Anatomy - Exam 4

Chapter 9

Functions of muscles

Movement, stability, control of openings, heat production, glycemic control

Movement muscle funtion

Move body parts and body contents (breathing, circulation, digestion)

Stability muscle function

Maintain posture by preventing unwanted movements and stabilize joints by maintaining tension

Control of openings muscle function

Control of sphincters

Heat production muscle function

Skeletal muscles produce 85% of our body heat

Glycemic control muscle function

Muscles absorb and store glucose which helps regulate blood sugar

Skeletal muscle

Long, cylindrical multinucleate cells with striations → voluntaryand is responsible for the movement of bones and maintenance of posture

Cardiac muscle

Branching, striated uninucleate cells that connect through intercalated discs → involuntary and responsible for pumping blood throughout the body

Smooth muscle

Spindle shaped cells with central nuclei and don’t have striations → involuntary and control movement in hollow organs

Skeletal muscle

Cardiac muscle

Smooth muscle

3 kinds of muscle

Skeletal, cardiac, and smooth

Myology

Study of the muscular system

What are muscles specialized for

Converting the chemical energy in ATP into mechanical energy for movement

Endomysium

Thin connective tissue around each muscle fiber → allows room for capillaries and nerve fibers, provides chemical environment for muscle fiber

Perimysium

Thick layer of connective tissue that wraps muscle fascicles → carries nerves, blood vessels, and stretch receptors

Fascicles

Bundles of muscle fibers wrapped together → wrapped by the perimysium

Epimysium

Fibrous sheath surrounding entire muscle → outer surface blends into fascia and inner projections (parts that move inward in between muscle fascicles) help form perimysium

Fascia

Sheet of connective tissue that covers entire muscle and separates neighboring muscles or muscle groups from each other and the subcutaneous tissue

What determines the strength of a muscle and the direction it pulls?

The orientation of the fascicles within the muscle

Fusiform muscles

Thick in the middle and taper at both ends → ex: biceps brachii

Parallel muscles

Uniform width (doesn’t taper) and has parallel fascicles → ex: sartorius

Triangular (convergent) muscles

Broad at one end and narrow at the other → ex: pectoralis major

Circular muscles (sphincters)

Form rings around body openings → ex: obicularis oris

Pennate muscles

Feather shaped and there are 3 types

• Unipennate

• Bipennate

• Multipennate

Unipennate muscles

Fascicles attach to one side of tendon and run at an angle → ex: extensor digitorium longus

• Think of feather with the tendon being the shaft and barbs only coming off one side

Bipennate muscles

Fascicles attach to both sides of a central tendon and run at an angle → ex: rectus femoris

• Think of a feather with the tendon being the shaft and barbs coming off both sides

Multipennate muscles

Fascicles attach to multiple tendons within the muscle and are arranged in several feather like sections → ex: deltoid

• Think of many feathers bundled together

Muscle compartment

Group of muscles that function together enclosed by fascia → also contains nerves and blood vessels that supply the muscle group

Intermuscular septa

Very thick fascia that separate muscle compartments from each other

Indirect attachment to bone

Tendons connect muscle to bone → muscle doesn’t directly touch the bone

Direct (fleshy) attachment to bone

Muscle seems to emerge directly from bone → little separation between muscle and bone

Aponeurosis

Flat, broad tendon → sheet-like tendon

Retinaculum

Band of thickened fascia around joints that helps stabilize tendons and maintain their proper position → holds tendons in place at joints

Intrinsic muscle

Muscle that originates and inserts in the same region → muscles located entirely within a specific structure (ex: hand, foot, tongue)

• Provide fine motor control and precise movement

Extrinsic muscle

Muscle that originates outside of the structure it acts on → finger movements are from extrinsic muscles in the forearm

Agonist

Muscle producing most of the force during a particular action → also known as the prime mover

• Ex: the biceps brachii is the agonist when flexing the elbow

Synergist

Muscle that assists the prime mover (agonist) → may contribute additional force, stabilize nearby joint, or modify the direction of movement

• Ex: the brachioradialis acts as a synergist to the biceps brachii during elbow flexion

Antagonist

Muscle that opposes the actions of the prime mover (agonist) → often relaxes while the agonist contracts

• Can contract to prevent excessive movement or stabilize the joint during movement

• Ex: the triceps brachii are the antagonist in flexing the elbow → when the biceps contract, the triceps relax to allow the movement

Antagonist pairs

Muscles that act on opposite sides of a joint → oppose each other’s actions during movement

• Ex: biceps brachii and triceps brachii

Fixator

Muscle that prevents movement of bone → stabilizes a joint or body part during an action

• Ex: the brachialis acts as a fixator in flexing the elbow by stabilizing the elbow joint

Innervation of a muscle

Refers to the nerve that stimulates a muscle

Spinal nerves innervate…

Muscles below the neck

Cranial nerves innervate…

Muscles of the head and neck → numbered CN I to CN XII

How much blood does the muscular system receive each minute at rest?

