Muscle Physiology Quiz

Muscle Physiology

• There are three types of muscle tissue in mammals: skeletal, cardiac, and smooth muscle.
• Skeletal muscle is composed of bundles of long, multinucleated cells with cross-striations.
• Cardiac muscle has cross-striations and is composed of elongated, branched cells that lie parallel to each other.
• Smooth muscle consists of fusiform cells without cross-striations.

Skeletal Muscle

• Skeletal muscle is composed of numerous bundles (fasciculi) of muscle fibers, which are made up of successively smaller subunits called myofibrils.
• Each muscle fiber is usually innervated by only one nerve ending, located near the middle of the fiber.
• Muscle fibers show cross-striations of alternating light and dark bands, with the darker bands called A bands and the lighter bands called I bands.
• The sarcomere is the unit of contraction, extending from Z line to Z line.

Structure of Myofibrils

• Each myofibril consists of two types of filaments: thick and thin myofilaments.
• Thick filaments contain several hundred myosin molecules, each with two globular heads and a long tail.
• Thin filaments are polymers made up of two chains of actin that form a long double helix.
• Tropomyosin and troponin molecules are also present in the thin filaments.

Sarco-tubular System

• The muscle fibrils are surrounded by structures made up of membranes, forming the sarcotubular system.
• The sarcotubular system consists of the T system and the sarcoplasmic reticulum.
• The T system provides a path for the rapid transmission of the action potential from the cell membrane to all the fibrils in the muscle.
• The sarcoplasmic reticulum is an important store of Ca2+ and also participates in muscle metabolism.

Neuro-Muscular Junction

• The axon supplying a skeletal muscle fiber approaches its termination, loses its myelin sheath, and divides into terminal buttons or end-feet.
• The end-feet contain many small, clear vesicles that contain acetylcholine, the transmitter at these junctions.
• The whole structure is known as the neuromuscular junction or motor end plate.

Excitation-Contraction Coupling

• Muscle fiber membrane depolarization normally starts at the motor end plate and initiates the contractile response.
• The sliding theory is the most acceptable theory that explains the contraction and relaxation response of skeletal muscle.
• Steps in contraction include:
  • Impulse arriving at the end of the motor neuron increases the permeability of its endings to Ca2+.
  • Acetylcholine is released from the vesicles and binds to receptors on the muscle fiber.
  • Binding of acetylcholine initiates an influx of Na+ that produces a depolarizing potential in the end plate.
  • Action potentials are conducted away from the end plate in both directions along the muscle fiber.
  • Ca2+ is released from the sarcoplasmic reticulum, and contraction occurs by the formation of cross-linkages between actin and myosin.
• Steps in relaxation include:
  • Ca2+ is pumped back into the sarcoplasmic reticulum by active transport.
  • Once the Ca2+ concentration outside the reticulum has been lowered sufficiently, the chemical interaction between myosin and actin ceases, and the muscle relaxes.

The Muscle Twitch

• A single action potential causes a brief contraction followed by relaxation, known as a muscle twitch.
• The twitch starts about 2 ms after the start of depolarization of the membrane, before repolarization is complete.
• The duration of the twitch varies with the type of muscle being tested.

Summation of Contractions

• Repeated stimulation before relaxation has occurred produces additional activation of the contractile elements and a response known as summation of contractions.
• With rapidly repeated stimulation, activation of the contractile mechanism occurs repeatedly before any relaxation has occurred, and the individual responses fuse into one continuous contraction, known as tetanus.

Types of Contraction

• Isometric contraction: contraction without an appreciable decrease in the length of the whole muscle, which does not do work.
• Isotonic contraction: contraction against a constant load, with approximation of the ends of the muscle, which does work.

Muscle Types

• Skeletal muscle is a heterogeneous tissue made up of fibers that vary in myosin ATPase activity, contractile speed, and other properties.
• Fibers fall roughly into two types, type I and type II.
• Red muscles contain many type I fibers, which are darker than other muscles due to their high content of myoglobin.
• White muscles contain mostly type II fibers, with less content of myoglobin, and are specialized for fine, skilled movement.

Energy Sources

• Muscle contraction requires energy, and muscle has been called "a machine for converting chemical energy into mechanical work."
• The immediate source of this energy is ATP, which is formed by the metabolism of carbohydrates and lipids.
• ATP is also formed by the hydrolysis of phosphoryl creatine.

The Oxygen Debt Mechanism

• Use of the anaerobic pathway causes accumulation of lactate in the muscles and produces an enzyme-inhibiting decline in pH.
• After a period of exertion, extra O2 is consumed to remove the excess lactate, replenish the ATP and phosphoryl creatine stores, which is known as oxygen debt.
• Trained athletes are able to increase the O2 consumption of their muscles to a greater degree than untrained individuals and are able to utilize FFA more effectively.

Types of Smooth Muscle

• Smooth muscle of each organ is distinctive from that of most other organs in several ways.
• Smooth muscle can be divided into two major types, multi-unit smooth muscle and unitary (or single-unit) smooth muscle.

Regulation of Contraction by Calcium Ions

• The initiating stimulus for most smooth muscle contraction is an increase in intracellular calcium ions.
• Calcium ions combine with calmodulin to cause activation of myosin kinase and phosphorylation of the myosin head.
• The calmodulin-calcium complex then joins with and activates myosin light chain kinase, a phosphorylating enzyme.
• The myosin light chain becomes phosphorylated, leading to contraction of the smooth muscle.

This quiz covers the three types of muscle tissue in mammals, including skeletal and cardiac muscle, based on morphological and functional characteristics.