Muscular system Flashcards
Reverse
A muscle fiber is a cell containing the usual cellular components (see Section 3.2), but special names have been assigned to some of these components (Table 13.1).
This is a muscle fiber
Reverse
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Summarize the role of the muscular and skeletal systems in movement.
Summarize the role of the muscular system in body temperature homeostasis.
13.5 Homeostasis
troponin molecule
During contraction of a muscle, calcium ions bind to the
Reverse
Simply put, the muscular system
The muscular system is involved with movement. This may be the movement of the entire organism (walking or running) or the movement of materials (blood, food) within the organism. The muscular system is made up of muscles. The structure of a muscle allows it to provide movement by contracting, or shortening.
Reverse
Cardiac muscle fibers are
striated and
Reverse
- vertebrates (see Section 23.3): internal vertebral column, a skeleton, and jointed appendages.
- Our skeletal muscles are attached to the skeleton, and their contraction causes the movement of bones at a joint.
- SCIENCE IN YOUR LIFE
- How many muscles are there in the human body?
- Most experts agree that there are over 600 muscles in the human body. The exact number varies, because some experts lump muscles together under one name and others differentiate them. The smallest of these, the stapedius, is a 1.27-mm-long muscle in the middle ear. The longest muscle is the sartorius, which starts at the hip and extends to the knee. The biggest muscle (in terms of mass) is the gluteus maximus, the muscle that makes up the majority of the buttocks.
Human beings are _____________ meaning we have a internal vertebral column, skeleton, and jointed appendages.
__________causes movement of bones at joints
Reverse
Skeletal Muscles of the Body
Humans belong to a class of animals called the vertebrates (see Section 23.3). Vertebrate animals possess an internal vertebral column, a skeleton, and jointed appendages. Our skeletal muscles are attached to the skeleton, and their contraction causes the movement of bones at a joint.
SCIENCE IN YOUR LIFE
How many muscles are there in the human body?
Most experts agree that there are over 600 muscles in the human body. The exact number varies, because some experts lump muscles together under one name and others differentiate them. The smallest of these, the stapedius, is a 1.27-mm-long muscle in the middle ear. The longest muscle is the sartorius, which starts at the hip and extends to the knee. The biggest muscle (in terms of mass) is the gluteus maximus, the muscle that makes up the majority of the buttocks.
Functions of Skeletal Muscles
The skeletal muscles of the body have a wide variety of functions, including the following:
Support. Skeletal muscle contraction opposes the force of gravity and allows us to remain upright.
Movements of bones and other body structures. Muscle contraction accounts not only for the movement of arms and legs but also for movements of the eyes, facial expressions, and breathing.
Maintenance of a constant body temperature. Skeletal muscle contraction causes ATP to break down, releasing heat, which is distributed throughout the body.
Movement of fluids in the cardiovascular and lymphatic systems. The pressure of skeletal muscle contraction keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels.
Protection of the internal organs and the stabilization of joints. Muscles pad the bones, and the muscular wall in the abdominal region protects the internal organs. Muscle tendons help hold bones together at joints.
Basic Structure of Skeletal Muscles
Skeletal muscles are well organized. A whole muscle contains bundles of skeletal muscle fibers called fascicles (Fig. 13.2). These are the strands of muscle we see when we cut red meat and poultry. Within a fascicle, each fiber is surrounded by connective tissue; the fascicle is also surrounded by connective tissue. Muscles are covered with fascia, a type of connective tissue that extends beyond the muscle and becomes its tendon. Tendons quite often extend past a joint before anchoring a muscle to a bone. Small, fluid-filled sacs called bursae (sing., bursa) can often be found between tendons and bones. The bursae act as cushions, allowing ease of movement.
Figure 13.2 Connecting muscle to bone. Connective tissue separates bundles of muscle fibers that make up a skeletal muscle. A layer of connective tissue covering the muscle contributes to the tendon, which attaches muscle to bone.
Skeletal Muscles Work in Pairs
In general, each muscle is concerned with the movement of only one bone. To simplify the discussion, we will focus on the movement of a single bone and no others. The origin of a muscle is on a stationary bone, and the insertion of a muscle is on a bone that moves. When a muscle contracts, it pulls on the tendons at its insertion and the bone moves. For example, when the biceps brachii contracts, it raises the forearm.
Skeletal muscles usually function in groups. Consequently, to make a particular movement, your nervous system does not stimulate a single muscle. Rather, it stimulates an appropriate group of muscles. Even so, for any particular movement, one muscle does most of the work and is called the agonist, or prime mover. While a prime mover is working, other muscles called synergists function as well. Synergists assist the agonist and make its action more effective.
When muscles contract, they shorten. Therefore, muscles can only pull; they cannot push. This means that muscles work in opposite pairs. The muscle that acts opposite to a prime mover is called an antagonist. For example, the biceps brachii and the triceps Page 261brachii are antagonists. The biceps flexes the forearm (Fig. 13.3a), and the triceps extends the forearm (Fig. 13.3b). If both of these muscles contracted at once, the forearm would remain rigid. Smooth body movements depend on an antagonist relaxing when a prime mover is acting.
Figure 13.3 Skeletal muscles often work in pairs. a. When the biceps brachii contracts, the forearm flexes. b. When the triceps brachii contracts, the forearm extends. Therefore, these two muscles are antagonistic. The origin of a skeletal muscle is on a bone that remains stationary, and the insertion of a muscle is on a bone that moves when the muscle contracts.
Not all skeletal muscles are involved in the movement of limbs. For example, the facial muscles (Fig. 13.4) produce the facial expressions that tell us about the emotions and mood of a person and therefore play an important role in our interactions with other people.
Figure 13.4 Facial expressions. Our many facial expressions are due to muscle contractions.
(both photos): ©McGraw-Hill Education/J.W. Ramsey, photographer
Names and Actions of Skeletal Muscles
Figure 13.5a, b illustrates the location of some of the major skeletal muscles and gives their actions. (Not all the muscles mentioned are featured in Figure 13.5, but most are.)
Figure 13.5 The major skeletal muscles of the human body. a. Anterior view. b. Posterior view.
When learning the names of muscles, considering what the names mean will help you remember them. The names of the various skeletal muscles are often combinations of the following terms used to characterize muscles:
Size. The gluteus maximus is the largest muscle that makes up the buttocks. The gluteus minimus is the smallest of the gluteal muscles. Other terms used to indicate size are vastus (huge), longus (long), and brevis (short).
Shape. The deltoid is shaped like a triangle. (The Greek letter delta has this appearance: ∆.) The trapezius is shaped like a trapezoid. Other terms used to indicate shape are latissimus (wide) and teres (round).
Location. The external oblique muscles are located outside the internal obliques. The frontalis muscle overlies the frontal bone. Other terms used to indicate location are pectoralis (chest), gluteus (buttock), brachii (arm), and sub (beneath).
