Anatomy and Physiology of Muscles: Types, Functions, Structure, and Contraction - Prof. Th, Study notes of Physiology

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Myology

Bio 250

Human Anatomy & Physiology

Types of Muscle Tissue

„ Skeletal, striated, voluntary

„ Visceral, smooth, involuntary

„ CardiacCardiac , heart muscle, involuntaryheart muscle involuntary

Functions of Muscle

„ Motion: e.g.walking, running, writing,

chewing, swallowing or breathing

„ Heat production: muscles make upp p

nearly half of body and are in a

constant state of activity producing heat

as by-product

„ Posture and body support: muscles give

form and stability to the body

Skeletal Muscles are Organs

„ composed of muscle,

nerve, vascular and

various types of

connective tissues

„ Connective tissue

coverings include:

„ Epimysium (surrounds entire muscle) „ Perimysium (surrounds fasciculi) „ Endomysium (surrounds each muscle cell i.e. muscle fiber)

Skeletal Muscle Cells

„ Transverse tubules

extend from

sarcolemma to

terminal cisternae

which connect to the

sarcoplasmic reticulum

„ Note triad

„ Unit of contraction

called a sarcomere

Review of Skeletal Muscle Structure

Neuromuscular Junction

„ Muscle fibers stimulated to contract by motor neurons

„ Motor end plate of a muscle fiber lies on one side of the myoneuralside of the myoneural junction „ In response to a nerve impulse, the end of the motor nerve secretes a neurotransmitter which binds to receptors sites and stimulates the sarcolemma

„ Eventually causes muscle contraction

Skeletal Muscle Contraction

„ Structural and functional unit of contraction called SARCOMERE „ Muscle fiber contraction results from a sliding movement within myofibrilsy „ Actin and myosin filaments slide past each other „ Sacromeres shorten and muscle fiber as a whole shortens

Stimulus for Muscle

Contraction

„ Muscle fiber is stimulated by acetylcholine at

myoneural (neuromuscular) junction

„ Acetylcholine binds to receptors on the

sarcolemma and initiates an action potential

„ Action potential travels along the sarcolemma

and enters sarcoplasm at T-tubules

„ T-tubules stimulate terminal cisternae to

release stored Calcium ions

Role of Calcium Ions

„ Calcium ions diffuse throughout sarcoplasm

„ Ions expose myosin binding sites on the actin

molecule

„„ Myosin cross bridges form with actinMyosin cross bridges form with actin

„ Activated myosin breaks down ATP

„ Energy released from ATP used to slide Actin

and Myosin filaments together

„ Sarcomere and muscle fiber shorten

„ This is called CONTRACTION

„ Note that ATP is needed to release myosin

heads from actin in order for relaxation to

occur

Review of Muscle Contraction

What happens to ADP?

„ ADP is rebuilt within the mitochondria

„ During periods of high muscle activity,

CREATINE PHOSPHATE is used toCREATINE PHOSPHATE is used to

restore some of the ADP to ATP

„ Activity: ADP + CP -> ATP + C

„ During rest periods, creatine is

recharged to creatine phosphate

„ At rest: C + ATP -> CP + ADP

What happens to acetylcholine?

„ An enzyme in the synaptic cleft called

ACETYLCHOLINESTERASE destroys

acetylcholine stopping the stimulation

of the receptors on the sarcolemmaf

„ Acetylcholine ->acetate + choline

„ This insures that one nerve stimulus

causes one muscle fiber contraction

„ Motor nerve rebuilds acetylcholine from

acetate acid and choline

Muscle Fatigue

„ Fatigued muscles lose ability to contract „ Usually due to accumulation of lactic acid

„ Athletes usually produce less lactic acid because of increased ability to supply oxygen and nutrients toincreased ability to supply oxygen and nutrients to muscles „ Physiological contracture results from lack of ATP needed to break myosin bridges and “pump” Ca++^ back into the sarcoplasmic reticulum

„ Rigor mortis- contraction of all the muscles of the body within a few hours of death. Lasts 15-72 hours depending on conditions like temperature. Lysosomal enzymes eventually break bridges.

Muscular Responses

„ Threshold stimulus vs sub-threshold

stimulus

„„ All or none responseAll or none response

„ Muscle fibers follow A or N response

„ Motor units follow A or N response

„ Muscles DO NOT follow A or N response

Recruitment of Motor Units

„ Low intensity of stimulation of muscle

stimulates small numbers of motor units

„„ High intensity of stimulation causesHigh intensity of stimulation causes

additional motor units to be RECRUITED

„ Muscles have graded responses

Twitch contraction

„ Myogram is a

recording of muscular

contraction

„ Twitch includes:

„ latent period „ contraction phase „ relaxation phase

„ Refractory period is

time following

contraction when

muscle remains

unresponsive to

stimulation