Joint+Receptors

=__ Joint Receptors __=

__**INTRODUCTION:**__

Joint receptors are found in the synovial junctions between bones. The receptors detect mechanical deformation within the capsule and ligaments. They serve two main purposes; to protect the joint from potentially injurious flexion and extension, and serve as propriocepters. There are four types of sensory endings that make up joint receptors; __free nerve endings__, __golgi type endings__, __ruffini endings__, and __paciniform endings__. Until recently, it was commonly believed that joint receptors fired only at the extremes of movement. Now, however, it has been proven that there is always some amount of receptors firing at any given point in the range of motion of that joint.

Figure 1

__**FUNCTIONAL ANATOMICAL REVIEW**:__

-Free nerve endings are found in the joint capsule and connective tissue surronding a synovial joint. -They are the most numerous type of receptor innervating the joint capsule and are also the smallest in size. -They are activated only by extreme mechanical or chemical irritation, they have a high threshold for activation, and they are slowly adapting. -They are innervated by Group III, unmyelinated fibers.
 * FREE NERVE ENDINGS**:

Figure 2

-Golgi type endings are only located in the ligaments of the joint. -The axon endings are closely intertwined with the collagen strands of the ligament, which straighten as the ligament is stretched, physically deforming the axon endings and causing the Golgi-type endings to fire. -They are active at the end of joint range, they have a high threshold of activation, and they are slowly adapting. - While their functional role is unknown, it has been suggested that they possibly play a protective role for the joint.
 * GOLGI TYPE ENDINGS**:

-Ruffini endings are found mainly in the joint capsule. -They are active both at rest and in movement, they are slowly adapting, and they have a low threshold for activation. -Ruffini endings sense and are stimulated by a stretching in the tissue and other tactile stimulation. -They are innervated by Group II fibers (medium myelinated fibers). They signal the position of the joint during movement since a flexing muscle causes a joint to move and for a receptor to be stretched, but also when the muscle is extended and at a more resting state. Thus, ruffini endings are capable of signaling static joint position, joint movements, and direction and speed of movements.
 * RUFFINI ENDINGS**:

Figure 3

-Paciniform endings are located in the periosteum near the articular attachments and the fibrous part of the joint capsule. -They are active at onset and termination of movement, they have a low threshold of movement, and they are rapidly adapting. -They are the largest cutaneous receptor and they sense quick mechanical deformation or vibrations. -They can only inform they body of joint movements, not static position, and they are particularly suited to signal movement velocity, as they have also been called "acceleration receptors".
 * PACINIFORM ENDINGS**:

=
In order to elicit an action potential and send information to the brain, the joint receptors must be stimulated by an energy source, or their adequate stimuli. When the stimulus is present, it places pressure on the receptor and causes physical deformation of the joint receptor ending. This triggers mechanically-gate ion channels in the receptor to open, allowing Na+ to enter into the cell. This depolarizes the receptor and triggers an action potential. This process, from stimulus to action potential, is call transduction.======

Figure 4

=
When joint receptors are stimulated, their corresponding action potentials travel along the axon and join the afferent nerve for that particular dermatone level. The nerve enters into the dorsal root of the spinal cord, synapses with the second-order neuron in the dorsal horn, and then decussates to the contralateral side of the column. After decussating, the second-order neuron ascends contralaterally in the anterolateral portion of the lateral column as either the spinothalamic tract or the spinoreticular tract. As new inputs come into the tract from higher spinal cord sections, lower inputs are pushed ventrally to allow room for the new information to travel upwards. The second-order neuron then synapses in the ventral posterolateral (VPL) of the thalamus with the third-order neuron, and that proceeds up to the somatosensory cortex to Broadmann’s area 3b.======

__SIGNS OF DYSFUNCTION:__
There can be many signs of dysfunction when it comes to joint receptors and cutaneous receptors alike. Some of the more common issues are listed here: Allodynia- Pain due to a stimulus that does not normally elicit pain. Analgesia- The absence of pain when a stimulus would normally elicit a painful response. Dysesthesia- An unpleasant abnormal sensation, whether spontaneous or evoked. Hyperalgesia- An increased response to a stimulus that is normally painful Paresthesia- An abnormal sensation, whether spontaneous or evoked.

