Course Handout - Biological Cybernetics
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First published online 08:28 BST 1st April 2003, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 18:00 14th January 2010
Earlier versions of this material appeared in Smith (1997; Chapter 4). It is simplified here and supported with hyperlinks.
Although this paper is reasonably self-contained, it is best read as a extension of our paper on Control Theory. To go directly to the earlier paper, click here, and to see the author's homepage, click here.
1 - Biological Cybernetics in General
We turn now to the topic of control in biological systems, that is to say, to the search for the control hierarchies and servomechanisms in the body's skeletomuscular system. Wiener himself, in the years following the publication of "Cybernetics", was personally active in this very field. As early as 1948 he was discussing feedback circuits in the body's motor control systems, concluding that the complexity of biological control processes demanded a "multiplicity of feedback", very little of which ever became conscious (Wiener, 1948). During the 1950s and 1960s, another pioneer, Larry Stark, made reasonable progress analysing the neural circuitry controlling the intrinsic and extrinsic muscles of the eyes (eg. Stark, 1968), and Ragnar Granit analysed the role of alpha and gamma motor neurons in skeletal muscle control (eg. Granit, 1970; see Sections 2 and 3 below). Walter (1961) even went so far as to accord the phenomenon of biological thermostasis - that is to say, homeostasis of body temperature - the greatest evolutionary significance. It was, he argued, a supreme event in natural history, because it made possible the survival of mammals on a cooling globe. With similar enthusiasm, Bennett (1979) has called feedback "the twentieth-century metaphor", and Gosling (1994, p7) calls control theory "the art of being and doing". Nevertheless, real advances have actually been few and far between. By and large, cybernetic analyses of biological systems have been too simplistic, and have failed more or less totally to explain the "vital spark" of life, as manifested in such unpredictable behaviours as play, curiosity, and creativity, and (above all) in the phenomenon of consciousness. Here, with an inbuilt glossary of terms, are some of the success stories.
2 - Alpha-Gamma Muscle Control
Granit (1970) has shown that the body's skeletomuscular system includes sophisticated spinal level control technology. The alpha-gamma muscle control system is a good example of this. This is a major biological servosystem involving widespread structures of the central and peripheral nervous systems, and responsible for managing posture and locomotion. Here is the basic vocabulary:
Mini Glossary - Muscles and Muscle Control Systems
Action Potential, Muscle: When a neuromuscular junction is repeatedly stimulated by incoming neural action potentials, the end-plate potentials eventually exceed the threshold for an action potential to be initiated within the muscle fibre instead. The active ions in this new type of action potential are calcium ions instead of sodium, and the resulting calcium ion influx is the key enabling factor in excitation-contraction coupling.
Alpha-Gamma Control System: An important biological servosystem involving structures of the central and peripheral nervous systems. Helps to manage bodily posture and locomotion from the level of the spinal segment, thus (a) making for very rapid muscular response when problems are encountered, and (b) generally reducing neural traffic up and down the spinal cord. (See alpha motor neuron and pyramidal tract.)
Alpha Motor Neurons: These are large lower motor neurons, situated in the ventral spinal grey. Their axons form the bulk of the motor root of the spinal nerve.
Annulospiral Endings: These are the sensory receptors of the intrafusal muscle fibres. They are wound spirally around the belly of each such muscle fibre, and are thus admirably placed to detect the "fatness" of that fibre at any moment in time. Their sensory information is conveyed back to the spinal segment by the sensory branch of the spinal nerve.
Antagonism: Type of muscle organisation where muscles groups oppose each other in moving a limb. When controlling such muscles it is necessary for one group to relax while the other is pulling. (See extensor muscle and flexor muscle.)
Corticobulbar Tracts: These are fibres derived from upper motor neurons whose lower motor neurons are situated in the cranial nerve nuclei of the medulla. (Compare corticospinal fibres.)
Corticospinal Tracts: These are fibres derived from upper motor neurons whose lower motor neurons are situated in the spinal grey. (Compare corticobulbar fibres.) This is therefore the major class of fibres found in the pyramidal tract.
Electro-Myograph (EMG): Electromyography is the technique of detecting the synchronised discharge of all the muscle fibres in a motor unit. This is made possible by the fact that this discharge creates an electrostatic field which can be picked up from some distance away by either an implanted or a skin surface electrode (the former being very sensitive, the latter less so but quicker and more convenient to apply). EMGs are much used in research and clinical medicine, as well as being available in kit form for personal amusement and biofeedback.
