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Chapter 29 The Skeletal and Muscular

Systems

Artificial limb: ©Jeff J Mitchell/Getty Images

Muscles Interact with the Skeleton

Section 29.1

Most animal movements require interaction between the muscular and skeletal systems.

Section 29.1

The muscular system provides motion. Muscle cells contract when stimulated by the nervous system.

Muscle

Section 29.1

The skeleton adds a firm supporting structure that muscles pull against.

Muscle

Muscle attachment to bone

Skeleton

Section 29.1

Scientists classify skeletons into three categories:• Hydrostatic skeleton• Exoskeleton• Endoskeleton

Skeletons Take Many Forms

Figure 29.1Jellyfish: © Gabriel Bouys/AFP/Getty Images; crab: © Photodisc/Getty Images RF; fish X-ray: © Jim Wehtje/Getty Images RF

Hydrostatic skeleton

Exoskeleton

Endoskeleton

Section 29.1

A hydrostatic skeleton is fluid constrained within a layer of flexible tissue. Many invertebrates have hydrostatic skeletons.

Figure 29.1

Jellyfish: © Gabriel Bouys/AFP/Getty Images

Section 29.1

An exoskeletonprotects an animal from the outside. Animals with exoskeletons must periodically molt.

Figure 29.2

Crab: © Photodisc/Getty Images RF

Section 29.1

An endoskeleton is an internal support structure. An endoskeleton grows with the animal and weighs less than an exoskeleton, but external soft tissues are not protected.

Figure 29.3

Clicker Question #1

The graph below depicts the growth of an animal over time. What sort of skeleton does the animal probably have?

A. endoskeletonB. exoskeletonC. hydrostatic skeleton

Flower: © Doug Sherman/Geofile/RF

Clicker Question #1

The graph below depicts the growth of an animal over time. What sort of skeleton does the animal probably have?

A. endoskeletonB. exoskeletonC. hydrostatic skeleton

29.1 Mastering Concepts

Describe similarities and differences among the three main types of skeletons.

Section 29.2

Bones are the organs of the vertebrate skeleton. They are grouped into two categories: •Axial skeleton (tan)•Appendicular skeleton (blue)

The Vertebrate Skeleton: An Overview

Figure 29.4

Section 29.2

The axial skeleton is located along the central axis of the body. It consists of:

• Bones in the head• Bones of the rib cage• The vertebral column

Figure 29.4

Section 29.2

The axial skeleton shields soft body parts. • The skull protects the brain

and the sense organs.• The ribs protect the heart and

lungs. • The vertebral column protects

the spinal cord.

Figure 29.4

Section 29.2

The appendicular skeleton consists of the appendages and the bones that support them.• The pectoral girdle connects

the forelimbs to the axial skeleton.

• The pelvic girdle attaches the hind limbs to the axial skeleton.

Figure 29.4

Section 29.2

The vertebrate skeleton consists of many bones.

Figure 29.4

Describe the two subdivisions of the human skeleton.

The Skull and Associated Bones

Sutures

• Sutures – Immovable joints that join skull bones together

• Form boundaries between skull bones

• Four sutures:

– Coronal – between parietal and frontal

– Sagittal– between parietal bones

– Lambdoid – between the parietal and occipital

– Squamous – between the parietal and temporal

Fontanels – usually ossify by 2 years of age

Frontal

(Coronal)

Sagittal

Squamous

Lambdoid

Sutures

•skull = 22 bones

•cranium = 8 bones: frontal, occipital, 2 temporals, 2 parietals, sphenoid and

ethmoid

•facial bones = 14 bones: nasals, maxillae, zygomatics, mandible, lacrimals,

palatines, inferior nasal conchae, vomer.

