Chapter 23: Nervous and Motor Systems

Actions, reactions, sensations, and addictions: meet your nervous system

Lecture by Danielle DuCharme, Waubonsee Community College

Learning Objectives

By the end of this chapter, you should be able to:

• Define what the nervous system is

• Describe how neurons work

• Show how our senses detect and transmit stimuli

Learning Objectives

By the end of this chapter, you should be able to:

• Understand how the muscular and skeletal systems generate movement.

• Diagram brain structure and associated function.

• Comprehend the effects of drugs on the body.

Heritable Sensory Autonomic Neuropathy

Gabby Gringas feels no pain.

Instead of being a benefit, this condition fails to alert Gabby of a dangerous situation.

Feeling pain is a necessary part of being a living animal.

The Nervous System

The nervous system is a network of cells that collects information about the organism’s internal and external environments, processes that information, and sends signals to effectors, muscles, and glands that are capable of responding to the information.

The Nervous System Has Three Critical Features

1. Receives input from the surrounding world.

2. Processes that information.

3. Initiates responses to the environment when necessary.

Take-home message 23.1

Present in all multicellular animals other than sponges, the nervous system is a network of cells that collects information about the organism’s internal and external environments, processes that information, and sends signals to muscles and glands in response to the information.

23.2 Neurons are the building blocks of all nervous systems. In all

vertebrates, the nervous system is divided into two components:

• The peripheral nervous system

• The central nervous system

Peripheral Nervous System

Network of sensory cells that receive information from the environment and…

the cells that transmit signals to effectors, the organism’s muscles and glands.

Central Nervous System

Made of the spinal cord and the brain.

The central nervous system processes information that it receives from sensory cells about the organism’s surroundings and sends out instructions to other nervous tissue to act in response.

The Neuron

The type of cell specialized for carrying electrical signals

The building block of all nervous systems

Nerves

Nerves are comprised of neurons bundled together with connective tissue.

These structures connect us to our world by enabling us to sense light, sound, touch, tastes, and smells.

Parts of a Neuron

The cell body contains all of the typical machinery of a eukaryotic cell.

Neurons have two specialized structures:• the dendrite• the axon

The Dendrite

The dendrite senses and responds to stimulation from outside the cell and sends that information toward the cell body of the neuron.

The Axon

The axon is a tube-like extension of the main cell body that transmits the signals picked up by the dendrite to the rest of the organism’s body.

Glial Cells

These non-neuronal cells function like a support staff to protect, insulate, and nourish the neurons.

Neurons Come in Three Types

1. Sensory Neurons2. Motor Neurons3. Interneurons

Overview of the Nervous System

Reflexes

Some signals bypass the central nervous system and travel directly from sensory neurons to the spinal cord, where they connect directly with motor neurons.

This is called a reflex.

Take-home message 23.2

In all vertebrates, the nervous system is divided into the peripheral nervous system and the central nervous system.

The neuron is a type of cell specialized for carrying electrical signals and is the building block of all nervous systems.

Take-home message 23.2

Each neuron is very small, but groups of neurons bundled together enable us to sense light, sound, touch, tastes, and smells and to respond to them.

23.3 Dendrites receive external stimuli.This occurs in two ways:

• Motor neurons and interneurons generally connect with and receive signals from other neurons.

• Sensory neuron dendrites, on the other hand, are modified to respond to a specific external stimulus such as a touch or sound, light, or a chemical.

Dendrite Action

Action Potential

At the cell body, signals from all of the dendrites of the neuron converge.

The cell body then integrates them.

If the sum total of signals coming in is positive, then the neuron initiates an action potential, an electrical signal that travels down its axon.

Gradations of Sensation

The intensity of the sensation an individual feels is modulated by the number of neurons that fire as a result of the stimulation.

Take-home message 23.3

Dendrites receive external stimuli in one of two ways.

Take-home message 23.3

Dendrites on motor neurons and interneurons generally connect with and receive signals from other neurons.

