Peripheral nerve fibres

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General

Formed from dendrites and axons

Structure

Each nerve fibre has a central core formed by the axon (aka: axis cylinder)

Axolemma:

Plasma membrane surrounding the axis cylinder
Axis cylinder is surrounded by a myelin sheath.

Each segment of the myelin sheath is formed by one Schwann cell.

Nodes of Ranvier.

Neurilemma

Outside the myelin sheath
Formed of a thin layer of Schwann cell cytoplasm and an external lamina (similar to the basal lamina of epithelium).
Important in the regeneration of peripheral nerves after their injury.

No neurilemma in oligodendrocytes --> no CNS regeneration

Fasciculus:

a bundle of nerve

The connective tissue components include: Endo = inner, Peri = around, Epi = outer

Epineurium

Dense connective tissue that surrounds entire nerve (fascicles and blood vessels).
Consists of

External epineurium:

Loose collagenous connective tissue that either encloses groups of nerve fascicles

Internal epineurium

Contains fat that cushions nerve fibres
If lost can cause pressure on nerve
Cushions fascicles from external pressure and trauma to prevent injury

Perineurium

Surrounds individual nerve fascicles
Perineurium (blood-nerve permeability barrier)

Blood nerve barrier

Capillaries in nerves are nonfenestrated
Capillary endothelial cells have tight junctions.
Continuous basal lamina around the capillary.
Reinforced by cell layers present in the perineurium.

Made up of layers of flattened cells separated by layers of collagen fibres.
The size, quantity, and ultrastructure of nerve fascicles vary greatly along the length of a given nerve
Defines the fascicular pattern of a given nerve:

Monofascicular
Oligofascicular
Polyfascicular

Function:

Controls diffusion of substances in and out of axons
The fascicular pattern of a peripheral nerve has important implications when trying to repair an injured or divided peripheral nerve.

Endoneurium

A layer of connective tissue surround each nerve fibre
Each nerve has a vascular network along its entire length including arteries, veins, and capillaries.
Thin, supportive connective tissue that ensheathes and supports individual myelinated nerve fibers.
Blood vessels travel through endoneurium
May be affected in Guillain-Barré syndrome.

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Peripheral nerve

Classification of Peripheral Nerve Fibres

Function

Efferent: spinal cord or brain → peripheral structures (muscle/gland)

Afferent: peripheral organs → brain or spinal cord.

Area of innervation

Somatic afferent fibres: Carry impulses from skin, bones, muscles, and joints to the CNS

Somatic efferent fibres: Carry impulses from CNS to the skeletal muscles

Visceral afferent fibres: Carry impulses from visceral organs and blood vessels to the CNS

Visceral efferent fibres: Carry impulses from CNS to the cardiac muscle, glands, and smooth muscles

Diameter and velocity of conduction

Classification of fibres in the peripheral nerves

Fibre type	Function	Sensory classification	Diameter (μm)	Velocity (m/s)	Myelination
A α	Muscle spindle, annulo-spiral ending	Ia	13–20	70–120	Myelinated
ㅤ	Golgi tendon organ	Ib	13–20	70–120	Myelinated
ㅤ	Somatic motor	–	13–20	70–120	Myelinated
A β	Muscle spindle, flower-spray ending	II	6–12	30–70	Myelinated
ㅤ	Touch, pressure	II	6–12	30–70	Myelinated
A γ	Motor to muscle spindles	–	3–6	15–30	Myelinated
A δ	Pricking pain, cold, touch	III	2–5	12–30	Myelinated
B	Preganglionic autonomic	–	1–5	3–15	Myelinated
C	Burning pain, temperature, itch, tickle	IV	0.2–1.5	0.5–2	Unmyelinated
ㅤ	Postganglionic autonomic	–	0.2–1.5	0.5–2	Unmyelinated

Presence of myelin sheath

Features	Myelinated	Unmyelinated
Number of schwann cells	Many	One
Number of axons	One	Many
Speed of conduction	Fast	Slow
Mesaaxon	Spiral around axon	Does not spiral around axons

Myelinated nerve

Myelin wraps and insulates axons: ↓ membrane capacitance, ↑ membrane resistance, ↑ space (length) constant, ↓ time constant.

CNS: Oligodendrocytes.
PNS: Schwann cells. COPS

Made by

In CNS (including CN II): oligodendrocytes;
In PNS (including CN III-XII): Schwann cells.

Function

Increases velocity of conduction (for a nerve fibre of the same diameter).

↑ conduction velocity of signals transmitted down axons → saltatory conduction of action potential at the nodes of Ranvier, where there are high concentrations of Na⁺ channels.

Reduces the energy of conduction.
Gives the white colour of white matter in brain and spinal cord.

Myelin formation

Lipid and sphingomyelin form the myelin sheath

Each Schwann cell forms a short segment of the myelin sheath. The figures to the right are transverse sections through the nerve fibre, at the corresponding stages

(A) Stages in the formation of the myelin sheath by a Schwann cell—the axon, which first lies near the Schwann cell; (B and C) Then it invaginates into its cytoplasm, and comes to be suspended by a mesaxon. (D and E) The mesaxon elongates and comes to be spirally wound around the axon

Unmyelinated nerve

Example

Postganglionic autonomic fibres
Fibres carrying “slow”, burning pain.

But are also surrounded by schwann cell

Unmyelinated axons invaginate into the cytoplasm of Schwann cells, but the mesaxon does not spiral around them

Several axons invaginates into the cytoplasm of one Schwann cell

Diagram of a cell with different types of cell types AI-generated content may be incorrect.

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