Anatomy of the human body notes and summary

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Anatomy of the human body notes and summary

Anatomy of the Human Body Notes A

Organization of the Human Body

Many people have compared the human body to a machine. Think about some common machines, such as drills and washing machines. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform an overall function. The human body is like a machine in all these ways. In fact, it may be the most fantastic machine on Earth.

Levels of Organization

The human machine is organized at different levels, starting with the cell and ending with the entire organism. At each higher level of organization, there is a greater degree of complexity. The levels are:

Atom

Compound or Molecule

Organelle

Cell

Tissue

Organ

Organ System

Organism

Cells

The most basic parts of the human machine are cells—an amazing 100 trillion of them by the time the average person reaches adulthood! Cells are the basic units of structure and function in the human body, as they are in all living things. Each cell carries out basic life processes that allow the body to survive. Many human cells are specialized in form and function. Each type of cell in the figure plays a specific role. For example, nerve cells have long projections that help them carry electrical messages to other cells. Muscle cells have many mitochondria that provide the energy they need to move the body.

Tissues

After the cell, the tissue is the next level of organization in the human body. A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues: epithelial, muscle, nervous, and connective tissues. These four tissue types make up all the organs of the human body.

The human body consists of these four tissue types.

Connective tissue is made up of cells that form the body’s structure. Examples include bone and cartilage.
Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the lining of the digestive tract. Epithelial tissue protects the body and its internal organs, secretes substances such as hormones, and absorbs substances such as nutrients.
Muscle tissue is made up of cells that have the unique ability to contract, or become shorter. Muscles attached to bones enable the body to move.
Nervous tissue is made up of neurons, or nerve cells, that carry electrical messages. Nervous tissue makes up the brain and the nerves that connect the brain to all parts of the body.

Organs and Organ Systems

After tissues, organs are the next level of organization of the human body. An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples of human organs include the brain, heart, lungs, skin, and kidneys. Human organs are organized into organ systems. An organ system is a group of organs that work together to carry out a complex overall function. Each organ of the system does part of the larger job.

Vocabulary

cell: Basic unit of structure and function of living things.
connective tissue: Tissue made up of cells that form the body’s structure, such as bone and cartilage.
epithelial tissue: Tissue made up of cells that line inner and outer body surfaces, such as skin.
muscle tissue: Tissue made up of cells that can contract; includes smooth, skeletal, and cardiac muscle tissue.
nervous tissue: Tissue made up of neurons, or nerve cells; carry electrical messages.
neuron: The structural and functional unit of the nervous system; a nerve cell.
organ: Structure composed of more than one type of tissue that performs a particular function.
organ system: Group of organs that work together performing a specific function.
tissue: Group of cells of the same kind that perform a particular function in an organism.

Summary

The human body is organized at different levels, starting with the cell.
Cells are organized into tissues, and tissues form organs.
Organs are organized into organ systems such as the skeletal and muscular systems.

Homestasis

What happens if stability is disrupted?

Remove one stone and the whole arch collapses. The same is true for the human body. All the systems work together to maintain stability or homeostasis. Disrupt one system, and the whole body may be affected.

Homeostasis

All of the organs and organ systems of the human body work together like a well-oiled machine. This is because they are closely regulated by the nervous and endocrine systems. The nervous system controls virtually all body activities, and the endocrine system secretes hormones that regulate these activities. Functioning together, the organ systems supply body cells with all the substances they need and eliminate their wastes. They also keep temperature, pH, and other conditions at just the right levels to support life processes.

Maintaining Homeostasis

The process in which organ systems work to maintain a stable internal environment is called homeostasis. Keeping a stable internal environment requires constant adjustments. Here are just three of the many ways that human organ systems help the body maintain homeostasis:

Respiratory system: A high concentration of carbon dioxide in the blood triggers faster breathing. The lungs exhale more frequently, which removes carbon dioxide from the body more quickly.
Excretory system: A low level of water in the blood triggers retention of water by the kidneys. The kidneys produce more concentrated urine, so less water is lost from the body.
Endocrine system: A high concentration of sugar in the blood triggers secretion of insulin by an endocrine gland called the pancreas. Insulin is a hormone that helps cells absorb sugar from the blood.