End-Plate Potential (EPP): The potential in a motor end-plate following the arrival of a neural action potential. This is not yet a muscular action potential, rather it is the equivalent to the EPSP in neurotransmission. (It is "graded" activity in other words, rather than all-or-none.) Typically, the EPP takes longer to decay (ca 10 msec) than do the traces of the acetylcholine which caused it (3 msec). It may therefore require the temporal summation of a succession of incoming impulses to produce a full muscular action potential.
Excitation-Contraction Coupling: Term used to describe the link between the action potential of muscle fibres, and the contraction of the myofibrils which then follows. This phenomenon results from sensitivity of the protein filaments within the sarcomere to a sudden influx of calcium ions.
Extensor Muscle: In an antagonistic muscle pair, the one which straightens the limb in question rather than bending it. (Compare flexor muscle.)
Extrafusal Muscle Fibres: These are the main muscle fibres making up a skeletal muscle. They provide the main contractile power for bodily posture and locomotion.
Extrapyramidal Tract: This is the ancillary spinal motor tract. It arises in a variety of basal ganglia, midbrain, and hindbrain locations, descends more ventrally in the spinal white than does the pyramidal tract, and carries involuntary muscle instructions to the gamma motor neurons.
Fibre Bundle (Muscle): The first level of muscle organisation above the muscle fibre. Consists of bundles of individual muscle fibres. These bundles are sometimes referred to as fasciculi, and consist of individual muscle fibres encased within a layer of connective tissue called the perimyseum.
Flexor Muscle: In an antagonistic muscle pair, the one which bends the limb in question rather than straightening it. (Compare extensor muscle.)
Gamma Motor Neurons: These are small lower motor neurons, situated in the ventral spinal grey. Their axons join those of the alpha motor neurons to form the motor root of the spinal nerve.
Golgi Tendon Organs: These are the sensory receptors of the muscle tendons. They detect the amount of tension in the tendons, and therefore of the entire muscle. Their sensory information is conveyed back to the spinal segment by the sensory branch of the spinal nerve. Excitation of the Golgi tendon organs excites inhibitory interneurons within the spinal grey which inhibit the alpha motor neurons which caused the muscle contractions in the first place. In the absence of any other effect, therefore, muscle stretch inhibits itself, thus serving as a valuable safety mechanism against cramp, tetany, etc.
Interneuron: One of several neurons in a neuronal circuit, usually relatively small and with only a short axonal travel. Frequently inhibitory by nature, so that they act as negative feedback devices (as, for example, with Renshaw cells). Inhibitory interneurons in the spinal grey are involved in the alpha-gamma control system, where they are responsible for reducing the output from a given alpha motor neuron whenever the muscle being controlled by that neuron is determined to be contracting too vigorously.
Intrafusal Muscle Fibres: An individual muscle fibre which is smaller and thinner than normal. These are small muscle fibres contained in the muscle spindles, and wound about by the annulospiral endings. These fibres are excited by the gamma motor neurons, and respond via the annulospiral endings. Functionally, these are the control fibres, whereas the extrafusal muscle fibres are the strength fibres. See under muscle spindle for mode of operation. Also optionally known as spindle fibres.
Lower Motor Neuron: Synonym for alpha motor neuron. (Compare upper motor neuron.)
Motor End-Plate: The motor axons of a spinal nerve branch into numerous telodendria as they approach the muscle they innervate. Where a single telodendrion touches the muscle fibre it flattens out to make a better contact. This enlargement is known as the motor end-plate, or neuromuscular junction. It is thus the nerve-muscle equivalent of a synaptic button.
Motor Root: One of the neural fibre bundles which emerge from the ventral horn of a given spinal segment and make up the motor element of that segment's spinal nerve. (Compare sensory root.)
Motor Unit: The bundle of muscle fibres innervated by a single motor end-plate. Can be of widely differing size: in eye muscles there are seven muscle fibres per nerve fibre, but in leg muscles this rises to 1700 muscle fibres per nerve fibre.
Muscle Contraction, Overview: There is a six-stage sequence of events during the initiation of a striate muscle contraction. These are (a) arrival of the neural action potential at the neuromuscular junction, (b) release of the acetylcholine neurotransmitter, (c) creation of the partial depolarisation in the muscle fibre (the end-plate potential), (d) triggering of the muscular action potential, (e) triggering of the calcium ion action potential within the tubule membranes of the sarcoplasmic reticulum, and (f) contraction of the sarcomeres making up each myofibril.