Direction of muscle fibers. The rectus abdominis is a longitudinal muscle of the abdomen (rectus means “straight”). The orbicularis oculi is a circular muscle around the eye. Other terms used to indicate direction are transverse (across) and oblique (diagonal).
Attachment. The sternocleidomastoid is attached to the sternum, clavicle, and mastoid process. The mastoid process is located on the temporal bone of the skull. The brachioradialis is attached to the brachium (arm) and the radius (forearm).
Number of attachments. The biceps brachii has two attachments, or origins, and is located on the arm. The quadriceps femoris has four origins and is located on the femur.
Action. The extensor digitorum extends the fingers, or digits. The adductor longus is a large muscle that adducts the thigh. Adduction is the movement of a body part toward the midline. Other terms used to indicate action are flexor (to flex or bend), masseter (to chew), and levator (to lift).Page 262
SCIENCE IN YOUR LIFE
Which muscles are best to use for intramuscular injections?
When giving intramuscular injections, health-care providers have to choose muscles that are sufficiently large and well developed to tolerate the injections. But they must also avoid muscles that contain large blood vessels or nerves, because an injection in these muscles could pierce a blood vessel or damage a nerve. Typically, they choose one of three preferred injection sites. The deltoid muscle on the upper arm is usually well developed in older children and adults. The vastus lateralis on the side of the thigh (part of the quadriceps group) is the best site for infants and young children. The gluteus medius is on the lower back, above the buttock. However, a clinician injecting into the gluteus medius must be careful to avoid the gluteus maximus (buttock) muscle. The body’s largest nerve, the sciatic nerve, lies underneath and within the gluteus maximus.
CHECK YOUR PROGRESS 13.1
State the three types of muscles in the human body and explain where each is found in the body.
Answer
Smooth muscle found in walls of internal organs; cardiac muscle of the walls of the heart; skeletal muscle attached to the skeleton.
Summarize the functions of skeletal muscles.
Answer
Support the body, make bones move, produce heat, increase fluid movement in cardiovascular and lymphatic vessels, protect internal organs, stabilize joints.
Explain how skeletal muscles work together to cause bones to move.
Answer
Skeletal muscles work in opposite groups, with one flexing and the other extending a joint.
CONNECTING THE CONCEPTS
For more information on the three types of muscles, refer to the following discussions:
Section 4.3 describes the general structure of cardiac, smooth, and skeletal muscle.
Section 5.3 examines the function of cardiac muscle in the heart.
Section 9.1 illustrates how smooth muscle lines the wall of the digestive tract.
In your life and check yourself
Reverse
Skeletal Muscles of the Body
Humans belong to a class of animals called the vertebrates (see Section 23.3). Vertebrate animals possess an internal vertebral column, a skeleton, and jointed appendages. Our skeletal muscles are attached to the skeleton, and their contraction causes the movement of bones at a joint.
SCIENCE IN YOUR LIFE
How many muscles are there in the human body?
Most experts agree that there are over 600 muscles in the human body. The exact number varies, because some experts lump muscles together under one name and others differentiate them. The smallest of these, the stapedius, is a 1.27-mm-long muscle in the middle ear. The longest muscle is the sartorius, which starts at the hip and extends to the knee. The biggest muscle (in terms of mass) is the gluteus maximus, the muscle that makes up the majority of the buttocks.
Functions of Skeletal Muscles
The skeletal muscles of the body have a wide variety of functions, including the following:
Support. Skeletal muscle contraction opposes the force of gravity and allows us to remain upright.
Movements of bones and other body structures. Muscle contraction accounts not only for the movement of arms and legs but also for movements of the eyes, facial expressions, and breathing.
Maintenance of a constant body temperature. Skeletal muscle contraction causes ATP to break down, releasing heat, which is distributed throughout the body.
Movement of fluids in the cardiovascular and lymphatic systems. The pressure of skeletal muscle contraction keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels.
Protection of the internal organs and the stabilization of joints. Muscles pad the bones, and the muscular wall in the abdominal region protects the internal organs. Muscle tendons help hold bones together at joints.
Basic Structure of Skeletal Muscles
Skeletal muscles are well organized. A whole muscle contains bundles of skeletal muscle fibers called fascicles (Fig. 13.2). These are the strands of muscle we see when we cut red meat and poultry. Within a fascicle, each fiber is surrounded by connective tissue; the fascicle is also surrounded by connective tissue. Muscles are covered with fascia, a type of connective tissue that extends beyond the muscle and becomes its tendon. Tendons quite often extend past a joint before anchoring a muscle to a bone. Small, fluid-filled sacs called bursae (sing., bursa) can often be found between tendons and bones. The bursae act as cushions, allowing ease of movement.
Figure 13.2 Connecting muscle to bone. Connective tissue separates bundles of muscle fibers that make up a skeletal muscle. A layer of connective tissue covering the muscle contributes to the tendon, which attaches muscle to bone.
Skeletal Muscles Work in Pairs
In general, each muscle is concerned with the movement of only one bone. To simplify the discussion, we will focus on the movement of a single bone and no others. The origin of a muscle is on a stationary bone, and the insertion of a muscle is on a bone that moves. When a muscle contracts, it pulls on the tendons at its insertion and the bone moves. For example, when the biceps brachii contracts, it raises the forearm.
Skeletal muscles usually function in groups. Consequently, to make a particular movement, your nervous system does not stimulate a single muscle. Rather, it stimulates an appropriate group of muscles. Even so, for any particular movement, one muscle does most of the work and is called the agonist, or prime mover. While a prime mover is working, other muscles called synergists function as well. Synergists assist the agonist and make its action more effective.
When muscles contract, they shorten. Therefore, muscles can only pull; they cannot push. This means that muscles work in opposite pairs. The muscle that acts opposite to a prime mover is called an antagonist. For example, the biceps brachii and the triceps Page 261brachii are antagonists. The biceps flexes the forearm (Fig. 13.3a), and the triceps extends the forearm (Fig. 13.3b). If both of these muscles contracted at once, the forearm would remain rigid. Smooth body movements depend on an antagonist relaxing when a prime mover is acting.
Figure 13.3 Skeletal muscles often work in pairs. a. When the biceps brachii contracts, the forearm flexes. b. When the triceps brachii contracts, the forearm extends. Therefore, these two muscles are antagonistic. The origin of a skeletal muscle is on a bone that remains stationary, and the insertion of a muscle is on a bone that moves when the muscle contracts.
Not all skeletal muscles are involved in the movement of limbs. For example, the facial muscles (Fig. 13.4) produce the facial expressions that tell us about the emotions and mood of a person and therefore play an important role in our interactions with other people.
