The intrafugal fibres contain specialised areas of contractibility innervated by efferent motor fibres referred to as gamma motor fibres (Matthews 1981). Each intrafugal fibre receives motor innervation from more than one gamma motor axon. This is termed polyneuronal innervation and is peculiar for the most part to the intrafugal fibres of muscle. (Polyneuronal innervation does occur in extrafugal fibres in neonates before functional maturation has occurred and occasionally in the early stages of reinnervation of muscle tissue following injury (Burke & Lance 1992).) These gamma motor fibres release acetylcholine at their synapses and arise from gamma motor neurons that live in lamina IX of the anterior grey matter of the spinal cord of the same segmental level as the alpha motor neurons supplying the extrafugal fibres from the homonymous muscle (Hunt 1974). The contractile elements of the nuclear bag fibres are rich in mitochondria and oxidative enzymes, whereas the nuclear chain fibres have more extensive sarcoplasmic reticulum and T-fibre development but fewer mitochondria and oxidative enzymes. This results in a slower contraction of nuclear bag fibres than nuclear chain fibres when the spindle is stimulated (Williams & Warwick 1984).

The gamma motor input to the intrafugal fibres is involved with adjusting the appropriate length of intrafugal fibres in order to maintain an accurate measure of the degree of contraction of the muscle. If the length of the intrafugal fibre was not constantly adjusted as the extrafugal fibres contracted, it would become slack and fail to accurately record further contraction or the static state of the muscle at that instant. Loading of the intrafugal fibres may occur by increasing the stretch on the extrafugal fibres or by increasing the activity of the contractile elements of the intrafugal fibres by increasing the gamma motor activity. Increasing the gamma motor activity results in a high specificity of muscle activity with a small amplitude of sway variation in maintaining extrafugal to intrafugal muscle length within a tight oscillating pattern. The degree of oscillation occurring in a muscle is referred to as gain. Thus with increasing gamma activation the specificity of movement or sensitivity increases and the gain decreases. This interaction provides for the occurrence of a reflex compensation for small irregularities of movement that may occur between the suprasegmental programming and the performance of the actual movement (Burke & Lance 1992). This modulation circuit is referred to as the motor servo mechanism (see Chapter 6). In order to ensure that this system performs properly there is an interactive connection between the alpha and gamma motor neurons that results in a co-activation of both systems simultaneously. So important is knowing the moment-to-moment state of our muscle position to the nervous system that nearly 30% of all descending efferent motor control systems are associated with the gamma motor modulation of spindle fibres, and the feedback information from these fibres is transmitted from the spinal cord to the suprasegmental areas via redundant pathways. As a general rule only information crucial to the nervous system is transmitted by more than one pathway. As previously mentioned, when information is transmitted through more than one pathway it is referred to as redundant transmission. To exemplify this point, the information from the muscle spindles below vertebral level of T1 travels up the spinal cord via the ipsilateral dorsal or posterior spinocerebellar tract and bilaterally in the ventral or anterior spinocerebellar tracts to the cerebellum. The axons of the posterior spinocerebellar tracts and the ipsilateral anterior spinocerebellar tracts pass through the ipsilateral inferior cerebellar peduncles to reach the ipsilateral cerebellum. The axons of the contralateral anterior spinocerebellar tracts cross back to the side of their origin in the superior cerebellar peduncles and thus maintain the rule that cerebellar input from muscle spindles occurs ipsilaterally. The paravertebral muscles of the spine have one of the highest concentrations of spindle receptors in the body and the upper cervical region of the spinal cord has the highest density of muscle spindle receptors in the spine (Kulkarni et al. 2001).

The muscle spindles of humans are usually well developed at birth. The development of the spindles in the fetus depends solely on the presence of the primary afferent axon. After the spindles have formed they can tolerate periods of denervation; however, they often undergo marked changes in function and reactivity even when reinnervation occurs by the original axon (Milburn 1973). The altered functional activity in muscle spindles following injury can contribute to asymmetries in cortical afferent input, resulting in hemispheric asymmetry. This presents one of the clinical challenges facing a patient following axon injuries or demyelination conditions.

These specialised receptors are found extensively in the collagen fibres of the muscle tendon junction. These receptors align themselves in series with the muscle fibres in such a way that stretching the tendon results in depolarisation of the receptors (Barker 1974). Some tendon organs are activated with even the weakest of contractions. The total discharge frequency of the organ increases with the strength of contraction of the homonymous muscle. The tendon organs are generally silent in relaxed muscle. There is some controversy as to when the tendon organs actually become activated. It has been the prevailing view that tendon organs become active whenever the muscle is stretched either by active contraction or by passive stretch (Matthews 1972). However, another opinion which states that tendon organs usually will not activate even if the muscle is stretched unless the muscle contracts during the stretch has recently arisen. Thus, the newest theory is that these organs monitor muscle contraction in conjunction with tendon tension and not just tendon tension in isolation (Houk & Crago 1980). One of the actions of stimulating the tendon organs of a muscle is a reflex inhibitory feedback stimulus on the alpha motor neurons controlling the homonymous muscle (Fig. 5.8).

