overview of pns

Question 1

how many sets of dendrites do sensory neurons have?

Answer 1

2 sets

Question 2

where is the cell body of a sensory neuron?

Answer 2

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

Question 3

where do sensory neurons enter the spinal cord?

Answer 3

in the dorsal root

Question 4

how do motor neurons exit the spinal cord?

Answer 4

via the ventral root

Question 5

where are motor cell bodies found?

Answer 5

in the grey matter in the ventral root

Question 6

how are spinal nerves formed?

Answer 6

from the fusion of dorsal and ventral roots

Question 7

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

Answer 7

sensory axons cannot regenerate in the spinal cord

Question 8

at what points can spinal nerves be damaged?

Answer 8

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

Question 9

at what spinal level is CSF sampling done?

Answer 9

L3/L4

Question 10

what is myelin?

Answer 10

sheath of fatty insulation wrapped around axons

Question 11

what cells produce myelin?

Answer 11

Schwann cells

Question 12

how do Schwann cells produce myelin?

Answer 12

Schwann cells wrap themselves around the axon multiple times

Multiple layers act as an electrical insulator around the nerve

Question 13

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

Answer 13

charge cant be stored so current is forced to the nodes

Question 14

explain how myelin can lead to a demyelinating disease?

Answer 14

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

Question 15

what happens in a demyelinating disease?

Answer 15

• 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

Question 16

what is a node of ranvier?

Answer 16

small gap in the myelin where the 2 sheaths meet

Question 17

what is the endoneurium?

Answer 17

thin protective membrane which surrounds individual sensory/motor nerve fibres

Question 18

what is the perineurium?

Answer 18

surrounds fascicles – groups of functionally related nerve fibres

Question 19

what is the epineurium?

Answer 19

thick connective tissues protecting several fascicles bundles together with blood vessels

Question 20

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

Answer 20

share

Question 21

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

Answer 21

have their own

Question 22

what are sensory receptors?

Answer 22

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

Question 23

what are the two types of sensory receptors?

Answer 23

free nerve endings

capsules

Question 24

what are free nerve endings?

Answer 24

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

form a fine nerve plexus in dermis and other tissues

Question 25

what stimuli do free nerve endings respond to?

Answer 25

chemical

Question 26

what are capsules?

Answer 26

specialised connective tissue surrounds nerve endings

Question 27

what does the capsule determine?

Answer 27

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

Question 28

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

Answer 28

motor to skeletal muscle

Question 29

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

Answer 29

sensory from muscle spindle

Question 30

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

Answer 30

sensory from Golgi tendon organ

Question 31

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

Answer 31

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

Question 32

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

Answer 32

motor to muscle spindles

Question 33

what is the function of A delta nociceptor/thermoreceptor?

Answer 33

fast pain from skin, muscle, joints, thermoreceptors

Question 34

what is the function of C nociceptor/thermoreceptor?

Answer 34

slow pain from skin, muscle, viscera, thermoreceptors

Question 35

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

Answer 35

amount of myelin decreases

Question 36

what are examples of encapsulated endings?

Answer 36

Meissner's corpuscles Pacinian corpuscles Ruffini's

Question 37

what does Meissner's corpuscles detect?

Answer 37

sensitive to touch

Question 38

what do Pacinian corpuscles detect?

Answer 38

vibration and pressure

Question 39

what does Ruffini's detect?

Answer 39

pressure and proprioception

Question 40

what is the main type of receptor in hairy skin?

Answer 40

hair follicle receptors - hybrid form of free and encapsulated endings

Question 41

what do hair follicle receptors respond to?

Answer 41

hair displacement

Question 42

what receptors does glabrous/non-hairy skin have?

Answer 42

Meissner’s corpuscles, Ruffini corpuscles and Pacinian corpuscles.

Question 43

what are capsules made up of?

Answer 43

connective tissue

Question 44

explain the process of a capsule forming

Answer 44

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

Question 45

what frequency do Pacinian corpuscles make nerve endings sensitive to?

Answer 45

high frequency > 50Hz

Question 46

how are encapsulated sensory axon endings activated?

Answer 46

by physical distortion of their terminal membrane

Question 47

explain the process of the initiation of sensory nerve fibres

Answer 47

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

Question 48

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

Answer 48

refractory period of the axon

Question 49

what are rapidly adapting capsules?

Answer 49

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

Question 50

name rapidly adapting capsules

Answer 50

Pacinian corpuscles | Meissner's corpuscles

Question 51

what are slow adapting capsules?

Answer 51

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

Question 52

name slow adapting capsules

Answer 52

Ruffini’s endings, Merkel’s disks

Question 53

what is the receptive field?

Answer 53

the area of skin innervated by a single nerve fibre

Question 54

where do we have small receptive fields?

Answer 54

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

Question 55

where do we have large receptive fields?

Answer 55

areas not used for tactile discrimination - limbs

Question 56

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

Answer 56

gets smaller

Question 57

how does the brain localise a stimulus?

Answer 57

by processing info from many fibres simultaneously

Question 58

what does a progressive loss of nerve fibres lead to?

Answer 58

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

Question 59

name a condition where there is progressive loss of nerve fibres

Answer 59

diabetic neuropathy

Question 60

what happens when a peripheral nerve is cut?

Answer 60

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

Question 61

explain how an axon repairs itself after a peripheral nerve injury

Answer 61

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

Question 62

at what rate do nerve fibres regenerate?

Answer 62

1.5mm/day

Question 63

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

Answer 63

increases