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Overview: Lines of Communication

• The cone snail kills prey with venom that disables neurons

• Neurons are nerve cells that transfer information within the body

• Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance)

© 2011 Pearson Education, Inc.

• Interpreting signals in the nervous system involves sorting a complex set of paths and connections

• Processing of information takes place in simple clusters of neurons called ganglia or a more complex organization of neurons called a brain


Concept 48.1: Neuron organization and

structure reflect function in information

transfer

• The squid possesses extremely large nerve cells

and has played a crucial role in the discovery of

how neurons transmit signals


Introduction to Information Processing

• Nervous systems process information in three

stages: sensory input, integration, and motor

output


Ganglia

Brain

Arm

Nerve Eye Mantle

Nerves with giant axons

Figure 48.2

• Sensors detect external stimuli and internal conditions and transmit information along sensory neurons

• Sensory information is sent to the brain or ganglia, where interneurons integrate the information

• Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity


• Many animals have a complex nervous system that consists of

– A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord

– A peripheral nervous system (PNS), which carries information into and out of the CNS

– The neurons of the PNS, when bundled together, form nerves


Figure 48.3

Sensor

Effector

Sensory input

Motor output

Integration

Peripheral nervous

system (PNS)

Central nervous

system (CNS)

Neuron Structure and Function

• Most of a neuron’s organelles are in the cell body

• Most neurons have dendrites, highly branched extensions that receive signals from other neurons

• The axon is typically a much longer extension that transmits signals to other cells at synapses

• The cone-shaped base of an axon is called the axon hillock


Figure 48.4

Nucleus

Dendrites Stimulus

Axon hillock

Cell

body Presynaptic

cell Signal

direction

Axon

Synapse

Neurotransmitter

Synaptic terminals

Postsynaptic cell

Synaptic

terminals

• The synaptic terminal of one axon passes

information across the synapse in the form of

chemical messengers called neurotransmitters

• A synapse is a junction between an axon and

another cell


• Information is transmitted from a presynaptic

cell (a neuron) to a postsynaptic cell (a neuron,

muscle, or gland cell)

• Most neurons are nourished or insulated by cells

called glia


Dendrites

Axon

Cell

body

Portion

of axon

Sensory neuron Interneurons Motor neuron

Figure 48.5

Figure 48.6

Glia

80 m

Cell bodies of neurons

Concept 48.2: Ion pumps and ion channels

establish the resting potential of a neuron

• Every cell has a voltage (difference in electrical

charge) across its plasma membrane called a

membrane potential

• The resting potential is the membrane potential

of a neuron not sending signals

• Changes in membrane potential act as signals,

transmitting and processing information


Formation of the Resting Potential

• In a mammalian neuron at resting potential, the

concentration of K+ is highest inside the cell,

while the concentration of Na+ is highest outside

the cell

• Sodium-potassium pumps use the energy of

ATP to maintain these K+ and Na+ gradients

across the plasma membrane

• These concentration gradients represent

chemical potential energy


• The opening of ion channels in the plasma

membrane converts chemical potential to electrical

potential

• A neuron at resting potential contains many open

K+ channels and fewer open Na+ channels; K+

diffuses out of the cell

• The resulting buildup of negative charge within the

neuron is the major source of membrane potential



Animation: Resting

Potential Right-click slide / select

“Play”

Table 48.1

Key Na

K

Sodium- potassium pump

Potassium channel

Sodium channel

OUTSIDE OF CELL

INSIDE OF CELL

Figure 48.7

Modeling the Resting Potential

• Resting potential can be modeled by an artificial membrane that separates two chambers

– The concentration of KCl is higher in the inner chamber and lower in the outer chamber

