Muscular System Lecture Exam (Anatomy)

Skeletal Muscle Tissue

(Striated) Moves bones, few muscles attach to skin and moves skin or other skeletal muscles. Activity is consciously and subconsciously controlled by neurons (part of somatic cell division of nervous system).

Striated

Alternating light and dark protein bands (striations) are seen when tissue is examined with microscope

Skeletal Muscle Tissue

(Striated) Moves bones, few muscles attach to skin and moves skin or other skeletal muscles. Activity is consciously and subconsciously controlled by neurons (part of somatic cell division of nervous system).

Striated

Alternating light and dark protein bands (striations) are seen when tissue is examined with microscope

Example of Subconscious Control in Skeletal Muscle

Diaphragm continues to alternately contract and relax so you don’t stop breathing. You also do not have to think about contracting skeletal muscles maintaining posture or stabilizing body positions.

Cardiac Muscle Tissue

(Striated) Forms heart wall, action is involuntary. Alternating contraction and relaxation of heart is due to natural pacemaker (autorhythmicity). (Not consciously controlled)

Autorhythmicity

Naturally built-in pace-making in the heart, several hormones and neurotransmitters can adjust heart rate by speeding or slowing pacemaker.

Smooth Muscle Tissue

(Nonstriated) Located in walls of hollow internal structures (blood vessels, airways and organs within abdominopelvic cavity). Also found in skin attached to hair follicles. Action is usually involuntary.

Which kinds of Muscle Tissue have Autorhythmicity?

Cardiac muscle tissue and smooth muscle tissue like the ones that propel food through digestive canal.

Cardiac and Smooth Muscle are regulated by…

Neurons part of the autonomic (involuntary) division of nervous system and by hormones released by endocrine glands.

Functions of Muscular Tissue

1. Producing body movements

2. Stabilizing body positions

3. Storing and moving substances within body

4. Generating heat

Storage/Transfer through Smooth Muscle

Sphincters prevent outflow of contents of hollow organs. Temporary storage of food in stomach or urine in urinary bladder is possible because the sphincter closes off outlets of these organs. Also moves food and substances such as bile and enzymes through digestive canal, pushes gametes (sperm and oocytes) through passageways of genital systems.

Storage/Transfer through Cardiac Muscle

Contractions in heart wall, pump blood through blood vessels of body. Contraction and relaxation of smooth muscle in walls of blood vessels help adjust their diameter and regulate rate of blood flow.

Storage/Transfer through Skeletal Muscle

Contractions promote flow of lymph plasma and aid return of blood in veins to the heart.

Thermogenesis

As muscular tissue contracts, it produces heat which is used to maintain body temperature. Shivering can increase rate of heat production.

Electrical Excitability

(Muscle and nerve cells) Ability to respond to stimuli by producing action potentials. Two main stimuli trigger action potentials in muscle: one is autorhythmic (heart pacemaker) and the other is chemical stimuli (neurotransmitters released by neurons, hormones or changes in pH).

Action Potentials

Electrical signals (impulses), rapid changes in membrane potential involving depolarization followed by repolarization. Related to nerve impulses and muscle fibers.

Contractility

Ability to contract forcefully when stimulated by nerve impulses. When skeletal muscle contracts it creates tension (force of contraction).

Extensibility

Ability to stretch within limits, without being damaged. Connective tissue within muscle limits the range and keeps it within contractile range of muscle cells. Smooth muscle is stretched whenever stomach fills food and cardiac muscle is stretched each time heart fills with blood.

Elasticity

Ability to return to original length and shape after contraction or extension.

Muscle Fibers

Skeletal muscles are composed of hundreds to thousands of ____

Connective Tissue

Surrounds and protects muscular tissue

Subcutaneous Tissue (Hypodermis)

Separates muscle from skin and is composed of areolar connective and adipose tissues.

Which Tissue provides pathway for nerves, blood vessels and lymph vessels to enter and exit muscles?

Subcutaneous Tissue

Adipose In Subcutaneous Tissue

Stores most of the body’s triglycerides, serves as an insulating layer, reducing heat loss, and protecting muscles from physical trauma.

Fascia

Dense sheet or broad band of irregular connective tissue that lines body wall and limbs and supports and surrounds muscles and other organs.

Other Functions of Fascia

Holds muscles with similar functions together, allows free movement of muscles, carries nerves, blood vessels and lymph vessels, and fills spaces between muscles.

What are the three Connective Tissue layers?

Epimysium, perimysium, endomysium

Epimysium

(Dense Irregular) Outer layer, encircling the entire muscle

Perimysium

(Dense Irregular) Surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called muscle fascicles.

Endomysium

(Reticular fibers) Penetrates the interior of each muscle fascicle and separates individual muscle fibers.

The three layers of connective tissue could…

extend beyond muscle fibers to form a ropelike tendon, that attaches to periosteum of bone

Example of Tendon

Calcaneal (Achilles) tendon of gastrocnemius muscle attaches to calcaneus (heel bone).

