The Tactile System
The tactile system or touch system refers to stimulation reaching the central nervous system from receptors in the skin. Since there are14 to 18 square feet of skin covering the adult body, it is obviously a large source of incoming information. Add the fact that there are dozens of each of three major types of receptors on every square inch of skin, and the possibilities for input are huge! The most primitive type of receptors register light touch, like a feather being brushed over the skin. Light touch is a primitive, but still powerful alarm system. There might be an insect crawling on me, or someone sneaking up on me; I need to look and see – I can’t pay attention to anything else until I know what is touching me, or might touch me!
The second type of receptors are for pressure touch, known as discriminative touch. This is much more important for learning than we realize because we do not have to look at or think about what our fingers or feet are pressing against; we recognize the “feel” of things. Sustained, or pressure touch also has an important role in countering or subduing alarm and anxiety. When a baby is fretful or over-stimulated, wrapping in a light blanket, – applying pressure touch, in other words, – usually results in his falling asleep.
The third set of skin receptors register heat, cold, and pain. Obviously it is important for survival that these receptors work efficiently, and that their input is organized and processed quickly so that appropriate action can be taken.
When the nervous system is immature, when development is delayed, problems occur which can be directly related to immature registration and processing of input from the skin. The connection between the skin and the nervous system is not strange. The nervous system is formed from the same layer of embryonic tissue as the skin. The most common symptom of immaturity is excessive sensitivity to light touch. The individual may describe feeling as if the skin had been rubbed raw where he/she was touched. The infant avoids being touched, cries when picked up, avoids being washed or shampooed. Parents feel rejected, and the stage is set for emotional problems added to the original sensory difficulty. Feeding problems often result because extreme sensitivity in an around the mouth leads to rejection of the nipple, and later to rejection of many foods.
Unfortunately, hypersensitivity to light touch is often accompanied by lack of normal sensitivity to heat, cold, and pain. The child may refuse to wear a jacket in the winter, and refuse to take it off during the hot summer. He may not cry at injuries that would make another child scream.
The contribution of tactile defensiveness to attention deficit disorder is one of its most common manifestations. The child who is constantly on the alert because something might be moving toward him, – might touch him – cannot attend to what the teacher is saying. The tactile system’s connections with hearing and vision means that hyper-alertness extends to other stimulations as well. The tactually defensive child often reacts reflexly by striking out in response to another child’s well-meaning touch. This is labeled “aggressiveness”, and one label leads to another.
The therapists’ strategy in dealing with tactile defensiveness is two-fold. First, it is important to protect the child as much as possible from sudden and unwanted touch. Secondly, various kinds of activities utilizing sustained pressure can be used to dampen down the over-reactivity.
The Vestibular System
Our world has three spatial dimensions. Accordingly, our vestibular system contains three semicircular canals in each labyrinth. They are approximately orthogonal to each other, and are called horizontal, anterior, and posterior canal. (Alternatively, they may be referred to aslateral, superior, and inferior, respectively.)
The canals are cleverly arranged in such a way that each canal on the left side has an almost parallel counterpart on the right side. Each of these three pairs works in a push-pull fashion: when one canal is stimulated, its corresponding partner on the other side is inhibited, and vice versa.
This push-pull system allows us to sense all directions of rotation: while the right horizontal canal gets stimulated during head rotations to the right (Fig 2), the left horizontal canal gets stimulated (and thus predominantly signals) by head rotations to the left.
Vestibulo-ocular reflex (VOR)
The vestibular system needs to be fast: if we want clear vision, head movements need to be compensated almost immediately. Otherwise our vision corresponds to a photograph taken with a shaky hand. To achieve clear vision, signals from the semicircular canals are sent as directly as possible to the eye muscles. This direct connection involves only three neurons, and is correspondingly called Three-neuron-arc (Fig 3). Using these direct connections, eye movements lag the head movements by less than 10 ms, one of the fastest reflexes in the human body. The automatic generation of eye movements from movements of the head is called vestibulo-ocular reflex, or short VOR.
