# data[].attributes.headline
What is a stimulus: internal and external

# data[].attributes.subject
What is a stimulus: internal and external

# data[].attributes.attachment_link.provider_name
Biology Articles, Tutorials & Dictionary Online

# data[].attributes.attachment_link.title
Stimulus - Definition and Examples - Biology Online Dictionary

# data[].attributes.attachment_link.description
Stimulus is any external or internal event that elicits a response or reaction from an organism. Learn more about stimulus and take the Quiz!

# data[].attributes.attachment_link.title
The Brain for Kids | Learn cool facts about the human brain

# data[].attributes.attachment_link.readable_text
A stimulus is a change in the environment (either external or internal) that is detected by a receptor

Receptors transform environmental stimuli into electrical nerve impulses

These impulses are then transmitted via neurons to the central nervous system where decision-making occurs

When a response is selected (consciously or unconsciously), the signal is transmitted via neurons to effectors

Effectors are organs (either muscles or glands) that produce a response to a stimulus

A response is a change in the organism resulting from the detection of a stimulus

Overview of the Stimulus-Response Pathway
Three types of neurons are required to transmit information via the stimulus-response pathway:

Sensory neurons transmit information from sensory receptors to the central nervous system (CNS)

Relay neurons (interneurons) transmit information within the CNS as part of the decision-making process

Motor neurons transmit information from the CNS to effectors (muscles or glands), in order to initiate a response

While these three types of neurons share common features, their basic structure may differ slightly according to their function

This includes their length, cell body (soma) position and the comparative distribution of dendrites and axon terminals

# data[].attributes.attachment_link.description
A stimulus is a change in the environment (either external or internal) that is detected by a receptor Receptors transform environmental stimuli into electrical nerve impulses These impulses are then transmitted via neurons to the central nervous system where decision-making occurs When a response is selected (consciously or unconsciously), the

# data[].attributes.body
Easier

# data[].attributes.headline
Nervous System: Responding to changes in environment

# data[].attributes.subject
Nervous System: Responding to changes in environment

# data[].attributes.attachment_link.title
Responding to changes in environment - How does the nervous system help us respond? - OCR 21st Century - GCSE Biology (Single Science) Revision - OCR 21st Century - BBC Bitesize

# data[].attributes.attachment_link.description
Revise the nervous system for OCR 21st Century with BBC Bitesize.

# data[].attributes.body
Stress Response

# data[].attributes.attachment_link.title
Understanding the stress response - Harvard Health

# data[].attributes.attachment_link.description
Research suggests that chronic stress is linked to high blood pressure, clogged arteries, anxiety, depression, addictive behaviors, and obesity.…

# data[].attributes.body
Easier: Text to Speech

# data[].attributes.attachment_link.readable_text
How Big Is the Eye?

The eye is about as big as a ping-pong ball and sits in a little hollow area (the eye socket) in the skull.

What Do Eyelids and Eyelashes Do?

The eyelid protects the front part of the eye. The lid helps keep the eye clean and moist by opening and shutting several times a minute. This is called blinking, and it's both a voluntary and involuntary action, meaning you can blink whenever you want to, but it also happens without you even thinking about it.

The eyelid also has great reflexes, which are automatic body responses, that protect the eye. When you step into bright light, for example, the eyelids squeeze together tightly to protect your eyes until they can adjust to the light. And if you flutter your fingers close (but not too close!) to your friend's eyes, you'll be sure to see your friend's eyes blink. Your friend's eyelids shut automatically to protect the eye from possible danger. And speaking of fluttering, don't forget eyelashes. They work with the eyelids to keep dirt and other unwanted stuff out of your eyes.

What's the Sclera?

The white part of the eyeball is called the sclera (say: SKLAIR-uh). The sclera is made of a tough material and has the important job of covering most of the eyeball. Think of the sclera as your eyeball's outer coat. Look very closely at the white of the eye, and you'll see lines that look like tiny pink threads. These are blood vessels, the tiny tubes that deliver blood, to the sclera.

What's the Cornea?

The cornea (say: KOR-nee-uh), a transparent dome, sits in front of the colored part of the eye. The cornea helps the eye focus as light makes its way through. It is a very important part of the eye, but you can hardly see it because it's made of clear tissue. Like clear glass, the cornea gives your eye a clear window to view the world through.

What's the Iris?

Behind the cornea are the iris, the pupil, and the anterior chamber. The iris (say: EYE-riss) is the colorful part of the eye. When we say a person has blue eyes, we really mean the person has blue irises! The iris has muscles attached to it that change its shape. This allows the iris to control how much light goes through the pupil (say: PYOO-pul).

What's the Pupil?

