KNPE 452 CH1 - Structure and function of exercising muscle

For unit 1 quiz 1

Smooth muscle

Involuntary, hollow organs , ex: bronchioles (lung) and blood vessels

Cardiac muscle

Involuntary, heart

Skeletal muscle

Voluntary, skeleton, requires nervous stimulation for contraction to occur

Mysium

Connective tissue sheaths that transfer force production

3 layers of connective tissue (CT)

Epimysium, perimysium, endomysium

Epimysium

Surrounds entire muscle

Perimysium

Surrounds each fasciculus

Endomysium

Surrounds each muscle fiber

Fasciculus

Bundle of muscle fibers

Tendon 

All 3 mysium coming together

Sarcolemma

Cell membrane of muscle fiber

Transverse tubule

Opening/well in sarcolemma, allows outside to be connected to the inside

Sarcoplasmic reticulum

Stores calcium, wraps around myofibrils

Myofibrils

Column-like, consisting of myofilaments (actin + myosin), myofilaments cause striations

Mitochondria

ATP is primarily produced

Sarcomere

Function unit of myofibril, extends from z line to z line

Myofilaments

Contractile proteins in sarcomere (actin and myosin)

Actin

Thin filament

Myosin

Thick filament, has a head

Contraction

Shortening at the sarcomere level, occurs through the entire myofibril

Myosin heads/crossbridge

Capable of forming a bond with actin molecule, can be energized by ATP

Tropomyosin

Cord like structure wrapped around actin

Troponin complex

Bound to tropomyosin

Tropomyosin @ rest

Inhibits myosin head from bonding/covers binding site on actin

α (Alpha) - motor neurons

Neurons that innervate skeletal muscle fibers (provides neuron stimulation)

Motor unit

An α-motor neuron + ALL skeletal muscle fibers it innervates

Neuromuscular junction

Crossroads where muscles and neurons meet (synapse)

Synapse

Site of communication between neuron and muscle

Innnervation ratio

The wide range of muscle fibers that are innervated within one given motor unit

Ion

Charged particle (Ex: sodium, potassium, calcium

Ligand-gated channel

Open in response to the binding of a chemical messenger (e.g. neurotransmitter), 

Voltage-gated channel

Open by changes in the electrical membrane potential near the channel (e.g. depolarization)

Depolarization

When a cell becomes positive

What is the inside charge of skeletal muscle fibers @ rest?

-90 mV

What does the inside charge of skeletal muscle change to in response to nervous stimulation

+30 mV (depolarization)

The skeletal muscle fiber has more of what on the outside?

Na

The skeletal muscle fiber has more of what on the inside?

K

Neuromuscular junction process - Step 1

Motor neuron action potential (AP) travels to synaptic terminal

AP is depolarizing along neuron

Neuromuscular junction process - Step 2

In response to AP voltage-gated Ca++ channels open

Ca++ flows down concentration gradient

Neuromuscular junction process - Step 3

Ca++ enters synaptic terminal causing the release of acetylcholine (ACh) (held in vesicle)

Neuromuscular junction process - Step 4

ACh binds to receptors on ligand-gated Na channels causing Na+ channels to open

Na flows down concentration gradient from outside to inside of the cell

Neuromuscular junction process - Step 5

Na+ enters muscle fiber causing sarcolemma to depolarize

Neuromuscular junction process - Step 6

Depolarization spreads along sarcolemma

Neuromuscular junction process - Step 7

Depolarization continues down transverse tubules

Cytosol/sarcoplasm

Liquid component of muscle fiber

Myosin heads are _________ at rest

Energized

When Ca++ binds to troponin it causes what

Conformation change in tropomyosin and exposes the cross bridge binding sites on actin

ATPase equation

ATP —ATPase—> ADP + Pi + energy

ATPase

Enzyme capable of breaking down ATP

Cross bridge cycle - Step 1

Ca++ binds to troponin

Cross bridge cycle - Step 2

Cross bridge sites become exposed and energized myosin heads are able to bind to actin

