Afferent nerve
n., plural: Afferent nerves
[ˈæ.fɜː.ənt nɜːv]
Definition: nerve that carries sensory nerve impulses from the periphery towards the central nervous system
Table of Contents
Afferent Nerve Definition
The word ‘aferent’ means “steering or conducting something towards a destination”. The afferent nerves are the messenger neurons that bring the information from different parts of the body to the central nervous system (CNS). In biology, the afferent nerves are defined as the projections of axons that carry the stimulus from the peripheral organs to the central nervous system. The information of the stimulus received by afferent nerves is known as ‘sensory data’.
Are the afferent nerves sensory nerves?
Yes, the afferent nerves are the sensory nerves that carry sensory impulses from the different organs to the CNS.
What are afferent and efferent nerves?
The human nervous system is basically composed of two parts:
- Central Nervous System (CNS) – consists of the brain and spinal cord
- Peripheral Nervous System (PNS) -a network of neurons that covers the whole body and connects the body with the central nervous system. The two-primary function of the peripheral nervous system are:
- To carry information from the brain to different parts of the body– in simple terms, Output
- To carry information from different parts of the body and transmit it to the brain and spinal cord– in simple terms, Input
Accordingly, to carry out this process of transmitting information ‘to’ and ‘fro’ from the central nervous system to the whole body, messenger neurons are required. Thus, there are two sets of messenger neurons:
- Efferent Neurons: the messenger neurons that carry information from the brain to different parts of the body –OR– the messenger neurons that transmit the ‘OUTPUT’ from the central nervous system
- Afferent Neurons: the messenger neurons that carry information from different parts of the body to the brain –OR– the messenger neurons that transmit the ‘INPUT’ to the central nervous system
This may be quite confusing. So, how do you remember the difference between afferent and efferent?
Take note of what they relay:
OUTPUT=EFFERENT NEURONS
INPUT= AFFERENT NEURONS
Afferent nerves bring the information or ‘the stimulus’ from the body to the CNS and then CNS process the information into a ‘reaction’. The ‘reaction’ is relayed as the input stimulus to the body by the efferent nerves.
The afferent pathway that brings the stimulus to the brain is known as the ‘ascending tract’ while the opposite pathway that carries information from the CNS to the body via the efferent nerves is known as the ‘descending tract’. Thus, the afferent division of the sensory nervous system brings the sensory stimulus from the afferent sensory nervous system. A simplified representation of the different functions carried out by the afferent and efferent nerves is represented in Figure 1.
This can be further understood with the help of a simple example.
Imagine. By accident, your finger touches the fire flame of a candle. So, what you will do? Obviously, you will instantly pull back your hand to protect your finger from burning. Have you ever given a thought, only your finger has burned, but how the whole body knows about it? And, why you instantly react and pull your whole hand back!
The reason is simple.
When the finger feels the pain due to candle flame, the afferent nerve endings of that area of your skin sense the stimulus, and immediately, they transmit the signal to the CNS. The CNS processes the information. And so to protect the whole finger from burning, it transmits the information via the efferent nerves to pull back your hand from the flame of the candle, thus, protecting your hand from burning.
Though this process is highly complex, the closed-loop pathway works to sense the stimulus and react to the stimulus. Hence, in case there is afferent nerve dysfunction, your brain will not be notified or get the information that your hand/finger is burning while, in case of efferent nerve dysfunction, your hand will not get information from CNS to pull it back. Thus, in the case of nerve dysfunction, the type of nerve affected will determine the consequence of it.
Furthermore, from this example, it is now easy to understand that the afferent nerves are the sensory
nerves that have the capability to perceive the stimulus. While the efferent nerves are only motor nerves and they simply carry the information from the brain to the tissues and cells.
Sensory neurons, by definition, are the neurons that pick up or detect the stimulus through their receptors by a process known as sensory transduction. These nerves are basically afferent nerves. On the other hand, motor neurons form the pathway for the transmission of impulses or information from the CNS to the rest of the body. These nerves are essentially efferent nerves.
In brief, the combination of afferent and efferent neurons governs the reflexes and essential parts of the sensory nervous system. Some of the common attributes or similarities between afferent and efferent nerves are as follows:
- Both afferent nerves, as well as efferent nerves, are an essential part of the nervous system
- Both nerves carry the nerve impulses
- Structurally, both types of nerves have the general neuronal structure: a cell body, dendron, and an axon
- Both nerves are connected to CNS
While there are similarities, there are, of course, certain differences that set them apart. The differences between afferent nerves and efferent nerves are enlisted in Table 1.
| Table 1: Afferent nerves vs Efferent nerves | ||
|---|---|---|
| Afferent Nerves | Efferent Nerves | |
| Type of neurons | Sensory neurons | Motor neurons |
| Purpose or neurons | Afferent signals i.e., sensory data or sensory impulses are carried from the body to the CNS | These nerves transmit information or motor impulses from CNS to different body parts |
| Functionality of neuron | These nerves essentially provide INPUT to the CNS | These nerves essentially transmit the OUTPUT function |
| Location of the neurons | The main cell body of the afferent nerves is found near the CNS | The main cell body of the efferent nerves is found/placed/located within CNS |
| Cellular structure | Have a short axon and one long dendron | Have long axon and have multiple short dendrons |
| Polarity of neuron | Unipolar | Multipolar |
| Ends of the neurons | Initiates at receptor | Terminates at effector |
| The cell body of the neuron | The cell body is round and smooth | The cell body is similar to a satellite |
| Location | Found in mouth, tongue, nose, and skin | Muscles and glands |
Afferent Nerve Structure
Structurally, the afferent or sensory neuron has dendrites, axon, axon terminal, and a cell body. This is similar to any neuronal cell. The majority of the sensory neurons have one axon that extends from the cell body and eventually forming two extensions: the axon and the dendrites. This is known as a pseudounipolar structure.