The pupil is the black circle in the center of the iris, which is really an opening in the iris, and it lets light enter the eye. To see how this works, use a small flashlight to see how your eyes or a friend's eyes respond to changes in brightness. The pupils will get smaller when the light shines near them and they'll open wider when the light is gone.

What's the Anterior Chamber?

The anterior (say: AN-teer-ee-ur) chamber is the space between the cornea and the iris. This space is filled with a special transparent fluid that nourishes the eye and keeps it healthy.

What's the Lens?

These next parts are really cool, but you can't see them with just your own eyes! Doctors use special microscopes to look at these inner parts of the eye, such as the lens. After light enters the pupil, it hits the lens. The lens sits behind the iris and is clear and colorless.

The lens' job is to focus light rays on the back of the eyeball - a part called the retina (say: RET-i-nuh).

The lens works much like the lens of a movie projector at the movies. Next time you sit in the dark theater, look behind you at the stream of light coming from the projection booth. This light goes through a powerful lens, which is focusing the images onto the screen, so you can see the movie clearly. In the eye's case, however, the film screen is your retina.

What Does the Retina Do?

Your retina is in the very back of the eye. It holds millions of cells that are sensitive to light. The retina takes the light the eye receives and changes it into nerve signals so the brain can understand what the eye is seeing.

What's the Ciliary Body?

The lens is suspended in the eye by a bunch of fibers. These fibers are attached to a muscle called the ciliary (say: SIL-ee-air-ee) body. It has the amazing job of changing the shape of the lens. That's right - the lens actually changes shape right inside your eye! Try looking away from your computer and focusing on something way across the room. Even though you didn't feel a thing, the shape of your lenses changed. When you look at things up close, the lens becomes thicker to focus the correct image onto the retina. When you look at things far away, the lens becomes thinner.

What's the Vitreous Body? And What's Vitreous Humor?

The biggest part of the eye sits behind the lens and is called the vitreous (say: VIH-tree-us) body. The vitreous body forms two thirds of the eye's volume and gives the eye its shape. It's filled with a clear, jelly-like material called the vitreous humor. Ever touch toy eyeballs in a store? Sometimes they're kind of squishy - that's because they're made to feel like they're filled with vitreous humor. In a real eye, after light passes through the lens, it shines straight through the vitreous humor to the back of the eye.

What Do Rods and Cones Do?

The retina uses special cells called rods and cones to process light. Just how many rods and cones does your retina have? How about 120 million rods and 7 million cones - in each eye!

Rods see in black, white, and shades of gray and tell us the form or shape that something has. Rods can't tell the difference between colors, but they are super-sensitive, allowing us to see when it's very dark.

Cones sense color and they need more light than rods to work well. Cones are most helpful in normal or bright light. The retina has three types of cones. Each cone type is sensitive to one of three different colors - red, green, or blue - to help you see different ranges of color. Together, these cones can sense combinations of light waves that let our eyes to see millions of colors.

Rods and cones process the light to give you the total picture. You're able to see that your friend has brown skin and is wearing a blue hat while he tosses an orange basketball.

Why Do Some People Need Glasses?

Sometimes someone's eyeball shape makes it difficult for the cornea, lens, and retina to work perfectly as a team. When this happens, some of what the person sees will be out of focus.

To correct this fuzzy vision, many people, including many kids, wear glasses. Glasses help the eyes focus images correctly on the retina and allow someone to see clearly. As adults get older, their eyes lose the ability to focus well and they often need glasses to see things up close or far away. Most older people you know - like your grandparents - probably wear glasses.

What's the Optic Nerve?

Think of the optic nerve as the great messenger in the back of your eye. The rods and cones of the retina change the colors and shapes you see into millions of nerve messages. Then, the optic nerve carries those messages from the eye to the brain!

The optic nerve serves as a high-speed telephone line connecting the eye to the brain. When you see an image, your eye "telephones" your brain with a report on what you are seeing so the brain can translate that report into "cat," "apple," or "bicycle," or whatever the case may be.

What Are the Lacrimal Glands? And Why Do Eyes Need Tears?

For crying out loud, the eye has its own special bathing system - tears! Above the outer corner of each eye are the lacrimal (say: LAK-ruh-mul) glands, which make tears. Every time you blink your eye, a tiny bit of tear fluid comes out of your upper eyelid. It helps wash away germs, dust, or other particles that don't belong in your eye.

Tears also keep your eye from drying out. Then the fluid drains out of your eye by going into the lacrimal duct (this is also called the tear duct). You can see the opening of your tear duct if you very gently pull down the inside corner of your eye. When you see a tiny little hole, you've found the tear duct.

Your eyes sometimes make more tear fluid than normal to protect themselves. This may have happened to you if you've been poked in the eye, if you've been in a dusty or smoking area, or if you've been near someone who's cutting onions.