– K+ diffuses down its gradient to the outer chamber

– Negative charge (Cl–) builds up in the inner chamber

• At equilibrium, both the electrical and chemical gradients are balanced


Figure 48.8

Inner chamber

90 mV 62 mV Outer chamber

Inner chamber

Outer chamber

140 mM

KCl

150 mM

NaCl 5 mM

KCl

15 mM

NaCl

Potassium channel

Sodium channel Artificial

membrane

K Na Cl

Cl

(a) Membrane selectively permeable to K

(b) Membrane selectively permeable to Na

EK 62 mV 90 mV ENa 62 mV 62 mV

Figure 48.8a

Inner chamber

90 mV Outer chamber

140 mM

KCl

5 mM

KCl

Potassium channel

Artificial membrane

K Cl

(a) Membrane selectively permeable to K

EK 62 mV 90 mV

• The equilibrium potential (Eion) is the membrane voltage for a particular ion at equilibrium and can be calculated using the Nernst equation

Eion = 62 mV (log[ion]outside/[ion]inside)

• The equilibrium potential of K+ (EK) is negative, while the equilibrium potential of Na+ (ENa) is positive


• In a resting neuron, the currents of K+ and Na+

are equal and opposite, and the resting potential

across the membrane remains steady


Figure 48.8b

62 mV Inner chamber

Outer chamber

150 mM

NaCl

15 mM

NaCl

Sodium channel

Na

Cl

(b) Membrane selectively permeable to Na

ENa 62 mV 62 mV

Group 1 material ends here

Concept 48.3: Action potentials are the

signals conducted by axons

• Changes in membrane potential occur because

neurons contain gated ion channels that open

or close in response to stimuli


Figure 48.9

Microelectrode

Voltage recorder

Reference electrode

TECHNIQUE

• When gated K+ channels open, K+ diffuses out,

making the inside of the cell more negative

• This is hyperpolarization, an increase in

magnitude of the membrane potential


Hyperpolarization and Depolarization

Stimulus

Threshold

Resting potential

Hyperpolarizations

Time (msec)

50

0

50

100 1 0 2 3 4 5

50

0

50

100

50

0

50

100

Time (msec)

1 0 2 3 4 5 Time (msec) 1 0 2 3 4 5 6

Threshold

Resting potential

Threshold

Resting potential

Stimulus Strong depolarizing stimulus

Action potential

Depolarizations

Me

mb

ran

e p

ote

nti

al

(mV

)

Me

mb

ran

e p

ote

nti

al

(mV

)

Me

mb

ran

e p

ote

nti

al

(mV

)

(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K

(b) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to Na

(c) Action potential triggered by a depolarization that reaches the threshold

Figure 48.10

Stimulus

Threshold

Resting potential

Hyperpolarizations

Time (msec)

50

0

50

100 1 0 2 3 4 5

Mem

bra

ne

po

ten

tial

(mV

)

(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K

Figure 48.10a

• Opening other types of ion channels triggers a

depolarization, a reduction in the magnitude of

the membrane potential

• For example, depolarization occurs if gated Na+

channels open and Na+ diffuses into the cell


Stimulus

Threshold

Resting potential

Depolarizations

Time (msec)

50

0

50

100 1 0 2 3 4 5

Me

mb

ran

e p

ote

nti

al

(mV

)

(b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na

Figure 48.10b

• Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus

• These are not the nerve signals that travel along axons, but they do have an effect on the generation of nerve signals


Graded Potentials and Action Potentials

• If a depolarization shifts the membrane potential sufficiently, it results in a massive change in membrane voltage called an action potential

• Action potentials have a constant magnitude, are all-or-none, and transmit signals over long distances

• They arise because some ion channels are voltage-gated, opening or closing when the membrane potential passes a certain level


Strong depolarizing stimulus

Threshold

Resting potential

Time (msec)

50

0

50

100 1 0 2 3 4 5

Mem

bra

ne

po

ten

tial

(mV

)

