Module 9

The movement of sodium and potassium creates a nerve impulse

A nerve impulse is used to stimulate tissues

Parts of a muscle breakdown image

(a) Part of a muscle attached by a tendon to a bone. (b) Enlargement of one muscle fiber. The muscle fiber contains several myofibrils. (c) A myofibril extended out the end of the muscle fiber. The banding patterns of the sarcomeres are shown in the myofibril.

Parts of a muscle breakdown image of contractile point

(d) A single sarcomere of a myofibril is composed of actin myofilaments and myosin myofilaments. The Z disk anchors the actin myofilaments, and the myosin myofilaments are held in place by titin molecules and the M line.

Effects of aging on skeletal muscle

-Reduced muscle mass - muscles get smaller -Increased time for muscle to contract in response to nervous stimuli - action potentials are not propagated as quickly -Reduced stamina -Increased recovery time - take longer to get the muscles ready for subsequent contractions -Loss of muscle fibers -Decreased density of capillaries in muscle

Muscle relaxation

Calcium moves back into the sarcoplasmic reticulum by active transport - which requires energy Calcium moves away from troponin-tropomyosin complex The complex re-establishes its position and blocks binding sites

Skeletal muscle structure

Composed of muscle cells (muscle cells aka muscle fibers), connective tissue, blood vessels, nerves Fibers are long, cylindrical, multinucleated Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length - if you expand the individual muscle fibers the whole muscle looks larger Develop from myoblasts; numbers remain constant Striated appearance due to light and dark banding - being striated creates a dark banding

General properties of muscle - what makes muscle tissue unique

Contractility: ability of a muscle to shorten with force - ability to contract - can shorten with power - how we lift things Excitability: capacity of muscle to respond to a stimulus (from our nerves) - starts with a stimulation from the nervous system - it is excitable Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree Elasticity: ability of muscle to recoil to original resting length after stretched

An action potential will depolarize and then repolarization needs to occur quickly because if it does not reset then the muscle cannot receive the next signal

This allows you to do the same action over and over again each contraction comes from a different depolarization

Powerstroke

This is when the actin slides over the myosin

Actin (thin) myofilaments

Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere. Composed of G actin monomers each of which has an active site Actin site can bind myosin during muscle contraction. Tropomyosin = an elongated protein that winds along the groove of the f actin double helix Troponin is composed of three subunits = one binds to actin, one binds to tropomyosin, and one binds to calcium ions

Treppe

When a rested muscle is stimulated repeatedly with maximal stimuli at a low frequency, which allows complete relaxation between stimuli, the second contraction produces a slightly greater tension than the first, and the third contraction produces greater tension than the second. After a few contractions, the levels of tension produced by all the contractions are equal.

For the release to occur ATP is needed

When the bridge is released the units slide back

The myofilaments are

actin and myosin Contractile proteins in muscle

The sarcoplasmic reticulum is a holding vessel for calcium

action potential stimulates calcium to diffuse from the sarcoplasmic reticulum into the sarcoplasm and those calcium ions will bind to troponin and this causes tropomyosin to move - when tropomyosin moves it exposes the active sites on the G actin molecules cause myosin to reach up and bind - at that point you have a contraction ( meeting of the heads - forming an active cross bridge)

Cardiac muscle

Found only in heart Striated - involuntary Each cell usually has one nucleus Has intercalated disks and gap junctions Autorhythmic cells - only need to stimulate one part of the cell and then the signal will propagate from cell to cell Action potentials of longer duration and longer refractory period (resting period) Calcium regulates contraction

The difference between f actin and g actin is

G actin has receptor sites = the myosin needs to come and make contact with the G actin

Actin myofilaments - Each actin myofilament is composed of three separate proteins: (1) globular (G) actin, (2) tropomyosin, and (3) troponin

G actin molecules are globular subunits that form a long chain of about 200 subunits. The chain of 200 G actin subunits forms into a strand called fibrous (F) actin. Each G actin subunit has an active site for myosin myofilament binding during muscle contraction. In a sense, we can think of the active sites on the G actin as receptor sites for a portion of the myosin myofilament, the myosin head. Tropomyosin is a long, fibrous protein that lies in the groove along the fibrous actin strand. In a relaxed muscle, tropomyosin is covering the active sites on the G actin subunits. A muscle cannot contract until the tropomyosin moves to uncover the active sites. Troponin consists of three subunits: (1) a subunit that anchors the troponin to the actin, (2) a subunit that prevents the tropomyosin from uncovering the G actin active sites in a relaxed muscle, and (3) a subunit that binds Ca2+.