Sensory neuron dendrites are modified to respond to a specific external stimulus such as a touch or sound, light, or a chemical.

23.4 The action potential propagates a signal down the axon.

In response to an action potential, axon terminals release the contents of vesicles into the extracellular matrix, potentially influencing adjacent cells.

The Axon

Myelin Sheaths

Axons are insulated, by a fatty coating called the myelin sheath, preventing the action potential from weakening as it travels down the axon.

The Lack of Myelin

The lack of myelin on an axon can be seen in babies when they first start trying to walk.

At that time, myelin hasn’t completely formed around all of their axons and their gross motor control isn’t very good.

Take-home message 23.4

Each neuron has one axon, a projection that leaves the cell body and can extend several feet or more.

At its end, an axon branches into numerous axon terminals (terminal buttons), positioned close to a muscle cell or gland or the dendrites of another neuron.

Take-home message 23.4

In response to an action potential, the axon terminals release chemicals into the extracellular matrix, potentially influencing adjacent cells.

At the Synapse, Neurons Interact with Other Cells

The end of an axon—the axon terminal or terminal button—is always right next to another neuron or a muscle cell or a gland.

The point where they meet is called a synapse.

At the Synapse, Several Things Occur…

1. Sacs called vesicles release neurotransmitters into the synaptic cleft.

2. The neurotransmitter diffuses to nearby receptor sites.

At the Synapse, Several Things Occur…

3. The neurotransmitter attaches to postsynaptic receptors.

4. Gates open in the postsynaptic cell membrane.

At the Synapse, Several Things Occur…

5. Open gates enable the signal to pass to the postsynaptic cell.

6. Neurotransmitter is released from the postsynaptic cell receptors.

7. Neurotransmitter is recycled or broken down.

The Synapse

Like Call Screening for Your Brain

Why would it be useful to inhibit activity and effectively block information from being passed along?

• This option is not just useful, but essential to the nervous system’s capacity to control which signals get through.

Continual Stimulation

With continuous stimulation, too, most neurons gradually reduce the amount of neurotransmitter they release, and thus reduce the strength of the signal.

Take-home message 23.5

At the synapse, a neuron interacts with other cells.

Take-home message 23.5

In response to an action potential, neurotransmitters are released into the synaptic cleft, diffuse, and may bind to receptors on an adjacent neuron, muscle cell, or gland, potentially stimulating an action potential, muscle contraction, or secretion.

Take-home message 23.5

Neurotransmitters may then be taken back in by the axon terminal or enzymatically broken down in the synaptic cleft.

23.6 There are many types of neurotransmitters.

About 25 neurotransmitters have been identified, each of which has a sort of personality based on its specific actions.

Neurotransmitters

Four neurotransmitters are particularly important:

1. Acetylcholine2. Glutamate3. Dopamine 4. Serotonin

Acetylcholine

Acetylcholine is the neurotransmitter released by motor neurons at the point where they synapse with muscle cells.

When enough acetylcholine binds to a muscle cell, the muscle contracts.

CurareCurare works by blocking the receptor sites where acetylcholine binds to muscle cells.

• Once it gets into an animal’s system, curare causes death from asphyxiation, because it makes it impossible for the skeletal muscles to contract.

Glutamate

This is an excitatory neurotransmitter.

It appears to be involved with learning and memory.

Dopamine

Dopamine is important in initiating and coordinating movement.

Dopamine is also one of the body’s chief happiness neurotransmitters.

Serotonin

Serotonin generally functions as an inhibitory neurotransmitter.

It affects appetite, sleep, anxiety, and mood and produces feelings of contentment and satiation when released.

Take-home message 23.6

About 25 neurotransmitters have been identified, each of which has several actions, depending on the synapse where it occurs.

Take-home message 23.6

Acetylcholine is the neurotransmitter released by motor neurons at the point where they synapse with muscle cells.

