26-1

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 26Coordination

by Neural Signaling

26-2

Most Animals Have a Nervous System That Allows

Responses to Stimuli

26-3

26.1 Invertebrates reflect an evolutionary trend toward bilateral symmetry

and cephalization

Invertebrate Nervous Organization In simple animals, such as sponges, the most common

observable response is closure of the osculum (central opening) Hydras (cnidarians) have a nerve net that is composed of

neurons Planarians, (flatworms) have a ladderlike nervous system In annelids (earthworm), arthropods (crab), and molluscs (squid)

the nervous system shows further advances Cephalization - concentration of ganglia and sensory

receptors in a head region Ganglion (pl. ganglia) - cluster of neurons

26-4

Figure 26.1A Evolution of the nervous system

26-5

Figure 26.1A Evolution of the nervous system (continued)

26-6

Vertebrate Nervous Organization Cephalization, and bilateral symmetry, results in paired sensory

receptors to gather information about environment Eyes, ears, and olfactory structures

Central nervous system (CNS) Spinal cord and brain and develops from an embryonic dorsal neural

tube Ascending tracts carry sensory information to the brain, and descending

tracts carry motor commands to the neurons in the spinal cord that control the muscles

Vertebrate brain divided into three parts Hindbrain - most ancient part and regulates motor activity below the

level of consciousness Midbrain - optic lobes are part of the midbrain and was a center for

coordinating reflexes involving the eyes and ears Forebrain - originally dealt mainly with smell. Later, the thalamus

evolved to receive sensory input from the midbrain and the hindbrain and to pass it on to cerebrum

Cerebrum integrates sensory and motor input and is particularly associated with higher mental capabilities

26-7

Figure 26.1B Organization of the vertebrate brain

26-8

26.2 Humans have well-developed central and peripheral

nervous systems Peripheral nervous system (PNS) consists of all the

nerves and ganglia that lie outside the CNS All signals that enter and leave the CNS travel through paired

nerves Those connected to spinal cord are spinal nerves Those attached to brain are cranial nerves

Somatic sensory axons (fibers) send signals from the skin and special sense organs

Visceral sensory fibers convey information from the internal organs

CNS and PNS must work in harmony to carry out three primary functions Receive sensory input Perform integration Generate motor output

26-9

Figure 26.2 Organization of the nervous system in humans

26-10

Neurons Process and Transmit Information

26-11

26.3 Neurons are the functional units of a nervous system

Neurons (nerve cells) receive sensory information and convey information to an integration center Three major parts

Cell body - contains a nucleus and a variety of organelles Dendrites - short, highly branched processes receive signals from

sensory receptors or other neurons and transmit them to cell body Axon - portion of the neuron that conveys information to another

neuron or to other cells Axons bundle together to form nerves and are often called nerve

fibers Axons are covered by a white insulating layer called the myelin

sheath Neuroglia - cells that provide support and nourishment

to the neurons Myelin sheath is formed from membranes of tightly spiraled

neuroglia In PNS, Schwann cells perform this function, leaving gaps

called nodes of Ranvier, or neurofibril nodes

26-12

Types of Neurons

Motor (efferent) neurons carry nerve impulses from CNS to muscles or glands Have many dendrites and a single axon Cause muscle to contract or glands to secrete

Sensory (afferent) neurons take nerve impulses from sensory receptors to the CNS Sensory receptors may be the end of a sensory neuron itself (a

pain or touch receptor), or may be a specialized cell that forms a synapse with a sensory neuron

Interneurons (association neurons) occur entirely within the CNS Parallel the structure of motor neurons and convey nerve

impulses between various parts of the CNS

26-13

Figure 26.3A Motor neuron

26-14

Figure 26.3B Sensory neuron

26-15

Figure 26.3C Interneuron

26-16

26.4 Neurons have a resting potential across their membranes

when they are not active Voltage, in millivolts (mV), is a measure of the electrical

potential difference between two points In the case of a neuron, the two points are the inside and the

outside of the axon Membrane potential - when an electrical potential

difference exists between the inside and outside of a cell When a neuron is not conducting an impulse, its resting

potential is about −65 mV Negative sign indicates that the inside of the cell is more

negative than the outside Potential is created by an unequal distribution of ions