And how about the last time you felt sad, scared, or upset? Your eyes got a message from your brain to make you cry, and the lacrimal glands made many, many tears.

How Can I Protect My Eyes?

Your eyes do some great things for you, so take these steps to protect them:

Wear protective goggles in classes where debris or chemicals could go flying, such as wood shop, metal shop, science lab, or art.

Wear eye protection when playing racquetball, hockey, skiing, or other sports that could injure your eyes.

Wear sunglasses. Too much light can damage your eyes and cause vision problems later in life. For instance, a lens could get cloudy, causing a cataract. A cataract prevents light from reaching the retina and makes it difficult to see.

The eyes you have will be yours forever - treat them right and they'll never be out of sight!

# data[].attributes.attachment_link.description
Ever wonder how your eyes work? This article for kids takes you from the pupil to the retina and beyond.

# data[].attributes.attachment_link.description
The basics of the nervous system are demonstrated through explanations and animations.

# data[].attributes.body
Easier: Text to Speech

# data[].attributes.attachment_link.readable_text
The Brain Is the Boss

When you're taking a big math test, you know that your brain is hard at work. But your brain is doing a lot more than just remembering formulas. Those sweaty palms you get as the test starts? That's your brain at work. The relief you feel when you know an answer's right? That's your brain too. And yes, your brain is even in charge when you take a minute to daydream about the big party on Friday night.

The brain may simply be the bossiest part of the body: It tells virtually every other part of your body what to do, all the time, whether you're aware of it or not. It controls what you think and feel, how you learn and remember, and the way you move. It also controls things you might not think about - like the beating of your heart and whether you feel sleepy or awake.

The Brain & Nervous System in Everyday Life

If the brain is like a central computer that controls all the functions of your body, then the nervous system is like a network that sends messages back and forth from the brain to different parts of the body. It does this via the spinal cord, which runs from the brain down through the back and contains threadlike nerves that branch out to every organ and body part.

When a message comes into the brain from anywhere in the body, the brain tells the body how to react. For example, if you accidentally touch a hot stove, the nerves in your skin shoot a message of pain to your brain. The brain then sends a message back telling the muscles in your hand to pull away. Luckily, this neurological relay race takes a lot less time than it just took to read about it!

How the Brain Works

Considering everything it does, the human brain is incredibly compact, weighing just 3 pounds. Its many folds and grooves, though, provide it with the additional surface area necessary for storing all of the body's important information.

The spinal cord, on the other hand, is a long bundle of nerve tissue about 18 inches long and ¾ inch thick. It extends from the lower part of the brain down through spine. Along the way, various nerves branch out to the entire body. These make up the peripheral nervous system.

Both the brain and the spinal cord are protected by bone: the brain by the bones of the skull, and the spinal cord by the set of ring-shaped bones called vertebrae that make up the spine. They're both cushioned by layers of membranes called meninges as well as a special fluid called cerebrospinal fluid. This fluid helps protect the nerve tissue, keep it healthy, and remove waste products.

The brain is made up of three main sections: the forebrain, the midbrain, and the hindbrain.

The Forebrain

The forebrain is the largest and most complex part of the brain. It consists of the cerebrum - the area with all the folds and grooves typically seen in pictures of the brain - as well as some other structures beneath it.

The cerebrum contains the information that essentially makes us who we are: our intelligence, memory, personality, emotion, speech, and ability to feel and move. Specific areas of the cerebrum are in charge of processing these different types of information. These are called lobes, and there are four of them: the frontal, parietal, temporal, and occipital.

The cerebrum has right and left halves, called hemispheres, which are connected in the middle by a band of nerve fibers (the corpus callosum) that enables the two sides to communicate.

Although these halves may look like mirror images of each other, many scientists believe they have different functions. The left side is considered the logical, analytical, objective side. The right side is thought to be more intuitive, creative, and subjective. So when you're doing a math problem you're using the left side, and when you're listening to music you're using the right side. Scientists think that some people are more "right-brained" or "left-brained" while others are more "whole-brained," meaning they use both halves of their brain to the same degree.

The outer layer of the cerebrum is called the cortex (also known as "gray matter"). Information collected by the five senses comes into the brain from the spinal cord to the cortex. This information is then directed to other parts of the nervous system for further processing. For example, when you touch the hot stove, not only does a message go out to move your hand but one also goes to another part of the brain to help you remember not to do that again.

In the inner part of the forebrain sit the thalamus, hypothalamus, and pituitary gland. The thalamus carries messages from the sensory organs like the eyes, ears, nose, and fingers to the cortex. The hypothalamus controls body temperature, thirst, appetite, sleep patterns, and other processes in our bodies that happen automatically. It also controls the pituitary gland, which makes the hormones that control our growth, metabolism, water and mineral balance, sexual maturity, and how we respond to stress.

