Lower motor neurone signs
Damage to a lower motor neurone, the nerve cell that connects the central nervous system to a muscle, leaves that muscle weak, floppy and in time wasted (loss of bulk).
Remember
Damage to a lower motor neurone leaves a weak, floppy muscle with reduced or absent reflexes that wastes with time, and fasciculations may be seen; tone and reflexes in an upper motor neurone lesion go the opposite way.
Memory aid
lower lesion, everything low; upper lesion, everything up
Use it as a quick sort: low tone and reduced reflexes point to a lower motor neurone, while high tone and brisk reflexes point to an upper motor neurone.
A lower motor neurone is the nerve cell that connects the central nervous system to a muscle. Their cell bodies are found in the ventral horn of the spinal cord (the part of the cord that holds them) and in cranial nerve motor nuclei. A lower motor neurone can be damaged anywhere from its cell body to the neuromuscular junction (the contact between nerve and muscle). The most common site is the axon, the neurone's long fibre, and an injury there is also known as a peripheral nerve injury.
The muscle is then denervated, meaning it has lost its nerve supply. In the muscles supplied by the damaged neurones you expect weakness and reduced reflexes or areflexia (absent reflexes), both due to denervation. In time there is wasting (loss of muscle bulk), and there is hypotonia (reduced tone, so the muscle feels floppy) because the small background contractions that give a muscle its tone are lost. There may also be fasciculations, brief visible twitches in part of the muscle.
Fasciculations are usually explained by the muscle up-regulating its nicotinic acetylcholine receptors, an attempt to compensate for denervation: receptors that normally cluster at the neuromuscular junction spread across the muscle fibre. The twitch itself is a spontaneous firing within the motor system, so the receptor change is one part of a more complex picture.
In the patellar reflex, tapping the tendon stretches the muscle spindle fibres (stretch sensors in the muscle), and the lower motor neurone makes the quadriceps contract. Damage to the ventral horn at the level of the quadriceps destroys those neurones, so the motor (outgoing) limb is lost and the knee jerk is reduced or absent.
Upper motor neurone damage gives the opposite picture: high tone, brisk reflexes and an extensor plantar response (an upgoing toe).
Key facts
- Weakness and reduced or absent reflexes
- Both are due to denervation, in the muscles supplied by the damaged neurones.
- Wasting
- Loss of muscle bulk, from loss of nutritional support from the neurone, which develops with time.
- Low tone
- The muscle feels floppy because the small background contractions that give it tone are lost.
- Fasciculations
- Brief visible twitches in part of a muscle, usually explained by up-regulation of nicotinic acetylcholine receptors (one part of a more complex picture), though not every patient has visible fasciculations.
- Most common site
- The axon, so a peripheral nerve injury is the most common way for a lower motor neurone to be damaged.
- Ventral horn damage at the level of the quadriceps
- It destroys the lower motor neurones themselves, so the knee jerk is reduced or absent and the quadriceps is weak with low tone.
Exam traps
- Spasticity (stiffness) with increased muscle tone does not fit lower motor neurone damage, because that damage reduces tone as the muscle loses the background activity that gives it tone.
- An extensor plantar response does not occur with lower motor neurone damage, because it is a sign of dysfunction of the corticospinal tract, which carries upper motor neurone axons.
- Weakness that worsens with repeated use and improves with rest does not separate lower from upper motor neurone damage, because it suggests a problem at the neuromuscular junction.
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