[MOD5LEC] Muscular System

What are the general functions of the skeletal muscular system?

Controls voluntary movement of the head, face, trunk, limbs, etc.

• Makes the maintenance of the position of the body possible

• Provides 85% of the body’s heat through glucose catabolism

• Accounts for approximately 40% of the body’s mass

• Participates in involuntary spinal cord reflexes

What are some important characteristics of skeletal muscle tissue?

It can also be referred to as voluntary muscle tissue or striated muscle tissue

• Muscles attach to various bones of the skeleton by tendons

What are universal characteristics of muscles?

• Responsiveness - all living cells are excitable

  • muscle cells respond to stimuli in a unique way by producing an electric current that travels along the plasma membrane and triggers contraction of the cell

• Conductivity - stimulation of a muscle cell produces more than just a local effect

  • local electrical change triggers a wave of excitation that travels rapidly along the cell and initiates processes leading to contraction

• Contractility - unique in their ability to shorten substantially when stimulated

  • enables muscle cells to exert tension on other tissue to which they are mechanically coupled to create movement

• Extensibility - able to stretch back after contraction

  • most cells rupture if they are stretched even a little, but skeletal muscle fibers can safely stretch to as much as 3 times their contracted length

• Elasticity - when muscle cells are stretched and released, they recoil to a shorter length

  • different from the ability to stretch, which is a trait of extensibility

Why are muscle and nerve cells regarded as electrically excitable cells?

Because their plasma membranes exhibit voltage changes in response to stimulation

• Electrical activity of cells is related to differences in the concentration of ions in the intracellular fluid (ICF) and extracellular fluid (ECF) adjacent to the plasma membrane

Explain why a membrane is polarized as a result of the concentration of ions in the intracellular and extracellular fluid

The ICF contains a greater concentration of negative anions than the ECF does, such as negatively charged proteins, nucleic acids, and phosphates

The ICF also contains a great excess of potassium ions (K⁺), whereas the ECF contains a great excess of sodium ions (Na⁺)

The difference in electrical charge from one point to another is called an electrical potential, or voltage

What is resting membrane potential (RMP)?

About 90 millivolts (mV), a number that is critically important to life

Resting membrane potential is maintained by the sodium-potassium pump

Sarcolemma [Microscopic Structure Of Skeletal Muscle Cells]

Specialized plasma membrane specific to skeletal muscle cells

Sarcoplasm [Microscopic Structure Of Skeletal Muscle Cells]

Specialized type of cytoplasm specific to skeletal muscle cells, that contains an abnormally higher amount of mitochondria than other cells for the high energy demands of muscle

Sarcoplasmic reticulum (SR) [Microscopic Structure Of Skeletal Muscle Cells]

Specialized smooth endoplasmic reticulum (ER) parallel to myofibrils

The sarcoplasmic reticulum of muscle cells stores calcium, which is vital in the process of muscle contraction and relaxation

Terminal cisternae

A perpendicular terminal, which is an enlarged portion of sarcoplasmic reticulum (SR)

• A reservoir for calcium ions

• Has gated channels in its membrane that can release a flood of calcium into the sarcoplasm

Transverse tubules or T-tubules [Microscopic Structure Of Skeletal Muscle Cells]

A hollow elongated tube

• Carries an electrical current from the extracellular fluid and sarcolemma to myofibrils

Triad [Microscopic Structure Of Skeletal Muscle Cells]

Refers to an arrangement of a T-tubule and terminal cisternae on each side

Myofibrils [Microscopic Structure Of Skeletal Muscle Cells]

The many threadlike rods that extend and make up the full length of a muscle fiber

• Fill up most of the muscle cell

• Composed of protein myofilaments

Myofilaments refer to two types of filaments; thick filaments and thin filaments

PICTURE: Thick filaments are purple, thin filaments are red

Thick filaments [Types Of Myofilaments]

A type of myofilament made of the protein myosin

• Myosin molecules are shaped like golf clubs, with a shaft-like tail and a globular head

• A thick filament consists of 200 to 500 myosin molecules, with their heads directed outward in a spiral array around the bundle

• Heads on one half of the thick filament angle to the left, while the heads on the other half angle to the right

There is an area in the middle of thick filaments where there is no myosin heads present, and is referred to as a bare zone or an H-zone

Thin filaments [Types Of Myofilaments]

A type of myofilament made of two intertwined strands. These strands are made out of the protein known as fibrous actin

• Fibrous actin is referred to as F-actin

• Each F-actin molecule is a string, which is made of bead-like subunits called globular actin or G-actin

• Each G-actin has an active site that binds to the heads of myosin molecules on thick filaments

Tropomyosin

A long strand-like protein that covers and blocks the active sites of thin filaments

• Tropomyosin occupies the spiral groove along the length of an F-actin molecule

Troponin

A globular molecule that binds with tropomyosin and G-actin at the active site

Troponin contains a calcium (Ca⁺⁺) receptor

How do tropomyosin and troponin work to regulate muscles?

