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Afferent Nerve Definition
The word afferent means ‘steering’. The messenger neurons in the body that bring the information from different parts of the body to the central nervous system are called afferent nerves. In biology, the axons of a neuron include some projection that helps in carrying the stimulus from the peripheral nervous system to the central nervous system, these projections are defined as afferent nerves.
The afferent nerves receive information in the form of stimuli, called sensory data. The afferent nerves are the sensory nerves that help in carrying information.
What is Afferent Nerve?
The human nervous system is classified into two parts:
1. Central Nervous System (CNS) – brain and spinal cord are part of the CNS.
2. Peripheral nervous system (PNS) – the PNS covers the whole body and connects the body to the central nervous system.
It performs two major functions in the body:
• Output, carrying information from the brain to other parts of the body.
• Input, transmit the information to the brain from different body parts.
The whole process of input and output requires messenger neurons that are of two types:
1. Efferent Neurons: These neurons help in process of transmitting ‘output’ from CNS to other body parts.
2. Afferent Neurons: These messenger neurons help is transmitting ‘input’ to the brain from various body parts.
The information is brought in form of stimulus from the body to CNS by afferent nerves. The information is then processed by CNS into a ‘reaction’. The reaction is then transmitted to the body by efferent nerves.
The ‘ascending tract’ is the pathway that brings the stimulus to the brain, whereas the ‘descending tract’ brings information from the CNS to the body. The functioning of the afferent and efferent nervous system can be explained by an example-
Imagine: If our finger touches the fire flame accidentally, in that condition, we instantly pull back our finger from burning. So, how our whole body knows about the burning finger?
Reason: The afferent nerve of that area senses the stimulus of pain and transmits the signal immediately to the CNS. The CNS then processes the information and transmits the information via efferent nerves to protect the finger from burning.
The whole process is highly complex, in which the closed-loop- pathway senses the stimulus. That means, that our brain will not get information about the finger in case of afferent nerve dysfunction. Now we understood that afferent nerves are the sensory nerves. Sensory neurons are defined as “the neurons that detect the stimulus by the process of sensory transduction”. These nerves are generally afferent whereas the transmission of impulse from the CNS to the body is done by efferent nerves.
There are some similarities between both the nerves:
• Both nerves are part of the nervous system.
• Both help is carrying the nerve impulses.
• Both are structurally similar consisting same body parts: a cell body, Dendron, and an axon.
However, there are several differences also that set them apart.
Afferent Nerve vs Efferent Nerve
| Afferent Nerves | Efferent Nerves |
| Sensory neurons | Motor neurons |
| Carry sensory impulses from the body to the CNS | Carry motor impulses from CNS to body parts |
| Responsible to provide INPUT to CNS. | Responsible to transmit OUTPUT from CNS |
| These nerves are composed of short axon and one long Dendron | They composed of multiple dendrons and long axon |
| Unipolar | Multipolar |
| It initiates at receptor | It terminates at affector |
Afferent Nerve Structure
The afferent nerve mainly consists of dendrites, axon, axon terminal, and a cell body. Most of the sensory neurons consist of a single axon that eventually forms two extensions: the exon and the dendrites. Thus the structure is called pseudounipolar structure.
The multiple projections of dendrites collect the stimulus and help in the formation of the initiation point of the neuronal cell. The neurons have long dendrites that pick up the stimulus. The cell body consists of a nucleus, cytoplasm, and other cellular organelles.
The cell body of pseudounipolar sensory neurons occurs in the side branch of the afferent nerve fiber. The axons end in the CNS and transmit the information from the body to CNS. The afferent nerves consist of relatively short axons.
The nerve fibers are packed in a myelin sheath to avoid any loss of signal or impulse. The myelin sheath is composed of Schwann cells. The axons are categorized into four types based on their diameter:
• A-α (thickest)
• A-β
• A-δ
• C- type fiber (thinnest)
The C-type fibers are non-myelinated nerve fibers while all the others are myelinated nerve fibers. The transfer of signal is also affected by the thickness of the nerve fiber. The transmission rate is fastest in the thickest axon afferent nerve while the rate is slowest in the thinnest axon afferent fibers.
Afferent Nerve Function
The major function of afferent nerves includes transmitting sensory impulses from the body parts to the CNS. The example of sensory impulses is pain, vibrations, temperature, light, moving stimuli, etc. The afferent nerves are present in different organs of the human body. The examples include:
Respiratory system: here, the coughing stimulation, breathing pattern, and airway autonomic neural tone are regulated by the afferent nerves.
Urinary system: the micturition from the urinary bladder involves afferent nerves that are present in the lower urinary tract. Cardiac muscle action also involves cardiac afferent nerves.
Afferent Nerve Dysfunction
The dysfunction in the afferent nerve can cause various problems such as loss of transmission of sensory impulses from the organs to the CNS because these nerves are responsible to carry impulses from the body. The reasons for afferent nerve dysfunction can be nerve damage due to physiologic disorders such as diabetes, auto-immune disorders, cancer, drugs, or infectious disease, etc.
The results of afferent nerve damage also include difficulty in sensing pain or temperature, maintaining balance with closed eyes or fastening buttons, etc.
Afferent Nerve Citations
- Enterocyte-afferent nerve interactions in dietary fat sensing. Diabetes Obes Metab . 2014 Sep;16 Suppl 1:61-7.
- Afferent nerve regulation of bladder function in health and disease. Handb Exp Pharmacol . 2009;(194):91-138.
- Spinal afferent nerve endings in visceral organs: recent advances. Am J Physiol Gastrointest Liver Physiol . 2016 Dec 1;311(6):G1056-G1063.
- Figures are created with BioRender.com
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