Afferent Nerves

Afferent nerves are responsible for our brain to experience what is on the outside of our bodies and interpret it. For example, the world is full of beauty and awe-inspiring sights and smells; to fully appreciate them, we need afferent nerves to relay these images, scents, and tastes.

Afferent nerves are axon projections carrying peripheral organ stimulus to the central nervous system. These message-relaying neurons bring external information such as touch, smell, hearing, and sight from various body parts to the CNS for analysis and processing.

There are over 86,000,000,000 nerves in the body! Without these sensory detectives, the stimulus we receive from external sources to the brain and central nervous system will not be processed. Afferent nerves are part of a complex sensory system connected to the brain so let us find out more.

What Is The Function Of Afferent Nerves?

The afferent nerves primarily transmit sensory impulses from organs such as the viscera, body surface, and muscles to the central nervous system. The sensory impulses can be any number of sensations such as –

• Temperature
• Light
• Pain
• Vibrations
• Noxious signals through nociceptors
• Moving stimuli

Nociceptors are unique afferent nerves capable of conducting signals that can potentially damage tissues inside the body.

Afferent nerves can be found in multiple organs as follows –

• Cardiac afferent nerves – These afferent nerves help regulate the cardiac muscle action.
• Respiratory system – Afferent nerves are present in the airways to help with the airway autonomic neural tone and the breathing pattern and regulate coughing stimulation.
• Urinary system – Afferent nerves are present in the lower urinary tract and are involved in the micturition of the bladder.

Afferent nerves form part of the nervous system of the body. The nervous system consists of the following two divisions and their processing bodies, namely –

• The central nervous system
• The peripheral nervous system
• The afferent nerves - CNS
• The efferent nerves - PNC

The nervous system is responsible for processing all sensory information in and outside the body through the brain. All the nerves play a role, but the afferent nerves are critical for relaying external sensory input.

The afferent nerves transport sensory information from the outside to the central nervous system, which can involve senses such as:

• Pain
• Sense of touch
• Hearing
• Taste
• Vision
• temperature

Where Are Afferent Nerves Located?

Because afferent nerves relay external sensory information, they need to be located in areas that gather this data. You will find afferent nerves in

• The skin
• The tongue
• The eyes
• The nose
• The mouth

All of these input areas are highly sensitive and critical for relaying the correct messages to the brain so that the body can react appropriately.

Are Afferent Nerves Sensory Nerves?

Afferent nerves are sensory nerves. They carry external sensory impulses from the organs like the skin, mouth, eyes, and nose to the central nervous system.

Afferent nerves will follow the sensory pathway. The pathway consists of a chain of neurons from the receptor organ to the cerebral cortex responsible for sensory perception. They relay what you see, smell, feel and taste into recognizable data.

The somatosensory stimuli activate a neuron chain from the peripheral first and finally end in the cerebral cortex.

The Difference Between Afferent And Efferent Nerves

The nervous system of the human body is comprised of two parts. These two parts each have a relaying mechanism that does different tasks. In addition, the afferent and efferent nerves each have other functions.

The CNS, or central nervous system, is made up of the brain and the spinal cord, while the PNS, or peripheral nervous system, is a vast neuron network that spans the entire body. The peripheral nervous system is the connection between the body and the central nervous system.

The peripheral nervous system has two functions, and they primarily carry important output information from the brain to the designated areas of the body. It is also responsible for carrying important input information from designated body areas to the spinal cord and the brain.

In order to continuously transmit this critical information to and from the central nervous system to the body and back, the brain needs the help of messenger neurons. These neurons are classified as –

Afferent Neurons - INPUT – These messenger neurons relay information from sensory-rich areas of the body like the eyes, mouth, skin, and nose to the brain. These are in input neurons to the central nervous system.

Efferent Neurons - OUTPUT – These messenger neurons relay information from the brain to designated areas in the body. These are output neurons to the central nervous system.

The difference between afferent neurons and efferent neurons is what information they relay and from where.

