Muscle Tissue Muscle Structure and Function

Types of Muscle Tissue Skeletal Muscle Tissue – moves the body by pulling on bones of the skeleton ▫Allows us to walk, move, pick up and throw objects  Voluntary – we can control Cardiac Muscle Tissue – pumps blood through the circulatory system ▫Involuntary – we can’t control Smooth Muscle Tissue – pushes material through the digestive tract and controls the diameter of small arteries. ▫Involuntary – we can’t control

Functions of Skeletal Muscle 1.Produce skeletal movement 2.Maintain posture and body positioning 3.Support of soft tissues 4.Guard entrances and exits 5.Maintain body temperature 6.Store nutrient reserves

Organization of Muscle Tissue From smallest structure to largest structure: ▫Muscle fiber (cell) Muscle fascicle (bundle of cells) Skeletal Muscle (organ)

Anatomy of Skeletal Muscle 3 layers of connective tissue ▫Epimysium  Surrounds entire muscle ▫Perimysium  Divides skeletal muscle into compartments called fascicles  Contains blood vessels and nerve fibers ▫Endomysium  Surrounds each individual muscle fiber  Contains capillaries that supply blood to fiber, satellite cells (stem cells that repair muscle cells), nerve fibers that control the muscle.

Anatomy of Skeletal Muscle The fibers of the epimysium, endomysium and perimysium are interwoven to form either a bundle (tendon) or a broad sheet (aponeurosis)

Skeletal Muscle Fibers (cells) Different than typical cells ▫Very large  Can run the length of your thigh (30cm) ▫Multi-nucleated  Contain hundreds of nuclei  Control production of enzymes and proteins necessary for muscle function

Skeletal Muscle Fibers (cells) Formed during development from the fusing of multiple embryonic cells (myoblasts) Some myoblasts don’t fuse ▫Called satellite cells in adult muscles  Repair damaged muscle

Skeletal Muscle Fibers (cells) Parts of a muscle fiber ▫Sarcolemma – cell membrane of muscle cells ▫Sarcoplasm – cytoplasm of muscle cells ▫Transverse tubules (T-tubules) – narrow tubes that carry the electric signal for contraction deeper into the cell

Skeletal Muscle Fibers (cells) Parts of a muscle fiber ▫Myofibrils  Consist of bundles of protein filaments  Thin filaments – composed of actin  Thick filaments - composed of myosin  Actively shortening component of muscle  Responsible for muscle contractions

Skeletal Muscle Fibers (cells) Sarcoplasmic reticulum (SR) ▫Similar to endoplasmic reticulum in regular cells ▫Tightly bound to the T-tubules ▫Forms a network around each myofibril ▫Stores Ca++ ions for muscle contractions  Up to 40,000 time the amount found in the sarcoplasm  A contraction begins when Ca++ ions are released into the sarcoplasm Video Clip

Skeletal Muscle Fibers (cells) Sarcomeres ▫Functional unit of skeletal muscle  Actual contracting unit ▫About 10,000 sarcomeres, end-to-end, make up a myofibril

Skeletal Muscle Fibers (cells) Each sarcomere has dark bands (A bands) and light bands (I) The A band ▫Made up of thick filaments (myosin) ▫3 subdivisions  M line – dark, central line where the thick filaments are connected to their neighbors  H zone – light area around the M line. Has thick filaments but no thin filaments  Zone of overlap – thin and thick filaments overlap

Skeletal Muscle Fibers (cells) The I band ▫Contains just the thin (actin) filaments ▫Extends from the A band from one sarcomere to the A band of the next ▫Z line – marks the boundary between adjacent sarcomeres The A and I bands are visible with a light microscope and are called striations, thus skeletal muscle is also known as striated muscle

Skeletal Muscle Fibers (cells) Thin Filaments ▫Contains strands of proteins (actin) ▫Has active sites that are used during muscle contraction

Skeletal Muscle Fibers (cells) Thick Filaments ▫Contains roughly 300 myosin molecules  The myosin tail is long and is bound to other myosin molecules in the thick filament  The free head projects out toward the nearest thin filament  When the head interacts with the thin filament during a contraction, it is called a cross-bridge

Skeletal Muscle Fibers (cells) Thick Filaments ▫Myosin molecules are arranged with their tails towards the M line ▫The heads are arranged in a spiral ▫H zone contains no myosin heads

Sliding Filament Theory When a muscle contracts: ▫H zones and I bands get smaller ▫Zones of overlap get larger ▫Z lines move closer together ▫Width of the A band remains constant This only makes sense if the thin filaments slide alongside the thick filaments toward the center of the sarcomere (M line) ▫This is known as the sliding filament theory

Muscle Tissue Muscle Contraction

When muscle fibers contract, they actively pull on the tendon fiber the way people pull on a rope. ▫This pull is called tension ▫It is an active force, so it requires energy For movement to occur, the tension must overcome the resistance of the object. ▫Resistance depends on the weight, shape, friction, and other factors. MUSCLES PULL…THEY DO NOT PUSH!!!!

