FREE ESSAY ON METABOLIC, MUSCULAR AND NERVOUS SYSTEMS

METABOLIC, MUSCULAR AND NERVOUS SYSTEMS

The immediate source of energy for muscular contraction is the high-energy phosphate
compound called adenosine triphosphate (ATP). Although ATP is not the only
energy-carrying molecule in the cell, it is the most important one, and without
sufficient amounts of ATP most cells die quickly. The three main parts of an ATP molecule
are: an adenine portion, a ribose portion, and three phosphates linked together. The
formation of ATP occurs by combining adenosine diphosphate (ADP) and inorganic phosphate
(Pi). This formation requires a large amount of energy to and it is called a high-energy
bond. In order for a muscle to contract, the enzyme ATPase breaks the ATP bond and
releases energy which is used to do work. ATP is the energy produced from the breakdown
of food into a useable form of energy required by cells.
Muscle cells store limited amounts of ATP. Therefore, because muscular exercise requires
a constant supply of ATP to provide the energy needed for contraction, metabolic pathways
must exist in the cell to be able to produce ATP rapidly. Muscle cells can produce ATP by
three metabolic pathways: creatine phosphate (CP), formation of ATP, formation of ATP
through the degragation of glucose or glycogen (glycolysis), and oxidative formation of
ATP. The formation of ATP through the CP pathway or glycolysis is called anaerobic
metabolism because they do not use oxygen. Oxidative formation of ATP by the use of
oxygen is called aerobic metabolism.
As rapidly as ATP is broken down to ADP and Pi during exercise, ATP is reformed through
the CP reaction. However, muscle cells only contain small amounts of CP, so the total
amount of ATP formed through this action is limited. The combination of stored ATP and CP
is called the ATP-CP system and provides energy for muscle contraction during short-term
high-intensity exercise. CP is reformed only while you are recovering from exercise. For
this process to occur, there has to be ATP present.
A second metabolic pathway capable of producing ATP rapidly without the involvement of
oxygen is called glycolysis. Glycolysis involves the breakdown of glucose or glycogen to
form two molecules of pyruvic acid or lactic acid. Glycolysis is an anaerobic pathway
used to transfer energy from glucose to rejoin Pi to ADP. Glycolysis produces a net gain
of two molecules of ATP and two molecules of pyruvic or lactic acid. Although the end
result of glycolysis is energy producing, you must add ATP at two points at the beginning
of the pathway. In conclusion, glycolysis is the breakdown of glucose or glycogen into
pyruvic or lactic acid with the net production of two or three ATP. This depends on
whether the pathway began with glucose or glycogen. Since oxygen is not directly involved
in glycolysis, the pathway is considered anaerobic. However, in the presence of oxygen in
the mitochondria, pyruvate can participate in the aerobic production of ATP. In addition
to being an anaerobic pathway capable of producing ATP without oxygen, glycolysis is the
first step in the aerobic degragation of carbohydrates.
Although several factors serve to control glycolysis, the most important rate-limiting
enzyme in glycolysis is phosphofructokinase (PFK). PFK is located near the beginning of
glycolysis. When exercise begins, ADP/Pi levels rise and enhance PFK activity, which
serves to increase the rate of glycolysis. In contrast, at rest when cellular ATP levels
are high, PFK activity is inhibited and glycolytic activity is slowed. Further, high
cellular levels of free fatty acids also inhibit PFK activity. Similar to the control of
the ATP-CP system, regulation of PFK activity operates through negative feedback. Another
important regulating enzyme in glycolysis is phosphorylase, which is responsible for
degrading glycogen to glucose. This reaction provides the glycolytic pathway with the
necessary glucose at the origin of the pathway. At the beginning of exercise, calcium is
released from the sarcoplasmic reticulum in muscle. This rise in sarcoplasmic calcium
concentration indirectly activates phosphorylase which immediately begins to break down
glycogen to glucose for entry into glycolysis. 
In addition, phosphorylase activity is stimulated by high levels of the hormone
epinephrine. Epinephrine, released at a faster rate during heavy exercise, results in the
formation of cyclic AMP. It is cyclic AMP, not epinephrine, that directly activates
phosphorylase. Therefore, the influence of epinephrine on phosphorylase is indirect. 
It is important to emphasize the interaction of anaerobic and aerobic metabolic pathways
in the production of ATP during exercise. Although it is common to hear someone speak of
aerobic versus anaerobic exercise, in reality the energy to perform most types of
exercise comes from a combination of anaerobic/aerobic sources. The contribution of
anaerobic ATP production is greater in short-term high-intensity activities, while
aerobic metabolism is mainly found in longer activities. In conclusion, the shorter the
activity, the greater the contribution of anaerobic energy production. The longer the
activity, the greater the contribution of aerobic energy production.
