Biology: An Interactive Tour
Biology: An Interactive Tour

Biology: An Interactive Tour

Lead Author(s): Robert Pozos

Student Price: Contact us to learn more

Biology: An Interactive Tour is for a non-majors audience in technology-enhanced learning and makes the complex world of biological science approachable and relatable.

What is a Top Hat Textbook?

Top Hat has reimagined the textbook – one that is designed to improve student readership through interactivity, is updated by a community of collaborating professors with the newest information, and accessed online from anywhere, at anytime.


  • Top Hat Textbooks are built full of embedded videos, interactive timelines, charts, graphs, and video lessons from the authors themselves
  • High-quality and affordable, at a significant fraction in cost vs traditional publisher textbooks
 

Key features in this textbook

Biology: An Interactive Tour introduces the content in a much more approachable way than traditional texts.

Includes homework sets with 30+ questions per chapter.

Embedded videos that apply biology concepts to the real world!

Comparison of Introduction to Biology Textbooks

Consider adding Biology: An Interactive Tour to your upcoming course. We’ve put together a textbook comparison to make it easy for you in your upcoming evaluation.

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

Pricing

Average price of textbook across most common format

Up to 40-60% more affordable

Lifetime access on any device

$63

E-book

$126

Hardcover print text only

$117

E-book

$166.95

Hardcover print text only

$95.95

E-book

$173.85

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

In-Book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Only available with supplementary resources at additional cost

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Built-In Interactive Assessment Questions

Assessment questions with feedback embedded throughout textbook

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Pricing

Average price of textbook across most common format

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Up to 40-60% more affordable

Lifetime access on any device

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

$63

E-book

$126

Hardcover print text only

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

$117

E-book

$166.95

Hardcover print text only

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

$95.95

E-book

$173.85

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

In-book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Only available with supplementary resources at additional cost

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Pearson

Marieb & Hoehn – Human Anatomy and Physiology, 10th Edition

Wiley

Gerard Tortoria & Bryan Dickerson, Principles of Anatomy & Physiology, 14th Edition

McGraw-Hill

Kenneth Saladin, Anatomy and Physiology: The Unity of Form and Function, 7th Edition

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Pearson

Marieb & Hoehn – Human Anatomy and Physiology, 10th Edition

Wiley

Gerard Tortoria & Bryan Dickerson, Principles of Anatomy & Physiology, 14th Edition

McGraw-Hill

Kenneth Saladin, Anatomy and Physiology: The Unity of Form and Function, 7th Edition

About this textbook

Lead Author

Robert PozosUniversity of Minnesota-Duluth School of Medicine

Robert Pozos has extensive experience studying human response to environments resulting in hypothermia and hyperthermia. He established the hypothermia laboratory at University of Minnesota-Duluth School of Medicine and was a part of the chief civilian scientists at Naval Health Research Center in San Diego where he evaluated the thermal effectiveness of military garb for combat operations.

Contributing Authors

Christina AlevrasUniversity of Saint Joseph

Marion McClaryFairleigh Dickinson University

Explore this textbook

Read the fully unlocked textbook below, and if you’re interested in learning more, get in touch to see how you can use this textbook in your course today.

Chapter 35: Muscles

Figure 35.1: Our muscles allow us to move and carry out actions that require great strength, as demonstrated by this weightlifter. [1]

What makes us move? What moves our food? Muscle! Mankind is ingenious in developing their skeletal muscles for different functions. Occasionally, drugs are used to enhance skeletal muscle which might adversely affect smooth muscle.

Muscles: This diagram demonstrates the four main concepts of the chapter.

35.1 The Body

Concept 35.1: The human body has three types of muscle: smooth, found in areas like our gut; cardiac, making up our heart; and skeletal, the muscle type that allows us to carry out actions that require strength and endurance.​


Figure 35.2: Three types of muscle: skeletal, cardiac and smooth all generate force.

There are three major groups of muscle: skeletal, smooth and cardiac. All three have neural reflexes that allow them to have an involuntary component.

