Muscle Types and Skeletal Muscle Overview

Muscle Types

• Muscles are responsible for movement in the body.
• There are three main types of muscle tissue: skeletal, cardiac, and smooth.
• Skeletal muscle is attached to bones and is under voluntary control.
• Cardiac muscle is found in the walls of the heart and is involuntary.
• Smooth muscle lines organs like the digestive tract and blood vessels and is also involuntary.

Skeletal Muscle

• Approximately 40% of the body is made up of skeletal muscle.
• There are over 650 skeletal muscles, with over 150 being surface (anatomical) muscles.
• Each skeletal muscle is composed of millions of muscle fibers bound together by connective tissue sheaths.
• Skeletal muscle is responsible for movement by contracting and pulling on bones.
• Muscles work in antagonistic pairs, where one muscle flexes a joint and the other extends it.
• The point of attachment that remains fixed during contraction is called the origin, while the point that moves is called the insertion.

Skeletal Muscle Structure

• Each skeletal muscle contains bundles of fascicles, which themselves are composed of numerous muscle fibers.
• Muscle fibers contain myofibrils, which are the contractile units of the muscle.
• Myofibrils contain sarcomeres, which are the basic repeating units of striated muscle.
• Skeletal muscle is often referred to as striated muscle due to its striped appearance under a microscope.

Cardiac Muscle

• Cardiac muscle is found exclusively in the heart.
• It is a type of striated muscle, but also shares characteristics with smooth muscle.
• Cardiac muscle is involuntary and exhibits high endurance.
• Its cells are striated and contain intercalated discs, which are specialized junctions that allow for rapid communication between cells.
• Each cardiac muscle cell has 1-2 centrally-placed nuclei.
• Cardiac muscle is innervated by the pacemaker, which regulates its rhythmic contractions.

Smooth Muscle

• Smooth muscle is found in the lining of internal organs like the digestive tract, trachea/bronchus, and blood vessels.
• It is involuntary and exhibits high endurance.
• It has a single, centrally-placed nucleus per cell and lacks striations.
• Smooth muscle is responsible for sustained contractions in the organs it lines.

Motor Unit

• The functional unit of skeletal muscle is the motor unit (MU), which consists of a single motor neuron and the group of muscle fibers it innervates.
• The number of muscle fibers per motor unit varies depending on the level of control required.
• Fewer fibers per motor unit lead to increased dexterity, while a higher ratio results in greater strength.
• The neuromuscular junction (NMJ) is the connection between a muscle fiber and its motor neuron.
• Each muscle fiber has one NMJ.

Muscle Contraction

• When a muscle fiber is stimulated at the NMJ, it contracts in an "all-or-nothing" response.
• The strength of a whole muscle contraction depends on the number of motor units activated.
• Maximal contraction occurs when all motor units are firing simultaneously.
• Asynchronous firing of motor units allows for a smooth and graded contraction.

Types of Muscle Contraction

• Muscles contract in two main ways: isometric and isotonic.
• During isometric contraction, the muscle length remains unchanged while tension increases.
• Isotonic contraction involves a change in muscle length while tension is maintained.
• Concentric isotonic contraction involves muscle shortening, while eccentric isotonic contraction involves muscle lengthening.

Muscle Physiology

• Whole muscle function is studied using laboratory preparations, such as the frog gastrocnemius muscle.
• When a muscle is stimulated by a single electric shock, it responds with a quick twitch.
• The duration of a muscle twitch varies between species, typically lasting about 0.1 seconds in frogs and 0.05 seconds in humans.

Muscle Twitch Phases

• A muscle twitch has three phases: a latent period, a contraction period, and a relaxation period.
• The latent period is the time between the stimulus and the first visible reaction.
• The contraction period is when the muscle shortens.
• The relaxation period occurs when the muscle returns to its original length.
• Following stimulation, there is a brief refractory period during which the muscle will not respond to further stimuli.

Muscle Summation

• A muscle can respond to a second stimulation while still contracting.
• Stimulating a muscle at a frequency shorter than the twitch time results in summation, where responses are superimposed upon one another.
• Summation in whole muscle can occur through multiple motor unit summation (recruitment) or wave summation.
• Multiple motor unit summation involves increasing the number of motor units activated.
• Wave summation occurs when the rate of contraction of individual motor units is increased.
• In practice, both types of summation occur simultaneously during muscle contraction.

Tetanus

• As the frequency of stimulation increases, twitches become superimposed in wave summation, leading to a state of maximal contraction called tetanus.
• Tetanus occurs at stimulation frequencies above 40 pulses per second.
• Further stimulation beyond tetanus will not result in significant additional muscle shortening.

This quiz covers the different types of muscle tissue, focusing on skeletal muscle. Learn about the characteristics, functions, and anatomy of skeletal muscles, along with their roles in movement and muscle coordination. Test your knowledge about the involuntary and voluntary muscle systems.