Take-home message 23.6

Glutamate is an excitatory neurotransmitter involved with learning and memory.

Dopamine is important in initiating and coordinating movement and in producing feelings of intense pleasure.

Take-home message 23.6

Serotonin, an inhibitory neurotransmitter, affects appetite, sleep, anxiety, and mood and produces feelings of contentment and satiation.

23.7 Sensory receptors are our windows to the world around us.

Because the head of the animal encounters new things in the environment first, it makes more sense to put the sensory equipment there.

The SensesThe process by which all of our senses work is basically the same:• A dendrite on a sensory neuron is stimulated

by some aspect of the outside world.

Sensory Outcomes

In response, the sensory neuron will either:

1. Fire an action potential itself, or2. Alter its rate of neurotransmitter

secretion

Take-home message 23.7

The process by which all of our senses work is basically the same.

A modified dendrite on a sensory neuron is stimulated by some aspect of the outside world, causing the sensory neuron to fire an action potential or to alter its rate of neurotransmitter secretion.

TasteWithin each taste bud are 60–80 sensory receptor cells, called chemoreceptors, which are stimulated when chemicals in food dissolve in saliva and bind to proteins on the cell surface.

Taste Chemoreceptors Form Five Groups

Particular taste receptors allow only specific food molecules, with exactly the right shape, to bind.

The five groups are: sweet, salty, sour, bitter, and umami.

Not All Animals Sense Taste in the Same Way!

Some insects have chemoreceptors on their legs.

Other animals have taste receptors on their antennae or tentacles.

Taste Can Fool the Brain

If a molecule that is not sugar has a chemical structure closely resembling sugar, it can stimulate the same taste-bud receptors and be perceived by the brain as sugar.

Take-home message 23.8

On your tongue, there are about 10,000 taste buds, each of which contains 60–80 chemoreceptors, which are stimulated when particular chemicals in food bind to receptor proteins on the cell surface.

Take-home message 23.8

Some animals have chemoreceptors on their antennae or tentacles, and the binding of chemicals to these receptors similarly triggers an action potential that delivers a taste sensation to the brain.

23.9 Smell: Receptors in the nose detect airborne chemicals.

Our sense of smell works in almost exactly the same way as our sense of taste. Airborne chemicals move through

mucus in the nasal cavity and bind to the smell receptors.

This triggers action potentials that shoot down the axon, all the way into the smell center of the brain, where the signal is perceived as a particular odor.

Smell

Sense of Smell in Animals

Dogs are among the most smell-sensitive vertebrates, having as many as 40 times more smell receptors than a human.

• Hence, their tremendous proficiency at detecting drugs or explosives.

Take-home message 23.9

Neurons that can detect smells have dendrites modified with tiny, hair-like projections covered with chemoreceptors, densely packed within the nasal cavity.

23.10 Vision: Seeing is the perception of light by the brain. Despite the great diversity of

species with light-sensing capabilities, the basic functioning of light-absorbing cells is quite consistent.

The functions of these cells can be broken down into five steps.

Vision

Three Different Types of Eyes Have Evolved

1. Eye cups• Simplest• Able to detect the presence and intensity of

light.

2. Compound eyes

2. Single-lens eyes

Three Different Types of Eyes Have Evolved

Compound Eyes

Made of multiple, separate light-sensing units.

Due to a broader range of light-sensitive pigments, insects can use these eyes to see wavelengths invisible to us.

Single-lens Eye• Light enters through the iris.• A lens focuses the light onto the retina.• Impulses are transmitted via the optic

nerve to photosensitive cells called rods and cones.

Eye Location

The farther apart the eyes are, the wider the animal’s field of vision.

Take-home message 23.10

Vision results from the stimulation of light-sensitive sensory neurons, called photoreceptor cells.

The photoreceptor cells have a variety of molecules, embedded within their membranes, that are chemically altered by light.

Take-home message 23.10

Signals are conveyed to the brain and interpreted as an image.