The Midbrain

The midbrain, located underneath the middle of the forebrain, acts as a master coordinator for all the messages going in and out of the brain to the spinal cord.

The Hindbrain

The hindbrain sits underneath the back end of the cerebrum, and it consists of the cerebellum, pons, and medulla. The cerebellum - also called the "little brain" because it looks like a small version of the cerebrum - is responsible for balance, movement, and coordination. The pons and the medulla, along with the midbrain, are often called the brainstem. The brainstem takes in, sends out, and coordinates all of the brain's messages. It is also controls many of the body's automatic functions, like breathing, heart rate, blood pressure, swallowing, digestion, and blinking.

How the Nervous System Works

The basic functioning of the nervous system depends a lot on tiny cells called neurons. The brain has billions of them, and they have many specialized jobs. For example, sensory neurons take information from the eyes, ears, nose, tongue, and skin to the brain. Motor neurons carry messages away from the brain and back to the rest of the body.

All neurons relay information to each other through a complex electrochemical process, making connections that affect the way we think, learn, move, and behave.

Intelligence, Learning, and Memory

When you learn things, messages travel from one neuron to another, over and over. Then the brain creates connections (or pathways) between the neurons, so things become easier and you can do them better and better.

In young children, the brain is highly adaptable. In fact, when one part of a young child's brain is injured, another part may learn to take over some of the lost function. But as we age, the brain has to work harder to make new neural pathways, making it more difficult to master new tasks or change established behavior patterns. That's why many scientists believe it's important to keep challenging your brain to learn new things and make new connections - it helps keep the brain active over the course of a lifetime.

Memory is another complex function of the brain. The things we've done, learned, and seen are first processed in the cortex, and then, if we sense that this information is important enough to remember permanently, it's passed inward to other regions of the brain (such as the hippocampus and amygdala) for long-term storage and retrieval. As these messages travel through the brain, they create pathways that serve as the basis of our memory.

Movement

Different parts of the cerebrum are responsible for moving different body parts. The left side of the brain controls the movements of the right side of the body, and the right side of the brain controls the movements of the left side of the body. When you kick a soccer ball with your right foot, for example, it's the left side of your brain that sends the message allowing you to do it.

Basic Body Functions

A part of the peripheral nervous system called the autonomic nervous system is responsible for controlling many of the body processes we almost never need to think about, like breathing, digestion, sweating, and shivering. The autonomic nervous system has two parts: the sympathetic and the parasympathetic nervous systems.

The sympathetic nervous system prepares the body for sudden stress, like if you see a robbery taking place. When something frightening happens, the sympathetic nervous system makes the heart beat faster so that it sends blood more quickly to the different body parts that might need it. It also causes the adrenal glands at the top of the kidneys to release adrenaline, a hormone that helps give extra power to the muscles for a quick getaway. This process is known as the body's "fight or flight" response.

The parasympathetic nervous system does the exact opposite: It prepares the body for rest. It also helps the digestive tract move along so our bodies can efficiently take in nutrients from the food we eat.

The Senses

Your eyes may watch as your best friend walks toward you - but without the brain, you wouldn't even recognize her. Pepperoni pizza sure is delicious - but without the brain, your taste buds wouldn't be able to tell if you were eating pizza or the box it came in. None of your senses would be useful without the processing that occurs in the brain.

Sight. Sight probably tells us more about the world than any other sense. Light entering the eye forms an upside-down image on the retina. The retina transforms the light into nerve signals for the brain. The brain then turns the image right-side up and tells us what we are seeing.

Hearing. Every sound we hear is the result of sound waves entering our ears and causing our eardrums to vibrate. These vibrations are then transferred along the tiny bones of the middle ear and converted into nerve signals. The cortex then processes these signals, telling us what we are hearing.

Taste. The tongue contains small groups of sensory cells called taste buds that react to chemicals in foods. Taste buds react to sweet, sour, salty, and bitter. Messages are sent from the taste buds to the areas in the cortex responsible for processing taste.

Smell. Olfactory cells in the mucous membranes lining each nostril react to chemicals we breathe in and send messages along specific nerves to the brain - which, according to experts, can distinguish between more than 10,000 different smells. With that kind of sensitivity, it's no wonder research suggests that smells are very closely linked to our memories.

Touch. The skin contains more than 4 million sensory receptors - mostly concentrated in the fingers, tongue, and lips - that gather information related to touch, pressure, temperature, and pain and send it to the brain for processing and reaction.

Things That Can Go Wrong With the Brain

Because the brain controls just about everything, when something goes wrong, it's often serious and can affect many different parts of the body. Inherited diseases, brain disorders associated with mental illness, and head injuries can all affect the way the brain works and upset the daily activities of the rest of the body.

