Muscle Fiber Types: Oxidative and Glycolytic

• Different muscle fiber types vary in twitch time, power development, and fatigue resistance.
• Fatigue-resistance depends on ATP production

Muscle Fiber Types

• Type I fibers are slow twitch and take a longer time to develop force and to relax
• Type IIa and IIb fibers are fast twitch and rapidly develop force and relax
• Type I fibers are limited in the ability to produce force rapidly.
• Type IIa and IIb fibers produce force very rapidly.
• Type I fibers are generally efficient and resistant to fatigue, with a high capacity for aerobic energy supply.
• Type IIa and IIb fibers are inefficient and highly fatiguing, with a low aerobic capacity.

Oxidative vs Glycolytic Fibers

• Oxidative fibers (Type I) rely on oxidative phosphorylation, yield more ATP, and fatigue slower than glycolysis.
• Glycolytic fibers (Type II) use glycolysis for ATP, and are said to be fatigable

Fiber Appearance

• Type I muscle fibers are red muscles, containing small muscle cells, large amounts of myoglobin, slow contractions and are less powerful
• Type IIa muscle fibers are also red muscles, are the least numerous and of an intermediate size
• Type IIb muscle fibers are white muscles, are the most prevalent, cause fast and powerful contractions and have less myoglobin

Twitches and Tetanus

• Wave summation occurs through temporal summation of twitches.
• Incomplete tetanus is a quivering contraction because stimulation occurs far enough apart that incomplete rest occurs.
• Tetanus involves a smooth, sustained contraction at maximal strength because of rapid successive action potential.

Motor Unit Recruitment

• Force of contraction is accomplished by recruitment of motor units.
• Threshold stimulus is the stimulus where the first observable contraction occurs.
• Maximal stimulus is the strongest stimulus where all the motor units are recruited.
• Multiple-fiber summation is the addition of motor neuron pools to increase tension and ensures constant force over time.
• Fibers in a motor unit are not clustered but dispersed throughout a muscle; stimulation of a motor unit will result in an evenly distributed contraction.
• Fatigue is the inability to maintain muscle tension.
• Asynchronous recruitment of motor units counters fatigue.
• Recruitment of fatigue-resistant muscles occurs first, and quickly fatiguing muscles last.

Contraction Types

• Isotonic contraction is where the force is kept constant, whilst muscle length changes
  • Concentric contraction is where muscles shorten with tension
  • Eccentric contraction is where muscles lengthen with tension
• Isometric contraction is where the muscle doesn't change length, so tension develops at constant muscle length

Muscular Adaptation and Training

• Hypertrophy involves enlargement of muscle fiber cross-sectional area following training.
• It Involves synthesis of the contractile proteins actin and myosin within the myofibril and increases the amount of myofibrils in the muscle fiber
• Hypertrophy is beneficial for power and force production.
• Type II fibers have greater potential for hypertrophy, which leads to increased force generation.

Detrimental Hypertrophy

• Can be detrimental to aerobic performance because there are fewer capillaries, and muscles fatigue more easily

Altering Muscle Fibers

• training can lead to change, but the muscle type remains unchanged
• Proportions of muscle fiber types are genetically determined.
• Training can lead to changes within subtypes.
  • e.g. High-intensity resistance training and aerobic endurance training can cause type IIb fibers to become more oxidative to type IIa fibers.
• Transitions from Type I to Type II appear less probable.

Resistance & Sprint Training

• Leads to increased cytoplasmic density, sarcoplasmic reticulum, and T-tubule density
• This accommodates muscle hypertrophy, enhances muscle function, and enables greater expression of strength.
• Enhanced calcium release assists in increasing speed and power production.
• Decreased blood and muscle pH during exercise
  • Aids the ability to tolerate the accumulation of H+ in the working muscle
  • Delays fatigue and produces greater muscular endurance.

Endurance training

• Increases the aerobic capacity of the skeletal muscle, leading to performance with less effort
• Improves maximal aerobic power, e.g. run the same distance faster with the same perceived effort.
• Increases the size of type I muscle fibers, but not as much as type II fibers. Increases the size and number of capillaries and mitochondria
• This enhances the cell's ability to receive and utilize oxygen to produce ATP via oxidation and resist fatigue.

Effects of Inactivity

• Sedentary leisure activities lead to skeletal muscle loss.
• Skeletal muscles that are not regularly stimulated atrophy.
  • Results in Reduction in muscle size, tone, and power.
  • Initially Reversible
  • Dying muscle fibers cannot be replaced.

