A & P chapter 9
explain what happens to the length of the A band, I band, and H zone during contraction
- H zone narrows - I band narrows - A bands doesn't narrow because the length of the myosin myofilaments does not change
describe the changes that occur in aging skeletal muscles
- by 80 years of age 50% of muscle mass is gone due to a loss of muscle fibers. Fast-twitch muscle fibers decrease in number more rapidly than slow-twitch fibers - can be dramatically slowed if physical activity is maintained, particularly strength training activity
describe the components of a muscle fiber
- connective tissue is critical to proper muscle function: connects muscles to other structures (ex. tendons), provides a pathway for nerves and blood vessels to reach individual muscle - sarcolemma: plasma membrane of a muscle cell - sarcoplasm: cytoplasm of a muscle cell - myofibrils: densely packed, rod-like contractile elements that extend from one end of the muscle fiber to the other, make up most of the muscle volume - myofilaments (actin and myosin) make up myofibrils - sarcomeres make up myofibrils and are the smallest individual contractile unit within a muscle
discuss the production of an action potential, including depolarization and repolarization
- depolarization results from an increase in the permeability of the plasma membrane to Na+ - if depolarization reaches threshold, an action potential is produced - results from the opening of many Na+ channels - inward movement of Na+ makes the inside of the membrane more positive - the repolarization phase of the action potential occurs when the Na+ channels close and K+ channels open briefly - Na+ movement into the cell stops and K+ movement out of the cell increases, causing repolarization
describe how the sliding filament model explains the contraction of muscle fibers
- during contraction, actin slides past myosin to shorten the sarcomeres - actin and myosin themselves do not change length, only position - during relaxation, sarcomeres lengthen because of an external force, like contraction of antagonistic muscles: muscles that produce the opposite effect
summarize the events of cross-bridge movement and the role they play in muscle contraction
- myosin head is in the cocked position - ADP and phosphates are attached to the head (from ATP breakdown, energy from the breakdown of ATP is stored in the myosin head) - when Ca2+ binds to troponin, tropomyosin moves to expose the active sites on the actin - myosin heads bind to the active sites on the actin myofilaments - a cross-bridge is formed - the phosphate is released from the myosin head - energy stored in the myosin head, is used to move the head at the hinge region of the molecule (movement at the hinge region is called the powerstroke)(movement of the head causes actin to slide past the myosin) - ADP is released from the myosin head - ATP binds to the myosin head and causes cross-bridge release (the myosin head separates from actin) - the ATP is broken down by myosin ATPase to ADP and phosphate which remain attached to the myosin head (hydrolysis reaction) - the myosin head returns to its cocked position following the recovery stroke (energy released from the breakdown of ATP is again stored in the myosin head) - if Ca2+ are still bound to troponin, cross-bridge formation and movement are repeated (cross-bridge cycling usually occurs many times during a dingle contraction)
state the conditions needed for muscle relaxation
- occurs as a result of active transport of Ca2+ back into the sarcoplasmic reticulum - as the Ca2+ concentration decreases in the sarcoplasm, Ca2+ diffuse away from the troponin - tropomyosin moves to cover the active sites on the actin molecules, preventing further cross-bridge formation
explain multiple-motor-unit recruitment
- the nervous system regulates muscle force by increasing the number of contracting motor units - whole muscles are composed of many motor units (hundreds in some) - as more motor units are stimulated, the force of contraction increases
explain the connection between the initial length of a muscle and the amount of tension produced
- the number of cross-bridges that can form determines the force of contraction, but the number of cross-bridges than can form is dependent on muscle length - muscle length determines sarcomere length - determines the amount of overlap between actin and myosin - maximal cross-bridge formation results in maximal contraction - muscle contracts with less than maximum force if its initial length is shorter or longer than optimal - crumpled muscle: myofilaments crumpled, cross-bridges can't contract - stretched muscle: not enough cross-bridge formation
state the all-or-none principle as it pertains to action potentials
Action potentials occur in an all-or-none fashion - subthreshold stimulus: produces no action potential - threshold stimulus: a stimulus at or above threshold will produce an action potential Characteristics of action potentials: - once an action potential begins, all of the ion channel changes proceed without stopping - occur in a very small area of the plasma membrane - do not affect the entire plasma membrane at one time - may propagate (travel) across plasma membranes. Stimulate the production of an action potential in an adjacent location.
