Reflexes

The nervous system and reflexes

The nervous system has two major parts, which then divide into their own systems. One major division is the central nervous system (CNS) which contains the brain and spinal cord. The other is the peripheral nervous system (PNS) which comprises pairs of nerves that originate from either the brain or the spinal cord.

The peripheral nervous system then divides into sensory neurones and motor neurones. The job of sensory neurones is to carry electrical signals known as nerve impulses from receptors towards the central nervous system. On the other hand, motor neurones carry nerve impulses away from the central nervous system to effectors.

The motor nervous system

The system of motor neurones (neurones that regulate muscle movement) is called the motor nervous system. It can be further split up into two categories: the voluntary nervous system and the autonomic nervous system.

The role of the voluntary nervous system is to carry nerve impulses from the CNS to muscle effectors. The ‘voluntary’ part stems from the fact that we consciously control these movements (i.e. we do them voluntarily). This nervous system is also known as the somatic nervous system, and the reflexes carried out by this system are called somatic reflexes. The effectors of somatic reflexes are muscles and include reflexes such as moving your leg away when you step on a sharp object.

The role of the autonomic nervous system is to carry nerve impulses to glands, smooth muscle, and cardiac muscle. It differs from the voluntary nervous system in that it is not under voluntary control. Everything done by the autonomic nervous system is done subconsciously. Reflexes carried out by this nervous system are called autonomic reflexes. These affect inner organs. Examples of autonomic reflexes include breathing, control of heart rate, and swallowing.

Somatic reflexes and autonomic reflexes are the two main types of reflexes.

The spinal cord

As discussed above, the CNS is made up of the brain and the spinal cord. But what actually is the spinal cord?

There is a bone structure called the vertebral column running vertically along our back. Inside this column is the spinal cord. The spinal cord is a long, thin, tubular structure of nervous tissue. Pairs of nerves emerge from intervals along the spinal cord.

Reflex arcs

A reflex arc is the pathway of neurones that are involved in a reflex. Reflex arcs only contain three neurones: the sensory, the relay, and the motor neurones. Let's take a look at a reflex action when stepping on a pin.

1. A pain receptor in the skin detects the stimulus (in this case the pin) and generates nervous impulses.
2. The receptor sends the nervous impulses to the spinal cord through the sensory neurone. In this scenario, the spinal cord acts as the coordinator.
3. The nervous impulses are then transmitted to the relay neurone (aka interneuron) in the spinal cord.
4. The relay neurone passes this information on to the motor neurone.
5. The nervous impulses travel through the motor neurone to the muscle in the leg, which acts as the effector.
6. Finally, the muscles contract, causing the leg to jerk up and away from the pin. This movement is known as the response.

Fig. 1 - The pathway of a reflex arc from the stimulus to the response

Reflex characteristics and their importance

The reflexes’ response is rapid, short-lived, localized, and totally involuntary. In this section, we will look at what these words mean and why they make reflexes so important for our survival.

Reflexes are involuntary

Reflexes being involuntary means that the decision-making part of our brain doesn’t need to be involved in acting. This allows the brain to focus on more complex processes in the body.

That being said, some nervous impulses may be sent to the brain, meaning that the brain has the power to override them. For example, the brain may override the reflex to move your hand from a hot plate if there is food on the plate that you don’t want to drop.

Unconscious and involuntary both have very similar meanings. Both words refer to actions that you have no biological control over, such as hiccuping and shivering. The difference between them is that unconscious also has a separate meaning on top of this, which means when you are asleep or passed out. Involuntary does not have this other meaning.

Reflexes are protective

Reflexes are not a learnt behaviour. We are simply born with them, so the same reflexes are present in all healthy humans. Some reflexes allow us to move away from danger, protecting ourselves and ensuring our survival.

Reflexes are rapid

Reflexes cause action to happen very quickly. This is because of the very short reflex arc pathway. This means that nervous impulses can travel from the receptor to the effector almost instantly. Furthermore, the fact that we do not have to decide about whether to act saves time, too. This is beneficial as it helps us respond to danger as quickly as possible to keep ourselves safe.

Reflexes are short-lived and localized

‘Localized’ means that the response only affects the parts of the body involved, and ‘short-lived’ means that the response does not last for a long time. This is beneficial for the situations where reflexes are used, as it allows us to resume normal behaviour as soon as the danger has passed. In the example above, the blinking response only affects your eyes and is over once the object has stopped coming towards you.

Examples of reflexes

Many types of reflexes are in our body. Four key examples of reflexes include:

• The stretch reflex.
• The flexor (withdrawal) reflex.
• The crossed-extensor reflex.
• The Golgi tendon reflex.

The stretch reflex

When a muscle is stretched, muscle spindle receptors will send nervous impulses to the spinal cord, causing the same muscle to contract. At the same time, the antagonistic muscle is relaxed.

Another common example of this reflex is the knee-jerk reflex. Doctors do this test on patients to test that specific spinal cord segments are working. It involves striking a tendon near the knee (called the patellar tendon) with a rubber hammer. This will cause the patellar tendon to pull on the leg’s quadriceps muscle (the effector). The stretch reflex will then counteract this stretch as the muscle ‘thinks’ it is stretching very rapidly. The response to this will be that the knee will jerk, which is why the reflex is called the ‘knee-jerk reflex’!

Fig. 2 - The knee-jerk reflex. The quadriceps and hamstring muscles work antagonistically. Therefore, for the knee to stretch, the hamstrings must relax while the quadriceps contract

Flexor (withdrawal) reflex

When a body part comes into something dangerous, such as touching a hot plate as mentioned above, the body part will reflexively pull away. This occurs because sensory receptors in the skin send nervous impulses to the spinal cord, which will cause the body part to contract and move away.

The sensory information for this type of reflex will also travel to the brain, so conscious decision-making can occur and override the reflex from happening.

Crossed-extensor reflex

As we saw above, stepping on a sharp object causes a withdrawal reflex of the leg. You might wonder why this doesn’t cause us to topple over, and the answer to this is the crossed-extensor reflex. When the withdrawal reflex of the injured leg happens, the opposite leg extends at the same time so that the person can appropriately shift their balance to the opposite foot during the reflex.

Golgi tendon reflex

This is a reflex that happens due to the Golgi tendon organs. Golgi tendon organs are types of sensory receptors that sense changes in muscle tension. They lie in between a muscle and its tendon.

When a muscle is stimulated by a stretch that is too much or too long, the Golgi tendon organs will detect this and send nervous impulses to the spinal cord that cause the stretched muscle to relax.

As a result, the Golgi tendon reflex is also referred to as the ‘inverse stretch reflex’ as it has the opposite effect to the stretch reflex. In the stretch reflex, the antagonistic (opposite) muscle is relaxed, whereas in the inverse stretch reflex, it is the muscle that is stretching that is relaxed.

Fig. 3 - Golgi tendon organs have a high threshold, and they function to protect the muscles and connective tissues from excessive stretching. They create a reflex inhibition of the muscle when it has reached its limit of contraction