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Brain and Behaviour > Overview of the Peripheral Nervous System > Flashcards

Overview of the Peripheral Nervous System Flashcards

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1
Q

what are axons

A

these are structures that go away from the cell body

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2
Q

describe sensory afferents structure

A
  • they have two sets of dendrites-like processes
  • one in the periphery and one in the spinal cord
  • the cell body is not in the CNS and is off the axon in the dorsal root ganglion
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3
Q

what are the sensory axons in the body usually known as

A

sensory nerve fibres

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4
Q

in sensory neurones what happens if the space between the cell body and spinal cord is damaged

A
  • Sensory neurones do not have the cell bodies in the spinal cord as if these nerves are damaged between the dorsal root ganglion and spinal cord they cannot regenerated and grow
  • may contribute factor to spinal paralysis, as sensory input is a major factor in voluntary movements
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5
Q

what are spinal nerves formed from

A

the fusion of dorsal and ventral roots

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6
Q

where do motor neurones exit

A
  • motor neurones exit in the ventral root, they have their cell bodies in grey matter in the ventral spinal root
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7
Q

where can spinal nerves be damaged

A
  • Spinal nerves can be damaged where they leave the spinal cord and pass through the intervertebral foramina, this can happen during disc herniation
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8
Q

describe how a lumbar puncture takes place

A
  • Sampling of cerebrospinal fluids is done by a lumbar puncture at L3/L4 The spinal cord stops at L1/2, so the needle cannot damage it (nerves get pushed out of the way)
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9
Q

what happens when you are born and as you grow

A
  • Spinal canal elongates
  • But the nerves still go through the same verebtral hole
  • Therefore, the lumbar and sacral sections of the vertebrae have to go a long way down before they exit
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10
Q

what produces myelin

A

Schwann cells

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11
Q

what neurones have myelin in them

A
  • Individual sensory & motor neuron axons
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12
Q

what happens in demyelinating diseases

A
  • Demyelinating diseases of peripheral nerves damage the myelin sheath and block conduction of action potentials
  • this can be due to an autoimmune condition as myelin as antigens on tit
  • can originally cause intermittent function and then can lead to a complete failure
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13
Q

describe how myelin is formed

A
  • Myelin is formed by the schwann cell wrapping itself many times around the axon gradually squeezing out the cytoplasm until multiple layers of cell membranes are left, the multiple layers of lipid membrane provide an electrically insulating layer around the nerve fibres so that current flow can occur at the nodes of ranvier
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14
Q

what does it mean when myelin sheath has low electrical capacitance

A
  • The myelin sheath also has a low electrical capacitance meaning charge cannot be stored on it. This also forces current to flow only at the nodes.
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15
Q

what would happen if the neurone was not surrounded by the epineurium

A
  • The neurone has no tensile strength therefore would break easily if they were not surrounded by the epineurium
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16
Q

describe the connective tissue of the neurones

A
  • Individual sensory or motor nerve fibres are surrounded by a thin protective membrane the endoneurium. Groups of functionally related nerve fibres are collected together into nerve fascicles; each fascicle is surrounded by perineurium.
  • A whole peripheral nerve consists of several fascicles bundled together with blood vessels and all surrounded by epineurium (the epineurial sheath).
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17
Q

what happens to unmyelnated neurones and Schwann cells

A
  • There are many unmyelinated axons that are in the Schwann cell, there are several unmyelinated axons in one Schwann cell
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18
Q

what type of receptors are in the skin

A
  • In the skin we have sensory receptors, these generate action potentials in the sensory nerve fibres
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19
Q

what are the two classes of sensory receptors

A
  • Bear Nerve Receptors and capsulated nerve endings
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20
Q

describe how capsulated nerve endings decide what kind of stress that they respond to

A
  • All axons will fire action potentials if they are bent, but the connective tissue capulse is like a mechanical filter and determines the kind of stress that the capulse responds to
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21
Q

what do free nerve endings do

A

the sensory nerve branches and ends up lying in the extracellular space between tissue cells
- these cause pain

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22
Q

what are the types of peripheral nerves in men

A 
Aalpha  fibres
ABeta fibres
Agamma fibres
Adelta fibres
C fibres
23
Q

describe Aalpha fibres

  • function
  • diameter
  • velocity
A

alpha motor neurone

function

  • motor to skeletal muscle
  • sensory from muscle spindle
  • sensory from Golgi tendon organ

diameter = 10 um
velocity - 60+ m/s

24
Q

Describe Abeta fibres

  • function
  • diameter
  • velocity
A

function

  • sensory from skin and visceral
  • sensory from secondary endings in muscle spindles
diameter = 6-10 um 
velocity = 36-60m/s
25
Q

describe Agamma fibres

  • function
  • diameter
  • velocity
A 
function 
- motor to muscle spindles
diameter = 5-8um
velocity = 30-48 m/s
26
Q

