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What are Myotomes? - Definition & Testing

Instructor: Rachel Torrens
Did you know that you have myotomes that you use everyday? In this lesson, discover what defines a myotome, its function, and how it can help a healthcare provider diagnose problems of the spine.

What is a Myotome?

Think of an outlet in your house. You have a plethora of electronic devices plugged into it. One cord leads to the lamp, another to the TV, and another to your phone charger. The outlet provides electricity to each of these devices, but the cord is what connects the devices to the power source.

This is similar to how different muscle groups receive messages from the central nervous system. Your spinal cord is like the outlet, providing necessary 'power' for the muscles to function properly. Coming out from the spinal cord are spinal nerves, like cords. Each spinal nerve gives function to a specific set of muscles called a myotome. A myotomes is therefore a set of muscles innervated by a specific, single spinal nerve.

You have 31 spinal nerves. Your vertebrae (the bones that make up your spine), each hold a spinal nerve. The nerves are categorized by the vertebra which house them. There are:

  • 8 cervical nerves
  • 12 thoracic nerves
  • 5 lumbar nerves
  • 5 sacral nerves
  • 1 coccygeal nerve

Spinal nerves are named based on the vertebrae.
vertebrae

Of these 31 spinal nerves, 16 lead to distinct myotomes that control voluntary muscle movement.

The Movements of Muscles

A myotome helps you to do everyday functions, such as typing an email or walking to the bathroom. So which spinal nerve controls which muscle groups? Before we delve into that, let's review some muscle terminology.

Flexion refers the angle of two bones meeting at a joint decreasing. For example, make a fist and pull it towards your shoulder. The angle between the forearm and upper arm bones decreases, so it's called elbow flexion.

Extension refers the angle of two bones meeting at a joint increasing. Push your fist away from your shoulder, and the angle between the forearm and upper arm bones increases. You've just performed elbow extension!

Muscles flex and extend.
muscle movement

For the next terms, envision an imaginary line down the middle of the body or body part. Abduction refers to movement of the body part(s) away from the mid-line. Look at your hand, spread your fingers apart. This is finger abduction because the fingers are moving outward from the hand's mid-line.

Adduction refers to movement of the body part(s) towards the midline. Squish all your fingers back together and you'll see finger adduction. Let's see what myotomes move!

Myotome Groupings

Myotomes Associated spinal nerve(s) Function of Innervated Muscle(s) Movement Observed
Cervical myotomes C1, C2 cervical flexion Touching chin to chest
C3 cervical side flexion Touching ear to shoulder
C4 shoulder elevation Shrugging motion
C5 shoulder abduction Raising both arms in the air
C6 elbow flexion, wrist extension Bending the wrist upward, as if signaling 'stop'
C7 elbow extension, wrist flexion Bending the wrist downward
C8 thumb extension 'Thumbs up' motion
Thoracic myotome T1 finger abduction and adduction Spreading fingers apart, squeezing them together
Lumbar myotomes L1, L2 hip flexion Knee moving toward the chest
L3 knee extension Straightening the lower leg so it's in line with upper leg
L4 ankle dorsiflexion Pulling the foot upwards towards the lower leg, flexing the toes
L5 big toe extension Sticking the big toe up in the air
Sacral myotomes S1 ankle plantar flexion Pointing the toes downward, straightening the foot so it is in line with the lower leg
S2 knee flexion Heel of the foot moving towards the buttocks

Testing of Myotomes

Neurologists assess the functions of these myotomes to pinpoint problems in the spine. By testing a patient's ability to perform a specific movement, the neurologist is assessing the health of not only the muscles, but the spinal nerve that innervates them!

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