1. Chapter 13
PNS

2. PNS
Cranial nerves
Spinal nerves

3. Sensory Receptors
Specialized cells that monitor specific conditions in the body or external environment
Activation of sensory receptors results in depolarizations that trigger impulses to the CNS
Sensation: activation of sensory receptor cells

4. Sensation vs. Perception
Activity in sensory cells
Perception:
Conscious awareness of a sensation (in cortex)

5. Signaling
Stimulation of a receptor produces action potentials along the axon of a sensory neuron
Action potentials are all the same so:
The frequency and pattern of action potentials contains information about the strength, duration, and variation of the stimulus

6. Senses General senses: temperature Special senses Olfaction (smell)
pain
touch
pressure
vibration
proprioception
Special senses
Olfaction (smell)
Vision (sight)
Gustation (taste)
Equilibrium (balance)
Hearing

7. General receptor types
Exteroceptors
Provide information about the external environment
Proprioceptors
Report the positions of skeletal muscles and joints
Interoceptors
Monitor visceral organs and functions

8. Modality
Your perception of the nature of a stimulus (its modality) depends on the path it takes inside the CNS, especially, where in the brain the information ends up.

9. Adaptation of Sensory Receptors
Adaptation occurs when sensory receptors are subjected to an unchanging stimulus
Receptor membranes become less responsive
Receptor potentials decline in frequency or stop
Receptors responding to pressure, touch, and smell adapt quickly
Pain receptors and proprioceptors do not exhibit adaptation

10. Spinal Nerves
Figure 13–6

11. Spinal Nerves 31 pairs one per segment on each side of the spine
dorsal and ventral roots join to form a spinal nerve
Carry both afferent (sensory) and efferent (motor) fibers = mixed nerves

12. Spinal Nerve Organization
Every spinal nerve is surrounded by 3 connective tissue layers that support structures and contain blood vessels (just like muscles)
Epineurium:
outer layer
dense network of collagen fibers
Perineurium:
middle layer
divides nerve into fascicles (axon bundles)
Endoneurium:
inner layer
surrounds individual axons

13. Peripheral Distribution of Spinal Nerves
start where dorsal and ventral roots unite (just lateral to the vertebral column), then branch and form pathways to destination

14. Spinal Nerves: Rami
The short spinal nerves branch into three or four mixed, distal rami
Small dorsal ramus
Larger ventral ramus
Rami communicantes at the base of the ventral rami in the thoracic region

15. Nerve Plexuses
All ventral rami except T2-T12 form interlacing nerve networks called plexuses
Plexuses are found in the cervical, brachial, lumbar, and sacral regions
Each resulting branch of a plexus contains fibers from several spinal nerves
Each muscle receives a nerve supply from more than one spinal nerve
Damage to one spinal segment (gray matter) cannot completely paralyze a muscle

16. Peripheral Distribution of Spinal Nerves
Motor
fibers
Figure 13–7a

17. Motor fibers: First Branch
From the spinal nerve, the first branch (blue):
carries visceral motor fibers to sympathetic ganglion of autonomic nervous system (More about this later)

18. Communicating Rami
Also called Rami Communicantes, means “communicating branches”
made up of gray ramus and white ramus together

19. Communicating Rami White Ramus: Gray Ramus Preganglionic branch
Myelinated axons (hence: white)
Going “to” the sympathetic ganglion
Gray Ramus
Unmyelinated nerves (so: gray)
Return “from” sympathetic ganglion
Rejoin spinal nerve, go to target organ

20. Dorsal and Ventral Rami
Both are somatic and visceral outflow to the body
Dorsal ramus:
contains somatic and visceral motor fibers that innervate the back
Ventral ramus:
larger branch that innervates ventrolateral structures and limbs

21. Peripheral Distribution of Spinal Nerves
Sensory
fibers
Figure 13–7b

22. Sensory Nerves
Dorsal, ventral, and white rami (but not gray) also carry sensory information in addition to motor efferent outflow.

23. Dermatomes Bilateral region of skin
Each is monitored by specific pair of spinal nerves
Figure 13–8

24. Peripheral Neuropathy
Regional loss of sensory or motor function
Due to trauma, compression, or disease

25. Reflexes

26. Reflexes
Rapid, automatic responses to specific stimuli coordinated within the spinal cord (or brain stem)
Occurs via interconnected sensory, motor, and interneurons
Can be a movement, like a knee jerk, or visceral, like pupil dilation or swallowing

27. Functional Organization of Neurons in the NS
Sensory neurons:
about 10 million that deliver information to CNS
Motor neurons:
about 1/2 million that deliver commands to peripheral effectors
Interneurons:
about 20 billion that interpret, plan, and coordinate signals in and out = information processors

28. The Reflex Arc The wiring of a single reflex
Begins at sensory receptor
Ends at peripheral effector (muscle, gland, etc)
Generally opposes original stimulus (negative feedback)

29. 5 Steps in a Neural Reflex
Step 1: Arrival of stimulus, activation of receptor
physical or chemical changes
Step 2: Activation of sensory neuron
graded depolarization
Step 3: Information processing by postsyn. cell
triggered by neurotransmitters
Step 4: Activation of motor neuron
action potential
Step 5: Response of peripheral effector

30. 5 Steps in a Neural Reflex
Figure 13–14

31. Classification of Reflexes
There are several ways to classify reflexes but most common is by complexity of the neural circuit: monosynaptic vs polysynaptic

32. Monosynaptic Reflexes
Have the least delay between sensory input and motor output:
e.g., stretch reflex (such as patellar reflex)
Completed in 20–40 msec
No interneurons involved

33. Monosynaptic Reflex
A stretch reflex
Figure 13–15

34. Muscle Spindles The receptors in stretch reflexes
Bundles of small, specialized muscle fibers
Sense passive stretching in a muscle

35. Polysynaptic Reflexes
More complicated than monosynaptic reflexes
Interneurons involved that control more than 1 muscle group
Produce either EPSPs or IPSPs
Examples: the withdrawal reflexes

36. Withdrawal Reflexes
Move body part away from stimulus (pain or pressure):
flexor reflex:
pulls hand away from hot stove
crossed extensor reflex
Strength and extent of response depends on intensity and location of stimulus

37. A Flexor Reflex
Figure 13–17

38. Key = Reciprocal Inhibition
For flexor reflex to work:
the stretch reflex of the antagonistic (extensor) muscles must be inhibited
reciprocal inhibition by interneurons in spinal cord causes antagonistic extensors to be inhibited

39. Reflex Arcs Crossed extensor reflexes:
involves a contralateral reflex arc
occurs on side of body opposite from the stimulus

40. Crossed Extensor Reflexes
Occur simultaneously and coordinated with flexor reflex
Example: flexor reflex causes leg to pull up:
crossed extensor reflex straightens other leg to receive body weight

41. The Crossed Extensor Reflex
Figure 13–18

42. Integration and Control of Spinal Reflexes
Though reflex behaviors are automatic, processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord

43. Reinforcement of Spinal Reflexes
Higher centers can reinforce spinal reflexes:
by stimulating excitatory neurons in brain stem or spinal cord
creating EPSPs at reflex motor neurons
facilitating postsynaptic neurons

44. Inhibition of Spinal Reflexes
Higher centers can inhibit spinal reflexes:
by stimulating inhibitory neurons
creating IPSPs at reflex motor neurons
suppressing postsynaptic neurons

45. Voluntary Movements and Reflex Motor Patterns
Higher centers of brain incorporate lower, reflexive motor patterns
Automatic reflexes:
can be activated by brain as needed
use few nerve impulses to control complex motor functions
e.g. walking, running, jumping