The afferent signals of the nervous system comprise the inputs into the brain that we use to measure our environment and our relation to it. These input signals tell us how something feels, it’s texture, movement, temperature, and whether it causes pain or not. Afferent signals are also used in proprioception, the brain’s awareness of the body’s position in space.

One of the hardest parts of losing an arm or leg is the lack of sensation and feedback that a prosthetic device provides. Without the ability to feel or even tell where their limb is in space, amputees are missing out on the incredible feedback the nervous system normally provides.

In order to understand how new advances in prosthetics with afferent feedback, a basic understanding of the nervous systems sensory system in limbs is required:

Receptor Background

Mechanoreceptors
Cutaneous receptors are mechanically-activated receptors on the skin that propagate an action potential to the spinal cord. They respond to stimulations such as texture, vibrations, and skin deformation. There are four basic types:
USEME.jpg
[7]



Merkel discsà Receptors located near the epidermal-dermal junction, that are slowly adapting and detects skin indentation


Ruffini endingsà Receptors located deep in the dermis, slowly adapting, and detect tactile stimu
lation and skin stretching

Pacinian corpusclesà Located deep in the dermis, rapidly adapting, and detect quick changes in mechanical deformation, such as vibration
Meissner’s corpuscleà Located in the folds of the epidermal-dermal junction, rapidly adapting, and detect rapid skin deformation such as “buzzing” or “fluttering”


Thermoreceptors
These are free nerve endings categorized as separate “warming” and “cooling” receptors and they detect changes in temperature according to their category.

Nociceptors
These free nerve endings detect pain, including tactile pain, muscle/tendon pain, and extreme temperatures.

Proprioceptive Receptors
Proprioception is the body’s ability to innately determine its position in space. Two of the main structures that measure this are the muscle spindles and the Golgi tendon organs of muscles. They send afferent signals reporting the levels of contraction or stretching for each muscle. Then this information is compiled to map
out the orientation of body parts.

Muscle Spindles are bundles of intrafusal fibers bound within a muscle. As the muscle stretches, the muscle spindle stretches as well, firing a signal via type Ia or type II afferent fibers towards ascending pathways to tell the brain that the muscle is stretched and the degree of its stretching.

Golgi tendon organs report the tension of musculotendinous junctions to higher cortical levels. As a muscle contracts or stretches the nerve endings in the Golgi tendon organ are distorted and an impulse is send to ascending pathways via a type Ib afferent
fiber to tell the somatosensory cortex how much tension is being put on the muscle. [6]


13-16_OperMuscSpnd.JPG
1_22.jpg
Muscle Spindle [1] Golgi Tendon Organ [4]




Ascending Pathways

Once cutaneous receptors and other sensory structures fire their action potentials, they need to travel up the ascending pathways towards the spinal cord then to the brain.

Direct Ascending Pathways

The direct ascending pathways have three neurons between the receptor and the somatosensory cortex: one that terminates prior to the spinal cord, one in the spinal cord, and a third that terminates in the thalamus.

Dorsal Columns (Spinobulbar Tract)
Transfers discriminative touch and proprioceptive signals
- Medial fasiculus gracilis transfers from the sacral and lumbar segments
- Lateral fasiculus cuneatus transfers from the thoracic and cervical segments

Spinothalamic Tract
Transfers feelings of pain, heat, and pressure

Indirect Ascending Pathways

Spinoreticular Tract
Transfers slow, dull and imprecise pain

Spinocerebellar Tract
Transfers proprioceptive information for the trunk and limbs

Somatosensory Cortex
After going through the thalamus, these ascending pathways ultimately make it to the fourth layer of the primary somatosensory cortex. Cutaneous input goes to areas 1 and 3b, muscle spindle and Golgi tendon organ input goes to area 3a, and information regarding the rotation of joints goes to area 2. [8]

Feeling with Prosthetics

Being unable to feel or experience proprioception with a prosthetic limb is concerning for amputees because it reduces the effectiveness of the limb as well as separates them from a key human experience. Attempts have been made in order to return the sensation of touch to amputees.

