Acknowledgments
6.1 INTRODUCTION
6.1.3 Electromyography
The electrical signal associated with the contrac-tion of a muscle is called an electromyogram or EMG. Electromyography, which is the study of EMG, has revealed some basic information. Vol-untary muscular activity results in an EMG that increases in magnitude with tension. However, other variables influencing the signal at any given time are velocity of shortening or length-ening of the muscle, rate of tension buildup, fatigue, and reflex activity.
Muscle tissue conducts electrical potentials somewhat similarly to axons of the nervous sys-tem. Motor unit action potential (m.u.a.p.) is an electrical signal generated in the muscle fibers because of the recruitment of fibers as the motor unit. Electrodes placed on the surface of a muscle or inside the muscle tissue will record the alge-braic sum of all m.u.a.p.’s being transmitted along the muscle fibers at that point in time.
Those motor units away from the electrode site
will result in a smaller m.u.a.p. than those of similar size near the electrode.
For a given muscle there can be a variable number of motor units, each controlled by a motor neuron through special synaptic junctions called motor end plates. An action potential trans-mitted down the motor neuron arrives at the motor end plate and triggers a sequence of elec-trochemical events. A quantum of acetylcholine (ACh) is released. It then crosses the synaptic gap (200–500 Å wide) and causes a depolarization of the postsynaptic membrane. Such a depolariza-tion can be recorded by a suitable microelectrode and is called an end plate potential (EPP). In nor-mal circumstances, the EPP is large enough to reach a threshold level and an action potential is initiated in the adjacent muscle fiber membrane.
The beginning of the m.u.a.p. starts at the Z-disc of the contractile element by means of an inward spread of the stimulus along the trans-verse tubular system. This results in a release of Ca2+ in the SR. Ca2+ rapidly diffuses to the con-tractile filaments of actin and myosin where ATP is hydrolyzed to produce ADP plus heat plus mechanical energy (tension). The mechani-cal energy manifests itself as an impulsive force at the cross-bridges of the contractile element.
The depolarization of the transverse tubular system and the SR results in a depolarization wave along the direction of the muscle fibers. It is this depolarization wave front and the subse-quent repolarization wave that are seen by the recording electrodes.
Two general types of EMG electrodes have been developed. Surface electrodes consist of disks of metal, usually silver/silver chloride, of about 1 cm in diameter. These electrodes detect the average activity of superficial muscles and give more reproducible results than do in-dwelling types. In-in-dwelling electrodes are required, however, for the assessment of fine movements or to record from deep muscles.
A needle electrode is a fine hypodermic needle with an insulated conductor located inside and bared to the muscle tissue at the open end of
the needle. The needle itself forms the other conductor.
In-dwelling electrodes are influenced by both waves that actually pass by their conducting surface and by waves that pass within a few millimeters of the bare conductor. The same is true for surface electrodes.
ATP is an important molecule for the life of living cells. It provides energy for various cellular activities such as muscular contraction, movement of chromosomes during cell division, movement of cytoplasm within cells, transporting substances across cell membranes, and putting together larger molecules from smaller ones during syn-thetic reactions. Structurally, ATP consist of three phosphate groups attached to an adenosine unit composed of adenine and five-carbon sugar ribose.
ATP is the energy reserve of living systems.
When a reaction requires energy, ATP can trans-fer just the right amount, because it contains two high-energy phosphate bonds. When the termi-nal phosphate group P is hydrolyzed by addition of a water molecule, the reaction releases energy.
This energy is used by the cell to power its activi-ties. The resulting molecule, after removal of the terminal phosphate groups, is ADP. This reaction may be represented as follows:
The energy supplied by the catabolism of ATP into ADP is constantly being used by the cell. Since the supply of ATP at any given time is limited, a mechanism exists to replenish it.