•skull forms a larger cranial cavity

-also forms the nasal cavity, the orbits, paranasal sinuses

mandible and auditory ossicles are the only movable skull bones

•cranial bones also: attach to membranes called meninges

-stabilize positions of the brain, blood vessels

-outer surface provides large areas for muscle attachment that

move the head or provide facial expressions

The Adult Skull

Bones of the Cranium

Frontal View

Frontal

Frontal View

Parietal

Frontal View

Temporal

Frontal View

Zygoma

Frontal View

Maxilla

Frontal View

Mandible

Frontal View

FrontalParietal

Temporal

Zygoma

Maxilla

Mandible

Frontal View

Lateral View

Frontal

Lateral View

Parietal

Lateral View

Temporal

Lateral View

Zygoma

Lateral View

Maxilla

Lateral View

Mandible

Lateral View

Occipital

Lateral View

Mastoid Process

Lateral View

External Auditory Meatus

Lateral View

Frontal

ZygomaMaxilla

Mandible

Parietal

Sphenoid

Temporal

Occipital

External Auditory Meatus

Mastoid Process

Lateral View

Figure 6.4 Sectional Anatomy of the Skull, Part I

14 Facial Bones

Nasal (2) Maxillae (2) Zygomatic (2)

Mandible (1) Lacrimal (2) Palatine (2)

Inferior nasal conchae (2) Vomer (1)

Section 29.3

Bones consist mostly of specialized cells suspended in a hard extracellular matrix.

Bone Structure and Function

Section 29.3

Bones are lightweight and strong because they are porous, not solid.

Section 29.3

Figure 29.6

The shaft of a long bone contains a marrow cavity, which houses red or yellow bone marrow.

Cartilage: © Chuck Brown/Science Source; bone:© Ed Reschke

Section 29.3

Figure 29.6

Red bone marrowis a nursery for blood cells and platelets.

Section 29.3

Figure 29.6

Yellow bone marrow replaces red marrow in adult limb bones. It does not produce blood, but can revert to red marrow if necessary.

Section 29.3

Figure 29.6

Bones also contain nerves and blood vessels.

Section 29.3

Figure 29.6

Bone cells are called osteocytes.They secrete a hard extracellular matrix that consists of collagen and minerals.

Section 29.3

Figure 29.6

Collagen adds flexibility, and the minerals add hardness and rigidity.

Section 29.3

Figure 29.6

Compact bone is hard and dense. Spongy bone is light and porous.

Section 29.3

Figure 29.6

The vertebrate skeleton also has cartilage, which serves as a shock absorber.

Section 29.3

Figure 29.7

Bone tissue replaces cartilage as an individual develops. In children, bone growth occurs at growth plates made of cartilage.

Section 29.3

Table 29.1

The vertebrate skeleton has several important functions.

Section 29.3

Table 29.1

Especially notable is the skeleton’s role in calcium homeostasis.

Section 29.3

Figure 29.9

Bones are a reservoir for calcium, which has many essential functions in the body. Thyroid and parathyroid hormones regulate calcium homeostasis.

If bones consistently lose more calcium than they add, then osteoporosisdevelops, in which bones become less dense.

Section 29.3

Figure 29.10Healthy bone: © Prof. P.M. Motta/Science Source; osteoporosis:© Dee Breger/Science Source

Section 29.3

Figure 29.10

Sometimes, disuse causes bones to become less dense.

Healthy bone: © Prof. P.M. Motta/Science Source; osteoporosis:© Dee Breger/Science Source

Section 29.3

Bone Meets Bone at a Joint

Figure 29.11

A joint is an area where two bones meet.

Section 29.3

Movable joints have fluid-filled capsules of connective tissue, allowing bones to move against each other with little friction.

Figure 29.11

Section 29.3

Bands of connective tissue help stabilize movable joints. Ligaments connect bone to bone. Tendons connect bone to muscle.

Figure 29.11

Clicker Question #2

A painful condition called “tennis elbow” is caused by inflammation of the tissue that connects a muscle to an arm bone. Tennis elbow therefore reflects a problem with a

A. joint.B. ligament.C. tendon.D. muscle fiber.E. bone.

Clicker Question #2

A painful condition called “tennis elbow” is caused by inflammation of the tissue that connects a muscle to an arm bone. Tennis elbow therefore reflects a problem with a

A. joint.B. ligament.C. tendon.D. muscle fiber.E. bone.

What are the main parts of a long bone?

