Function of the Muscular System

Characteristics of Muscles Four common: Contractibility Excitability or irritability Extensibility Elasticity Collectively produce a veritable mechanical device capable of complex, intricate movements

Contractibility Quality possessed by no other body tissue; muscle become shorter and thicker causing muscle movement When a muscle shortens or contracts, it reduces the distance b/t the parts of its contents or the space it surrounds Contraction of muscles that connect a pair of bones brings the attachment points closer together, thus causing the bone to move When cardiac muscles contract, they reduce the area in the heart chambers, pumping blood from the heart into the blood vessels Smooth muscles surround blood vessels and the intestines, causing the diameter of these tubes to decrease upon contraction

Excitability or Irritability Characteristic of both muscle and nervous cells (neurons) Ability to respond to certain stimuli by producing electric signals called action potentials (impulses)

Extensibility Ability to be stretched When we bend our forearm, the muscles on the back of it are extended or stretched

Elasticity Ability to return to original length when relaxing

Muscle Attachments and Functions A muscle must be able to exert its force upon a moveable object Must be attached to bones for leverage in order to have something to pull against MUSCLES ONLY PULL, NEVER PUSH Muscles are attached to the bones of the skeleton by nonelastic cords called tendons Bones are connected at __________. pg 125

Attachments (figure 7-4 pg 126) Muscles are attached at both ends Attachment may be to bones, cartilage, ligaments, tendons, skin, and sometimes to each other Origin: part of a skeletal muscle that is attached to a fixed structure or bone; it moves least during muscle contraction Insertion: other end, attached to a movable part; moves the most during a muscle contraction Belly: central body of the muscle

continued Muscles of the body are arranged in pairs Primer mover: produces movement in a single direction Antagonist: produces movement in the opposite direction Known as an antagonist pair

Upper Arm (figure 7-4 pg 126) Upper arm muscles are arranged in antagonist pairs The muscle located on the front part of the upper arm is the biceps One end of the biceps is attached to the scapula and humerus (its origin) When the biceps contract, these 2 bones remain stationary The opposite end of the biceps is attached to the radius (its insertion); this bone moves upon contraction of the biceps

continued The muscle on the back of the upper arm is the triceps Demonstration pg 126, 2nd paragraph Another group of muscles, called the synergists, help to steady a movement or stabilize joint activity

Movement Muscles move bones by pulling on them Groups of muscles usually contract to produce a single movement

Contraction of Skeletal Muscle Must be stimulated by nerve impulses to contract Single contraction is called a twitch A motor neuron (nerve cell) stimulates all of the skeletal muscles within a motor unit Motor neuron plus all the muscle fibers it stimulates Neuromuscular junction: junction b/t the motor neuron’s fiber (axon), which transmits the impulse, and the muscle cell’s sarcolemma (muscle cell membrane) Gap b/t the axon and the end of the muscle cell is the synaptic cleft

continued When the nerve impulses reach the end of the axon, the chemical neurotransmitter acetylcholine is released Diffuses across the synaptic cleft (carries impulse across synaptic cleft) A muscle cell, when stimulated properly, contracts all the way The muscle cell relaxes until it is stimulated by the next release of acetylcholine See figure 7-5 page 128

Muscle Fatigue Caused by an accumulation of lactic acid in the muscles During vigorous exercise, the blood is unable to transport enough oxygen Causes the muscles to contract anaerobically Which means? Impedes muscular contraction, causing fatigue and cramps Oxygen debt: after exercise, the amount of oxygen needed by the muscle to change lactic acid back to glucose Diaphragm: dome-shaped muscle; separates abdominal and thoracic cavity; aids in breathing

Muscle Tone Muscles should be slightly contracted and ready to pull Achieved through proper nutrition and regular exercise Muscle contractions may be isotonic or isometric When muscles contract and shorten (when we walk) When the tension in a muscle increases but the muscle does not shorten (tensing the abdominals) Atrophy: muscles that become weak and flaccid; shrink (waste away) from disuse Hypertrophy: muscles that become enlarged from overexercise; the size of the muscle fiber enlarges

How Exercising and Training Change Muscles Alters size, structure, and strength of a muscle Muscles that have been injured can regenerate only to a certain degree If damage is extensive, muscle tissue is replaced by connective (scar) tissue Causes the muscle to become tougher

Effects of Training of Muscle Efficiency Improved coordination of all muscles involved in a particular activity Improvement of the respiratory and circulatory system to supply the needs of an active muscular system Elimination or reduction of excess fat Improved joint movement involved with that particular muscle activity

Effects of Training on Muscle Strength Capacity to do work Increased with proper training and can have the following effects on skeletal muscles: Increase in muscle size Improved antagonistic muscle coordination, where antagonistic muscles are relaxed at the right moment and do not interfere with the functioning of the working muscle Improved functioning in the cortical brain region where the nerve impulses that start muscular contraction originate