The particular wavelength perceived depends on which version of the light-sensitive molecules in the photoreceptor cell membranes is stimulated.

23.11 Hearing: Sound waves are collected by the ears and stimulate auditory neurons. Sound waves, tiny fluctuations in air

pressure, are collected and amplified by the ears, which then pass the information to the brain.

Hearing is a result of the stimulation of mechanoreceptors, specialized neurons with receptors that respond to mechanical pressure.

Hearing Works in a Six-Step Process

Hearing Loss

Long-term exposure to loud noises, including music, can be damaging to hearing because such stimulation can wear out the inner ear.

Ears Aren’t Necessary for Hearing

Insects can “hear” with antennal hairs.

Insects can also generate communicative sounds by beating their wings or rubbing their wings together.

Bats Have a Unique System of Hearing

Echolocation is a sensitive call-and-response system of hearing.

• They do this by emitting high-pitched squeaks, and then waiting to hear the sounds as they bounce off surfaces around them.

Take-home message 23.11

Hearing occurs when sound waves cause the eardrum to vibrate, moving tiny bones that pass on the vibrations to the inner ear, where they bend hair cells and thus trigger a pattern of action potentials that varies according to the wavelengths of the sound. This is interpreted by the brain as sound.

23.12 Touch: The brain perceives pressure, temperature, and pain.

Touch is actually a class of sensations generated by numerous different types of sensory neurons that are sensitive to: pressure (mechanoreceptors), temperature (thermoreceptors), or pain (pain receptors)

The Sense of Touch

“Phantom Pain”Most amputees report that they sometimes experience pains that come from where their amputated limb once was.• These sensations are

real and occur when neurons in the brain that used to represent touch receptors in the lost limb are firing.

Take-home message 23.12

Touch is a class of sensations generated by mechanoreceptors, thermoreceptors, and pain receptors located throughout the body.

Take-home message 23.12

Stimulation of these receptors causes a change in the shape of the sensory neuron’s membrane, altering its permeability, generating action potentials, and causing the perception of touch by the brain.

23.13 Other senses help animals negotiate the world.

1. Balance and Motion2. Electricity3. Magnetism4. Heat

Magnetism and Migration

A wide range of species, from birds to eels and sharks to migrating beluga whales, and even some bacteria, are able to detect this magnetic field and use it to help them navigate.

Seeing Heat

Pythons, vipers, and some other snakes can “see” heat differences in their environments without having to touch it, using “pit organs” located on either side of the head.

Take-home message 23.13

Among animals, there are several senses in addition to the five found in humans, including those that perceive and respond to balance and motion, electricity, magnetism, and heat production in the animal’s environment.

23.14 Muscles generate force through contraction.There are three types of muscle tissue:

1. Skeletal muscle2. Cardiac muscle3. Smooth muscle

Skeletal Muscle is Moved by the Nervous System

Muscle Vocabulary

Each muscle fiber is a single cell, but with multiple nuclei and with numerous myofibrils.

The sarcomeres are composed of large numbers of long filaments, made from the proteins actin and myosin.

The Sarcomere Shortens ina Four-Step Process

1. Detach2. Reach3. Reattach4. Pull-back

Fast-Twitch vs. Slow-Twitch

When a muscle contracts, the duration between a contraction and a relaxation is called a twitch.

• There are slow-twitch and fast-twitch fibers.

Take-home message 23.14

Muscle tissue—including skeletal, cardiac, and smooth muscle—is made up of elongated cells capable of generating force when they contract.

Take-home message 23.14

A muscle fiber is a single cell, containing myofibrils that shorten with the making and breaking of links between parallel actin and myosin filaments.

23.15 Skeletal systems enable movement, among several other important functions.

The Vertebrate Skeleton Features an Axial and Appendicular Skeleton

Breakdown Can Occur in the Skeletal System

Osteoporosis is a disease that occurs as the density of bone is reduced and the chemical composition of the bones changes.