Here are some of the problems that can affect the brain:

Brain tumors. A tumor is an abnormal tissue growth in the brain. A tumor in the brain may grow slowly and produce few symptoms until it becomes large. Or a tumor can grow and spread rapidly, causing severe and quickly worsening symptoms.

Brain tumors can be benign or malignant. They usually grow in one place and may be curable through surgery if they're located in a place where they can be removed without damaging the normal tissue near the tumor. A malignant tumor is cancerous and more likely to grow rapidly and spread.

Cerebral palsy (CP). Cerebral palsy is the result of a developmental defect or damage to the brain before or during a child's birth, or during the first few years of life. CP affects the motor areas of the brain. A person with cerebral palsy may have average intelligence or can have severe developmental delays or mental retardation.

Cerebral palsy can affect body movement in many different ways. In mild cases, a person may have minor muscle weakness in the arms and legs. In other cases, there may be more severe motor impairment - a person may have trouble talking and performing basic movements like walking.

Epilepsy. Epilepsy is a condition of the nervous system that can lead people to have seizures. Partial seizures involve specific areas of the brain, and symptoms vary depending on the location of the seizure activity. Other seizures, called generalized seizures, involve a larger area of the brain and usually cause uncontrolled movements of the entire body and loss of consciousness. Although in many cases doctors don't know what causes it, epilepsy can be related to brain injury, tumors, or infections. The tendency to develop epilepsy may be inherited in families.

Headaches. Of the many different types of headaches, some of the more common are:

tension headache (the most common type) is caused by muscle tension in the head, neck, and shoulders

migraine is an intense, recurring headache with an unclear cause

cluster headache is often considered to be a form of migraine

Migraines happen with or without warning and may last for a few hours or up to several days. The tendency to develop migraines may be inherited in families. A person who has migraines may get one after being exposed to a "trigger" (such as a particular food), and may feel dizzy, numb, nauseated, and sensitive to light, and may see flashing zigzag lines before their eyes.

Meningitis and encephalitis. These are infections of the brain and spinal cord that are usually caused by bacteria or viruses. Meningitis is an inflammation of the coverings of the brain and spinal cord, and encephalitis is an inflammation of the brain tissue. Both conditions may result in permanent injury to the brain.

Mental illness. Mental illnesses involve a wide range of problems in how people think and function. Experts now know that certain mental illnesses are linked to structural or chemical problems in the person's brain. Some mental illnesses are inherited, but even though researchers know that these illnesses run in families, they often can't pinpoint what causes them in the first place. Injuries to the brain and chronic drug or alcohol abuse also can trigger some mental illnesses.

Signs of chronic mental illnesses such as bipolar disorder or schizophrenia may first show up in childhood. Mental illnesses that can be seen in teens include depression, eating disorders such as bulimia or anorexia nervosa, obsessive-compulsive disorder (OCD), and phobias.

Head injuries. Head injuries fit into two categories: external (usually scalp) injuries and internal injuries. Internal injuries may involve the skull, the blood vessels within the skull, or the brain. Fortunately, most falls or blows to the head usually injure the scalp only, which is usually more frightening than threatening. An internal head injury is usually more serious because the skull serves as the protective helmet for the delicate brain.

Concussions are a type of internal head injury that causes the temporary loss of normal brain function. Repeated concussions can lead to permanent brain injury. Playing sports is one of the most common ways teens get concussions. The right protective gear, worn the right way, can help people avoid concussions. But even helmets don't prevent all concussions. If you get hit on the head while playing sports and think there's a chance you have a concussion, stop playing right away and let a coach or parent know what's going on.

# data[].attributes.attachment_link.description
If the brain is a central computer that controls all the functions of the body, then the nervous system is like a network that relays messages back and forth to different parts of the body. Find out how they work in this Body Basics article.

# data[].attributes.attachment_link.title
Rocketlit Science Reading for stimuli, sensory neuron, interneuron, motor neuron, reflex

# data[].attributes.attachment_link.description
Leveled, non-fiction, and print-friendly science article - In this article, students are introduced to how reflexes work in the body. The process starts with sensory neurons that sense stimuli and connect with interneurons to decide what to do. Motor neurons are activated without the brain's involvement in a reflex, because action is needed before any thoughts or associations need to be accessed.

# data[].attributes.attachment_link.readable_text
Introduction to the Nervous System

The nervous system controls bodily function by gathering sensory input, integrating that information internally, and communicating proper motor output.

Learning Objectives

Describe the hierarchical structure of the nervous system

Key Takeaways

Key Points

The nervous system is the body's main communication system; it gathers, synthesizes, and uses data from the environment.

The most basic unit of the nervous system is the neuron, which serves as both a sensor and communicator of internal and external stimuli.

The nervous system can be broken down into two major parts-the central nervous system and the peripheral nervous system.

