MUSCLE PHYSIO

Epimysium

outermost dense CT, surrounds entire muscle

Perimysium

surrounds fascicles

Fascicles definition

bundle grouping multiple muscle fibers

Endomysium

thin areolar (loose) CT surrounds muscle fibers, separates muscle fibers from eachother

Where does muscle excitation occur?

muscle fibers, where axons innervate muscles

Electrical component of muscle

sarcolemma, transverse tubules, sarcoplasmic reticulum, sarcoplasm

Mechanical component of muscle

myofibril, actin, myosin, sarcomere

Sarcolemma

specialized membrane covering muscle fiber embedded with receptors for chemicals that neurons release

Transverse tubules

invaginations of sarcolemma, connects extracellular environment to inside of muscle fiber, near SR

Sarcoplasm

where mitochondria and glycogen granules (stores energy) can be found

Myofibril

inside muscle fiber, contains light (I) and dark (A) band

H zone

area WITHOUT actin filaments in A band (when at rest), only myosin

What happens to the H zones during contraction

disappears when actin and myosin overlap

Role of troponin and tropomyosin

lock myosin head in place

Titin

holds myosin to Z disc

A band

contains both actin and myosin, dark band

I band

contains only actIn

Troponin

has 3 binding sites (actin, calcium, tropomyosin) at end of tropomyosin, holds tropomyosin in place

Myosin head

ATPase enzyme that splits ATP (after it binds), aka crossbridge

Sarcomere

structural, functional, contracticle unit of skeletal muscle, inside myofibril

Describe how myofibril looks during contraction

A band covered by 2 Z discs

Neuromuscular junction

synapse between neuron and muscle fiber

Motor unit

consists of 1 neuron and all skeletal muscle fibers it stimulates

Axon

extension of neuron reaches muscle, branches out to other axon terminals

Nerve impulses

electrical signals that motor neurons carry

Presynaptic neuron is usually a _

Axon terminal

Axon terminal

Forms junctions with sarcolemma, containing synaptic vesicle (Ach)

Acetylcholine

Specific neurotransmitter for muscle contraction, binds to ligand gated ion channels, acetic acid + choline

Synaptic cleft

gap between axon terminal and muscle fiber, filled with interstitial fluid, where ach is released

Ach receptor location

on membrane of sarcolemma

How is sliding filament initiated?

Ca2+ triggers myosin head to bind to actin

What is the electrical properties of skeletal muscle based on

Movement of ions across membrane

Ligand vs. voltage gated ion channel

Ligand for ACh, voltage for Na+, K+, Ca+

Resting membrane potential definition

Amount of energy stored due to charge difference between inside and outside of plasma

Action potential frequency definition

Action potential produced per unit of time

Phases of muscle contraction

Muscle twitch → Latent → contraction → relaxation → refractory

How do motor units vary

number of muscle fibers and motor units

sensitivity to stimulus

Describe motor units that have: DELICATE AND PRECISE MOVEMENT

Many motor units, few muscle fiber

Describe motor units that have: POWERFUL BUT LESS PRECISE CONTRACTION

Few motor units, many muscle fiber

Activity of ions in membrane at rest

K+ leaks out cell faster than Na+ entering, and negative charge trapped inside due to permeability

Describe tetanus or tetanic contraction

Sustained muscle contraction during high rate of action potential, caused by stimuli with increasing frequency

Wave summation

increased tension via increased contraction

Describe tension

elastic parts of sarcoplasm stretch + CT of muscle stretch

Unfused tetanus

incomplete, partial relaxation between contraction

Fused tetanus

rapid production of action potential, relaxation no longer occurs

Length and tension relationship

length increases, tension increases

Size and force relationship

size increases, force increases

3 energy sources

phosphocreatine, glycolysis, oxidative metabolism

What happens during relaxation period after contraction

Ca2+ actively pumped back tp SR, tropomyosin covers binding sites

Sliding filament theory

myosin head sliding actin towards M line

What happens during period of contraction

Cross bridge starts, TENSION INCREASES

What happens during relaxation period

Calcium pumped back to SR, cross bridge ends, TENSION DECREASES

Properties of muscle

contractility, excitability, extensibility, elasticity

Sources of ATP in skeletal muscle

myokinase, creatine kinase, anaerobic respiration, aerobic respiration

Muscle fatigue definition

Reduced work capacity, temporary

Muscle tone

small tension in muscle due to weak, involuntary contraction of motor units

Isotonic contraction

same tension, length changes

Isometric contraction

same length, tension changes

Describe muscle twitch

1. Happens at low stimulus, all or none, lets muscle rest

1. Brief contraction and relaxation

2. Not long enough to generate tension in action

What happens when muscle fibers are higher

Greater force generated

Describe large diameter muscle fiber

More myofibril, more cross bridges formed

What is tension influenced by

Initial length of muscle fiber

Active tension

Force of muscle lifting object (contracts)

