Somatosensory receptors

Definitions & Key takeaways

Somatosensory receptors are specialized sensory nerves that respond to various stimuli, including vision, hearing, taste, and smell, as well as general somatic senses which make up the somatosensory system, which is involved in the sense of touch, proprioception, pain, and temperature. They are located throughout the body, including the skin, muscles, tendons, joints, and internal organs. There are several different types of somatosensory receptors, including: Mechanoreceptors which respond to stimuli such as touch, pressure, and vibrations. Thermoreceptors which respond to changes in temperature. Nociceptors which respond to harmful stimuli, such as extreme temperatures or tissue damage, transmitting pain signals to the brain. Proprioceptors which respond to changes in the position and movement of body parts. Chemoreceptors which respond to specific chemical signals such as taste buds.

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Introduction0:00–0:52

Broadly speaking, the nervous system can be split into an afferent or sensory division and an efferent or motor division.
The afferent division brings sensory information from the outside world into the brain. Sensory information involves special senses - like vision, hearing, taste, and smell - as well as general somatic senses which make up the somatosensory system, which is involved in the sense of touch, proprioception, pain, and temperature.
These sensations are felt by sensory neurons all over the body. These sensory neurons have receptors that are classified according to the stimulus they respond to - there are mechanoreceptors for touch and proprioception, nociceptors for pain, and thermoreceptors for temperature.

Somatosensory pathways0:52–2:09

Now, neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and nerve fibers, which are projections that extend out from the neuron cell body.
These are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons. Where two neurons come together is called a synapse, and that’s where one end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.
The somatosensory pathways are made up of a relay of four neurons. The first neuron is called the first order neuron or sensory neuron, which has the sensory receptors and converts stimuli from the outside world into an impulse that can be passed through a synapse to the next neuron in series.
Next is the second order neuron, and it may have its cell body in the spinal cord or up in the brainstem. The second order neuron then takes the impulse to the third order neuron, which has its cell body in the thalamus.
Finally, the third order neuron takes the impulse to the fourth order neuron, which has its cell body further up in the sensory cortex of the brain.

Sensory neurons2:09–3:36

Now let’s zoom into first order or sensory neurons. First order neurons are called pseudounipolar neurons, which means that they don’t have separate dendrites and axons; instead, there’s only one axon that extends out from the cell body, and it has two branches: a peripheral branch and a central branch.
The peripheral branch goes from the cell body - located at the dorsal root ganglia right next to the spinal cord - to the peripheral tissues.
Every first order neuron has a receptive field, which is the area that it receives sensory input from. The end of the peripheral branch is full of sensory receptors and ion channels.
When a stimulus from the outside world hits the sensory receptors - for instance if somebody pokes you - ion channels open and close and that allows ions to flow in and out of the neuron.
Overall, if enough positive charge flows into the cell, that’s called depolarization. And if the first order neuron reaches a certain threshold of depolarization, another set of ion channels that are voltage-gated sense it and flip open, letting even more positive charge enter the cell.
This triggers an action potential that’s sent through the peripheral branch and back to the central branch, and from there it goes to the spinal cord.

Receptive fields3:36–4:58

Now, receptive fields vary in size, and the smaller the receptive field, the higher the resolution, which means that stimuli can be localized or identified more precisely.
As an example, we have smaller receptive fields in the fingertips than at the back. This is very important to read Braille, which is a method of reading through touch - specifically feeling raised dots with the tip of the index finger.
Also, to understand that two stimuli are distinct, the receptive fields of two sensory neurons need to be separated by at least one neuron’s receptive field.
This third neuron will send a negative signal between the two positive ones, allowing two-point discrimination. Now, in the case of a strong stimulus, the nearby sensory neurons might get mildly activated by deformation of the surrounding skin.
To help minimize these collateral stimuli, the neuron with the strongest activation will activate inhibitory interneurons, which are neurons that project onto surrounding first order neurons to suppress their activity.
This is called lateral inhibition, because it helps the precise localization of a stimulus by defining its boundaries, and that helps us recognize objects.

Receptor adaptation4:58–6:03

Two more features - the strength and duration of a stimulus - are encoded by the frequency of nerve firing. A strong stimulus - like boiling water - will make a first order neuron fire at a really high frequency.
But sensory neurons also have a tendency to adapt - which means that if a stimulus doesn’t change for a while, sensory neurons get used to it and stop firing.
Receptors are either fast adapting or phasic, or they’re slow adapting or tonic. Fast adapting receptors have a burst of activity when a stimulus starts, but stop firing soon afterwards.
As an example, if you were to put a watch on your wrist, a bunch of sensory neurons would start firing due to a change in pressure.
But after a few seconds, these neurons would stop firing, and you might even forget about the watch. On the other hand, slow adapting receptors convey the duration of the stimulus, since they keep firing.
In terms of reading Braille, slow adapting receptors help a person feel the raised dots without losing sensitivity. All right, so the end of the peripheral branch of first order neurons is full of sensory receptors that are classified according to the stimulus they respond to - there are mechanoreceptors, thermoreceptors, and nociceptors.