6

(c) Action potential triggered by a depolarization that reaches the threshold Action

potential

Figure 48.10c

Generation of Action Potentials: A Closer

Look

• An action potential can be considered as a series of stages

• At resting potential

1. Most voltage-gated sodium (Na+) channels are closed; most of the voltage-gated potassium (K+) channels are also closed


OUTSIDE OF CELL

INSIDE OF CELL

Inactivation loop

Sodium channel

Potassium channel

Threshold

Resting potential

Time M

em

bra

ne p

ote

nti

al

(mV

)

50

100

50

0

Na

K

Key

1

1

Resting state

Figure 48.11-1

• When an action potential is generated

2. Voltage-gated Na+ channels open first and Na+

flows into the cell

3. During the rising phase, the threshold is

crossed, and the membrane potential increases

4. During the falling phase, voltage-gated Na+

channels become inactivated; voltage-gated K+

channels open, and K+ flows out of the cell


OUTSIDE OF CELL

INSIDE OF CELL

Inactivation loop

Sodium channel

Potassium channel

Threshold

Resting potential

Time M

em

bra

ne p

ote

nti

al

(mV

)

50

100

50

0

Na

K

Key

2

1

1

2

Resting state

Depolarization

Figure 48.11-2

OUTSIDE OF CELL

INSIDE OF CELL

Inactivation loop

Sodium channel

Potassium channel

Action potential

Threshold

Resting potential

Time M

em

bra

ne p

ote

nti

al

(mV

)

50

100

50

0

Na

K

Key

2

1

3

1

2

3

Resting state

Depolarization

Rising phase of the action potential

Figure 48.11-3

OUTSIDE OF CELL

INSIDE OF CELL

Inactivation loop

Sodium channel

Potassium channel

Action potential

Threshold

Resting potential

Time M

em

bra

ne p

ote

nti

al

(mV

)

50

100

50

0

Na

K

Key

2

1

3 4

1

2

3

4

Resting state

Depolarization

Rising phase of the action potential Falling phase of the action potential

Figure 48.11-4

5. During the undershoot, membrane

permeability to K+ is at first higher than at rest,

then voltage-gated K+ channels close and

resting potential is restored


OUTSIDE OF CELL

INSIDE OF CELL

Inactivation loop

Sodium channel

Potassium channel

Action potential

Threshold

Resting potential

Time M

em

bra

ne p

ote

nti

al

(mV

)

50

100

50

0

Na

K

Key

2

1

3 4

5

1

2

3

4

5 1

Resting state Undershoot

Depolarization

Rising phase of the action potential Falling phase of the action potential

Figure 48.11-5

Figure 48.11a

Action potential

Threshold

Resting potential

Time

Mem

bra

ne p

ote

nti

al

(mV

)

50

100

50

0

1

2

3

4

5 1

• During the refractory period after an action

potential, a second action potential cannot be

initiated

• The refractory period is a result of a temporary

inactivation of the Na+ channels


Conduction of Action Potentials

• At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane

• Action potentials travel in only one direction: toward the synaptic terminals


• Inactivated Na+ channels behind the zone of

depolarization prevent the action potential from

traveling backwards


Figure 48.12-3

K

K

K

K

Na

Na

Na

Action potential

Axon

Plasma membrane

Cytosol

Action potential

Action potential

2

1

3

Evolutionary Adaptation of Axon Structure

• The speed of an action potential increases with

the axon’s diameter

• In vertebrates, axons are insulated by a myelin

sheath, which causes an action potential’s

speed to increase

• Myelin sheaths are made by glia—

oligodendrocytes in the CNS and Schwann

cells in the PNS


Axon Myelin sheath

Schwann cell

Nodes of Ranvier

Node of Ranvier

Layers of myelin

Axon

Schwann cell Nucleus of Schwann cell

0.1 m

Figure 48.13

• Action potentials are formed only at nodes of

Ranvier, gaps in the myelin sheath where

voltage-gated Na+ channels are found

• Action potentials in myelinated axons jump

between the nodes of Ranvier in a process

called saltatory conduction


Cell body

Schwann cell

Depolarized region (node of Ranvier)