Aponeurosis

Broad, flat sheet extended from connective tissue elements.

Example of Aponeurosis

Epicranial aponeurosis on top of skull between frontal and occipital bellies of the occipitofrontalis muscle.

Sarcolemma

Plasma membrane of muscle fiber, located above multiple nuclei of skeletal muscle fibers.

T Tubules (transverse)

Thousands of tiny tube-shaped invaginations of sarcolemma, which tunnel in from surface toward center of each muscle fiber.

What travels along sarcolemma and through T tubules?

Muscle action potentials

Sarcoplasm

(Contains myoglobin) Cytoplasm of muscle fiber includes substantial amount of glycogen. Glycogen can be used for synthesis of ATP.

Myoglobin

Red-colored protein, found only in muscle, binds oxygen molecules that diffuse into muscle fibers from interstitial fluid. It releases oxygen when needed by mitochondria for ATP production.

Muscular Hypertrophy

Muscle growth after birth by enlargement of existing muscle fibers.

What causes Hypertrophy?

Increased production of myofibrils, mitochondria, sarcoplasmic reticulum and other organelles. It results from very forceful, repetitive muscular activity such as strength training.

Fibrosis

Replacement of muscle fibers by fibrous scar tissue

Muscular Atrophy

Decrease in size of individual muscle fibers as a result of progressive loss of myofibrils.

Microscopic Organization of Skeletal Muscle

During embryonic development, many myoblasts fuse to form one skeletal muscle fiber. After fusion, skeletal muscle fiber loses the ability to undergo cell division, but satellite cells retain this ability. The sarcolemma of the fiber encloses sarcoplasm and myofibrils, which are striated. Sarcoplasmic reticulum wraps around each myofibril. Thousands of T tubules, filled with interstitial fluid, invaginate from the sarcolemma toward the center of the muscle fiber.

Contractile Elements of Skeletal Muscle

Myofibrils, filaments, sarcoplasmic reticulum and sarcomeres.

Contractile Proteins

Generate force during contraction

Regulatory Proteins

Help switch the contraction process on and off

Structural Proteins

Keep thick and thin filaments in proper alignment, give myofibril elasticity and extensibility, and link myofibrils to sarcolemma and extracellular matrix.

What are the Two Contractile Proteins?

Myosin and actin

Myosin

Main component of thick filaments, functions as motor protein in all three types of muscle tissue. Tails of these point towards the M line in the center of a sarcomere. The two projections of each of these molecules are the heads.

Myosin Heads have Two Binding Sites…

An actin binding site and an ATP binding site.

Motor Proteins

Pull various cellular structures to achieve movement by converting the chemical energy in ATP to mechanical energy (production of force).

Actin

Main components of thin filaments, individual molecules join to form the filament that is twisted into a helix. Each of these molecules have a myosin binding site, where myosin head can attach.

Z Discs

Narrow, plate-shaped regions of dense material that separate one sarcomere from the next.

A Band

Dark, middle part of sarcomere that extends entire length of thick filaments and includes those parts of thin filaments that overlap thick filaments.

I Band

Lighter, less dense area of sarcomere that contains remainder of thin filaments but no thick filaments. A Z disc passes through center of each I band.

H Band

Narrow region in center of each A band that contains thick filaments but no thin filaments.

M Line

Region in center of H zone that contains proteins that hold thick filaments together at center of sarcomere.

Myofibril Banding Pattern in Relaxed Muscle:

H, I, and A bands all maintain length.

Myofibril Banding Pattern in Partially Contracted Muscle:

H and I bands are narrowed while the A band remains the same, Z disc contracts

Myofibril Banding Pattern in Maximally Contracted Muscle:

H and I bands are completely contracted, A band remains the same, Z disc fully contracted.

Contraction Cycle Steps

- ATP Hydrolysis

- Attachment of myosin to actin

- Power stroke

- Detachment of myosin from actin

What Stars and Stops Muscle Contraction?

Increase in Ca2+ starts it and decrease stops it

Beginning of Muscle Contraction:

As a muscle action potential propagates along the sarcolemma and into the T tubules, it causes the release of Ca2+ from the SR into the sarcoplasm and this triggers muscle contraction.

Where does Excitation-Contraction Coupling Occur?

Triads of the skeletal muscle fiber

Triads Consist of:

A T tubule, and two opposing terminal cisterns of the sarcoplasmic reticulum, which are mechanically linked together by two groups of integral membrane proteins

Voltage-Gated Ca2+ Channels

Located in T tubule membrane (cluster of four = Tetrads). Serve as voltage sensors that trigger the opening of the Ca2+ release channels.

Ca2+ Release Channels

Present in the terminal cisternal membrane of SR. When skeletal muscle fiber is at rest, the part of the Ca2+ release channel that extends into the sarcoplasm is blocked by a given cluster of voltage-gated Ca2+ channels, preventing Ca2+ from leaving the SR. When skeletal muscle fiber is excited and an action potential travels along the T tubule, the voltage-gated Ca2+ channels detect the change in voltage and undergo a conformational change that ultimately causes the Ca2+ release channels to open.

