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Reflex Actions


Reflex actions are involuntary and instantaneous movements in response to stimuli.

 

What is a reflex action?

A reflex or a reflex action is an instant movement in response to a stimulus.

 

Some natural pathways called reflex arcs act in an impulse before the impulse reaches the brain, and this makes a reflex or a reflex action.

The pathway of a reflex actionThe pathway of a reflex action

 

 

Reflex actions examples

Have you noticed that sometimes your body reacts to something automatically, even before you think about it?

 

For example;

 

What if a ball comes flying at your face?

Your eyelids blink shut quickly and you may even scrunch up your eyes or bring your hands up to your face for extra protection.

These quick movements are called reflexes and they protect your body from harm without you even having to think about it.

 

Sequence of a reflex action

 

Reflexes or reflex actions take place in your body without your brain being involved.

 

You don’t need to think for a reflex action to happen.

 

What is your reaction if you touch something hot or sharp?

You automatically pull your hand away. This is a withdrawal reflex. This type of a movement happens even before your brain has received information from the sense organs that there is a problem.

 

Reflex actions - Reflex arcWhat is your reaction if you touch something hot or sharp?

 

How does a reflex action work?

 

Many reflexes are controlled by the spinal cord. Signals travel to the spinal cord along nerves from the receptors of the sense organs. But, even before the information reaches the brain, a signal zaps back out from the spinal cord that tells the effectors, which is the muscles to move. This overall sequence is called the reflex arc.

 

Reflex actions - Reflex arc diagramHow does a reflex action work – Diagram of Reflex Arc

 

Next we are going to learn about the reflex arc in detail. But, before that let’s learn what the spinal cord (Also read Human Nervous System), receptors and effectors are.

 

What is the spinal cord?

 

The spinal cord is made up of bundles of nerves that connect the brain to other parts of the body. It runs down from the brain through a canal in the center of the bones of the spine (backbone). These bones which surround and protect the spinal cord are called vertebrae.

 

Reflex actions through spinal cordReflex actions through the spinal cord

 

What are receptors?

 

There are specialised groups of cells in each of your sense organ, which are the ear, eye, nose, tongue and the skin. These specialised groups of cells can detect changes or stimuli in the environment. Each organ has receptors sensitive to particular kinds of stimulus.

 

Location of the receptors

Receptors sensitive to

Eyes

Light

Ears

Sound

Nose

Odour (Chemicals in the air)

Tongue

Taste (Flavours / Chemicals in your food)

Skin

Temperature, touch, pressure, pain etc.

 

What are effectors?

Effectors can be muscles and glands. There job is to produce a specific response to a detected stimulus.

 

Steps of the reflex arc

  • A receptor detects a stimulus. E.g. – A receptor in the skin detects the change in temperature
  • Electrical impulses are sent by the sensory neurons to relay neurons located in the spinal cord. Here, the relay neurons connect sensory neurons to motor neurons.
  • Electrical impulses are sent by the motor neurons to an effector.
  • A response is produced by the effector E.g. – Hand moves away as the muscle contracts

 

Remember, The signals of a reflex action only travel through relay neurones in the spinal cord. They do not travel through the brain.

 

The pathway of a reflex actionThe pathway of a reflex action

 

Reflex arc flow chartReflex Arc Flow Chart

 

Examples of reflex actions

 

  • Sneezing and coughing
  • Knee-jerk reflex
  • Pupil reflex
  • Sweating
  • Jerk occurs by touching a hot or sharp object
  • Reflexes take place by seeing an object suddenly coming to you

 

Sneezing and coughing

 

Sneezing and coughing are also types of reflexes. When harmful substances get into the nose and lungs the body uses them to clear dust and particles out of the airways. This is why it is difficult to control and stop, once you feel that you are about to sneeze.

 

Reflex action: Sneezing and coughing

Stimulus: Harmful substances get into the nose and lungs

Receptors found in: Nose

Response: Sudden contraction of internal muscles causing a violent exhalation to blow irritant material out.

 

Reflex actions examples - SneezingReflex actions examples – Sneezing

 

Knee-jerk reflex

Cross your legs and tap just below the knee. Your knee jumps forwards even though your brain hasn’t told it to move.

 

Reflex action: Knee-jerk

Stimulus: Tapping the knee

Receptors found in: Skin

Response: Due to sudden contraction of front thigh muscles, the lower leg kicks forwards

 

Reflex actions examples - Knee JerkReflex actions examples – Knee Jerk

 

Pupil reflex

 

Reflex action: Pupil reflex

Stimulus: Light intensity

Receptors found in: Eye

Response: The diameter of your eye pupil is adjusted by the Iris muscles, wide in dim light, narrow in bright light

 

Reflex actions examples - Eye pupil reflex in bright and dim lightReflex actions examples – Eye pupil reflex in bright and dim light

 

Sweating

 

Reflex action: Sweating

Stimulus: Increased body temperature

Receptors found in: Skin

Response: Increased sweat is released to cool body surface down.

 

ReflexReflex actions examples – Sweating

 

A Baby’s Reflexes

 

A baby’s brain is not developed to think and do something. So, babies are born with many reflexes that they need to survive. These are also called instinctive behaviours that are not learned but that they know automatically.

 

See if you have noticed the following reflexes on your baby sibling?

 

Rooting reflex – A baby automatically roots around to find its mother’s breast

Suck reflex – A baby starts to suck mother’s milk when its mouth touches mother’s breast

Grasp reflex – It grasps your finger or anything it touches or put in its hand

Startle reflex – When a baby is startled by a loud sound or movement, in response to the change, the baby throws back its head, stretches out its arms and legs, cries, then pulls the arms and legs back in.

 

Babies also don’t have the control of their bladder. This is why their bladder automatically works and make them urinate when it’s full.

 

Reflexes in sports

 

Runners and swimmers need good reflexes as they need to react quickly to the start gun. Tennis players may have less than half a second to react to a ball coming their way.