Afferent vs. efferent nerve fibers

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Patients with spinal cord injury have deficits in their motor and sensory systems. What exactly does that mean from a biological point of view?

Our nervous system - a complex machine
Our nervous system is divided into two parts. The central nervous system includes the brain and the spinal cord. The peripheral nervous system consists of a network of neurons, which spans the organs, the muscles and the body. The neurons in both systems work together to help us think, survive and act on the world around us.

How does this system work?
The nervous system works on the principle of input and output, perception and (re) action. Living beings are able to sense what is going on in their surroundings and do something in reaction to that. Let’s take a simple example: if a car is about to hit you, you jump out of the way. This easy action is more complex than it seems. Your eyes saw the car, your brain figured out that this is a dangerous and told your muscles of your leg to jump out of the way. Another example: if the flame of a candle burns your finger, you draw back your hand immediately. You sensed and then acted.

It’s important to know that the nervous system is connected within all our body’s activity. For example it always receives information on the exact position of a limb, without having to look at it, scanning the flexion and stretching of the joints and muscles. This sense is essential for a body’s movement, for example during sport, and is sometimes referred to as the sixth sense. Based on this permanent feedback the nervous system can control the body activity, either voluntarily (muscle movement) or involuntarily (heart beat).

Simple components for a complex assembly
Our nervous system has different types of neurons that are constantly at work. Neurons that receive information from our sensory organs (e.g. eye, skin) and transmit this input to the central nervous system are called afferent neurons. Neurons that send impulses from the central nervous system to your limbs and organs are called efferent neurons.

Therefore, as the afferent neurons convey the sensory stimulus to the brain (like burning sensation of a candle), the efferent neurons convey the motor stimulus to the muscles (moving the hand away from the candle). To sum it up: Afferent = Receive and Efferent = Act.

The tracts that are conveying sensations up to the brain are also referred to as the ascending tracts. Going in the opposite direction than the ascending tracts, the tracts linking the brain to all the muscles and organs of the body are called descending tracts.

Which types of fibers are damaged after a spinal cord injury determine the individual failures. If motor (= efferent) fibers are destroyed, you are not able to lift your leg, because the command can’t be transmitted from the brain to the muscles in the leg. If sensory (= afferent) fibers are affected, you and your brain won’t be notified by the sensory organs, e.g. if somebody strikes your leg. In fact, mostly a combination of efferent and afferent fibers is damaged after a spinal cord injury.

But there is more …
As described earlier, the nervous system can be seen as a “closed loop” system of sensation, decision and reactions. Depending on the complexity of the reaction and the involved muscle groups/parts of the body, different levels of the central nervous system are involved.

In some cases the closed loop does not require the intervention of higher levels like the brain. Afferent fibers are also directly connected to efferent fibers. The patellar reflex, also known as the knee-jerk reflex, is a good example. This simple test that many undertook during medical examination reveals a reflex that is essential in maintaining the posture and balance, thus allowing a person to walk without having to think about every single step.

When the reaction is more complex, the intervention of higher levels of the central nervous system is required. The receptor in the finger detects the heat coming from the candle. This information travels to the spinal cord and through several neuronal relays the muscles of the arm are innervated, the hand is retracted and the brain senses the pain. Even more complex is the escape from the car: the eye detects the car and transmits this information to the brain. The brain then elaborates an appropriate response (jumping out of the way) and sends the corresponding motor action to the muscles.

To summarize, whether and to what extent afferent and/or efferent fibers are damaged after a spinal cord injury determines whether the patients have deficits in e.g. feeling and holding a posture and/or in commanding the muscles.

Graphics: Dr. Vieri Failli