Take-home message 23.15

Among animals, three distinct types of skeletal systems occur: hydrostatic skeletons, exoskeletons, and endoskeletons.

Take-home message 23.15

The vertebrate skeletal system can enable movement, provide support, offer protection, provide a site for production of important blood and immune cells, and serve as a reservoir of minerals, including calcium.

23.16 The brain is organized into several distinct regions.

There are three principal regions of the brain:

1. The hindbrain2. The midbrain3. The forebrain

The Hindbrain

The top of the spinal cord is the beginning of the brain. Here, the spinal cord expands into the hindbrain.

• Three important structures are located in the hindbrain: the medulla, the pons, and the cerebellum.

The Midbrain

The midbrain, along with the medulla and pons of the hindbrain, makes up the brainstem.

The midbrain helps filter and evaluate the importance of each signal.

The Forebrain

In humans, the forebrain is the largest region of the brain.

It includes two control and relay structures, the thalamus and hypothalamus, as well as the cerebrum.

Responsible for most of what we consider “higher” thought, including perception, memory, language, intelligence, and personality.

The Pleasure Center

In the 1950s, James Olds inserted a tiny electrode into the hypothalamus of a rat’s brain.

• The animal seemed to experience great pleasure.

The Cerebral Cortex and Phineas Gage

The cerebral cortex is the most sophisticated part of the brain.

Gage suffered from severe damage to part of his frontal cortex.• It became clear that

something in Gage’s personality had changed dramatically as a result of the injury.

Regions of the Cerebral Cortex

Corpus Callosum

The brain is divided into a left and a right hemisphere, connected by a broad, thick band of neurons, called the corpus callosum.

Left-Brain vs. Right-Brain

The left-hemisphere is the site of areas specializing in language, logic, and mathematical skills.

The right hemisphere is more commonly home to larger areas dedicated to emotions, intuitive thinking, and artistic expression.

Limbic System

Within the forebrain, near the center of the brain, is a set of structures that together make up the limbic system.

The Limbic System

It includes parts of the cerebral cortex and two structures deeper within the center of the brain, the hippocampus and the amygdala.

It is responsible for many of our physiological drives and instincts.

Take-home message 23.16

There are several distinct regions in the brain, each of which is the control center for various activities in the body.

Take-home message 23.16

The hindbrain regulates basic physiological functions—such as respiration, heart rate, and digestion—and coordinates numerous types of motor activity.

The midbrain, part of the brainstem, helps filter and evaluate the importance of sensory and motor information.

Take-home message 23.16

The forebrain is the largest region of the brain, responsible for most of what we consider higher thought, including perception, memory, language, intelligence, and personality.

23.17 Measuring brain activity helps us understand how tasks are allocated.

Brain imaging technologies capture pictures of the brain as it functions, revealing activity patterns. These include:

PET (positron emission tomography) fMRI scanning (functional magnetic

resonance imaging)

PET ScansIn a PET scan, radioactive glucose is injected into the subject. The radioactivity can

then be traced in the brain as the subject performs a task.

Because areas with more metabolic activity require more fuel, more of the radioactive glucose moves to those areas.

fMRI Scans

In an fMRI scan, a giant magnet detects the relative amounts of oxygenated blood and deoxygenated blood in various parts of the brain.

When parts of the brain are active, more blood flows to them and the sensitive magnet can pick up the change.

Interesting Discoveries from Measuring Brain Activity

Seeing your favorite foods activates brain reward centers.

Altruistic behavior activates reward centers.

People with Alzheimer’s disease exhibit significantly reduced brain activity.

Take-home message 23.17

PET scans and fMRI scans make it possible to detect and visualize metabolic activity in brain tissue and to see how the brain works during various tasks.

These technologies help us understand how tasks are processed by different regions of the brain and how individuals vary in their patterns of brain activity for similar tasks.

23.18 Specific brain areas are involved in the processes of learning, language, and memory.

Language is simply the means of communication between individuals of any species.

