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CHAPTER 2 THE NEUROMUSCULAR SYSTEM 2.1 INTRODUCTION Voluntary movement in human body is made possible by the co-ordinated functioning of the Central Nervous System, the muscles and the bones. In normal people the brain controls the voluntary movement of the extremities. The structure and the organisation of muscles, and the mechanism of voluntary control over these muscles are the subject matter of this chapter. 2.2 THE TYPES OF MUSCLE CELLS In human body, three different types of muscle cells can be identified on the basis of structure and contractile properties. They are 1. Smooth muscle 2. Cardiac muscle and 3. Skeletal muscle. The Smooth muscles surround such hollow chambers as the stomach, intestinal tract, the urinary bladder etc and the Cardiac muscle is the muscle of the heart. Both these types of muscles are not normally under direct conscious control. Skeletal muscles as the name implies, are attached to the bones of the body and are under voluntary control. 23 THE MAJOR ELEMENTS IN THE NEUROMUSCULAR SYSTEM The elements that are involved in the control of voluntary movement are the skeletal muscles and the neural control loops consisting of the Central Nervous System and the peripheral nerves. These are explained below (AJ.Vander etal 1975). 23.1 The Skeletal Muscle Skeletal muscle is the largest tissue in the body, accounting for 40 to 45 % of the total body weight. Each muscle cell is cylindrical having a diameter of 10 to 100 micro-meters and may be upto 1 foot long. A single muscle cell is known as a muscle fiber. A muscle is a number of muscle fibers bound together by connective tissue. Surrounding the individual muscle fibers is a network of connective tissues through which blood vessels and nerve fibers pass to the muscle fibers. Generally, each end of the whole muscle is attached to bone by a bundle of collagen fibers known as tendons. The forces generated by the contracting muscles is transmitted by the connective tissues and tendons to the bones. A muscle fiber is composed of a number of independent cylindrical elements in the cytoplasm of the fiber known as myo-fibrils. Each myo-fibril is about 1 to 2 micrometer in diameter and continues through the length of the muscle fiber. They consist of smaller myo-filaments which form a regular repeating pattern along the length of the muscle fiber. One unit of this repeating pattern is known as sarcomere. It is the functional unit of the contractile system in muscle, and the events occurring in the sarcomere are duplicated in the other sarcomeres along the myo-fibrils. Each sarcomere contains two types of myo-filaments, the thick and the thin as shown in Figure 2.1. The thick filaments are located in the central region of the sarcomere. The orderly parallel arrangement gives rise to the dark bands, known as A bands. These thick filaments contain the protein known as myosin. The thin filaments contain the protein actin and are attached to either end of the sarcomere to a structure known as the Z-line. Two successive Z lines define the limits of one sarcomere. The Z lines consist of short elements which inter-connect the thin 1 7 MUSCLE MUSCLE FIBER ! ZONE LINE BAN D BAND 4 - >Z 2 J SARCO»|ERE ^ MYOFIBRIL \ «*»*»%*«**1... .. / sit ACTIN THIN FILAMENT MYOSIN THICK FI LAMENT FIGURE 2.1 THE MUSCLE STRUCTURE filaments from the adjoining sarcomeres and thus provide an anchoring point for the thin filaments. The thin filaments extend from the Z lines towards the centre of the sarcomere where they overlap on the thick filaments. The I band represents the region between the ends of the A bands of two adjoining sarcomeres. This band contains that portion of the thin filaments which do not overlap with the thick filaments and is bisected by the Z line. The changes in the sarcomere structure found at different muscle lengths is shown in Figure 2.2. As the muscle becomes shorter, the thick and thin filaments slide past each other, but the lengths of the individual thick and thin filaments do not change. The width of A band remains constant, corresponding to the constant length of the thick filaments. The I band narrows as the thick filaments approach the Z line. As the thin filaments move past the thick filaments, the width of the H-zone, between the ends of the thin filaments become smaller and may disappear altogether when the thin filaments meet at the centre of the sarcomere. With further shortening, new banding patterns appear as their filaments from opposite ends of the sarcomere begin to overlap. These changes in banding pattern during contraction is called sliding- filament theory of muscle contraction. It has been found that Calcium promotes the contraction and Magnesium inhibits the same. 23.2 The Neural Control Mechanism Muscle contraction is enabled by stimulation of muscle fibers by impulses arriving from the central nervous system to the extremities. The elements involved in it and the control mechanism are explained below. The basic unit of the Nervous System is the individual nerve cell, the Neuron. About 10% or so of the cells in the nervous system are the neurons, the remaining are the glial cells, which probably sustain the neurons metabolically and support them physically. The brain and the spinal cord, together form the Central Nervous System. 