This page provides a summary of the main types of afferent axons in peripheral nerves for future reference. There is no need to memorize the list at this point, since we will be elaborating on each of them at some point. Because axons are long and thin, they are often called "fibers", although this is unfortunate, redundant terminology.
The figure to the right shows the main types of afferents from the skin:
- The C fibers are the unmyelinated afferents. Certain pain sensors, along with warm and cold sensors, fall in this category. They conduct from about 0.5 to 2.0 meters/sec
- The A-delta fibers are the smallest and slowest of the myelinated afferents. These are thinly myelinated and give rise to sharp, quick pain, and some crude touch and temperature sensations. They conduct action potentials between about 5 to 30 meters/sec
- The A-beta fibers are large, fast, myelinated afferents. The Meisnner's corpuscles and Pacinian corpuscles belong to this class. These are rapidly adapting touch afferents. Also, Merkel's discs, which we could not see in lab, belong to this category. These are slowly adapting touch afferents. The axons conduct between 35 and 75 meters/sec. (Touch afferents from hair follicles, which we are not studying, also are A-beta.)
There are two major types of afferents from muscles we will be talking about. (There are also less specialized receptors that sense joint position and movement.)
- Muscle spindles are found scattered throughout each skeletal muscle. Each consists of a small group of small muscle fibers enclosed in a connective tissue capsule. These monitor muscle length. The principle afferent axons are classified as A-alpha fibers (type Ia), which conduct from 80 to 120 meters/sec. Axons of the muscle spindles are the fastest axons in the body. (As shown, there is also a second, slower afferent in the muscle spindle we will not be studying.)
- Golgi tendon organs lie at the junction between a muscle and its tendons. The sensory dendrites are embedded in a connective tissue structure. The axons are classified as A-alpha fibers (type Ib). These monitor muscle tension. These conduct a little slower than the muscle spindle afferents.
Neurotransmitters of Afferent Neurons
The C fibers release glutamate. In this case, it can have both fast actions by binding to a five subunit ligand gated ion channel and slow actions actions by binding to an NMDA receptor. Also, these neurons release peptides, such as subtance P. More in lecture.
All of the myelinated afferent neurons release glutamate as their neurotransmitter, which acts as a fast neurotransmitter binding to the five subunit ligand-gated ion channel.