Summary

Kinesthesia. II. Sensory stimulation

At the stimulation level kinesthetic, raw sensory data arise throughout the body from receptors in:

• muscles,
  
• tendons,
  
• joints,
  
• skin,
  
• vestibular apparatus,
  
• eyes,
  
• ears, and
  
• from 'central feedback' of interior knowledge of motor commands (efference).
  

Various sensory receptors and an internal knowledge of motor commands contribute data which is derived into kinesthetic perceptions. This section will briefly review the functioning of each type of receptor, the stimulation which it responds to, and the type(s) of information the receptors provide. Evidence for an internal knowledge of motor commands (referred to here as “efferent data”) is also noted.

A fundamental characteristic of receptor function is the rate of “adaptation” of a sensory receptor response to a stimulus which is steady and continual (Sherrick and Cholewaik, 1986, p. 111-6). These are classified as generally two types: 1) Quickly adapting receptors stop responding to a continual stimulus very soon are are therefore efficient in sensing rapidly changing stimulation such as quick movements; 2) Slowly adapting receptors maintain their response to a continual stimulus for a long period and are therefore efficient in sensing continuous, unchanging stimulation such as a maintained bodily position.

Muscle Spindles; Primary and Secondary Endings.

Muscles are composed of hundreds of individual long slender muscle fibres which connect to tendon filaments at either end which in turn attach to bones. The large main muscle fibres are referred to as extrafusal fibres and produce the muscle’s strength from their force of contraction.

Modified muscle fibres of the sensory spindle organs are referred to as intrafusal fibers and are arranged in parallel to the longer, thicker extrafusal fibres. This arrangement allows the intrafusal fibres to shorten and lengthen together with the extrafusal fibres but without carrying any of the burden of force. A muscle spindle receptor, within the thin intrafusal fibres, has two types of sensory endings known as primary and secondary.

Secondary spindle endings increase their response linearly as the muscle length increases throughout the range of the muscle (Matthews and Stein, 1969) and so they function analogously to slowly adapting receptors. The secondary spindles provide data about the overall length of the muscle but are not sensitive to small quick changes in the muscle length (Rothwell, 1987, pp. 76-87).

Primary spindle endings are sensitive to much smaller muscle length increases but their response does not increase regularly with muscle length (Matthews and Stein, 1969). Rothwell (1987, pp. 77-79, 86-87, 97) reviews how primary endings are thought to respond to “cross-bridges” which link parallel intrafusal fibres. The cross-bridges are stiff, when the fibres slide apart (as the muscle lengthens) beyond some critical point the cross-bridges break and reform at the new muscle length. Because of this they are sensitive to very small changes in muscle length (motion) and then quickly return to a static level of response once the new length is arrived at. This pattern of response occurs irrespective of the overall muscle length. They are so sensitive to muscle length changes that they may even respond to arterial pulse or respiratory movements. However, the response of the primary endings to static positions is low, “only 10 per cent of spindles show any discharge at all at a comfortable rest position of the hand” (p. 97). This behaviour of quick responses to small stimuli changes, followed with an immediate return to a neutral response level is analogous with quickly adapting receptors.

From these patterns of responses, it is believed that the primary endings sense velocity of muscle change-of-length and muscle length, while the secondary endings sense only muscle length (Clark and Horch, 1986; Rothwell, 1987, pp. 74-104).

Tendon Receptors.

Slowly adapting Golgi tendon organs are located at muscle-tendon junctions and are composed of a capsule enclosing several tendon filaments. These are attached end-to-end with muscle fibres and tendon filaments (in series) and this arrangement allows the Golgi tendon organs to respond to muscle-tendon tension regardless of muscle length. Muscle length and tension are separate, for example in an isometric contraction the muscles contract (increased muscle-tension) but limbs do not move (identical muscle-length). Golgi tendon organs have been shown to increase their response to increases in muscle tension very accurately (Crago et al., 1982).

Two other types of receptors in muscles and tendons seem to be of a lesser importance and have not been widely studied. Paciniform corpuscles are mostly found near the Golgi tendon organs are are sensitive to vibrations. Free nerve endings are found throughout the muscle and tendon structures and seem to be sensitive to mechanical pressure and pain stimulation (Clark and Horch, 1986; Rothwell, 1987, pp. 74-104).

Joint receptors.

Slow adapting Golgi sensory receptors (similar to Golgi tendon organs), are found in the ligaments which connect bone to bone and form the outer layer of the joint capsule. Slow adapting Ruffini receptors and quick adapting paciniform corpuscles, similar to those found in skin, are also found in the tendon material of the joint capsule. Free nerve endings are found throughout the joint connective tissue (McCloskey, 1978, pp. 766-767).

The functioning of joint receptors is debated by physiologists (for reviews see Clark and Horch, 1986; and McCloskey, 1978). Skoglund’s (1956) findings that cat knee joint receptors are selectively activated by certain positions of joint angle led most researchers to believe that the slow adapting Golgi and Ruffini receptors within the ligaments around the joint respond to being stretched. Contrary to this other researchers (Burgess and Clark, 1969; Clark, 1975; Clark and Burgess, 1975; Grigg, 1975) found that most cat knee joint receptors respond only to extreme joint angles. Still other researchers (Carli et al., 1979) found that cat hip joint receptors responded at all angles with the same increasing rate of response with increased flexion or extension of the joint.