35.1.1 Skeletal Muscle

There are approximately 600 skeletal muscles that move the joints. They are responsible for your posture, running, writing, dancing, etc. Skeletal is the only muscle type that can be controlled voluntarily.

35.1.2 Smooth Muscle

Smooth muscle comprises all of the intestinal tract as well as all of the blood vessels except for the capillaries, which have no smooth muscle. It is responsible for the movement of the food through the intestinal tract as well as directing blood from one part of your body to another. It has an intrinsic neural control in the intestinal tract and is controlled by the autonomic nervous system.

35.1.3 Cardiac Muscle

Cardiac muscle is the most important of the muscle groups since it propels the blood throughout the body. Cardiac cells generate force for a much longer period of time than do skeletal muscles. The heart must work continuously and not get fatigued for years. It has intrinsic innervation so it can contract spontaneously but is also controlled by the autonomic nervous system. 

Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


All three systems are presented separately, but in the real world, they all work together. As you run, consider the following:

  • Skeletal muscle contracts
  • Blood vessels (smooth muscle) send more blood to your skeletal muscle
  • Cardiac muscle contracts faster and with more force 
  • Smooth muscle in your gastrointestinal tract decreases its activity


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Question 35.04

Which of the following muscles do not generate force?

A

Skeletal

B

Smooth

C

Cardiac

D

All of them generate force


35.2 Skeletal Muscle

Concept 35.2: Skeletal muscle controls the movement of our bones. It has striations generated by the overlap of contractile proteins and is under voluntary, conscious control.​

35.2.1 Skeletal Muscle Function & Location

For you to be reading this page- requires skeletal muscle contraction. Your head must be pointed towards the monitor, and for some of you, your eyeballs will be moving to read the page, your back muscles are contracting to maintain your posture, etc. Movement occurs at the joints of our bones but is caused by the ATP powered contraction of muscles that control the joint.

Figure 35.3: Different configurations of skeletal muscle for different movement

Skeletal muscle is designed to be under voluntary control for the execution of different kinds of movements. It is remarkable in that you can make fast or slow movements and can also train the muscle to perform complex actions. Although it has a voluntary component, skeletal muscle also can generate movement involuntarily. In the biological world, different animals use voluntary muscle in different ways.

35.2.2 Skeletal Muscle Anatomy

Skeletal muscle looks striped or "striated" – the fibers contain alternating light and dark bands (striations) like horizontal stripes

Slow Twitch Muscle (Also Called Type I):

  • has many tiny blood vessels called capillaries (and so looks red)
  • has many mitochondria (sites of energy production)
  • has myoglobin (the oxygen-transporting and storage protein of the muscle)
  • uses oxidative phosphorylation for ATP production. 
  • doesn’t fatigue easily (can sustain aerobic activity)
  • can contract slowly

Fast Twitch Muscle (Type II)

  • has fewer mitochondria and less myoglobin than slow twitch
  • can contract more quickly than Type I
  • can contract with more force than slow twitch muscle
  • can sustain only short, anaerobic bursts of activity 
  • uses glycolysis for ATP generation


Question 35.05

Which qualities best describe a slow twitch/type 1 muscle? Select all that apply.

A

Many mitochondria

B

Few mitochondria

C

Used to sustain aerobic activity

D

Tires easily


Question 35.06

Which qualities best describe fast twitch / type II muscle?

A

Many mitochondria

B

Few mitochondria

C

Tires easily

D

Fastest muscle type in humans

E

Used for aerobic activity


35.2.2.3 Skeletal Muscle is Striated

Muscle is broken down into the following components:  muscle fibers,  single muscle fiber, myofibril, single sarcomere, which is composed of different proteins called myosin and actin.

Figure 35.4: Skeletal muscle divides into the thin and thick protein filaments called actin and myosin respectively.​

35.2.2.4 The Sarcomere

A sarcomere is defined as the segment between two neighboring Z-lines. A Z-line is a thin dark line of protein to which the actin filaments are attached. The I-band is the zone of thin filaments that is not superimposed by thick filaments. An A-band contains the entire length of a single thick filament (myosin filament). The H-band is the zone of the thick filaments (myosin filaments) that is not superimposed by the thin filaments. Finally, inside the H-zone is a thin m-line formed of cross-connecting elements of the cytoskeleton. So where do actin and myosin fit into this picture?