Due to the activity of the sodium-potassium pump, which moves three sodium ions (Na+) out of the neuron for every two potassium ions (K+), it moves into the neuron

26-17

Figure 26.4 Resting potential: More Na+ outside the axon and more K+ inside the axon. Inside is −65 mV, relative to the outside

26-18

26.5 Neurons have an action potential across axon membranes

when they are active Action potential - rapid change in polarity across axonal

membrane as nerve impulse occurs An action potential uses two types of gated ion channels

in the axon membrane First, a gated ion channel allows sodium (Na+) to pass into the

axon Another gated ion channel allows potassium (K+) to pass out of

the axon If a stimulus causes the axon membrane to depolarize to

threshold, an action potential occurs in an all-or-none manner

26-19

Figure 26.5A An action potential can be visualized as voltage changes over time

26-20

Depolarization and Repolarization

Sodium Gates Open When an action potential begins, the gates of the

sodium channels open, and Na+ flows into the axon Membrane potential changes from −65 mV to +40 mV

Called depolarization because inside axon changes from negative to positive

Potassium Gates Open Gates of potassium channels open, and K+ flows out

of axon Action potential changes from +40 mV back to −65 mV

Called repolarization because inside of axon becomes negative again as K+ exits the axon

26-21

Figure 26.5B Action potential begins: Depolarization to +40 mV as Na+ gates open and Na+ moves to inside the axon

26-22

Figure 26.5C Action potential ends: Repolarization to −65 mV as K+ gates open and K+ moves to outside the axon

26-23

26.6 Propagation of an action potential is speedy

In nonmyelinated axons, action potential travels down an axon one section at a time, at a speed of about 1 m/second As soon as an action potential has moved on, the previous

section undergoes a refractory period, during which the Na+ gates are unable to open

The action potential cannot move backward and instead always moves down an axon toward its terminals

After refractory period, the sodium potassium pump restores the previous ion distribution by pumping Na+ to outside the axon and K+ to inside the axon

In myelinated axons, gated ion channels that produce action potential are concentrated at the nodes of Ranvier Ion exchange only at the nodes makes the action potential travel

faster in nonmyelinated axons called saltatory conduction

26-24

Figure 26.6 Saltatory conduction

26-25

26.7 Communication between neurons occurs at synapses

Every axon branches into many fine endings, tipped by a small swelling, called an axon terminal Each terminal lies very close to the dendrite (or the

cell body) of another neuron Region of close proximity is called a synapse

At synapse, membrane of first neuron is presynaptic The membrane of the next neuron is the postsynaptic Small gap between the neurons is the synaptic cleft

A nerve impulse cannot cross a synaptic cleft Transmission across a synapse is carried out by

molecules called neurotransmitters, which are stored in synaptic vesicles

26-26

Figure 26.7 Synapse structure and function

26-27

26.8 Neurotransmitters can be stimulatory or inhibitory

Acetylcholine (ACh) and norepinephrine (NE) are well-known neurotransmitters in both the CNS and the PNS In the PNS, ACh excites skeletal muscle but inhibits cardiac

muscle ACh has either an excitatory or inhibitory effect on smooth

muscle or glands In the CNS, NE is important to dreaming, waking, and

mood Serotonin, another neurotransmitter, is involved in

thermoregulation, sleeping, emotions, and perception Clearing of Neurotransmitter from a Synapse

Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft

Short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of postsynaptic membranes

26-28

26.9 Integration is a summing up of stimulatory and inhibitory signals

A single neuron can have many synapses all over its dendrites and the cell body A neuron is on the receiving end of many excitatory and

inhibitory signals An excitatory neurotransmitter produces a signal that drives the

neuron closer to threshold An inhibitory neurotransmitter produces a signal that drives the

neuron further from threshold Integration is the summing up of excitatory and

inhibitory signals If a neuron receives many excitatory signals the axon will

transmit a nerve impulse If a neuron receives both inhibitory and excitatory signals, the

summing up of these signals may prohibit the axon from reaching threshold and firing