Afferent and efferent nerves must work together to regulate and govern essential sections of the body's sensory nervous system and reflexes. The most apparent similarities between efferent and afferent nerves are –

• Afferent and efferent nerves are connected to the central nervous system
• Afferent and efferent nerves carry impulses
• Afferent and efferent nerves are an essential and crucial part of the central nervous system.
• Afferent and efferent nerves have the same general neuron structure: cell body, axon, and dendron.

There are many similarities but also notable differences as follows –

Afferent Nerve Overview – Sensory Neurons

• Purpose of afferent nerves – These nerves carry sensory data and impulses to the central nervous system
• Neuron functionality – Providing input to the central nervous system
• Cellular structure – A long dendron and short axon
• Location - The main cell body is found near the central nervous system
• Neuron Polarity – Unipolar neuron
• Neuron ends – Starts at the receptor
• The appearance of the cell body – The cell body is smooth and round
• Afferent nerve location – Found in the skin, nose, mouth, tongue

Efferent Nerve Overview – Motor Neurons

• Purpose of efferent nerves –  These nerves carry motor impulses or information from the central nervous system to various body parts 
• Neuron functionality – Providing output from the central nervous system
• Cellular structure – Multiple short dendrons and a long axon
• Location - The main cell body is embedded in the central nervous system
• Neuron Polarity – Multipolar neuron
• Neuron ends – Terminates at the effector
• The appearance of the cell body – The cell body has the appearance of a satellite.
• Efferent nerve location – Found in the muscles and glands

How Does The Nervous System Work?

The entire nervous system is a closed loop, and the central nervous system is primary while it carries the decision-making function. Afferent and efferent nerves carry out the sensation and reaction functions.

The afferent nerves are tasked with bringing stimulus from the sensory points, the mouth, eyes, skin, nose, and tongue, to the central nervous system. The central nervous system then analyses and processes this information accordingly. The efferent nerves are then responsible for relaying the input or reaction stimulus to the body.

The ascending tract is the pathway the afferent nerves use to bring stimuli to the brain. The descending tract is the pathway that the efferent nerves use to get the analyzed information through the central nervous system to the brain.

The afferent nerves are the nervous system division that brings the sensory input stimulus from the afferent SNS or sensory nervous system.

How Does The Afferent Nerve React To Stimuli?

It's easier to understand how afferent nerves react to stimuli and the role of the afferent nerve if you look at some day-to-day scenarios in which the afferent nerves are activated. This is because our bodies rely on afferent nerves much more than we realize.

Here is one example of how afferent nerves help protect the body-

Say, for example, you cook a meal and pre-heat the oven. Most people know that the elements inside the stove are pretty close to the oven rack, but you may still get burnt no matter how careful you are.

As you place the oven dish inside and accidentally let your wrist touch the top element, you burn your skin. Think for a minute that as soon as you burn your wrist, your whole body is aware that you got injured. Yet, the body immediately reacts to pull back instantly from the heat. Why is that?

Here is what happens – as soon as the skin on the wrist felt the pain of the burn, the afferent nerve endings in the skin on your wrist pick up the stimulus. They immediately transmit a signal to the central nervous system, which then processes the information.

To prevent further injury and prolonged exposure to the red hot element, the central nervous system sends a signal to the efferent nerves to immediately pull away from the heat source. This is a self-preservation mechanism the body uses to protect itself from harm.

This is a complex and highly advanced process in the closed-loop pathway system of sensing and reacting to stimuli. In the unfortunate event that a person suffers from afferent nerve dysfunction, the brain will not be notified that the body is being injured.

If a person should suffer from efferent nerve dysfunction, their hand will not get the response from the central nervous system that it must pull away, and the burn injury will be severe. If a person has a nerve dysfunction, any injury will be more significant.

Using an example such as hand burning, it is practical and easy to deduce that the afferent nerves are sensory nerves responsible for perceiving the stimulus. In contrast, efferent nerves are motor nerves and carry information from the brain to the cells, tissues, muscles, and organs.