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Figure 35.5: Muscle contraction: Actin filaments (thin filaments) are the major components of the I-band and extend into the A-band. Myosin filaments are bipolar meaning that they have two heads and extend throughout the A-band. They are cross-linked at the center by the M-band. (Note: ATP is needed for these reactions. Not shown)


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


35.2.3 Skeletal Muscle Contraction

Figure 35.6: The various metabolic pathways for the muscle to generate ATP for contraction​​

 Muscles generate ATP, the energy needed for contraction, in  three different ways: 

  • The first source of energy is creatine phosphate which donates a phosphate to reconstitute ADP to become ATP.
  • The second source is glycogen from your liver that is released and converted into glucose where it is used in glycolysis to either produce energy directly or to be shunted to the oxidative phosphorylation pathway.
  • The third source of energy is oxidative phosphorylation in which oxygen is needed to produce the most ATP. This is a critical step. Notice how proteins and fatty acids can be converted into metabolites that feed into the oxidative phosphorylation pathway. 


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.

A couple of major points: for sprinters or those who generate a great amount of muscle force quickly- glycolysis is the best way for them to generate energy. For marathon runners, oxidative phosphorylation is the preferred method.

35.2.3.1 Signals Needed for Contraction

The Neuromuscular Junction: For skeletal muscles to contract, they require a signal from neurons in the spinal cord. The motor neuron activates a certain group of muscle fibers. This is called a motor unit. The neuron connects to the skeletal muscle by way of unique junctions called neuromuscular junctions. These neuromuscular junctions are the bridge between the neural control system and the skeletal muscle. If the motor neuron does not generate an electrical signal to cause the muscle to contract, the fibers will wither. In patients with no signals coming from the motor neuron, the muscles will be very skinny.

Figure 35.7: Motor units are the functional components of the muscle. Motor neurons from the ventral part of the spinal cord control select muscle fibers allowing for a wide range of movements.

Skeletal muscle contraction is controlled by neural signals. An electrical signal travels from the spinal cord to the skeletal muscle and to its smaller component parts to make them contract. The name of the neuron in the spinal cord that sends the action potential to the muscle is called the alpha motor neuron. This neuron determines whether or not the motor unit that it innervates will contract. The neuron communicates with the motor unit by way of the neuromuscular junction.

The Action Potential: The action potential from the alpha motor neuron travels to the neuromuscular junction. When the action potential reaches the junction, it causes the calcium ion to flow inward through specific openings called calcium channels. These channels are activated by the action potential which is nothing more than a traveling voltage signal. Hence these channels are called voltage-gated calcium channels which means that a voltage must open the channels for calcium to enter.

The Calcium Influx: The calcium influx will cause vesicles located at the end of the alpha motor neuron to release acetylcholine. In other words, the vesicles are like water balloons containing acetylcholine. The calcium will cause the vesicle to bind to the alpha motor neuron causing a release of acetylcholine. The water balloons burst and release their contents. This chemical is now between the alpha motor neuron and the neuromuscular junction in a region called the cleft. Imagine that you interlaced your fingers. The right-hand fingers are the extensions of the alpha motor neuron and the fingers from the left hand are the neuromuscular junction. The space between your fingers is the space into which acetylcholine is dumped from one neuron to another. Once acetylcholine is released, it will diffuse across the space to combine with a protein called the acetylcholine receptor that is located on the surface of the neuromuscular membrane.

The Endplate Potentials: The movement of ions at the endplate area will cause the generation of small voltages that are called end plate potentials. The potentials vary in amplitude. If they are small and localized to one region, they are called miniature endplate potentials. If they begin to depolarize other areas and spread, they are called end plate potentials. The end plate potentials will cause an action potential at the postsynaptic site triggering a muscle contraction.