The dendrites have multiple projections that collect the signals or the stimulus and form the initiation point of the neuronal cell. Afferent neurons are characterized by long dendrites. The sensory impulse or the stimulus picked up by the dendrite travels through the afferent nerve fiber to reach the cell body. The cell body has the nucleus, cytoplasm, and other cellular organelles.
Pseudounipolar sensory neurons characteristically have the cell body in the side branch of the afferent nerve fiber. The sensory impulse or the information from the cell body then travels through the axons to reach the CNS. The axons are the projection from the cell body that ends in the CNS. In afferent nerves, axons are characteristically short in length and for the terminal point of the neuronal cell.
To avoid any loss of signal or impulse, the nerve fibers are packed within a fatty cushion, called a myelin sheath. Myelin sheath, which is made up of Schwann cells, protects, insulates the nerve fiber along with serving the purpose of providing nutrition to the nerve fibers.
Sensory nerves are considered afferent nerves. For instance, an external stimulus detected by the body results in sensation when it is processed in the central nervous system through the afferent nerve pathways. Both afferent and efferent nerves are made of chains of neurons.
Based on the diameter of the axon of the afferent nerve fiber, there are four categories:
- A-α- Thickest axon afferent nerve fiber
- A-β
- A-δ
- C- type fiber- Thinnest axon afferent nerve fiber
A-α, -β,- δ are myelinated nerve fibers while C type fibers are non-myelinated nerve fibers. The thickness of the nerve fiber also affects the speed of transfer of stimulus or signals. The thickest axon afferent nerve fiber transmits the stimulus at the fastest speed while the thinnest axon afferent fibers transmit at the slowest speed.
Afferent Nerve Function
What is the function of afferent nerves? The primary function of the afferent nerves is to transmit sensory impulses from the organs like the body surface, viscera, and muscles to the CNS. These sensory impulses could range from pain, vibrations, temperature, light, moving stimuli, and even noxious signals through nociceptors. The nociceptors are the special afferent nerves that could have the capability to conduct signals that could be potentially damaging the tissues.
Afferent nerves are present in a variety of organs. Examples:
- Respiratory system: the afferent nerves that are present in the airways regulate coughing stimulation, breathing pattern, and airway autonomic neural tone.
- Urinary system: the afferent nerves are also present in the lower urinary tract and are involved in the micturition from the urinary bladder.
- Cardiac afferent nerves are involved in cardiac muscle action as well.
Afferent Nerve Dysfunction
As we are aware that afferent nerves carry the sensory impulse from a variety of the organs like urinary tract, respiratory system, skin, etc. Thus, afferent nerve dysfunction can result in loss of transmission of sensory impulses from these organs to the CNS resulting in dysfunctional organs. Afferent nerve dysfunction results from nerve damage that may have occurred as a result of physiologic disorders, like diabetes (a most common cause of nerve damage), auto-immune disorders (e.g. Guillain-Barré syndrome, lupus, and inflammatory bowel disease), cancer, nutritional deficiency, trauma, drugs, or even an infectious disease. Depending upon the afferent nerves and the organs involved, the afferent nerve damage can result in difficulty in sensing pain or temperature, walking, maintaining balance with closed eyes or fastening buttons, etc.
Try to answer the quiz below to check what you have learned so far about afferent nerves.
References
- Koop, L.K., Tadi, P. (2020) Neuroanatomy, Sensory Nerves. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK539846/
- de Groat, W. C., & Yoshimura, N. (2009). Afferent nerve regulation of bladder function in health and disease. Handbook of experimental pharmacology, (194), 91–138. https://doi.org/10.1007/978-3-540-79090-7_4
- Undem, B.J., Carr, M.J. (2001). Pharmacology of airway afferent nerve activity. Respir Res 2, 234. https://doi.org/10.1186/rr62
- Kopp U. C. (2015). Role of renal sensory nerves in physiological and pathophysiological conditions. American journal of physiology. Regulatory, integrative and comparative physiology, 308(2), R79–R95. https://doi.org/10.1152/ajpregu.00351.2014
- Sonne J, Lopez-Ojeda W. (2020). Neuroanatomy, Cranial Nerve. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; Available from https://www.ncbi.nlm.nih.gov/books/NBK470353/
- Sanvictores T, Tadi P. (2020). Neuroanatomy, Autonomic Nervous System Visceral Afferent Fibers and Pain. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Available from: https://www.ncbi.nlm.nih.gov/books/NBK560843/
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