Light chains means not as many amino acids = actin

Heavy chains means many more amino acids = myosin

Fast twitch oxidative glycolytic fibers (type IIa) (FOG)

Intermediate fiber diameter High myoglobin content Many mitochondria Many capillaries Intermediate aerobic metabolic capacity and high anaerobic metabolic capacity Intermediate fatigue resistance Fast myosin ATPase activity High glycogen concentration Fibers are most abundant generally in lower limbs Endurance activities in endurance-trained muscles

Muscle fatigue

Is a decreased capacity to work and reduced efficiency of performance - decreased ability to contract Types - Psychological = depends on emotional state of the individual - Muscular = results from ATP depletion - Synaptic = occurs in neuromuscular junction due to lack of acetylcholine

Smooth muscle muscle type

Locations: Walls of hollow organs, blood vessels, eyes, glands, and skin Cell shape: Spindle-shaped (15-200 μm in length, 5-8 μm in diameter) Nucleus: Single, centrally located Gap junctions join some visceral smooth muscle cells together No striations Involuntary control Some smooth muscle is capable of spontaneous contraction Function: Moving food through the digestive tract, emptying the urinary bladder, regulating blood vessel diameter, changing pupil size, contracting many gland ducts, moving hair, and having many other functions

Myosin (thick) myofilament

Many elongated myosin molecules shaped like golf clubs Molecule consists of myosin heavy chains wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally Myosin heads = golf clubs = can bind to active sites on the actin molecules to form cross bridges = they attach to the rod portion by a hinge region that can bend and straighten during contraction ATPase enzyme activity breaks down adenosine triphosphate (ATP) which releases energy - part of the energy is used to bend the hinge region of the myosin molecule during contraction - breaking down ATP turns it to ADP releasing a phosphate Need energy for muscle to contract and for them to release

Synapse: axon terminal resting in an invagination of the sarcolemma

Neuromuscular junction (NMJ): - Presynaptic terminal: axon terminal with synaptic vesicles - Synaptic cleft: space - Postsynaptic membrane or motor end plate sarcolemma: where nerve ending is approximating the muscle

Skeletal muscle in microscope

Nucleus Striations Skeletal muscle fiber

Parts of a muscle image

Perimysium Muscle fascicles Endomysium (surrounding muscle fibers) Nuclei Capillary Sarcoplasmic reticulum Z disk Z disk A I I Mitochondrion Muscle fiber Epimysium (muscular fascia) Bone Transverse (T) tubule Sarcolemma (plasma membrane) Myofibrils Actin myofilament Myofibril Myosin myofilament Sarcomere Sarcomere Cross-bridge Actin myofilament Myosin myofilament Z disk Z disk Titin M line Skeletal muscle Tendon

Physiological contracture and Rigor Mortis

Physiological contracture = state of fatigue where due to lack of ATP neither contraction nor relaxation can occur Rigor Mortis = after death where the body stiffens = Development of rigid muscles several hours after death -> calcium leaks into sarcoplasm and attaches to myosin heads and cross bridges form - rigor ends as tissues start to deteriorate

Muscles are primarily made of

Protein

Types of muscle tissue

Skeletal: - Responsible for locomotion, facial expressions, posture, respiratory movements, and other types of body movement - it is voluntary muscle Smooth: - Walls of hallow organs, blood vessels, eye, glands, and skin - Some functions = propel urine, mix food in digestive tract, dilate/ constrict pupils, regulate blood flow - In some locations it is autorhythmic - Controlled involuntarily by endocrine and autonomic nervous system Cardiac: - found in the heart = major source of movement of blood - autothythmic - controlled involuntarily by endocrine and autonomic nervous systems

Slow and fast fibers

Slow twitch oxidative type I - Contract more slowly, smaller in diameter, better blood supply, more mitochondria, more fatigue resistant than fast twitch, large amount of myoglobin (myoglobin is a blood pigment) - Postural muscles, more in lower than upper limbs "dark meat of chicken" Fast twitch Type II - Respond rapidly to nervous stimulation, contain myosin that can break down ATP more rapidly than in Type I, less blood supply, fewer and smaller mitochondria than slow twitch (not as much ATP - so they cant work as long) - Lower limbs in sprinter, upper limbs of most people "white meat of chicken" - Comes in oxidative and glycolytic forms

Function of neuromuscular junction

Synaptic vesicles - Neurotransmitter: substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane. -> Acetylcholine is a neurotransmitter or ligand Acetylcholinesterase: A degrading enzyme in synaptic cleft. Prevents accumulation of ACh - helps break up ACh

There is not direct contact between the end of the muscle and the surface of the muscle fiber

The action potential is propagated down to the muscle and then it has to get to the muscle using synaptic vesicles to transmit neurotransmitters The post synaptic membrane is the muscle and the presynaptic membrane refers to everything on the nerve end and everything in between is the synaptic cleft

The whole muscle is referred to as

The belly of the muscle

Voltage-gated ion channels and the action potential - image

1) Resting membrane potential. Voltage-gated Na+ channels (pink) and some, but not all, K+ channels (purple) are closed. K+ diffuses down its concentration gradient through the open leak K+ channels, making the inside of the plasma membrane negatively charged compared to the outside. 2) Depolarization. Voltage-gated Na+ channels are open. Na+ diffuses down its concentration gradient through the open voltage-gated Na+ channels, making the inside of the plasma membrane positively charged compared to the outside 3) Repolarization. Voltage-gated Na+ channels are closed, and Na+ movement into the cells stops. More voltage-gated K+ channels open. K+ movement out of the cell increases, making the inside of the plasma membrane negatively charged compared to the outside, once again.