Figure 13.4 Facial expressions. Our many facial expressions are due to muscle contractions.
(both photos): ©McGraw-Hill Education/J.W. Ramsey, photographer
Names and Actions of Skeletal Muscles
Figure 13.5a, b illustrates the location of some of the major skeletal muscles and gives their actions. (Not all the muscles mentioned are featured in Figure 13.5, but most are.)
Figure 13.5 The major skeletal muscles of the human body. a. Anterior view. b. Posterior view.
When learning the names of muscles, considering what the names mean will help you remember them. The names of the various skeletal muscles are often combinations of the following terms used to characterize muscles:
Size. The gluteus maximus is the largest muscle that makes up the buttocks. The gluteus minimus is the smallest of the gluteal muscles. Other terms used to indicate size are vastus (huge), longus (long), and brevis (short).
Shape. The deltoid is shaped like a triangle. (The Greek letter delta has this appearance: ∆.) The trapezius is shaped like a trapezoid. Other terms used to indicate shape are latissimus (wide) and teres (round).
Location. The external oblique muscles are located outside the internal obliques. The frontalis muscle overlies the frontal bone. Other terms used to indicate location are pectoralis (chest), gluteus (buttock), brachii (arm), and sub (beneath).
Direction of muscle fibers. The rectus abdominis is a longitudinal muscle of the abdomen (rectus means “straight”). The orbicularis oculi is a circular muscle around the eye. Other terms used to indicate direction are transverse (across) and oblique (diagonal).
Attachment. The sternocleidomastoid is attached to the sternum, clavicle, and mastoid process. The mastoid process is located on the temporal bone of the skull. The brachioradialis is attached to the brachium (arm) and the radius (forearm).
Number of attachments. The biceps brachii has two attachments, or origins, and is located on the arm. The quadriceps femoris has four origins and is located on the femur.
Action. The extensor digitorum extends the fingers, or digits. The adductor longus is a large muscle that adducts the thigh. Adduction is the movement of a body part toward the midline. Other terms used to indicate action are flexor (to flex or bend), masseter (to chew), and levator (to lift).Page 262
SCIENCE IN YOUR LIFE
Which muscles are best to use for intramuscular injections?
When giving intramuscular injections, health-care providers have to choose muscles that are sufficiently large and well developed to tolerate the injections. But they must also avoid muscles that contain large blood vessels or nerves, because an injection in these muscles could pierce a blood vessel or damage a nerve. Typically, they choose one of three preferred injection sites. The deltoid muscle on the upper arm is usually well developed in older children and adults. The vastus lateralis on the side of the thigh (part of the quadriceps group) is the best site for infants and young children. The gluteus medius is on the lower back, above the buttock. However, a clinician injecting into the gluteus medius must be careful to avoid the gluteus maximus (buttock) muscle. The body’s largest nerve, the sciatic nerve, lies underneath and within the gluteus maximus.
CHECK YOUR PROGRESS 13.1
State the three types of muscles in the human body and explain where each is found in the body.
Answer
Smooth muscle found in walls of internal organs; cardiac muscle of the walls of the heart; skeletal muscle attached to the skeleton.
Summarize the functions of skeletal muscles.
Answer
Support the body, make bones move, produce heat, increase fluid movement in cardiovascular and lymphatic vessels, protect internal organs, stabilize joints.
Explain how skeletal muscles work together to cause bones to move.
Answer
Skeletal muscles work in opposite groups, with one flexing and the other extending a joint.
CONNECTING THE CONCEPTS
For more information on the three types of muscles, refer to the following discussions:
Section 4.3 describes the general structure of cardiac, smooth, and skeletal muscle.
Section 5.3 examines the function of cardiac muscle in the heart.
Section 9.1 illustrates how smooth muscle lines the wall of the digestive tract.
In your life and check yourself
Reverse
Skeletal Muscles of the Body
Humans belong to a class of animals called the vertebrates (see Section 23.3). Vertebrate animals possess an internal vertebral column, a skeleton, and jointed appendages. Our skeletal muscles are attached to the skeleton, and their contraction causes the movement of bones at a joint.
SCIENCE IN YOUR LIFE
How many muscles are there in the human body?
Most experts agree that there are over 600 muscles in the human body. The exact number varies, because some experts lump muscles together under one name and others differentiate them. The smallest of these, the stapedius, is a 1.27-mm-long muscle in the middle ear. The longest muscle is the sartorius, which starts at the hip and extends to the knee. The biggest muscle (in terms of mass) is the gluteus maximus, the muscle that makes up the majority of the buttocks.
Functions of Skeletal Muscles
The skeletal muscles of the body have a wide variety of functions, including the following:
Support. Skeletal muscle contraction opposes the force of gravity and allows us to remain upright.
Movements of bones and other body structures. Muscle contraction accounts not only for the movement of arms and legs but also for movements of the eyes, facial expressions, and breathing.
Maintenance of a constant body temperature. Skeletal muscle contraction causes ATP to break down, releasing heat, which is distributed throughout the body.
Movement of fluids in the cardiovascular and lymphatic systems. The pressure of skeletal muscle contraction keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels.
Protection of the internal organs and the stabilization of joints. Muscles pad the bones, and the muscular wall in the abdominal region protects the internal organs. Muscle tendons help hold bones together at joints.
Basic Structure of Skeletal Muscles
Skeletal muscles are well organized. A whole muscle contains bundles of skeletal muscle fibers called fascicles (Fig. 13.2). These are the strands of muscle we see when we cut red meat and poultry. Within a fascicle, each fiber is surrounded by connective tissue; the fascicle is also surrounded by connective tissue. Muscles are covered with fascia, a type of connective tissue that extends beyond the muscle and becomes its tendon. Tendons quite often extend past a joint before anchoring a muscle to a bone. Small, fluid-filled sacs called bursae (sing., bursa) can often be found between tendons and bones. The bursae act as cushions, allowing ease of movement.
Figure 13.2 Connecting muscle to bone. Connective tissue separates bundles of muscle fibers that make up a skeletal muscle. A layer of connective tissue covering the muscle contributes to the tendon, which attaches muscle to bone.
Skeletal Muscles Work in Pairs
In general, each muscle is concerned with the movement of only one bone. To simplify the discussion, we will focus on the movement of a single bone and no others. The origin of a muscle is on a stationary bone, and the insertion of a muscle is on a bone that moves. When a muscle contracts, it pulls on the tendons at its insertion and the bone moves. For example, when the biceps brachii contracts, it raises the forearm.
Skeletal muscles usually function in groups. Consequently, to make a particular movement, your nervous system does not stimulate a single muscle. Rather, it stimulates an appropriate group of muscles. Even so, for any particular movement, one muscle does most of the work and is called the agonist, or prime mover. While a prime mover is working, other muscles called synergists function as well. Synergists assist the agonist and make its action more effective.