Figure 5.8 Segmental reflex activity. Both intrafusal and extrafusal muscle fibres stretched; spindles activated. Reflex via Ia fibres and alpha motor neurons causes secondary contraction (basis of stretch reflexes, same as knee jerk). Stretch is too weak to activate Golgi tendon organs.

The sensory organs of Golgi tendon organs give rise to type Ib afferent fibres, which are myelinated but slightly smaller in diameter than the type Ia fibres of the muscle spindles. The fibres intertwine between the collagenous fibres of the tendon muscle junction where they are unmyelinated and highly branching. As they emerge from the collagen fibre network they bundle together and form a single myelinated axon. Usually, each individual tendon organ gives rise to one type Ib axon (Schoultz & Swett 1974; Ghez & Gorden 1995). Although the receptor information from the Golgi tendon organs does not reach the somatosensory cortex directly, and thus is not consciously perceived, the information supplied by the Golgi tendon organs is very important in the integration of inhibition of motor activity. This is especially apparent when learning how to perform a new motor function. For example, when learning how to hit a tennis ball, the player learns after many attempts not only how to perform the coordinated actions of the muscles involved in the correct swing, but also the ’feel’ of the muscle tensions involved in the correct swing (see Chapter 6).

These receptors are present on all parts of the hands and feet, on the forearm, the lips, and the tip of the tongue. The corpuscles consist of a capsule and a core portion. The capsule is formed by layers or lamellae between which substantial amounts of collagen have been laid down. The core consists of epidermally derived cells and nerve fibres. Each corpuscle is supplied by several large, heavily myelinated nerve (Aα) fibres and a few small unmyelinated (C) fibres. These receptors are low threshold, rapidly adapting and thus provide information concerning the change of mechanical pressure on the overlying skin. Interestingly, these receptors are formed in abundance at birth and over our lifetimes reduce in number by some 80% unless constantly maintained by stimulation (Cauna 1966) (Fig. 5.9).

Figure 5.9 Receptors of the skin. A variety of peripheral receptors including Ruffini endings, Pacinian corpuscles, free nerve endings, and Merkle discs are shown. Note the axons of all of these receptors contribute to the formation of a peripheral nerve fibre.

These receptors are sensitive to changes in pressure in a variety of tissues in the body. Pacinian corpuscles are found on the palmar aspects of the hands and feet, the genital organs of both sexes, and buried deep in most muscle tissues. These receptors are extremely sensitive to mechanical disturbances and are particularly sensitive to vibration (Gray & Sato 1953). In muscle tissue they give feedback on the internal pressure changes inside the muscle fibres during both stretch and contraction.

Each corpuscle consists of a capsule and a central core containing a nerve fibre. The capsule is lamellar in nature, with the lamellae separated by layers of collagen. The amounts of collagen deposited between the lamellae increases with age, decreasing the sensitivity of the receptors. Each corpuscle is supplied by one thickly myelinated nerve fibre of the Aα type (Williams & Warwick 1984) (see Fig. 5.9).

These receptors are found immediately under the epidermis or around the base of certain hair follicles. The nerve fibres expand at their periphery into flattened disc-like structures that come into close association with a non-neural type of cell called the Merkel cell. The Merkel cells have a number of interdigitating processes that contact other neighbouring cells. The nerve/Merkel cell units together are called Merkel discs and are sensitive to vertical pressure. The receptors are slow adapting and transmit information via large, myelinated (Aα) afferent fibres (English 1977) (see Fig. 5.9).

The Ruffini system of receptors including the Ruffini endings and Ruffini nuclei are found in each and every joint complex and in the dermis of hairy skin. These receptors respond to changes in stress levels in collagen. In joint capsules they are very sensitive to capsular deformation and may be activated by changes in the degree of arc about a joint. In most cases involving peripheral joints they can detect a 1° change of arc about the joint. In the case of certain types of receptors in the upper cervical spine as little as a 0.4° change of arc has been shown to cause activation. This is probably due to the high concentration of joint receptors in the upper cervical spine region which has been shown to contain the highest concentration of joint receptors in the spine (Vele 1970). These receptors are slowly adapting and utilise large, myelinated (Aα) afferent fibres for transmission (Chambers et al. 1972) (see Fig. 5.9).

A complex system of specialised receptors that supply feedback concerning the static and dynamic position of joints has been described by Wyke (1966). His description involves a classification of four types of joint receptors, which are described as follows.

