Pain from one body region perceived from different region
Since visceral and somatic pain fibers travel in same nerves the brain assumes stimulus from commonly felt areas, like skeletal muscles or skin
Ex., left arm pain (commonly felt in life) is perceived during heart even though the pain is actually from the heart (hardly ever felt in life!).

20. © 2013 Pearson Education, Inc.
Figure Map of referred pain.
Lungs and
diaphragm
Heart
Gallbladder
Liver
Appendix
Stomach
Pancreas
Small intestine
Ovaries
Colon
Kidneys
Urinary
bladder
Ureters
© 2013 Pearson Education, Inc.

21. © 2013 Pearson Education, Inc.
Regeneration of a nerve axon in a peripheral nerve. (1 of 4)
Endoneurium
Schwann cells
The axon
becomes
fragmented at
the injury site. 1
Droplets
of myelin
Fragmented
axon
Site of nerve damage
© 2013 Pearson Education, Inc.

22. © 2013 Pearson Education, Inc.
Figure Regeneration of a nerve fiber in a peripheral nerve. (2 of 4)
Macrophages
clean out the dead
axon distal to the
injury. 2
Schwann cell
Macrophage
© 2013 Pearson Education, Inc.

23. © 2013 Pearson Education, Inc.
Figure Regeneration of a nerve fiber in a peripheral nerve. (3 of 4) 3
Aligning Schwann cells
form regeneration tube
Axon sprouts,
or filaments, grow
through a
regeneration tube
formed by
Schwann cells.
Fine axon sprouts
or filaments
© 2013 Pearson Education, Inc.

24. © 2013 Pearson Education, Inc.
Figure Regeneration of a nerve fiber in a peripheral nerve. (4 of 4)
Schwann cell
New myelin
sheath forming
The axon
regenerates and a
new myelin sheath
forms. 4
Single enlarging
axon filament
© 2013 Pearson Education, Inc.

25. Regeneration of damaged axons:summary
If a cell body of a damaged nerve cell is still
intact, peripheral axon might regenerate
The axon fragments (Wallerian degeneration)
Macrophages clean dead axon
The Schwann cells are still in place and form a “regeneration tube” or tunnel.
Axon filaments grow through regeneration tube
Axon regenerates; new myelin sheath form
© 2013 Pearson Education, Inc.

26. Regeneration of CNS neurons
Most CNS neurons never regenerate if damaged
CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration
Astrocytes at injury site form scar tissue containing chondroitin sulfate that blocks axonal regrowth
Treatment
Neutralizing growth inhibitors, blocking receptors for inhibitory proteins, destroying chondroitin sulfate promising
© 2013 Pearson Education, Inc.