Failure of Homeostasis

Many homeostatic mechanisms such as these work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do, cells may not get everything they need, or toxic wastes may accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease or even death.

Vocabulary

endocrine system: Organ system of glands that release hormones into the blood.
homeostasis: The process of maintaining a stable environment inside a cell or an entire organism.
hormone: a chemical messenger molecule
nervous system: Organ system that carries electrical messages throughout the body.

Summary

All of the organ systems of the body work together to maintain homeostasis of the organism.
If homeostasis fails, death or disease may result.

Human Skeletal System

The human skeleton is an internal framework that, in adults, consists of 206 bones. You can learn more about bones in the animation “Bones Narrated”: http://medtropolis.com/virtual-body/

In addition to bones, the skeleton also consists of cartilage and ligaments:

Cartilage is a type of dense connective tissue, made of tough protein fibers, that provides a smooth surface for the movement of bones at joints.
A ligament is a band of fibrous connective tissue that holds bones together and keeps them in place.

The skeleton supports the body and gives it shape. It has several other functions as well, including:

protecting internal organs
providing attachment surfaces for muscles
producing blood cells
storing minerals
maintaining mineral homeostasis.

Maintaining mineral homeostasis is a very important function of the skeleton, because just the right levels of calcium and other minerals are needed in the blood for normal functioning of the body. When mineral levels in the blood are too high, bones absorb some of the minerals and store them as mineral salts, which is why bones are so hard. When blood levels of minerals are too low, bones release some of the minerals back into the blood, thus restoring homeostasis.

Vocabulary

bone: Hard tissue in most vertebrates that consists of a collagen matrix, or framework, filled in with minerals such a calcium.
cartilage: Dense connective tissue that provides a smooth surface for the movement of bones at joints; involved in endochondral ossification.
ligament: Band of fibrous connective tissue that holds bones together.
mineral homeostasis: Having the proper levels of minerals in the blood for normal functioning of the body.

Summary

The adult human skeleton includes 206 bones and other tissues.
The skeleton supports the body, protects internal organs, produces blood cells, and maintains mineral homeostasis.

Structure of Bones

It's common to think of bones as not living. But bones are very much living. In fact, you are constantly making new bone tissue. That means that you are also constantly getting rid of bone. Bone is full of blood and nerves and all sorts of cells and proteins, making it an extremely complex living tissue.

Many people think of bones as being dead, dry, and brittle. These adjectives correctly describe the bones of a preserved skeleton, but the bones in a living human being are very much alive. The basic structure of bones is bone matrix which makes up the underlying rigid framework of bones and is composed of both compact bone and spongy bone. The bone matrix consists of tough protein fibers, mainly collagen, that become hard and rigid due to mineralization with calcium crystals. Bone matrix is crisscrossed by blood vessels and nerves and also contains specialized bone cells that are actively involved in metabolic processes.

Bone Cells

There are three types of specialized cells in human bones: osteoblasts, osteocytes, and osteoclasts. These cells are responsible for bone growth and mineral homeostasis.

Osteoblasts make new bone cells and secrete collagen that mineralizes to become bone matrix. They are responsible for bone growth and the uptake of minerals from the blood.
Osteocytes regulate mineral homeostasis. They direct the uptake of minerals from the blood and the release of minerals back into the blood as needed.
Osteoclasts dissolve minerals in bone matrix and release them back into the blood.

Bones are far from static, or unchanging. Instead, they are dynamic, living tissues that are constantly being reshaped. Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break it down.

Bone Tissues

Bones consist of different types of tissue, including compact bone, spongy bone, bone marrow, and periosteum.

Compact bone makes up the dense outer layer of bone. Its functional unit is the osteon. Compact bone is very hard and strong.
Spongy bone is found inside bones and is lighter and less dense than compact bone. This is because spongy bone is porous.
Bone marrow is a soft connective tissue that produces blood cells. It is found inside the pores of spongy bone.
Periosteum is a tough, fibrous membrane that covers and protects the outer surfaces of bone.

