Nerve

Key Components of a Nerve

Neuron: - The fundamental unit of a nerve. - Consists of a cell body, axon, and dendrites.

Cell Body: - Gives rise to axons and dendrites.

Dendrites: - Thin sensory processes that receive input from other neurons.

Axon: - The main route for conducting signals to tissues. - Axon size: 0.2 to 20 μm.

Glial Cells: - Support neurons and form the myelin sheath. - CNS Glial Cells: Oligodendrocytes (myelin production), astrocytes, and microglia. - PNS Glial Cells: Schwann cells.

Myelination: - Myelinated Axons: One Schwann cell per axon internode. - Unmyelinated Axons: Bundled together with no myelin (Remak Bundle).

Myelin: - A lipid and protein-rich sheath laid down in the PNS, forming the neurilemma (not present in CNS myelinated axons).

Nodes of Ranvier: - Gaps between Schwann cells, essential for saltatory conduction.

Nerve Structure

  • Axons group to form nerve fibres.
  • Fibres group to form bundles.
  • Bundles group to form fascicles.
  • Fascicles group to form a nerve trunk.

Layers of a Nerve: 1. Endoneurium: Surrounds grouped axons (nerve fibres), contains collagen and blood vessels. 2. Perineurium: Surrounds fascicles, prevents excessive diffusion and oedema. 3. Epineurium: A dense collagen layer that surrounds groups of fascicles, forming a supportive sheath with blood vessels.

Blood Supply to Nerves

Nerves receive blood through intrinsic and extrinsic plexuses: - Extrinsic Plexuses: Segmental vessels in the epineurium. - Intrinsic Plexuses: Longitudinal vessels, mainly in the endoneurium, with a blood-nerve barrier.

Biomechanics of Nerves

  • Nerves exhibit viscoelastic properties.
  • Susceptible to traction injuries; ischemia occurs at 15% strain, rupture at around 20%.
  • Nerves rely on ATP-dependent transport, making them vulnerable to ischemia, anoxia, and hypothermia.

Action Potentials

  • The axon’s resting membrane potential is -70mV, maintained by Na+ and K+ channels.
  • Action potential involves the opening of Na+ channels, followed by K+ channels, creating electrical currents essential for nerve function.

Nodes of Ranvier play a crucial role in saltatory conduction, allowing faster signal transmission with less energy consumption.

Synapse and Neuromuscular Junction (NMJ)

  • The NMJ is where the nerve communicates with muscle fibres.
  • Neurotransmitters like acetylcholine are released into the synaptic cleft, passing on the action potential.
  • Residual neurotransmitters are metabolised and removed from the cleft.

Embryology of the Nervous System

The nervous system arises from the ectoderm (one of the three germ layers). The neural tube forms the CNS and motor neurons, while the neural crest forms sensory neurons, the PNS, and basal ganglia.

Organisation of the Nervous System

  • CNS: Includes 12 paired cranial nerves and 31 paired spinal nerves.
    • Spinal nerves divide into dorsal (sensory) rami and ventral (motor) rami.
  • Sensory Nerve Receptors:
    1. Mechanoreceptors (e.g., Meissner, Pacinian corpuscles),
    2. Thermoreceptors (temperature),
    3. Nociceptors (pain).

Autonomic Nervous System

The autonomic nerves originate from the grey matter of the spinal cord (T1 to L2 levels). They synapse in the paravertebral ganglia before relaying signals to viscera or traveling along the sympathetic trunk.

Nerve Fibre Types

Fibre Type Example Myelination Diameter (µm) Velocity (m/s)
Aα Motor (large diameter) Myelinated 20 100
Aβ Organised sensory (hair) Myelinated 10 50
Aδ Pain & Temperature Myelinated 5 25
B Autonomic Pre-Ganglionic Myelinated 5 10
C Autonomic Post-ganglionic, Slow Pain Unmyelinated 1 2

Peripheral Nerve Injury

Peripheral nerve injuries may result from crush, traction, laceration, or compression. Their classification involves histological analysis and time to assess the severity:

Seddon Classification

Type Features Sunderland Grade Prognosis
Neuropraxia No Wallerian degeneration, conduction block Grade 1 Good
Axonotmesis Wallerian degeneration, axonal damage Grade 2-4 Moderate
Neurotmesis Complete nerve transection Grade 5 Poor

Pathology of Nerve Injury

In Wallerian degeneration, both the proximal and distal parts of the axon undergo significant changes. The proximal axon undergoes atrophy, while the distal part degenerates completely. Schwann cells assist in regeneration, with regrowth occurring at 1mm per day.

Motor End Plate

The motor end plate plays a crucial role in muscle innervation. Over time, without re-innervation, the motor end plate degenerates, followed by muscle spindle degradation.

Recovery Order of Sensory Nerves: 1. Pressure sense 2. Protective pain 3. Moving touch 4. 2-point discrimination 5. Normal sensation

Factors Affecting Nerve Recovery

  • Patient Factors: Age.
  • Injury Factors: High energy, associated vascular injury, crush or traction injury.
  • Surgical Factors: Delay in repair, infection, repair under tension.

Management Principles

Non-operative Treatment:

  • For neuropraxia and most axonotmesis injuries.

Surgical Repair:

  • Performed in clean, vascularised beds with stable skeletal structures. Nerve grafting may be needed for gaps. Autografts (sural nerve) or conduits can be used for reconstruction.

Outcomes:

  • Depend on factors such as patient age, injury location, and surgical timing.
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