Lower Motor Neurons

Hi :) In this post we'll take a look at how lower motor neurons work. We'll discuss neuromuscular transmission, motor unit recruitment, the patellar tendon and withdrawal reflexes, as well as muscle spindles and Golgi tendon organs. We'll also take a look at some of the signs of lower motor neuron dysfunction as well as some important definitions. Enjoy! 

Neuromuscular Transmission

Several steps are involved in the transmission of an action potential across a neuromuscular junction:

1. First, an Action Potential arrives at the presynaptic terminal of the motor neuron.
2. This causes voltage-gated Calcium channels to open which leads to an influx of calcium ions into the cell.
3. The entry of calcium triggers the release of Acetylcholine via exocytosis into the synaptic cleft.
4. Acetylcholine then binds to nicotinic acetylcholine receptors (nAChRm) on the motor end plate which causes ligand-gated channels to open.
5. This causes the flow of sodium ions into the muscle cell which generates an end-plate potential (EPP).
6. The end-plate potential causes the generation of an action potential which spreads via voltage gated sodium channels and ultimately leads to muscle contraction.
7. The neuromuscular transmission is stopped by acetylcholinesterase which degrades acetylcholine to choline and acetate.
8. Choline is taken up by the sodium dependent transporters.  

Lower Motor Neurons

Lower motor neurons (LMNs) are found in the grey matter of the spinal cord and innervate skeletal muscles. LMNs provide coordination between muscle groups which is required for organised movement. The cell bodies of LMNs are located in the ventral horn of the spinal cord while their axons exit via the ventral root. They reach muscle, where their axons branch so that each muscle fibre is innervated by the axon of a single neuron, through peripheral nerves.

Interestingly, the fibres of the LMNs are arranged somatotopically in the spinal cord. The cell bodies of the neurons which innervate the trunk and proximal limbs are found arranged ventromedially in the spinal cord. The neurons which innervate the muscles of the distal limb have cell bodies which are located more dorsolaterally in the spinal cord.

Motor Unit Recruitment

A motor unit consists of a motor neuron and all the muscle-fibres it innervates. Individual muscle fibres are innervated by a single motor neuron but a motor neuron may innervate more than one muscle fibre.

Force Generation

The generation of force in muscles is dependent on several factors. The number of motor units that are recruited plays an important effect on force as the more muscle fibres are active, the more force is generated. The force that a muscle produces is also dependent on the force generated by individual muscle fibres. Three factors affect this:

1. Frequency of stimulation: a high frequency of stimulation allows cytoplasmic calcium ion concentrations to build up and this leads to the summation of contractions.
2. Fibre Diameter: this is dependent on the number of sarcomeres in parallel.
3. Resting Fibre Length: this is the overlap of thick and thin filaments in sarcomeres.

Muscle fibre type also affects how much force is generated, three types exist:

1. Slow Muscle Fibres: these contract slowly and generate small forces.
2. Fast-Fatigable: These contract quickly and generate large forces.
3. Fast-Fatigue Resistant Fibres: these are an intermediate between the slow and fast-fatigable fibres.

The Patellar Tendon Reflex

This is the simplest type of reflex as it only involves one synapse at the spinal cord. The reflex involves several steps:

1. The tap of the hammer stretches the tendon of the extensor muscles which stretches sensory receptors.
2.  a) The sensory neuron synapses with the motor neuron in the spinal cord and excites it.
   b) The sensory neuron also synapses with an interneuron in the spinal cord which in turn synapses with, and inhibits, the motor neuron innervating the flexors of the leg.
3. a) The motor neuron conducts the action potential to the extensor muscle fibres and causes them to contract.
   b) The flexor muscles relax because their motor activity has been inhibited.
4. The leg extends.

The Withdrawal Reflex

This is a basic reflex that helps to avoid harmful stimuli. A harmful stimulus, such as a pin prick, causes electrical impulses to be transmitted along sensory nerve fibres to the spinal cord. Here they synapse with an interneuron (also known as local circuit neurons) which subsequently synapses with a motor neuron. The motor neuron innervates the muscles near the source of the stimulus and causes them to flex in order to move the body part away from the noxious stimulus. This is known as a polysynaptic reflex because more than one synapse occurred between neurons. Interestingly, spinal reflexes will continue to occur even if that spinal cord segment has been isolated from the rest of the central nervous system.

Crossed Extensor Reflex

In some situations, such as stepping on a nail, activation of nociceptors may stimulate the crossed extensor reflex. Once the nociceptors are activated, afferent neurons send impulses to the spinal cord where they synapse with interneurons. These interneurons synapse with extensors and flexors on the ipsilateral leg (the one that stood on the nail) causing the flexors to contract and the extensors to relax. This removes the harmful stimulus. Some of the interneurons synapse with flexors and extensors on the contralateral leg (the one that didn’t step on the nail) and causes them flexors to relax and the extensors to contract. This causes the leg to extend which supports the body. This reflex is important for the maintenance of posture and locomotion.  