Mechanoreceptors6:03–9:09

Mechanoreceptors can be divided into two broad groups: mechanoreceptors for touch, or mechanosensors - which respond to pressure or deformation of the skin - and mechanoreceptors for proprioception, or proprioceptors - which receive input about body position from muscles and joints.
Now, mechanosensors are divided into four main classes: Meissner corpuscles, Merkel discs, Ruffini corpuscles, and Pacinian corpuscles.
Meissner or tactile corpuscles are encapsulated receptors - which means they are surrounded by a capsule made of connective tissue - located in the dermis of hairless skin, such as the fingertips, and sense vertical indentations on the skin, such as the raised dots in Braille.
They are fast adapting receptors and have small receptive fields to allow good resolution. Merkel or tactile discs are non-encapsulated receptors found in the epidermis of hairless skin and have small receptive field as well.
But unlike Meissner corpuscles, Merkel discs are slow adapting receptors. Ruffini or bulbous corpuscles are encapsulated receptors in the dermis of hairy and hairless skin - and they detect skin stretching – and they’re in joints where they can detect joint rotation.
They’re also slow adapting receptors and have big receptive fields with poorly defined boundaries, so they have poor resolution.
Finally, pacinian or lamellar corpuscles are encapsulated receptors located deep in the dermis or in the subcutaneous tissue of hairy and hairless skin, and they sense vibration.
They’re fast adapting receptors and have big receptive fields. Now, there are three types of proprioceptors: the muscle spindle, the Golgi tendon organ, and the joint receptors.
The muscle spindle is a spindle-shaped receptor - which means it’s wide in the middle and tapers at both ends - found throughout the perimysium, which is a sheath of connective tissue surrounding skeletal muscle cells.
Muscle spindles detect when a muscle is being stretched. Golgi tendon organs are receptors located in tendons, close to the skeletal muscle insertion.
They sense when a tendon is being stretched, which indirectly senses the force exerted by the muscle. Finally, joint receptors are basically Ruffini corpuscles in the joints.
They provide information on joint position and motion by sensing stretch in the articular capsules that enclose synovial joints, and are especially important for judging the position of the fingers.

Thermoreceptors9:09–10:24

Thermoreceptors are slowly adapting receptors that detect changes in skin temperature. Thermoreceptors are transient receptor potential channels or TRP ion channels, which means that they open when activated, allowing positively charged ions like sodium and potassium to flow through them.
Transduction of heat involves a subclass of TRP channels called TRPV channels, which also get activated by spicy food. TRPV channels get activated from 32 to 48 Celsius degrees, or 90 to 120 degrees Fahrenheit, and are located deep in the dermis.
On the other hand, transduction of cold involves a different class of thermoreceptors called TRPM8 channels, which also get activated by compounds like menthol.
TRPM8 channels get activated from 10 to 40 Celsius degrees, or 50 to 104 degrees Fahrenheit, and are located in the superficial dermis.
At extremely cold or extremely hot temperatures outside these ranges, thermoreceptors become inactive, and nociceptors become active.

Nociceptors10:24–11:11

Nociceptors respond to potentially damaging stimuli by triggering pain as a protective mechanism. There are three main nociceptor classes: thermal - which sense extremely cold or hot temperatures - mechanical - which sense excess pressure and deformation - and polymodal - which sense a combination of the two.
As it turns out, we all have pretty much the same pain threshold of depolarization - that is, we begin to perceive pain at roughly the same stimulus intensity.
But what varies widely between people is pain tolerance. So when we say that someone is sensitive to pain, we mean that the person has a low pain tolerance rather than a low pain threshold.
Alright, as a quick recap, sensory receptors are found in the first order neuron or sensory neuron to convert external stimuli into sensory information that can be sent up to the brain.

Review11:11–11:52

Sensory receptors are classified according to the stimulus they respond to into mechanoreceptors for touch and proprioception, nociceptors for pain, and thermoreceptors for temperature.
The most important factors for sensory coding are the receptive field of each sensory neuron for stimulus resolution, lateral inhibition of surrounding sensory neurons for precise localization, and different adaptation speed to convey stimulus strength and duration.