Muscle Fibres: Muscle cells. Striate muscle fibres are of two types. Red muscle fibres contain plentiful supplies of myoglobin, a variant of haemoglobin, because they have to satisfy the metabolic demands of large muscle movement. White muscle fibres contain less myoglobin and fatigue more easily. The proportion of each depends upon biological need (eye muscles are mainly white, for example), but can be affected by training. The main subcellular components of a muscle fibre are the myofibrils and the sarcoplasmic reticulum. (Not to be confused with nerve fibres!)
Muscle Spindles: These are small bundles of intrafusal muscle fibres scattered in moderate numbers throughout the main bulk of a muscle. They make no real contribution to the power of that muscle's contraction, but serve instead as contraction sensors. The information they provide is one of the key factors in the alpha-gamma control system.
Nerve Fibres: A collection of axons en route from their source neurons to their appointed destination. (Not to be confused with muscle fibres!)
Neuromuscular Junction (NMJ): The junction between an efferent nerve axon and a skeletal muscle. Much like a synapse, but involving an alpha motor neuron and a motor unit rather than two neurons. Also known as the motor end-plate.
Perimyseum: The connective tissue wrapped around a bundle of muscle fibres.
Pyramidal Tract: This is the direct corticospinal tract. It arises in the primary motor cortex, as a bundling together of axons from the upper motor neurons. By the time it reaches the medulla, it contains a million or so fibres, 90% of which decussate (cross over) and travel down the contralateral (or "crossed") lateral corticospinal tract of the spinal cord. The pyramidal tract is conventionally believed to carry voluntary muscle instructions to the alpha motor neurons. (See also corticospinal fibres, and compare corticobulbar fibres.)
Renshaw Cell: An inhibitory interneuron found in the spinal grey, and associated in two ways with an alpha motor neuron. Firstly, it receives an excitatory collateral from the alpha neuron's axon as it emerges from the motor root, so that is "kept informed" of how vigorously that neuron is firing. Secondly, it sends its own inhibitory axon to synapse with that alpha neuron. The rate of discharge of the Renshaw cell is thus broadly proportional to the rate of discharge of the associated motor neuron, and the rate of discharge of the motor neuron is broadly inversely proportional to the rate of discharge of the Renshaw cell. Renshaw cells thus act as "limiters", or "governors", on the alpha motor neuron system, thus helping to prevent muscular damage from tetanus.
Sensory Root: One of the six or so fibre bundles which emerge from the dorsal horn of a given spinal segment and make up the sensory element of that segment's spinal nerve. (Compare motor root.)
Spinal Grey: The mass of neuron cell bodies making up the central core of the spinal cord throughout most of its length. Shoes a distinctive butterfly shape if transected, the tips of the wings of which being known as the dorsal horns (dorsally) or the ventral horns (ventrally). The alpha motor neurons are located in the ventral horns. (Compare spinal white.)
Spinal Nerve: A bilaterally symmetrical pair of peripheral nerves arising from each spinal segment, and responsible for the body's sensory, skeletomuscular and visceral operation.
Spinal Segment: A section of spinal cord corresponding more or less to a single vertebra, from which a single bilateral pair of spinal nerves originates.
Spinal White: The spinal cord's ascending and descending axon tracts. So called because the myelin sheathing of each component axon gives the tissue a white colouring upon dissection. In spinal cross-sections, the white matter can be seen to be channeled into the "flutings" provided by the horns of the spinal grey. (Compare spinal grey.)
Spindle Fibres: See intrafusal muscle fibres.
Tetanus (1): State of constant muscle tension caused (a) by incoming excitations arriving so quickly that their individual twitches overlap and merge ("fused tetanus") (see end-plate potential), or (b) pathological state such as tetanus (2) or strychnine poisoning.
Tetanus (2): Name of disease (commonly known as "lock-jaw") characterised by severe and life-threatening tetanus (1) caused by action of toxins produced by the bacteria responsible.
Tetany: Muscle spasm (uncontrolled overcontraction) caused by insufficient calcium ions available at the muscle end-plate.
Tremor: Minute oscillations of a muscle which accompany its contraction. Their frequency is in the range 8 - 12 Hz, and their amplitude about 1% that of the main muscle contraction.