Muscles & Attachments

• Muscles are attached to bone.
• The origin is the muscle's proximal attachment, usually the more stationary attachment, often attached via “fleshy" attachments, where the Epimesium attaches to periosteum and distributes force
• The insertion is muscle's distal attachment, usually the more mobile attachment attached via "fibrous" attachments through Tendons that insert into the bone and focuses force

Force generation

• Creating movement requires all muscles to activate
• All movements happen because of multiple muscles action
• Agonist: main muscle or muscle group directly involved in creating a movement
• Antagonist muscles or muscle group opposes to agonist
  • Their role is to Stabilizing a joint during the movement. -Slowing down the limb at the end of fast movement
  • Antagonist muscles need to be relaxed while the agonist muscle creates movement.
• Reciprocal inhibition is when the Central Nervous System, CNS signals agonist contraction and relaxation of the antagonist
• Synergist muscles stabilize the body during movement but are not directly responsible for moving

Muscle fiber alignment

• Two fiber alignments in muscles, pennate and non-pennate
• Muscle fibers attach obliquely to tendon within pennate muscles
• High pennation angle means there little muscle force is transmitted to the tendon
• Pennate generate greater force due to greater density of crossbridges/volume of muscle
• Non pennate muscle fibers are parallel to the line between point of origin and insertion
• Non pennate muscles produce higher velocities due to a greater number of sarcomeres in a row

Naming muscles

• Human body has more than 500 skeletal muscles
• Muscles are named by their location, position, structure size, shape, origin, insertion or action
• Their names can refect:
  • Orientation of the muscle fibers, this would rectus muscles.
  • Size, brevis, longus, major, minor, or vastus muscles.
  • Shape of the muscle, deltoid, rhombus, or quadratus. -Their action, Abductor, adductor, depressor, extensor, flexor, levator, opponens.

Smooth muscle

• Smooth muscles have less material, they are spindle-shaped, uninucleate, and smaller than skeletal muscles lacking a sarcomere
• They have dense bodies scattered throughout the cell body with same protein as Z lines
• Their myosin to actin ratio means they Create greater cross-bridges than skeletal muscles
• Myosin and actin create a diamond-shaped lattice, which contains Fewer thick and thin filaments than in skeletal muscle
• Thick and thin filaments are in the muscle but they have a different constitution
• Smooth muscle Contains intermediate filaments which are part of the cytoskeleton as well as non-contractile

Smooth Muscle Mechanics

• Connected to each other by mechanical junctions and sometimes electrically coupled gap junctions
• Has A well defined sarcoplasmic reticulum storing Ca2+
• Has proximity to cell membrane specifically invaginations called calveolae but No T-tubules
• Found in walls of hollow organs, often arranged in Two sheets (syncytium)
• In most cases, two sheets run parallel to the long axis of the organ or run circumferentially
• Branch together in bundles rather than skeletal fibers that run parallel to eachother
• Responsible for involuntary movements, regulating bloodflow movement of materials along digestive and urinary passageways, changing resistance to airflow in respiratory passageways or cause uterus contraction/relaxation

Smooth muscle contraction

• Stimuli opening voltage-gated Ca2+ channels. Ca2+ enters primarily from the interstitial fluid
• Ca2+ binds to calmodulin to form a Ca2+-calmodulin complex
• Ca2+-calmodulin complex activates MLCK phosphorylating myosin head, activating myosin - a relatively slow process
• Activated myosin heads bind to thin filaments to form crossbridges
• Myosin ATPase hydrolyzes ATP, providing the energy for the power stroke
• Repeating this process transfers force to anchoring filaments thereby shortening the smooth muscle

Smooth muscle relaxation

• Involves removing the Ca2+ and dephosphorylating the MLC with myosin phosphatase

Smooth Muscle Features

• Exhibits a stress-relaxation response, allows a hollow organ to fill or expand slowly to accommodate a greater volume without strong contraction
• Is capable of functioning efficiently even when twice or half its resting length
• Is hyperplastic certain smooth muscle fibers that can divide to increase their numbers
• Uteruses for example
• In terminal branches of the autonomic nervous system travel along multiple smooth muscle cells
  • They release neurotransmitters from bulbs (varicosities) diffusing to many cells
• Innervation generally assists contractions but also in adjusting the rate and strength of contraction
• Multiunit smooth muscle is made up of discrete motor units that function independently
  • Has neuromuscular junctions muscles, an example are muscles in large blood vessels in lungs, hair follicles, & in the eye (lens and iris)
• Single unit smooth has electrically connected with gap junctions contracting as a single coordinated unit
  • Single unit muscles Self-excitable, a resting potential that fluctuates without outside influences.
  • Occurs as spontaneous depolarization.
  • Only small portion of cells within a syncytium specializies to be self excitable
  • Once initiated, the impulse is conducted throughout remaining nonself excitablecells