Second: Movement in man
Movement in the human beings:
In the human beings, movement depends on the presence of 3 systems:
1- Skeletal system to support the limbs.
2- Muscular system for contraction and relaxation to move the limbs.
3- Nervous system gives the order to control the muscles movement.
The muscular system
The muscular system is a group of body muscles by which different parts of body can move.
The unit structure of the muscular system is the muscle.
The muscle consists of a muscular tissue have the ability to
contract and relax causing motion, the muscle usually known as flesh.
The number of muscles in man is about 620 muscles or more.
Functions of muscles:
1) Movement,(change in the position of certain organ in relation to the others).
2) Transportation from one place to another.
3) Movement of the blood inside the blood vessels and keep the
normal blood pressure by contraction of the smooth involuntary muscles
in the wall of the blood vessels.
4) Maintain body position in sitting or standing by the muscles of neck, trunk and lower limbs.
Structure of the muscle
The muscle consists of a large
number of units called muscle cells or muscle fibres.
Each muscle fibre (cell) consists of from 1000 to 2000 myofibrils(very small fibres)
Myofibrils arranged longitudinally and parallel to the longitudinal axis of the muscle.
Each muscle fibre (cell) contains a large number of nuclei (multinucleated).
Each muscle fibre (cell) consists of: cytoplasm (sarcoplasm), cell membrane (sarcolenima) , muscle fibres.
It consists of a living cytoplasm called sarcoplasm surrounded by cell membrane called sarcolenima.
Each group of muscle fibres are collected to form muscle bundle which surrounded by a membrane called perimycium.
Each muscle fibre consists of:
1- Alternation of dark bands (discs) and light bands that gives the muscle fibre its striated appearance.
2- The light band is called I -band and bisected by a dark line
called Z- zone and formed of a thin protein filaments called actin.
3. The dark band is called A-band and bisected by a light area
called H-zone and formed of another thick protein filaments called
myosin.
4. The distance between each successive two Z-lines is called sarcomere.
The dark bands and light bands are present in the skeletal
muscles and cardiac muscles therefore they are called striated muscles.
The dark bands and light bands are not present in the smooth muscles therefore they are called unstriated muscles.
Muscle contraction
The skeletal system, nervous system and the muscular system are three systems coordinate together to perform the body movements
a) The skeletal (bony) system:
It is suitable sites, which connect the muscles, and support the
moving limbs therefore the joints play an important role in the
movement.
b) The nervous system:
It gives the orders (nerve impulse) for muscles to contract and relax.
c) The muscular system:
It is responsible for movement.
Most of the body muscles are under the control of will ,so they are called voluntary muscles (skeletal, striated muscles).
The other muscles are not under the control of will, so they are called involuntary muscles.
Questions need answers
a) How does the muscle contract?
b) What are the role of nerve impulse and the physiology of muscle response to nervous stimulation?
c) How all these parts coordinate together?
1. In skeletal muscles, the outer surface of the muscle fibre
membrane is (+ve) positively charged while the inner surface is (-ve)
negatively charged, that form potential difference due to the difference
of the ions concentrations between outside and inside the membrane.
2. The stimulus for muscle contraction is the motor impulses that coming from the brain and spinal cord through the motor nerve
3. When the motor impulses reach the synapse, the vesicles
release the chemical transmitters (neurotransmitters) acetylcholine
through the synaptic cleft between the nerve fibre and the muscle fibre.
4. The calcium pump play an important role in the process of
releasing the chemical transmitters ( neurotransmitters ) as
acetylcholine.
4. When the neurotransmitter reaches the membrane of the muscle
fibre change its permeability so, the sodium ions pass through the
membrane causing depolarization (+ve inside and -ve outside) this leads
to muscle contraction
5. After a part of a second the potential difference of the
muscle fibre membrane returns to its normal state “repolarization” due
to the action of
Cholinesterase enzyme, which destroys acetylcholine.
6. The membrane permeability to ions returns to the resting state
it is now ready to be stimulated and respond again and so on
Mechanism of muscle contraction
The theory of sliding filaments (Huxely theory)
It is the most acceptable theory for muscle contraction .
Huxely depends on the microscopic structure of the muscle fibre
which consists of myofibrils and each myofibrils consists of thin actin
filaments and thick myosin filaments.
Huxely compare between the muscle fibre in a state of contraction and in a state of relaxation using electron microscope.
Huxely concluded that, the protein filaments slide over each
other due to the presence of transverse links extended from the myosin
filaments and attach to the actin filaments.
In presence of calcium ions and energy, the transverse links
act as hooks that pull the actin filaments from both sides towards each
other leading to muscle contraction.
Huxely theory does not explain the contraction of unstriated
smooth muscle although scientific reports suggest that the protein
filaments in smooth muscle are almost similar to that in skeletal
muscles.
Biology - chapter 1| Movement in man
The Motor Unit
The motor unit is the unit of function of the skeletal muscles
The muscle contraction is the sum of all motor units forming the muscle.
Each motor unit consists of a group of muscle fibres - and the nerve fibre supplying them.
When the nerve fibre enters the muscle, it divides into a large
number of branches, which supply a group of muscle fibres (from 5 to
100).
The neuromuscular junction
It is the point of connection between terminal branch of the nerve fibre and the motor end plate of the muscle fibre.
Repeated rapid contraction of muscle leads to its fatigue due to
the inability of the blood to supply the muscle rapidly with the oxygen
needed for respiration and energy production.
Accordingly the muscle converts glycogen to glucose which
oxidized anareobically (shortage of oxygen) to produce rapid energy
(2ATP) to allow the muscle to contract, causing the accumulation of
lactic acid.
Lactic acid causes muscle fatigue and stop moving to give the
chance for the muscle to be supplied by enough amount of oxygen to
perform aerobic cellular respiration, which produces a large amount of
enengy (38 ATP).
