Physiology of the Muscular System Chapter 11

Anatomy & Physiology

Functions

• Movement

• Heat production

• Posture

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

Neuromuscular Junction

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

Energy for Muscle Contractions

• ATP: adenosine triphosphate

• CP: creatine phosphate

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

Smooth muscle• Single nuclei, no T tubules

• Calcium for contraction comes from outside the cell

• 2 types: visceral & multiunit

• Visceral: found in digestive, urinary, reproductive tracts in hollow organs

• Multiunit: found in vessels, arrector pili