The central nervous system, the main data center of the body, includes the brain and spinal cord.

The peripheral nervous system includes all of the neurons that sense and communicate data to the central nervous system.

The peripheral nervous system can be further divided into the autonomic system, which regulates involuntary actions, and the somatic system, which controls voluntary actions.

Key Terms

central nervous system: In vertebrates, the part of the nervous system comprising the brain, brainstem, and spinal cord.

peripheral nervous system: The part of the nervous system comprising a large system of nerves that are linked to the brain and spinal cord; this system is divided into the autonomic and somatic nervous systems.

neuron: A cell of the nervous system, which conducts nerve impulses; consisting of an axon and several dendrites. Neurons are connected by synapses.

innervate: To supply nerves to a tissue.

The nervous system allows organisms to sense, organize, and react to information in the environment. The basic unit of the nervous system is the neuron. Synapses form between the neurons, allowing them to communicate to other neurons or other systems in the body. The general flow of information is that the peripheral nervous system (PNS) takes in information through sensory neurons, then sends it to the central nervous system (CNS) to be processed. After processing, the CNS "tells" the PNS what to do-what muscles to flex, whether the lungs need more oxygen, which limbs need more blood, any number of biological processes-and the PNS makes it happen through muscle control. The neurons responsible for taking information to the CNS are known as afferent neurons, while the neurons that carry the responses from the CNS to the PNS are known as efferent neurons.

The nervous system can be divided into two major parts-the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System

The central nervous system includes the spinal cord and the brain. The brain is the body's main control center. The main function of the CNS is the integration and processing of sensory information. It synthesizes sensory input to compute an appropriate motor response, or output.

Peripheral Nervous System

The peripheral nervous system includes a large system of nerves that are linked to the brain and spinal cord. It is comprised of sensory receptors, which process changes in internal and external stimuli and communicate that information to the CNS. The PNS can be further subdivided into the autonomic nervous system and the somatic nervous system.

Autonomic Nervous System

The autonomic nervous system regulates involuntary actions such as internal-organ function and blood-vessel movement. It supplies nerves to ("innervates") cardiac and smooth muscle tissue. The autonomic nervous system is made of two components, which work in opposition to one another: the sympathetic nervous system, responsible for the body's "fight-or-flight" response to danger, and the parasympathetic nervous system, which calms the body back down.

Somatic Nervous System

The somatic nervous system controls voluntary movements such as those in the skin, bones, joints, and skeletal muscles.

Both of these systems within the PNS work together with the CNS to regulate bodily function and provide reactions to external stimuli.

The Central Nervous System (CNS)

The central nervous system is made up of the brain and spinal cord, which process sensory input and provide instructions to the body.

Learning Objectives

Describe the structural elements of the central nervous system

Key Takeaways

Key Points

The central nervous system (CNS) and the peripheral nervous system (PNS) comprise the entirety of the body's nervous system, which regulates and maintains its most basic functions.

The CNS is the main control center of the body-it takes in sensory information, organizes and synthesizes this input, then provides instructions for motor output to the rest of the body.

The CNS is made up of the brain and spinal cord.

The brain is the main data center of the body, consisting of the cerebrum (which regulates higher-level functioning such as thought) and the cerebellum (which maintains coordination).

The brain stem includes the midbrain, pons, and medulla, and controls lower-level functioning such as respiration and digestion.

The spinal cord connects the brain and the body's main receptors, and serves as a conduit for sensory input and motor output.

Key Terms

cerebrum: In humans it is the largest part of the brain and is the seat of motor and sensory functions, as well as the higher mental functions such as consciousness, thought, reason, emotion, and memory.

spinal cord: A thick, whitish cord of nerve tissue that is a major part of the central nervous system. It extends from the brain stem through the spine, with nerves branching off to various parts of the body.

cerebellum: Part of the hindbrain in vertebrates. In humans it lies between the brainstem and the cerebrum, and plays an important role in sensory perception, motor output, balance, and posture.

brain stem: The part of the brain that connects the spinal cord to the forebrain and cerebrum.

Introduction to the Central Nervous System

The central nervous system (CNS) is one of the two major subdivisions of the nervous system. The CNS includes the brain and spinal cord, which together comprise the body's main control center. Together with the peripheral nervous system (PNS), the CNS performs fundamental functions that contribute to an organism's life and behavior.

Activity of the CNS

The nervous system has three main functions: gathering sensory information from external stimuli, synthesizing that information, and responding to those stimuli. The CNS is mainly devoted to the "information synthesizing" function. During this step in the process, the brain and spinal cord decide on appropriate motor output, which is computed based on the type of sensory input. The CNS regulates everything from organ function to high-level thought to purposeful body movement. Thus, the CNS is commonly thought of as the control center of the body.

