Axons

Axon Structure

-       Each neuron has 1 axon that starts at cone-shaped area (axon hillock)

-       In some neurons, axons are short/absent

-       In others, axon comprises almost entire length of cell

-       Some axons are over 1-meter long

-       Long axons called (nerve fibers)

-       Axons have occasion branches called (axon collaterals)

-       Axons branch profusely at their end (terminus) (can have as many as 10,000 terminal branches)

-       Distal endings called (axon terminals) or (terminal boutons)

Axon Function

-       Conducting region of neuron

-       Generates nerve impulses, transmits them along axolemma (cell membrane)

-       Axon rely on cell bodies to renew proteins and membranes (quickly decay if cut or damaged)

-       Axons have efficient internal transport mechanisms (molecules and organelles are moved along axons by motor proteins and cytoskeletal elements)

-       Movement occurs in both directions (Anterograde & Retrograde)

Anterograde

AWAY from cell body (Ex. Mitochondria, cytoskeletal elements, membrane components, enzymes)

Retrograde

TOWARD cell body (Ex. Organelles to be degraded, signal molecules, viruses, and bacterial toxins)

Axolemma

Neuron cell membrane

Axon Hillock

1 axon that starts at cone-shaped area

Myelin sheath

Composed of myelin, a whitish, protein-lipid substance

Myelin Function

Protect and electrically insulate axon increase speed of nerve impulse transmission

Myelinated Fibers

Segmented sheath surrounds most long or large-diameter axons

Unmyelinated Fibers

Do not contain sheath

Conduct impulses more slowly

Thin fibers not wrapped in myelin

Surrounded by Schwann cells but no coiling

1 cell may surround 15 different fibers

Myelination in PNS

- Formed by Schwann cells

- Wraps around axon in jelly roll fashion

- 1 Cell forms one segment of myelin sheath

- Outer collar of perinuclear cytoplasm (neurilemma): peripheral bulge containing nucleus and most of cytoplasm

- Plasma membranes have less protein

- No channel or carriers, so good electrical insulators

- Interlocking proteins bind adjacent myelin membranes

Myelin Sheath Gaps

- Gaps between adjacent Schwann cells

- Sites where axon collaterals can emerge

- Formerly called “nodes of Ranvier”

Myelin sheaths in CNS

- Formed by processes of oligodendrocytes, not whole cells

- Each cell can wrap up to 60 axons at once

- Myelin sheath is present

- No outer collar of perinuclear cytoplasm

- Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia

Nodes of Ranvier

“Myelin sheath gaps”

Gap between adjacent Schwann cells

Sit where axon collaterals can emerge

Collaterals

Occasional branches

Terminals

- Region that secretes neurotransmitters, which are releases into extracellular space

- can excite or inhibit neurons it contacts

- Carries on many conversations w/ different neurons at same time

Homeostatic Imbalance

Certain viruses/bacterial toxins damage neural tissue by retrograde axonal transport

Ex. Polio, rabies, HSV, & tetanus toxin