Figure 35.8: Acetylcholine (red balls) activates the acetylcholine receptor to generate an end plate potential that triggers an action potential. In the case of fewer acetylcholine receptors (AChRs), the acetylcholine will not cause the generation of an action potential only an end plate potential.​



Ch35_NmuscularJunctionNew.jpg
Figure 35.9: Acetylcholine receptors when activated will cause a sodium influx which results in an endplate potential.



Figure 35.10: Action potentials at the axon terminal cause the release of acetylcholine which causes an electrical signal at the motor end plate to release calcium which activates the union of actin and myosin.


Question 35.11

The neuron will communicate with its muscle cell at the NMJ using an action potential. Sort the sequence of steps involved in the generation of this action potential.

A

Sodium ions rush through the membrane

B

Axon Terminal

C

Acetylcholine binds to the receptor on the motor end plate

D

Action Potential is triggered

E

Acetylcholine is released into the synapse


Acetylcholine and Depolarization: So far the sequence is that the motor neuron will send an action potential to the neuromuscular junction which will cause the release of acetylcholine which will trigger end plate potentials which if large enough will trigger an action potential.

Figure 35.11: Miniature endplate potential is due to the small release of acetylcholine whereas an endplate potential is due to the release of a large amount of acetylcholine which may trigger an action potential if it reaches threshold voltage.

Once acetylcholine attaches to its receptor, it is important that it is removed quickly or it will continue to cause the activation of the receptor, leading to continuous electrical discharges. An enzyme, called acetylcholine esterase, which is located in the synaptic cleft, will cleave acetylcholine so that it is metabolized into acetate and choline. Choline is returned to the postsynaptic region to be used in the synthesis of new acetylcholine molecules.

Calcium Release in the Muscle Causes the Actin and Myosin to Generate Force: Once the acetylcholine attaches to the acetylcholine receptor, it causes a change in the membrane so that it causes sodium ion to rush in triggering an action potential. Thus, we have had two action potentials: one coming from the alpha motor neuron and the other from the neuromuscular junction. The action potential at the neuromuscular junction now traverses the muscle fiber causing the T-tubules to be depolarized. Depolarization means that the positive and negative signals have reversed at the membrane. The depolarization opens up another set of calcium channels which happen to be in close proximity to the sarcoplasmic reticulum which has their own set of calcium release channels. Once the voltage-dependent calcium channels interact with the calcium release channels on the sarcoplasmic reticulum, it releases calcium.

The calcium from the sarcoplasmic reticulum binds to a protein on the actin called troponin C which is on the thin filaments of the myofibrils. Troponin is like a can opener. It now interacts with tropomyosin which normally does not allow myosin to connect with actin and causes tropomyosin to move, unlocking the blocking sites. Subsequently, myosin binds to actin and then it releases ADP and inorganic phosphate which is associated with the power stroke of myosin. Myosin pulls itself along actin by way of the power strokes. Finally, a new molecule of ATP unbinds the myosin from the actin so that myosin can do another power stroke.

Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Figure 35.12: Action potential produced by the motor end plate has an effect on the release of calcium which triggers the union of actin and myosin.​

A summary of the sliding filament explanation of skeletal muscle contractions is as follows:

During muscle contraction, the thin actin filaments slide over the thick myosin. When calcium is present the blocked active site of the actin clears.

  • Step 1: Myosin head attaches to actin (High energy ADP + P configuration).
  • Step 2: Power stroke: myosin head pivots pulling the actin filament toward the center.
  • Step 3: The cross-bridge detaches when a new ATP binds with the myosin.


Question 35.13

Sort the steps that lead to muscle contraction.