Section 29.4

Muscle Contraction Requires ATP

As we’ve seen, movement relies on interaction between bones and muscles.

Section 29.4

Muscles have many other functions too.

Table 29.2

Section 29.4

The human body has more than 600 skeletal muscles, which generate voluntary movements.

Figure 29.12

Section 29.4

Pairs of muscles work together to generate body movements.

Figure 29.12

Section 29.4

Muscles have a hierarchical organization.

Figure 29.14

Section 29.4

Muscles consist mainly of bundles of muscle fibers.

Figure 29.10

Section 29.4

Each muscle fiber is a single cell.

Figure 29.10

Section 29.4

Each muscle cell is packed with myofibrils.

Figure 29.10

Section 29.4

Figure 29.10

Myofibrils are divided into functional units called sarcomeres.

Section 29.4

Figure 29.10

Each sarcomere contains thick and thin protein filaments.

Clicker Question #3

If you arrange the following items in order from largest to smallest, which item is THIRD on the list?

myofibril, muscle fiber, sarcomere, muscle, actin subunit

A. muscle fiberB. sarcomereC. muscleD. actin subunitE. myofibril

Clicker Question #3

If you arrange the following items in order from largest to smallest, which item is THIRD on the list?

myofibril, muscle fiber, sarcomere, muscle, actin subunit

A. muscle fiberB. sarcomereC. muscleD. actin subunitE. myofibril

Section 29.4

When a muscle cell contracts, its sarcomeres become shorter.

Figure 29.15

Section 29.4

According to the sliding filament model, sarcomeres contract because the thin actin filaments slide between the thick myosin filaments.

Figure 29.15

Section 29.4

How does this sliding action occur?

Figure 29.15

Section 29.4

Before the thick and thin filaments of a sarcomere can slide passed one another, the muscle cell must be stimulated.

Figure 29.16

Section 29.4

An action potential travels along a motor neuron. When it reaches the axon terminal, the cell releases neurotransmitters.

Figure 29.16

Section 29.4

Neurotransmitters bind to the muscle cell, starting an electrical wave that propagates along the muscle cell membrane.

Figure 29.16

Section 29.4

As the electrical wave spreads down T tubules, it causes the nearby sarcoplasmic reticulum to release calcium into the sarcomere.

Figure 29.16

Section 29.4

The presence of calcium allows filaments to slide. Let’s look more closely at a sarcomere.

Figure 29.16

Section 29.4

Figure 29.17

Muscle contraction at a molecular scale:

Section 29.4

In step 1, actin and myosin filaments are near each other but not touching. The muscle has received a signal from the nervous system.

Figure 29.17

Section 29.4

Calcium binds to a thin-filament protein called troponin, causing it to change shape and pull on another protein called tropomyosin.

Figure 29.17

Section 29.4

With tropomyosin pulled aside, binding sites on actin are exposed.

Figure 29.17

Section 29.4

In step 2, myosin binds to exposed binding sites on actin, forming cross bridges.

Figure 29.17

Section 29.4

In step 3, the cross bridge changes shape from straight to bent. This “power stroke” pulls on the actin filament.

Figure 29.17

Section 29.4

A new ATP molecule comes in at step 4. It binds to the cross bridges, which causes them to separate from the actin binding sites.

Figure 29.17

Section 29.4

In step 5, hydrolysis of ATP provides the energy to return myosin to its straight conformation.

Figure 29.17

Section 29.4

Myosin is now ready to bind to actin again.

Figure 29.17

Section 29.4

Once the electrical stimulation from the nervous system subsides, calcium will release from troponin, and myosin can no longer bind to actin.

Figure 29.17

Clicker Question #4

Amyotrophic lateral sclerosis (ALS), also called Lou Gehrig's disease, is a disorder in which motor neurons are destroyed. How would this disorder affect muscle function?

A. Muscles fail to contract because ATP is not available.B. Muscles fail to contract because they do not receive a signal to do so.C. Muscles are continually contracted because ATP is not available.D. Muscles are continually contracted because they do not receive a signal to relax.