Twitch: A sudden but unsustained contraction of one or many muscle fibres.
Upper Motor Neuron: A relatively large neuron in Layer V of the primary motor cortex (Brodmann's Area 4). The axons pass down through the corona radiata to the internal capsule, then down through the cerebral peduncles of the midbrain to form the pyramidal tract.
3 - The Alpha-Gamma Spinal Servomechanism
The essence of the alpha-gamma system is that the spinal cord simultaneously conveys two sets of descending instructions to the ventral grey of the spinal segment from which the muscles in question are innervated. The first of these - the power signal - descends from the neocortex via the pyramidal tract, and synapses with the alpha motor neurons. The axons of these neurons - the alpha fibres - emerge via the ventral root of the spinal nerve, travel to the destination muscle, and cause contraction of the extrafusal muscle fibres (which form the bulk of the muscle). Unfortunately, alpha control of this sort is not very sophisticated, and can go wrong in one of two ways. At one extreme, the muscle tension might turn out to be insufficient for the job, and at the other extreme it might turn out to be too much. In the first instance your limb either stays where it is or moves too slowly, and in the other instance it moves away too violently; and in both instances you are at risk of injury, if not of loss of life.
The second type of descending instruction helps remove this risk. These signals - the control signals - descend from the extrapyramidal system via the extrapyramidal tract, and synapse with the gamma motor neurons at the appropriate spinal level. From the brainstem downwards, therefore, they run in parallel with the pyramidal alpha system. Their axons too - the gamma fibres - emerge via the ventral root of the spinal nerve, and travel to the destination muscle. Here, however, instead of exciting the extrafusal muscle fibres, they enter the muscle spindles and excite the intrafusal muscle fibres. This arrangement is important, because it provides a means of sensing how appropriately the alpha muscle contractions are proceeding. The argument at this juncture goes as follows:
Question: Are the extrafusal muscle fibres and the spindle fibres contracting at the same speed, and, if not, are the spindle fibres contracting slower or faster than the others?
Possible Answers: There are three mutually exclusive possible answers to this question:
(a) Answer = YES: If the two types of fibre are contracting at the same speed, then the muscle as a whole is deemed to be contracting at the right speed, and no remedial action is necessary. But keep checking at regular intervals just in case.
(b) Answer = NO, with gamma slower: If the intrafusal muscle fibres are contracting slower than the extrafusal, then the extrafusal are contracting too quickly. This means that the limb loading has been overestimated, and that it would be wise to slow the movement down. This is done by feeding back inhibition from the Golgi tendon organs to the alpha motor neurons, thus subtracting locally from that being fed down the pyramidal tract.
(c) Answer = NO, with gamma faster: If the intrafusal fibres are contracting faster than the extrafusal, then the extrafusal are contracting too slowly. This means that the limb loading has been underestimated, and that it would be wise to speed the movement up. This is done by feeding back excitation from the spindle fibre to the alpha motor neuron, thus adding locally to that being fed down the pyramidal tract.
Thus, notwithstanding the fact that it is the alpha system which says "go" to a given muscular contraction, it is the gamma system which says how strongly to go. Moreover, it is the gamma system which is then best placed to bring about automatic changes to those contractions. If the gamma signal became rhythmic, for example, then the muscle contractions would also become rhythmic even if the alpha signal remained unchanged. All of which is totally consistent with the conventional neuroanatomical opinion that the neocortical pyramidal system serves voluntary movement whilst the extrapyramidal system concerns itself with involuntary fine coordination and synchronisation.
There is also evidence for a hunting mechanism (see earlier this chapter) in the tremor which accompanies the normal contraction of muscle. Lippold (1971) found a slight oscillation at approximately 10 Hz superimposed upon the main contractions of skeletal muscles, and argued that this was a valuable property of the stretch reflex arc because it allowed more rapid response than would have been the case if this particular circuit were more effectively damped. As he puts it, reflex action must involve "a necessary compromise between speed of response on the one hand and a certain degree of overshooting, or inaccuracy, on the other" (p70).
The anatomical features of the alpha and gamma systems are shown diagrammatically in the associated paper on the Pyramidal and Extrapyramidal Motor Systems.
