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Flashcards in overview of pns Deck (63):
1

how many sets of dendrites do sensory neurons have?

2 sets

2

where is the cell body of a sensory neuron?

on a t-junction off of the axon in the dorsal root ganglia

3

where do sensory neurons enter the spinal cord?

in the dorsal root

4

how do motor neurons exit the spinal cord?

via the ventral root

5

where are motor cell bodies found?

in the grey matter in the ventral root

6

how are spinal nerves formed?

from the fusion of dorsal and ventral roots

7

what happens to sensory axons when dorsal roots are severed between dorsal root ganglion and the spinal cord?

sensory axons cannot regenerate in the spinal cord

8

at what points can spinal nerves be damaged?

where they leave the spinal cord and pass through the IV foramina

9

at what spinal level is CSF sampling done?

L3/L4

10

what is myelin?

sheath of fatty insulation wrapped around axons

11

what cells produce myelin?

Schwann cells

12

how do Schwann cells produce myelin?

Schwann cells wrap themselves around the axon multiple times
Multiple layers act as an electrical insulator around the nerve

13

what is meant by myelin having a 'low electrical capacitance'?

charge cant be stored so current is forced to the nodes

14

explain how myelin can lead to a demyelinating disease?

myelin has antigenic proteins on its surface
can trigger an autoimmune response
leads to demyelinating disease of peripheral nerves

15

what happens in a demyelinating disease?

- Schwann cell dies so there’s a larger node of Ranvier.
- AP can’t jump the gap  causes intermittent conduction as node gets wider
- As the autoimmune disease dies down, the myelin regenerates
- Nerve isn’t damaged

16

what is a node of ranvier?

small gap in the myelin where the 2 sheaths meet

17

what is the endoneurium?

thin protective membrane which surrounds individual sensory/motor nerve fibres

18

what is the perineurium?

surrounds fascicles – groups of functionally related nerve fibres

19

what is the epineurium?

thick connective tissues protecting several fascicles bundles together with blood vessels

20

do small axons have their own Schwann cells or do they share?

share

21

do grown axons each have their own Schwann cell or do they share?

have their own

22

what are sensory receptors?

how the peripheral branches of the sensory nerve fibres end in the skin/muscle

23

what are the two types of sensory receptors?

free nerve endings
capsules

24

what are free nerve endings?

sensory nerve branches end up lying in the extracellular space between tissue cells

form a fine nerve plexus in dermis and other tissues

25

what stimuli do free nerve endings respond to?

chemical

26

what are capsules?

specialised connective tissue surrounds nerve endings

27

what does the capsule determine?

the kind of stimulus the nerve ending will be sensitive to e.g. slow pressure, vibration, stretch

28

what is the function of an A-alpha alpha-motoneuron?

motor to skeletal muscle

29

what is the function of A-alpha 1a/muscle spindle afferent?

sensory from muscle spindle

30

what is the function of A-alpha 1b/golgi tendon afferent?

sensory from Golgi tendon organ

31

what is the function of A-beta general sensory afferents?

sensory from skin, viscera etc.
sensory from secondary endings in muscle spindles

32

what is the function of A-gamma gamma-motoneurons?

motor to muscle spindles

33

what is the function of A delta nociceptor/thermoreceptor?

fast pain from skin, muscle, joints, thermoreceptors

34

what is the function of C nociceptor/thermoreceptor?

slow pain from skin, muscle, viscera, thermoreceptors

35

how does the amount of myelin change as you go from A-alpha to A-delta?

amount of myelin decreases

36

what are examples of encapsulated endings?

Meissner's corpuscles
Pacinian corpuscles
Ruffini's

37

what does Meissner's corpuscles detect?

sensitive to touch

38

what do Pacinian corpuscles detect?

vibration and pressure

39

what does Ruffini's detect?

pressure and proprioception

40

what is the main type of receptor in hairy skin?

hair follicle receptors - hybrid form of free and encapsulated endings

41

what do hair follicle receptors respond to?

hair displacement

42

what receptors does glabrous/non-hairy skin have?

Meissner’s corpuscles, Ruffini corpuscles and Pacinian corpuscles.

43

what are capsules made up of?

connective tissue

44

explain the process of a capsule forming

nerve fibre grows into tissue bare and unencapsulated
cytokines released from bare end of the nerve fibre
causes local connective tissue cells to form a capsule around it

45

what frequency do Pacinian corpuscles make nerve endings sensitive to?

high frequency > 50Hz

46

how are encapsulated sensory axon endings activated?

by physical distortion of their terminal membrane

47

explain the process of the initiation of sensory nerve fibres

when axon is bent, Na+ ions enter through mechanically sensitive sodium channels in their membrane --> depolarisation (receptor potential)
receptor potential triggers action potential
stronger receptor potential = high AP frequency

48

what is the maximum frequency of action potential firing limited by?

refractory period of the axon

49

what are rapidly adapting capsules?

only respond at the beginning of a stimulus – they fatigue after a second or to a sustained steady stimulus

50

name rapidly adapting capsules

Pacinian corpuscles
Meissner's corpuscles

51

what are slow adapting capsules?

continue firing to a sustained stimulus but at a gradually reducing rate

52

name slow adapting capsules

Ruffini’s endings, Merkel’s disks

53

what is the receptive field?

the area of skin innervated by a single nerve fibre

54

where do we have small receptive fields?

on skin we use for tactile discrimination – e.g. our skin. Allows us to localise stimuli

55

where do we have large receptive fields?

areas not used for tactile discrimination - limbs

56

how does the size of the discriminative field change the more distal you go?

gets smaller

57

how does the brain localise a stimulus?

by processing info from many fibres simultaneously

58

what does a progressive loss of nerve fibres lead to?

leads to a progressive worsening in ability to localise stimuli as there is less and less overlap

59

name a condition where there is progressive loss of nerve fibres

diabetic neuropathy

60

what happens when a peripheral nerve is cut?

the distal part disconnected from its cell body and degenerates
distal Schwann cells unwrap themselves from dead fragments
divide to form a continuous line of cells lining the distal endoneurial sheaths
Proximal cut ends form growth cones
grow back down inside the sheaths guided by chemical factors (cell adhesion molecules) on the surface of Schwann cells

61

explain how an axon repairs itself after a peripheral nerve injury

microtubules transport growth-related materials down to the growth cone
actin filaments at the edge of the cone extend out in filopodia
tips of filopodia attach to the tissues
actin contracts - pulls cone towards denervated tissue
schwann cells proliferate behind the growth cone
start to wrap myelin around nerve fibre

62

at what rate do nerve fibres regenerate?

1.5mm/day

63

how does the likelihood of successful regeneration change as a nerve injury gets more distal?

increases