Type I or tonic receptors have a low threshold and are slow adapting. These receptors are active both at rest and during changes of joint arc. Thus when the joint arc changes, these receptors show an initial burst of activity and continue to fire at a reduced but specific frequency until another change in joint angle occurs. These types of receptors are multibranched, encapsulated Ruffini ending type receptors. These receptors appear to be the most abundant type of joint receptor and are most highly concentrated in the weight-bearing joints where static postural information is crucial. It is thought that the information transferred by these receptors may reach conscious perception (Skoglung 1973).

Type II joint receptors are similar to Pacinian corpuscles but much smaller in size and have a low threshold but are rapidly adapting. Their activity increases at both the initiation and cessation of movement. They are highly sensitive to movement and pressure changes in the capsule and are thought to detect duration of movement. The information from these receptors is thought not to reach conscious awareness (Skoglung 1973).

Type III receptors have a high threshold and are slowly adapting to stimuli. They are inactive in non-moving joints and fire only at extremes of joint motion. They are similar in structure to the Golgi tendon organs of tendons and cause a reflex inhibition of the homonymous muscle when highly stimulated (Williams & Warwick 1984) (Fig. 5.10).

Figure 5.10 Supraspinal modulation of reflex control. Intrafusal as well as extrafusal fibres contract; spindles activated, reinforcing contraction stimulus via 1a in accord with resistance. Tendon organ activated, causing relaxation if load is too great.

Type IV receptors are nociceptive in function and have a high threshold and do not adapt. These types of receptors are free nerve endings that invade the synovial layers, blood vessels, and fat pads surrounding the joint and are normally inactive. They seem to be most sensitive to excessive joint movement and all causes of damaging stimuli that result in the perception of joint pain (Wyke 1966; Newton 1982; Fitz-Ritzon 1988; McLain & Picker 1998; Eriksen 2004).

This diverse variety of receptors in the joint capsule allows these receptors to supply the nervous system with a continuous input stream regarding joint position at any given moment in time. Some of the input from this system of receptors can be volitionally perceived although most of the time we are not consciously aware of the position of each of our joints (Fitz-Ritzon 1988).

Free nerve endings are found in all types of connective tissue including dermis, fascia, ligaments, tendon sheaths of blood vessels, meninges, joint capsules, periosteum, perichondrium, Haversian systems of bone, and endomysial tissue of all muscles. Nociceptive fibres may be depolarised by a number of different modalities termed nociceptive stimuli such as alterations in pH (acid or base environment), the presence of chemicals or collagenous deformation such as those associated with swelling and inflammation, heat, physical damage, and some chemicals normally contained inside of cells.

Three different types of nociceptors can be distinguished by the various stimuli to which they respond. These are mechanical, thermal, and polymodal receptor types (Gardner et al. 2000). Mechanical receptors are responsive to sharp penetration of objects into the skin or tissues. They are also sensitive to pinching or squeezing actions. The axons are mechanical nociceptors, are myelinated, and thus are the fastest conducting fibres of the nociceptors receptors. Thermal receptors are excited by extremes of temperature. The two different types of thermal receptors respond to cold and heat. Polymodal receptors respond to a variety of stimuli including mechanical, chemical, and temperature extremes. These receptors evoke the sensation of deep burning pain when their activation becomes perceived consciously.

Some of the nociceptive input may reach the somatosensory cortex where it is consciously perceived most commonly as pain (Cauna 1966). Nociceptive information is conveyed from the spinal cord to the thalamus and then to the sensory cortex. The spinal tracts that convey this information to the thalamus or hypothalamus include the spinothalamic, spinoreticular, spinomesencephalic, cervicothalamic, and spinohypothalamic tracts. The most prominent nociceptive tract is the spinothalamic tract of which both anterior and lateral divisions participate in nociceptive signal transmission (Basbaum & Jessell 2000).

A good illustration of the chemical activation of nociceptors involves the build-up of lactic acid inside a working muscle. When a muscle contracts in an anaerobic environment, lactic acid is produced as a by-product. When this substance builds to a threshold value usually due to a lack of circulation caused by sustained muscle contraction, the nociceptors fibres depolarise and fire a series of action potentials along the peripheral branch of the primary nociceptive afferent neuron’s axon which arrive eventually in the dorsal horn of the spinal cord. Here the fibre synapses with a variety of cells in the interneuron pools of the substantia gelatinosa. Some of these neurons send inhibitory stimuli back to the muscle while others project via the contralateral lateral spinothalamic pathways to the ventral posterior lateral nucleus of the thalamus. Here, depending on the central integrative state of the thalamus, further stimuli may or may not be relayed to the somatosensory cortex where perception of the pain may occur.