A

ATP enables for cross-bridge formation

B

Tropomyosin is moved out of the way no longer blocking actin and myosin

C

Calcium is released from the Sarcoplasmic Reticulum

D

Myosin pulls actin towards the center of the sarcomere

E

Maximum overlap between actin and myosin produce maximum shortening of sarcomere


The collective bending of numerous myosin heads on either side of the M line (all in the same direction), combine to move the actin filament relative to the myosin filament. This results in muscle contraction and ultimately force. Imagine the myosin fibers moving along the actin protein which pulls the actin together. Thus, the ends of the sarcomere are drawn together resulting in muscle contraction. ATP is key to this entire process. Once myosin attaches to actin, what will cause its release so that myosin can attach again to actin? ATP! ATP allows myosin to detach from actin and then ATP is chemically broken into ADP and phosphate to re-energize the myosin head so that it can re-attach to actin.

Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Figure 35.13: Sequence of muscle contraction​


Question 35.15

Which two things would happen if there was no ATP available following cross-bridge formation?

A

The calcium would allow for actin to be released from myosin

B

The cross-bridge would not be relieved

C

The muscle would remain in its contracted state

D

The muscle would relax

E

The cross-bridge would release


35.2.4 Isometric vs. Isotonic Muscle Contractions

35.2.4.1 Isometric Contraction

Isometric exercise or contraction involves a static contraction of a muscle without any visible movement in the angle of the joint. A good example of this kind of exercise would be arm wrestling in which the two opponents are contracting their arm muscles but neither one is generating any movement. Image needed to show force production. 

35.2.4.2 Isotonic Contraction

Muscle generates force and to do that it contracts. Hence, the ultimate result of the muscle function is to generate force. In an isotonic contraction, tension remains unchanged and the muscle's length changes. Lifting an object at a constant speed is an example of isotonic contractions. Weightlifting is a great example of isotonic contraction. There are two types of isotonic contractions (1) concentric and (2) eccentric.

35.2.4.3 Concentric Contraction

In a concentric contraction, the muscle tension increases to meet the resistance or load, then remains the same as the muscle shortens; whereas in eccentric, the muscle lengthens due to the resistance being greater than the force the muscle is producing. A biceps curl is an example of a concentric isotonic contraction.

35.2.4.4 Eccentric Contraction

Running downhill is an example of eccentric contraction of the leg muscles since the muscles are being stretched while they are being contracted. You can think of eccentric contraction as a way to slow down the motion. Muscle injury, shakes, also called tremors, and soreness are selectively associated with eccentric contraction. Muscle strengthening may be greatest doing exercises that involve eccentric contractions.

Figure 35.14: Types of muscle contractions. During isotonic contractions, muscle length changes to move a load. During isometric contractions, muscle length does not change because the load exceeds the tension the muscle can generate.​


Figure 35.15: Phases of contraction of skeletal muscle. A single muscle twitch has a latent period when there is no increase in tension followed by a contraction period when tension increases and a relaxation period when tension decreases.

35.2.5 Muscle Tone

Skeletal muscles are rarely completely relaxed, or flaccid. Even if a muscle is not producing movement, it is slightly contracted due to the small amount of acetylcholine being released at the neuromuscular junction. In addition, the stretch reflex is also involved in maintaining posture since as the muscle stretches, it will contract. The events at the neuromuscular junction and the muscle spindle reflex produce a certain degree of muscle contraction called muscle tone. The tension produced by muscle tone allows muscles to continually stabilize joints and maintain posture.

Muscle tone is accomplished by a complex interaction between the nervous system and skeletal muscles that results in the activation of a few motor units at a time, most likely in a cyclical manner. In this manner, muscles never fatigue completely, as some motor units can recover while others are active.

Question 35.16

You are sitting in class and sense yourself getting sleepy. Which of the following is the reason for why you don't just collapse out of your chair as you get sleepier and sleepier?

A

Muscle tone

B

Muscle fatigue

C

Depolarization

D

Acetylcholine


35.2.6 Muscle Fatigue

When the muscle is no longer able to generate the amount of force required- it is fatigued. This fatigue state is due to either a lack of glycogen or glucose for sprinters or lack of oxygen for marathon runners. Notice in the diagram that lactic acid is produced by way of glycolysis and is considered by some to be the chemical cause of fatigue. Lactic acid is likely not the only culprit causing fatigue. 

