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The physiology of motor systems

9.1 Introduction

Intrauterine movement is detectable by ultrasound from a very early stage of gestation and is felt by the mother for the first time between 16 and 20 weeks ('quickening' of the fetus). By the time of birth a baby has the capability for some coordinated movement. During the first 2 years of life, as the brain and spinal cord continue to develop and mature, the child learns to defy gravity, by first sitting, then standing, and later to walk, run, jump, and climb. At the same time, the capacity to perform rhe precise movements needed for complex manipulations and speech is acquired. In short, coordinated purposeful movement is a fundamental aspect of human existence.

The simplest form of motor act controlled by the nervous system is called a reflex. This is a rapidly executed, automatic, and stereotyped response to a given stimulus. As reflexes are not under the direct control of the brain, they are described as involuntary motor acts. Nevertheless, most reflexes involve coordination between groups of muscles and this is achieved by inter­connections between various groups of spinal neurons. The neurons forming the pathway taken by the nerve impulses responsible for a reflex make up a reflex arc. Many voluntary motor acts are guided by the intrinsic properties of reflex arcs but are modified by commands from the brain and from sensory inputs.

Two kinds of voluntary motor function can be distinguished: (1) the maintenance of position (posture) and (2) goal-directed movements. They are inextricably linked in practice. A goal-directed movement will only be performed successfully if the moving limb is first correctly positioned. Similarly, a posture may only be maintained if appropriate compensatory movements are made to counteract any force tending to oppose that posture.

Physiological understanding is still insufficient to allow a full explanation of the events occurring within the central nervous system during the execution of a voluntary movement. There is no doubt that voluntary movement is impaired whenever there is an interruption of the afferent pathways to the brain arising from sense organs such as the labyrinth, eyes, proprioceptors, and mechanoreceptors. However, what happens between the arrival of afferent information and the execution of an appro­priate movement is unclear. Similarly, the processes of motor learning and the relationship between the 'will' to carry out a movement and the movement itself, remain poorly understood.

Investigation of the ways in which the CNS controls move­ment is far more difficult than the study of sensory systems. There are a number of reasons for this. First, movements them­selves are difficult to describe in a precise, quantitative manner. Secondly, experimentation on anaesthetized animals can give little useful information regarding whar is, by nature, a volun­tary process. Thirdly, the many complex moror parhways operate in parallel, making it hard to assign particular roles to each. Furthermore, every motor action resulrs in sensory feedback which may modify it further.

Despite these difficulties, it is possible to define some key questions that must be addressed if we are to make progress towards a greater understanding of motor systems:

  1. What structural components of the central and peripheral
    nervous systems participate in the maintenance of posture
    and movement of the head, trunk, and limbs?

  1. How are reflexes organized within the spinal cord?

  1. How do 'higher centers' influence the fundamental motor
    patterns contained within the spinal cord?

  2. How is information from the peripheral sense organs
    used to plan and refine both postural mechanisms and
    voluntary movements?

9.2 The hierarchical nature of motor control systems

The term motor system refers ro the neural pathways that control the sequence and pattern of muscle contractions. The structures responsible for the neural control of posture and movement are distributed throughout the brain and spinal cord, as indicated in Fig. 9.1.

As skeletal muscles can only contract in response to excitation of the motoneurons that supply them, all motor acts depend on neural circuits that eventually impinge on the ^-motoneurons that form the output of the motor system. As discussed in Chapter 7, each motoneuron supplies a number of skeleral muscle fibers and an α-motoneuron together with the skeletal muscle fibers it innervates constitute a motor unit which is the basic element of motor conrrol. For this reason α-motoneurons are often referred to as the final common pathway of the motor


9 The physiology of motor systems

Fig. 9-1 A survey of the motor systems of the body, showing possible interactions between them.

system. The processes that underlie excitation-contraction cou­pling in muscle are discussed in Chapter 7.

