CUTANEOUS RECEPTORS
Laura Ogren

Introduction

Cutaneous receptors are sensory receptors located in the dermis, which is a layer of the skin. These receptors are responsible for sensations of touch, pressure, heat, cold, and pain. They are classified as mechanoreceptors which are associated with pressure, thermoreceptors which are associated with temperature, and nociceptors which are associated with pain (Brodal).

Functional Anatomy

There are five mechanoreceptors, free nerve endings, Meissner's corpuscles, Pacinian Corpuscles, Merkel's disks, and Ruffini's corpuscles. Table 1 shows where each receptor is located, what their function is in detecting a stimulus, and lastly what their rate of adaptation is. The strength of the stimulus is determined by the rate of action potential discharge triggered by the receptor potential. This is where the rate of adaptation comes into play for these receptors. Rapidly adapting receptors fire when a stimulus is first presented but then stop if the stimulation continues. Whereas, slowly adapting receptors will sustain discharge if the stimulation continues. The size of the receptive field of a cutaneous receptor is also important in order to discriminate between tactile stimuli. If the receptor has a large receptive field the receptor would not be able to discriminate between two stimuli placed on the same area. Also the more receptors located in one area of skin allow that area to be more sensitive to stimuli (Brodal, Purves).

Table 1
Receptor Type
Location
Function
Rate of Adaptation
Free Nerve Endings
All skin
Temperature, Pain,
Crude Touch

Slow
Meissner's Corpuscles
Glaborous skin
Touch, Pressure (dynamic)
Rapid
Pacinian Corpuscles
Subcutaneous tissue,
Interosseous membranes,
Viscera
Deep pressure,
Vibration (static)
Rapid
Merkel's Disks
All skin, Hair Follicles
Touch, Pressure (static)
Slow
Ruffini's Corpuscles
All skin
Stretching of skin
Slow
(Purves)
Figure 1
external image cutaneous%2Breceptors.jpg
http://1.bp.blogspot.com/_Z2lCle-AtKQ/SGlNcGbVASI/AAAAAAAAA_s/2zIw6J37Sfc/s1600/cutaneous%2Breceptors.jpg

Input Pathways
The afferent pathway for mechanosensory information from the upper and lower body is the dorsal column-medial lemniscus system represented in Figure 2 by the green pathway. When the first-order neurons enter the spinal cord carrying information from peripheral mechanoreceptors, they bifurcate into ascending and descending branches. These branches send collateral branches to several spinal segments; some penetrate the dorsal horn of the cord and synapse in a region called Rexed’s laminae III-V. These axons ascend ipsilaterally through the dorsal columns to the lower medulla, they then contact the second-order neurons in the gracile and cuneate nuclei (together known as the dorsal column nuclei). The second-order neurons send their axons to the somatic sensory portion of the thalamus. Axons from the dorsal column nuclei project to the dorsal portion of the lower brainstem, which forms the internal arcuate tract. Axons from the internal arcuate tract cross the midline to form a new tract called the medial lemniscus. Axons carrying information from the lower limbs are located ventrally and information from upper limbs are located dorsally, until they pass the pons and the midbrain where they rotate 90 degrees so that the information from the lower limbs is located medially, and the upper limbs laterally. These axons then reach the ventral posterior lateral nucleus of the thalamus where they connect with the third-order neurons.

Sensory information from the face goes through the trigeminal somatic sensory system (Figure 3b). Low-threshold mechanorecption in the face is mediated by first-order neurons in the trigeminal ganglion. Most of the axons carrying information from low-threshold mechanorecptors terminate in the principal nucleus which corresponds to the dorsal column nuclei along with sensory information from the rest of the body.

The spinothalamic tract (Figure 2 represented by the red lines and 3a) mediates pain and temperature sensations. Axons of the nociceptor nerve cells enter the spinal cord via the dorsal roots. The first-order neurons ascend and descend on Lissauer's tract, once they reach the dorsal horns. Once the axons are within the dorsal horn they give off branches that contact neurons in Rexed's laminae. Axons of the second-order neurons in the dorsal horn cross the midline and ascend to the brainstem and thalamus, and synapse with third-order neurons at the ventral posterior lateral nucleus (VPL), and project to the somatosensory cortex (Blumenfeld, Greenstein, Purves).



Figure 2
external image fig05.jpg
Figure 3

external image U5F2.gif

Function in the control of normal movement
Reflex arcs allow for immediate withdrawal from dangerous stimuli. They are able to process the information in the spinal cord before being sent to the brain to allow for a quicker response to the stimuli. Sensory receptors in the skin will detect a painful stimulus and send the message to the interneuron in the spinal cord which will synapse with a motor neuron that carries the message to the effector which will elicit the response and withdrawal from the stimulus (Gardner).external image reflex_arc.jpg


Signs of dysfunction
A unilateral lesion of the spinothalamic tract will affect the cutaneous receptors because this lesion will produce a sensory loss of touch, pressure, and vibration below the lesion on the same side. These lesions will affect pain and temperature as well because their pathway crosses the midline so loss of sensory information will appear on the opposite side of the sensory loss (Purves).

