Receptors, Effectors, and the Reflex Arc

A reflex is a fast, automatic response to a stimulus that does not need conscious thought, and the pathway it follows is the reflex arc: receptor → sensory neuron → relay neuron → motor neuron → effector. In brief: when you touch something hot, receptors in the skin detect the heat, a sensory neuron carries the signal to the spinal cord, a relay neuron passes it to a motor neuron, and the motor neuron makes a muscle (the effector) pull your hand away. The brain is informed afterward.

Reflexes protect the body, because they are faster than voluntary responses. This guide explains the types of receptors and effectors, describes the parts of a reflex arc, compares reflexes with voluntary actions, works through the knee-jerk reflex and the withdrawal reflex, and explains the typical mistakes.

What are receptors?

Receptors are cells or nerve endings that detect stimuli, which are changes in the environment, and convert them into electrical signals. Each type is sensitive to a specific kind of stimulus.

• Photoreceptors (rods and cones in the retina) detect light.
• Mechanoreceptors detect touch, pressure, stretch and sound vibrations in the skin, muscles and ear.
• Thermoreceptors detect heat and cold.
• Chemoreceptors detect chemicals, as in taste buds, the nose and the carotid arteries, which detect carbon dioxide.
• Nociceptors detect painful, damaging stimuli.
• Proprioceptors in muscles and tendons detect the position and movement of body parts.

What are effectors?

Effectors carry out the response. In the human body, there are two kinds. Muscles respond by contracting, as when the arm muscles pull the hand away. Glands respond by secreting, as when the sight of food makes the salivary glands release saliva or when adrenal glands release adrenaline in a fright.

What is a reflex?

A reflex is a rapid, involuntary and stereotyped response to a stimulus. It is inborn and does not have to be learned, and it involves a minimum number of neurons. The same stimulus always produces the same response. Reflexes allow the body to respond very quickly to dangerous situations and to maintain posture and basic body functions without using the brain’s attention.

Conditioned reflexes, such as Pavlov’s dogs salivating when a bell rings, are learned and are not simple reflexes. They are covered in the article on classical conditioning.

What are the parts of the reflex arc?

1. Receptor: detects the stimulus and starts an impulse in a sensory neuron.
2. Sensory neuron: carries the impulse along its axon to the spinal cord, entering via the dorsal root.
3. Relay neuron (interneuron): located in the gray matter of the spinal cord. It connects the sensory neuron to the motor neuron through synapses. In a few reflexes, such as the knee jerk, there is no relay neuron, and the sensory neuron connects directly to the motor neuron.
4. Motor neuron: carries the impulse from the spinal cord, leaving through the ventral root, to the effector.
5. Effector: the muscle or gland that produces the response.

At the same time, other relay neurons send signals up the spinal cord to the brain. That is why you become aware of the pain after you have already pulled your hand away. The brain can also modify some reflexes, for example when you decide to hold a hot plate to avoid dropping it.

Reflex and voluntary actions compared

Worked example: the withdrawal reflex and the knee jerk

The withdrawal reflex

You accidentally touch a hot stove with your fingertip.

1. Thermoreceptors and nociceptors in the fingertip detect the heat and start impulses.
2. The sensory neuron carries them to the spinal cord.
3. A relay neuron in the spinal cord connects to a motor neuron serving the biceps muscle.
4. The motor neuron sends impulses to the biceps, which contracts and pulls the arm away. The triceps is inhibited by another relay neuron, so that it relaxes.
5. Meanwhile, signals travel to the brain, and you feel pain.

The knee-jerk reflex

A doctor taps the tendon below the kneecap with a small hammer. The tap stretches the thigh muscle (quadriceps). Stretch receptors in the muscle send an impulse through a sensory neuron to the spinal cord, where it synapses directly with a motor neuron that returns to the same muscle. The quadriceps contracts, and the lower leg kicks forward. Doctors use it to test whether the nerve pathway between the muscle and the spinal cord is intact. An absent or exaggerated response can suggest a problem with the nervous system.

Using a speed of 70 m/s and a path of about 1.0 meter in total (0.5 m each way), the conduction time is about 1.0 ÷ 70 ≈ 0.014 s, or 14 ms, plus around 0.5 to 1 ms at the synapse. The reflex takes about 15 to 20 ms, which is about ten times quicker than a typical voluntary reaction.

What are other examples of reflexes?

• Pupil reflex: the pupils narrow in bright light to protect the retina. The circular muscles of the iris are the effectors.
• Blinking: the eyelids close when something approaches the eye or touches the cornea.
• Coughing and sneezing: clear the airways of irritants.
• Swallowing, gagging and the suckling reflex in newborn babies.
• Grasp reflex: a baby grips a finger placed in its palm.

How was the reflex discovered?

René Descartes proposed in the seventeenth century that some actions are mechanical responses to stimuli. The idea of the reflex arc was developed through the work of Marshall Hall in the 1830s and Charles Sherrington in the early twentieth century. Sherrington described how neurons coordinate in the spinal cord, introduced the word synapse, and shared the 1932 Nobel Prize in Physiology or Medicine. Ivan Pavlov studied conditioned reflexes in dogs around 1900.

Which mistakes are common?

Mistake 1: Saying that reflexes go through the brain first

Many reflexes are processed in the spinal cord, and the brain is informed at the same time or later.

Mistake 2: Mixing up the order of the neurons

The order is sensory, relay, motor. The sensory neuron brings the signal in and the motor neuron takes it out.

Mistake 3: Confusing receptors and effectors

Receptors detect stimuli. Effectors, muscles and glands, make the response.

Mistake 4: Thinking all reflexes are simple and cannot be changed

Some can be controlled or modified by the brain, and conditioned reflexes can be learned.