When muscles contract, they shorten. Therefore, muscles can only pull; they cannot push. This means that muscles work in opposite pairs. The muscle that acts opposite to a prime mover is called an antagonist. For example, the biceps brachii and the triceps Page 261brachii are antagonists. The biceps flexes the forearm (Fig. 13.3a), and the triceps extends the forearm (Fig. 13.3b). If both of these muscles contracted at once, the forearm would remain rigid. Smooth body movements depend on an antagonist relaxing when a prime mover is acting.
Figure 13.3 Skeletal muscles often work in pairs. a. When the biceps brachii contracts, the forearm flexes. b. When the triceps brachii contracts, the forearm extends. Therefore, these two muscles are antagonistic. The origin of a skeletal muscle is on a bone that remains stationary, and the insertion of a muscle is on a bone that moves when the muscle contracts.
Not all skeletal muscles are involved in the movement of limbs. For example, the facial muscles (Fig. 13.4) produce the facial expressions that tell us about the emotions and mood of a person and therefore play an important role in our interactions with other people.
Figure 13.4 Facial expressions. Our many facial expressions are due to muscle contractions.
(both photos): ©McGraw-Hill Education/J.W. Ramsey, photographer
Names and Actions of Skeletal Muscles
Figure 13.5a, b illustrates the location of some of the major skeletal muscles and gives their actions. (Not all the muscles mentioned are featured in Figure 13.5, but most are.)
Figure 13.5 The major skeletal muscles of the human body. a. Anterior view. b. Posterior view.
When learning the names of muscles, considering what the names mean will help you remember them. The names of the various skeletal muscles are often combinations of the following terms used to characterize muscles:
Size. The gluteus maximus is the largest muscle that makes up the buttocks. The gluteus minimus is the smallest of the gluteal muscles. Other terms used to indicate size are vastus (huge), longus (long), and brevis (short).
Shape. The deltoid is shaped like a triangle. (The Greek letter delta has this appearance: ∆.) The trapezius is shaped like a trapezoid. Other terms used to indicate shape are latissimus (wide) and teres (round).
Location. The external oblique muscles are located outside the internal obliques. The frontalis muscle overlies the frontal bone. Other terms used to indicate location are pectoralis (chest), gluteus (buttock), brachii (arm), and sub (beneath).
Direction of muscle fibers. The rectus abdominis is a longitudinal muscle of the abdomen (rectus means “straight”). The orbicularis oculi is a circular muscle around the eye. Other terms used to indicate direction are transverse (across) and oblique (diagonal).
Attachment. The sternocleidomastoid is attached to the sternum, clavicle, and mastoid process. The mastoid process is located on the temporal bone of the skull. The brachioradialis is attached to the brachium (arm) and the radius (forearm).
Number of attachments. The biceps brachii has two attachments, or origins, and is located on the arm. The quadriceps femoris has four origins and is located on the femur.
Action. The extensor digitorum extends the fingers, or digits. The adductor longus is a large muscle that adducts the thigh. Adduction is the movement of a body part toward the midline. Other terms used to indicate action are flexor (to flex or bend), masseter (to chew), and levator (to lift).Page 262
SCIENCE IN YOUR LIFE
Which muscles are best to use for intramuscular injections?
When giving intramuscular injections, health-care providers have to choose muscles that are sufficiently large and well developed to tolerate the injections. But they must also avoid muscles that contain large blood vessels or nerves, because an injection in these muscles could pierce a blood vessel or damage a nerve. Typically, they choose one of three preferred injection sites. The deltoid muscle on the upper arm is usually well developed in older children and adults. The vastus lateralis on the side of the thigh (part of the quadriceps group) is the best site for infants and young children. The gluteus medius is on the lower back, above the buttock. However, a clinician injecting into the gluteus medius must be careful to avoid the gluteus maximus (buttock) muscle. The body’s largest nerve, the sciatic nerve, lies underneath and within the gluteus maximus.
CHECK YOUR PROGRESS 13.1
State the three types of muscles in the human body and explain where each is found in the body.
Answer
Smooth muscle found in walls of internal organs; cardiac muscle of the walls of the heart; skeletal muscle attached to the skeleton.
Summarize the functions of skeletal muscles.
Answer
Support the body, make bones move, produce heat, increase fluid movement in cardiovascular and lymphatic vessels, protect internal organs, stabilize joints.
Explain how skeletal muscles work together to cause bones to move.
Answer
Skeletal muscles work in opposite groups, with one flexing and the other extending a joint.
CONNECTING THE CONCEPTS
For more information on the three types of muscles, refer to the following discussions:
Section 4.3 describes the general structure of cardiac, smooth, and skeletal muscle.
Section 5.3 examines the function of cardiac muscle in the heart.
Section 9.1 illustrates how smooth muscle lines the wall of the digestive tract.
In your life and check yourself
Reverse
Skeletal Muscles of the Body
Humans belong to a class of animals called the vertebrates (see Section 23.3). Vertebrate animals possess an internal vertebral column, a skeleton, and jointed appendages. Our skeletal muscles are attached to the skeleton, and their contraction causes the movement of bones at a joint.
SCIENCE IN YOUR LIFE
How many muscles are there in the human body?
Most experts agree that there are over 600 muscles in the human body. The exact number varies, because some experts lump muscles together under one name and others differentiate them. The smallest of these, the stapedius, is a 1.27-mm-long muscle in the middle ear. The longest muscle is the sartorius, which starts at the hip and extends to the knee. The biggest muscle (in terms of mass) is the gluteus maximus, the muscle that makes up the majority of the buttocks.
Functions of Skeletal Muscles
The skeletal muscles of the body have a wide variety of functions, including the following:
Support. Skeletal muscle contraction opposes the force of gravity and allows us to remain upright.
Movements of bones and other body structures. Muscle contraction accounts not only for the movement of arms and legs but also for movements of the eyes, facial expressions, and breathing.
Maintenance of a constant body temperature. Skeletal muscle contraction causes ATP to break down, releasing heat, which is distributed throughout the body.
Movement of fluids in the cardiovascular and lymphatic systems. The pressure of skeletal muscle contraction keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels.
Protection of the internal organs and the stabilization of joints. Muscles pad the bones, and the muscular wall in the abdominal region protects the internal organs. Muscle tendons help hold bones together at joints.
Basic Structure of Skeletal Muscles
Skeletal muscles are well organized. A whole muscle contains bundles of skeletal muscle fibers called fascicles (Fig. 13.2). These are the strands of muscle we see when we cut red meat and poultry. Within a fascicle, each fiber is surrounded by connective tissue; the fascicle is also surrounded by connective tissue. Muscles are covered with fascia, a type of connective tissue that extends beyond the muscle and becomes its tendon. Tendons quite often extend past a joint before anchoring a muscle to a bone. Small, fluid-filled sacs called bursae (sing., bursa) can often be found between tendons and bones. The bursae act as cushions, allowing ease of movement.