Vocabulary

bone marrow: Soft connective tissue in spongy bone; produces blood cells.
bone matrix: Rigid framework of bone that consists of tough protein fibers and mineral crystals.
collagen: Protein fibers in the extracellular matrix of bone and cartilage.
compact bone: Dense outer layer of bone that is very hard and strong.
osteoblast: Type of bone cell; makes new bone cells and secretes collagen.
osteoclast: Type of bone cell; dissolves minerals in bone and releases them back into the blood.
osteocyte: Type of bone cell; regulates mineral homeostasis by directing the uptake of minerals from the blood and the release of minerals back into the blood as needed.
osteon: The functional unit of compact bone.
periosteum: Tough, fibrous membrane that covers the outer surface of bone.
spongy bone: Light, porous inner layer of bone that contains bone marrow.

Summary

Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break down bone. These processes help maintain mineral homeostasis.
Bone tissues include compact bone, spongy bone, bone marrow, and periosteum.

Smooth, Skeletal, and Cardiac Muscles

The muscular system consists of all the muscles of the body. Muscles are organs composed mainly of muscle cells, which are also called muscle fibers. Each muscle fiber is a very long, thin cell that can do something no other cell can do. It can contract, or shorten. Muscle contractions are responsible for virtually all the movements of the body, both inside and out. There are three types of muscle tissues in the human body: cardiac, smooth, and skeletal muscle tissues.

Smooth Muscle

Muscle tissue in the walls of internal organs such as the stomach and intestines is smooth muscle. When smooth muscle contracts, it helps the organs carry out their functions. For example, when smooth muscle in the stomach contracts, it squeezes the food inside the stomach, which helps break the food into smaller pieces. Contractions of smooth muscle are involuntary. This means they are not under conscious control.

Skeletal Muscle

Muscle tissue that is attached to bone is skeletal muscle. Whether you are blinking your eyes or running a marathon, you are using skeletal muscle. Contractions of skeletal muscle are voluntary, or under conscious control. Skeletal muscle is the most common type of muscle in the human body.

Cardiac Muscle

Cardiac muscle is found only in the walls of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Cardiac muscle contains a great many mitochondria, which produce ATP for energy. This helps the heart resist fatigue. Contractions of cardiac muscle are involuntary, like those of smooth muscle. Cardiac muscle, like skeletal muscle, is arranged in bundles, so it appears striated, or striped.

Vocabulary

cardiac muscle: Involuntary, striated muscle found only in the walls of the heart.
muscle fiber: Long, thin muscle cell that has the ability to contract, or shorten.
muscular system: Human body system that includes all the muscles of the body.
skeletal muscle: Voluntary, striated muscle that is attached to bones of the skeleton and helps the body move.
smooth muscle: Involuntary, nonstriated muscle that is found in the walls of internal organs such as the stomach.
striated: Striped appearance of cardiac and skeletal muscle.

Summary

There are three types of human muscle tissue: smooth muscle (in internal organs), skeletal muscle, and cardiac muscle (only in the heart).

Skeletal Muscles

Skeletal Muscles

There are well over 600 skeletal muscles in the human body. Skeletal muscles vary considerably in size, from tiny muscles inside the middle ear to very large muscles in the upper leg.

Structure of Skeletal Muscles

Each skeletal muscle consists of hundreds or even thousands of skeletal muscle fibers. The fibers are bundled together and wrapped in connective tissue. The connective tissue supports and protects the delicate muscle cells and allows them to withstand the forces of contraction. It also provides pathways for nerves and blood vessels to reach the muscles. Skeletal muscles work hard to move body parts. They need a rich blood supply to provide them with nutrients and oxygen and to carry away their wastes.

Skeletal Muscles and Bones

Skeletal muscles are attached to the skeleton by tough connective tissues called tendons. Many skeletal muscles are attached to the ends of bones that meet at a joint. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move.

Muscles can only contract. They cannot actively extend, or lengthen. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. For example, the biceps and triceps muscles of the upper arm work in opposition to bend and extend the arm at the elbow.

Use It or Lose It

In exercises such as weight lifting, skeletal muscle contracts against a resisting force. Using skeletal muscle in this way increases its size and strength. In exercises such as running, the cardiac muscle contracts faster and the heart pumps more blood. Using cardiac muscle in this way increases its strength and efficiency. Continued exercise is necessary to maintain bigger, stronger muscles. If you don’t use a muscle, it will get smaller and weaker—so use it or lose it.