Fortunes have been made by companies that have sold creatine to enhance strength, oxygen delivery systems to be given to athletes to help minimize fatigue, and hot or cold baths to minimize fatigue. None of these products have scientific data to support their claims. My favorite is oxygenated drinks. As you drink these solutions, the amount of oxygen in your system is increased - according to the claim. Nothing could be further from the truth. Humans do not get their oxygen from their stomach or gut. It is a great marketing gimmick to sell oxygenated drinks but it is scientifically not possible to get oxygen from a drink into your lungs or bloodstream. 


Question 35.17

Which of the following generates force in an aerobic environment?

A

Slow twitch

B

Fast twitch

C

Both fast and slow twitch

D

Neither one since all muscle requires oxygen


35.3 Smooth Muscle

Concept 35.3: Smooth muscle is found in areas like the gut and the walls of the human arteries. Unlike skeletal muscle, it is not striated and is under involuntary control.​

35.3.1 Smooth Muscle Function and Location

Smooth muscle in contrast to striated muscle is not striated, as one would guess from the name, meaning that you do not see nice lines in the cellular structure and it is not under voluntary control. A unique feature about smooth muscle is that it is able to contract without any external electrical signal. It is by nature “twitchy”. However, for the smooth muscle to work in a coordinated fashion, it needs neural innervation. Thus, the spontaneous nature of the smooth muscle to contract is controlled by neural innervation. Interestingly, this neural innervation is not voluntary. The nervous system that controls smooth muscle is called the Autonomic Nervous System. Although much attention is paid to the skeletal muscle, smooth muscle is in many ways just as important as striated muscle. Smooth muscle is found:

  • Within the walls of blood vessels of large and small arteries and veins and is called vascular smooth muscle. Smooth muscle controls the diameter of these vessels.
  • Lymphatic vessels which contract
  • In the urinary bladder where it plays a major role in contracting the bladder
  • In the uterus in which it contracts to expel the fetus
  • In male and female reproductive tracts in which it causes erection and ejaculation
  • In the gastrointestinal system, which is the main explanation for peristalsis
  • In the respiratory tract, in which it causes the bronchi to constrict
  • Arrector pili of skin, in which it causes “goose bumps “and hair to stand up
  • In the pupil of the eye, in which it causes the pupil to open or close


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Question 35.19

You are experiencing a significant amount of stress that has caused your blood pressure to rise. This is directly correlated to the constriction of your blood vessels. Which muscle type is the culprit here?

A

Skeletal muscle

B

Smooth muscle

C

Cardiac muscle


Smooth muscle is involved in digestion, reproduction, directing blood flow throughout your body so when you exercise, the blood flows to your muscles and not to your intestinal tract, etc. It is very important and usually never acknowledged much. Smooth muscle is more difficult to study than skeletal muscle since the latter has a joint that moves and it is, therefore, easier to study function. Smooth muscle does not move any joints and can generate force at different parts of the structure.

35.3.1.1 Smooth Muscle Anatomy

Most smooth muscle is of the single-unit variety so that the whole sheet of smooth muscle contracts or relaxes, but there is the multiunit smooth muscle in the trachea, the large elastic arteries, and the iris of the eye. Single unit smooth muscle, however, is most common and lines blood vessels, the urinary tract, and the digestive tract. Within single-unit smooth muscle tissues, the nervous system innervates a single cell within a sheet or bundle and the action potential is propagated to neighboring cells such that the whole bundle contracts as a single unit which is called a syncytium. In contrast, in multiunit smooth muscle tissues, the nerves that are responsible for enervating the multiunit smooth muscle affect individual cells in the layer.

Figure 35.16: Two major types of smooth muscle, multi-unit and single unit

35.3.2 Smooth Muscle Contraction

Smooth muscle contraction occurs in a similar process as skeletal muscle contraction.

Smooth muscle fibers are spindle-shaped (wide in the middle and tapered at both ends, somewhat like a football) and have a single nucleus; they range from about 30 to 200 μm (thousands of times shorter than skeletal muscle fibers. They do not have striations and sarcomeres but have actin and myosin, and thick and thin filaments. These thin filaments are anchored by dense bodies. A dense body is analogous to the Z-discs of skeletal and cardiac muscle fibers and is fastened to the sarcolemma. This arrangement causes the entire muscle fiber to contract in a manner whereby the ends are pulled toward the center, causing the midsection to bulge in a corkscrew motion.

