Physiology
Sensory Fibres
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Get access- Conduction velocity of nerve fibres is increased by
- Myelination
- Increased diameter
- Increased temperature
- Increased external sodium ions
- Decreased serum calcium
- A-alpha fibres: muscle spindle afferents and skeletal muscle efferents
- Largest diameter of any nerve fibre
- A-gamma fibres: intrafusal fibres of muscle spindles
- A-beta (type II) fibres: mechanoreceptors
- Fast
- Myelinated
A-delta (type III) fibres: mechanical “fast” nociceptors (cold, sharp acute injury pain)
- Thinner
- Slower than above
- Myelinated
C (type IV) fibres: polymodal “slow” nociceptors (heat, dull inflammatory pain)
- Thin
- Slowest
- Unmyelinated
All sensory fibres enter the spinal cord via the dorsal roots with cell bodies lying just before it in the dorsal root ganglia
- Mechanoreceptors and proprioceptors:
- Ascend in the dorsal columns
- Synapse and then decussate at the level of the medulla
- Nociceptors and temperature sensation:
- Travel in the anterolateral, spinothalamic system
- Ascend or descend 1-2 spinal segments before synapsing
- A-delta synapse in lamina I
- C fibres synapse in lamina II
Second order neurones decussate at the level of synapse and then ascend to the thalamus and synapse