Figure 13.2 Connecting muscle to bone. Connective tissue separates bundles of muscle fibers that make up a skeletal muscle. A layer of connective tissue covering the muscle contributes to the tendon, which attaches muscle to bone.
Skeletal Muscles Work in Pairs
In general, each muscle is concerned with the movement of only one bone. To simplify the discussion, we will focus on the movement of a single bone and no others. The origin of a muscle is on a stationary bone, and the insertion of a muscle is on a bone that moves. When a muscle contracts, it pulls on the tendons at its insertion and the bone moves. For example, when the biceps brachii contracts, it raises the forearm.
Skeletal muscles usually function in groups. Consequently, to make a particular movement, your nervous system does not stimulate a single muscle. Rather, it stimulates an appropriate group of muscles. Even so, for any particular movement, one muscle does most of the work and is called the agonist, or prime mover. While a prime mover is working, other muscles called synergists function as well. Synergists assist the agonist and make its action more effective.
When muscles contract, they shorten. Therefore, muscles can only pull; they cannot push. This means that muscles work in opposite pairs. The muscle that acts opposite to a prime mover is called an antagonist. For example, the biceps brachii and the triceps Page 261brachii are antagonists. The biceps flexes the forearm (Fig. 13.3a), and the triceps extends the forearm (Fig. 13.3b). If both of these muscles contracted at once, the forearm would remain rigid. Smooth body movements depend on an antagonist relaxing when a prime mover is acting.
Figure 13.3 Skeletal muscles often work in pairs. a. When the biceps brachii contracts, the forearm flexes. b. When the triceps brachii contracts, the forearm extends. Therefore, these two muscles are antagonistic. The origin of a skeletal muscle is on a bone that remains stationary, and the insertion of a muscle is on a bone that moves when the muscle contracts.
Not all skeletal muscles are involved in the movement of limbs. For example, the facial muscles (Fig. 13.4) produce the facial expressions that tell us about the emotions and mood of a person and therefore play an important role in our interactions with other people.
Figure 13.4 Facial expressions. Our many facial expressions are due to muscle contractions.
(both photos): ©McGraw-Hill Education/J.W. Ramsey, photographer
Names and Actions of Skeletal Muscles
Figure 13.5a, b illustrates the location of some of the major skeletal muscles and gives their actions. (Not all the muscles mentioned are featured in Figure 13.5, but most are.)
Figure 13.5 The major skeletal muscles of the human body. a. Anterior view. b. Posterior view.
When learning the names of muscles, considering what the names mean will help you remember them. The names of the various skeletal muscles are often combinations of the following terms used to characterize muscles:
Size. The gluteus maximus is the largest muscle that makes up the buttocks. The gluteus minimus is the smallest of the gluteal muscles. Other terms used to indicate size are vastus (huge), longus (long), and brevis (short).
Shape. The deltoid is shaped like a triangle. (The Greek letter delta has this appearance: ∆.) The trapezius is shaped like a trapezoid. Other terms used to indicate shape are latissimus (wide) and teres (round).
Location. The external oblique muscles are located outside the internal obliques. The frontalis muscle overlies the frontal bone. Other terms used to indicate location are pectoralis (chest), gluteus (buttock), brachii (arm), and sub (beneath).
Direction of muscle fibers. The rectus abdominis is a longitudinal muscle of the abdomen (rectus means “straight”). The orbicularis oculi is a circular muscle around the eye. Other terms used to indicate direction are transverse (across) and oblique (diagonal).
Attachment. The sternocleidomastoid is attached to the sternum, clavicle, and mastoid process. The mastoid process is located on the temporal bone of the skull. The brachioradialis is attached to the brachium (arm) and the radius (forearm).
Number of attachments. The biceps brachii has two attachments, or origins, and is located on the arm. The quadriceps femoris has four origins and is located on the femur.
Action. The extensor digitorum extends the fingers, or digits. The adductor longus is a large muscle that adducts the thigh. Adduction is the movement of a body part toward the midline. Other terms used to indicate action are flexor (to flex or bend), masseter (to chew), and levator (to lift).Page 262
SCIENCE IN YOUR LIFE
Which muscles are best to use for intramuscular injections?
When giving intramuscular injections, health-care providers have to choose muscles that are sufficiently large and well developed to tolerate the injections. But they must also avoid muscles that contain large blood vessels or nerves, because an injection in these muscles could pierce a blood vessel or damage a nerve. Typically, they choose one of three preferred injection sites. The deltoid muscle on the upper arm is usually well developed in older children and adults. The vastus lateralis on the side of the thigh (part of the quadriceps group) is the best site for infants and young children. The gluteus medius is on the lower back, above the buttock. However, a clinician injecting into the gluteus medius must be careful to avoid the gluteus maximus (buttock) muscle. The body’s largest nerve, the sciatic nerve, lies underneath and within the gluteus maximus.
CHECK YOUR PROGRESS 13.1
State the three types of muscles in the human body and explain where each is found in the body.
Answer
Smooth muscle found in walls of internal organs; cardiac muscle of the walls of the heart; skeletal muscle attached to the skeleton.
Summarize the functions of skeletal muscles.
Answer
Support the body, make bones move, produce heat, increase fluid movement in cardiovascular and lymphatic vessels, protect internal organs, stabilize joints.
Explain how skeletal muscles work together to cause bones to move.
Answer
Skeletal muscles work in opposite groups, with one flexing and the other extending a joint.
CONNECTING THE CONCEPTS
For more information on the three types of muscles, refer to the following discussions:
Section 4.3 describes the general structure of cardiac, smooth, and skeletal muscle.
Section 5.3 examines the function of cardiac muscle in the heart.
Section 9.1 illustrates how smooth muscle lines the wall of the digestive tract.
In your life and check yourself
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Naming skeletal muscles
Names and Actions of Skeletal Muscles
Figure 13.5a, b illustrates the location of some of the major skeletal muscles and gives their actions. (Not all the muscles mentioned are featured in Figure 13.5, but most are.)
Figure 13.5 The major skeletal muscles of the human body. a. Anterior view. b. Posterior view.
When learning the names of muscles, considering what the names mean will help you remember them. The names of the various skeletal muscles are often combinations of the following terms used to characterize muscles:
Size. The gluteus maximus is the largest muscle that makes up the buttocks. The gluteus minimus is the smallest of the gluteal muscles. Other terms used to indicate size are vastus (huge), longus (long), and brevis (short).
Shape. The deltoid is shaped like a triangle. (The Greek letter delta has this appearance: ∆.) The trapezius is shaped like a trapezoid. Other terms used to indicate shape are latissimus (wide) and teres (round).