The motoneurons are found in the brainstem and spinal cord and rheir excitability is influenced by neural pathways that may be local circuits or that atise in a variety of brain areas. Thus there is a kind of hierarchical arrangement of so-called 'motor centers' from the spinal cord up through to the cerebral cortex. A vast array of reflexes are controlled by neural circuits within

the spinal cord and these reflex circuits form a system that organizes the basic motor patterns of posture and movement. Superimposed on these local circuits are influences from higher centets in the brain. Postural control is exerted largely at the level of the brainstem, while goal-directed movements require the participation of the cerebral cortex. The basal ganglia and cerebellum both play an important role in moror control, though neither is connected directly with the spinal motoneurons. Instead, they influence the motor cortex by way of the thalamic nuclei.

^ 9.3 Organization of the spinal cord

The α-motoneurons (or somatic motor neurons) are large neurons whose cell bodies lie in clumps (sometimes called motor nuclei) within the ventral horn of the spinal cord or in the brain­stem. Each innervates a motor unit that may consist of anything between 12 and 1500 skeletal muscle fibers. a-Motoneurons have long dendrites and receive many synaptic connections (see Fig. 9.2b). These include afferents from interneurons and pro­prioceptors as well as descending paths from higher levels of the CNS. The axons of α-motoneurons collect in bundles that leave the ventral horn and pass through the ventral white matter of the spinal cord before entering the ventral root. Some axons send off branches that turn back into the cord and make excitatory synaptic contact with small interneurons called Rensbaw cells. These cells, in turn, have short axons which synapse with the pool of motor neurons by which they are stimulated. These synapses are inhibitory and bting about recurrent, or feedback inhibition.

The ventral horn motor neurons show an orderly topo­graphical arrangement within the spinal cord, as illustrated in Fig. 9.2a. Motor neurons supplying the muscles of the trunk are situated in the medial ventral horn, while those supplying more distal muscle groups tend to be situated dorsolaterally. Muscles that flex the limbs (flexors) are under the control of neurons that lie dorsal to those that control the muscles that extend the limbs (extensors).

Fig. 9-2 (a) A transverse section of the spinal cord. The localization of motor neurons corresponding to various groups of muscles are indicated, with flexors represented more dorsally while extensors are represented ventrally within the cord. The muscles of the trunk are represented medially and the extremities are represented laterally, (b) An α-motoneuron in rhe ventral horn of the spinal cord, illustrating the elaborate dendritic tree. Although not shown in this diagram, numerous synaptic connections are made with these dendrites.

^ 9.3 Organization of the spinal cord


The spinal cord receives afferent input from proprioceptors in muscles and joints

For movements to be carried out in a functionally appropriate way, it is essential for sensory and motor information to be integrated. All the neural structures involved in the execution of movements are continually informed of the position of the body and of the progress of the movement by sensory receptors within the muscles and joints, which provide informa­tion regarding the position of our limbs and their move­ments relative to each other and to our surroundings. These mechanoreceptors are called proprioceptors and the information they provide is used to control muscle length and posture (Section 9.8).

Fig. 9-3 The basic organization of a muscle spindle. These sensory organs lie in parallel with the extrafusal muscle fibers and are therefore adapted to monitor muscle length. Note that it is innervated by both motor and sensory nerve fibers.

The main proprioceptors are the muscle spindles and Golgi tendon organs. Both provide information with regard to the state of the musculature. Muscle spindles lie within the muscles between (in parallel with) the skeletal muscle fibers and can therefore respond to muscle length and its rate of change. Golgi tendon organs lie within the tendons and are in series with the contractile elements of the muscle. They are sensitive to the force generated within that muscle during contraction.

Muscle spindles

Although muscle spindles are found in most skeletal muscles, they are particularly numerous in those muscles that are respons­ible for fine motor control, such as those of the eyes, neck, and hands. The basic organization of a muscle spindle is illustrated in Fig. 9.3. The muscle spindle consists of a small bundle of modified muscle fibers innervated both by sensory and motor neurons. The muscle fibers of the muscle spindles are called intrafusal fibers, while those of the main body of the muscle are the extrafusal fibers. The nerve endings and intrafusal fibers of the muscle spindles are enclosed within a capsule.