A phosphate group is added to ADP to manu-facture more ATP. The reaction may be repre-sented as follows:
The energy required to attach phosphate groups to ADP to make ATP is provided by breakdown of glucose in the cellular respiration process, which has two phases:
1. Anaerobic. In the absence of oxygen, glucose is partially broken down by the glycolysis
(6.1) ATP→ADP+P+Energy.
(6.2) ADP+P+Energy→ATP.
6.1 INTRODUCTION 143
process into pyruvic acid. Each glucose that is converted into a pyruvic acid molecule yields two molecules of ATP.
2. Aerobic. In the presence of oxygen, glucose is completely broken down into carbon dioxide and water. These reactions generate heat and ATP molecules from each glucose molecule.
A muscle fiber is about 100 µm in diameter and consists of fibrils about 1 µm in diameter.
Fibrils in turn consist of filaments about 100 Å in diameter. These further are of smaller units of molecular chains called actin, myosin, and elastic elements. Electron micrographs of fibrils show the basic mechanical structure of the interacting actin and myosin filaments. The darker and wider myosin protein bands are interlaced with the lighter and smaller actin protein bands, as seen in electron micrographs. The space between them consists of a cross-bridge structure where the tension is created and elongation/contrac-tion takes place. The term contractile element is used to describe the part of the muscle that gen-erates the tension, and it is this part that shortens and lengthens as positive or negative work is done. The sarcomere, which is a basic length of the myofibril, is the distance between the Z-discs. It can vary from 1.5 µm at full shortening to 2.5 µm at resting length to about 4 µm at full lengthening.
The structure of the muscle is such that many filaments are in parallel and many sarcomere elements are in series to make up a single con-tractile element. Consider a motor unit of a cross-sectional area of 0.1 cm2 and a resting length of 10 cm. The number of sarcomere con-tractile elements in series would be 10 cm/2.5 µm = 40,000 and the number of filaments (each with an area of 10−8 cm2) in parallel would be 0.1/10−8= 107. Thus the number of contractile elements of sarcomere length packed into this motor unit would be 4 x 1011.
The active contractile elements are contained within the fascia. These tissue sheaths enclose the muscles, separating them into layers and
groups and ultimately connecting them to the tendons at either end. The mechanical character-istics of connective tissue are important in the overall biomechanics of the muscle. Some of the connective tissue is in series with the con-tractile element; some is in parallel. These tissues are modeled as springs and viscous dampers for modeling purposes.
Each muscle has a finite number of motor units (motor neuron plus muscle fibers it inner-vates), each of which is controlled individually by a separate nerve ending. Excitation of each unit is an all-or-none event. The electrical indi-cation is a motor unit action potential with the mechanical result being a tension twitch. An increase in tension can be accomplished in two ways: by increasing the stimulation rate for the motor unit or by the excitation (recruitment) of an additional motor unit.
It is now generally accepted that the motor units are recruited according to the size princi-ple, which states that the size of the newly recruited motor unit increases with the tension level at which it is recruited. This means that the smallest unit is recruited first and the largest unit last. In this manner, low-tension movements can be achieved in finely graded steps. Conversely, those movements requiring high forces but not needing fine control are accomplished by recruit-ing the larger motor units.
Successive recruitment can be described as follows: The smallest motor unit (MU-1) is recruited first, usually at an initial frequency ranging from about 5–13 Hz. Tension increases as MU-1 fires more rapidly until a certain ten-sion is reached, at which MU-2 is recruited.
Here MU-2 starts firing at its initial low rate, and further tension is achieved by the increased firing of both MU-1 and 2. At a certain tension, MU-1 reaches its maximum firing range (15–60 Hz) and therefore generates its maxi-mum tension. This process of increasing tension reaching new thresholds and recruit-ing another larger motor unit continues until maximum voluntary contraction is reached.
At that point, all motor units will be firing at their maximum frequencies. For a detailed discussion of mammalian muscles, the reader is refered to Bobet and Stein [1] and Ding et al.
[2, 3].
In the following section we present a brief review of electroactive polymers (EAP) as artifi-cial muscles, in general.