Stimulus frequency and whole muscle contraction

As the frequency of action potentials increase, the frequency of contraction increases; Incomplete tetanus: muscle fibers partial rest between contractions Complete tetanus = no rest between contractions Multiple-wave summation = muscle tension increases as contraction frequencies increase

Physiological basis for each phase of the twitch

Lag ■ The action potential arrives at the motor end-plate; acetylcholine is released from the motor neuron. ■ Acetylcholine binds to ligand-gated Na+ channels in the motor end-plate generating an action potential along the sarcolemma. ■ The action potential propagates along the sarcolemma and down the T tubules. ■ The action potential causes voltage-gated Ca2+ channels in the sarcoplasmic reticulum to open. Contraction ■ Ca2+ diffuses out of the sarcoplasmic reticulum into the sarcoplasm and binds to troponin. ■ The active sites on the actin myofilament are exposed and the myosin heads bind to them. ■ Cross-bridge cycling begins and continues as long as Ca2+ is in the sarcoplasm. Relaxation ■ Ca2+ is pumped from the sarcoplasm into the sarcoplasmic reticulum. ■ Ca2+ levels in the sarcoplasm decline and cross bridge cycling comes to a stop.

Skeletal muscle muscle type

Location: attached to bones Cell shape: Very long and cylindrical (1 mm-4 cm, or as much as 30 cm, in length, 10 µm-100 μm in diameter) Nucleus: Multiple nuclei: peripherally located Has striation Voluntary and involuntary control (involuntary = reflexes) Not capable of spontaneous contraction Function: Body

The nervous system controls muscle contractions through action potentials

Membrane voltage difference across membrane (polarized) Inside cell is more negative due to the accumulation of large protein molecules that do not move in and out of the cell - There is more potassium (K+) on the inside than the outside of the cell - K+ leaks out but does not completely leave the cell because negative proteins hold some back Outside the cell is more positive - and there is more Na+ (sodium) outside the cell than there is inside - the sodium potassium pump (Na+/ K+ pump) maintains this situation which requires a constant use of energy The outside and inside being different areas of charge must exist for action potential to occur

Functions of the muscular system

Movement of the body Maintenance of posture Respiration - ventilate Production of body heat Communication Constriction of organs and vessels Contraction of the heart

Two actins relate to a single myosin

The linear protein is tropomyosin, troponin and f actin make actin Myosin is made up of these heads that make contact with the actin

Connective tissue holds muscle tissue together

The muscle tissue (fibers) is held together, grouped and separated by connective tissue

Troponin interacts with actin and myosin

Troponin will bind to G actin, calcium and tropomyosin (the linear protein)

Muscle contractions

Isometric = no change in length but tension increases - seen in postural muscles of the body Isotonic = change in length but tension is constant -> concentric is when the muscle shortens aka when the muscle overcomes opposing resistance and muscle shortens -> Eccentric is when tension is maintained but muscles lengthen Muscle tone = overall constant tension by muscles for long periods of time

Smooth muscle

Not striated, fibers are smaller than those in skeletal muscle Spindle-shaped; single, central nucleus - no banding More actin than myosin

Regulation of smooth muscle

-Innervated by autonomic nervous system (automatic - happens without your control) -Neurotransmitters are acetylcholine and norepinephrine -Hormones can stimulate smooth muscle important ones are epinephrine and oxytocin -Receptors present on plasma membrane; which neurotransmitters or hormones bind determines response

Functional properties of smooth muscle

-Some visceral muscle exhibits autorhythmic contractions - Esophagus stays collapsed until food is put into it (smooth muscle tone) - smooth muscle has constant tension -Tends to contract in response to sudden stretch but not to slow increase in length -Exhibits relatively constant tension: smooth muscle tone -Amplitude of contraction remains constant although muscle length varies

Shortening sarcomere

1) Actin and myosin myofilaments in a relaxed muscle and a contracted muscle are the same length - myofilaments do not change length during muscle contraction 2) During contraction, actin myofilaments at each end of the sarcomere slide past the myosin myofilaments toward each other. As a result, the Z disks are brought closer together, and the sarcomere shortens 3) As the actin myofilaments slide over the myosin myofilaments, the H zones and the I bands narrow. The A bands, which are equal to the length of the myosin myofilaments, do not narrow, because the length of the myosin myofilaments does not change 4) In a fully contracted muscle, the ends of the actin myofilaments overlap at the center of the sarcomere and the H zone disappears