This reflex, combined with the push-pull principle described above, forms the physiological basis of the Rapid head impulse test or Halmagyi-Curthoys-test: when the function of your right balance system is reduced by a disease or by an accident, quick head movements to the right cannot be sensed properly any more. As a consequence, no compensatory eye movements are generated, and the patient cannot fixate a point in space during this rapid head movement.
The mechanics of the semicircular canals can be described by a damped oscillator. If we designate the deflection of the cupula with θ, and the head velocity with , the cupula deflection is approximately
α is a proportionality factor, and s corresponds to the frequency. For humans, the time constants T1 and T2 are approximately 3 ms and 5 s, respectively. As a result, for typical head movements, which cover the frequency range of 0.1 Hz and 10 Hz, the deflection of the cupula is approximately proportional to the head-velocity (!). This is very useful, since the velocity of the eyes must be opposite to the velocity of the head in order to have clear vision.
Signals from the vestibular system also project to the Cerebellum (where they are used to keep the VOR working, a task usually referred to as Learning or Adaptation) and to different areas in the cortex. The projections to the cortex are spread out over different areas, and their implications are currently not clearly understood.
Diseases of the vestibular system can take different forms, and usually induce vertigo and instability, often accompanied by nausea. The most common ones are vestibular neuritis, also called Labyrinthitis, and BPPV. In addition, the function of the vestibular system can be affected by tumors on the cochleo-vestibular nerve, an infarct in the brain stem or in cortical regions related to the processing of vestibular signals, and cerebellar atrophy. Less severe, but often also with large consequences, is vertigo caused by the intake of large amounts of alcohol.
BPPV, which is short for Benign Paroxysmal Positional Vertigo, is probably caused by pieces that have broken off from the Otoliths, and have slipped into one of the semicircular canals. In most cases it is the posterior canal that is affected. In certain head positions, these particles push on the cupula of the canal affected, which leads to dizziness and vertigo. This problem occurs rather frequently, often after hits to the head or after long bed rest. The tell-tale sign of BPPV are vertigo attacks which repeatably appear when the head is brought into a specific orientation. In most cases BPPV can be eliminated (for the patient in an almost miraculous way) by lying down, bringing the head in the right orientation, and sitting up quickly.
The Proprioceptive System
Proprioception (from Latin proprius, meaning “one’s own” and perception) is the sense of the position of parts of the body, relative to other neighbouring parts of the body. Unlike the six exteroception human senses of sight, taste, smell, touch, hearing, and balance, that advise us of the outside world, proprioception is a sense that provides feedback solely on the status of the body internally. It is the sense that indicates whether or not your body is moving with required effort, as well as where the various parts of the body are located in relation to each other.
Kinesthesia is another term that is often used interchangeably with proprioception. Some users differentiate the kinesthetic sense from proprioception by excluding the sense of equilibrium or balance from kinesthesia. An inner ear infection, for example, might impact the sense of balance. This would impact the proprioceptive sense, but not the kinesthetic sense. The infected person would be able to walk, but only by using the person’s sense of sight to maintain balance; the person would be unable to walk with his/her eyes closed.
Kinesthesia is a key component in muscle memory and hand-eye coordination, and training can improve this sense. The ability to effortlessly swing a golf club, or catch a baseball requires a finely tuned sense of the position of the joints, so that the eyes can concentrate on the ball and let the kinesthetic sense handle moving the body as needed to meet the ball.
The proprioceptive sense is believed to be composed of information from sensory neurons located in the inner ear (motion and orientation) and in the joints and muscles (stance). There are specific nerve receptors for this form of perception, just like there are specific receptors for pressure, light/dark, temperature, sound, and other sensory experiences.