Myelin sheath Axon

Figure 48.14

Concept 48.4: Neurons communicate with

other cells at synapses

• At electrical synapses, the electrical current

flows from one neuron to another

• At chemical synapses, a chemical

neurotransmitter carries information across the

gap junction

• Most synapses are chemical synapses


• The presynaptic neuron synthesizes and

packages the neurotransmitter in synaptic

vesicles located in the synaptic terminal

• The action potential causes the release of the

neurotransmitter

• The neurotransmitter diffuses across the

synaptic cleft and is received by the

postsynaptic cell


Presynaptic cell

Postsynaptic cell

Axon

Presynaptic membrane

Synaptic vesicle containing neurotransmitter

Postsynaptic membrane

Synaptic cleft

Voltage-gated Ca2 channel

Ligand-gated ion channels

Ca2

Na

K

2

1

3

4

Figure 48.15

Generation of Postsynaptic Potentials

• Direct synaptic transmission involves binding of

neurotransmitters to ligand-gated ion channels

in the postsynaptic cell

• Neurotransmitter binding causes ion channels to

open, generating a postsynaptic potential


• Postsynaptic potentials fall into two categories

– Excitatory postsynaptic potentials (EPSPs)

are depolarizations that bring the membrane

potential toward threshold

– Inhibitory postsynaptic potentials (IPSPs) are

hyperpolarizations that move the membrane

potential farther from threshold


• After release, the neurotransmitter

– May diffuse out of the synaptic cleft

– May be taken up by surrounding cells

– May be degraded by enzymes


Summation of Postsynaptic Potentials

• Most neurons have many synapses on their

dendrites and cell body

• A single EPSP is usually too small to trigger an

action potential in a postsynaptic neuron


Figure 48.16

Postsynaptic neuron

Synaptic terminals of presynaptic neurons

5

m

Figure 48.17

Terminal branch of presynaptic neuron

Postsynaptic neuron

Axon hillock

E1

E2

E1

E2

E1

E2

E1

E2

I I I I

0

70

Me

mb

ran

e p

ote

nti

al (m

V)

Threshold of axon of postsynaptic neuron

Resting potential

Action potential

Action potential

I E1 E1 E1 E1 E1 E2 E1 I

Subthreshold, no summation

(a) (b) Temporal summation (c) Spatial summation Spatial summation of EPSP and IPSP

(d)

E1

• If two EPSPs are produced in rapid succession, an effect called temporal summation occurs


Terminal branch of presynaptic neuron

Postsynaptic neuron

Axon hillock

E1

E2

E1

E2

I I

0

70

Mem

bra

ne p

ote

nti

al (m

V)

Threshold of axon of postsynaptic neuron

Resting potential

Action potential

E1 E1 E1 E1

Subthreshold, no summation

(a) (b) Temporal summation

Figure 48.17a

• In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

• The combination of EPSPs through spatial and temporal summation can trigger an action potential


E1

E2

E1

E2

I I

Action potential

I E1 E2 E1 I

(c) Spatial summation Spatial summation of EPSP and IPSP

(d)

E1

Figure 48.17b

• Through summation, an IPSP can counter the effect of an EPSP

• The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential

Modulated Signaling at Synapses

• In some synapses, a neurotransmitter binds to a

receptor that is metabotropic

• In this case, movement of ions through a

channel depends on one or more metabolic

steps


• Binding of a neurotransmitter to a metabotropic

receptor activates a signal transduction pathway

in the postsynaptic cell involving a second

messenger

• Compared to ligand-gated channels, the effects

of second-messenger systems have a slower

onset but last longer


Neurotransmitters

• There are more than 100 neurotransmitters,

belonging to five groups: acetylcholine, biogenic

amines, amino acids, neuropeptides, and gases

• A single neurotransmitter may have more than a

dozen different receptors


Table 48.2

Acetylcholine

• Acetylcholine is a common neurotransmitter in vertebrates and invertebrates

• It is involved in muscle stimulation, memory formation, and learning

• Vertebrates have two major classes of acetylcholine receptor, one that is ligand gated and one that is metabotropic