Cross bridge cycle - Step 3

ADP + Pi released from cross bridge

Cross bridge cycle - Step 4

Results in a “power stroke” of cross bridge and the sarcomere shortens

Cross bridge cycle - Step 5

ATP binds to myosin cross bridge and the cross bridge releases actin

Cross bridge cycle - Step 6

Myosin ATPase breaks down ATP and the myosin head becomes re-energzed

Length of the sarcomere at rest

4.0 μm

Length of the sarcomere during contraction

2.7 μm

A band

Isotropic = looks different throughout, contains acting and myosin, only extends myosin

I band

Isotropic = looks the same throughout, contains only actin

H zone

“Helle”, contains only myosin

Type 1 muscle fiber

~50% of fibers in average muscle, peak tension in 110ms, slow twitch/slow oxidative

Twitch

Contractile response to a single action potential

Type 2 muscle fiber

Type 2a ~25% of fibers in an average muscle

Type 2x ~25% of fibers in an average muscle

Peak tension in 50ms, fast twitch/fast oxidative, glycolytic (fog fibers)

How type 1 fibers produce ATP

Utilize oxygen, oxidative in nature

How type 2x fibers produce ATP

Glycolysis

How type 2a fibers produce ATP

Both oxygen and glycolysis, intermediate fibers (“best of both worlds”)

Type 1 fibers during exercise

High aerobic endurance: can maintain exercise for prolonged periods, require oxygen for ATP production, recruited during low-intensity aerobic exercise (daily activities)

Efficiently produces ATP from fat and carbohydrates

Type 2 fibers during exercise - general

Poor aerobic endurance, fatigues quickly

Produces ATP anaerobically

Type 2a fibers during exercise

More force, faster fatigue than type 1

Short, high-intensity endurance events (1,600 m run)

Type 2x fibers during exercise

Seldom used for everyday activities

Short, explosive sprints (100m)

Fiber type determinants

Genetic factors

Training factors (Can induce small (10%) change in fiber type [type 2 → type 1, type 2x → type 2a'])

Aging (muscles lose type 2 motor units)

Fiber type

Type of myosin within the muscle fiber

Muscle fiber recruitment

Also called motor unit recruitment

Method for altering force production (less force production: fewer or smaller motor units, more force production: more or larger motor units, type 1 motor units smaller than type 2)

Muscle fiber recruitment order

Type 1 → type 2a → type 2x

Orderly recruitment

Recruit minimum number of motor units needed (smallest [type 1], midsized [type 2a], largest [type 2x])

Recruited in the same order each time

Size principle

As force requirements increase, there is an orderly recruitment of progressively larger motor units directly related to size of α-motor neuron

Predominant fiber type in endurance athletes

Type 1

Predominant fiber type in sprinters

type 2

Whats predicts athletic success

Fiber type, motivation, training habits, muscle size

Types of muscle contraction

Static (isometric) contraction

Dynamic contraction

Static (isometric) contraction

Same length, muscle produces force but does not change length (ex: wall sit)

Joint angle doesn’t change

Myosin cross-bridges form and recycle, no sliding

Dynamic contraction

Muscle produces force and changes length

Joint movement produced

Dynamic contraction subtypes

Concentric contraction

Eccentric contraction

Concentric contraction

Muscle shortens while producing force (ex: biceps curl)

Most familiar type of contraction

Sarcomere shortens, filaments slide toward center

Eccentric contraction

Muscle lengthens while decreasing force (ex: lowering weight down)

Cross-bridges form but sarcomere shortens

Generation of force: increases in force production

Motor unit recruitment

Frequency of stimulation (rate coding)

Length-tension relationship

Speed-force relationship

Motor unit recruitment

Type 2 motor units = more force

Type 1 motor units = less force

Fewer small fibers versus more larger fibers

Frequency of stimulation (rate coding)

Rate of action potentials send down motor neurons to depolarize sarcolemma, leading to Ca++ release, and crossbridge formation

Length-tension relationship

Optimal sarcomere length = optimal overlap

Too short or too stretched = little or no force develops

Speed-force relationship

Also called the “force-velocity relationship”

Maximal force development decreases at higher speeds during concentric muscle actions