Clicker Question #4

Amyotrophic lateral sclerosis (ALS), also called Lou Gehrig's disease, is a disorder in which motor neurons are destroyed. How would this disorder affect muscle function?

A. Muscles fail to contract because ATP is not available.B. Muscles fail to contract because they do not receive a signal to do so.C. Muscles are continually contracted because ATP is not available.D. Muscles are continually contracted because they do not receive a signal to relax.

Describe how sliding filaments shorten a sarcomere.

Section 29.5

Muscle Fibers Generate ATP in Multiple Ways

As we’ve seen, ATP is important for muscle contraction.

Figure 29.17

Section 29.5

A resting muscle cell generates ATP in aerobic respiration.

Figure 29.18

Section 29.5

During activity, muscle cells replenish ATP with creatine phosphate, a molecule that donates a phosphate to ADP.

Section 29.5

But creatine phosphate supply is depleted in less than a minute of continuous exercise.

Section 29.5

After creatine phosphate is gone, aerobic respiration continues to produce ATP if muscles are receiving enough oxygen.

Section 29.5

Otherwise, the cells switch to fermentation, an anaerobic pathway of ATP production that generates lactic acid as a byproduct.

Section 29.5

Muscle cell metabolism also explains why a dead body “goes stiff.” After death, cells no longer produce ATP. Actin cannot separate from myosin, resulting in rigor mortis.

Describe the role of creatine phosphate in muscle metabolism.

Section 29.4

Muscles Are Organized into Motor Units

Motor neurons send impulses from the central nervous system to muscle cells, causing them to contract.

Figure 29.19

Section 29.4

One motor neuron typically synapses with several muscle cells, forming a motor unit that contracts together.

Figure 29.19

Section 29.4

Motor units may be small (consisting of few muscle cells) or large. Smaller motor units produce fine, precise movements.

Figure 29.19

Section 29.4

The nervous system stimulates of many motor units in concert to produce smooth, coordinated movements.

Figure 29.19

Section 29.6

Muscle Fiber Types Influence Athletic Performance

Figure 29.20

Skeletal muscles contain two types of cells, distinguished by how quickly they contract and tire.

Section 29.6

Figure 29.20

Slow-twitch fibers are small, use energy slowly, and have high endurance.

Section 29.6

Figure 29.20

Fast-twitch fibers are large, use bursts of energy, and tire quickly.

Section 29.6

Figure 29.20

The proportion of slow- and fast-twitch fibers in an individual’s muscles is mostly genetically determined. However, it is possible to alter the ratio through exercise.

Section 29.6

Figure 29.20

Exercise also increases

• the size of muscle cells• the efficiency of muscle cell

metabolism • blood flow to muscles• bone strength

Clicker Question #5

An athlete supplements her diet with creatine phosphate. How might her athletic performance improve?

A) She might be more competitive in long triathlons. B) She might be able to run a marathon much more quickly.C) She might be able to sprint for slightly longer.D) More than one of these choices is correct.

Clicker Question #5

An athlete supplements her diet with creatine phosphate. How might her athletic performance improve?

A) She might be more competitive in long triathlons. B) She might be able to run a marathon much more quickly.C) She might be able to sprint for slightly longer.D) More than one of these choices is correct.

Why do endurance sports require a high proportion of slow-twitch muscle fibers, whereas power sports require more fast-twitch muscle fibers?

Section 29.7

Investigating Life: Did a Myosin Gene Mutation Make Humans Brainier?

Myosin genes encode proteins abundant in muscle cells. A myosin gene expressed in jaw muscles is mutated in human DNA but active in related primates.

Figure 29.22

The myosin mutation explains why humans have small and weak jaw muscles compared to our relatives, but how does it relate to our intellectual abilities?

Figure 29.22Section 29.7

Large jaw muscles have larger attachment areas on the skull than small jaw muscles, which might put a constraint on brain size.

Weak jaw muscles might have lifted this constraint on brain size, changing the course of human evolution.

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