They act as a “switch” to determine when a muscle can and cannot contract

Tropomyosin molecules have troponin bound to them at regular intervals

A-bands

Refers to the darkest area of a muscle fiber, where myosin and actin myofilaments overlap

These are the primary reason that cross-striations are visible

H-band

The area of an A-band that is only composed of myosin (thick filament)

I-bands

Lightest area between A-bands composed of actin only

Z-discs bisect the light colored I-bands

Sarcomere

The functional unit of the myofibril

• Refers to the portion of a myofibril from one Z-disc to the next

• Constitutes one contractive unit

Muscles shorten because the individual sarcomeres shorten, thus pulling the Z-discs closer together

Z-disks

Dark peripheral boundaries of sarcomere

Striations

The appearance given off by actin and myosin fibers, which are organized into geometrically precise arrays

• Striated muscle consists of dark A-bands alternating with lighter I-bands

What is the structure of skeletal muscle from its smaller units to its larger units?

A bundle of muscle fibers is a fascicle

Each muscle fascicle is surrounded by perimysium on all sides

A group of muscle fascicles is collectively surrounded by epimysium

Epimysium is then covered by fascia

What are the types of fascia?

• Subcutaneous fascia - lies deep to the skin

• Subserous fascia - covers organs and body cavities

• Deep fascia - surrounds and penetrates whole muscles

Describe what a neuron is and what structures they’re made of

A neuron is a nerve cell that the nervous system uses to communicate

• Soma - control center of the neuron; contains the nucleus

• Dendrites - branching structures only found on the soma that receive signals from other neurons

• Axon - emerges from the axon hillock of the soma, specialized for rapid conduction of nerve signals to points far from the soma

Nerve impulses only travel in one direction through a neuron:

Dendrites → through the soma → to the axon hillock → out through the axon → other neurons or effectors

What is a motor unit? [Neuromuscular Junctions]

One nerve fiber and all the muscle fibers innervated by it

• There is an average of 200 muscle fibers that connect to a motor neuron

• The muscles behave as a single unit

What is the location of a neuromuscular junction? What is the function of a motor neuron? [Neuromuscular Junctions]

Neuromuscular junctions are located wherever motor neurons meet muscle fibers

The function of motor neurons is to relay nerve impulses to a muscle fiber, where it then becomes a motor impulse

What structures make up a motor neuron? [Neuromuscular Junctions]

Synaptic knob - contains synaptic vesicles

Synaptic vesicles - contain the neurotransmitter acetylcholine (ACh)

Describe the synaptic cleft [Neuromuscular Junctions]

The gap between a motor neuron and a muscle fiber at a neuromuscular junction

• The muscle fiber contains about 50 million acetylcholine (ACh) receptors

• Junctional folds - infoldings of a muscle cell’s sarcolemma that increases the surface area and allows more ACh receptor sites to be present

Acetylcholine is released from the synaptic knob of motor neurons and travels to the surface of the muscle (sarcolemma) to be retrieved by acetylcholine receptors

What are the four major phases of muscle contraction and relaxation?

Excitation - action potentials in the nerve fiber lead to action potentials in the muscle fiber

Excitation-Contraction Coupling - the events that link the action potentials on the sarcolemma to activation of the myofilaments, preparing them to contract

Contraction - muscle fibers develop tension and may shorten

Relaxation - when its work is the done, a muscle fiber relaxes and returns to resting length

Excitation [Phases Of Muscle Contraction And Relaxation]

Action potentials in the nerve fiber lead to action potentials in the muscle fiber

• The dendrites of a motor neuron are stimulated

• An action potential (nerve impulse) travels along the neurilemma of an axon to the synaptic end-foot

• Synaptic vesicles in the synaptic knob release acetylcholine (ACh)

• ACh diffuses across the synaptic cleft towards a muscle fiber

• ACh binds to an ACh-receptor site and stimulates a motor impulse

Acetylcholine receptors are ligand-gated ion channels

SUMMARY: A nerve impulse travels through a motor neuron to the synaptic knob, prompting the synaptic vesicles to release ACh that move across the synaptic cleft and bind to ACh receptors to stimulate a motor impulse

During the excitation phase, what happens when ACh bind to ACh receptors? [Excitation]

Sodium (Na⁺) quickly flows into the muscle cell and potassium (K⁺) flows out

• Results in the reversing of polarity of sarcolemma

• Voltage jumps from -90 mV to a peak of +75 mV as sodium (Na⁺) enters

• Voltage drops back to -90 mV as potassium (K⁺) exits

ACh is a ligand, and the ACh receptors on a muscle cell are ligand-gated ion channels; open or close when a ligand binds to it

During the excitation phase, what do voltage-gated ion channels do? [Excitation]

• Some are specific to sodium (Na⁺) and allow it to leave the cell

• Some are specific to potassium (K⁺) and allow it to leave the cell

• The movement of these ions creates an action potential, which excites the muscle fiber

Voltage-gated ion channels open or close in response to a change of voltage

Excitation-Contraction Coupling [Phases Of Muscle Contraction And Relaxation]

The events that link the action potentials on the sarcolemma to the activation of the myofilaments, preparing them to contract

• Wave of action potentials spread from the endplate of a synaptic knob in all directions