Glossary of terms
First-order neurons- Initiate the sensory process.
Free nerve endings- Where some receptors end, as unmyelinated and myelinated fibers.
Meissner’s corpuscles- Encapsulated mechanoreceptors that detect fine touch and pressure.
Pacinian corpuscles- Encapsulated mechanoreceptor that detect high-frequency vibrations.
Merkel’s disks- Encapsulated mechanoreceptors that detect fine touch and pressure.
Ruffini’s corpuscles- Encapsulated mechanoreceptors that detect stretching of skin.
Thermoreceptors- Cutaneous receptors that detect changes in temperature.
Nociceptors- Cutaneous receptors for the detection of harmful stimuli.
Second-order neurons- Receive sensory information, process it, and deliver already processed information to the brain.
Slowly adapting- Continue to respond throughout the duration of the stimulus.
Rapidly adapting- May respond at the onset and possibly at the offset of the stimulus.
Receptive field- Region of a receptor surface that causes a sensory nerve cell to respond.

Relevant links or suggesting readings
1. Reflex Arcs
This link shows an animation and walk through of the reflex arc.

2. Cutaneous Receptors and Kinesthesia at the Index Finger, Elbow and Knee
This study shows that vibration and skin stretch has an affect on the cutaneous receptors at the joints of the finger, elbow, and knee and kinesthesia.

3. Do spike insoles enhance postural stability and plantar-surface cutaneous sensitivity in the elderly?
Cutaneous receptors have been shown to help in the control of posture. A study by Paulluel E, Nougier V, Olivier I showed that the use of spiked insoles aided in unperturbed stance in a goup of elderly people.

4. Spinal Cord Injury Reveals Unexpected Function of Cutaneous Receptors
This links to a study done in cats where cutaneous receptor sensation was lost in the hind paws and the cats were unable to step with their hind paws because they were not receiving any cutaneous input.

Quiz questions & answers

Mulitple Choice

1. Which receptors are involved in detecting pain?
A. Thermoreceptors
B. Nociceptors
C. Mechanoreceptors
D. None of the above

2. What are the two rapidly adapting receptors?
A. Meissner's corpuscles & Pacinian corpuscles
B. Ruffini's corpuscles & Free nerve endings
C. Pacinian corpuscles & Merkel's disks
D. Merkel's disks & Ruffini's corpuscles

3. The spinothalamic pathway is the output pathway for _.
A. Mechanoreceptors of the face
B. Mechanoreceptors of the upper and lower body
C. Nociceptors
D. Thermoreceptors

4. Receptive information for the lower body enter the spinothalamic tract at the
.
A. Lumbar spinal cord
B. Throacic spinal cord
C. Pons
D. Caudal medulla

True/False
1. Merkel's disks are slowly adapting.
2. Meissner's corpuscles are located in all the skin and it's layers.
3. The trigeminal pathway is used for mechanoreceptive information from the head to be sent to the central nervous system.
4. Information from the reflex arc needs to be sent to the brain first before it is able to withdraw from a stimulus.
5. A unilateral lesion of the spinothalamic tract will show sensory loss on the opposite side of the body for pain and temperature.

Essay/Short Answer:
1. Describe the trigeminal somatic sensory pathway and what information it is receiving.
2. Diagram the withdrawal reflex arc and explain how it works.
3. Explain the difference between rapidly adapting and slowly adapting receptors.
4. Explain the differences between mechanoreceptors, thermoreceptors, and nocireceptors and determine which receptor is part of which group.

Answers:
Multiple Choice
1. B
2. A
3. C
4. A
True/False
1. True
2. False, glaborous skin
3. True
4. False, it goes to the spinal cord first and withdraws
5. True


References
1. Blumefeld, H. Neuroanatomy Through Clinical Cases. Sunderland:Sinauer Associates, Inc. 2002, p. 268.
2. Brodal P. The Central Nervous System: Structure and Function. Oxford: Oxford University Press, 2004, p. 139.
3. Gardner, E. Fundamentals of Neurology. Philadelphia, PA: W.B. Saunders Company, 1993, p. 221-25, 233.
4. Greenstein, B., & Greenstein, A. Color Atlas of Neuroscience Neuroanatomy and Neurophysiology. Stuttgart, NY: Thieme, 2000, p. 136, 158-60.
5. Central Pain Pathways The Spinothalamic Tract. In: Neuroscience. 2001. [Available at http://www.ncbi.nlm.nih.gov/​bookshelf/​br.fcgi?book=neurosci&part=a682].
6. Purves, D. et al. (Eds.). Neuroscience. Sunderland, MA: Sinauer Associates, Inc, 2004, p. 189-217.
7. Introduction to Clinical Neurology. From University of Florida College of Medicine. 2009. [Available at http://medinfo.ufl.edu/​year2/​neuro/​review/​images/​fig05.jpg].