Location. The external oblique muscles are located outside the internal obliques. The frontalis muscle overlies the frontal bone. Other terms used to indicate location are pectoralis (chest), gluteus (buttock), brachii (arm), and sub (beneath).
Direction of muscle fibers. The rectus abdominis is a longitudinal muscle of the abdomen (rectus means “straight”). The orbicularis oculi is a circular muscle around the eye. Other terms used to indicate direction are transverse (across) and oblique (diagonal).
Attachment. The sternocleidomastoid is attached to the sternum, clavicle, and mastoid process. The mastoid process is located on the temporal bone of the skull. The brachioradialis is attached to the brachium (arm) and the radius (forearm).
Number of attachments. The biceps brachii has two attachments, or origins, and is located on the arm. The quadriceps femoris has four origins and is located on the femur.
Action. The extensor digitorum extends the fingers, or digits. The adductor longus is a large muscle that adducts the thigh. Adduction is the movement of a body part toward the midline. Other terms used to indicate action are flexor (to flex or bend), masseter (to chew), and levator (to lift).Page 262
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13.1 Overview of the Muscular System
LEARNING OUTCOMES
Upon completion of this section, you should be able to
List the three types of muscle tissue and provide a function for each.
Describe the general structure of a skeletal muscle.
Recognize how skeletal muscles are named.
How much do you know
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Thoracic vertebrae
Know this rib cage shit
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Download all those photos
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Write study guide
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myofilaments
thick- myosin
actin- thin
striations
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Inside a skeletal muscle cell, a set of small cylinders called _______ are assembled into larger cylinders called _______.
myofilaments; myofibrils
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This muscle is found in blood vessels of the body
What is smooth muscle
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Immediately after the cross-bridge is formed and ADP and phosphate are released, what happens to the actin filament?
It is pulled closer towards the center of the sarcomere, causing muscle contraction.
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Muscle contractions are made by
actin filaments sliding past myosin filaments.
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LEARNING OUTCOMES
Upon completion of this section, you should be able to
Summarize the role of the muscular and skeletal systems in movement.
Summarize the role of the muscular system in body temperature homeostasis.
In this section, our discussion centers on the contribution of the muscular system to homeostasis (Fig. 13.13). In many cases, the muscular system works closely with the skeletal system—for example, in the protection of body parts and movement.
Figure 13.13 The muscular system and homeostasis. The muscular system works with these body systems to assist in homeostasis.
Both Systems Produce Movement
Movement is essential to maintaining homeostasis. The skeletal and muscular systems work together to enable body movement. This is most evidently illustrated by what happens when skeletal muscles contract and pull on the bones to which they are attached, causing movement at joints. Body movement of this sort allows us to respond to certain types of changes in the environment. For instance, if you are sitting in the sun and start to feel hot, you can get up and move to a shady spot.
The muscular and skeletal systems work for other types of movements that are just as important for maintaining homeostasis. Contraction of skeletal muscles associated with the jaw and tongue allows you to grind food with the teeth. The rhythmic smooth muscle contractions of peristalsis move ingested materials through the digestive tract. These processes are necessary for supplying the body’s cells with nutrients. The ceaseless beating of your heart, which propels blood into the arterial system, is caused by the contraction of cardiac muscle. Contractions of skeletal muscles in the body, especially those associated with breathing and leg movements, aid in the process of venous return by pushing blood back toward the heart. This is why soldiers and members of marching bands are cautioned not to lock their knees when standing at attention. The reduction in venous return causes a drop in blood pressure, which can result in fainting. The pressure exerted by skeletal muscle contraction also helps squeeze interstitial fluid into the lymphatic capillaries, where it is referred to as lymph.
Both Systems Protect Body Parts
The skeletal system plays an important role by protecting the soft internal organs of your body. The brain, heart, lungs, spinal cord, kidneys, and liver and most of the endocrine glands are shielded by the skeleton. In particular, the nervous and endocrine organs must be defended so they can carry out activities necessary for homeostasis.
The skeletal muscles pad and protect the bones, and the tendons and bursae associated with skeletal muscles reinforce and cushion the joints. Muscles of the abdominal wall offer additional protection to the soft internal organs. Examples of these muscles include the rectus abdominis and external oblique muscles illustrated in Figure 13.5.Page 275
Muscles Help Maintain Body Temperature
The muscular system helps regulate body temperature. When you are very cold, smooth muscle constricts inside the blood vessels supplying the skin. Thus, the amount of blood close to the surface of the body is reduced. This helps conserve heat in the body’s core, where vital organs lie. If you are cold enough, you may start to shiver. Shivering is caused by involuntary skeletal muscle contractions. This is initiated by temperature-sensitive neurons in the hypothalamus of the brain. Skeletal muscle contraction requires ATP, and using ATP generates heat. You may also notice that you get goose bumps when you are cold. This is because arrector pili muscles contract. These tiny bundles of smooth muscle attached to the hair follicles cause the hairs to stand up. This is not very helpful in keeping humans warm, but it is quite effective in our furrier fellow mammals. Think of a cat or dog outside on a cold winter day. Its fur is a better insulator when standing up than when lying flat. Goose bumps can also be a sign of fear. Although a human with goose bumps may not look very impressive, a Page 276frightened or aggressive animal whose fur is standing on end looks bigger and (it is hoped) more intimidating to a predator or rival.
CHECK YOUR PROGRESS 13.5
Summarize the importance of movement in homeostasis.
Answer
Muscle movement allows body movement in response to environmental change. It is also necessary for breathing, peristalsis, moving gametes, and childbirth. Also, it moves fluid in blood and lymph vessels, ureters, and the urinary bladder.
Summarize how the muscular system works to maintain body temperature.
Answer
Smooth muscles in blood vessels at body surfaces can constrict, diverting blood internally to conserve heat; contraction of skeletal muscle and involuntary shivering can produce heat.
Explain how the muscular system interacts with the digestive system.
Answer
Muscle contraction accounts for chewing and peristalsis; the digestive system absorbs nutrients needed for muscle contraction.
CONNECTING THE CONCEPTS
For more information on calcium and body temperature homeostasis, refer to the following discussions:
Section 4.8 explores how the body maintains homeostasis using feedback mechanisms.
Figure 4.18 examines how the hypothalamus is involved in body temperature regulation.
Section 16.3 describes how the thyroid and parathyroid glands are involved in calcium homeostasis.
CONCLUSION
There are several different classes of muscular dystrophy. In the most common types of muscular dystrophy, symptoms occur very early in life. In Kate’s case, the relatively late onset of the disease suggested she had a rarer form called Becker muscular dystrophy. For Kate, the good news was that this is a much slower-progressing form of the disease, with most patients living well into their thirties. Furthermore, many of the symptoms of Becker muscular dystrophy can be controlled with medication. Becker muscular dystrophy is known to cause heart problems later in life, but researchers are actively studying whether it may be possible to use gene therapy (see Section 22.4) to replace the defective dystrophin gene. In the interim, patients of Becker muscular dystrophy, such as Kate, are recommended to regularly exercise to slow the loss of muscle tissue over time.