Vocabulary

joint: Place where two or more bones of the skeleton meet.
muscle fiber: Long, thin muscle cell that has the ability to contract, or shorten.
tendon: Tough connective tissue that attaches skeletal muscle to bones of the skeleton.

Summary

Skeletal muscles are attached to the skeleton and cause bones to move when they contract.

Muscle Contraction

What makes a muscle contract?

It starts with a signal from the nervous system. So it starts with a signal from your brain. The signal goes through your nervous system to your muscle. Your muscle contracts, and your bones move. And all this happens incredibly fast.

Muscle contraction occurs when muscle fibers get shorter. Literally, the muscle fibers get smaller in size. To understand how this happens, you need to know more about the structure of muscle fibers.

Structure of Muscle Fibers

Each muscle fiber contains hundreds of organelles called myofibrils. Each myofibril is made up of two types of protein filaments: actin filaments, which are thinner, and myosin filaments, which are thicker. Actin filaments are anchored to structures called Z lines. The region between two Z lines is called a sarcomere. Within a sarcomere, myosin filaments overlap the actin filaments. The myosin filaments have tiny structures called cross bridges that can attach to actin filaments.

Sliding Filament Theory

The most widely accepted theory explaining how muscle fibers contract is called the sliding filament theory. According to this theory, myosin filaments use energy from ATP to “walk” along the actin filaments with their cross bridges. This pulls the actin filaments closer together. The movement of the actin filaments also pulls the Z lines closer together, thus shortening the sarcomere.

When all of the sarcomeres in a muscle fiber shorten, the fiber contracts. A muscle fiber either contracts fully or it doesn’t contract at all. The number of fibers that contract determines the strength of the muscular force. When more fibers contract at the same time, the force is greater.

Muscle contraction is described by the sliding-filaments model:

ATP binds to a myosin head and is converted to ADP and Pi, which remain attached to the myosin head.
Ca2+ exposes the binding sites on the actin filaments. Ca2+ binds to the troponin molecule causing tropomyosin to expose positions on the actin filament for the attachment of myosin heads.
Cross bridges between myosin heads and actin filaments form. When attachment sites on the actin are exposed, the myosin heads bind to actin to form cross bridges.
ADP and Pi are released, and sliding motion of actin results. The attachment of cross bridges between myosin and actin causes the release of ADP and Pi. This, in turn, causes a change in shape of the myosin head, which generates a sliding movement of the actin toward the center of the sacromere. This pulls the two Z discs together, effectively contracting the muscle fiber to produce a power stroke.
ATP causes the cross bridges to unbind. When a new ATP molecule attaches to the myosin head, the cross bridge between the actin and myosin breaks, returning the myosin head to its unattached position.

Without the addition of a new ATP molecule, the cross bridges remain attached to the actin filaments. This is why corpses become stiff with rigor mortis (new ATP molecules are unavailable).

Muscles and Nerves

Muscles cannot contract on their own. They need a stimulus from a nerve cell to “tell” them to contract. Let’s say you decide to raise your hand in class. Your brain sends electrical messages to nerve cells, called motor neurons, in your arm and shoulder. The motor neurons, in turn, stimulate muscle fibers in your arm and shoulder to contract, causing your arm to rise. Involuntary contractions of cardiac and smooth muscles are also controlled by nerves.

Vocabulary

actin: The monomeric subunit of microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the myofibril in muscle cells.
cross bridge: Structure of myosin filament that attaches to actin filament.
motor neuron: Type of neuron that carries nerve impulses from the central nervous system to muscles and glands.
myofibril: Organelle of muscle fiber; composed of actin and myosin filaments.
myosin: Filamentous protein involved in muscle contraction; forms thick filaments of myofibril in muscle cells.
sacromere: Region of myofibril between two Z-lines.
sliding filament theory: Theory that explains muscle contraction by the sliding of myosin filaments over actin filaments within muscle fibers.
Z line: Region of sarcomere where actin filaments are attached.

Summary

According to the sliding filament theory, a muscle fiber contracts when myosin filaments pull actin filaments closer together and thus shorten sarcomeres within a fiber.
When all the sarcomeres in a muscle fiber shorten, the fiber contracts.