Amino Acids

• Amino acid neurotransmitters are active in the

CNS and PNS

• Known to function in the CNS are

– Glutamate

– Gamma-aminobutyric acid (GABA)

– Glycine


Biogenic Amines

• Biogenic amines include

– Epinephrine

– Norepinephrine

– Dopamine

– Serotonin

• They are active in the CNS and PNS


Neuropeptides

• Several neuropeptides, relatively short chains of amino acids, also function as neurotransmitters

• Neuropeptides include substance P and endorphins, which both affect our perception of pain

• Opiates bind to the same receptors as endorphins and can be used as painkillers


Figure 48.18

Radioactive naloxone

Drug

Protein mixture

Radioactive naloxone and a test drug are incubated with a protein mixture.

Proteins are trapped on a filter. Bound naloxone is detected by measuring radioactivity.

EXPERIMENT

RESULTS

2

1

Drug Opiate Concentration That Blocked

Naloxone Binding

Morphine

Methadone

Levorphanol

Phenobarbital

Atropine

Serotonin

Yes

Yes

Yes

No

No

No

6 109 M

2 108 M

2 109 M

No effect at 104 M

No effect at 104 M

No effect at 104 M

Radioactive naloxone

Drug

Protein mixture

Radioactive naloxone and a test drug are incubated with a protein mixture.

Proteins are trapped on a filter. Bound naloxone is detected by measuring radioactivity.

EXPERIMENT

2

1

Figure 48.18a

RESULTS

Drug Opiate Concentration That Blocked

Naloxone Binding

Morphine

Methadone

Levorphanol

Phenobarbital

Atropine

Serotonin

Yes

Yes

Yes

No

No

No

6 109 M

2 108 M

2 109 M

No effect at 104 M

No effect at 104 M

No effect at 104 M

Figure 48.18b

Gases

• Gases such as nitric oxide and carbon monoxide

are local regulators in the PNS


Overview: The Body’s Long-Distance

Regulators

• Animal hormones are chemical signals that are

secreted into the circulatory system and

communicate regulatory messages within the

body

• Hormones reach all parts of the body, but only

target cells have receptors for that hormone

• Insect metamorphosis is regulated by hormones


• Two systems coordinate communication throughout the body: the endocrine system and the nervous system

• The endocrine system secretes hormones that coordinate slower but longer-acting responses including reproduction, development, energy metabolism, growth, and behavior

• The nervous system conveys high-speed electrical signals along specialized cells called neurons; these signals regulate other cells