1.24 L of blood per minute at rest (about 25% of total cardiac output) → during exercise more blood is pumped to the muscles (muscles get a higher percentage of the cardiac output)

How much blood does the muscular system receive during heavy exercise?

~11.6 L per minute (about 75% of total cardiac output) → during strenuous activity, blood flow to muscles increases significantly to meet oxygen demands and the muscles receive more of the total cardiac output

Universal characteristics of muscle

• Contractility

• Excitability (responsiveness)

• Extensibility

• Elasticity

• Conductivity

Contractility

Ability of a muscle to shorten (contract) when stimulated

Excitability (responsiveness)

Ability of a muscle to respond to chemical signals, stretch, and electrical charges across the plasma membrane → this characteristic enables muscle fibers to receive stimuli and initiate a contraction

Extensibility

Muscles are capable of being stretched between contractionsand can extend beyond their resting length without damage

Elasticity

Ability of a muscle to return to its original rest length after being stretched

Conductivity

Local electrical excitation sets off a wave of excitation that travels along the muscle fiber

Striations

Alternating light and dark transverse bands in skeletal muscle → result from the arrangement of internal contractile proteins (actin and myosin filaments)

Sarcolemma

Plasma membrane of a muscle fiber

Sarcoplasm

Cytoplasm of a muscle fiber that contains organelles and myofibrils, as well as other components vital for muscle contraction

Myofibrils

Long protein cords occupying most of sarcoplasm inside muscle fiber (cell) → made of repeating sarcomeres lined up end to end

Glycogen

Carbohydrate (polysaccharide) stored in muscle fibers to provide energy for exercise → acts as a ready-to-use fuel source

Myoglobin

Red protein in muscle cells that binds oxygen, facilitating oxygen storage and transport during muscle contraction.

Red, oxygen-binding protein found in muscle cells → stores oxygen inside muscle fibers and releases it when the muscle needs it like during exercise

• Source of SOME oxygen needed for muscle activity

Myoblasts

Stem cells that fused to form each muscle fiber early in development

Satellite cells

Unspecialized stem cells (myoblasts) that remain between the muscle fiber and endomysium (connective tissue around each muscle fiber)

• Play a role in regeneration of damaged skeletal tissue

• Outside of sarcolemma

Mitochondria location muscle fiber (cell)

Packed into spaces between myofibrils

Sarcoplasmic reticulum (SR)

Smooth ER that forms network around each myofibril and stores calcium ions, playing a crucial role in muscle contraction

Terminal cisterns

Dilated sacs of sarcoplasmic reticulum (SR) that store calcium ions and release them to initiate muscle contraction → located and A-band and I-band junctions on myofibrils

• Forms “triad” with transverse tubule → 2 terminal cisterns with 1 transverse tubule in the middle

Transverse tubules (T tubules)

Inward “tunnels” of the sarcolemma that run through the muscle fiber and create the triad with terminal cisterns → openings of transverse tubules found in sarcolemma

• Carry electrical signals (action potentials) from the surface quickly to the interior of the muscle fiber → makes sure all parts contract simultaneously

Triad

Structure formed by T tubule and two terminal cisterns associated with it → one T tubule with a terminal cistern on each side

• ensures rapid uniform conduction at the A-I junction

Thick filament

Myofilament made of several hundred myosin molecules → each molecule is shaped like a gold club where two chains are intertwined to form a shaft like tail with a double globular head at the end

• Think of intertwined chains as shaft of club and the 2 globular heads as the clubhead

• Heads on half on the thick filament angle to the left and the heads on the other half angle to the right with a bare zone with no heads in the middle

Thin filament

Myofilament made of 2 intertwined fibrous (F) actin strands → fibrous (F) actin strand made of string of globular (G) actin subunits each with an active site that can bind to myosin head

• Has tropomyosin molecules and troponin molecules

Tropomyosin molecules

Long protein that runs along actin strands and block the myosin binding sites at rest → each tropomyosin molecule blocks 6-7 active sites

Troponin molecules

Small calcium binding protein on each tropomyosin molecule → when calcium binds to troponin, the tropomyosin molecules move and uncover the myosin binding sites on actin strands

Elastic filament

Myofilament made of titin proteins (huge, springy proteins) that provide elasticty, prevent overstretching, and anchor thick filaments → run through the core of thick filament and anchor it to Z-disc and M-line

• Helps maintain muscle alignment

• When the muscle is stretch titin stretches and then pulls the thick filament back into position