Skin

What is integumentary? Because the organs of the integumentary system are external to the body, you may think of them as little more than “accessories,” like clothing or jewelry. But the organs of the integumentary system serve important biological functions. They provide a protective covering for the body and help the body maintain homeostasis.

The Skin

The skin is the major organ of the integumentary system, which also includes the nails and hair. In fact, the skin is the body’s largest organ, and a remarkable one at that. Consider these skin facts. The average square inch (6.5 cm2) of skin has 20 blood vessels, 650 sweat glands, and more than a thousand nerve endings. It also has an incredible 60,000 pigment-producing cells. All of these structures are packed into a stack of cells that is just 2 mm thick, or about as thick as the cover of a book.

Although the skin is thin, it consists of two distinct layers, called the epidermis and the dermis.

Epidermis

The epidermis is the outer layer of skin, consisting of epithelial cells and little else. For example, there are no nerve endings or blood vessels in the epidermis. The innermost cells of the epidermis are continuously dividing through mitosis to form new cells. The newly formed cells move up through the epidermis toward the skin surface, while producing a tough, fibrous protein called keratin. The cells become filled with keratin and die by the time they reach the surface, where they form a protective, waterproof layer called the stratum corneum. The dead cells are gradually shed from the surface of the skin and replaced by other cells.

The epidermis also contains melanocytes, which are cells that produce melanin. Melanin is the brownish pigment that gives skin much of its color. Everyone has about the same number of melanocytes, but the melanocytes of people with darker skin produce more melanin. The amount of melanin produced is determined by heredity and exposure to UV light, which increases melanin output. Exposure to UV light also stimulates the skin to produce vitamin D. Because melanin blocks UV light from penetrating the skin, people with darker skin may be at greater risk of vitamin D deficiency.

Dermis

The dermis is the lower layer of the skin, located directly beneath the epidermis. It is made of tough connective tissue and attached to the epidermis by collagen fibers. The dermis contains blood vessels and nerve endings. Because of the nerve endings, skin can feel touch, pressure, heat, cold, and pain. The dermis also contains hair follicles and two types of glands.

Hair follicles are the structures where hairs originate. Hairs grow out of follicles, pass through the epidermis, and exit at the surface of the skin.
Sebaceous glands produce an oily substance called sebum. Sebum is secreted into hair follicles and makes its way to the skin surface. It waterproofs the hair and skin and helps prevent them from drying out. Sebum also has antibacterial properties, so it inhibits the growth of microorganisms on the skin.
Sweat glands produce the salty fluid called sweat, which contains excess water, salts, and other waste products. The glands have ducts that pass through the epidermis and open to the surface through pores in the skin.

Functions of the Skin

The skin has multiple roles in the body. Many of these roles are related to homeostasis. The skin’s main functions are preventing water loss from the body and serving as a barrier to the entry of microorganisms. In addition, melanin in the skin blocks UV light and protects deeper layers from its damaging effects.

The skin also helps regulate body temperature. When the body is too warm, sweat is released by the sweat glands and spreads over the skin surface. As the sweat evaporates, it cools the body. Blood vessels in the skin also dilate, or widen, when the body is too warm. This allows more blood to flow through the skin, bringing body heat to the surface, where it radiates into the environment. When the body is too cool, sweat glands stop producing sweat, and blood vessels in the skin constrict, or narrow, thus conserving body heat.

Skin Problems

In part because it is exposed to the environment, the skin is prone to injury and other problems. Two common problems of the skin are acne and skin cancer.

Acne is a condition in which red bumps called pimples form on the skin due to a bacterial infection. It affects more than 85 percent of teens and may continue into adulthood. The underlying cause of acne is excessive secretion of sebum, which plugs hair follicles and makes them good breeding grounds for bacteria.

“ABCDs of Skin Cancer. A brown spot on the skin is likely to be a harmless mole, but it could be a sign of skin cancer. Unlike moles, skin cancers are generally asymmetrical, have irregular borders, may be very dark in color, and may have a relatively great diameter.”

