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 contraction Motor 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