Download CHAPTER 2 THE NEUROMUSCULAR SYSTEM

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
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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
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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
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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
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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
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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
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FIGURE 2-6 NEUROMUSCULAR JUNCTION
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Muscle action potential
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27
FIGURE 2-7 THE MAJOR ELEMENTS OF NEURO MUSCULAR CONTROL LOOPS
28
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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
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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.