26-29

Figure 26.9 Synaptic integration

26-30

APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 26.10 Drugs that interfere with neurotransmitter

release or uptake may be abused Alcohol

Acts as a depressant on many parts of the brain where it affects neurotransmitter release or uptake

Increases the action of GABA, which inhibits motor neurons, and increases the release of endorphins

Nicotine Binds to neurons, causing the release of dopamine, neurotransmitter

that promotes a sense of pleasure and is involved in motor control In the PNS, nicotine is a stimulant by mimicking acetylcholine increasing

heart rate, blood pressure, and muscle activity Club and Date Rape Drugs

Structure of methamphetamine is similar to that of dopamine, and its stimulatory effect mimics that of cocaine

Ecstasy has an overstimulatory effect on neurons that produce serotonin, which, like dopamine, elevates our mood

Cocaine Powerful stimulant in CNS, interferes with re-uptake of dopamine at

synapses Result is a rush of well-being that lasts from 5 to 30 minutes

26-31

Heroin, Marijuana and Treatment for Addictive Drugs

Heroin Highly addictive drug that acts as a depressant in the nervous system Come from opium poppy plant, thus called opiates

Opiates depress breathing, block pain pathways, cloud mental function, and cause nausea

Marijuana (THC) THC may mimic the actions of anandamide, a neurotransmitter When THC reaches CNS, person experiences euphoria, with alterations

in vision and judgment In heavy users, hallucinations, anxiety, depression, body image distortions,

paranoia, and psychotic symptoms can result Treatment for Addictive Drugs

Mainly consists of behavior modification Heroin addiction can be treated with synthetic opiate compounds, such

as methadone that decrease withdrawal symptoms and block heroin’s effects

New treatment techniques include the administration of antibodies to block the effects of cocaine and methamphetamine

26-32

Figure 26.10 Drug use

26-33

The Vertebrate Central Nervous System (CNS) Consists of the

Spinal Cord and Brain

26-34

26.11 The human spinal cord and brain function together

CNS consists of the spinal cord and the brain, where sensory information is received and motor control is initiated Spinal cord and brain are wrapped in three protective membranes known as

meninges Spaces between meninges filled with cerebrospinal fluid that protects CNS

Spinal Cord - bundle of nervous tissue enclosed in vertebral column Extends from base of the brain to the vertebrae just below the rib cage Two main functions

Center for many reflex actions, automatic responses to external stimuli Provides a means of communication between the brain and the spinal nerves, which

leave the spinal cord Central portion of gray matter and a peripheral region of white matter

Gray matter consists of cell bodies and unmyelinated fibers Myelinated long fibers of interneurons that run together in bundles called tracts

give white matter its color Tracts connect the spinal cord to the brain

Brain Ventricles - brain contains four interconnected chambers called ventricles Two lateral ventricles are inside the cerebrum Third ventricle is surrounded by the diencephalon, and the fourth ventricle lies

between the cerebellum and the pons

26-35

Figure 26.11 The human brain

26-36

26.12 The cerebrum performs integrative activities

Cerebrum - largest portion of the brain in humans Last center to receive sensory input and carry out integration before

commanding voluntary motor responses Cerebral Hemispheres - brain is divided into two halves

Each hemisphere receives information from and controls the opposite side of the body

The two are connected by a bridge of tracts within the corpus callosum The Cerebral Cortex

A thin, but highly convoluted, outer layer of gray matter that covers the cerebral hemispheres

Primary motor area is in the frontal lobe and is where voluntary commands to skeletal muscles begin

Primary somatosensory area is in the parietal lobe and sensory information from the skin and skeletal muscles arrives here

Basal Nuclei Integrate motor commands, ensure proper muscle groups activated Huntington disease and Parkinson disease are believed to be due to

malfunctioning basal nuclei

26-37

Figure 26.12 The lobes of a cerebral hemisphere

26-38

26.13 The other parts of the brain have specialized functions

Hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle Hypothalamus forms the floor of the third ventricle Thalamus consists of two masses of gray matter located in the sides and

roof of the third ventricle It is on the receiving end for all sensory input except smell

