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brain and behaviour > overview of pns > Flashcards

overview of pns Flashcards

1
Q

how many sets of dendrites do sensory neurons have?

A

2 sets

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2
Q

where is the cell body of a sensory neuron?

A

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

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3
Q

where do sensory neurons enter the spinal cord?

A

in the dorsal root

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4
Q

how do motor neurons exit the spinal cord?

A

via the ventral root

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5
Q

where are motor cell bodies found?

A

in the grey matter in the ventral root

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6
Q

how are spinal nerves formed?

A

from the fusion of dorsal and ventral roots

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7
Q

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

A

sensory axons cannot regenerate in the spinal cord

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8
Q

at what points can spinal nerves be damaged?

A

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

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9
Q

at what spinal level is CSF sampling done?

A

L3/L4

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10
Q

what is myelin?

A

sheath of fatty insulation wrapped around axons

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11
Q

what cells produce myelin?

A

Schwann cells

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12
Q

how do Schwann cells produce myelin?

A

Schwann cells wrap themselves around the axon multiple times

Multiple layers act as an electrical insulator around the nerve

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13
Q

what is meant by myelin having a ‘low electrical capacitance’?

A

charge cant be stored so current is forced to the nodes

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14
Q

explain how myelin can lead to a demyelinating disease?

A

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

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15
Q

what happens in a demyelinating disease?

A
  • 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
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16
Q

what is a node of ranvier?

A

small gap in the myelin where the 2 sheaths meet

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17
Q

what is the endoneurium?

A

thin protective membrane which surrounds individual sensory/motor nerve fibres

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18
Q

what is the perineurium?

A

surrounds fascicles – groups of functionally related nerve fibres

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19
Q

what is the epineurium?

A

thick connective tissues protecting several fascicles bundles together with blood vessels

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20
Q

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

A

share

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21
Q

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

A

have their own

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22
Q

what are sensory receptors?

A

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

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23
Q

what are the two types of sensory receptors?

A

free nerve endings

capsules

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24
Q

what are free nerve endings?

A

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

form a fine nerve plexus in dermis and other tissues

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25
Q

what stimuli do free nerve endings respond to?

A

chemical

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26
Q

what are capsules?

A

specialised connective tissue surrounds nerve endings

27
Q

what does the capsule determine?

A

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

28
Q

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

A

motor to skeletal muscle

29
Q

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

A

sensory from muscle spindle

30
Q

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

A

sensory from Golgi tendon organ

31
Q

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

A

sensory from skin, viscera etc.

sensory from secondary endings in muscle spindles

32
Q

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

A

motor to muscle spindles

33
Q

what is the function of A delta nociceptor/thermoreceptor?

A

fast pain from skin, muscle, joints, thermoreceptors

34
Q

what is the function of C nociceptor/thermoreceptor?

A

slow pain from skin, muscle, viscera, thermoreceptors

35
Q

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

A

amount of myelin decreases

36
Q

what are examples of encapsulated endings?

A

Meissner’s corpuscles
Pacinian corpuscles
Ruffini’s

37
Q

what does Meissner’s corpuscles detect?

A

sensitive to touch

38
Q

what do Pacinian corpuscles detect?

A

vibration and pressure

39
Q

what does Ruffini’s detect?

A

pressure and proprioception

40
Q

what is the main type of receptor in hairy skin?

A

hair follicle receptors - hybrid form of free and encapsulated endings

41
Q

what do hair follicle receptors respond to?

A

hair displacement

42
Q

what receptors does glabrous/non-hairy skin have?

A

Meissner’s corpuscles, Ruffini corpuscles and Pacinian corpuscles.

43
Q

what are capsules made up of?

A

connective tissue

44
Q

explain the process of a capsule forming

A

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
Q

what frequency do Pacinian corpuscles make nerve endings sensitive to?

A

high frequency > 50Hz

46
Q

how are encapsulated sensory axon endings activated?

A

by physical distortion of their terminal membrane

47
Q

explain the process of the initiation of sensory nerve fibres

A

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
Q

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

A

refractory period of the axon

49
Q

what are rapidly adapting capsules?

A

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

50
Q

name rapidly adapting capsules

A

Pacinian corpuscles

Meissner’s corpuscles

51
Q

what are slow adapting capsules?

A

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

52
Q

name slow adapting capsules

A

Ruffini’s endings, Merkel’s disks

53
Q

what is the receptive field?

A

the area of skin innervated by a single nerve fibre

54
Q

where do we have small receptive fields?

A

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

55
Q

where do we have large receptive fields?

A

areas not used for tactile discrimination - limbs

56
Q

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

A

gets smaller

57
Q

how does the brain localise a stimulus?

A

by processing info from many fibres simultaneously

58
Q

what does a progressive loss of nerve fibres lead to?

A

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

59
Q

name a condition where there is progressive loss of nerve fibres

A

diabetic neuropathy

60
Q

what happens when a peripheral nerve is cut?

A

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
Q

explain how an axon repairs itself after a peripheral nerve injury

A

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
Q

at what rate do nerve fibres regenerate?

A

1.5mm/day

63
Q

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

A

increases

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