Contractile proteins

Myosin and actin proteins are responsible for the work of contraction

Regulatory proteins

Tropomyosin and troponin act like switch that determines when fiber can and can’t contract

Dystrophin

Protein that links actin in outermost myofilaments to membrane proteins that link to endomysium (connective tissue around muscle fibers) → transfers forces of muscle contraction to connective tissue leading to tendon

• Acts as shock absorber

Striations are alternating…

A-bands (dark) and I-bands (light)

A-band

Dark bands of myofibrils where thick (myosin) filaments overlap with thin (actin) filaments

• H band = middle of the A-band and its not as dark because it contains only thick filaments, with no overlap from thin filaments (also where there are no myosin heads from thick filaments)

• M-line = middle of H band

I-band

Light bands of myofibrils that contain only thin (actin) filaments and no thick (myosin) filaments, located on either side of the A-band

• Also contains Z-disc

Z-disc

Dense protein plate that acts as an anchor for thin and elastic filaments, connecting them between adjacent sarcomeres

Sarcomere

Segments of myofibrils from z-disc to z-disc → functional contractile unit of muscle fiber

Muscle cells shorten because…

their individual sarcomeres shorten → Z-discs are pulled closer together as thick and thin filaments slide past each other during contraction

• neither thick or thin filaments change length during muscle shortening, only the amount of overlap between them changes

Structural hierarchy of skeletal muscle from biggest to smallest

1. Muscle

2. Muscle fascicles

3. Muscle fibers

4. Myofibrils

5. Sarcomeres

6. Myofilaments

Sliding filaments start when…

Myosin heads bind to actin to form cross bridges→ thick filaments pull the thin filaments toward the center of the sarcomere (H band and M line), causing contraction

• Z-discs are pulled closer to M-line → I-bands shorten because overlap increases → H band (area with only thick filaments) disappears → A bands move closer together but their lengths stay the same

Denervation atypohy

Shrinkage of paralyzed muscle when nerve remains disconnected

Somatic motor neurons

Nerve cells whose bodies are int he brainstem and spinal cord that serve skeletal muscles

• Their axons that lead to the skeletal muscle = somatic motor fibers

Motor unit

One nerve fiber and all the muscle fibers innervated by it → muscle fibers of one motor unit are dispersed throughout the muscle

• Produce weak contraction over wide area and provide ability to sustain long term contraction as motor units take turns contracting

• Effective contraction usually requires contraction of several motor units at once

Small motor units

Fine degree of control → 3-6 muscle fibers per neuron (ex: eye and hand muscles)

Average motor units

~200 muscle fibers per neuron

Large motor units

More strength than control → up to 1,000 muscle fibers per neuron (ex: leg muscles)

• Powerful contractions by large motor units

Muscle action potential

1. Sodium gates open in plasma membrane of muscle fibers and sodium flows into cell down gradient

2. Depolarization → inside of plasma membrane becomes more positive because of more Na+

3. Sodium gates close and potassium gates open

4. Potassium rushes out of cell and repolarization occurs → membrane becomes negative again because of loss of positive ions

5. Potassium channels close

Difference between resting membrane potential (RMP) and action potential

RMP is seen in waiting excitable cell while action potential is a quick event seen in a stimulated excitable cell

Acetylcholine (ACh)

Initiates muscle contraction when released from synaptic vesicles of axon terminal (through exocytosis) and binds to receptors on muscle cell membrane, allowing sodium influx and triggering an action potential

Acetylcholinesterase (AChE)

Breaks down ACh, allowing for muscle relaxation

Excitation of muscle fibers

1. Action potential arrives at axon terminal of neuromuscular junction and ACh is released from synaptic vesicles

2. ACh binds to receptors on sarcolemma (motor end plate) and open ligand-gated Na⁺ channels resulting in depolarization/end plate potential (more positive = less negative charge)

3. Action potiential initiated in muscle fiber → it travels along the sarcolemma and T tubules

4. Terminal cisterns of SR release calcium which binds to troponin molcules, tropomyosin moves and exposes myosin binding sites on actin strands

5. Myosin binds to actin (formation of cross bridge) and contraction begins

Relaxation after contraction

1. Nervous stimulation stops, so ACh stops being released

2. ACh is broken down by AChE

3. Calcium is reabsorbed by terminal cisterns of SR → troponin loses calcium and tropomyosin moves back to blocking myosin binding sites

4. Tension decreases and muscle returns to resting state

Spastic paralysis

State of continual contraction of the muscles becuase AChE can’t degrade ACh → possible suffocation

• Some pestisides contain cholinesterase inhibitors

• Ex: lockjaw

Flaccid paralysis

State in which the muscles are limp and cannot contract → Botulism prevents ACh release