Function of the neuromuscular junction

1) An action potential (orange arrow) arrives at the presynaptic terminal and causes voltage-gated Ca2+ channels in the presynaptic membrane to open. 2) Calcium ions enter the presynaptic terminal and initiate the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles. 3) (ACh) acetylcholine is released into the synaptic cleft by exocytosis. 4) (ACh) acetylcholine diffuses across the synaptic cleft and binds to ligand-gated Na+ channels on the motor end-plate. 5) Ligand-gated Na+ channels open and Na+ enters the muscle fiber, causing the motor end-plate to depolarize. If depolarization passes threshold, an action potential is generated along the sarcolemma. 6) (ACh) acetylcholine unbinds from the ligand-gated Na+ channels, which then close. 7) The enzyme acetylcholinesterase, which is attached to the motor end-plate, removes acetylcholine from the synaptic cleft by breaking it down into acetic acid and choline. 8) Choline is symported with Na+ into the presynaptic terminal, where it can be recycled to make ACh (acetylcholine). Acetic acid diffuses away from the synaptic cleft. 9) (ACh) acetylcholine is re-formed within the presynaptic terminal using acetic acid generated from metabolism and from choline recycled from the synaptic cleft. acetylcholine (ACh) is then taken up by synaptic vesicles

Action potential propagation in a muscle fiber image

1) An action potential in a local area of the plasma membrane is indicated by the orange band. Note the reversal of charge across the plasma membrane. 2) The action potential is a stimulus that causes another action potential to be produced in the adjacent plasma membrane. 3) The action potential propagates along the plasma membrane (orange arrow).

Action Potentials and Muscle Contraction - Action potentials are propagated down the T tubules and stimulate Ca2+ release from the sarcoplasmic reticulum.

1) An action potential that was produced at the neuromuscular junction is propagated along the sarcolemma of the skeletal muscle. The depolarization also spreads along the membrane of the T tubules. 2) The depolarization of the T tubule causes voltage-gated Ca2+ channels in the sarcoplasmic reticulum to open, resulting in an increase in the permeability of the sarcoplasmic reticulum to Ca2+, especially in the terminal cisternae. Calcium ions then diffuse from the sarcoplasmic reticulum into the sarcoplasm. 3) Calcium ions released from the sarcoplasmic reticulum bind to troponin molecules. The troponin molecules bound to G actin molecules are released, causing tropomyosin to move, and to expose 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.

A Summary of Skeletal Muscle Contraction

1) An action potential travels along an axon membrane to a neuromuscular junction. 2) Voltage-gated Ca2+ channels open and Ca2+ enters the presynaptic terminal. 3) Acetylcholine is released from presynaptic vesicles. 4) Acetylcholine stimulates ligand-gated Na+ channels on the motor end-plate to open. 5) Na+ diffuses into the muscle fiber, initiating an action potential that travels along the sarcolemma and T tubule membranes. 6) Action potentials in the T tubules causes opening of voltage-gated Ca2+ channels in the sarcoplasmic reticulum releasing Ca2+. 7) On the actin, Ca2+ binds to troponin, which moves tropomyosin and exposes myosin head binding sites. 8) ATP molecules on myosin heads are broken down to ADP and P, which releases energy needed to move the myosin heads. 9) The heads of the myosin myofilaments bend (power stroke), causing the actin to slide past the myosin. As long as Ca2+ is present, the cycle repeats

Breakdown of ATP and Cross-Bridge Movement During Muscle Contraction

1) Exposure of active sites. Before cross-bridges cycle, Ca2+ binds to the troponins and the tropomyosins move, exposing active sites on actin myofilaments. 2) Cross-bridge formation. The myosin heads bind to the exposed active sites on the actin myofilaments to form cross-bridges, and phosphates are released from the myosin heads. 3) Power stroke. Energy stored in the myosin heads is used to move the myosin heads (small, dark blue arrow), causing the actin myofilaments to slide past the myosin myofilaments (dark blue arrow), and ADP molecules are released from the myosin heads (black arrow). 4) Cross-bridge release. An ATP molecule binds to each of the myosin heads, causing them to detach from the actin. 5) Hydrolysis of ATP. The myosin ATPase portion of the myosin heads break down ATP into ADP and phosphate (P), which remain attached to the myosin heads. 6) Recovery stroke. The heads of the myosin molecules return to their resting position (small, dark blue arrow), and energy is stored in the heads of the myosin molecules. If Ca2+ is still attached to troponin, cross-bridge formation and movement are repeated (return to step 2). This cycle occurs many times during a muscle contraction. Not all cross-bridges form and release simultaneously.