• Motor impulse travels into a muscle fiber via T-tubules

• Motor impulses stimulate the sarcoplasmic reticulum to release calcium (Ca⁺⁺) ions

• Ca⁺⁺ ions diffuse out of terminal cisternae into the sarcoplasm

• Ca⁺⁺ ions bind to troponin molecules causing them to change shape. This change in shape causes tropomyosin molecules to be pushed out of the way and exposes the active sites on G-actin molecules

SUMMARY: Action potentials travel into a muscle cell through the T-tubules, and stimulate the sarcoplasmic reticulum to release Ca⁺⁺ ions. These ions diffuse through terminal cisternae into the sarcoplasm, and bind to troponin. Troponin changes shape and pushes tropomyosin molecules, and exposes the active sites on G-actin molecules, thus allowing muscle contraction to take place

When the active sites on G-actin molecules are blocked, muscles cannot contract

Contraction [Phases Of Muscle Contraction And Relaxation]

Muscle fibers develop tension and may shorten

• Cross-bridges form between myosin heads and active sites

• Power stroke; ATP breaks the cross-bridge and allows myosin heads to pull actin molecules towards the center of the sarcomere

• Sarcomere and thus the muscle fiber shortens and contraction has occurred

Relaxation [Phases Of Muscle Contraction And Relaxation]

When its work is done, the muscle fiber relaxes and returns to resting length

• Nerve signals stop arriving at the neuromuscular junction

• Synaptic knob stops releasing acetylcholine (Ach)

• ACh disassociates or leaves its receptor

• Acetylcholinesterase (AChE) stimulates the break down of ACh

• Ca⁺⁺ ions are actively transported away from the troponin molecules they were bound to, and move back into the sarcoplasmic reticulum

• Cross-bridges between myosin and actin myofilaments are broken

• The active sites on tropomyosin molecules are once again blocked

• Muscle fibers lengthen; relaxation has occurred

Muscle twitch

Refers to a single muscular contraction and relaxation

Threshold stimulus

Minimal stimulus required for a single muscle twitch

Latent period

Delay of about 2 milliseconds between the onset of a stimulus and the onset of the twitch

All-or-none response (phenomenon)

Muscle fibers may respond to their fullest extent when exposed to only threshold stimulus

Graded muscle response

An entire muscle will contract to varying degrees depending on the nature of the stimulus

Recruitment

A given amount of muscle fibers or motor units that can be activated by a stimulus

Treppe

Increased strength of stimuli causes an increase in the strength of contractions

Maximal stimulus

Stimulus causing maximum contraction of a muscle

Supramaximal stimulus

Any stimulus greater than maxial stimulus

Summation

When an increased frequency of stimuli does not allow a muscle to completely relax before the next contraction occurs; wave summation

This results in fluttering contractions; incomplete tetany

Complete tetany

When muscle contraction lacks even a partial relaxation, and contractions fuse into a smooth, prolonged contraction

Muscle tone

The small amount of sustained muscular contraction occurring in most muscles at rest

• Necessary for maintaining posture, breathing, etc.

ATP (adenosine triphosphate)

All muscle contraction depends on ATP; no other source can serve in its place

• The supply of ATP depends on the availability of oxygen, glucose, and fatty acids

The two main pathways through which ATP is created are:

Aerobic respiration - cells produce ATP in the presence of oxygen

and

Anaerobic fermentation - cells produce ATP without oxygen, but produce much less; generates lactic acid

Describe how ATP is produced and utilized when the body needs immediate energy (PART 1)

In examples such as an 100 meter dash:

• Myoglobin in a muscle fiber supplies oxygen so that aerobic respiration can occur in a limited amount

• Oxygen supply is quickly depleted

• Until the respiratory and cardiovascular systems catch up with the heightened oxygen demand, the muscle meets most of its ATP requirements by borrowing phosphate groups (P_i) from other molecules and transferring them to ADP

Describe how ATP is produced and utilized when the body needs immediate energy (PART 2)

Two enzymes control the phosphate group (P_i) transfer to ADP

• Myokinase - breaks off a P_i from one ADP to another ADP, converting the latter to ATP

• Creatine kinase - obtains P_i from a phosphate-storage molecule, creatine phosphate

  • donates it to ADP to make ATP

ATP and creatine phosphate are collectively called the phosphagen system, and provide nearly all the energy used for short bursts of intense activity

Describe how ATP is produced and utilized when the body needs short-term energy (CONNECTED TO NEXT SLIDE)

When the phosphagen system is exhausted, the muscles transition to anaerobic fermentation until the heart and lungs can begin to produce more oxygen required for aerobic respiration

• During this period, muscles obtain glucose from the blood and their own stored glycogen

• The pathway from glycogen to lactic acid, called the glycogen-lactic acid system, produces enough ATP for about 30 to 40 seconds of maximum activity

Describe how ATP is produced and utilized when the body needs long-term energy (CONNECTED TO PREVIOUS SLIDE)

After 40 seconds or so, the respiratory and cardiovascular systems have caught up and deliver oxygen to the muscles fast enough for aerobic respiration to begin taking up ATP production

• Typically another 30 ATP per glucose

• It is a very efficient means of meeting the ATP demands of prolonged exercise