Structure of the Central Nervous System

The CNS is comprised of the brain, brain stem, and spinal cord.

Brain

The brain is found in the cranial cavity and consists of the cerebrum and cerebellum. It houses the nerve centers responsible for coordinating sensory and motor systems in the body. The cerebrum, or the top portion for the brain, is the seat of higher-level thought. It is comprised of two hemispheres, each controlling the opposite side of the body. Each of these hemispheres is divided into four separate lobes:

the frontal lobe, which controls specialized motor control, learning, planning, and speech;

the parietal lobe, which controls somatic or voluntary sensory functions;

the occipital lobe, which controls vision;

the temporal lobe, which controls hearing and some other speech functions.

The cerebellum is located underneath the backside of the cerebrum, and governs balance and fine motor movements. Its main function is maintaining coordination throughout the body.

Brain Stem

The brain stem is connected to the underside of the brain. It consists of the midbrain, pons, and medulla. The midbrain is found in between the hindbrain and the forebrain. It regulates motor function and allows motor and sensory information to pass from the brain to the rest of the body. The pons houses the control centers for respiration and inhibitory functions. The medulla also helps regulate respiration, as well as cardiovascular and digestive functioning.

Spinal Cord

The spinal cord connects the brain and brain stem to all of the major nerves in the body. Spinal nerves originate from the spinal cord and control the functions of the rest of the body. Impulses are sent from receptors through the spinal cord to the brain, where they are processed and synthesized into instructions for the rest of the body. This data is then sent back through the spinal cord to muscles and glands for motor output.

The Peripheral Nervous System (PNS)

The peripheral nervous system connects the central nervous system to environmental stimuli to gather sensory input and create motor output.

Learning Objectives

Discuss the organization of the peripheral nervous system

Key Takeaways

Key Points

The peripheral nervous system (PNS) provides the connection between internal or external stimuli and the central nervous system to allow the body to respond to its environment.

The PNS is made up of different kinds of neurons, or nerve cells, which communicate with each other through electric signaling and neurotransmitters.

The PNS can be broken down into two systems: the autonomic nervous system, which regulates involuntary actions such as breathing and digestion, and the somatic nervous system, which governs voluntary action and body reflexes.

The autonomic nervous system has two complementary parts: the sympathetic nervous system, which activates the "fight-or-flight-or-freeze" stress response, and the parasympathetic nervous system, which reacts with the "rest-and-digest" response after stress.

The somatic nervous system coordinates voluntary physical action. It is also responsible for our reflexes, which do not require brain input.

Key Terms

afferent: Leading toward the central nervous system.

efferent: Leading away from the central nervous system.

polysynaptic reflex: Involves at least one interneuron between the sensory and motor neurons.

monosynaptic reflex: Involves a single synapse between the sensory neuron that receives the information and the motor neuron that responds.

somatic nervous system: The part of the peripheral nervous system that transmits signals from the central nervous system to skeletal muscle and from receptors of external stimuli to the central nervous system, thereby mediating sight, hearing, and touch.

autonomic nervous system: The part of the nervous system that regulates the involuntary activity of the heart, intestines, and glands, including digestion, respiration, perspiration, metabolism, and blood-pressure modulation.

sympathetic nervous system: The part of the autonomic nervous system that raises blood pressure and heart rate, constricts blood vessels, and dilates the pupils in situations of stress.

parasympathetic nervous system: One of the divisions of the autonomic nervous system; located between the brain and the spinal cord; slows the heart and relaxes the muscles.

The peripheral nervous system (PNS) is one of the two major components of the body's nervous system. In conjunction with the central nervous system (CNS), the PNS coordinates action and responses by sending signals from one part of the body to another. The CNS includes the brain, brain stem, and spinal cord, while the PNS includes all other sensory neurons, clusters of neurons called ganglia, and connector neurons that attach to the CNS and other neurons.

Divisions of the Peripheral Nervous System

The PNS can also be divided into two separate systems: the autonomic nervous system and the somatic nervous system.

Autonomic Nervous System

The autonomic nervous system regulates involuntary and unconscious actions, such as internal-organ function, breathing, digestion, and heartbeat. This system consists of two complementary parts: the sympathetic and parasympathetic systems. Both divisions work without conscious effort and have similar nerve pathways, but they generally have opposite effects on target tissues.

The sympathetic nervous system activates the "fight or flight" response under sudden or stressful circumstances, such as taking an exam or seeing a bear. It increases physical arousal levels, raising the heart and breathing rates and dilating the pupils, as it prepares the body to run or confront danger. These are not the only two options; "fight or flight" is perhaps better phrased as "fight or flight or freeze," where in the third option the body stiffens and action cannot be taken. This is an autonomic response that occurs in animals and humans; it is a survival mechanism thought to be related to playing dead when attacked by a predator. Post-traumatic stress disorder (PTSD) can result when a human experiences this "fight or flight or freeze" mode with great intensity or for large amounts of time.