There are two different types of intrafusal fibers, the nuclear bag fibers and nuclear chain fibers, so-called because of the arrangement of their nuclei. They are shown diagrammatically in Fig. 9-4. Nuclear bag fibers have a cluster of nuclei near their mid-point. Nuclear chain fibers are smaller, and have a single row of nuclei near their mid-point. The central regions of both bag and chain fibers contain no myofibrils and are the most elastic parts of the fibers so that this central region stretches preferentially, when the muscle spindle is stretched.

Fig. 9.4 Nuclear bag and chain fibers of a muscle spindle, with their sensory and motor nerve supplies.

The afferent fibers of muscle spindles are of two types. Each spindle receives fibers from a primary afferent (group la fibers). These wind around the middle section of both bag and chain fibers forming so-called annulospiral endings (or primary sensory endings). Many muscle spindles are also innervated by one or


9 The physiology of motor systems

Fig. 9-5 Responses of primary (la) and secondary (II) muscle spindle afferent fibers to muscle stretch. Note the very intense period of activity of the primary ending during the stretch.

more afferent fibers of group II (see Chapter 6 for the classification of nerve fibers). They terminate more peripherally than the primary endings, almost exclusively on the nuclear chain fibers. They are known as secondary sensory endings (or flower-spray endings because of their multiple branched nature).

The two types of afferent nerve endings respond to muscle stretch in different ways. The rate of firing of both kinds is pro­portional to the degree of stretch of the muscle spindle at any moment. Although the primary (la) fibers respond in proportion to the degree of stretch, they are much more sensitive to rapid changes in muscle length, for this reason they are classed as rapidly adapting or dynamic endings. The secondary endings are nonadapting and are said to be static endings. The differing nature of the responses to stretch of the primary and secondary endings is illustrated in Fig. 9-5.

The motor neurons innervating the intrafusal fibers are known as γ-motoneurons to distinguish them from the large α-motoneurons which innervate the extrafusal fibers. The cell bodies of the y-motoneurons lie in the ventral horn of the spinal cord and their axons, which are also known as fusimotor fibers, leave the spinal cord via the ventral roots. The fusimotor fibers have diameters in the range 3—6 μ-m and conduction velocities of 15—30ms-1. The α-motoneurons have large-diameter fibers which range in size between 15 and 20 pim and have conduction velocities of 70—120 m s_1 (see Table 6.2).

Within the muscle, the fusimotor fibers branch to supply several muscle spindles and, within these, branch further to supply several intrafusal fibers. The γ-motoneurons innervate both the nuclear chain and nuclear bag fibers and bring about contraction of the peripheral regions of the muscle spindles. Note, however, that contraction of the intrafusal fibers is too weak to affect movements of the muscle as a whole, rather the tension in the intrafusal fibers regulates the sensitivity of the spindles. When the intrafusal fibers contract in response to fusimotor stimulation, their sensory endings are stimulated by the stretch. The excitation of the la fibers that results is super­imposed on the firing that results from the degree of stretch imposed by the extrafusal muscle fibers.

Fig. 9-6 The Golgi tendon organ, (a) The basic organization of a Golgi tendon organ, (b) The response of a Golgi tendon organ to tension in the muscle. The upper trace shows the tension in the muscle to which the tendon organ is attached and the lower trace shows the firing pattern in the Ib fiber supplying the receptor. Note that the Golgi tendon organ lies in series with the muscle and so is adapted to monitor muscle tension.

To sum up, muscle spindles may be stimulated in two ways:

  1. by stretching the entire skeletal muscle; or

  2. by causing the intrafusal fibers to contract while the
    extrafusal muscle fibers remain at the same length.

In either case, stretching a muscle spindle will increase the rate of discharge of the group la and group II afferent nerve fibers to which it is connected.

Golgi tendon organs

The Golgi tendon organs are mechanoreceptors that lie within the tendons of muscles immediately beyond their attachments to the muscle fibers (Fig. 9.6). Around 10 or 15 muscle fibers are usually connected in series to each Golgi tendon organ, which is then stimulated by the tension produced by this bundle of fibers. Impulses are carried from the tendon organs to the central nervous system (particularly the spinal cord and the cerebellum) by group Ib afferent fibers.