Muscle Spindles

Muscle spindles are groups of specialised skeletal muscle fibres (known as intrafusal muscle fibres) contained in a capsule and arranged in parallel with striated (or extrafusal) muscle fibres. They are attached to the rest of the muscle by connective tissue and do not contribute the force of muscle contraction. In addition they are innervated by different neurons compared to extrafusal fibres: intrafusal fibres receive Ia and II sensory and γ motor innervation while extrafusal fibres receive α motor innervation.


Muscle spindles have three components:

• Specialised intrafusal muscle fibres whose ends are contractile while its centre is non-contractile.
• Type Ia (primary) and Type II (secondary) sensory fibres which originate from non-contractile regions of the intrafusal fibres. 
• Myelinated γ motor neurons which innervate the contractile polar regions of the intrafusal fibres.

Function:

The function of muscle spindle fibres is to provide information about the muscle length and changes in muscle length (which is velocity or the speed of contraction).  The nerves of the spindle allow it to do this. At the spinal cord, the sensory neurons of the spindle synapse with γ motor neurons that innervate the same spindle and α motor neurons that innervate the same muscle that the spindle is situated in.

The alpha motor neurons cause the extrafusal muscle fibres to contract while the gamma motor neurons cause the intrafusal polar regions to contract too. This allows the spindle fibres to always be under tension while the muscle is contracting which allows more information about the length and velocity of the contraction to be sent to the CNS.

But how do the spindles actually provide information about length and changes in length? The lengthening and shortening of skeletal muscle fibres involves two phases: the dynamic phase, when the muscle length is changing, and the static phase, when the muscle has stabilised at the new length after lengthening/shortening. Separate components of the muscle spindles signal each of these phases.

Now, there are two types of intrafusal spindle fibres and a typical spindle will contain: 2-3 large nuclear bag fibres and about 5 nuclear chain fibres. The nuclear bag fibres may be dynamic or static. The primary (Type Ia) sensory neurons innervate the centres of all the fibres. The secondary (Type II) sensory fibres innervate the chain and static bag fibres. The dynamic γ motor neurons innervate the contractile regions of the dynamic bag fibres while the static γ motor neurons innervate the contractile region of the chain and static bag fibres.

The sensory neurons respond through stretch-sensitive ion channels. Primary afferent fibres respond to muscle length but more strongly to changes in length (velocity) while secondary afferents provide information mainly about the steady-state length of muscle. Dynamic γ motor neurons are able to control the sensitivity to velocity by increasing the dynamic sensitivity of the endings of the primary sensory neurons. The sensitivity to length can be controlled by static γ neurons which may increase the tonic level of activity in primary and secondary endings and decrease the dynamic sensitivity of primary endings.

This allows the CNS to control the sensitivity to velocity or length to suit the activity of the muscles. Static γ motor neurons are active when the muscle length changes slowly and predictably (such as when standing or walking) while dynamic gamma neurons are active when length changes rapidly and unpredictably (such as when shaking a paw or balancing).

This video explains it all really well:

Golgi Tendon Organs

Golgi Tendon Organs (GTOs) provide information about the change in muscle tension. They are afferent nerve endings covered by a capsule and located at the junction between the muscle and tendon. Importantly, they are arranged in series with extrafusal fibres and this allows them to sense tension. GTOs are innervated by Type Ib sensory neurons and at the spinal cord they inhibit α motor neurons which innervate the muscle that the GTO is situated in. This prevents the muscle from generating too much tension.

So, to sum this all up, spindles provide sensory information about the length and change in length (velocity) of skeletal muscle while Golgi tendon organs provide information about muscle tension. In addition, the spindles provide a feedback system which monitors and maintains the length and tone of muscles. The Golgi organ system provides a feedback system which monitors and maintains muscle force. Both of these systems receive information from multiple sources including sensory receptors in the skin and descending spinal motor pathways from the brain. Both of these systems also send information to the brain through ascending spinal pathways which are part of the proprioception system.     

LMN Dysfunction

This section is quite important as it often comes up in exams! The signs that one would see in an animal that has a lower motor neuron problem include:

• Weakness/paralysis: This is because motor neurons to the muscles aren’t working and the muscles are unable to contract.
• Hyporeflexia: this refers to reduced or absent flexor reflexes and occurs because the nerves in the reflex circuit are not working.
• Neurogenic Atrophy: muscles waste away because the alpha motor neurons no longer communicate with them.
• Muscle fasciculation: muscles contract slowly in a disordered fashion.
• Flaccidity/hypotonia: this is because spindle fibre activity is inhibited.

Important Definitions

The following definitions all regard motor neurons.

• Paresis (-paretic): a partial deficit in motor function
• Paraparesis: when both pelvic limbs are affected
• Tetra- or quadriparesis: when all four limbs are affected
• Hemiparesis: when the thoracic and pelvic limb on the same side of the body are affected.
• Monoparesis: when only one limb is affected.
• Paralysis: the complete loss of voluntary movements
• The same prefixes apply as for paresis.

That’s it for this post, if you have any questions please let me know :)