Concept 45.1: Hormones and other

signaling molecules bind to target receptors,

triggering specific response pathways

• Endocrine signaling is just one of several ways

that information is transmitted between animal

cells


Intercellular Communication

• The ways that signals are transmitted between

animal cells are classified by two criteria

– The type of secreting cell

– The route taken by the signal in reaching its

target


Endocrine Signaling

• Hormones secreted into extracellular fluids by

endocrine cells reach their targets via the

bloodstream

• Endocrine signaling maintains homeostasis,

mediates responses to stimuli, regulates growth

and development


Figure 45.2

(a) Endocrine signaling

Blood vessel Response

Response

Response

Synapse

Response

Response

(b) Paracrine signaling

(c) Autocrine signaling

Neuron

(d) Synaptic signaling

Neurosecretory cell

Blood vessel

(e) Neuroendocrine signaling

Paracrine and Autocrine Signaling

• Local regulators are molecules that act over

short distances, reaching target cells solely by

diffusion

• In paracrine signaling, the target cells lie near

the secreting cells

• In autocrine signaling, the target cell is also the

secreting cell


Figure 45.2a

(a) Endocrine signaling

Blood vessel Response

Response

Response

(b) Paracrine signaling

(c) Autocrine signaling

Synaptic and Neuroendocrine Signaling

• In synaptic signaling, neurons form specialized

junctions with target cells, called synapses

• At synapses, neurons secrete molecules called

neurotransmitters that diffuse short distances

and bind to receptors on target cells

• In neuroendocrine signaling, specialized

neurosecretory cells secrete molecules called

neurohormones that travel to target cells via the

bloodstream


Figure 45.2b

Synapse

Response

Response

Neuron

(d) Synaptic signaling

Neurosecretory cell

Blood vessel

(e) Neuroendocrine signaling

Signaling by Pheromones

• Members of the same animal species sometimes

communicate with pheromones, chemicals that

are released into the environment

• Pheromones serve many functions, including

marking trails leading to food, defining territories,

warning of predators, and attracting potential

mates


Endocrine Tissues and Organs

• In some tissues, endocrine cells are grouped

together in ductless organs called endocrine

glands

• Endocrine glands secrete hormones directly into

surrounding fluid

• These contrast with exocrine glands, which have

ducts and which secrete substances onto body

surfaces or into cavities


Figure 45.4 Major endocrine glands:

Hypothalamus

Pineal gland

Pituitary gland

Thyroid gland

Parathyroid glands (behind thyroid)

Adrenal glands (atop kidneys)

Pancreas

Ovaries (female)

Testes (male)

Organs containing endocrine cells:

Thymus

Heart

Liver

Stomach

Kidneys

Small intestine

Chemical Classes of Hormones

• Three major classes of molecules function as

hormones in vertebrates

– Polypeptides (proteins and peptides)

– Amines derived from amino acids

– Steroid hormones


• Lipid-soluble hormones (steroid hormones) pass

easily through cell membranes, while water-

soluble hormones (polypeptides and amines) do

not

• The solubility of a hormone correlates with the

location of receptors inside or on the surface of

target cells


Lipid-soluble (hydrophobic) Water-soluble (hydrophilic)

Polypeptides Steroids

0.8 nm Insulin Cortisol

Amines

Epinephrine Thyroxine

Figure 45.5

Cellular Response Pathways

• Water- and lipid-soluble hormones differ in their paths through a body

• Water-soluble hormones are secreted by exocytosis, travel freely in the bloodstream, and bind to cell-surface receptors

• Lipid-soluble hormones diffuse across cell membranes, travel in the bloodstream bound to transport proteins, and diffuse through the membrane of target cells


Figure 45.6-2

Lipid- soluble hormone

SECRETORY CELL

Water- soluble hormone

VIA BLOOD

Signal receptor

TARGET CELL

OR

Cytoplasmic response Gene

regulation

(a) (b)

Cytoplasmic response Gene

regulation

Signal receptor

Transport protein

NUCLEUS

Pathway for Water-Soluble Hormones

• Binding of a hormone to its receptor initiates a

signal transduction pathway leading to

responses in the cytoplasm, enzyme activation,

or a change in gene expression



Animation: Water-Soluble Hormone Right-click slide / select”Play”

• The hormone epinephrine has multiple effects

in mediating the body’s response to short-term

stress

• Epinephrine binds to receptors on the plasma

membrane of liver cells

• This triggers the release of messenger

molecules that activate enzymes and result in

the release of glucose into the bloodstream


Figure 45.7-2

Epinephrine

G protein

Adenylyl cyclase

G protein-coupled receptor

GTP

ATP

cAMP Second messenger

Inhibition of glycogen synthesis

Promotion of glycogen breakdown

Protein kinase A

Pathway for Lipid-Soluble Hormones

• The response to a lipid-soluble hormone is

usually a change in gene expression

• Steroids, thyroid hormones, and the hormonal

form of vitamin D enter target cells and bind to

protein receptors in the cytoplasm or nucleus

• Protein-receptor complexes then act as

transcription factors in the nucleus, regulating

transcription of specific genes



Animation: Lipid-Soluble Hormone Right-click slide / select”Play”