Vocabulary

acne: Condition in which red bumps called pimples form on the skin; due to a bacterial infection.
dermis: Lower layer of the skin; made of tough connective tissue; contains blood vessels, nerve endings, hair follicles, and glands.
epidermis: Outer layer of skin; consists mainly of epithelial cells and lacks nerve endings and blood vessels.
hair follicle: Structure in the dermis of skin where a hair originates.
integumentary system: Human body system that includes the skin, nails, and hair.
keratin: Tough, fibrous protein in skin, nails, and hair.
melanin: Brown pigment produced by melanocytes in the skin; gives skin most of its color and prevents UV light from penetrating the skin.
melanocyte: Cell that produces melanin; found in the epidermis.
sebaceous gland: Gland in the dermis of skin; produces sebum.
sebum: An oily substance produced by sebaceous glands; waterproofs the hair and skin.
skin cancer: A disease in which skin cells grow out of control; mainly caused by excessive exposure to UV light.
stratum corneum: The outer protective, waterproof layer of the skin.
sweat glands: Gland in the dermis of skin; produce the salty fluid called sweat.
vitamin D: Vitamin that is synthesized when sun exposure is adequate; needed for bone health.

Summary

The skin consists of two layers: the epidermis, which contains mainly epithelial cells, and the dermis, which contains most of skin’s other structures, including blood vessels, nerve endings, hair follicles, and glands.
Skin protects the body from injury, water loss, and microorganisms. It also plays a major role in maintaining a stable body temperature.
Common skin problems include acne and skin cancer.

Nervous System

A small child darts in front of your bike as you race down the street. You see the child and immediately react. You put on the brakes, steer away from the child, and yell out a warning, all in just a split second. How do you respond so quickly? Such rapid responses are controlled by your nervous system. The nervous system is a complex network of nervous tissue that carries electrical messages throughout the body. It includes the brain and spinal cord, the central nervous system, and nerves that run throughout the body, the peripheral nervous system. To understand how nervous messages can travel so quickly, you need to know more about nerve cells.

Nerve Cells

Although the nervous system is very complex, nervous tissue consists of just two basic types of nerve cells: neurons and glial cells. Neurons are the structural and functional units of the nervous system. They transmit electrical signals, called nerve impulses. Glial cells provide support for neurons. For example, they provide neurons with nutrients and other materials.

Neuron Structure

The cell body contains the nucleus and other cell organelles.
Dendrites extend from the cell body and receive nerve impulses from other neurons.
The axon is a long extension of the cell body that transmits nerve impulses to other cells. The axon branches at the end, forming axon terminals. These are the points where the neuron communicates with other cells.

Myelin Sheath

The axon of many neurons has an outer layer called a myelin sheath. Myelin is a lipid produced by a type of a glial cell known as a Schwann cell. The myelin sheath acts like a layer of insulation, similar to the plastic that encases an electrical cord. Regularly spaced nodes, or gaps, in the myelin sheath allow nerve impulses to skip along the axon very rapidly.

Types of Neurons

Neurons are classified based on the direction in which they carry nerve impulses.

Sensory neurons carry nerve impulses from tissues and organs to the spinal cord and brain.
Motor neurons carry nerve impulses from the brain and spinal cord to muscles and glands.
Interneurons carry nerve impulses back and forth between sensory and motor neurons.

Vocabulary

axon: Long extension of the cell body of a neuron; transmits nerve impulses to other cells.
axon terminal: Branches at the end of an axon of a neuron; points where the neuron communicates with other cells.
cell body: Central part of a neuron; contains the nucleus and other cell organelles.
central nervous system (CNS): One of two main divisions of the nervous system that includes the brain and spinal cord.
dendrite: Extension of the cell body of a neuron; receives nerve impulses from other neurons.
glial cell: A cell that provides support for a neuron.
interneuron: Neuron that carries nerve impulses back and forth between sensory and motor neurons.
motor neuron: Neuron that carries nerve impulses from the central nervous system to muscles and glands.
myelin: A lipid produced by a Schwann cell; forms the myelin sheath.
myelin sheath: Lipid layer around the axon of a neuron; allows nerve impulses to travel more rapidly down the axon.
nerve impulse: Electrical signal transmitted by the nervous system.
nervous system: Body system that carries electrical messages throughout the body.
neuron: Nerve cell; structural and functional unit of the nervous system.
peripheral nervous system (PNS): One of two major divisions of the nervous system; consists of all the nervous tissue that lies outside the central nervous system.
Schwann cell: A type of glial cell responsible for producing myelin.
sensory neuron: Neuron that carries nerve impulses from tissue and organs to the spinal cord and brain.

