Video: Sensory receptors

After a long, tiring week, I like to decompress with a warm cup of coffee, a nice book, and some relaxing tunes. It's a very sensory experience -- hearing the melody of the music, seeing the letters on the page, feeling the softness of the paper and the warmth of the mug, smelling and tasting the coffee -- all of these are sensory modalities which is stimuli that get picked up by our sensory system.

Some of them are special while some are more general somatic sensations. These reach our conscious awareness. Visceral sensations, like change in blood pressure, are more subconscious. They are sensed and processed in the brainstem such that we can't perceive them actually happening. All of these sensations are picked up by sensory receptors, our starting point of the sensory system. And in this tutorial, we'll learn more about the different kinds of sensory receptors.

Sensory receptors are specialized cells whose job is to take sensory stimuli and convert them into electrical signals, sending them along sensory neurons to reach the central nervous system -- that's the brain and the spinal cord -- for processing, thus, they act as transducers. And that process of converting a stimulus into an electrical impulse is sensory transduction.

Sensory receptors vary in their structure, location, and function. Based on their appearance, they could be nonencapsulated, encapsulated, or specialized receptor cells. The nonencapsulated ones don't have a capsule at their ends like with free nerve endings. More complex receptors have neuron terminals with a capsule around them like the lamellar corpuscles and muscle spindles. In both these situations, the neuron terminal forms a part of the receptor.

The special senses like taste and hearing can have specialized receptor cells associated with sensory neurons forming synapses. They use neurotransmitters to communicate with these neurons.

The location of the receptor depends on what stimulus it's meant to be picking up. If we classify them based on location, there are three kinds. The ones intended for more external stimuli, like cutaneous sensations, are the exteroceptors while the ones for internal stimuli coming from visceral organs are interoceptors. Receptors in muscles and joints that keep the brain aware of the position of different parts of our bodies, like our arms and legs, are proprioceptors.

Each of these receptors is sensitive to a particular type of stimulus. For instance, thermoreceptors are sensitive to temperature; that's their preferred modality. Think of it as a personal preference.

If we were to look at the receptors from a functional perspective, we could classify them into mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, and nociceptors. Of these, the mechanoreceptors are the largest category. These receptors respond to mechanical stimuli. They get physically deformed by the stimuli. Most of them are in the skin and called cutaneous mechanoreceptors which detect tactile sensations like touch and pressure.

The nonencapsulated ones are the free nerve endings which sense crude non-discriminative touch and pressure. But as we'll see later on, they also respond to pain and temperature.

There are receptors associated with other cutaneous structures such as hair follicle bulbs. These are called hair follicle endings or hair end organs which detect mechanical bending of the hair follicle. Some neurons end as expanded discs associated with special cells called tactile epithelial cells or Merkel cells, together forming an epithelial tactile complex, also known as the Merkel cell-neurite complex.

The encapsulated receptors are the tactile or Meissner's corpuscles, which are touch receptors. Deeper down are the bulbous or Ruffini corpuscles, sensitive to dermal stretch, and the lamellar or Pacinian corpuscles, which responds to high-frequency vibration.

Mechanoreceptors also detect position sense. These are the proprioceptors located in muscles, tendons, and joints. Muscles have two important receptors, both encapsulated. They are the muscle spindle and tendon organ. The muscle spindles detect the length of the muscle while the tendon organs are more responsive to muscle tension. Together, they give the central nervous system information about the movement and position of body parts. This is known as proprioception.

Internally, there are baroreceptors located in the carotid sinus and arch of the aorta. They detect stretch of the vessel when arterial pressure changes; thus, they are stretch mechanoreceptors and are important for the regulation of blood pressure.

Another interesting area where we have mechanoreceptors are our ears. Surprising, isn't it? Both hearing and balance require mechanoreceptors. Receptors like cochlear hair cells of the inner ear and the vestibular hair cells of the semicircular canals that respond to sound and changing head positions are both getting mechanically deformed and thus activated. This makes them mechanoreceptors.

The eyes, however, have photoreceptors. They are the only area with photoreceptors. The best known ones are the rod cells and cone cells in the retina that responds to the stimulus of light. The rod cells help us see in dim light and the cone cells in brighter light. Cone cells are also needed for color vision. They take that light energy, and with transduction, send electrical signals which will then travel along the optic pathway to the brain to help us see.

The other two special senses, taste and smell, have gustatory and olfactory receptors. These are chemoreceptors. They respond to chemical stimuli. The gustatory sensory epithelial cells are located in the taste buds of the tongue and other areas of the oral cavity. Tastants that dissolve in saliva are their stimulants. Similarly, the odorants that dissolve in the mucus of the nose are stimuli for the olfactory sensory neurons located high up in the roof of the nasal cavity.