Figure 45.8-2

EXTRACELLULAR FLUID

Hormone (estradiol)

Estradiol (estrogen) receptor Plasma

membrane

Hormone-receptor complex

NUCLEUS

DNA

CYTOPLASM

Vitellogenin mRNA

for vitellogenin

Multiple Effects of Hormones

• The same hormone may have different effects on

target cells that have

– Different receptors for the hormone

– Different signal transduction pathways


Different receptors Same receptors but different intracellular proteins (not shown)

Different cellular responses

Different cellular responses

Epinephrine Epinephrine Epinephrine

receptor receptor receptor

Glycogen deposits

Vessel dilates.

Vessel constricts.

Glycogen breaks down and glucose is released from cell.

(a) Liver cell (b) Skeletal muscle blood vessel

Intestinal blood vessel

(c)

Figure 45.9

Signaling by Local Regulators

• Local regulators are secreted molecules that link

neighboring cells or directly regulate the secreting

cell

• Types of local regulators

– Cytokines and growth factors

– Nitric oxide (NO)

– Prostaglandins


• In the immune system, prostaglandins promote

fever and inflammation and intensify the

sensation of pain

• Prostaglandins help regulate aggregation of

platelets, an early step in formation of blood clots


• The endocrine and nervous systems generally

act coordinately to control reproduction and

development

• For example, in larvae of butterflies and moths,

the signals that direct molting originate in the

brain


Coordination of Neuroendocrine and

Endocrine Signaling

• In insects, molting and development are controlled by a combination of hormones

– A brain hormone (PTTH) stimulates release of ecdysteroid from the prothoracic glands

– Juvenile hormone promotes retention of larval characteristics

– Ecdysone promotes molting (in the presence of juvenile hormone) and development (in the absence of juvenile hormone) of adult characteristics


Brain

Neurosecretory cells

Corpora cardiaca

Corpora allata PTTH

Prothoracic gland

Ecdysteroid

Juvenile hormone (JH)

Low JH

EARLY LARVA LATER

LARVA PUPA ADULT

Figure 45.10-3

Concept 45.2: Feedback regulation and

antagonistic hormone pairs are common

in endocrine systems

• Hormones are assembled into regulatory

pathways


Simple Hormone Pathways

• Hormones are released from an endocrine cell,

travel through the bloodstream, and interact with

specific receptors within a target cell to cause a

physiological response


• For example, the release of acidic contents of the

stomach into the duodenum stimulates endocrine

cells there to secrete secretin

• This causes target cells in the pancreas, a gland

behind the stomach, to raise the pH in the

duodenum


Pathway Example

Stimulus Low pH in

duodenum

Endocrine cell

S cells of duodenum secrete the hormone secretin ( ).

Hormone

Blood vessel

Target cells

Pancreas

Response Bicarbonate release

Neg

ati

ve f

eed

back

Figure 45.11

• In a simple neuroendocrine pathway, the stimulus

is received by a sensory neuron, which stimulates

a neurosecretory cell

• The neurosecretory cell secretes a

neurohormone, which enters the bloodstream

and travels to target cells


Pathway

Example

Stimulus Suckling

Sensory neuron

Po

sit

ive f

eed

back

Hypothalamus/ posterior pituitary

Neurosecretory cell

Neurohormone

Blood vessel

Target cells

Response

Posterior pituitary secretes the neurohormone oxytocin ( ).

