Neurophysiology basics

Surface sensors

Signal processing

Neurophysiology basics

What are motor neurons?

Motor neurons are split into two types. The upper motor neuron connects the brain  to the spinal cord, and the lower motor neuron travels from the spinal cord to the muscle. The two types of motor neuron work in relay, whereby signals from the upper motor neuron are communicated to the lower motor neuron via a synapse in the spinal cord.

The destination of the lower motor neuron dictates the level at which this synapse occurs. For leg muscles, this is in the thoracic part of the spinal cord; but for tongue muscles, this is in the brainstem (before the spinal cord even starts).

Many motor neurons are therefore extremely long (e.g. over one metre). Consequently, it takes huge amounts of energy to ensure the correct proteins and molecules are in the correct part of the cell at the correct time. If any of this complex system goes awry, the motor neuron is vulnerable to deterioration.

What is a motor unit?

A motor unit comprises the lower motor neuron, its terminal branches and the muscle fibres it connects to. When activated, the lower motor neuron sends signals to the muscle fibres, telling them to contract.

The way these motor units adapt to the underlying disease process can be tracked over time.

Motor units share several similarities with trees in a forest

Long distance transport

The axons of lower motor neurons cover vast distances for their size. From the spinal cord to the muscle is the equivalent of a tube train travelling from London to New York! Transporting the necessary proteins from the cell body in the spinal cord to the nerve terminal in the muscle is therefore an onerous and energy-consuming task.

Variation in size

If you were to cut a peripheral nerve in cross-section, neurons of all types (motor and sensory) would be seen tightly aligned. The wider ones are the lower motor neurons, able to transmit signals at the fastest speeds.

Terminal branching

As axons reach the muscle, they begin to branch off in order to connect to many muscle fibres simultaneously. These projections are long and plentiful, enabling a single motor unit to cover a large volume of muscle. This ensures the force generated by a single motor unit is sufficiently spread out.

Compensatory sprouting

Lower motor neurons have the capacity to create new branches to compensate for the loss of neighbouring motor neurons. This sprouting process has evolved as a protective mechanism, so that the connections with muscle fibres can be maintained. This helps to maintain muscle strength despite the loss of motor neurons.

It’s estimated that a half of motor units can be lost before muscle weakness develops

Why does weakness develop?

In motor neuron disease, this compensatory mechanism can only go on for so long. Once half of the original pool of motor units have died (due to the relentless nature of the disease), a tipping point is reached.

The sprouting process is no longer sufficient to maintain connections with all muscle fibres. Consequently, muscle fibres begin to fail leading to muscle wasting. It’s the failure and loss of muscle fibres that leads to weakness.

What is a fasciculation?

A fasciculation is an involuntary muscle twitch, which arises from the spontaneous firing of a single motor unit. Usually, motor units are instructed by the brain when to become active and when to remain inactive. This precision is essential to ensure motor units fire simultaneously, thereby activating sufficient muscle fibres to generate a certain muscular force for a specific activity.

In some conditions, like motor neuron disease and benign fasciculation syndrome, motor neurons can become hyperexcitable, which can lead to random, solitary firing. Here’s a summary of what we know about this process:

What causes a fasciculation?  When the tightly regulated control of sodium and potassium ions across the neuronal membrane is impaired, the neuron can become hyperexcitable and is more likely to fire spontaneously. When it does, the electrical signal travels along the axonal branches deep within the muscle. This in turn leads to the activation of multiple muscle fibres, which we are able to detect with high-density surface EMG. Each motor unit produces its own ‘fingerprint’ so we can recognise when the same motor unit fires again and again.

A fasciculation in 3D

Surface sensors

What is high-density surface electromyography?

SPiQE is based on muscle recordings captured by high-density surface electromyography (or ‘HDSEMG’ for short). This relatively modern technique has many advantages over alternative, more traditional methods like needle electromyography.

It’s non-invasive and painless

It can be repeated on multiple occasions for longer durations

It provides enriched spatial information

Arranged in a square grid covering 40 square centimetres, 64 channels simultaneously record the underlying muscle activity. It’s like 64 eyes watching from slightly different angles, building up a comprehensive 3D picture of the electrical changes taking place. This is what makes HDSEMG so powerful.

How are the sensors applied to the skin?

Before application of the sensor, the skin is prepared by gently wiping away excess oils and dry skin. The sensor is then stuck to the skin with a mild adhesive pad. The use of an electrode gel improves the electrical contact between the skin and individual electrodes. After the recording, the sensor peels away easily from the skin.

Neither electrical stimulation nor invasive needles are necessary for the technique

Who supplies the sensors?

The surface sensors are provided by Twente Medical Systems International (TMSi). This company also supplies the Refa-64 recording device, which amplifies the signals before transmitting them to a computer.

Signal processing

Take a look at this 2-minute video, which provides a basic explanation of SPiQE’s analytical process.