Human Anatomy & Physiology
The Muscular System
PowerPoint^® Lectures created by Suzanne Pundt, University of Texas at Tyler
Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved
Second Edition
Module 9.1 Overview of Skeletal Muscles 
Structure of a Skeletal Muscle 
Skeletal muscles are not made of muscle cells alone
•	Nerves, blood vessels, and connective tissues are also essential to the job of skeletal muscle.
Structure of a Skeletal Muscle 
•	Each individual muscle cell (fiber) is surrounded by thin connective tissue called endomysium 
•	Several (between 10 and 100) muscle cells are bundled together into a fascicle by the connective tissue of the perimysium
•	All fascicles that make up a muscle are, in turn, enclosed in an outer fibrous connective tissue wrapping (epimysium)
Structure of a Skeletal Muscle 
•	These interconnected fibrous connective tissues taper down and connect to tendons or other connective tissues; attach muscle to bone or other structure to be moved
•	Example of Structure-Function Core Principle; makes sure muscle pulls as a unit even if some muscle cells are not pulling with same strength as others
Structure of a Skeletal Muscle 
Structure of a Skeletal Muscle 
•	Motor unit – describes motor neuron-muscle cell interaction that controls muscle contraction; example of Cell-Cell Communication Core Principle
•	Consists of a single motor neuron and all muscle cells it connects to
•	Some motor units have only a few muscle cells, whereas others have many
•	Fewer number of muscle cells in a motor unit = more precise movements of that muscle when it contracts
Structure of a Skeletal Muscle 
The pattern in which fascicles are arranged greatly affects the appearance and function of a skeletal muscle 

Following are different fascicle patterns: 

Structure of a Skeletal Muscle 
Fascicles and Muscle Shapes (continued):
•	Parallel – evenly spaced fascicles attaching to a tendon that is about the same width, often produces a straplike muscle, like sartorius in thigh
Structure of a Skeletal Muscle 
Fascicles and Muscle Shapes (continued):
•	Convergent – broad muscle that uniformly tapers into a single tendon
Structure of a Skeletal Muscle 
Fascicles and Muscle Shapes (continued):
•	Pennate – has fibers and fascicles that attach to the tendon at an angle, giving it a feather-like appearance
•	Unipennate – pennate variation; features 
fascicles that are only attached to one side 
of associated tendon
Structure of a Skeletal Muscle 
Fascicles and Muscle Shapes (continued):
•	Bipennate – pennate variation; features fascicles angle out from both sides of its tendon
Structure of a Skeletal Muscle 
Fascicles and Muscle Shapes (continued):
•	Multipennate – pennate variation; usually composed of several areas where fascicles angle away from connective tissue separators, looking like several feathers joined together. Ex, deltoid.
Structure of a Skeletal Muscle 
Fascicles and Muscle Shapes (continued):
•	A circular muscle encircles a structure, such as the opening of the eye, to close it when it contracts. Circular muscles are often referred to as sphincters.
Structure of a Skeletal Muscle 
Fascicles and Muscle Shapes (continued):
•	Spiral –may wrap around a bone or have the twisted appearance of a towel wrung out to dry. Ex, supinator muscle in forearm.
Structure of a Skeletal Muscle 
Fascicles and Muscle Shapes (continued):
•	Fusiform – tend to be thicker in their belly (middle region) and tapered at their ends. Ex, biceps brachii
Structure of a Skeletal Muscle 
Structure of a Skeletal Muscle 
•	Naming muscles 
•	Many muscles are named for their shape or appearance and others for their size or position
•	Some are named for structural considerations, such as the number of separate sections or heads with individual tendons providing multiple points of attachment.
•	Some named for size (major, minor, longus, brevis)
•	Some named for location (directional words)
•	Some names reveal the structures they attach to
•	Some named for what they do (their actions)
•	A number of muscle names have historical origins 
Structure of a Skeletal Muscle
Structure of a Skeletal Muscle
Functions of Skeletal Muscles 
•	Muscle contractions are involved in more than just movement of bones at a joint; other functions of muscle contraction include:
•	Muscle tissue produces heat as a waste product; can be used as a homeostatic mechanism for maintaining body temperature
•	Contraction of the diaphragm muscle is a vital function associated with breathing
Functions of Skeletal Muscles 
•	Movement and Other Functions
•	Skeletal muscles attached to skin allow for facial expression; muscles in throat assist with swallowing
•	Sphincters composed of skeletal muscle allow conscious control over opening and closing of body openings
Muscle Knots 
•	Muscle knots (myofascial trigger points) – can occur when groups of muscle fibers, or fascicles, get “stuck” in a painful contraction that will not relax. The entire muscle does not necessarily go into sustained contraction, so the cause of the pain is not always obvious. The pain and stress on joints and connective tissues can be severe. 