Aerobic Respiration is the metabolic process that generates ATP in association with a
chemiosmotic process driven by a respiratory chain that depends on the use of oxygen as
the ultimate ele ctron acceptor. Water is the ultimate reduced end product and this
process occurs in the mitochondria where ATP is made by oxidative phosphorylation. In
mitochondria tricarborylic acid cycle activity and fatty acid oxidation provide most of
the reducing equivalents that fuel this process but reducing equivalents released by
metabolite oxidation reactions in the cytosol can be shuttled into mitochondria to supply
a small proportion of ATP needs.
The abdominal muscles help to maintain the trunk, maintain posture and compress the
contents of the abdomen. There are four different sets of abdominal muscles involved. The
first is the rectus abdominus. This is the straight muscle of the abdomen. It is medial,
and it is divided into segments laterally by connective tissue. The rectus abdominus
flexes and rotates the trunk and compresses the abdomen. 
The external obliques are the most superficial of the lateral muscles. Its fibres run
obliquely from the ribs to the linea alba. The linea alba is the midline seam of
connective tissue which binds all of the abdominal muscles. The external obliques flex
and laterally flexes the trunk, and compresses the abdomen. The internal obliques are
deep to the external obliques. The fibres run at right angles to the externals, which
increases the strength of the muscular abdominal wall. The internal obliques flex and
laterally flexes the trunk, and as well assists in compressing the abdomen. The
transversus abdominus is the deepest of the lateral muscles. Its fibers run transversely
from the ribs and top of the ox coxa to the linea alba. The only function it has is to
compress the abdomen. When performing a regular crunch exercise, you can hit all four of
the abdominal muscles discussed. There is no abdominal exercise that is better than the
rest, but it is important that you switch exercises every so often. The reason for this
is due to the fact that each exercise hits the abdomen in a different way and in order to
prevent your muscles from adapting, you must not only increase intensity, but the
exercise as well.
Muscular contraction is a complex process involving a number of cellular proteins and
energy production systems. The final result is a sliding of attin over myosin, which
causes the muscle to shorten and therefore develop tension. The process of muscular
contraction is best explained by the sliding filament theory of contraction; muscle
fibres contract by a shortening of their myofibrils, which results in a reduction of
distance from Z line to Z line. As the sarcomeres shorten in length, the A bands do not
shorten but move closer together. However, the I bands decrease in length. Filament
sliding occurs due to the action of the numerous cross-bridges extending out like arms
from myosin and attaching on the actin filament. The head of the myosin cross-bridge is
oriented in opposite directions on either end of the sarcomeres. This orientation of
cross-bridges is such that when they attach to actin on each side of the sarcomeres they
can pull the actin from each side towards the center.
The energy from contraction comes from the breakdown of ATP by the enzyme ATPase. The
breakdown of ATP to ADP and Pi and the release of energy serves to energize the myosin
cross bridges. The ATP released energy is used to cock the myosin cross-bridges, which in
turn pull the actin molecules over myosin and shortens the muscle. A single contraction
cycle, or power stroke of all the cross-bridges in a muscle would shorten the muscle by
one percent of its resting length. Since the muscles can shorten up to sixty percent of
their resting length, it is clear that the contraction cycle must be repeated over and
over again. In order for this to occur, the cross-bridges must detach from actin after
each power stroke, resume their original position and then re-attach to actin for another
power stroke.
Relaxed muscles are easily stretched which demonstrates that at rest, actin and myosin
are not attached. The regulation of a muscle contraction is a function of two proteins
called troponin and tropomyosin, which are located on the actin molecule. The actin
filament is formed from many smaller protein pub units arranged in a double row and
twisted. Tropomyosin is a thin molecule that lives in a grove between the double row of
actin. Troponin is attached directly to the tropomyosin. They work together to regulate
the attachment of the actin and myosin cross-bridges. In a relaxed muscle, tropomyosin
blocks the active sides on the actin molecule where the myosin cross-bridges must attach
in order for contraction to occur. The trigger of contraction to occur is linked to the
release of stored calcium from the sarcoplasmic reticulum. Most of this calcium is stored
within expanded portions of the sarcoplasmic reticulum. In a relaxed muscle the
concentration in the saroplasm is very low. However, when a nerve impulse arrives at the
mernomuscular junction it travels down the transverse tubules to the sarcoplasmic
reticulum and causes a release of calcium. Some of this calcium binds to troponin, which
causes a position change in tropomyosin such that the active sites on the actin are
uncovered. The energy released from the breakdown of ATP cocks the myosin cross-bridges.