Image - measuring the resting membrane potential

1) In a resting cell, there is a higher concentration of K+ (purple circles) inside the plasma membrane and a higher concentration of Na+ (pink circles) outside the plasma membrane. Because the membrane is not permeable to negatively charged proteins (green) they are isolated to inside of the plasma membrane. 2) There are more K+ leak channels than Na+ leak channels. In the resting cell, only the leak channels are opened; the gated channels (not shown) are closed. Because of the ion concentration differences across the plasma membrane, K+ diffuses out of the cell down its concentration gradient and Na+ diffuses into the cell down its concentration gradient. The tendency for K+ to diffuse out of the cell is opposed by the tendency of the positively charged K+ to be attracted back into the cell by the negatively charged proteins. 3) The sodium-potassium pump helps maintain the differential levels of Na+ and K+ by pumping three Na+ out of the cell in exchange for two K+ into the cell. The pump is driven by ATP hydrolysis. The resting membrane potential is established when the movement of K+ out of the cell is equal to the movement of K+ into the cell.

Smooth muscle contraction page 306

A hormone binds to its receptor and activates a G protein mechanism. An α subunit opens the ligandgated Ca2+ channel in the plasma membrane. Calcium ions diuse through the Ca2+ channels and combine with calmodulin. A Ca2+-calmodulin complex binds to myosin kinase and activates it. Activated myosin kinase attaches phosphate from ATP to myosin heads to activate the contractile process. A cycle of cross-bridge formation, movement, detachment, and cross-bridge formation occurs. Relaxation occurs when myosin phosphatase removes phosphate from myosin.

The nuclei are on the periphery of the skeletal muscle fiber

A muscle fiber has many Myofibrils in it - the treads that contained in a myofibril are filaments (thin and thick filaments) The thin and thick threads align themselves that provides a look of striation an individual area of striation is referred to as a sarcomere = a sarcomere is the structural and functional unit of the muscle because within the sarcomere you can see contraction occur

Nerves and blood vessel supply

A neuron is the basic cell of the nervous system it stimulates Motor neurons: stimulate muscle fibers to contract - Nerve cells with cell bodies in brain or spinal cord - axons extend to skeletal muscle fibers through nerves Axons branch so that each muscle fiber is innervated - axons give muscles the information to contract The neuromuscular junction is where the nervous system meets the muscular system (in reference to skeletal muscle) - there is not actually a physical connection between the muscles and the neurons there is actually a little gap between them called a synapse - this is where communication takes place Capillary beds surround muscle fibers - the blood brings oxygen and nutrients

Physiology of skeletal muscle - muscle twitch

A simple muscle contraction is referred to as a muscle twitch - Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers Phases of a simple muscle twitch -> lag or latent phase - contraction (the actual power stroke) - relaxation

Actin and Myosin Proteins in a Smooth Muscle Cell Bundles of contractile myofilaments containing actin and myosin are anchored at one end to dense areas in the plasma membrane and at the other end, through dense bodies, to intermediate filaments. The contractile myofilaments are oriented with the long axis of the cell; when actin and myosin slide over one another during contraction, the cell shortens.

Actin myofilament Myosin myofilament Dense bodies in sarcoplasm Dense area attached to sarcolemma Intermediate filaments Myofilaments

Sliding filament model

Actin myofilaments slide over myosin to shorten sarcomeres - the actin and myosin do not change length - shortening sarcomeres are responsible for skeletal muscle contraction During relaxation - sarcomeres lengthen because of some external force, like the contraction of antagonistic muscles Antagonistic muscles are muscles that produce opposite effects

Muscle length vs tension

Active tension = force applied to an object to be lifted muscle contracts Stretched muscle = not enough cross bridging - this is a pulled muscle Crumpled muscle - haven't created a large enough H zone - myofilaments crumpled and cross bridges can't contract Passive tension = tension applied to a load when a muscle is stretched not simulated Total tension = active + passive

Stimulus strength and motor unit response

All or none law for muscle fibers = if you reach the threshold needed to stimulate the muscle then contraction of equal force will occur in response to each action potential Sub-threshold stimulus = no action potential = no contraction Stronger than threshold; action potential; contraction equal to that with threshold stimulus meaning - anything stronger than threshold will still elicit a threshold response

Muscle length and tension - The length of a muscle before it is stimulated influences the muscle's force of contraction. As the muscle changes length, the sarcomeres also change length

At the normal resting length of a muscle, the sarcomeres are also at an optimal length. The muscle produces maximum tension in response to a maximal stimulus at this length. At muscle/sarcomere length 1, the muscle is not stretched, and the tension produced when the muscle contracts is small because there is too much overlap between actin and myosin myofilaments. The myosin myofilaments run into the Z disks, and the actin myofilaments interfere with each other at the center of the sarcomere, reducing the number of cross-bridges that can form. At muscle/sarcomere length 2, the muscle is optimally stretched, and the tension produced when the muscle contracts is maximal because there is optimal overlap of actin and myosin myofilaments, so the number of cross-bridges that can form is maximal. At muscle/sarcomere length 3, the muscle is stretched severely, and the tension produced when the muscle contracts is small because there is little overlap between actin and myosin myofilaments, and few cross-bridges can form.

