Action Potential and Myelination

what is an action potential

highly localized depolarization

What happens in an action potential

Na+ and positive charges diffuse down axon along the membrane
- allows for propagation of depolarization

Electrochemical impulse

wave of depolarization sweeping from soma to axon

Action potential response type

all or nothing
- no way to stop once triggered

what threshold is crossed for an action potential to be triggered

-55 mV

Amplitude of ALL action potentials

100 mV

action potentials do not occur in

dendrites

A stronger stimulus increases

-frequency of action potentials
-number of axons stimulated

Absolute Refractory Period

time following an action potential in which membrane cannot respond to stimulus

Absolute Refractory Period results in

inactivation of Na+ voltage gated channels

Channels must return to resting state

before stimulus has an effect
this prevents retrograde propagation of action potential

time of absolute refractory period determined by

axon diameter

large axon ARF period

0.4 msec
up to 1000 impulses per second

small axon ARF period

4 msec periods
250 impules per second

Relative Refractory Period

time period pf transition to resting state following absolute refractory period

ion channel status during Relative Refractory Period

K+ channels are still open
axon is after-yperpolarized
some Na+ channels are closed and some are inactivated.

A strong stimulus may induce new action potential however,

must be strong enough to overcome after-hyperpolarization without being able yo open all of the Na+ channels

Diffusion

alone is insufficient to conduct a signal

Why is Diffusion insufficient alone?

neurons have high internal resistance and leak ions

decrement

the progressive loss of signal amplitude

Cable properties

allow for diffusion 1-2 mm in dendrites
-NOT IN AXONS

propagation

allows axons to conduct an impulse
POSITIVE FEEDBACK LOOP

action potentials self regenerate through

more Na+ ions enter the system in each segment

all-or-nothing response means

action potential is regenerated without decrement

Continuous conduction

A relatively slow type of neuronal action potential conduction

the thicker and warmer the signal

the faster the signal

Myelin Sheath

insulates neurons to conduct impulses faster

Neurilemma

covers axons in PNS
- nucleus and cytoplasm of Schwann cells

Neurilemma is not

myelination

Myelin Sheath covers

PNS and CNA axons

Myelination increases

from birth to maturity

Myelin Sheath contributes to

increased fine motor control

Myelin Sheath of PNS

formed by up to 100 layers of Schwann cell membrane

Myelin Sheath is internal to

neurilemma

Gaps created by schwann cells

nodes of ranvier

how much of an axon can one schwann cell wrap

1mm

CNA myelin Sheath

Formed by oligodendrocytes - one cell can myelinated multiple axons

Myelinated axons present as

white matter

the CNS does not contain

neurilemma

Myelination

insulates axons preventing ion movement across the membrane

Nodes of Ranvier

only site of action potentials
only axon membrane exposed
highly concentrated area of Na+ channels

space between nodes of Ranvier

1-2mm

function of 1st node

generates ionic currents in cytosol and extracellular fluid to depolarize 2nd node

Saltatory Conduction

much faster leaps over long stretches of axon

saltatory conduction saves energy through

opening and closing fewer channels save Na+/K+ pump from using ATP

action potentials require what amount of total energy used by CNS

47%

Myelinated Fiber types

A and B fibers

A fibers diameter

5-20 micrometers

A fibers speed \`

12-130 m /sec

A fibers function

-sensory neurons
touch, pressure, pain, proprioception, thermal
- skeletal motor neurons

B fibers diameter

2-3 micrometers

B fibers speed

15 m/sec

which fiber has a longer absolute refractory period

B fibers

B fibers function

afferent sensory neurons
-autonomic preganglionic motor neurons

Unmyelinted fiber types

C fibers

C fibers diameter

0.5 - 1.5 micrometers
0.5 - 2 m/sec

fiber type with longest absolute refractory period

C fibers

C fibers function

sensory neurons
- touch, pressure, thermal, pain
autonomic

where are C fibers found

autonomic postganglionic motor neurons

Multiple Sclerosis

autoimmune demyelination of CNS

Multiple Sclerosis symptoms

muscle weakness
abnormal sensation
double vision

Nerve regeneration occurs mostly in

PNS

what cell promotes nerve regeneration

neurilemma

neurilemma and nerve regeneration

forms regeneration tube and guides new axon to correct connection

neurilemma secretes

neurotrophins

Nerve Regeneration in CNS

way less common due to higher risk of misalignment

no neurilemma \=

no regeneration tube

Nogo in myelin sheath inhibits

axon regeneration

gial cells function

fill trauma sites

Neurotrophins may stimulate

axon collateral growth