Overview of Skeletal Muscle Contraction Skeletal muscle fibers are activated by neurons (nerve cells) ▫Activated by stimulation of the sarcolemma Excitation-contraction coupling occurs next ▫Calcium ions are released from sarcoplasmic reticulum Calcium ions trigger interactions between thick and thin filaments, resulting in fiber contraction and the consumption of energy in the form of ATP Tension is produced.

Control of Skeletal Muscle Skeletal muscle only contracts under control of the nervous system ▫Communication that occurs between muscle and nerve takes place at what is known as a neuromuscular junction (NMJ) Each muscle fiber is controlled by a neuron at a single NMJ midway along its length.

Control of Skeletal Muscle The neuron branches when it reaches the muscle ▫At the end of each branch, there is a synaptic terminal  Contains the neurotransmitter Acetylcholine (Ach) The synaptic cleft is the narrow space between the synaptic terminal and the sarcolemma. The sarcolemma surface of the synaptic cleft is known as the motor end plate The synaptic terminal and the sarcolemma contain acetylcholinesterase (AChE) ▫Breaks down ACh

Control of Skeletal Muscle Stimulation of the muscle occurs through 5 steps. 1.Arrival of action potential  Electrical impulse arrives at synaptic terminal 2.Release of ACh  The action potential triggers the release of ACh into the synaptic cleft 3.ACh binds at the Motor End Plate  ACh molecules diffuse across cleft and bind to receptors on Motor End Plate  Increases the sarcolemma’s permeability of sodium ions, and sodium ions rush into the sarcolemma

Control of Skeletal Muscle 4.Appearance of action potential in the sarcolemma  The rush of sodium ions causes an action potential in the sarcolemma  Travels inward via the T-tubules 5.Return to initial state  ACh is broken down by AChE.  ACh is recycled

Excitation-Contraction Coupling The step between the generation of the action potential in the sarcolemma and the start of a muscle contraction is called excitation- contraction coupling. The action potential in sarcolemma triggers the release of calcium ions(Ca ++ ) from the sarcoplasmic reticulum.

Excitation-Contraction Coupling Remember that the thin filament has active sites on it. ▫At rest, these are covered After the Ca++ is released from the sarcoplasmic reticulum, the active site is uncovered, allowing the myosin head to bind. Muscle contraction now begins

The Contraction Cycle The myosin head is already energized, ready to act. Step 1: Exposure of Active Sites  Ca++ binds to troponin, exposing the active sites Step 2: Formation of Cross-Bridges  The myosin heads bind to the exposed active sites Step 3: Pivoting of myosin heads  When at rest, the myosin head points away from the M line. Myosin head is “cocked”  After cross-bridge formation, the head pivots toward the M line as energy is released (called the power stroke)

The Contraction Cycle Step 4: Detachment of Cross-Bridges  When another ATP binds to the myosin head, it detaches from the active site  Active site can now form another cross-bridge Step 5: Reactivation of Myosin  Occurs when myosin head splits the ATP  This energy is used to re-cock the myosin head This cycle can be repeated several time each secondThis cycle can be repeated several time each second

The Contraction Cycle Each power stroke shortens the sarcomere by 1 percent ▫Because all sarcomeres contract together, the entire muscle shortens at the same rate. To better understand how tension is produced in a muscle fiber, think of a tug-of-war.

Relaxation Duration of contraction depends on: ▫Duration of stimulation at NMJ ▫Presence of free Ca++ in the sarcoplasm ▫Availability of ATP

Relaxation If one action potential arrives at the NMJ, Ca++ levels in the sarcoplasm will quickly return to normal. Two mechanisms are involved in this process: ▫Active transport of Ca++ across the cell membrane into the extracellular fluid ▫Active transport of Ca++ into the SR  This one is much more important

Relaxation As the Ca++ levels in the sarcoplasm fall, ▫Active sites are re-covered. The contraction ends

Return to Resting Length Since muscle can’t actively lengthen, outside forces must lengthen the muscle ▫Opposing muscle contractions ▫Gravity

Rigor Mortis Within a few hours after death, muscle fibers run out of ATP ▫The SR can not pump Ca++ out of the sarcoplasm, triggering a sustained contraction ▫Myosin heads don’t detach from active sites. Rigor mortis lasts until the Z-lines are broken down hours after rigor mortis sets in.