Proprioception is tested by American police officers using the field sobriety test where the subject is required to touch his nose with his eyes closed. People with normal proprioception may make an error of no more than 2 cm. People with severely impaired proprioception may have no clue as to where their hands (or noses) are without looking.
Proprioception is what allows someone to learn to walk in complete darkness without losing balance. During the learning of any new skill, sport, or art, it is usually necessary to become familiar with some proprioceptive concerns specific to that activity. Without the appropriate integration of proprioceptive input, an artist would not be able to brush paint onto a canvas without looking at the hand as it moved the brush over the canvas; it would be impossible to drive an automobile because a motorist would not be able to steer or use the foot pedals while looking at the road ahead; we could not touch type or perform ballet; and one would not even be able to walk without literally “watching where you put your feet”.
The proprioceptive sense can be sharpened through study of many disciplines. The Alexander Technique uses the study of movement to directly enhance kinesthetic judgment of effort and location. Juggling trains reaction time and spatial location and efficient movement. Standing on a wobble board is often used to retrain or increase proprioception abilities, particularly as physical therapy for ankle or knee injuries. Standing on one leg (stork standing) and various other body position challenges are also used, in such disciplines as Yoga. Several studies have shown that the efficacy of these types of training are challenged by closing the eyes, because the eyes give invaluable feedback to establishing the moment to moment information of balance.
Oliver Sacks once reported the case of a young woman who lost her proprioception due to a viral infection of her spinal cord. At first she was not able to move properly at all. Later she relearned by using her sight (watching her feet) and vestibulum (or inner ear) only. She eventually acquired a stiff and slow movement, which is believed to be the best possible in the absence of this sense. She could not judge effort involved in picking up objects.
David Bohm introduced the concept of “proprioception of thought.” His ideas suggest that other people’s point of view are needed to be able to compensate for the inevitable self-deceptive assumptions of thinking. He wrote about proprioception in Thought As a Systemand his theories of “Dialogue.”
Apparently, temporary loss or impairment of proprioception may happen periodically during growth, mostly during adolescence. Possible experiences include: suddenly feeling that feet or legs are missing from your mental self-image; the need to look down at arms, hands, legs, etc. to convince yourself that they are still there; falling down while walking, especially when attention is focused upon something other than the act of walking (e.g., looking at a person who started talking or reading a billboard).
The proprioceptive sense can become confused because humans will adapt to a continuously-present stimulus; this is called habituation or desensitization. The effect is that it seems as though proprioceptive sensory impressions disappear, just as a scent seems to disappear when a person smells it for a prolonged period of time. One practical advantage of this is that unnoticed actions or sensation continue in the background while an individual’s attention can move to another concern. The Jordan Technique addresses these issues.
People who have a limb amputated may still have a sense of that limb; this is termed a phantom limb. This phenomenon is not limited to one sensation, however. Phantom sensations can occur that are perceived as movement, pressure, pain, itching, or hot/cold as well. (Note: The work of V. S. Ramachandran indicates that despite popular belief, the phantom limb phenomenon is actually the result of neural signal bleed through the brain’s sensory maps, rather than from stimulation of nerves.)
There is one known case of a person losing her entire proprioceptive sense, which is one of the cases discussed in Oliver Sacks’ book The Man Who Mistook His Wife for a Hat.
Temporary impairment has also been known to occur due to an overdose of vitamin B6 (pyridoxine and pyridoxamine). Most of the impaired function discontinues shortly after the intake of vitamins returns to normal. Impairment can also be caused by cytotoxic factors such as chemotherapy.
It has been proposed that even common Tinnitus and the attendant hearing frequency-gaps masked by the perceived sounds may cause erroneous proprioceptive information to the balance and comprehension centers of the brain, and precipitating mild confusion.
Permanent impairment: Proprioception is also reduced in patients who suffer from joint hypermobility or Ehlers-Danlos Syndrome (a genetic condition that results in weak connective tissue throughout the body).