Targeted Reinnervation
Some of the earlier attempts at giving cutaneous sensation to a missing hand involved targeted reinnervation of afferent fibers that had once belonged to the hand. Two patients in one study had upper arm amputations and therefore used prosthetic devices. For the first patient, a 54 year old male, it had been 9 months since his injury, but his afferent nerves were intact. The patient underwent a surgery to attach the receiving end of these nerves along his pectoralis muscle. The subcutaneous fat was removed in order to maximize the EMG signal. Within 5 months the patient started to perceive sensation in his missing hand, when this area of his chest was touched. The perceived sensation was mapped using skin indentation with 300 grams of applied force.The Merkel discs on his chest were acting as hand Merkel discs.

F2.medium.gif

The other patient, a 24 year old female, had a similar case. Fifteen months after her injury she was given the procedure to reinnervate afferent fibers to her chest. For some reason or another (maybe the longer wait time after injury) she wasn’t able to register the skin indentations properly. The indentation of her chest Merkel discs register as a tingling sensation in her “hand”, increasing along with the applied force.

The female patient experience different sensations based on location of the stimulus according to the new, synthesized somatotopic mapping. One spot on her chest gave her the feeling that her fourth finger was being bent back, indicating that muscle spindles and Golgi tendon organs were being simulated. Another spot made it feel like the skin on her second finger and the webbing between her first and second fingers were being stretched, activating fictional Ruffini endings.

The sensitivity of each patient’s new “hand” sensory system were measured and the threshold for sensation was less than 8.5 grams of applied force for 50% of all measured thresholds on the male subject. It was less than 2 grams of applied force for the female patient. Average temperature thresholds were tested for areas affecting the missing limb. The thresholds for a warm sensation, cold sensation, and heat pain threshold were 28.8 ± 0.1°C, 36.3 ± 0.2°C, and 45.4 ± 0.9°C, respectively for the male patient. The female patient’s thresholds were slightly more responsive with the thresholds for heat sensation, cold sensation, and heat pain being 31.0 ± 0.2°C, 32.9 ± 0.0°C, and 40.1 ± 0.7°C.

In these two examples of reinnervating afferent fibers, originally for the hands, onto the pectoral region we see that not only can these fibers heal and function properly, but they naturally map themselves somatotopically, similarly to the somatosensory cortex. They even can differentiate stimuli to a degree approaching that of a normal hand. It’s interesting to see how an amputee can experience this sensation of touch and perceive it in their missing hand, but it doesn’t do much good for the feedback of the hand. If the chest needs to be touched for the hand to feel, then the process is fairly useless. The main point of these procedure was to confirm that afferent nerves can be dormant for long periods of time and still recover and topographically map themselves. [2]

In another example done several years later, a prosthetic hand was made that had pressure sensors in order to activate afferent signals, much like the pectoral region from the previous scenarios. The main difference is with the receptors of the patient’s hand; the feedback their getting is relevant.

Afferent Signals and Perceived Ownership

F1.medium.gif
Another hindrance of not having cutaneous feedback in an artificial limb is the disconnect between the amputee and their prosthetic. Without feedback it is merely viewed as a tool, that isn’t actually part of the person. One study was done to access the effectiveness of cutaneous feedback giving the amputee ownership of an artificial hand. The test was described as the rubber hand illusion, and basically, a rubber hand would be placed in front of the subject. As the rubber hand was being touched, afferent signals would be administered to the subject’s residual limb. Various situations were tested including having the signal and the hand touch coincide somatotopically, having the signal and hand touch not coincide, having one without the other, and several other situations. The subjects claimed more of a sense of ownership over the hand when the signal matched the location of the hand touch. [3]

Proprioception with Prosthetics
Another shortcoming of prosthetic devices is the lack of feedback they provide the user in terms of the limb’s location in space. Without feedback telling the somatosensory cortex joint angles and “muscle” tension, there is no way to determine the position of a prosthetic device, aside from visual input.