You'll hear the word chemoreceptors being more classically used when talking about the respiratory system, where we have central and peripheral chemoreceptors that detect changes in oxygen, carbon dioxide, and pH levels in blood. They are important for respiratory regulation.

Osmoreceptors are located in areas like the vascular organ of the lamina terminalis and the subfornical organ in the brain, and like the name suggests, they sense changes in the osmolality of plasma and are necessary for maintaining fluid balance.

Our next group is the thermoreceptors. They are usually free nerve endings and they sense temperature. There are cold receptors and warm receptors but they've got their limits. Not all thermal sensations are pleasant; some of them are painful. At very high temperatures or very low temperatures, it's actually the pain receptors that get stimulated. That's our last category, the nociceptors, which are usually free nerve endings that respond to stimuli which are capable of damaging tissue. These are considered noxious stimuli as they are strong and potentially harmful mechanical, chemical, or extreme thermal stimuli. This information gets carried by sensory neurons to the brain which may be perceived as pain.

It's important to note that pain is subjective and quite complex but the perception of pain that results from the activation of a nociceptor is called nociceptive pain; thus, we have lots of different kinds of sensory modalities which serve as stimuli for a variety of receptors.

These receptors, regardless of their sensory modalities, when stimulated, experience a change in membrane potential. This is called a receptor potential which could be depolarizing or hyperpolarizing. These are graded potentials.

In a lot of sensory cells, when the stimulus arrives, it opens ion channels on the neuron cell membrane. For example, in mechanoreceptors, the mechanically-gated channels open in response to deformation. The entry of ions like sodium changes the membrane potential in the neuron.

If the stimulus is strong enough, the depolarizing receptor potential brings the membrane potential up to threshold, triggering the opening of voltage-gated sodium channels, setting off action potentials in the sensory neuron. That's one way in which receptors can convert a stimulus into an electrical signal. This process is called sensory transduction.

Action potentials are all or none in nature. If the receptor potential brings the membrane potential up to threshold, there will be an action potential which can reach the brain and, thus, the stimulus is perceived. No threshold, no action potential, and the stimulus goes unnoticed. This avoids overwhelming the brain with weak stimuli that don't warrant its attention.

The receptor potentials, however, are graded potentials. They can add up as the stimulus strength increases. This in turn would increase the number or frequency of action potentials traveling along that sensory neuron. This is temporal summation, which we've looked at in earlier lessons. This is one way in which the brain can perceive an increase in stimulus intensity.

The other way is with spatial summation where a stronger stimulus can spread over a large area, recruiting more receptors, and thus, sensory neurons from these zones.

Just like increasing stimulus strength, a longer duration of the stimulus also generates more action potentials. But if the stimulus continues for a long time, after a while, these receptors couldn't be bothered with responding. They adapt. Some do that slowly and these are called slowly-adapting or tonic receptors. Some get bought faster. They're rapidly adapting or phasic receptors.

The bulbous or Ruffini corpuscles and tendon organs are examples of slowly-adapting receptors. They respond quickly at first and keep firing slowly for as long as the stimulus is present, important for the continuous monitoring of the stimulus. On the other hand, the lamellar corpuscle is an example of a very rapidly adapting receptor. When a stimulus like pressure deforms the capsule, it responds and adapts very quickly. It stops responding unless something else happens like if pressure is released. It keeps the brain informed of the dynamic changes happening.

These kinds of receptors respond when the stimulus changes, making them better suited to sense more quickly changing stimuli like how the lamellar corpuscle is good for detecting high-frequency vibration. Thus, based on adaptation, we have tonic and phasic receptors which differ in their action potential firing rate in response to a long-lasting stimulus.

But if the action potentials themselves aren't any different, how does the central nervous system distinguish between sensations? One reason is that even though the mechanisms are the same, the pathways, or lines, are different. When a thermoreceptor sensitive to warmth gets stimulated, the brain will always perceive it as warmth, thanks to dedicated pathways called labeled lines that connect with the specific areas in the brain. The label is the sensory modality and line is the neuronal connection to the central nervous system.

Based on this principle, there will be separate lines for different sensations like vision, hearing, and for somatic sensors as well like touch and vibration. So these receptors are just the first step on this sensory journey. Our next step involves the sensory pathways.

That concludes this tutorial on the sensory receptors. Make sure to check out our other study units and articles on the nervous system.