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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