It involves two areas of the brain:• Broca’s Area• Wernicke’s Area

Changing Accent

In 1999, an American woman named Tiffany Roberts had a stroke.

Upon recovery, she had developed a British accent!

It appeared that parts of her language production areas had been damaged by the stroke.

Memory The amygdala seems

to associate emotional feelings and sensory input with memories.

The hippocampus is also important in the storage and retrieval of memories, as well as the transfer of short-term memories.

A Good Memory . . .

Each fall, red squirrels hide more than 3,000 acorns and other nuts.

Come winter, they are able to remember the locations of more than 80% of those nuts and recover them for food.

Take-home message 23.18

Learning and memory, along with the use of complex language, are among the most sophisticated functions of the brain.

Take-home message 23.18

Although brain science is still in its infancy, we are gaining a clearer picture of the brain structures—primarily, for language, in the left frontal and left temporal lobes and, for learning and memory, in the amygdala and hippocampus—that are chiefly responsible for these functions.

23.19 Our nervous system can be tricked by chemicals.

Drugs—whether recreational or therapeutic, whether found in nature or made in the laboratory—can work by mimicking neurotransmitters.

Drugs

Examples include:

Cocaine Prozac and Zoloft Morphine and

Heroin Nicotine

Do human drug addictions and dependencies reflect differences

in our genes?The allele of the D4DR gene found among one-third of the smokers in a study was the same one that appears to cause individuals to exhibit a variety of personality traits associated with risk-taking and novelty-seeking behavior.

Take-home message 23.19

Our brain’s signaling system can be tricked by drugs, whether recreational or therapeutic, that mimic neurotransmitters.

Take-home message 23.19

Such drugs—including cocaine, Prozac, heroin, and nicotine—can produce euphoric sensations, can reduce depression, and can block pain, but the effects often come with significant health risks.

23.20 A brain slows down when it needs to sleep. Caffeine wakes it up.Worldwide, more caffeine-containing tea is consumed every day than any beverage other than water.

• A close second is coffee.

Caffeine Safety

Surprisingly, caffeine seems to be safe for most people. Despite considerable searching for ill

effects, there is no clear evidence that moderate consumption of caffeine increases our risk of anything beyond the occasional jitters.

Take-home message 23.20

Normal neural activity leads to a buildup of cellular waste products.

One of these, adenosine, fills adenosine receptors on nearby neurons, reducing the likelihood that a cell will initiate an action potential and causing fatigue.

Take-home message 23.20

Caffeine binds to the adenosine receptors, without reducing their likelihood of firing, thus blocking the fatigue-inducing message of adenosine.

23.21 Alcohol interferes with many different neurotransmitters.

Alcohol is a great neurotransmitter impersonator, fooling at least four different receptor molecules.

Alcohol

1. By blocking receptors for glutamate, one of the brain’s chief excitatory neurotransmitters, alcohol slows our reaction times and slurs our speech.

2. Acting like cocaine—but much weaker—alcohol blocks dopamine reuptake.

Alcohol Blocks Pain . . .

3. Resembling morphine and heroin in this respect, at a greatly reduced magnitude, alcohol spurs our body to produce a little opiate-like high.

. . . and Makes Us Happier

4. Much like Prozac, alcohol modifies and increases the efficiency of our serotonin receptors, increasing the contentment that accompanies serotonin release at synapses in the brain.

Effects of Alcohol

For most people, moderate alcohol consumption is pleasant and does not have significant health risks.

For some, however, alcohol abuse and alcoholism can lead to serious problems.

Long-Term Effects of Alcohol

These include increasing the risk for some types of cancer, increasing the risk of liver disease, and increasing the likelihood of harm to the fetus during pregnancy.

Take-home message 23.21

Alcohol affects the functioning of multiple neurotransmitters—including glutamate, endorphins, dopamine, and serotonin—slowing reaction times, slurring speech, blocking pain, and increasing contentment.