The cat knee has also been shown to respond sensitively to pressure into the joint capsule, leading some researchers (Clark, 1975; Clark and Burgess, 1975) to hypothesise a pressure response (rather than a stretch response) for joint receptors. The quickly adapting paciniform corpuscles probably respond to high speed vibrations as they do in the skin. (General references for joint receptors; Clark and Horch, 1986; Rothwell, 1987, pp. 74-104.)

Skin Receptors.

Free nerve endings are close to hair follicles and stimulated by movements of bodily hairs (resulting from bodily moves or external forces). Slow adapting Merkel disks are close to the surface of the skin, respond only to vertical skin pressure (ie. pressure into the body, not lateral stretch of the skin), and may maintain their response to a constant pressure for up to ten minutes. Quickly adapting Meissner corpuscles are also close to the surface of the skin and sensitive to pressure but will cease responding in seconds.

Slow adapting Ruffini sensory endings are deeper in the skin and demonstrate a directional specific response to stretching of the skin. One direction of stretch will elicit a response, but a stretch at a right angle to that direction will elicit no response (Knibestol, 1975). Quick adapting Pacinian corpuscles are also deep in the skin and respond to stimuli in an area “almost as large as the whole palm in some cases” (Rothwell, 1987, p. 99). Its sensory ending is surrounded by concentric rings which eliminate low frequency vibration and so they respond only to rapid vibrations. (General references for skin receptors; Clark and Horch, 1986; and Rothwell, 1987, pp. 74-104.)

Receptors in skin provide kinesthetic data about the stretching and bending of the skin during movement and within poses. Skin kinesthesia may be especially important in areas of dense skin receptor populations such as the hands, feet, face, and mouth (Moberg, 1983; see below).

Vestibular Receptors (Labyrinth).

The vestibular system is the non-auditory part of the inner ear. There is one vestibular system for each ear (bilateral). Each system is composed of two parts, the otolith organs and the semi-circular canals. Both the otolith organs and the canals consist of chambers filled with a think “endolymph” fluid. When the head moves through space the inertia of the heavy endolymph fluid causes it to lag behind the movement and thus push against a gelatinous membrane connected to tiny hairs which are connected to nerve endings. When a steady velocity is reached the endolymph fluid stabilises in its chambers and so the sensory response stops. Because of this, the vestibular system responds to accelerating or decelerating changes in speed but not to constant speed.

The otolith organs consist of two sack-shaped chambers, the utricle and the saccule, each filled with endolymph fluid. The hairs connected to nerve fibres are arranged on the floor of the utricle and around the wall of the saccule. Because of this symmetrical arrangement of hairs a rotary acceleration around a vertical axis passing through the head causes opposing forces in the otoliths which cancel each other out and therefore cause no sensation. A linear acceleration through space will cause the endolymph to push unevenly on the hairs and elicit a sensory response. The nerves of the otolith organ also have a constant discharge which continually indicates the direction of gravity.

The semi-circular canals consist of three ring-shaped chambers, each forming a compete circuit of endolymph fluid approximately 3-4mm in diameter. The three canals are oriented at approximate 90° angles from each other so that one canal is roughly parallel to the frontal, medial, and horizontal planes of the body. This mutually perpendicular arrangement allows rotation around any axis to be registered in at least one of the canals, however there is little response from purely linear motion.

In each canal there is one cupula which is the gelatinous projection into the endolymph fluid which is connected to the sensory hairs. When the head undergoes a rotary acceleration the inertia of the heavy endolymph fluid causes it to lag behind the motion and push against the cupula which elicits the sensory response. If the rotary speed is constant after a short time the fluid will stabilise in the canals and the nerves will stop responding. If the motion is then abruptly decelerated the fluid will continue moving and push against the cupula in the opposite direction. This can cause the sensation of turning in the opposite direction accompanied by post rotary nystagmus (see below). (General references for vestibular receptors; Kapit and Elson, 1977; Howard, 1986.)

Visual Receptors.

The visual-motor system plays an important part in kinesthesia by sensing visual field motion and vision of the body moving (general references; Hood and Finkelstein, 1986; Hallett, 1986; Westheimer, 1986).

Each eye is roughly spherical. At the front of the eye the cornea bulges forward which serves to gather electromagnetic light rays into itself and thus expand the visual field.* The light which is collected by the cornea passes through the adjustable opening of the pupil and into the oval shaped lens.
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* The visual field consists of the spatial range of electromagnetic radiation reaching the retina at any one moment. This is also known as the visual array or the optic array.
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The retina is a layer of photo-sensitive sensory receptor cells covering the interior surface of the eye. There are two types of visual receptor cells, approximately 120 million rods and 6.5 million cones in each eye. Cones occur in high density in the central fovea region of the retina (more than 140,000 / mm2), low density in the peripheral region of the retina (less than 10,000 / mm2) and are sensitive to high intensity light (daylight brightness), colour vision, and fine detail. Rods occur in low density in the central fovea region (virtually 0), high density in the peripheral region of the retina (from 50,000-160,000 / mm2) and are sensitive to low intensity light (night-light brightness).