13.5 Homeostasis
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motor neurons
branches to muscle fibers- if one motor neuron- doesn’t have to deal with so many- fine motor control
playing the piano- braiding hair- typing on keyboard
large motor units- one somatic neuron- invade a whole bunch of cells - at the same time- gross- bulk movements- quadriceps- lifting weights
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For example, the plasma membrane is called the sarcolemma (sarco means “muscle”); the cytoplasm is the sarcoplasm; and the endoplasmic reticulum is the sarcoplasmic reticulum.
The plasma membrane of a muscle fiber is called:
The cytoplasm is called:
The endoplasmic reticulum is called
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Label this diagram
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parts of sarcolema- close to reticulum- terminal -
myofibrals- bundles of filaments- contractal
gives skeletal- alternating light and dark bands
- striations
sarcoplasm
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’’
The region where the end of an axon from a neuron comes into close contact with a muscle fiber is called
neuromuscular junction.
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Muscular disorders include spasms, convulsions, cramps, and facial tics.
Muscular system injuries include strains, sprains, tendinitis, and bursitis.
Diseases of the muscular system include myalgias (fibromyalgia); muscular dystrophy (Duchenne muscular dystrophy); myasthenia gravis; and cancer of the muscles (sarcomas).
13.4Muscular Disorders
My walls are composed of compact bone.
What is medullar cavity?
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actin filaments sliding past myosin filaments.
Muscle contraction is caused by
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Skeletal muscle is involuntary
False.
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Drink water
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Animation: Sarcomere contraction
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View the animation below, and then answer the following questions to test your knowledge of the concept.
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H zones and I bands
Which of the following statements about H zones and I bands is true?
Multiple Choice
H zones contain only myosin, whereas I bands contain only actin.Correct
H zones contain only actin, whereas I bands contain only myosin.
H zones contain only myosin, whereas I bands contain both actin and myosin.
H zones contain both actin and myosin, whereas I bands contain only actin.
H zones and I bands each contain both actin and myosin.
H zones - only myosin, I bands- only actin
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This type of muscle tissue has tapered spindle like cells.
Smooth muscle
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Muscle fibers are stimulated to contract by motor neurons whose axons are grouped together to form nerves. The axon of one motor neuron can stimulate from a few to several muscle fibers of a muscle, because each axon has several branches (Fig. 13.7a). Each branch of Page 266an axon ends in an axon terminal that lies in close proximity to the sarcolemma of a muscle fiber. A small gap, called a synaptic cleft, separates the axon terminal from the sarcolemma (Fig. 13.7b). This entire region is called a neuromuscular junction.
Neuromuscular contraction action
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Three types of muscles
Classify the following descriptions based on the type of muscle they represent.
Explanation
Review muscle types in section 13.1 and figure 13.1.
Hmmm- Muscles PLQ
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What is the part of a sarcomere that contains only thin filaments?
I band
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Integrate those notes
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Muscular System Back
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Study this diagram.
Smooth muscle fibers are shaped like narrow cylinders with pointed ends. Each has a single nucleus (uninucleated). The cells are usually arranged in parallel lines, forming sheets. Striations (alternating light and dark bands) are seen in cardiac and skeletal muscle but not in smooth muscle. Smooth muscle is located in the walls of hollow internal organs (see Section 9.1) and blood vessels (see Section 5.2) and causes these walls to contract. Contraction of smooth muscle is involuntary, occurring without conscious control. Although smooth muscle is slower to contract than skeletal muscle, it can sustain prolonged contractions and does not fatigue easily.
Cardiac muscle forms the heart wall (see Section 5.3). Its fibers are generally uninucleated, striated, and tubular. Branching allows the fibers to interlock at intercalated discs. The plasma membranes at intercalated discs contain gap junctions (see Section 3.5) that permit contractions to spread quickly throughout the heart wall. Cardiac fibers relax completely between contractions, which prevents fatigue. Contraction of cardiac muscle is rhythmic. It occurs without outside nervous stimulation and without conscious control. Thus, cardiac muscle contraction is involuntary.
Skeletal muscle fibers are tubular, multinucleated, and striated and make up the skeletal muscles attached to the skeletal system (see Chapter 12). Fibers run the length of the muscle and Page 260can be quite long. The myocytes of skeletal muscle are commonly called muscle fibers. Skeletal muscle is voluntary; we can decide to move a particular part of the body, such as the arms and legs.
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These looked tapered
Smooth muscle cells
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Skeletal muscle
through tendons, muscles attached through bone or skin
long cylindrical cells- muscle fibers- description- super long- more than one nucleus- control centers- need this protein in which area- actin and myosin- striated appearance
- voluntary
- move both bone and skin
- somatic motor neurons individually stimulated
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Smooth, skeletal, cardiac
functions: skeletal muscle-
how muscles are named- variety- location, shape, function
major muscles of human body
skeletal muscles- muscle fibers- structured- overall shape
how contract- and different types do
sending messages to muscles
atp- contractions take place- where comes from
muscle fibers- fast twitch and slow twitch fibers
muscular conditions diseases and disorders
how skeletal and muscular system work together to maintain homeostasis
Themes
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Where are the fuel sources for muscle contraction? • Stored in the muscle • Glycogen • Lipid • In the blood • Glucose • Fatty acids 0 1 2 3 4 60 50 40 30 20 10 0 Exercise time (hr) muscle triglycerides plasma fatty acids blood glucose muscle glycogen Percentage of energy expenditure Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 13.10 The sources of energy for muscle contraction. 13.3 Whole Muscle Contraction 2 What are the sources of ATP for muscle contraction? • Limited amounts of ATP
Muscles3
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Names and Actions of Skeletal Muscles
Muscles are named for their size, shape, location, direction of fibers, number of attachments, and action.
How are muscles named? What are the 6 different ways?
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Muscles Have Motor Units
A muscle contains motor units: several fibers under the control of a single motor axon.
Motor unit contraction is described in terms of a muscle twitch, summation, and tetanus.
The strength of muscle contraction varies according to recruitment of motor units.
In the body, a continuous slight tension, called muscle tone, is maintained by muscle motor units that take turns contracting.
What is motor unit contraction?
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Structures
pre-fixes myo, mis, sarco- flesh
myo and mis- muscle
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Orbicularis oris:
This is the kissing muscle.
movement, both externally and internally.
myocytes.
Smooth muscle is involuntary and occurs in walls of internal organs.
