Muscular System Notes

Types of Muscle

Cardiac, Smooth, Skeletal

Functions of Muscles - Movement

Externally visible movements (e.g., lifting arm).
Internal movements (e.g., breathing, moving contents within the digestive tract, pumping blood).

Functions of Muscles - Stability

Prevents unwanted movement.
Maintains posture.
Enables movement of one bone while keeping another one still.

Functions of Muscles - Control of body openings and passages

Ring-shaped sphincter muscles regulate movement of content from one area to the next (e.g., digestive tract).

Functions of Muscles - Heat generation

Skeletal muscle produces up to 30% of body heat at rest and up to 40 times as much during exercise.

Functions of Muscles - Glycemic control

Regulation of blood glucose. Skeletal muscle plays a significant role in stabilizing blood sugar levels by absorbing a large share of it.

Skeletal Muscle Fibers

Voluntary, Striated (alternating light and dark bands), Striations result from the overlapping arrangement of internal proteins that enable contraction.

Structure of the Muscle Fiber

Long, slender shape. Multiple nuclei. Myofibrils (bundles of contractile proteins). Numerous mitochondria, network of smooth ER, glycogen, and red oxygen-binding pigment (myoglobin). Plasma membrane is the sarcolemma. Sarcolemma has transverse (T) tubules. Smooth ER is sarcoplasmic reticulum (SR).

Function of T Tubules

Carry an electrical current from the surface of the cell to the interior when the cell is stimulated.

Sarcoplasmic reticulum (SR)

Forms a web around each myofibril and T tubules. Have dilated sacs called terminal cisterns. SR is a reservoir for calcium ions. Releases flood of calcium into the cytosol to activate the contraction process.

Myofilaments

Thick Filaments, Thin Filaments

Thick Filaments

Made of several hundred proteins called myosin. Myosin head is shaped like a golf club.

Thin Filaments

About half as wide as thick filaments. Composed mainly of intertwined strands of a protein called actin.

Tropomyosin and Troponin

Molecular “switch” that either allows or inhibits muscle contraction.

Striations

Dark bands (A bands) where thick and thin filaments overlap. Light bands (I bands) consisting of only thin filaments, intersected by Z discs. Sarcomere (segment from one Z disc to the next).

Sarcomere

Muscle shortens when contracting because sarcomeres shorten and pull Z discs closer to each other. The sarcomere is the functional unit of muscle.

Motor neurons

Nerve cells that stimulate skeletal muscles. Located in the brain and spinal cord. Axons transmit electrical signals to the muscles. One motor neuron + all muscle fibers supplied by it are called a motor unit.

Synapse

Where an axon meets another cell.

Neuromuscular junction

Where an axon meets a skeletal muscle fiber.

Synaptic vesicles

Contain acetylcholine (ACh).

Acetylcholine (ACh)

ACh (a neurotransmitter) diffuses across the synaptic cleft and binds to ACh receptors on the surface of the muscle fiber, stimulating contraction. Acetylcholinesterase (AChE) breaks down ACh, stopping stimulation.

Muscle Excitation

Process of converting an electrical nerve signal to an electrical signal in the muscle fiber.

Steps of Muscle Excitation

Step 1: Nerve signal arrives, ACh released.
Step 2: ACh binds to receptors, ions move, exciting sarcolemma.
Step 3: Action potential spreads, muscle fiber “excited”.

Preparing for Contraction

Excitation is initiates a chain of events that allows myosin and actin to interact. 1) Calcium released. 2) Calcium binds to troponin. 3) Tropomyosin shifts, exposes active sites on actin.

Contraction

Muscle fiber develops tension and may shorten due to the sliding filament model. Thick and thin filaments slide across each other.

Steps of Contraction

Step 1: Myosin binds ATP, hydrolyzes to ADP + phosphate.
Step 2: Myosin binds to actin (cross-bridge).
Step 3: Myosin releases ADP and phosphate, pulls thin filament (power stroke).
Step 4: New ATP binds, myosin releases actin.

Contraction

Cycle repeats, consuming ATP. Thin filament pulls Z disc, sarcomeres shorten.

Relaxation

1. Nerve signal stops, ACh release stops. 2. AChE breaks down ACh.3. SR reabsorbs calcium. 4. Troponin-tropomyosin complex blocks myosin binding.

Muscle Twitch

Minimum contraction exhibited by a muscle cell. A single cycle of contraction and relaxation.

Isometric contraction

Tension rises but muscle does not shorten.

Isotonic contraction

Tension rises, muscle shortens.

Concentric contraction

Muscle shortens as it maintains tension.

Eccentric contraction

Muscle maintains tension as it lengthens.

Muscle Metabolism

All muscle contraction requires ATP. Can be generated by: Anaerobic fermentation or Aerobic respiration.

Anaerobic Fermentation

Glucose to lactate, 2 ATP produced, no oxygen used. Good for short bursts of exercise. Low ATP yield.

Aerobic Respiration

Glucose to CO2 and H2O, produces 30 ATP. More efficient than anaerobic fermentation. Requires oxygen.

Causes of Muscle Fatigue

Depletion of glycogen and blood glucose. Leakage of calcium from SR. Accumulation of K^+ in ECF.

Factors of Endurance

Muscles supply of myoglobin and glycogen. Density of blood capillaries. Number of mitochondria. Maximal rate of oxygen uptake. Will power.

Types of Muscle Fibers

Slow-twitch, Fast-twitch

Slow-Twitch Fibers

Respond slowly, resistant to fatigue, adapted for aerobic respiration.

Fast-Twitch Fibers

Respond quickly, fatigue quickly, adapted for anaerobic respiration.

Resistance training

Muscle cells increase in size.