The fovea is a small area on the retina which contains a high density of cone photoreceptors. This is the retinal location where visual stimuli can be seen in greatest detail and so is where visual images fall when a person fixates her vision on a point in space. The size of the fovea can encompass stimuli which fills approximately 0.5° of visual angle,* or about the same visual angle occupied by a view of the moon from earth (Westheimer, 1986, p. 4.6).
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* Visual angle refers to the amount of space filled by an object within the visual field, measured in angular degrees with the angle’s vertex at the centre of the eye (Sedgwick, 1986).
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Monocular focus, also called accommodation, is accomplished by the ciliary muscle adjusting circumferential tension around the lens, thus allowing the lens to bend the light rays in variable amounts. This adjustment of the lens’ shape takes approximately 0.6 sec. to complete. The lens’ accommodation bends the diverging light rays and converges them onto the retina at the back of the eye. When diverging light rays from a single point in space are converged by the lens into a single point on the retina, than this point is in monocular focus.

Binocular focus, also called vergence, is the only type of eye movement when the eyes do not follow parallel pathways. To keep a stimulus in binocular focus (ie. its image falling on the fovea of each eye) the two eyes either rotate closer together or farther apart in response to a stimulus moving closer or farther from the observer respectively.

Two types of data can be distinguished which contribute to visual kinesthesia. These can be referred to as visual field motion and vision-of-the-body moving.

- Visual field motion.

Early analysis of kinesthesia from visual field motion (Gibson, 1958; 1966) discussed how the visual field will appear to move across the retina when an organism travels or turns through space. Patterns of visual field motion, or “optical flow” become associated with the self-motion which usually produces them. For example, when travelling forward the entire visual field will expand and appear to move past on the sides while the point travelled toward remains in the centre of the visual field while gradually becoming larger (G. J. Andersen, 1986; Gibson, 1966).

Other cues within the visual array will add to the details available about the visual field motion (Sedgwick, 1986). “Motion parallax” refers to how visual stimuli close to the observer move across the visual field faster than visual stimuli far from the observer. “Occlusion” refers to how visual stimuli farther from the observer will sometimes disappear behind visual stimuli closer to the observer.

Visual nystagmus will also occur when the visual field flows across the retina. This is a basic orienting reflex which helps stabilise the perception of the visual world when the head is in motion. There are two phases of nystagmus which alternate slow phase, fast phase, slow phase etc.

During the slow phase of nystagmus the eyes remain fixated on a location in the exterior environment while the head is in motion. Thus, in the slow phase the eyes are rotating in the head in the opposite direction as the rotation of the head. This stabilises the perception of the exterior environment while the body is in motion.

During the fast phase of nystagmus the eyes quickly catch-up with the head, re-centering the eye in its socket and fixating on a new location in the exterior environment. Thus, in the fast phase the eyes are rotating in the same direction as the head motion.

When the body turns around the vertical axis the visual field flows across the retina right-to-left (or vice versa) and “horizontal nystagmus” occurs with the eyes also moving right and left. When the body rotates around the lateral axis (eg. somersaults) the visual field flows across the retina top-to-bottom (or vice versa) and “vertical nystagmus” occurs with the eyes also rotating up and down (also called “doll’s eye reflex”). When the body rotates around the sagittal axis (eg. cartwheels) the visual field rotates around the retina clockwise or counter clockwise and “torsional nystagmus” occurs with the eyes also torqueing clockwise or counter clockwise. (General reference for nystagmus; Hallett, 1986.)

The motor commands for the eye movements will effect the characteristics of the nystagmus. Focusing and fixating on objects in the visual field results in less frequent, higher speed and larger distance quick phases of nystagmus than non-focused staring (Honrubia et al., 1968). When the eyes have a visual fixation goal the sequence of motion is as follows:

1. The eyes begin moving 20 msec before the head begins moving.
   
2. The eyes reach the target before the head reaches the target.
   
3. The head completes its movement to the target while the eyes remain stable and fixated at the target via a compensatory reverse motion relative to the head.
   

1. The head begins moving while the eyes begin slow phase of nystagmus in the opposite direction, remaining stable in the environment.
   
2. The head continues moving while the eyes alternate slow and fast phases of nystagmus.
   
3. The head completes its movement to a new position.
   
4. The eyes catch-up to the head with the nystagmus fast phase.
   

[Spotting] is a term given to the movement of the head and focusing of the eyes in pirouettes [and other turning movements] . . . In these turns the dancer chooses a spot in front and as the turn is made away from the spot, the head is the last to leave and the first to arrive as the body completes the turn. This rapid movement . . . prevents the dancer from becoming dizzy. (Grant, 1982, p. 113)