Summary

Neurons are the structural and functional units of the nervous system. They consist of a cell body, dendrites, and axon.
Neurons transmit nerve impulses to other cells.
Types of neurons include sensory neurons, motor neurons, and interneurons.

Nerve Impulses

How does a nervous system signal move from one cell to the next?

It literally jumps by way of a chemical transmitter. Notice the two cells are not connected, but separated by a small gap. The synapse. The space between a neuron and the next cell.

Nerve Impulses

Nerve impulses are electrical in nature. They result from a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules.

Resting Potential

When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane. It uses energy in ATP to pump positive sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. As a result, the inside of the neuron is negatively charged, compared to the extracellular fluid surrounding the neuron. This is due to many more positivly charged ions outside the cell compared to inside the cell This difference in electrical charge is called the resting potential.

Action Potential

A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron. The reversal of charge is called an action potential. It begins when the neuron receives a chemical signal from another cell. The signal causes gates in sodium ion channels to open, allowing positive sodium ions to flow back into the cell. As a result, the inside of the cell becomes positively charged compared to the outside of the cell. This reversal of charge ripples down the axon very rapidly as an electric current.

In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin. As a result, the action potential jumps along the axon membrane from node to node, rather than spreading smoothly along the entire membrane. This increases the speed at which it travels.

The Synapse

The place where an axon terminal meets another cell is called a synapse. The axon terminal and other cell are separated by a narrow space known as a synaptic cleft. When an action potential reaches the axon terminal, the axon terminal releases molecules of a chemical called a neurotransmitter. The neurotransmitter molecules travel across the synaptic cleft and bind to receptors on the membrane of the other cell. If the other cell is a neuron, this starts an action potential in the other cell.

Vocabulary

action potential: Reversal of electrical charge across the membrane of a resting neuron that travels down the axon of the neuron as a nerve impulse.
ion: Electrically charged atoms or molecules.
nerve impulse: Electrical signal transmitted by the nervous system.
neurotransmitter: Chemical that carries a nerve impulse from one nerve to another at a synapse.
resting potential: Difference in electrical charge across the plasma membrane of a neuron that is not actively transmitting a nerve impulse.
sodium-potassium pump: Active transport protein; exchanges sodium ions for potassium ions across the plasma membrane of animal cells.
synapse: Junction where an axon terminal meets another cell.
synaptic cleft: Space between the axon terminals of one cell and the receptors of the next cell.

Summary

A nerve impulse begins when a neuron receives a chemical stimulus.
The nerve impulse travels down the axon membrane as an electrical action potential to the axon terminal.
The axon terminal releases neurotransmitters that carry the nerve impulse to the next cell.

The Heart and Circulatory System

What's the most active muscle in the body?

The human heart. An absolutely remarkable organ. Obviously, its main function is to pump blood throughout the body. And it does this extremely well. On average, this muscular organ will beat about 100,000 times in one day and about 35 million times in a year. During an average lifetime, the human heart will beat more than 2.5 billion times.

The Circulatory System

The circulatory system can be compared to a system of interconnected, one-way roads that range from superhighways to back alleys. Like a network of roads, the job of the circulatory system is to allow the transport of materials from one place to another. The materials carried by the circulatory system include hormones, oxygen, cellular wastes, and nutrients from digested food. Transport of all these materials is necessary to maintain homeostasis of the body. The main components of the circulatory system are the heart, blood vessels, and blood.

The heart is a muscular organ in the chest. It consists mainly of cardiac muscle tissue and pumps blood through blood vessels by repeated, rhythmic contractions. The heart has four chambers: two upper atria (singular, atrium) and two lower ventricles. Valves between chambers keep blood flowing through the heart in just one direction.

Blood Flow Through the Heart

Blood flows through the heart in two separate loops

Blood from the body enters the right atrium of the heart. The right atrium pumps the blood to the right ventricle, which pumps it to the lungs. This loop is represented by the blue arrows in the diagram above.
Blood from the lungs enters the left atrium of the heart. The left atrium pumps the blood to the left ventricle, which pumps it to the body. This loop is represented by the red arrows in the diagram above.