Image = Depolarization and the Action Potential in Skeletal Muscle

Depolarization is a change of the charge difference across the plasma membrane, making the charge inside the cell more positive and the charge outside the plasma membrane less positive. Once threshold is reached, an action potential is produced. During the depolarization phase of the action potential, the membrane potential changes from approximately -85 mV to approximately +20 mV. During the repolarization phase, the inside of the plasma membrane changes from approximately +20 mV back to -85 mV Na+/K+ pump returns the membrane to the resting membrane potential

Action potential phases

Depolarization: inside of the plasma membrane becomes less negative If the change reaches the threshold then depolarization can occur Repolarization is the return to resting membrane potential - note that during repolarization, the membrane potential drops lower than its original resting potential, and then rebounds - this is because sodium plus potassium together are higher, but then the sodium potassium pump restores the resting potential All or none principle = it is like a camera flash or toilet flushing -> when action potentials occur they are going to go all the way and complete the message Propagation = spread from one location to another - action potential does not move along the membrane, new action potential at each successive location Frequency - the number of action potentuals produced per unit of time

Fast and Slow twitch fibers

Distribution of fast-twitch and slow-twitch - Most muscles have both varies for each muscle Effects of exercise: change in size of muscle fibers Hypertrophy: increase in muscle size seen mostly in IIB - Increased in myofibrils - Increase in nuclei due to fusion of satellite cells - Increase in strength due to better coordination of muscles, increase in production of metabolic enzymes, better circulation, less restriction by fat Atrophy: decrease in muscle size - Reverse except in severe situations where cells die

Heat production

Exercise: metabolic rate and heat production increase Post- exercise: metabolic rate stays high due to oxygen debt - that's why you breathe heavy after you run Excess heat loss because of vasodilation and sweating Shivering: uncoordinated contraction of muscle fibers resulting in shaking and heat production - muscles attached to hair follicles spontaneous contraction causes shivering

Treppe

Graded response Occurs in muscle rested for prolonged period Each subsequent contraction is stronger than previous until all equal after few stimuli Possible explanation: more and more calcium remains in sarcoplasm and is not all taken up into the sarcoplasmic reticulum The little lift before you lift something heavy

Skeletal muscle fiber

Have several nuclei just inside the sarcolemma - on the periphery - in the plasma membrane of the muscle fiber (the plasma membrane of a muscle fiber is called the sarcolemma) Cell packed with myofibrils within cytoplasm (sarcoplasm = cytoplasm of a muscle fiber) Threadlike = Composed of protein threads called myofilaments: thin (actin) and thick (myosin) - the interaction of actin and myosin brings about muscle contraction Sarcomeres = highly ordered repeating units of myofilaments - this is what creates the striations

Physiology of skeletal muscle fibers

In a resting cell/ resting membrane the charges are overwhelming negative in the cell and overwhelmingly positive outside the cell - there is more potassium on the inside than there is sodium on the outside - there are large negatively charged proteins that are always inside the cell which is what allows the cell to be negative The sodium and potassium are dynamic and have the ability to move across the membrane At a resting state the membrane potential is around -85 mV • For skeletal muscle to contract - Activation (at neuromuscular junction) • Must be nervous system stimulation • Must generate action potential in sarcolemma - Excitation-contraction coupling • Action potential propagated along sarcolemma • Intracellular Ca2+ levels must rise briefly

During depolarization sodium moves into the cell - the sodium moving in (positively charged) plus the potassium that is already there (positively charged) creates a positive region

In repolarization the sodium potassium pump is working to get the sodium out and potassium is leaving which causes the membrane to become way negative Then some potassium will be pumped back in to make the membrane go back to its resting potential of -85

Fast twitch glycolytic fibers (Type IIb) (FG)

Largest fiber diameter Low myoglobin content Few mitochondria Few capillaries Low aerobic metabolic capacity and highest anaerobic metabolic capacity Low fatigue resistance Fast myosin ATPase activity High glycogen concentration Fibers are most abundant generally in upper limbs Rapid, intense movements of short duration

Connective tissue coverings of muscle

Layers: - Epimysium = connective tissue that surrounds a whole muscle - it surrounds many fascicles - outer connective tissue layer - Perimysium = denser connective tissue surrounding a group of muscle fibers - each group is called a fasciculus - a fasciculus is a grouping of muscle fibers - Endomysium = loose connective tissue with reticular fibers - endo means within Muscular fascia: connective tissue sheet - external to epimysium - holds muscles together and separates them into functional groups

