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Typical cells Muscle cell=fiber
Plasma membrane Sarcolemma
Cytoplasm Sarcoplasm
Endoplasmic reticulum
Sarcoplasmic reticulum (SR)
Many mitochondria
Multiple nuclei
Muscle cell structures not found in other cells
• Myofibrils: bundles of very fine fibers• Thick and thin myofilaments: very fine
fibers that make up myofibrils• Sarcomere: segment of myofibril
between two Z lines; contractile unit• T tubules: run transversely across
sarcoplasm at right angle to long axis of the cell; transmit electrical impulses thru cell
Myofilaments• 4 protein molecules that make up
myofilaments: Myosin, actin, tropomyosin, troponin
• Thin filaments: actin, tropomyosin, troponin
• Thick filaments: mostly myosin
Contraction• Motor neuron: nerve cell that
stimulates muscle cells• Neuromuscular junction: motor
neuron connects to sarcolemma• This connection is a synapse in
which there is a narrow gap across which the neurotransmitter, acetylcholine, is released
Mechanism of contractionMotor neuron acetylcholine binds to receptors on sarcolemma impulse
travels along sarcolemma thru T tubules to SR Ca released from SR binds to troponin exposing actin molecules in
thin filaments myosin in thick filaments bind to actin and pull thin filaments to center of sarcomere
Mechanism of Relaxation• Ca pumped back into SR shuts
down the contraction troponin can again block actin’s active site
Glucose & Oxygen
• Glucose stored in form of glycogen in muscle
• Excess oxygen molecules in sarcoplasm bound to myoglobin
Anaerobic respiration
• Allows body to avoid use of oxygen in short term
• Produces lactic acid
• Accumulation of lactic acid in muscles causes burning sensation
Motor unit
• One motor neuron plus the muscle fibers it attaches to
• The fewer the number of fibers supplied by one motor neuron the more precise the movements that can be produced.
Myography• Force from the contraction of a
muscle is recorded as a line that rises & falls as muscle contracts & relaxes
• For a muscle to contract, an electrical stimulus of enough intensity (threshold stimulus) is applied to muscle
Twitch contraction• Quick jerk of muscle• M. doesn’t contract
at moment of stimulus
• 3 phases:– Latent period– Contraction phase– Relaxation phase
Treppe: Staircase Phenomenon
• Gradual step like increase in strength of contractions that can be observed in a series of twitch contractions
Tetanus• If a series of stimuli come in a rapid
enough succession, muscle doesn’t have time to relax completely
• Smooth, sustained contractions
Tonic contraction(or muscle tone)
• Continual partial contraction in a muscle
• Important for maintaining posture
• Flaccid: less tone than normal
• Spastic: more tone than normal
Isotonic contraction “equal tension”
• Tone or tension within a muscle remains the same, length of muscle changes
• The muscle shortens
Isometric contraction “same length”
• Muscle length remains the same while muscle tension increases
• The muscle is unable to shorten
Cardiac muscle• Striated involuntary
• Cardiac m. fibers form strong electrically coupled junctions: intercalated discs
• Branching of individual fibers
• T tubules are larger than skeletal muscle
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