1.2D: Myelination and Transmissive Segment

Steps in Myelination/formation of the myelin sheath

1. neurolemmocyte begins to wrap around portion of axon

2. its plasma membrane begins to form layers as it continues wrapping

3. overlapping of layers form myelin sheath

   1. eventually the necleus of the neurolemmocyte and cytoplasm are pushed off to the side called neurilemma

neurilemma

the neurolemmocyte cytoplasm and nucleus that are pushed off to the side after the formation of myelin sheath

of which nervous system does neurolemmocytes myelinate

in PNS

what myelinates axons in CNS

oligodendrocyte

continuous conduction

occurs as action potentials travel down the entire axon of unmyelinated axons

each adjacent reigion of axon goes through all phases of action potentials sequentially

action potentials are unidirectional… from axon hillock to synaptic knob due to absolute refractory period

sodium influx that occurs at one region stimulates depolarization down the next region down the line where sodium voltage gated channels were resting

where does continuous conduction occur

as action potentials travel down the entire axon of unmyelinated axons

where to action potentials travel to and from

axon hillock to synaptic knob

saltatory conduction

only occurs in myelinated axons

• action potentials occur specifically in unmyelinated regions that have a lot of voltage gated na+ and k+ channels

  • myelinated regions are insulted areas which have a limited numer of voltage gated na+ and k+ channels

• sodium entering myelinated regions diffuses through the axoplasm until it stimulates threshold at the voltage gated channels present on the adjacent neurofibril node

• The reduced areas that undergo action potentials (neurofibril nodes) along the axon in saltatory conduction allows these signals to travel faster than continuous conduction where the action potentials must travel down the entire length of the axon membrane

where does saltatory conduction only occur

myelinated axons

where do action potentials occur in saltatory conduction

unmyelinated regions

why do saltatory conduction action potentials only occur in unmyelinated regions

large number of voltage gated na+ and k+ channels are present

why are there a limited number of gated na+ and k+ channels in myelinated regions

they are insulated by the myelin sheath and cannot exist there

how does sodium enter the axoplasm of the myelinated regions

diffuses through axoplasm

what does sodium entering the axoplasm of myelinated regions do

diffuses until it stimulates threshold at voltage gated channels present on the next neurofibril node

neurofibirl node

what myelin sheath covers but leaves gaps in

what do the reduced areas that undergo action potentials in saltatory conduction allow

they allow action potentials to travel faster than continuums conduction where action potentials must travel down the entire length of axon membrane

Transmissive segment