9.4 Reflex action and reflex arcs



  1. The basic element of motor control is the motor unic. The cell bodies
    of α-raotoneurons are topographically arranged within the ventral
    horn of the spinal cord. Their axons innervate skeletal muscle fibers.
    The cell bodies receive numerous synaptic connections from pro­
    prioceptors and from higher levels of the CNS, including the
    brainstem, basal ganglia, cerebellum and motor cortex.

  2. Proprioceptors are mechanoreceptors situated within muscles and
    joints. They provide the CNS with information regarding muscle
    length, position, and tension (force).

3- Muscle spindles lie in parallel with extrafusal muscle fibers. They are innervated by γ-motoneutons (efferents) and by group la and group II afferent fibers. The afferents respond to muscle stretch while γ-efferent activity regulates the sensitivity of the spindles.

4. Golgi tendon organs respond to the degree of tension within the muscle. Group Ib afferent fibers relay this information to the CNS (in particular the spinal cord and cerebellum).

result, the group la afferents arising in the muscle spindles within the quadriceps increase their rate of firing to com­municate the stretch to the spinal cord. As they enter the spinal cord, the afferent fibers branch, some enter the gray matter of the cord and make monosynaptic contact with the α-motoneurons supplying the quadriceps muscle, causing them to discharge in synchrony. The resulting contraction of this muscle abruptly extends the lower leg (hence the name 'knee jerk'). Collaterals of the la fibers make synaptic contact with inhibitory interneurons which in turn inhibit the antagonistic (flexor) muscles of the knee joint.

The stretch reflex arc is illustrated diagrammatically in Fig. 9.7a. The reflex is lost if the lower lumbar dorsal roots of the spinal cord (through which the afferents from the quadriceps pass) are damaged. A similar reflex occurs when the Achilles tendon is struck (the ankle-jerk reflex). In this case there is a plantar flexion of the foot produced by contraction of the calf muscles.

^ 9.4 Reflex action and reflex arcs

Reflexes represent the simplest form of irritability associated with the nervous system. Reflex arcs include at least two neurons, an afferent, or sensory, neuron and an efferent, or motor, neuron. The fiber of the afferent neuron carries information about the environment from a receptor towards the central nervous system, while the efferent fiber transmits nerve impulses from the central nervous system to an effector. Reflexes may be (and often are) subject to modulation by activity in the central nervous system.

In a simple reflex arc there are two neurons and just one synapse. Such reflexes are therefore known as monosynaptic reflexes. Other reflex arcs have one or more neurons interposed between the afferent and efferent neurons. These neurons are called mterneurons or internuncial neurons. If there is one interneuron, the reflex arc will have rwo synaptic relays and the reflex is called a disynaptic reflex. If there are two interneurons, there will be three synaptic relays so that the associated reflex would be tri-synaptic. If many interneurons are involved, the reflex would be called & polysynaptic reflex. Examples are the stretch reflex (mono­synaptic), the withdrawal reflex (disynaptic), and the scratch reflex (polysynaptic).

The knee jerk is an example of a dynamic stretch reflex

A classic example of a stretch reflex (also known as the myotactic reflex) is the knee-jerk or tendon-tap reflex, which is used routinely in clinical neurophysiology as a tool for the diagnosis of certain neurological conditions. Hinge joints such as the knee and ankle are extended and flexed by extensor and flexor muscles which act in an antagonistic manner. A sharp tap applied to the patellar tendon stretches the quadriceps muscle. The stretch stimulates the 'dynamic' nuclear bag receptors of the muscle spindles. As a

Fig. 9.7 (a) The stretch reflex arc. Note that this reflex arc comprises only two neurons and one synapse. It is therefore a monosynaptic reflex, (b) The basic flexor (withdrawal) reflex arc. In this case there are three neurons and two synapses in the basic arc. The reflex in its simplest form is therefore disynaptic.

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