Pineal gland, which secretes the hormone melatonin, is located in the diencephalon

Cerebellum lies under the cerebrum and is separated from the brain stem by the fourth ventricle

Receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts, and it also receives motor output from the cerebral cortex

Brain stem contains the midbrain, the pons, and the medulla oblongata Midbrain acts as a relay station for tracts passing between the cerebrum

and the spinal cord or cerebellum Pons contains bundles of axons traveling between cerebellum and rest of CNS Medulla oblongata contains a number of reflex centers for regulating

heartbeat, breathing, and blood pressure

26-39

Figure 26.13 The reticular activating system

26-40

26.14 The limbic system is involved in memory and learning as well as in emotions

Limbic system - complex network of tracts and nuclei that incorporates portions of the cerebral lobes, the basal nuclei, and diencephalon Blends higher mental functions and primitive emotions Two significant structures

Hippocampus - well situated in brain to make the frontal lobe aware of past experiences stored in various sensory areas

Amygdala - in particular, adds emotional overtones Learning and Memory

Memory is the ability to hold a thought in mind or recall events from the past

Frontal lobe is active during short-term memory A gradual extinction of brain cells, particularly in the

hippocampus, appears to be the underlying cause of Alzheimer disease (AD)

26-41

Figure 26.14 The limbic system (in purple)

26-42

The Vertebrate Peripheral Nervous System (PNS) Consists of Nerves

26-43

26.15 The peripheral nervous system contains cranial and spinal nerves

The peripheral nervous system (PNS) lies outside the central nervous system and contains nerves, bundles of axons Cranial nerves are attached to the brain

Some are motor nerves that contain only motor fibers, and others are mixed nerves that contain both sensory and motor fibers

Spinal nerves are attached to spinal cord A spinal nerve separates the axons of sensory neurons from

the axons of motor neurons Cell body of a sensory neuron is in the dorsal root ganglion Each spinal nerve serves the particular region of the body in

which it is located

26-44

Figure 26.15A Anatomy of a nerve

26-45

Figure 26.15B Ventral surface of brain showing the attachment of the cranial nerves (yellow)

26-46

26.16 In the somatic system, reflexes allow us to respond quickly to stimuli

Somatic system nerves serve the skin, joints, and skeletal muscles Includes nerves that take

Sensory information from external sensory receptors in the skin and joints to the CNS

Motor commands away from the CNS to the skeletal muscles Acetylcholine (ACh) active in somatic system

Reflexes - involuntary responses to stimuli Involve either the brain or just the spinal cord Enable the body to react swiftly to stimuli that could

disrupt homeostasis

26-47

Figure 26.16 A reflex arc showing the path of a spinal reflex

26-48

26.17 In the autonomic system, the parasympathetic and sympathetic divisions control the actions of internal organs

Autonomic system - automatically and involuntarily regulates the activity of glands and cardiac and smooth muscle

Divided into parasympathetic and sympathetic Parasympathetic Division

Includes a few cranial nerves as well as axons that arise from the last portion of the spinal cord

Promotes all the internal responses we associate with a relaxed state

Example: causes the pupil of the eye to constrict, promotes digestion of food, and retards the heartbeat

Sympathetic Division Axons arise from portions of the spinal cord Important during emergency situations and is associated with fight or

flight Example: accelerates the heartbeat and dilates the bronchi, while at the

same time it inhibits the digestive tract

26-49

Figure 26.17 Autonomic system

26-50

Connecting the Concepts:Chapter 26

The human nervous system has just three functions: sensory input, integration, and motor output

The central nervous system (CNS) carries out the function of integrating incoming data The brain allows us to perceive our environment, to reason, and to

remember After sensory data have been processed by the CNS, motor output

occurs Muscles and glands are the effectors that allow us to respond to the

original stimuli The human peripheral nervous system (PNS) contains nerves that

carry sensory input to the CNS and motor output to the muscles and glands

There is a division of labor among the nerves The cranial nerves serve the face, teeth, and mouth; below the head,

there is only one cranial nerve, the vagus nerve All body movements are controlled by spinal nerves, and this is why

paralysis may follow a spinal injury