2. The compound action potential. Conductive properties of various nerve fibers
Learning objectives
- What is a nerve?
- What is a compound action potential?
- What are the different types of nerve fibres in order from highest to lowest conduction velocity?
- Which nerve fibres are myelinated and which aren’t?
The compound action potential
What is a nerve?
A nerve is a bundle of the axons of many nerve cells. Most nerves are made up of axons from both sensory (afferent) and motor (efferent) neurons. For peripheral nerves, the bodies of the sensory neurons are in the dorsal root ganglia next to the spinal cord, while the bodies of the motor neurons are in the anterior horn of the spinal cord. Their axons meet and form a nerve.
An axon is sometimes called a nerve fibre.
Action potentials in nerves
One action potential travels down the axon of only one nerve cell. However, human nerves are not comprised of single axons.
If we stimulate one end of a nerve with electricity, many axons will be depolarized, each of which will conduct an action potential along the nerve. Each individual action potential will form a compound action potential. Different axons can have different properties, like different diameters, thresholds, and degrees of myelination. The amplitude of the compound action potential is the sum of the amplitude of each individual action potential. If we put electrodes further down on the nerve, we can detect the compound action potential.
The measurement of the compound action potential is used to diagnose injuries to nerves.
Conductive properties of various nerve fibres
Properties influencing conduction velocity
The conduction velocity of action potentials is mostly determined by two factors: the diameter of the axon, and the degree of myelination. The larger the diameter of the axon, and the more myelinated an axon is, the higher the conduction velocity. Recall that myelin is an insulator.
Different types of nerve fibres
There are six different types of nerve fibres, each with different properties. Most nerves consist of most of these. They are described in the table below:
| Nerve fibre | Myelinated? | Characteristics | Size | Conduction velocity |
| A-alpha (Aα) | Yes | Sensory: Muscle spindles
Motor: Alpha motoneuron |
15 µm | 60 – 120 m/s |
| A-beta (Aβ) | Sensory: mechanoreceptors in the skin | 8 µm | 30 – 60 m/s | |
| A-gamma (Aγ) | Motor: Gamma motoneuron (muscle spindle) | 5 µm | 2 – 30 m/s | |
| A-delta (Aδ) | Sensory: “Fast” pain | 3 µm | ||
| B fibres | Partly | Motor: Preganglionic sympathetic fibres | < 3 µm | 3 – 15 m/s |
| C fibres | No | Sensory: Pain
Motor: Postganglionic sympathetic fibres |
1 µm | ~1 m/s |
As you can see, some nerve fibre types can be found in both sensory and motor neurons. The cell body of all sensory neurons is in the dorsal root ganglia, while the cell body of all motor neurons are in the anterior horn of the spinal cord.
A-alpha nerve fibres are the axons of alpha motoneurons. These are the neurons which stimulate skeletal muscles, causing them to contract. They’re the thickest nerve fibres, and they’re myelinated, giving them the highest conduction velocity. The axons of the sensory nerves which innervate the muscle spindle are also of the A-alpha type.
A-beta are the axons of the sensory neurons which innervate the mechanoreceptors in the skin. They’re the second thickest fibres and therefore also have the second highest conduction velocity.
A-gamma are the axons of the gamma motoneurons. These neurons innervate the muscle spindle, causing it to contract.
A-delta are the axons of some of the sensory neurons which innervate pain receptors. Some pain receptors are innervated by these “fast” nerve fibres, while some are innervated by the much slower C type fibres.
B fibres are the axons of the preganglionic sympathetic fibres. These are the nerve fibres of special sympathetic neurons in the lateral horn of the spinal cord, which innervate the sympathetic ganglia and the adrenal medulla. These fibres are only partially myelinated, giving them a lower conduction speed than the fully myelinated fibres.
C fibres are unmyelinated, giving them the slowest conduction velocity. Non-“fast” pain receptors, as well as postganglionic sympathetic fibres, are C fibres. These are also the thinnest nerve fibres.