•	Causes – not completely understood; happen in response to diverse activities, repetitive, muscle-stressing exercises (like running) can cause microscopic tears and local damage (and subsequent repair cycles), or sudden muscle use in those with poor posture or sedentary lifestyle; diet and ion imbalance may contribute
Muscle Knots
•	Palpation is often necessary to identify location of trigger point
•	Treatment – relax muscle fibers in trigger points; massage therapy, anti-inflammatory medications, and muscle relaxants are helpful; regular, gentle stretching activities (like yoga) may be useful
•	Prevention – moderate exercise and a healthy diet
Functions of Skeletal Muscles 
•	Teamwork by muscles is needed for most movements, or actions. Most movements at joints involve several muscles, each with a particular job to do.
•	Agonists (prime movers) provide most force required for a given movement
•	Antagonists, usually on opposite side of joint from its agonist partner, tends to oppose and slow the action
Functions of Skeletal Muscles 
•	Functional groups of muscles (continued):
•	Synergists-work together with the agonists. Helps supplement the force of the agonist and help guide the movement to minimize unwanted deviations. 
•	Fixators-hold a bone in place, an anchoring function that makes movement more efficient and reduces risk of injury. 
Functions of Skeletal Muscles 
Functions of Skeletal Muscles 
•	Muscle origin and insertion – skeletal muscles begin and end at distinct anatomical locations
•	Origin – anchoring point on a bone, where skeletal muscle “originates from”
•	Insertion – insertion end of muscle is attached to the bone or structure that will be moved when the muscle contracts
Functions of Skeletal Muscles 
Functions of Skeletal Muscles 
A lever system includes three components: load or resistance (weight or object you are trying to move), the force is the effort applied to the lever, and the fulcrum is the pivot or hinge point
In moving the body, bones act as levers. The load is the weight of the body part being moved, the force is the effort generated by muscle contractions, and the fulcrum is the joint at which movement occurs:
Functions of Skeletal Muscles 
•	First-class lever – fulcrum sits between load and applied force; load moved in opposite direction than applied force
Functions of Skeletal Muscles 
•	Second-class lever – fulcrum is at one end of lever, applied force is near other end; load is somewhere in between; load moved in same direction as applied force
Functions of Skeletal Muscles 
•	Third-class lever – fulcrum and applied force are close to one another at same end of lever; load is near other end; load moved in same direction as applied force
Functions of Skeletal Muscles 
•	Lever system where fulcrum is located farther away from applied force works at a mechanical advantage; allows a relatively small force to move a large load over a short distance.
•	Lever system where fulcrum is located nearer to applied force and farther from the load moved is at a mechanical disadvantage; although third class levers have a disadvantage in terms of force, they exchange it for advantages in speed of movement and distance through which the load is moved. 
Functions of Skeletal Muscles 
•	Lever system where fulcrum is located farther away from applied force works at mechanical advantage; allows small force to move large load over short distance
Functions of Skeletal Muscles 
•	Lever system where fulcrum is located close to applied force and load is further away works at mechanical disadvantage; reduces load it can move; however, load can be moved faster over greater distance
Study Boost: How to Tell the Three Types of Levers Apart
•	All three types of levers differ in terms of which component is in middle of lever: 
•	First-class – fulcrum is in middle
•	Second-class – load is in middle
•	Third-class – force is in middle
Concept Boost: Understanding Lever Systems and Mechanical Advantage 
•	Good seesaws are adjustable 3–4 notches so position of fulcrum can be moved off center, compensating for weight differences in partners
•	Lighter person would move fulcrum position farther away to overcome the heavier weight that must be moved at the partner’s end of seesaw; her longer end of seesaw moves in a greater arc distance but as a payoff, moves the heavier partner (conferring a mechanical advantage)
Concept Boost: Understanding Lever Systems and Mechanical Advantage 
•	Conversely, the heavier partner moves a shorter distance on his end and produces greater (and faster) movement at the lighter partner’s end (though at a mechanical disadvantage). 
•	Placing fulcrum closer to or farther from applied force has a lot to do with whether lever system is built for strength or for speed
Concept Boost: Understanding Lever Systems and Mechanical Advantage