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من طرف KASIR السبت أكتوبر 18, 2008 7:29 pmMuscular System
Motion
Heat production
Maintenance of posture
Muscle tissue has four main properties: Excitability (ability to respond to stimuli), Contractibility (ability to contract), Extensibility (ability of a muscle to be stretched without tearing) and Elasticity (ability to return to its normal shape).
Through contraction, the muscular system performs three important functions:
Through contraction, the muscular system performs three important functions:
- Motion - walking, running etc.
- Heat production - maintain normal body temperature
- Maintenance of posture - standing, sitting etc.
Motion
To understand how the muscles combine with the skeleton in providing motion we must look at the basic mechanics of movement. The main framework of the body is covered by muscle, whose function is to permit movement. We know that to move or lift a load against another force, it is easier to use levers, and it is this principle which the musculoskeletal system adopts and which we must examine.
The component parts that are used in a lever are as follows:
The component parts that are used in a lever are as follows:
- Lever - nearly always the bone
- Fulcrum - pivot point of the lever, which is usually the joint
- Muscle Force - force that draws the opposite ends of the muscles together
- Resistive Force - force generated by a factor external to the body (e.g. gravity, friction etc.) that acts against muscle force
- Torque - the degree to which a force tends to rotate an object about a specified fulcrum
There are different types of levers dependent upon the position of fulcrum, effort and resistive force.
First Class lever: Muscle force and resistive force is on different sides of the fulcrum e.g. the head resting on the vertebral column. As the head is raised, the facial portion of the skull is the resistance, the fulcrum is between the atlas and occipital bone, and the effort is the contraction of the muscles of the back.
Second Class lever: Muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through the level longer than that through which the resistive force acts - e.g. raising the body up onto the toes. The body is the resistance, the ball of the foot is the fulcrum, and the effort is the contraction of the calf muscle.
Third Class lever: Muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through the lever shorter than that through which the resistive force acts - e.g. adduction of the thigh. The weight of the thigh is the resistance, the hip joint is the fulcrum, and the contraction of the adductor muscle is the effort.
Most of the limbs of the human body are articulated by third class levers.
First Class lever: Muscle force and resistive force is on different sides of the fulcrum e.g. the head resting on the vertebral column. As the head is raised, the facial portion of the skull is the resistance, the fulcrum is between the atlas and occipital bone, and the effort is the contraction of the muscles of the back.
Second Class lever: Muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through the level longer than that through which the resistive force acts - e.g. raising the body up onto the toes. The body is the resistance, the ball of the foot is the fulcrum, and the effort is the contraction of the calf muscle.
Third Class lever: Muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through the lever shorter than that through which the resistive force acts - e.g. adduction of the thigh. The weight of the thigh is the resistance, the hip joint is the fulcrum, and the contraction of the adductor muscle is the effort.
Most of the limbs of the human body are articulated by third class levers.
Heat production
Muscle contractions produce heat and as much as 70% of body heat is produced by energy produced in muscle tissue. Blood is an essential element in temperature control during exercise, taking heat from the body core and working muscles and redirecting it to the skin when the body is overheating. When the internal heat of the body reaches too low a level thermoreceptors in the skin relay a message to the hypothalamus in the brain. In response to this signal, the skeletal muscles contract and relax in an involuntary manner (shivering) increasing muscle activity to generate heat. In turn, muscles are also responsive to exterior heat - cold air increases muscle tone, and hot conditions have a relaxing effect on muscles.
Maintenance of posture
As well as enabling movement, muscles also maintain posture and body position. Sensory receptors in the muscles monitor the tension and length of the muscles and provide the nervous system with crucial information about the position of the body parts, therefore enabling posture to be maintained. Muscles are never completely at rest, nor do they actually have to shorten in length when they contract. The tension or tone produced as a result of these contractions between various opposing groups of muscle helps us remain in a static position, even when we are asleep.
KASIR