This energized cross-bridge then attaches to the active sites on actin and contraction
occurs. 
Attachment of fresh ATP to the myosin cross-bridges allows the cross-bridge to detach and
re-attach to another active site on an actin molecule. This contraction cycle is repeated
as long as free calcium is available to bind the troponin and ATP is available to provide
the energy. The signal to stop contraction is the absence of the nerve impulse at the
neuromuscular junction. When this occurs, an energy requiring calcium pump located within
the sarcoplasmic reticulum begins to move the calcium back into the sarcoplasmic
reticulum. This removal of calcium from troponin causes tropomyosin to move back to cover
the binding sites on the actin molecule and cross-bridge interaction ceases.
It is possible for skeletal muscle to exert force without the joint angle changing. This
might occur when an individual pushes against the wall of a building. Muscle tension
increases bu t the wall does not move, so neither does the body part that applies to the
force. This is called an isometric contraction. Isometric contractions maintains a static
body position during periods of standing or sitting. In contrast most types of exercise
involve contractions that result in movement of body parts. This is called an isor
isotonic contraction. Tension within the muscle increases but the joint angle changes as
the body parts move.
Skeletal muscle can be divided into three types of fibers. These are: fast-twitch fibers
(fast-glycolytic), low-twitch fibers (slow oxidative), and intermediate fibers (fast
oxidative glycolytic). Fast-twitch fibers have a small number of mitochondria, a limited
capacity for aerobic metabolism, and are less resistent to fatigue than slow-twitch
fibers. However, fast-twitch fibers are rich in glycogen stores and glycolytic enzymes,
which provide them with a large anaerobic capacity. In addition, fast-twitch fibers
contain more myofibrils and ATPase than slow-twitch fibers, and are therefore able to
contract more rapidly and develop more force than the slow-twitch fibers.
Slow-twitch fibers contain larger numbers of mitochondria and are surrounded by more
capillaries than fast-twitch fibers. In addition, slow-twitch fibers contain higher
concentrations of the red pigment myoglobin. The high concentration of myoglobin, and the
high content of mitochondrial enzymes provide slow-twitch fibers with a high capacity for
aerobic metabolism and a high resistance to fatigue.
Intermediate fibers contain biochemical and fatigue characteristics that are somewhere
betweeen fast-twitch and slow-twitch fibers.
The amount of force exerted during muscular contraction is dependent on a number of
factors. These include the types and the number of motor units recruited, the initial
length of the muscle, and nature of the neural stimulation of the motor units. Variations
in the strength of contraction within an entire muscle depends on the number of muscle
fibers that are stimulated to contract. If only a few motor units are recruited, the
force is small. If more motor units are stimulated the force is increased. As the
stimulus is increased, the force of contraction is increased due to the recruitment of
additional motor units. The peak force generated by muscle decreases as the speed of
movement increases. However, the amount of power generated by a muscle group increases as
a function of movement velocity. The muscle spindle functions as a length detector in
muscle. Golgi tendon organs continuously monitor the tension developed during muscular
contraction. In essence, Golgi tendon organs serve as safety devices that help prevent
excessive force during muscle contractions.
The 206 bones of your body protect and support your organs and allow movement. Bones are
living, changing structures that require adequate calcium and weight-bearing exercise to
build and maintain their density and strength. Bones are joined together by different
types of joints: fixed joints (as in the skull), hinged joints (as in the fingers), and
ball-and-socket joints (as in the shoulders and hips). The bones function as a lever. The
bones of the upper and lower limbs push and pull, with the help of muscles. 
Bones are also a calcium store. 97% of the body's calcium is stored in bone. Here it is
easily available and turns over fast. In pregnancy the demands of the fetus for calcium
require a suitable diet and after menopause hormonal control of calcium levels are
impaired which can cause brittleness and a chance for osteoporosis to occur. In addition,
bones are a marrow holder. This is secondary to produce maximum strength for minimum
weight. The cavities produced in unstressed areas are used for marrow, or in some places
just for storage. Around the outside is a layer of strong, hard, heavy compact bone. In
the middle is a branching network of trabecular bone which usually follow lines of force.
Marrow sits in the interconnecting cavities between those plates or rods of bone.