Describe A delta fibres

  • function
  • diameter
  • velocity
A 
function 
- fast pain from skin, muscle and joints
- thermoreceptors 
diameter =1-6um 
velocity= 6-36m/s
27
Q

describe C fibres

  • function
  • diameter
  • velocity
A 
function 
- slow pain from skin, muscle and viscera 
- thermoreceptors 
diameter = less than 1um
velocity = less than 1 m/s
28
Q

what are the encapsulated endings found in the skin and what do they do

A
  • meisnner corpuscles – touch
  • pancinain corpuscles- vibration
  • ruffinis corpuscles- pressure
29
Q

what are the encapsulated endings found in the muscle and what do they do

A
  • muscle spindles

- golgi tendon organs

30
Q

where do free nerve endings form a nerve plexus

A
  • these form a very fine nerve plexus in the dermis and many other tissues
31
Q

what do free nerve endings respond to

A
  • these free nerve endings respond to chemical stimuli – change in pH or chemcials like peptides in the extracellular space
32
Q

what type of receptors are encapsulated and free nerve endings

A
  • enacapualted- mechanoreceptors

- free nerve endings – chemoreceptors

33
Q

what are the two types of skin

A

Glabrous skin

hairy skin

34
Q

what does glabrous skin respond to

A

has a variety of different receptors that respond to pressure, light touch, vibration

35
Q

what does hairy skin respond to

A

has mostly hair follicle receptors which are a hybrid form between free and encapsulated endings. They respond to hair displacement.

36
Q

what are the receptors in glabrous skin

A

Glabrous (non-hairy) skin contains several types of receptor eg:, Meissner’s corpuscles, Ruffini corpuscles and Pacinian corpuscles

37
Q

How do encapsulated corpuscles form

A
  • The capsule is made of connective tissue. When the nerve fibre first grows into the tissue it is bare and unencapsulated.
  • Cytokines released from the bare end of the nerve fibre cause local connective tissue cells to form a capsule around it as specified by the cytokines.
38
Q

how to capsules act as a mechanical filter

A

For example, the capsule of a Pacinian corpuscle makes the nerve ending selectively sensitive to high frequency (>50 Hz) vibration. On the other hand the capsule of a Ruffini ending makes the nerve fibre sensitive to pressure on the skin.

39
Q

how are encapsulated sensory axons activated

A
  • Encapsulated sensory axon endings are activated by physical distortion of their terminal membrane
  • They have mechanically sensitive sodium channels in their membrane – when the axon is bent it allows sodium ions to enter the cell which becomes depolarised, the depolarisation is not an action potential it is called a receptor potential, this tirggers an action potential or train of action potentials in the nerve fibres
  • It triggers an action potential near the brain at the first node of Ranvier
  • The receptor potential triggers action potentials in the nerve fibre.
40
Q

what is the difference between light pressure and strong pressure in the amount of action potentials generated

A
  • (In the case of a slowly adapting receptor) weak pressure on the skin bends the axon slightly and produces a small receptor potential which triggers action potentials at a low rate during the pressure.
  • Stronger pressure increases the frequency of action potentials; it takes less time to start each action potential
41
Q

what is maximum frequency limited by

A

by the refractory period of the axon

42
Q

what are the rapidly adaptive receptors

A

pacinaina corpsules, Meissner corspcules

43
Q

what are the slowly adaptive receptors

A

ruffini endings and merkels discs

44
Q

describe how rapidly adapting versus slow adaptive receptors work

A
  • Rapidly adapting receptors only respond at the beginning of a stimulus: they ‘fatigue’ after a second or so to a sustained steady stimulus.
  • Slowly adapting receptors will continue firing to a sustained stimulus but at a gradually reducing rate
45
Q

what is a receptive field

A

The area of skin innervated by a single nerve fibre is its receptive field (RF).

46
Q

describe how receptive fields vary in size

A
  • the further distal you are the smaller the receptive field
    – Glabrous skin has a high density of receptors therefore you have small receptor fields
  • Small receptor fields improve your abaility to localise something on your skin
47
Q

where are the receptive fields large

A
  • Receptive fields are larger on our proximal limbs, back and abdomen, areas not used for tactile discrimination
48
Q

How do cut nerves repair themselves

A
  • The distal schwann cells unwrap themselves from the dead fragemnets and divide to form a continuous line of cells lining the distal endoneurial sheaths.
  • The proximal cut ends of the nerve fibres form growth cones and start to grow back down inside the sheaths guided by chemical factors (cell adhesion molecules or ‘CAM’s) on the surface of the Schwann cells
49
Q

when a nerve is cut why do you suture the two parts together

A
  • If a nerve is cut you suture the two parts together – the physical joining sends out guide signals that guide the growth cones down the right way through the epineurium, if you don’t suture a nerve then the growth cone is not guided correctly back to the target tissue
50
Q

what are the symptoms of peripheral nerve injury

A
  • LOSS of reflexes
  • Hyposensitivity (anaesthesia)
  • Muscle weakness (paralysis)
  • Atrophy
  • Hyporeflexia (areflexia)
  • Possibly neuropathic pain – phantomic limb pain – growth cones come down and axons clump together, caused by C fibres
51
Q

what are the roles of microtubules in regernerating nerves

A
  • Microtubules in the regrowing axon transport growth-related materials down to the growth cone.
  • At the edge of the cone there are actin filaments which extend out in filopodia.
  • The tips of the filopodia attach (adhere) to the tissue and then the actin contracts to pull the cone along towards the denervated tissue.
52
Q

How does regeneration do

A
  • Behind the growth cone Schwann cells proliferate and start to wrap myelin around the nerve fibre.
  • The nerve fibres are initially very thin and slow conducting.
  • With time they enlarge but may never reach their original diameter.
  • . The further distal a nerve injury, the more likely there is to be successful regeneration.
53
Q

how long does nerve rengeration take

A
  • Nerve fibres regenerate at about 1.5 mm/day.

- Nerves may take months or even years to fully recover

Brain and Behaviour (34 decks)

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