Tropomyosin

(Regulatory protein) component of thin filament; when skeletal muscle fiber is relaxed, tropomyosin covers myosin-binding sites on actin molecules, thereby preventing myosin from binding to actin.

Troponin

(Regulatory protein) component of thin filament; when calcium ions (Ca2+) bind to troponin, it changes shape; this conformational change moves tropomyosin away from myosin-binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin.

Structural Proteins

Proteins that keep thick and thin filaments of myofibrils in proper alignment, give myofibrils elasticity and extensibility, and link myofibrils to sarcolemma and extracellular matrix.

Titin

Structural protein that connects Z disc to M line of sarcomere, thereby helping to stabilize thick filament position; can stretch and spring back unharmed, accounts for much of the elasticity and extensibility of myofibrils.

a-Actinin

Structural protein of Z discs that attaches to actin molecules of thin filaments and to titin molecules.

Myomesin

Structural protein that forms M line of sarcomere; binds to titin molecules and connects adjacent thick filaments to one another.

Nebulin

Structural protein that wraps around entire length of each thin filament; helps anchor thin filaments to Z discs and regulates length of thin filaments during development

Dystrophin

Structural protein that links thin filaments of sarcomere to integral membrane proteins in sarcolemma, which are attached in turn to proteins in connective tissue matrix that surrounds muscle fibers; thought to help reinforce sarcolemma and help transmit tension generated by sarcomeres to tendons.

Levels of Organization:

1.Filaments (myofilaments)

2.Myofibril

3.Muscle Fiber

4.Muscle Fascicle

5.Skeletal Muscle

Rigor Mortis

Muscles cannot contract or stretch after death, begins 3-4 hours after death and lasts about 24 hours.

Rigor Mortis Process

Cellular membranes become leaky. Calcium ions leak out of the sarcoplasmic reticulum into the sarcoplasm and allow myosin heads to bind to actin. ATP synthesis ceases shortly after breathing stops, however, so the cross-bridges cannot detach from actin.

Neuromuscular Junction

The synapse between a somatic motor neuron and a skeletal muscle fiber.

Synapse

Region where communication occurs between two neurons, or between a neuron and a target cell, in this case, between a somatic motor neuron and a muscle fiber.

Synaptic Cleft

Small gap in synapses, separating two cells.

Neurotransmitter

Chemical messengers sent by first cell to the second across the gap, to send action potentials.

How does a Nerve Impulse (Nerve action potential) elicit a Muscle action potential?

1.Release of Acetylcholine

2.Activation of ACh receptors

3.Production of muscle action potential

4.Termination of ACh activity

ACh is broken down by…

Acetylcholinesterase (AChE)

Acetylcholine

Neurotransmitter released at the neuromuscular junction.

Acetylcholinesterase

This enzyme is located on the extracellular side of the motor end plate membrane. AChE breaks down ACh into acetyl and choline, products that cannot activate the ACh receptor.

Muscle Fibers have three ways of Producing ATP:

1.Creatine Phosphate

2.Anaerobic Glycolysis

3.Aerobic Respiration

Creatine Phosphate (ATP Production)

Excess ATP is used to synthesize this energy rich molecule found in muscle fibers. Creatine kinase (CK) catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming this molecule and ADP.

Creatine

Small, amino acid–like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers.

Where is a Creatine Phosphate Molecule more plentiful than ATP?

(3-6 times) In the sarcoplasm of a relaxed muscle fiber.

Which Process of ATP Production is the Fastest and Most likely the first source of energy?

Creatine Phosphate

Anaerobic Glycolysis (ATP Production)

Pyruvic acid generated from glycolysis is converted to lactic acid, due to lack of oxygen during heavy exercise. Breakdown of glucose gives rise to lactic acid when oxygen is low or absent.

Each Molecule of Glucose Catabolized via Anaerobic Glycolysis yields…

2 molecules of lactic acid and 2 molecules of ATP.

What does Pyruvic Acid formed by Glycolysis in the cytosol do?

It enters mitochondria, where it undergoes a series of oxygen-requiring reactions called aerobic respiration that produce a large amount of ATP.

What is Pyruvic acid converted to?

Lactic acid

Where does lactic acid produced by Anaerobic Glycolysis go?

It diffuses out of the skeletal muscle fiber into the blood.

Liver Cells can…

Take up some of the lactic acid molecules from the bloodstream and convert them back to glucose, this conversion also reduces the acidity of the blood.

Anaerobic glycolysis also…

Produces fewer ATPs than aerobic respiration, but it is faster and can occur when oxygen levels are low or absent.

Aerobic Respiration (ATP Production)

When oxygen is present, pyruvic acid formed by glycolysis enters mitochondria, where it undergoes a series of oxygen-requiring reactions (the Krebs cycle and the electron transport chain) that produce ATP, carbon dioxide, water, and heat.

Each molecule of glucose catabolized under aerobic conditions…

Yields about 30 or 32 molecules of ATP.