Anterior horn cell (alpha motor neurone, lower motor neurone)
Third-order neurone / LMN · ipsilateral LMN
Alpha motor neurones in lamina IX of the ventral grey matter are the final common pathway: they sum corticospinal, reticulospinal, vestibulospinal and segmental reflex input and send their axons out through the ventral root, spinal nerve, plexus and peripheral nerve to the neuromuscular junction.
Traced from the start
- Primary motor cortex (precentral gyrus, Brodmann area 4)
- Upper motor neurone axon: corona radiata, posterior limb of internal capsule, crus cerebri, basis pontis and medullary pyramid
- Pyramidal (motor) decussation at the cervicomedullary junction
- Proprioceptors (muscle spindle, Golgi tendon organ, joint receptors)
- Lateral corticospinal tractAnterior corticospinal tract (uncrossed 10-15%)Ia afferent monosynaptic collateral (stretch reflex arc)
- Anterior horn cell (alpha motor neurone, lower motor neurone)
Detail
- Location
- Rexed lamina IX; medial group for axial and lateral group for limb muscles
- Enlargements
- Cervical C5-T1 (upper limb) and lumbosacral L2-S3 (lower limb)
- Exit
- Ventral root, spinal nerve, plexus, peripheral nerve
When it goes wrong
Any lesion of the anterior horn cell, ventral root or peripheral nerve (LMN lesion)
Flaccid weakness, wasting, fasciculations, hypotonia, reduced or absent reflexes and a flexor or absent plantar response
Poliomyelitis or spinal muscular atrophy
Asymmetrical (polio) or symmetrical proximal (SMA) pure LMN weakness with no sensory loss
Brown-Sequard hemisection
Ipsilateral LMN weakness confined to the muscles of the lesioned segment
Syringomyelia extending into the anterior horns
Wasting and weakness of the small hand muscles with absent upper limb reflexes
Practise this structure
1 question in the bank tagged Lower Motor Neurone.