The Afferent Nerve Structure

In similarity with other neuronal cells, the afferent neuron has a cell body, axon, dendrites, and an axon terminal. Most sensory neurons have a single axon extending from the cell body and end up forming two extensions which are known as a pseudounipolar structure –

• The axon
• Multiple dendrites

The dendrite's multiple projections collect signals forming initiation points of neuronal cells. Afferent nerves are known for having long dendrites. When dendrites pick up the stimulus or sensory impulse, it must travel through the afferent nerve fiber before reaching the cell body.

Cell bodies consist of cellular organelles, a nucleus, and cytoplasm. Typically on the side branches of afferent nerve fibers, you will find the pseudounipolar sensory neurons cell body.

Sensory impulses from the cell body will travel through axons to reach the central nervous system. Afferent nerves have characteristically short axons.

Nerve fibers are packed within a fatty layer or myelin sheath to help avoid signal loss. The myelin sheath comprises Schwann cells and insulates and protects the nerve fibers. The myelin sheath also provides critical nutrition to nerve fibers.

Are All Afferent Nerves The Same Size?

Because of the different sensory regions or areas where you find afferent nerves, it makes sense that they will not be the same size. The chains of neurons that make up the afferent nerves may be thin or thick depending on the location.

Afferent nerves are divided into four categories based on the diameter of the efferent nerve axon. The categories are as follows -

• Aα- Thickest axon afferent nerve fiber -12 to 20 µm - Myelinated
• Aβ – Medium axon afferent nerve fiber – 6-12  µm -  Myelinated
• Aδ – Thin axon afferent nerve fiber – 1-6 µm -  Myelinated
• C-type fiber- Thinnest axon afferent nerve fiber 0.2-1.5 µm – Non-myelinated

The diameter of the nerve fibers will determine how fast or slow they transmit the sensory signals. The thicker the axon, the faster it will transfer; the thinner the axon, the slower it will transfer the signals.

What Are Afferent Nerve Dysfunctions?

Afferent nerves carry sensory impulses from various organs in the body, such as the mouth, urinary tract, skin, and respiratory system. Therefore, if an afferent nerve were to experience a loss of signal, it would result in a malfunctioning organ.

Dysfunction of the afferent nerves could be due to nerve damage and be the result of conditions or diseases such as –

• Auto-immune disorders such as lupus, IBS, Guillain- Barré syndrome, Multiple Sclerosis
• Cancer
• Diabetes
• Drug abuse
• Infectious diseases
• Toxins
• Trauma such as amputation or a crushing injury

Afferent nerve dysfunction can result in the person experiencing trouble feeling pain, temperature, or sensation. They may experience difficulty walking, keeping balance, dressing, or sleeping because the damaged afferent nerves misinterpret the stimuli.

Misinterpreting stimuli by the afferent nerves can result in other disorders where they experience heightened pain. For example, in the case of hyperalgesia or developing CRPS – Complex Regional Pain Syndrome.

Can  Afferent Nerve Dysfunction Be Cured?

In most cases, any nerve damage is likely to be permanent and cannot be fully cured. However, several treatment options are available that significantly reduce the symptoms and effects on the body.

Nerve damage is usually progressive, and that's why you must seek medical help for symptoms early on to possibly prevent permanent damage.

Nerve dysfunction can be treated as follows –

• Pain medication or anesthetic infusions in the injured area
• Dietary changes
• Physiotherapy
• Splints, braces, and pressure bandages
• Exercise
• Laser therapy
• Electrical stimulus therapy

Conclusion

The afferent nerves are critical for relaying crucial sensory information to the central nervous system. A well-maintained nervous system means the body will function optimally, so if you notice any signs that a sensory impulse is out of synchronization, seek medical help as soon as possible.

Afferent nerves are the ones that tell your body about all the good and beautiful things in life, so it's worth looking after.