1 9 Thin ------ 1 myofilament Thick myofilament FIGURE 2-2 CHANGES IN BANDING PATTERN RESULTING FROM THE MOVEMENTS OF THICK AND THIN FILAMENTS PAST EACH OTHER DURING CONTRACTION 20 The neuron can be divided structurally into three parts, each associated with a particular function. (1) The dendrites and the cell body, (2) The axon and (3) The axon terminals. The dendrites form a series of highly branched cell outgrowths connected to the cell body and may be looked upon as an extension of the cell membrane of the neuron cell body. The dendrites and cell body are the site of most of the specialised functions (Figure 2.3). The cell body also contains the nucleus and is responsible for maintaining the metabolism of the neuron and for its growth and its repair. The neurons can be divided into three classes: afferent, efferent and inter-neurons. Afferent and efferent neurones lie largely outside the skull or vertebral column; and inter-neurons lie within the Central Nervous System. At their peripheral endings afferent neurons have receptors, which, in response to various physical or chemical changes in their environment, cause electric potentials to be generated in the afferent neuron. The afferent neurons carry information from the receptors into the brain or the spinal cord. Efferent neurons transmit the final integrated information from the Central Nervous System out to the muscles or the glands. Efferent neurons which innervate the skeletal muscles are called motor neurons. The third group of nerve cells, the inter-neurons both originate and terminate within Central Nervous System. The inter-neurons and their connections in large part account for thoughts, feelings, learning etc. 233 The Resting and Action Potentials It has been found that all cells of the body exhibit a membrane potential oriented such that the inside of the cell is negative with respect to the outside. This potential is called the Resting Potential and is about -70 mv for a neuron. During periods when nerve and muscle cells appear to be physiologically active, the membrane potential undergoes rapid alteration, suddenly changing from -70 to 30 millivolts and then rapidly returning to its original value. This rapid change of membrane potential which may last about a millisecond is called an 21 FIGURE 2*3 A NEURON 22 Action Potential. Of all types of cells in the body only nerve and muscle cells are capable of producing Action Potentials (Figure 2.4). Such excitable membranes besides generating action potentials are able to transmit them along their surfaces. Thus the Action Potential is the signal which is transmitted from one part of the nerve or muscle cell to another. An Action Potential triggers, by local current flow, a new one at an adjacent area of membrane. The old Action Potential provides the electric stimulus that depolarises the new membrane site to just past its threshold potential. Normally an Action Potential in a nerve or muscle fiber travels along the fiber at speed characteristic of the fiber type. The larger the fiber diameter, the faster is the Action Potential propagation, because a large fiber offers less resistance to local current flow. Myelinization is the second factor influencing the propagation velocity. Myelin is a fatty covering present around most membranes. Myelin electrically insulates the membrane; making it more difficult for current to flow between intra and extracellular fluid compartments. The Action Potential occur only where the myelin coating is interrupted (called the nodes of Ranvier) and the membrane is exposed to the extracellular fluid. Thus the Action Potential appears to jump from one node to next as it propagates along the myelinated fiber, and for this reason this method of propagation is called saltatory conduction. The membrane of the nodes adjacent to the active nodes is brought to threshold faster and undergoes an Action Potential sooner than if myelin were not present Measured speeds range from a few centimeters per second in the slowest nerve and muscle fibers to 100 m/sec in fast fibers. As stimulus amplitude in neuron is increased from zero, at constant duration, no Action Potential is seen so long as amplitude remains below a critical value, the threshold. Above this value, an Action Potential is seen, and its amplitude is independent of stimulus strength. The Action Potential is therefore referred to in physiological terms as "all or none", since it is either obtained at full amplitude, or not at all. Membrane potential (m V ) 23 Time (msec) FIGURE 2-4 CHANGES IN MEMBRANE POTENTIAL DURING AN ACTION POTENTIAL 24 23.4 Synapses A synapse is an anatomically specialized junction between two neurons where the electrical activity in one neuron influences the excitability of the second. Most synapses occur between an axon terminal of one neuron and the cell body or dendrites of a second. The neurons conducting information toward synapses are called pre-synaptic neurons and those conducting information away are called post synaptic neurons. Figure 2.5 shows how in a multi-neuronal pathway, a single neuron can be postsynaptic to one group of cells and, at the same time, presynaptic to another. Postsynaptic neuron has thousands of synaptic junctions on the surface of its dendrites or cell body so that information from hundreds of presynaptic nerve cells converges upon it A single motor neuron in the spinal cord probably receives some 15,000 synaptic endings. Each activated synapse produces a small electric signal, either excitatory or inhibitory. If the postsynaptic neuron reaches threshold and generates a response, Action Potentials are transmitted out along its axon to the terminal branches, which diverge to influence the excitability of many other cells. In this manner, postsynaptic neurons function as neural integrators, their output reflects sum of all incoming bits of information arriving in the form of excitatory and inhibitory synaptic inputs. 23.5 The Motor-End-Plates Skeletal muscles are excited by stimulation through nerve fibers. The axonal process of a nerve fiber forms a junction with a skeletal muscle membrane known as neuromuscular junctions. The nerve cells which form myo- neural junctions with skeletal muscles are known as motor neurons and the cell bodies of these neurons are located in the brain and spinal cord (Arthur C.Guton et al. 1977). FIGURE 2-5 SYNAPSES 26 As the motor neuron approaches the muscle, it divides into many branches, each of which forms single myo- neural junction with a muscle fiber. The combination of the motor neuron and the muscle fibers it innervates is known as a motor unit. The region of the muscle membrane which lies directly under terminal portion of the axon has special properties and is known as motor-end-plate. 