Figure 35.17: Smooth muscle contraction demonstrating the change in shape due to the arrangement of the dense bodies.


Figure 35.18: Smooth muscle plays many roles in various tissues. A) It controls sphincters which are normally contracted but can relax; B) it maintains a baseline level of contraction in blood vessels, which can be increased; C) it exhibits a range of relaxation in the stomach and intestinal tissue; D) it is normally relaxed in esophageal and urinary bladder tissue but can contract.​


35.3.2.1 Innervation

The smooth muscle in arteries is innervated primarily by sympathetic fibers, whereas the smooth muscle in other tissues can have both sympathetic and parasympathetic innervations. In the gastrointestinal tract, smooth muscle is innervated by the enteric nervous system. The smooth muscle cells of some tissues, such as the uterus, have no innervation. Circulating hormones such as epinephrine will cause the smooth muscle in blood vessels to contract, decreasing the diameter of the blood vessel (vasoconstriction).

Question 35.20

You are looking at your rival in a card game. His pupils are dilated. This dilation is due to which of the following?

A

Smooth muscle activation

B

Skeletal muscle activation

C

Cardiac muscle activation–indirectly

D

No muscle is involved in pupil dilation


35.4 Cardiac Muscle

Concept 35.4: Cardiac muscle is what makes up the wall of the heart. It contracts to push the blood from the heart to the rest of the body. It is under involuntary control.​

Cardiac muscle is the last of the major muscle groups. We have discussed striated and smooth and have saved the most important for last. The heart must continuously beat to propel blood into the aorta to supply the entire body with oxygenated blood. The cell involved in this continuous contraction is the cardiac muscle cell. It is an involuntary striated muscle cell found in the walls of the heart, specifically the myocardium. Cardiac muscle is spontaneously active similar to smooth muscle, but it is also under the control of nervous systems. The Autonomic Nervous System controls the heart rate and the strength of contraction of the cardiac muscle. The entire muscle group acts together. When the muscle contracts it results in a wrapping motion so as to propel the blood from one chamber to the other.

Figure 35.19: Cardiac muscle corkscrew arrangement allows for pressure to be generated in the chambers.

35.4.1 Cardiac Muscle Anatomy

Compared to the giant cylinders of skeletal muscle, cardiac muscle cells are much shorter with much smaller diameters. Cardiac muscle has striations due to the precise arrangement of the myofilaments and fibrils that are organized in sarcomeres along the length of the cell. These contractile elements are identical to skeletal muscle.

35.4.1.1 Cardiomyocytes

Typically, cardiomyocytes have a single, central nucleus. Cardiac muscle cells branch freely. A junction between two adjoining cells is marked by a critical structure called an intercalated disc, which helps support the synchronized contraction of the muscle. The cell membranes (sarcolemma) from adjacent cells bind together at the intercalated discs in an area called a gap junction which allow the passage of ions between the cells and helps to synchronize the contraction. Desmosomes are also a component of the intercalated disc that holds the cardiac cells together during contraction. 


Question 35.21

Which of the following allow for synchronized contraction of the heart?

A

Branching of the heart cells

B

Intercalated discs

C

Sarcolemmas bind to one another at the intercalated discs

D

All of the above allow for synchronized contraction of the heart


Figure 35.20: Cardiac muscle showing the intercalated disc and gap junction​

35.4.1.2 T (Transverse) Tubules

T (transverse) tubules penetrate from the surface plasma membrane, the sarcolemma, to the interior of the cell, allowing the electrical impulse to reach the interior. The T tubules are only found at the Z discs, whereas in skeletal muscle, they are found at the junction of the A and I bands. Therefore, there is one-half as many T tubules in cardiac muscle as in skeletal muscle.