How does active tension drop

Muscle stretched beyond ideal length, actin and myosin overlap less, reducing contraction strength

Passive tension

Force applied to load when muscle stretches but not contracting

Describe Muscle fiber 

1. Long cells, make up muscles

2. Multinucleated from fusion of myoblast precursor cells

Cardiac muscle [1shape, 2striation, 3nuclei, 4control, 5location, 6function, 7key features]

 Branched, short

2. Yes

3. Single, central nucleus

4. Involuntary

5. Heart walls

6. Pump blood

7. Intercalated disc

Skeletal muscle [1shape, 2striation, 3nuclei, 4control, 5location, 6function, 7key features]

1. Long, cylindrical

2. yes

3. Multinucleate, peripherally placed

4. Voluntary

5. Attached to bone

6. Locomotion

Smooth muscle [1shape, 2striation, 3nuclei, 4control, 5location, 6function, 7key features]

1. Spindle-shaped

2. No striations

3. Single, central nucleus

4. Involuntary

5. Around walls of hollow organs

6. Peristalsis, vasoconstriction

7. Sustained contraction

What are the two tissues outside sarcolemma

1. External lamina - deeper, thinner

2. Endomysium - thicker

Sarcoplasmic reticulum 

1. Surrounds myofibril

2. Stores, releases Ca2+ into sarcoplasm

Actin filament

1. Thin

2. Held by Z disc

Myosin filament

1. Thick

2. Held by M line

Hierarchy

Muscle (covered by Epimysium) → Fascicle (covered by Perimysium) → Muscle fiber (covered by Endomysium) → Myofibril → Sarcomere

Describe structure of myosin

2 myosin-heavy chains wounded together with heads at the end

Latent period

Lag during stimulus and beginning of contraction

Tropomyosin

1. protein along groove of F actin double helix

2. Binds to actin, blocks myosin binding site on actin (unless displaced by troponin)

PROCESS OF EXCITATION-CONTRACTION COUPLING

1. Action potential (at neuromuscular junction) reaches axon terminal

2. Voltage gated Ca2+ channels open, Ca2+ enters

3. Ca2+ influences Ach to be released from synaptic vesicles in synaptic cleft (fuses with presynaptic membrane, released via exocytosis) 

4. Ach binds to receptors on sarcolemma of muscle fiber 

5. Na+ channels on sarcolemma open, depolarization, triggers action potential of muscle

   1. Action potential spreads from sarcolemma to T tubules

   2. Triggers Ca2+ release from SR (increases permeability of sarcoplasmic reticulum to Ca2+, allows Ca2+ to diffuse to sarcoplasm)

   3. Ca2+ binds to troponin, displaces tropomyosin, exposes myosin binding sites on actin

   4. CROSS BRIDGE: Myosin head (with ADP + Pi attached) attaches to actin, Phosphate (P) released

   5. POWER STROKE: Actin pulled inwards towards M-line, ADP released

   6. CROSS BRIDGE RELEASE: Myosin detaches from actin when ATP binds to myosin

   7. HYDROLYSIS ATP: ATP hydrolyzed by ATPase part of myosin head, still attached tho

   8. RECOVERY STROKE: May contract again if Ca2+ still attached to troponin

Resting membrane potential of muscle

-90 mV

Factors of muscle fatigue

1. Acidosis

2. ATP depletion

3. Oxidative stress

4. inflammation

Factors for force of contraction of muscle

1. Neuronal activity

2. Stimulation frequency

3. Muscle fiber diameter and length during contraction

Neuronal activity

1. Muscle contracts only after it’s activated by ACh

2. Muscle tone determined by neurons in brain & spinal cord

3. Muscle responds to stimuli in graded fashion

Describe high stimulus of muscle

1. Treppe: Muscle fiber contracts with more force each time until maximum force is reached and muscle can no longer relax

2. Wave summation → Incomplete tetanus (unfused) → Complete tetanus (fused)