There are ways to give feedback to the brain on whether or not a hand is open or closed. One group decided to test this technology by attaching the palmaris longus muscle of one of the authors to an experimental “prosthetic” device. This device was hidden from the subject’s view and would squeeze objects of various sizes. Within s
Capture.PNG
everal trials, the subject could determine the size of the object with 100% accuracy. This experiment was only using a one afferent input to one muscle ratio, so only the general sizes of the objects could be determined. The authors suspect that if more connections were made, such as at least one per finger, amputees could determine the shape and hardness of held objects. [5]






Quiz
1. Which afferent receptor perceives muscle tension?
a) Merkel discs
b) Golgi tendon organs
c) Nociceptors
d) Pascinian corpuscles

2. Which ascending pathway transfer proprioceptive information for upper extremities?
a) Spinoreticular tract
b) Spinocerebellar
c) Lateral fasiculus cuneatus
d) Medial fasiculus gracilis
e) B and C
f) A and D

3. When afferent input was reinnervated from old hand nerves to the chest, how did the location of the chest stimulation correspond to the imaginary hand?
a) The locations on the chest corresponded to locations on the hand.
b) The locations on the chest corresponded to levels of force on the hand.
c) The locations on the chest corresponded to different temperatures on the hand.
d) The locations on the chest corresponded to different textures on the hand.

4. With what success rate could subjects determine object size with artificial "muscle spindles"?
a) 10%
b) 25%
c) 80%
d) 100%

Answers after references

References
[1] "CH 13 Proprioception and Types of Reflexes." APSU Biology. N.p., n.d. Web. <http://www.apsubiology.org/anatomy/2010/2010_Exam_Reviews/Exam_4_Review/CH_13_Proprioception_and_Types_of_Reflexes.htm>.
[2] Kuiken, Todd A., Paul D. Marasco, Blair A. Lock, R. Norman Harden, and Julius Dewald. "Redirection of Cutaneous Sensation from the Hand to the Chest Skin of Human Amputees with Targeted Reinnervation." Proceedings of the National Academy of Sciences. N.p., 11 July 2007. Web. Dec. 2016. <http://www.pnas.org/content/104/50/20061.full>.
[3] Marasco, Paul D., Keehoon Kim, James Edward Colgate, Michael A. Peshkin, and Todd A. Kuiken. "Robotic Touch Shifts Perception of Embodiment in Targeted Reinnervation Amputees." Oxford University Press. Brain: A Journal of Neurology, 20 Jan. 2011. Web. Dec. 2016. <http://brain.oxfordjournals.org/content/134/3/747.full>.
[4] "Motor Systems I: Lower Systems." Integrative Neuroscience. N.p., n.d. Web. <http://www.utdallas.edu/~tres/integ/mot1/display1_22.html>.
[5] Nambu, Seiji, Mitsuhiko Ikebuchi, Masashi Taniguchi, Choong Sik Park, Takahiro Kitagawa, Shigeyoshi Nakajima, and Tatsuya Koike. "Advantages of Externally Powered Prosthesis with Feedback System Using Pseudo-cineplasty." Journal of Rehabilitation Research and Development 51.7 (2014): 1095-102. 7 Nov. 2014. Web. <http://www.rehab.research.va.gov/jour/2014/517/pdf/jrrd-2013-03-0067.pdf>.
[6] "Neuroscience Online: An Electronic Textbook for the Neurosciences." Neuroscience UTH. UTHealth, 2015. Web. Dec. 2016. <http://neuroscience.uth.tmc.edu/toc.htm>.
[7] Purves, D., GJ Augustine, and D. Fitzpatrick, eds. Neuroscience. 2nd ed. Sunderland, MA: Sinauer Associates, 2001. Print.
[8] Valenta, Jiři, and Pavel Fiala. "Central Nervous System : Overview of Anatomy." EBSCO Host. Karolinum Press, 2012. Web. Dec. 2016. <http://www.worldcat.org/title/central-nervous-system-overview-of-anatomy/oclc/884588770>.
Answers:1. B2. E3. A4. D