A lower motor neuron (LMN) is a multipolar neuron which connects the upper motor neurone (UMN) to the skeletal muscle it innervates. As discussed in the UMN article, an UMN may synapse directly or indirectly, via interneurons, onto a LMN.
This article shall consider the location of LMNs and the different types, as well as the classical signs and symptoms that are found when they are damaged.
Introduction to LMNs
The cell body of a LMN lies within the ventral horn of the spinal cord or the brainstem motor nuclei of the cranial nerves which have motor modalities. Therefore, the cell body of a LMN lies within the central nervous system (CNS).
The axon of a LMN exits the CNS and forms the somatic motor part of the peripheral nervous system (PNS). Finally, the LMN terminates on the muscle fibres which it innervates. The combination of the LMN and these fibres is known as a motor unit. It is important to note that although one LMN will innervate several muscle fibres, a single muscle fibre is innervated by only one LMN. The bridging gap between the LMN’s axon terminal and the muscle fibre it supplies is known as the neuromuscular junction (NMJ). It is here that the motor neuron releases the neurotransmitter acetylcholine, which causes firing of an action potential in the receiving muscle fibre.
The term LMN is often used interchangeability with α-motor neuron – but when clinicians refer to a LMN syndrome they are referring to damage to α-motor neurons. There are also another type of LMNs, known as γ-motor neurons.
Types of LMNs
As aforementioned, α-motor neurons are the type of LMNs which, when damaged, produce the characteristic clinical signs of a LMN syndrome. Within the spinal cord, the cell bodies of these neurons originate in laminae VIII and IX of the ventral horn and are somatotopically organised. That is to say, neurons which innervate distal musculature are located lateral to those which innervate axial muscles, and neurons which innervate extensors are ventral to those which innervate flexors.
The function of α-motor neurons is to cause contraction of the muscle fibres they innervate. It has been described as ‘the final common pathway’, as α-motor neurons are essential for muscle contraction. This can either been under voluntary control, through the action of UMNs, or through eliciting the myotatic stretch reflex, as α-motor neurons form the efferent portion of the reflex arc. Therefore, there can be no coordinated muscle contraction if the α-motor neurons are not functioning
γ-motor neurons have a key function in the regulation of muscle tone and maintaining nonconscious proprioception. Although γ-motor neurons fall under the umbrella term LMN, a LMN syndrome results from damage to α-motor neurons only.
γ-motor neurons also arise from laminae VIII and IX in the ventral horn of the spinal cord. These innervate fibres that form the contractile parts of the muscle spindles in skeletal muscle. In addition, whilst α-motor neurons receive input from both muscle spindle Ia sensory afferents and UMNs, γ-motor neurons are solely under control from the UMNs. These fibres are important in signalling the length and velocity of a muscle. The function of γ-motor neurons is to keep the fibre taut by causing contraction of its polar ends. Maintaining tension in these fibres is necessary for preserving sensitivity to muscle stretch by muscle spindles.
To illustrate their importance, consider the myotatic stretch reflex. Upon stretching the muscle, (for example through a patellar tendon tap) the muscle spindle will stretch and sensory afferent fibres will fire. This in turn will cause the α-motor neurons to fire and lead to muscle contraction through the reflex arc. The contraction of muscle causes the intrafusal fibres to become slack, reducing Ia afferent firing. A slack fibre attenuates the firing of muscle spindle afferents and they are no longer sensitive to stretch. Therefore, if the muscle were to be stretched again, there would no firing and no α-motor neuron activation.
The γ-motor neuron is essential in resetting the sensitivity of the muscle spindle by contracting both ends of the fibre. This makes the fibre taut and the muscle spindle sensitive to stretch once again. In voluntary movement, both the γ-motor neurons and α-motor neurons are activated simultaneously by UMNs. This maintains the muscles spindles sensitivity to stretch upon movement.
Sensitivity to the stretching of muscle spindles allows for information on muscle length and velocity to be relayed to the cerebellum via the various ascending spinocerebellar tracts. It is for this reason that γ-motor neurons play an important role in nonconscious proprioception. Finally, the firing of γ-motor neurons is directly proportional to the tone of a muscle. This is observed in pathological states. Tone is increased through increasing γ-motor neuron firing as muscle spindles become hypersensitive to stretch. This hypersensitivity in turn causes greater activation and recruitment of α-motor neurons via the reflex arc, creating a stiff muscle on passive movement.
As previously alluded to, both α and γ motor neurons have important roles in regulating voluntary movement, reflexes and tone. Therefore, when either are damaged, the clinical signs present reflect impairments in these areas. A LMN syndrome is the term used by clinicians to describe the collection of signs and symptoms present when a patient damages α-motor neurons. This damage can occur anywhere between the origins of the LMN in the ventral horn or brainstem nuclei and its termination on a muscle. The signs and symptoms of a LMN syndrome include:
- Hyporeflexia/ areflexia – Since the efferent portion of the reflex arc is damaged, eliciting the myotatic stretch reflex will produce decreased or absent reflexes depending on the extent of that damage.
- Hypotonia/ atonia – Tone is a product of the contraction of the extrafusal fibres in response to the stretch of a muscle. Therefore, loss of α-motor neurons leads to reduced or absent muscle tone.
- Flaccid muscle weakness or paralysis – Depending on the extent of the lesion, α-motor neuron damage means muscles will receive a weakened signal to contract or no signal at all when attempting to elicit voluntary movement.
- Fasciculations – When α-motor neurons are damaged, they can fire spontaneous action potentials, causing contractions in the fibres of the motor unit. This can be seen as small involuntary muscle twitches, often compared to having ‘a bag of worms under the skin’.
- Muscle atrophy – The loss of neurotrophic factors from the α-motor neuron nerve terminal, which typically support the muscle, causes atrophy. This is different to the disuse atrophy which may been observed in an UMN syndrome.
It is important to differentiate between weakness of a muscle as a result of damage to the nerve, disease of the NMJ or disease of the muscle itself. In primary muscle disease, there is no sensory loss, which would be present if the nerve was damaged. In addition, weakness is symmetrical and reflexes are often lost later than in nerve damage.
Clinical Relevance – Spinal Muscular Atrophy
Spinal Muscular Atrophy (SMA) is an autosomal recessive inherited disorder resulting in the loss of motor neurones within the ventral horn of the spinal cord and motor nuclei of the cranial nerves in the pons and medulla. It is believed that the incidence is approximately 1 in 6,000-10,000 live births and it is the most common genetically determined cause of neonatal death.
The presentation of SMA consists of the signs and symptoms seen in a LMN syndrome. Since the motor modalities of the cranial nerves arising from the pons and medulla are also affected, there is often an accompanying bulbar palsy.
There are four types of SMA (I-IV) which vary in severity. The age of onset is under 6 months and the patient will suffer from weakness, severe hypotonia and hyporeflexia. Ninety-five percent of patients die before the age of two years. SMA IV is the least severe, with patients often presenting with progressive muscle weakness in their 30s. Patients with this variant have a normal life expectancy.