2.4 THE MUSCLE EXCITATION The terminal ends of motor axon contain membrane- bound vesicles resembling the synaptic vesicles found at the synaptic junctions (Figure 2.6). These vesicles contain the chemical transmitter Acetylcholine (ACh). When an Action Potential in the motor axon arrives at the myo- neural junction, it depolarizes the nerve membrane and releases Acetylcholine into the space separating the nerve and muscle membranes. The Acetylcholine diffuses across the extra cellular cleft between the nerve and muscle membrane and combines with receptor sites on the motor-end-plate membrane; depolarizing the end-plate which results in the muscle excitation. The motor-end-plate membranes also contain the enzyme Acetylcholine-esterase which destroys Ach. The molecules of Ach released from the motor-neuron-endings have a life time of only about 5 milli secs before they are destroyed by this enzyme. Once Ach is destroyed, the muscle membrane returns to its Resting Potential. 2.5 THE MAJOR ELEMENTS OF NEUROMUSCULAR CONTROL LOOPS A fundamental property of skeletal muscle is that it is capable of producing active force only through contraction. To get bidirectional movement, muscles must be arranged in antagonistic pairs in which the opposing forces are controlled in the neural system by relatively precise timing relationship that stimulate contraction. Figure 2.7 schematically shows the important elements of the reflex loops which govern the activity of muscle (J.H.U.Brown et al. 1973). The signals to 4 4 V IU -V S' Se — fix 9 8 Vesicles containing acetylcholine Nerve action ] ] Acetylcholine > /J T r 1 ; ^ depolarized — FIGURE 2-6 NEUROMUSCULAR JUNCTION A r a tv . muscle membrane flow between end plate and Muscle action potential obogo*p° q,9qOooq 6 o c0 ^ °c Nerve axon ■Jivy unj yoiow its 6u . t*U f,U J W. f U JLT ---^----T C. ) y t (f trelea U . Md J *v t u f u r . ~ Current Acetylcholine esterase Acetylcholine bindinq site " Muscle action potential 27 FIGURE 2-7 THE MAJOR ELEMENTS OF NEURO MUSCULAR CONTROL LOOPS 28 29 contract are directed to the muscle from the Alpha-motor-neuron located in the ventral horn of the grey matter of the spinal cord. To achieve delicate movement there are feedback loops to notify the spinal cord and brain about muscle length (spindle receptors), muscle tension (Golgi organs), joint position and general orientation (skin receptors and visual feedback). Voluntary control of muscle is obtained from the cortex and relayed to the muscle over the Alpha and Gamma motor neurons which innervate the interfusal muscle fibers where the appropriate receptor is located. The extra-fusal muscle fibers are the primary units of contraction which produce the active force required to perform movement. An Alpha-motor-neuron sends its axon to several muscle fibers. All fibers activated by the same axon form a motor unit. If a single neural pulse is travelling along the axon, it branches off to different muscle fibers, releases Acetylcholine at the motor end plate, depolarizes the muscle fiber membrane, which results in the contraction of muscle fiber. Among the force producing extra-fusal muscle fibers lies a group of fibers called Muscle Spindles whose contribution to overall muscle force is negligible but whose capability to sense muscle length is of primary importance. The sensors for length are the Annulo-spiral and Flower spray endings, which are located in the equatorial part of the interfusal fibers. They relay signals to the spinal cord through la and II type nerve fibers. In addition to afferent innervation the spindle receives two types of efferent fibers; the T-plate and T-trail fibers. The la and II type fibers increase their firing frequency if the muscle gets extended or if the firing increases, since this causes the contraction of the polar region of the Intra-fusal fiber. The Golgi- Tendon organs are very sensitive to muscle contraction. Endings of afferent nerve fibers are wrapped around collagen bundles of the tendon, which are slightly bowed in the resting state. When the skeleto-motor fibers of the attached muscle contract, they pull on the tendon, straightening the collagen bundles and distorting the receptor endings of the 30 afferent nerves. The receptors fire in relation to the increasing force or tension generated by a contracting muscle. Then activity results in the initiation of inhibitory post-synaptic potentials in the motor neurons of the contracting muscles. Some of the Golgi-Tendon organs have high thresholds and respond only when the tension is very high. These high threshold receptors function as safety valves, inhibiting the action when the force it generates is great enough to damage the limb. 2.6 PARALYSIS A muscle or motor unit is paralyzed if its neural connection to the brain is interrupted. A disconnection in the motor neuron is called a lower motor neuron lesion; an analogous disconnection higher in the spinal cord or brain is named upper motor neuron lesion. In both the cases, the contractibility of the muscle is preserved, but after a period of disuse the muscle atrophies. However, atrophy is much delayed in the case of upper motor neuron lesion (Ruch, T.C et al. 1960). Surface EMG signals are the electrical potentials, appearing over the surface of the skin lying over the concerned muscle groups, when the muscles contract It is picked up using special silver-silver chloride electrodes interfaced with proper electrode jelly and placed firmly on the skin surface. The next two chapters describe some studies made on the properties of the EMG.