Figure 5

__SUMMARY:__
There is no joint position at which all joint receptors are silent Different receptors respond over different portions of the total range of movement Multiple receptors are activated in overlapping ranges By monitoring which receptors are activated at the same time, the CNS can determine joint position. In general, each ending discharges at its maximal steady rate when the joint is at some particular angle & less rapidly when the angle is increase or decrease -Only one is firing maximally, others 75%, others 35%, and cause overlapping regions. Joint receptors with a Group I and II afferent fibers (Golgi’s and Ruffini’s endings) are responsive only to deformation of the joint capsule or ligaments.

__GLOSSARY:__
- __Free nerve endings__- located in the joint capsule and connective tissue surrounding it, they are activated by extreme mechanical or chemical irritation - __Golgi type endings__- located in the ligaments of the joint, their axon endings are intertwined with the collagen fibers of the ligaments, physically deforming when the ligament is stretched. - __Ruffini endings__- located in the joint capsule, activated by stretching and tactile stimulation, active when joint is at rest and in motion, also signals speed of movements. - __Paciniform endings__- located in the periosteum near the articular attachments, active at onset and termination of movement, they signal movement velocity and acceleration.

__SUGGESTED READINGS:__

 * 1) Journal article dealing with the contributions of joint receptors to movement: __Joint Receptor Contributions to Reflexive and Kinesthetic Response__- Roberta A. Newton __http://ptjournal.apta.org/cgi/content/abstract/62/1/22__
 * 2) Journal article concerning the kinesthetic information provided by the joint receptors: __Joint Receptors and Kinesthesia__- U. Proske http://www.springerlink.com/content/m63j72t622614u86/
 * 3) Journal article with information on a behavioral theory that suggests position and timing of limb movement can be equated to the slowly adapting joint receptors: __Feedback theory of how joint receptor regulate the timing and positioning of a limb__- Jack A. Adams http://psycnet.apa.org/journals/rev/84/6/504/Edit
 * 4) A website that gives general information about proprioceptive receptors from Neuroscience Online:http://neuroscience.uth.tmc.edu/s2/ii2-5.html

__QUIZ:__
1) Joint receptors are found in what type of joints? A. Fibrous B. Cartilagenous C. Synovial D. Suture

2) What 2 main purposes do joint receptors serve? A. Protection from injury, joint movement B. Protection from injury, proprioception C. Proprioception, joint movement D. Joint movement, force production

3) How many different types of joint receptors are there? A. 3 B. 4 C. 5 D. 6

4) Which receptor is only found in the ligaments of the joint? A. Golgi type endings B. Ruffini endings C. Paciniform endings D. Free nerve endings

5) Which is the largest sensory organ in the body? A. Free nerve endings B. Golgi type endings C. Ruffini endings D. Paciniform endings

6) Free nerve endings endings sense quick mechanical deformation or vibrations. True / False

7) Ruffini endings are capable of signaling static joint position. True / False

8) Group II fibers are myelinated fibers. True / False

9) Golgi type endings are quickly adapting. True / False

10) Free Nerve Endings are the least active in the mid range of joint motion. True / False

11) How does the CNS determine joint position?

12) What type of stimulus is needed to fire ruffini endings?

__Answer Key:__
1. C 2. B 3. B 4. A 5. D 6. False, paciniform endings do 7. True 8. True 9. False, slowly adapting 10. True 11. By monitoring which receptors are activated at the same time 12. Stretching in the tissue or tactile stimulation

__REFERENCES:__
Brodal, P. __The Central Nervous System: Structure and Function. 3rd Ed.__ Oxford University Press, Oxford, 2004.

Tillmann, B.N. __Atlas of Human Anatomy, Clinical Edition.__ Mud Puddle Books, Inc., New York, 2007.

Rose, D.J., and R.W. Christina. __A Multilevel Approach to the Study of Motor Control and Learning, 2nd Ed.__ Pearson Education, San Francisco, 2006.

Blumenfeld, H. __Neuroanatomy through Clinical Cases.__ Sinauer Associates, Inc., Sunderland, 2002.