explain the role of ion channels in the production of an action potential
Ion channels assist with the production/spread of action potentials: - when a cell is stimulated, the permeability characteristics of the plasma membrane change as a result of the opening of certain ion channels - diffusion of ions across these channels changes the charge across the plasma membrane and produces an action potential - Ligand-gated channels or voltage-gated channels help produce action potentials Ligand: molecule that binds to a receptor located in the plasma membrane Receptor: protein of glycoprotein with a receptor site to which a ligand can bind Gates open in response to a ligand biding to a receptor that is part of the ion channel: - motor neurons that supply skeletal muscle release the neurotransmitter acetylcholine (ACh) - ACh binds to ligand-gated Na+ channels in the membranes of muscle fibers - Na+ channels open, allowing Na+ to enter the cell
recognize the phases of a muscle twitch and the events that occur in each phase
Lag (latent) phase - time between application of the stimulus to the motor neuron and the muscle contraction Contraction phase - contraction occurs Relaxation phase - relaxation occurs
describe the structure of a neuromuscular junction, and explain how an action potential is transmitted across the junction
Presynaptic terminals: - axonal endings - located in an invagination of the sarcolemma (space between is the synaptic cleft) - synaptic vesicles contain ACh Motor end-plate: - specific part of the sarcolemma in the area of the synapse - contains ACh receptors *check notes for how action potential is transmitted across the junction*
describe the resting membrane potential, and how it is generated and maintained
The nervous system stimulates muscles to contract through electrical signals called action potentials Resting membrane potential: - charge difference across the plasma membrane - the inside of the plasma membrane is negative as compared to the outside in a resting cell (Na+/K+ pump maintains this difference) - must exist for action potentials to occur An action potential is a reversal of the resting membrane potential so that the inside of the plasma membrane becomes positive (depolarizes)
identify the myofilaments and describe the structure and location of each
actin: - thin myofilament (consists of two helical polymer strands of F actin, tropomyosin, and troponin) - G (globular) actin (contains the active sites to which myosin heads attach during contraction) - troponin (molecules are attached at specific intervals along the actin myofilaments and have Ca2+ binding sites, also attach to tropomyosin) - tropomyosin (molecules located along the groove between the twisted strands of F actin, covers the active sites on G actin when Ca2+ is not bound to troponin) myosin: - thick myofilament (composed of myosin molecules) - myosin head (contains an ATPase, which breaks down ATP) - hinge region (enables the head to move) - rods (attach to each other and are arranged so that the heads of the myosin molecules are located at each end of the myofilament)
summarize what occurs in treppe
an increase in the force of contraction during the first few contractions of a rested muscle. When a muscle hasn't been used in a while, the first couple movements need to build up tension
describe the structure of a motor unit
consists of a single motor neuron and all of the muscle fibers that it innervates
explain the four functional properties of muscle tissue
contractility: - the ability to shorten forcibly - muscular contraction is an active process in which muscle cells generate the forces causing muscle to shorten - muscular relaxation is a passive process and results from forces outside of the muscle itself excitability: - the ability to receive and respond to stimuli - muscles normally contract as a result of stimulation by nerves extensibility: - the ability to be stretched or extended - muscles can stretch beyond their normal resting length and still contract elasticity: - the ability to recoil and resume the original resting length after being stretched
explain the events of excitation-contraction coupling
excitation-contraction: mechanism by which an action potential in the sarcolemma causes the contraction of a muscle fiber 1. an action potential (produced at the presynaptic terminal of the neuromuscular junction) is propagated along the sarcolemma of the skeletal muscle. 2. depolarization of the T-tubule causes gated Ca2+ channels in the SR to open, resulting in an increase in the permeability of the SR to Ca2+, especially in the terminal cisternae. Ca2+ then diffuse from the SR into the sarcoplasm 3. Ca2+ released from the SR bind to troponin molecules. The troponin molecules that are bound to G actin molecules are released, causing tropomyosin to move, and exposing the active sites on G actin. 4. Once active sites on G actin molecules are exposed, the heads of the myosin myofilaments bind to them to form cross-bridges. Movement of the cross-bridges results in contraction.