Cardiac muscle is involuntary and occurs in walls of the heart. Cardiac muscle has intercalated discs that permit rapid contraction.
Skeletal muscle is voluntary, and contains bundles of cells called muscle fibers arranged into fascicles, usually attached by tendons to the skeleton.
Skeletal muscle is involved in support, movement, and protection. Skeletal muscles are connected to bones by tendons. Bursae provide a cushion between the muscle and bone.
Names and Actions of Skeletal Muscles
Muscles are named for their size, shape, location, direction of fibers, number of attachments, and action.
Overview of Muscular System
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Skeletal muscle cells
Long rod-shaped cells;
actin and myosin- microfilaments
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Axon terminals contain synaptic vesicles filled with the neurotransmitter acetylcholine (ACh). Nerve signals travel down the axons of motor neurons and arrive at an axon terminal. The signals trigger the synaptic vesicles to release ACh into the synaptic cleft (Fig. 13.7c). When ACh is released, it quickly diffuses across the cleft and binds to receptors in the sarcolemma. Now, the sarcolemma generates electrical signals that spread across the sarcolemma and down the T tubules. Recall that the T tubules lie adjacent to the sarcoplasmic reticulum, but the two structures are not connected. Nonetheless, signaling from the T tubules causes the release of Ca2+ from the sarcoplasmic reticulum, which leads to sarcomere contraction, as explained in Figure 13.8.
ACH is released when
Label the cranium
Memorize this
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Connecting muscles to bone- Check out this diagram
Figure 13.2 Connecting muscle to bone. Connective tissue separates bundles of muscle fibers that make up a skeletal muscle. A layer of connective tissue covering the muscle contributes to the tendon, which attaches muscle to bone.
Skeletal Muscles Work in Pairs
In general, each muscle is concerned with the movement of only one bone. To simplify the discussion, we will focus on the movement of a single bone and no others. The origin of a muscle is on a stationary bone, and the insertion of a muscle is on a bone that moves. When a muscle contracts, it pulls on the tendons at its insertion and the bone moves. For example, when the biceps brachii contracts, it raises the forearm.
Skeletal muscles usually function in groups. Consequently, to make a particular movement, your nervous system does not stimulate a single muscle. Rather, it stimulates an appropriate group of muscles. Even so, for any particular movement, one muscle does most of the work and is called the agonist, or prime mover. While a prime mover is working, other muscles called synergists function as well. Synergists assist the agonist and make its action more effective.
When muscles contract, they shorten. Therefore, muscles can only pull; they cannot push. This means that muscles work in opposite pairs. The muscle that acts opposite to a prime mover is called an antagonist. For example, the biceps brachii and the triceps Page 261brachii are antagonists. The biceps flexes the forearm (Fig. 13.3a), and the triceps extends the forearm (Fig. 13.3b). If both of these muscles contracted at once, the forearm would remain rigid. Smooth body movements depend on an antagonist relaxing when a prime mover is acting.
Figure 13.3 Skeletal muscles often work in pairs. a. When the biceps brachii contracts, the forearm flexes. b. When the triceps brachii contracts, the forearm extends. Therefore, these two muscles are antagonistic. The origin of a skeletal muscle is on a bone that remains stationary, and the insertion of a muscle is on a bone that moves when the muscle contracts.
Not all skeletal muscles are involved in the movement of limbs. For example, the facial muscles (Fig. 13.4) produce the facial expressions that tell us about the emotions and mood of a person and therefore play an important role in our interactions with other people.
Figure 13.4 Facial expressions. Our many facial expressions are due to muscle contractions.
(both photos): ©McGraw-Hill Education/J.W. Ramsey, photographer
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Muscular system front
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Neuromuscular junctions
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muscular system works with skeletal system—for example, in the protection of body parts and movement.
see: Figure 13.13 The muscular system and homeostasis. The muscular system works with these body systems to assist in homeostasis.
Produce Movement
- essential to maintaining homeostasis. skeletal and muscular systems work together to enable body movement. This is most evidently illustrated by what happens when skeletal muscles contract and pull on the bones to which they are attached, causing movement at joints. Body movement of this sort allows us to respond to certain types of changes in the environment. For instance, if you are sitting in the sun and start to feel hot, you can get up and move to a shady spot.
- The muscular and skeletal systems work for other types of movements that are just as important for maintaining homeostasis. Contraction of skeletal muscles associated with the jaw and tongue allows you to grind food with the teeth. The rhythmic smooth muscle contractions of peristalsis move ingested materials through the digestive tract. These processes are necessary for supplying the body’s cells with nutrients. The ceaseless beating of your heart, which propels blood into the arterial system, is caused by the contraction of cardiac muscle. Contractions of skeletal muscles in the body, especially those associated with breathing and leg movements, aid in the process of venous return by pushing blood back toward the heart. This is why soldiers and members of marching bands are cautioned not to lock their knees when standing at attention. The reduction in venous return causes a drop in blood pressure, which can result in fainting. The pressure exerted by skeletal muscle contraction also helps squeeze interstitial fluid into the lymphatic capillaries, where it is referred to as lymph.
- Both Systems Protect Body Parts
- The skeletal system plays an important role by protecting the soft internal organs of your body. The brain, heart, lungs, spinal cord, kidneys, and liver and most of the endocrine glands are shielded by the skeleton. In particular, the nervous and endocrine organs must be defended so they can carry out activities necessary for homeostasis.
- The skeletal muscles pad and protect the bones, and the tendons and bursae associated with skeletal muscles reinforce and cushion the joints. Muscles of the abdominal wall offer additional protection to the soft internal organs. Examples of these muscles include the rectus abdominis and external oblique muscles illustrated in Figure 13.5.Page 275
- Muscles Help Maintain Body Temperature
- The muscular system helps regulate body temperature. When you are very cold, smooth muscle constricts inside the blood vessels supplying the skin. Thus, the amount of blood close to the surface of the body is reduced. This helps conserve heat in the body’s core, where vital organs lie. If you are cold enough, you may start to shiver. Shivering is caused by involuntary skeletal muscle contractions. This is initiated by temperature-sensitive neurons in the hypothalamus of the brain. Skeletal muscle contraction requires ATP, and using ATP generates heat. You may also notice that you get goose bumps when you are cold. This is because arrector pili muscles contract. These tiny bundles of smooth muscle attached to the hair follicles cause the hairs to stand up. This is not very helpful in keeping humans warm, but it is quite effective in our furrier fellow mammals. Think of a cat or dog outside on a cold winter day. Its fur is a better insulator when standing up than when lying flat. Goose bumps can also be a sign of fear. Although a human with goose bumps may not look very impressive, a Page 276frightened or aggressive animal whose fur is standing on end looks bigger and (it is hoped) more intimidating to a predator or rival.
*
13.5 Homeostasis
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