Biology chapter 1 Movement in man

Biology - chapter 1| Movement in man
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).