Muscle Physiology and Types

Glycolysis (Anaerobic Metabolism):
- Takes place in the sarcoplasm (cytoplasm of muscle fibers).
- Glucose is broken down to form two molecules of ATP and two molecules of pyruvic acid (pyruvate).
- In skeletal muscles at rest, there is a significant store of glycogen (animal starch, a polysaccharide).
Aerobic Respiration (Citric Acid Cycle/Krebs Cycle):
- Occurs in the mitochondria.
- Pyruvic acid (from glycolysis) and fatty acids are used to produce ATP.

Resting Muscles:
- Use fatty acids for metabolism.
- Store glycogen and creatine phosphate.
- Enough ATP is present to initiate contraction, but more must be produced to sustain it.
Peak Performance/Active Muscles:
- Pyruvic acid accumulates because glycolysis is faster than aerobic metabolism.
- Pyruvic acid is temporarily converted to lactic acid (lactate) to buffer the cellular environment, which functions best in a slightly alkaline condition.

Aerobic respiration is oxygen-consuming but slower than glycolysis.
After intense activity, the body remains in an oxygen debt, requiring elevated respiratory and heart rates to process pyruvic and lactic acid and remove waste products.
It takes time for the muscular system and body to return to a resting state.

Skeletal muscle contractions produce significant heat.
85% of the heat required for maintaining normal body temperature comes from skeletal muscle contractions.
Heat is not a waste product but essential for homeostasis.

Hormones crucial for the growth and maintenance of muscle tissue:
- Growth hormone (pituitary gland).
- Testosterone (major male sex hormone, present in smaller amounts in women).
- Thyroid hormones (affect body temperature, fatigue, and muscle maintenance).
- Epinephrine (increases muscle activity and helps maintain mass).

Force, tension, and power are used interchangeably to describe muscle contraction.
Endurance is how long a muscle can maintain activity and depends on physical condition and the type of muscle fibers involved.

Three types: fast, slow, and intermediate, differing functionally and structurally.
Fast Fibers:
- Rapid, powerful contractions.
- Fatigue quickly.
- High glycogen content for ATP production via glycolysis.
- Relatively few mitochondria.
Slow Fibers:
- Slower contractions, less force.
- Fatigue-resistant.
- Numerous mitochondria for ATP production.
- Contain myoglobin (like hemoglobin) to reversibly bind and release oxygen.
- Appear darker due to myoglobin and have a high stored fat content to feed aerobic metabolism.
Intermediate Fibers:
- Mix of fast and slow fiber characteristics.
- Better vascular supply than fast fibers, but don't have a lot of myoglobin.
- Not as powerful as fast fibers but more fatigue-resistant.
All skeletal muscles have these fiber types in varying proportions, influencing muscle appearance and characteristics (red vs. white muscles).

Hypertrophy:
- Increase in muscle size due to increased activity.
- Skeletal muscle fibers do not undergo cell division; existing cells enlarge.
- Increase in myofibrils, mitochondria, starch, and fat storage within muscle fibers.
Atrophy:
- Decrease in muscle size due to inactivity or neuromuscular disease.
- Loss of tone and myofibrils.
- Scar tissue replaces functional parts of muscle fibers.
Fibrosis (increased fibrous connective tissue) occurs with aging, reducing muscle elasticity and stress tolerance.

Anaerobic:
- High-level, short bursts (e.g., 50-meter dash, weightlifting).
- Activates fast fibers.
- Brief and intense.
Aerobic:
- Long-duration (e.g., running a 5k).
- Depends on mitochondrial activity and slow muscle fibers.
- Low-level activity over a longer period.

Striated, like skeletal muscle, with areas of white and dark bands.
Most cardiac muscle fibers have a single nucleus.
Cardiac muscle fibers can divide under extreme stress conditions.
Relatively small cells, typically with a single nucleus.
T-tubules are short.
Triads (T-tubule and paired terminal cisternae) are not present.
Cardiac muscle fibers have a very short rest period and are almost totally dependent upon mitochondrial activity; make a lot of ATP over the long haul.
Connect via intercalated discs, containing:
- Gap junctions: allow ions to move from one fiber to another, enabling rapid electrical impulse transmission.
- Desmosomes: strong attachments that keep cells intact and prevent separation.
Cardiac muscle fibers, an impulse caused the contraction lasts $250$ milliseconds. So a tenfold difference between the activity, same impulse.

Sinoatrial (SA) node: natural pacemaker that initiates the cardiac cycle.
Automaticity: the heart can continue beating on its own, even without nerve or hormonal input.
Contractions last ten times longer than in skeletal muscle fibers.

Examples: arrector pili muscle (integumentary system), walls of arteries and veins, respiratory, digestive, urinary, and reproductive systems
Spindle-shaped cells, always a single nucleus, good ability to regenerate.
Regulates blood pressure in the cardiovascular system; contraction reduces lumen size, raising blood pressure, while relaxation lowers it.
No striations.
Same contractile proteins (actin and myosin) as skeletal and cardiac muscle but oriented differently.
Thin myofilaments connect to dense bodies, transmitting force throughout the network.

Multiunit:
- Motor units (motor neuron and connected muscle fibers), but each cell can connect to multiple neurons.
- Dependent on motor neuron firing.
Visceral:
- No connection to the nervous system.
- Rhythmic activity regulated by pacesetter cells.
- Smooth muscle tone results from activity in both multiunit (nerve/hormone) and visceral (pacemaker cell) smooth muscle.

Vasomotor tone maintains blood pressure.
From medulla in brainstem, a constant, low level of nerve impulses keeps the smooth muscles slightly contracted.
Neurological damage can cause loss of tone, leading to dilation, plummeting blood pressure, and potential fatality.