Muscular System

3 types of muscle tissue

1. skeletal

2. smooth

3. cardia

skeletal muscle characteristics

• striated appearance - parallel/banded look

• involved in voluntary movement

• fibers with multi-nucleated cells (due to their length) - undergo myogenesis

what is myogenesis?

Formation of multinucleated muscle fibers through fusion of myoblasts

eimysium

surrounds the muscle fiber that keeps everything together

myofibril

contained in each muscle fiber, they’re the cellular structures inside of a muscle fiber that are responsible for its ability to contract

Myofibrils

long protein filaments that are organized into an even small level of structure

sarcomeres

functional unit of muscle fiber

• individual contractile units

• functional units of skeletal muscle

Sarcoplasmic Reticulum

Stores Ca²⁺ and surrounds myofibrils

• endoplasmic reticulum of a muscle cell

T-tubules

helps to initiate muscle contraction, made of muscle fibers plasma membrane, allow us to spread electrical signals throughout the inside of the muscle cell

sarcoplasm

functions as the cytoplasm in muscles

sarcolemma

plasma membrane of muscle cells → can propagate action potentials

Neuromuscular Interaction

nerve cells release acetylcholine to open ion channels that depolarize the sarcolemma

Sarcomere

the smallest functional unit of skeletal muscle, and are the structural unit of myofibrils.

Myofibrils

composed of chains of sarcomeres → make up a sarcomere

What is a sarcomere made of?

thick (myosin) and thin (actin filaments)

components of sarcomere

Z line, M line, I band, H zone, A band

Z line

anchors the actin filament and defines the outer boundaries of a single sarcomere

M line

middle of the myosin filaments; passes through the center of the sarcomere

I band

• region containing only actin (thin filaments)

• shortens during muscle contraction

H zone

region containing only myosin

A band

• entire length of a single myosin filament

• does not shorten during contraction

what happens to filaments when a sarcomere contracts?

the thin and thick filaments become more overlapped compared to when they are relaxed

skeletal muscle doesn’t undergo…

mitosis to create new muscle cells

What do muscle cells do to grow?

hypertrophy, the cell size increases, the number of nuclei then increases, the diameter of muscle fibers increase, therefore the number of sarcomeres and myofibrils increases.

Muscle Contraction

serves to develop tension within the muscle

• can squeeze blood and lymph vessels → aiding in circulation

3 types of muscle contraction

concentric, eccentric, isometric

concentric

muscle fibers shorten

eccentric

muscle fibers lengthen

isometric

muscle length is constant

Isotonic contractions

determined by movement → concentric and eccentric contractions

Skeletal muscle types

Type I, Type IIA, Type IIB

Type I skeletal muscle fibers

slow twitch fibers

• high endurance

• lots of myoglobin + mitochondria

• aerobic metabolism

• appear red

• small diameter

Type IIA skeletal muscle fibers

Fast twitch fibers

• balance of power + endurance

• intermediate myoglobin + mitochondria amount

• aerobic + anaerobic metabolism

• appear reddish-pink

• intermediate diameter

Type IIB skeletal muscle fibers

fast twitch fibers

• high power

• low myoglobin + mitochondria

• anaerobic metabolism (glycolysis)

• appear white

• large diameter

Action potential travels down the motor neuron and ….

arrives at the neuromuscular junction

Acetylcholine is released from the motor nuuron…

and binds to the acetylcholine receptors on the motor end plate

Sodium rapidly enters….

the muscle cell, triggering depolarization after ion channel is opened

An action potential is generated in the….

motor end plate, spreading across the sarcolemma and propagating down the T-tubules

Voltage-gated calcium channels on the….

sarcoplasmic reticulum are triggered to open

calcium then rushes…

out of the sarcoplasm reticulum

once calcium leaves sarcoplasm it…

binds to troponin this causes a conformational change which allows myosin and actin filaments to bind together

Sliding Filament Model

changes occurring with the thick and thin filaments

• 1st step: Ca²⁺ binds troponin → moves tropomyosin → beginning of myosin-actin interaction

Step 1 of sliding filaments model

binding of myosin to acit

• binding of Ca²⁺ exposes attachment sites

• myosin head forms a cross-bridge with Actin filament

step 2 of sliding filaments model

P_i is released → Power stroke

• P_i is released

• actin pulled towards middle of sarcomere

step 3 of sliding filament model

ADP is released → low-energy myosin

• ADP is released

• Myosin → low-energy form

Step 4 of sliding filament model

New ATP → unbinding of cross bridges

• new ATP binds to myosin head

• unbinding of myosin and actin

• without new ATP → cross bridges remain attached

Step 5 sliding filament model

ATP hydrolyzed → myosin head cocked back

• ATP is hydrolyzed (ATP → ADP + Pi)

• Myosin position is reset

Motor Units

a motor neuron and its innervated muscle fibers

One motor neuron can…

synapse with multiple muscle fibers

each muscle fiber synapses…

with only one motor neuron

Force of contraction is proportional to:

• size and number of motor units used (spatial summation)

• frequency of action potentials (temporal summation)

What is recruitment?

• when a greater quantity of muscle fibers are activated by the brain

• smaller motor units activated before larger ones

Intricate movements use

small motor units

large movements use

large motor units

Sub-threshold stimulus

so low, an action potential isn’t generated, therefore no muscle contraction is produced

threshold stimulus

bare minimum to get a response from a muscle fiber for a muscle contraction, used 1 motor unit

sub-maximal stimulus

greater than bare minimum, less than maximum threshold

as sub-maximal stimulus increases…

motor unit recruitment increases

Maximal stimulus

all motor units respond

Types of muscle response

• simple twitch

• temporal summation

• tetanus

• tonus

simple twitch

response of a single muslce fiber to a brief stimulus

3 stages of simple twitch response

• Latent period - stimulation of AP to onset of contraction, AP spreads on sarcolemma -? Ca²⁺ ions released

• Contraction - sliding filament model, muscle generates tension

• relaxation (refractory period) - muscle relaxes - unresponsive to stimulus

Temporal summation

contractions combine to become increasingly stronger → increasing frequency of action potential leads to more force being generated

Tetanus

rapid contractions to where maximum tension is generated and no relaxation occurs

Tonus

constant state of contraction → muscle is never completely ‘relaxed’

Example of tonus

• body posture

• balance and coordination

• generating reflexes

What is smooth muscle?

Mainly involuntary → found in lining of bladder, uterus, digestive tract, blood vessel walls

• one central nucleus, lacks striations, and stimulated by the autonomic nervous system

Types of smooth muscle

• single unit

• multi-unit

Single unit smooth muscle

• connected by gap junctions

• contract as a single-unit

• visceral organs: stomach, uterus, bladder

• less precise control

Multi-unit smooth muscle

• each fiber is innervated by a neuron

• can contract independently

• found in iris and bronchioles

• more precise control

Cardiac muscle

specialized muscle cells of the heart

• one or two central nuclei + has striations

Important characteristics of cardiac muslces

• Cells separated by intercalated discs - contain gap junctions + desmosomes, allow for electrical transmission of action potentials to other cardiac cells

• branched fibers

• lots of mitochondria

• involuntarily contractions

• not connected to bone - forms a net that contracts upon itself

• grows by hypertrophy