Muscular System
Muscular System
Muscles – organs composed of specialized cells that use the chemical energy stored in nutrients to contract.
Three Types of Muscles
• Skeletal
• Smooth
• Cardiac
Structure of Skeletal Muscle
• Connective Tissue Coverings
o Fascia – layer of dense connective tissue that surrounds and separates each muscle
o Tendons – attach muscle to bone
o Epimysium – surrounds each whole muscle
o Perimysium – separate muscle cells into small compartments called fascicles
o Endomysium – layer of connective tissue in which each muscle fiber in a fascicle lies
• Skeletal Muscle Fibers
o Single, long, cylindrical muscle cells
o Contract in response to stimulation and relaxes when the stimulation ends
o Cytoplasm contains myofibrils that play a role in muscle contraction (made of actin and myosin) – produce striations
o Sarcoplasmic and transverse tubules (membranous channels) activate muscle contraction when the fiber is stimulated
• Neuromuscular junctions
o Site where a motor neuron (axon from nerve cell) and muscle fiber meet
o Nerve impulse sent from brain – reaches motor neuron end – releases neurotransmitters (chemicals) – this action stimulates the muscle fiber to contract
• Motor Unit – a motor neuron and all of the muscle fibers it controls
Energy Sources for Contraction
• ATP supplies the energy for muscle fiber contraction – limited supply – must be regenerated
• Creatine phosphate – molecule that stores excess energy – regenerates ATP from ADP and phosphate
Oxygen supply
• Pigment called myoglobin stores oxygen in muscle tissue
• Oxygen deficiency – lactic acid build up
Muscle Fatigue
• When a muscle loses its ability to contract during strenuous activity
• Causes – accumulation of lactic acid – lowers pH which prevents muscles from contracting
Muscle Responses
• Threshold stimulus – minimal strength required to cause a contraction
• All or none response – muscles do not contract partially
Recording a Muscle Contraction
• Myogram – recording of an electronically stimulated muscle contraction
• Twitch – single contraction that lasts only a fraction of a second
• Latent period – delay between the time the stimulus was applied and the time the muscle responded
Sustained Contractions
• Muscle tone – response to nerve impulses that originate repeatedly from the spinal cord and stimulate a few muscle fibers (ex. Posture)
Smooth Muscles
• Elongated with tapering ends – lack striations
• Two types
1. Multiunit Smooth muscle – fibers occur separately rather than in sheets (ex. Blood vessels and iris of the eye)
2. Visceral Smooth muscle – occurs in sheets – these fibers can stimulate one another – display rhythmicity (ex. Peristalsis of intestines)
Smooth Muscle Contraction
• Slower to contract and relax than skeletal muscle
• Can maintain a forceful contraction longer with a given amount of ATP
• Two neurotransmitters and hormones responsible for contractions
Cardiac Muscle
• Found only in the heart
• Self exciting and rhythmic
• Extra calcium is supplied to allow for longer contractions
Skeletal Muscle Actions
• Origin – immovable end of muscle
• Insertion – movable ends
• Prime mover (agonist) – muscle doing the majority of the work
• Synergists – helper muscles
• Antagonists – opposing muscles
Major Skeletal Muscles
• Named according to size, shape, location, action, number of attachments, or direction of fibers.
• See Table
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