differentiate incomplete tetanus and complete tetanus (multiple-wave summation)
incomplete tetanus - muscle fibers relax partially between the contractions complete tetanus - action potentials produced so rapidly that no relaxation occurs between them
distinguish between isotonic contractions and concentric/eccentric phases
isotonic contractions - change in muscle length but no change in muscle tension concentric contractions - isotonic contractions in which muscle tension increases and the muscle shortens (dumbell towards body) eccentric contractions - isotonic contractions in which tension is maintained as the muscle lengthens (dumbell away from body)
describe the three types of muscular fascia and their locations
muscular fascia - sheets of connective tissue that separate and compartmentalize muscles epimysium - dense collagenous connective tissue that surrounds the entire muscle perimysium - fibrous connective tissue that surrounds groups of muscle fibers known as fascicles (bundles) endomysium - fine sheath of connective tissue composed of reticular fibers that surrounds each muscle fiber
distinguish between physiological and psychological fatigue and compare the mechanisms involved
psychological fatigue: - most common type of fatigue, involves the Central Nervous System - muscle capable of functioning but the individual perceives that additional muscular work is not possible physiological/muscular fatigue: - results from ATP depletion - without adequate ATP levels, cross-bridges and ion channels do not function properly. Muscle tension declines
summarize the functions of the muscular system
respiration - skeletal muscles of the thorax responsible for the movements necessary for respiration communication - skeletal muscles are involved in all aspects of communication (writing, speaking, facial expression, etc.) constriction of organs and vessels - contraction of smooth muscle within the walls of organs and vessels causes constriction
summarize the major characteristics of skeletal, smooth, and cardiac muscle (structure, function, location)
skeletal: - responsible for most body movement - locomotion, facial expressions, posture, respiratory movements, etc. - maintains posture - stabilizes joints - generates heat - the nervous system consciously controls the functions of skeletal muscles - voluntary - attached to the bones - composed of skeletal muscle cells/fibers called myocytes - striations and multinucleated smooth: - most widely distributed type of muscle in the body - found in the walls of hollow organs and tubes, interior of the eye, and the walls of blood vessels - contracts involuntarily (under unconscious control) - regulates flow through blood vessels - help to maintain blood pressure - squeezes or propels substances through organs (ex. food, feces, etc.) cardiac: - located only in the heart - contractions provide the force to move blood through the vascular system - Autorhythmic - contract spontaneously at somewhat regular intervals - nervous or hormonal stimulation is not always required for contraction - involuntary (unconscious control)
distinguish between fast-twitch oxidative, fast-twitch glycolytic, and slow-twitch muscle fibers, energy sources used by each type, and functions for which each type is best adapted
slow-twitch oxidative (SO)(type I): fatigue resistant, use aerobic energy sources, high mitochondrial density, highly vascularized, contain myoglobin, maintain posture and are involved with prolonged exercise, long-distance runners generally have a higher percentage of SO fibers fast-twitch glycolytic (FG)(type IIb): highly fatiguable, use anaerobic respiration, high glycogen concentration, produce powerful contractions of short durations, sprinters usually have a higher percentage of FG fibers fast-twitch oxidative glycolytic (FOG)(type IIa): fatigue resistance intermediate between SO and FG fibers, low mitochondrial density, use aerobic and anaerobic respiration, support moderate-intensity endurance exercises (like basketball, soccer, etc.)
explain how whole muscles respond in a graded fashion and how the force of contraction can be increased (multiple motor-unit recruitment, multiple-wave summation)
strength of contraction may vary from weak to strong based on the requirements multiple-motor unit summation: - increased strength of stimuli increases the number of muscle fibers that respond and contract - individual motor units and individual muscle fibers follow the all-or-none principle while whole muscles respond to stimuli in a graded manner - motor units in different muscles do not always contain the same number of muscle fibers (meaning smaller muscles that perform delicate movements contain motor units with few muscle fibers) - strength of contraction of a whole muscle depends on recruitment of motor units multiple-wave summation: - increased frequency of stimuli increases the force of contraction of as the muscle doesn't have time to fully relax - incomplete tetanus (partial relaxation) or complete tetanus (no relaxation)