A joint is formed by the meeting of two or more bones. A joint can allow full movement
(synovial), little movement (cartilagenous), or no movement (fibrous). With immovable
joints, bones are joined by cartilage (ex: rib meets sternum) or a series of dove tailed
edges (ex: skull). Slightly movable joints are where bones are joined by ligaments only
(ex: where tibia and fibula meet) or by ligaments and fibrous cartilage (ex: between
vertebrae). Freely movable joints are where both ends of the bone are covered with
cartilage and surounded by a fibrous capsule. This capsule is lined with smooth tissue
called synovial membrane which secretes a fluid to lubricate the joint. This type of
joint is strengthened by ligaments and is the most common type of joint. There are six
different types of freely movable joints:
1) Pivot - bone rotates on a fibrous ring;
2) Saddle - thumb joints, the articular surfaces fit together concave
to convex;
3) Condyloid - convex surfaces fit into concave, free movement, 
but no rotation (ex: wrist);
4) Gliding - vertebrae of spine, two nearly flat surfaces glide over 
each other;
5) Hinge - joint movement is in one plane (ex: elbow);
6) Ball-and-socket - the shoulder and hip joints are the only ball 
and socket joints in the body. Bone head fits into cup-like
cavity, movement is allowed in any direction. They are the
most freely movable synovial joints.
Types of movement of synovial joints:
- flexion - decreasing the angle between two bones
- extension - increasing the angle between two bones
- abduction - moving the bone away from the midline
- adduction - moving the bone towards the midline
- rotation - moving the bones around a central axis
- circumduction - complete circular movement
- elevation - raising a part of the body
- depression - lowering a part of the body
The nervous system is the body's means of perceiving and responding to events in the
internal and external environments. Receptors capable of sensing touch, pain,
temperature, and chemical stimuli send information to the central nervous system (CNS)
concerning changes in our environment. The CNS responds by either voluntary movement or a
change in the rate of release of some hormone from the endocrine system, depending on
which response is appropriate. The nervous system is divided into two major divisions,
the central nervous system and the peripheral nervous system. The central nervous system
includes the brain and the spinal cord, and the peripheral nervous system includes the
nerves outside the central nervous system.
Nerve cells are called neurons and are divided anatomically into the cell body,
dendrites, and axon. Axons are covered by schwann cells, with gaps between these cells
called nodes of ranvier. Neurons are specialized cells that respond to physical or
chemical changes in their environment. At rest, nerve cells are negatively charged in the
anterior when compared to the electrical charge outside the cell. This difference in ele
ctrical charge is called the resting membrane potential. A neuron fires due to a stimulus
changing the permeability of the membrane, allowing sodium to enter at a high rate,
depolarizing the cell. When the depolarization reaches threshold, an action potential or
nerve impulse is initiated. 
Repolarization occurs immediately following depolarization due to an increase in membrane
permeability to potassium, and a decreased permeability to sodium. Neurona communicate
with other neurons at junctions called synapsis. Synaptic transmission occurs when
sufficient amounts of a specific neurotransmitter are released from the presynaptic
neuron. Upon release, the neurotransmitter binds to a receptor on the post synaptic on
the postsynaptic membrane. An excitatory transmitter increases neuronal permeability to
sodium and results in excitatory postsynaptic potentials. However, some transmitters are
inhibitory and cause the neuron to become more negative or hyperpolarized. This
hyperpolarization of the membrane is called an inhibitory postsynaptic potential.
Propriosceptors are position receptors located in joint capsules, ligaments, and muscles.
The three most abundant joint and ligament receptors are free nerve endings, golgi-type
receptors, and pacinian corpuscles. These receptors provide the body with a conscious
means of recognition of the orientation of body parts as well as feedback relative to the
rates of limb movement. 
Reflexes provide the body with a rapid unconscious means of reacting to some stimuli. The
vestibular apparatus is responsible for maintaining general equilibrium and is located in
the inner ear. Specifically, these receptors provide information about linear and angular
acceleration.
The spinal cord plays an important role in voluntary movement due to groups of neurons
capable of controlling certain aspects of motor activity. The spinal mechanism by which a
voluntary movement is translated into appropriate muscle action is termed spinal tuning.
The brain can be divided into three parts: the brain stem, the cerebrum, and the
cerebellum. The motor cortex controls motor activity with the aid of input from
subcortical areas. The cerebellum receives feedback from proprioceptors after movement
has begun and sends information to the cortex concerning possible corrections of that
particular movement pattern. 
The basal ganglia are neurons involved in organizing complex movements and the initiation
of slow movements. The premotor cortex operates in conjunction with the motor cortex to
refine complex motor actions and may be important in acquisition of motor skills.
The autonomic nervous sytem is responsible for maintaining the constancy of the body's
internal environment and can be separated into two divisions: the sympathetic division
and the parasympathetic division. In general, the sympathetic portion tends to excite an
organ, while the parasympathetic portion tends to inhibit the same organ.