35.4.1.3 Sarcoplasmic Reticulum

In cardiac cells,  the sarcoplasmic reticulum stores few calcium ions, so most of the calcium ions must come from outside the cells. The result is a slower onset of contraction.

35.4.1.4 Mitochondria

In cardiac cells, mitochondria are plentiful, providing energy for the contractions of the heart. However, they require an extensive network of blood vessels to ensure an adequate supply of oxygen. "Heart attacks" or a decrease in cardiac muscle contraction can be caused by a decrease in blood supply to the cardiac muscle. 

35.4.2 Cardiac Muscle Contraction

Cardiac muscle cells contract as a group (syncytium) with long refractory periods followed by brief relaxation periods. The refractory period is when an additional muscle contraction cannot occur, and it can be absolute or relative. Obviously, for survival purposes, the heart muscles must contract together and not have another beat superimposed, since that would minimize the ability to pump blood. The action potential precedes the contraction.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Figure 35.21: Electrical signal from the cardiac muscle precedes the tension (force) developed by the muscle.

35.4.2.1 Energy Needed for Contraction

Cardiac muscle metabolism is entirely aerobic. Oxygenated hemoglobin from the lungs is brought to the heart via the coronary arteries. Heart cells also store appreciable amounts of oxygen in myoglobin. Circulating oxygenated hemoglobin and oxygen attached to myoglobin supply sufficient oxygen to the heart, even during peak performance.

35.4.2.2 Innervation

Unlike skeletal and smooth muscle, cardiac muscle can contract independently due to its inherent nervous system. However, its rate is controlled by the sympathetic system, which increases the rate and the strength of contraction, and the parasympathetic system via the vagus nerve causes a decrease in heart rate. Normally the heart rate is under the control of the vagus. Circulating epinephrine secreted from the adrenal medulla will cause an increase in heart rate and strength of contraction.

Question 35.23

A friend has a severe allergy to nuts. She accidentally ingests some in a piece of cake and starts to go into circulatory shock. How would an epinephrine pen help her?

A

It would cause an increase in heart rate and blood pressure

B

It would cause a decrease in heart rate and blood pressure


Question 35.24

In a boxing match, you are fighting to beat your rival. You seem to be winning and your heart rate is increasing due to which of the following?

A

Sympathetic activation

B

Parasympathetic activation

C

Both sympathetic and parasympathetic activation

D

Somatic activation


Figure 35.22: Chapter overview of muscles.​


35.5 Vocabulary Questions


Vocabulary Question 35.01

Neurons that receive light waves in the eye. These perceive the intensity or the amplitude of the light wave.


Vocabulary Question 35.02

Neurons that receive light waves in the eye. These perceive the frequency of the light.


Vocabulary Question 35.03

Responsible for processing information about static and moving objects and pattern recognition.


Vocabulary Question 35.04

The part of your brain that controls movement.


Vocabulary Question 35.05

Responsible for planning and selecting the appropriate movements.


Vocabulary Question 35.06

Responsible for the control and execution of voluntary movements.


Vocabulary Question 35.07

The system that connects your muscles that are under voluntary control to your central nervous system.


Vocabulary Question 35.08

Bundles of contractile units that contain the machinery to contract the muscle fibers.


Vocabulary Question 35.09

One contractile unit of the myofibril that consists of thick and thin filaments.


Vocabulary Question 35.10

A protein that composes the thin filaments of the sarcomere.


Vocabulary Question 35.11

A protein that composes the thick filaments of the sarcomere.


Vocabulary Question 35.12

The protein responsible for the movement of skeletal muscles.


Vocabulary Question 35.13

These signal equally when you watch someone do a particular task as they do when you actually perform the task yourself.


Vocabulary Question 35.14

A structure in your temporal lobe that helps translate signals from sensory inputs to responses in your autonomic nervous


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.


Locked Content
This Content is Locked
Only a limited preview of this text is available. You'll need to sign up to Top Hat, and be a verified professor to have full access to view and teach with the content.

35.6 Image Credits

[1] Image courtesy of Kyle Johnson in the Public Domain.