Lecture 6 - Somatosensory Receptors
1. Merkel’s discs, hair follicles, Meissner and Pacinian Corpuscles and Ruffini’s endings are all sensitive
They contain mechanoreceptors that activate ion channels when their shape is changed.
2. Some of these receptors, such as hair follicles, are nerves that are wrapped around the root of a hair.
Others, such as the corpuscles, are nerves contained within capsules.
3. These nerves run from the skin all the way to the spinal cord.
4. Nerve endings are located in several places.
a. Superficial dermis
– Meissner’s Corpuscle and Merkel’s discs.
Both are very sensitive to fine
mechanical stimuli and also have a very small receptive field to allow for acuity.
Between these two,
Meissner’s are phasic.
This means that they adapt very slowly and respond to the duration of the
On the other hand, Merkel’s are tonic.
They adapt rapidly, respond to the onset and offset
of a stimulus.
They let the individual know about fluctuations in stimuli.
– Pacinian Corpuscles and Ruffini’s endings.
Both are sensitve to deep pressure and have a
very large receptive field to receive more information.
Between these two, Pacinian are phasic like
On the other hand, Ruffini’s are tonic like Merkel’s discs.
c. Hair follicles
– phasic with very small receptive fields.
d. Free Nerve Endings
– very tiny.
They run into the dermal layer, close to the epidermis.
They have a
very large receptive field because they are highly branched.
These are sensitive to temperature (hot
or cold) and pain, not pressure like the others.
Some nerves are so tiny they are not myelinated
because this promotes action potentials.
Encoding Stimulus Modality
(Online Slide 1)
1. Law of Specific Nerve Energies
– Each nerve is specialized (due to ion channels, etc.) to receive and
transmit a specific stimulus.
For example, mechanoreceptors deliver information about pressure,
olfactory receptors about scent, thermoreceptors about temperature, etc.
2. Principle of Labeled Lines
The route of delivery of the stimulus has no role in what the stimulus ends
For example, you can excite a mechanoreceptor in any way, but it will only deliver the signal
it was built to deliver.
It will not translate any information about light or temperature, just pressure.
Therefore, the sensation is determined by the receptor pathway, not the stimulus itself.
Spinal Cord And Brain