Muscle Tissue Tension Production

Tension depends on the amount of pivoting cross-bridges. There is no mechanism to regulate the amount of tension by changing the number of contracting sarcomeres ▫When Ca++ is released, it is released from all SR in the muscle fiber ▫Muscle fiber is either “on” or “off”

Tension Production Tension at the muscle fiber level does vary. It depends on: ▫The fiber’s resting length at the time of stimulation ▫The frequency of stimulation

Length-Tension Relationship The amount of tension depends on the number of cross-bridges along the length of the fiber. ▫Depends on the degree of overlap between thick and thin fibers. ▫When a fiber is stimulated to contract, only the myosin heads in the zone of overlap can bind to the active sites.  The more myosin heads in the zone, the more tension…To a point

Length-Tension Relationship A sarcomere works most efficiently within an optimal range If the sarcomere is stretched, there will be less myosin heads in the zone of overlap. If the sarcomere is compressed/shortened, it has less room to shorten because the actin filaments are already close to the M line.

Length-Tension Relationship

Frequency of Stimulation Twitch ▫A single stimulation producing a single contraction ▫Lasts milliseconds depending on the muscle ▫Can be divided into 3 phases  Latent period – begins at stimulation, lasts 2 msec  Action potential sweeps across sarcolemma, and Ca++ is released from SR  NO TENSION IS PRODUCED YET

Frequency of Stimulation  Contraction phase – tension rises to a peak  Cross-bridges are forming  Ends about 15msec after stimulation  Relaxation phase – lasts about 25msec  Ca++ levels fall  Active sites are being covered  Decrease in cross-bridges  Tension falls back to resting levels

Frequency of Stimulation

Treppe ▫When the muscle is stimulated immediately after the relaxation phase has ended.  The resulting contraction will produce slightly more tension than the first  This will continue for the first contractions, then tension levels off  Tension rises because there is extra Ca++ left over from previous stimulus. SR doesn’t have enough time to pump all Ca++ back in.

Frequency of Stimulation Wave Summation ▫If a second stimulus arrives before the relaxation phase has ended, a second, more powerful contraction occurs.  A stimulus of greater of 50 per second will produce wave summation Incomplete Tetanus ▫If stimulation continues and the muscle is never allowed to relax completely, tension will rise until it reaches a peak 4X that of treppe. ▫This is incomplete tetanus

Frequency of Stimulation Complete Tetanus ▫Occurs when a higher stimulation frequency eliminates the relaxation phase completely.  SR doesn’t have time to reclaim the Ca++ at all.

Frequency of Stimulation

Motor Units Amount of tension a muscle produces as a whole is the sum of the tensions generated by the individual muscle fibers ▫You control the amount of tension in skeletal muscle by controlling the number of stimulated muscle fibers Motor Unit - All the muscle fibers controlled by a single motor neuron ▫The smaller the unit, the finer the control you have over the muscle.  Eye muscles have motor units of 4-6 muscle fibers  Leg muscles may have motor units consisting of muscle fibers.

Motor Units When a movement is performed, the smallest motor units in the muscle are activated ▫These muscle fibers contract slowly As the movement continues, larger, faster, more powerful motor units are activated. ▫Tension production rises steeply Recruitment – smooth steady increase in tension by increasing the number of activated motor units

Muscle Tone Muscle tone – the resting tension in skeletal muscle ▫Activated muscle units are changed so that they may relax and recover Resting muscle tone: ▫Stabilizes positions of bones  Ex. – balance and posture ▫Prevents sudden, uncontrolled changes in the positions of bones and joints ▫Higher resting tone accelerates the recruitment process because some of the motor units are already activated

Types of Muscle Contraction Isotonic Contraction ▫Tension rises and the muscle changes length  Muscle can shorten AND lengthen while activated ▫Concentric Contraction – tension EXCEEDS resistance and muscle shortens  Ex: flexion of elbow ▫Eccentric Contraction – tension is LESS than the load and the muscle elongates  Due to pull of another muscle, gravity, etc.  Ex: extension of elbow

Types of Muscle Contraction Isometric Contraction ▫Muscle length does not change  Tension produced never exceeds resistance  Ex: holding a bag of groceries, standing upright