Heartbeat

Unlike skeletal muscle, cardiac muscle contracts without stimulation by the nervous system. Instead, specialized cardiac muscle cells send out electrical impulses that stimulate the contractions. As a result, the atria and ventricles normally contract with just the right timing to keep blood pumping efficiently through the heart. You can watch an animation to see how this happens at this link: http://www.nhlbi.nih.gov/health/dci/Diseases/hhw/hhw_electrical.html.

Vocabulary

atrium (plural, atria): upper chamber of the heart
blood: Fluid connective tissue that circulates throughout the body through blood vessels.
blood vessel: Vessel that transports blood; includes the arteries, veins, and capillaries.
circulatory system: Organ system consisting of the heart, blood vessels, and blood that transports materials around the body.
heart: Muscular organ in the chest that that pumps blood through blood vessels when it contracts.
ventricles: lower chambers of the heart

Summary

The heart contracts rhythmically to pump blood to the lungs and the rest of the body.
Specialized cardiac muscle cells trigger the contractions.

Blood Vessels

Blood vessels form a network throughout the body to transport blood to all the body cells. There are three major types of blood vessels: arteries, veins, and capillaries.

Blood vessels include arteries, veins, and capillaries.

Arteries are muscular blood vessels that carry blood away from the heart. They have thick walls that can withstand the pressure of blood being pumped by the heart. Arteries generally carry oxygen-rich blood. The largest artery is the aorta, which receives blood directly from the heart.
Veins are blood vessels that carry blood toward the heart. This blood is no longer under much pressure, so many veins have valves that prevent backflow of blood. Veins generally carry deoxygenated blood. The largest vein is the inferior vena cava, which carries blood from the lower body to the heart. The superior vena cava brings blood back to the heart from the upper body.
Capillaries are the smallest type of blood vessels. They connect very small arteries and veins. The exchange of gases and other substances between cells and the blood takes place across the extremely thin walls of capillaries.

Blood Vessels and Homeostasis

Blood vessels help regulate body processes by either constricting (becoming narrower) or dilating (becoming wider). These actions occur in response to signals from the autonomic nervous system or the endocrine system. Constriction occurs when the muscular walls of blood vessels contract. This reduces the amount of blood that can flow through the vessels. Dilation occurs when the walls relax. This increases blood flows through the vessels.

Constriction and dilation allow the circulatory system to change the amount of blood flowing to different organs. For example, during a fight-or-flight response, dilation and constriction of blood vessels allow more blood to flow to skeletal muscles and less to flow to digestive organs. Dilation of blood vessels in the skin allows more blood to flow to the body surface so the body can lose heat. Constriction of these blood vessels has the opposite effect and helps conserve body heat.

Blood Vessels and Blood Pressure

The force exerted by circulating blood on the walls of blood vessels is called blood pressure. Blood pressure is highest in arteries and lowest in veins. When you have your blood pressure checked, it is the blood pressure in arteries that is measured. High blood pressure, or hypertension, is a serious health risk but can often be controlled with lifestyle changes or medication. You can learn more about hypertension by watching the animation at this link: http://www.healthcentral.com/high-blood-pressure/introduction-47-115.html.

Vocabulary

aorta: The largest artery; receives blood directly from the heart.
artery: Type of blood vessel that carries blood away from the heart toward the lungs or body.
blood pressure: Force exerted by circulating blood on the walls of blood vessels.
blood vessel: Vessel that transports blood; includes the arteries, veins, and capillaries.
capillary: Smallest type of blood vessel that connects very small arteries and veins.
constriction: Narrowing of the blood vessels; occurs when the muscular walls of blood vessels contract.
dilation: Widening of the blood vessels; occurs when the walls of blood vessels relax.
hypertension: High blood pressure.
inferior vena cava: The vein that receives blood directly from the heart.
superior vena cava: The vein that brings blood back to the heart from the upper body.
vein: Type of blood vessel that carries blood toward the heart from the lungs or body.

Summary

Arteries carry blood away from the heart, veins carry blood toward the heart, and capillaries connect arteries and veins.

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