Ion channels

Ligand gated: - ligands are molecules that bind to receptor - a receptor is a protein or glycoprotein with a receptor site - Ex: neurotransmitters (a neurotransmitter is a substance that can stimulate the ligand gated channel) - the gate is closed until the neurotransmitter attaches to the receptor; when ach (acetylcholine) attaches to a receptor on a muscle cell the sodium gate opens and sodium moves into the cell due to the concentration gradient Voltage gated: - gates open and close in response to small voltage changes across the plasma membrane

Cardiac muscle muscle type

Location: Heart Cell shape: Cylindrical and branched (100-500 μm in length, 12-20 μm in diameter) Nucleus: Single, centrally located Intercalated disks join cells to one another Have striations Involuntary control Capable of spontaneous contractions Function: Pumping blood; contractions provide the major force for propelling blood through blood vessels

Excitation- contraction coupling

Mechanism where an action potential causes muscle fiber contraction Involves: sarcolemma - Transverse tubules (T tubules) are the invaginations of the sarcolemma that receive the action potential - The terminal cisternae which is a holding area - and a sarcoplasmic reticulum which is the smooth ER of a muscle cell - the triad consists of the T tubule and two adjacent terminal cisternae - Calcium (Ca2+) - Troponin is a globular protein with three binding sites

Motor units

Motor units = a single motor neuron and all of the muscle fibers innervated by it (innervate = capable of sending a message to that structure) Motor unit numbers - Large muscles have motor units with many muscle fibers innervated - small muscles that make delicate movements contain motor units with few muscle fibers

Whole Skeletal Muscle Structure: Connective Tissue, Innervation, and Blood Supply

Muscular fascia (surrounds individual muscles and groups of muscles) Muscle fiber Artery Nerve Vein Capillary Fascicle Axon of motor neuron Synapse or neuromuscular junction Artery Vein Nerve Epimysium (surrounds muscles) Perimysium (surrounds fascicles) Endomysium (surrounds muscle fibers)

Motor unit image - (a) A motor unit consists of a single motor neuron and all the muscle fibers its branches innervate. The muscle fibers shown in dark pink are part of one motor unit, and the muscle fibers shown in light pink are part of a different motor unit. In this figure, the motor neuron is innervating the dark pink muscle fibers. (b) Photomicrograph of motor units

Neuromuscular Muscle fibers junction Axon of motor Myofibrils neuron Axon branches Capillary Axons of motor neurons Neuromuscular junctions Muscle fibers

Image the neuromuscular junction

Presynaptic terminal Synaptic vesicles Motor end-plate (sarcolemma) Synaptic cleft (b) Mitochondrion Muscle fiber Myofibrils Axon branch Presynaptic terminal Neuromuscular junction Capillary (a) Skeletal muscle fiber Neuromuscular junction Axon branch (c) Sarcolemma Sarcoplasm

Structure of actin and myosin image = (a) The sarcomere consists of actin (thin) myofilaments, attached to the Z disks, and myosin (thick) myofilaments, suspended between the actin myofilaments. (b) Actin myofilaments are composed of F actin (chains of purple spheres), tropomyosin (blue strands), and troponin (red spheres and rod). Myosin myofilaments are made up of many golf-club-shaped myosin molecules, with all the heads pointing in one direction at one end and the opposite direction at the other end. (c) G actin molecules (purple spheres), with their active sites (yellow), tropomyosin, and troponin, make up actin myofilaments. Myosin molecules (green) are golf-club-shaped structures composed of two molecules of heavy myosin wound together to form the rod portion and double globular heads. Four small, light myosin molecules are located on the heads of each of the myosin molecules.

Sarcomere Cross-bridge Actin myofilament Myosin myofilament Z disk Z disk G actin molecules Tropomyosin Troponin Titin Binds to G actin Binds to Ca2+ Binds to tropomyosin Active sites Myosin (thick) myofilament Myosin molecule Coiled portion of the two α helices Myosin light chains Hinge region of myosin Myosin head Rod portion Two myosin heavy chains Active sites Troponin Tropomyosin F actin molecules

Organization of sarcomeres

Sarcomere M line I band A band H zone Z disk Sarcomere Actin myofilament Myofibrils Myosin myofilament I band A band Z disk H zone M line Z disk Actin myofilaments only Myosin myofilaments surrounded by actin myofilaments Myosin myofilaments only Rod portion of myosin myofilaments and M line Mitochondria Sarcomere Actin myofilament Myosin myofilament Cross sections through regions of the sarcomeres The arrangement of I and A bands, H zones, Zdisks and M lines in sarcomeres