4 - Head/Eye Muscle Control
The oculomotor control system serves a variety of biologically essential behaviours such as food search and predator avoidance (Galiana, 1990). It therefore needs to be every bit as complicated as the skeletomuscular system it is helping to guide. This functionality is provided by having a complex of feedforward, predictive, and feedback control loops at work. To start with, there are mechanisms controlling the automatic focussing of the lens, binocular vergence, and the automatic stopping down of pupillary aperture. There are then additional mechanisms to control the automatic positioning of the eye relative to the head as the head moves relative to both the body and the external world. These latter mechanisms place heavy information processing demands on the vestibular system. This is the system which processes the information provided by the semicircular canals of the inner ear (the "labyrinth"), the body's balance detectors. Information from the semicircular canals travels to the brainstem down the vestibular branch of the vestibulocochlear nerve (CN VIII). Here it links in via the vestibular nuclei of the lower pons to the cerebellum and a host of other components of the extrapyramidal system.
5 - Stammering
Lee (1951) pioneered a technique of replaying a person's speech to that person's own ears, subject to a variable time delay. He found that there were two types of common effect. Either subjects slowed down and raised their voices, or else they began to speak haltingly, repeating syllables in a form of "artificial stutter". The same phenomenon emerged with skilled tympanists reading a drum-beat, and for the key presses of skilled morse operators. Lee gives the following specific examples:
aluminum..... degrades to aluminum-num.....
ten-nine-eight-seven.... degrades to ten-nine-nine-eight-seven.....
Lee interpreted these findings as evidence of a multiple loop control hierarchy, with four levels of feedforward and feedback. The top control level releases individual thoughts for action, and then monitors that action for successful progress and completion. The second control level does the same for the words expressing the thought. The third does the same for the syllables making up each word. And the fourth does the same for the phonemes making up each syllable. It is confusion at the hand-over between the second and the third level which presumably causes the aluminum-num syllable repetition. There were no single-phoneme repetitions.
6 - Other Control System Pathologies
Other clinical syndromes where control system deficiencies have been suggested from time to time are:
(a) Parkinsonism: The motor disorders which characterise Parkinson's disease are conventionally attributed to disorders of muscle control circuitry. Wiener himself likened Parkinsonian tremor to the oscillations of under-"damped" control loops (Wiener, 1950), Flowers (1978) blames lack of prediction, Harrington and Haaland (1991) blame "central processing deficits", and Dinnerstein, Frigyesi, and Lowenthal (1962) blame slower than normal proprioceptive feedback for a variety of the standard Parkinsonian symptoms, such as rigidity, slowness, and lack of coordination.
(b) Dyspraxia: This is an inability to initiate voluntary movements despite intact muscles and motor systems, and is a common correlate of CVAs. The suspicion here is that a major feedforward pathway or mechanism is unserviceable.
(c) Dysarthria: This is an inability to deliver voluntary movements cleanly once initiated. Thus when speech becomes slurred after a CVA, even to the extent of falsely suggesting drunkenness. The suspicion here is that the initiating feedforward is intact, but the subsequent fine control and sequencing is faulty.
(d) Dyslexia: This is an inability to process visually presented text efficiently. Whilst this is at first sight a perceptual problem, the very complexity of the oculomotor control system (see above) makes it a motor problem as well. You cannot read if you cannot control the movement of your eyes. When reading this text, for example, your eyes will be fixating after every eight characters (about every one and a half words) (Rayner and Pollatsek, 1989), and many authorities (typically Pavlidis, 1981/1985) believe that developmental dyslexia can be explained by defects in sequencing these fixations for maximum information uptake. Developmental dyslexics do appear to have eye movement patterns which differ from those of normal readers (Rayner and Pollatsek, 1989). However, this factor per se has not been strongly confirmed. Indeed, Rayner and Pollatsek place greater store in Stein and Fowler's (1982, 1984) findings of "vergence control" problems in dyslexics. Vergence movements are those which keep both eyes pointing at the same centre of attention. In normal readers, the two eyes move "conjugately", that is to say, they track at the same speed and in the same direction. Stein and Fowler's data suggests that about one in six cases of developmental dyslexia can improve reading performance with treatment of this problem in isolation.
(e) Learning Difficulties: Many categories of learning difficulty present with an inability (amongst other things) to communicate effectively at a pragmatic level. This can be alleviated to a greater or lesser extent by training at what Williamson (1992) describes as "backchannel" skills. These include a wide variety of both vocal and nonvocal responses, such as nods, shakes, grunts, facial expressions, etc., whose function is to feed back to a speaker the extent to which his/her utterances are being understood.
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