The parasympathetic nervous system activates a "rest and digest" or "feed and breed" response after these stressful events, which conserves energy and replenishes the system. It reduces bodily arousal, slowing the heartbeat and breathing rate. Together, these two systems maintain homeostasis within the body: one priming the body for action, and the other repairing the body afterward.

Somatic Nervous System

The somatic nervous system keeps the body adept and coordinated, both through reflexes and voluntary action. The somatic nervous system controls systems in areas as diverse as the skin, bones, joints, and skeletal muscles. Afferent fibers, or nerves that receive information from external stimuli, carry sensory information through pathways that connect the skin and skeletal muscles to the CNS for processing. The information is then sent back via efferent nerves, or nerves that carry instructions from the CNS, back through the somatic system. These instructions go to neuromuscular junctions-the interfaces between neurons and muscles-for motor output.

The somatic system also provides us with reflexes, which are automatic and do not require input or integration from the brain to perform. Reflexes can be categorized as either monosynaptic or polysynaptic based on the reflex arc used to perform the function. Monosynaptic reflex arcs, such as the knee-jerk reflex, have only a single synapse between the sensory neuron that receives the information and the motor neuron that responds. Polysynaptic reflex arcs, by contrast, have at least one interneuron between the sensory neuron and the motor neuron. An example of a polysynaptic reflex arc is seen when a person steps on a tack-in response, their body must pull that foot up while simultaneously transferring balance to the other leg.

# data[].attributes.attachment_link.description
The nervous system controls bodily function by gathering sensory input, integrating that information internally, and communicating proper motor output. Describe the hierarchical structure of the nervous system Key Points The nervous system is the body's main communication system; it gathers, synthesizes, and uses data from the environment.

# data[].attributes.attachment_link.title
Human Physiology - Somatic Nervous System

# data[].attributes.body
Sensory Processes

# data[].attributes.attachment_link.readable_text
Learning Objectives

Identify the general and special senses in humans

Describe four important mechanoreceptors in human skin

Explain in what way smell and taste stimuli differ from other sensory stimuli

Trace the path of sound through the auditory system to the site of transduction of sound

Identify the structures of the vestibular system that respond to gravity

Trace the path of light through the eye to the point of the optic nerve

Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration. Vestibular sensation, which is an organism's sense of spatial orientation and balance, proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track kinesthesia (limb movement) are part of somatosensation. Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (such as light, or sound, or the position of the body) into an electrical signal in the nervous system. This process is called sensory transduction.

There are two broad types of cellular systems that perform sensory transduction. In one, a neuron works with a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus. Stimulation of the sensory receptor activates the associated afferent neuron, which carries information about the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment: this neuron constitutes the sensory receptor. Free nerve endings can be stimulated by several different stimuli, thus showing little receptor specificity. For example, pain receptors in your gums and teeth may be stimulated by temperature changes, chemical stimulation, or pressure.

Reception

The first step in sensation is reception, which is the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor's receptive field. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with the body. For the sense of hearing, a stimulus can be a moderate distance away. For vision, a stimulus can be very far away; for example, the visual system perceives light from stars at enormous distances.

Transduction

The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor, and the change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? In this example, a type of receptor called a mechanoreceptor (as shown in Figure 1) possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. Recall that in the nervous system, a positive change of a neuron's electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.

Sensory receptors for different senses are very different from each other, and they are specialized according to the type of stimulus they sense: they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound; they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.

Encoding and Transmission of Sensory Information

Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor's afferent axons encode one type of stimulus, and this segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system, and electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus-a sound.

The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials, and reducing the stimulus will likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the CNS, and the brain will further process incoming signals.

Perception

Perception is an individual's interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system, in the brain. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that particular stimulus and synapse with particular neurons in the brain or spinal cord.

All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex. Recall that the thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex ( Figure 2) dedicated to processing that particular sense.

How are neural signals interpreted? Interpretation of sensory signals between individuals is largely similar; however, there are some individual differences. A good example of this is individual tolerances to a painful stimulus, such as dental pain, which certainly differ.

# data[].attributes.attachment_link.description
Identify the general and special senses in humans Describe four important mechanoreceptors in human skin Explain in what way smell and taste stimuli differ from other sensory stimuli Trace the path of sound through the auditory system to the site of transduction of sound Identify the structures of the vestibular system that respond to gravity Trace the path of light through the eye to the point of the optic nerve Senses provide information about the body and its environment.

# data[].attributes.attachment_link.title
Restoring the sense of touch in amputee...nals of nervous system -- ScienceDaily

# data[].attributes.body
Reactions and Reflexes