Sarcomeres: Z Disk to Z Disk

Sarcomere: Basic functional unit of a muscle fiber Z disk: a filamentous network of protein that serves as attachment for actin myofilaments (network means they arent completely parallel) Striated appearance: - contributed to by the Z disks - I bands: area from a Z disk to the ends of a thick filament - A bands: length of thick filaments (how long is the myosin) - H zone: region in an A band where actin and myosin do not overlap (this is where the potential is for actin and myosin to come together) - M line: Middle of the H zone; delicate filaments holding myosin in place (when you are contracting how close are you getting to the M line) (also helps to stabilize) In muscle fibers, A and I bands of parallel myofibrils are aligned

T tubules and sarcoplasmic reticulum image

Sarcoplasmic reticulum Triad Terminal cisterna Transverse tubule (T tubule) Terminal cisterna Capillary Mitochondrion Myofibril A band I band Sarcolemma

Membrane Potentials in Smooth Muscle Smooth muscle exhibits three patterns of action potentials.

Slow waves of depolarization Action potentials in smooth muscle superimposed on a slow wave of depolarization Action potential with prolonged depolarization (plateau)

Slow twitch oxidative fibers (type I) (SO)

Smallest fiber diameter High myoglobin content Many mitochondria Many capillaries High aerobic metabolic capacity and low anaerobic metabolic capacity High fatigue resistance Slow myosin ATPase activity Low glycogen concentration Fibers are generally most abundant in postural muscles and more in lower limbs than upper limbs Maintenance of posture and performance of endurance activities

Smooth muscle tissue is made up of sheets or bundles of spindle-shaped cells, with a single nucleus in the middle of each cell

Smooth muscle histology

Wave Summation

Stimuli 1-4 increase in frequency. For each stimulus, the arrow indicates the start of stimulation. Stimulus frequency 1: A single action potential arriving at a muscle fiber causes twitches that completely relax before the next action potential arrives. Stimulus frequencies 2-3: As the action potential frequency increases, muscle fibers only partially relax before the next action potential arrives and the muscle fiber contracts again; this results in incomplete tetanus. Stimulus frequency 4: Action potential frequency can increase to the point where the muscle fiber does not relax at all before the next action potential arrives, causing the muscle fiber to contract continuously; this results in complete tetanus.

Sarcomere shortening

The sarcomere is the individual unit When the muscle is relaxed the H zone is wide open When a contraction starts myosin heads have made contact with actin and now the myosin is pulling on the actin and the H zone is reducing - this will continue until there is no ability to bring the actin anymore this means that the H zone disappears

Types of physiological muscle response

Treppe - Tension produced increases for the first few contractions in response to a maximal stimulus at a low frequency in a muscle that has been at rest for some time. Increased tension may result from the accumulation of small amounts of Ca2+ in the sarcoplasm for the first few contractions or from an increasing rate of enzyme activity. Wave summation - Summation results when many action potentials are produced in a muscle fiber. ■ Contraction occurs in response to the first action potential, but there is not enough time for relaxation to occur between action potentials. ■ Because each action potential causes the release of Ca2+ from the sarcoplasmic reticulum, the ion levels remain elevated in the sarcoplasm to produce a tetanic contraction. ■ The tension produced as a result of wave summation is greater than the tension produced by a single muscle twitch. The increased tension results from the greater concentration of Ca2+ in the sarcoplasm and the stretch of the elastic components of the muscle early in contraction. Tetanus of muscles - Tetanus of muscles results from wave summation; frequency of stimulus is higher than for treppe. ■ Incomplete tetanus occurs when the action potential frequency is low enough to allow partial relaxation of the muscle fibers. ■ Complete tetanus occurs when the action potential frequency is high enough that no relaxation of the muscle fibers occurs. Multiple-motor-unit recruitment - Each motor unit responds in an all-or-none fashion. A whole muscle is capable of producing an increasing amount of tension as the number of motor units stimulated increases. Isometric contractions - A muscle produces increasing tension as it remains at a constant length; this is characteristic of postural muscles that maintain a constant tension without changing their length. Isotonic contractions - A muscle produces a constant tension and shortens during contraction; this is characteristic of finger and hand movements. ■ In concentric contractions, a muscle produces tension as it shortens; this is characteristic of biceps brachii curl exercises. ■ In eccentric contractions, a muscle produces tension as it resists lengthening; this is characteristic of slowly descending a flight of stairs.

Types of smooth muscle

Visceral or unitary: cells in sheets they function as a unit together - numerous gap junctions (the cells become more autorhythmic) - waves of contraction - often autorhythmic - esophagus Multiunit: cells or groups of cells that act as independent units - sheets [blood vessels] - bundles [arrector pili (the ones attached to hair follicles) and iris] single cells [capsule of spleen]

Contraction occurs on the microscopic level in

muscles

Threshold

the level of stimulation required to trigger a neural impulse You can have stimulation but if it is not strong enough to reach the threshold (and is in turn subthreshold) then it will not bring about an action potential