Smooth muscle in breasts

Milk release

Figure 45.12

• A negative feedback loop inhibits a response by

reducing the initial stimulus, thus preventing

excessive pathway activity

• Positive feedback reinforces a stimulus to

produce an even greater response

• For example, in mammals oxytocin causes the

release of milk, causing greater suckling by

offspring, which stimulates the release of more

oxytocin


Feedback Regulation

Insulin and Glucagon: Control of Blood

Glucose

• Insulin (decreases blood glucose) and glucagon

(increases blood glucose) are antagonistic

hormones that help maintain glucose

homeostasis

• The pancreas has clusters of endocrine cells

called pancreatic islets with alpha cells that

produce glucagon and beta cells that produce

insulin


Body cells take up more glucose.

Insulin

Beta cells of pancreas release insulin into the blood.

Liver takes up glucose and stores it as glycogen.

Blood glucose level declines.

Blood glucose level rises.

Homeostasis: Blood glucose level

(70–110 mg/m100mL)

STIMULUS: Blood glucose level rises

(for instance, after eating a carbohydrate-rich meal).

Liver breaks down glycogen and releases glucose into the blood.

Alpha cells of pancreas release glucagon into the blood.

Glucagon

STIMULUS: Blood glucose level

falls (for instance, after skipping a meal).

Figure 45.13

Figure 45.13a-2

Body cells

take up more

glucose.

Insulin

Beta cells of

pancreas

release insulin

into the blood.

Liver takes

up glucose

and stores it

as glycogen. Blood glucose

level declines.

Homeostasis:

Blood glucose level

(70–110 mg/100 mL)

STIMULUS:

Blood glucose level rises

(for instance, after eating a

carbohydrate-rich meal).

Figure 45.13b-2

Blood glucose

level rises.

Homeostasis:

Blood glucose level

(70–110 mg/100 mL)

Liver breaks

down glycogen

and releases

glucose into

the blood.

Alpha cells of pancreas

release glucagon into

the blood. Glucagon

STIMULUS:

Blood glucose level

falls (for instance, after

skipping a meal).

Target Tissues for Insulin and Glucagon

• Insulin reduces blood glucose levels by

– Promoting the cellular uptake of glucose

– Slowing glycogen breakdown in the liver

– Promoting fat storage, not breakdown


• Glucagon increases blood glucose levels by

– Stimulating conversion of glycogen to glucose in

the liver

– Stimulating breakdown of fat and protein into

glucose


Diabetes Mellitus

• Diabetes mellitus is perhaps the best-known

endocrine disorder

• It is caused by a deficiency of insulin or a

decreased response to insulin in target tissues

• It is marked by elevated blood glucose levels


• Type 1 diabetes mellitus (insulin-dependent) is an

autoimmune disorder in which the immune

system destroys pancreatic beta cells

• Type 2 diabetes mellitus (non-insulin-dependent)

involves insulin deficiency or reduced response of

target cells due to change in insulin receptors


Concept 45.3: The hypothalamus and

pituitary are central to endocrine regulation

• Endocrine pathways are subject to regulation by

the nervous system, including the brain


Coordination of Endocrine and Nervous

Systems in Vertebrates

• The hypothalamus receives information from the

nervous system and initiates responses through

the endocrine system

• Attached to the hypothalamus is the pituitary

gland, composed of the posterior pituitary and

anterior pituitary


• The posterior pituitary stores and secretes

hormones that are made in the hypothalamus

• The anterior pituitary makes and releases

hormones under regulation of the hypothalamus


Pineal gland

Cerebellum

Spinal cord

Cerebrum

Thalamus

Hypothalamus

Pituitary gland

Posterior pituitary

Anterior pituitary

Hypothalamus

Figure 45.14

Posterior Pituitary Hormones

• The two hormones released from the posterior

pituitary act directly on nonendocrine tissues

– Oxytocin regulates milk secretion by the

mammary glands

– Antidiuretic hormone (ADH) regulates

physiology and behavior


Neurosecretory cells of the hypothalamus

Neurohormone

Posterior pituitary

Hypothalamus

Axons

Anterior pituitary

HORMONE

TARGET

ADH Oxytocin

Kidney tubules

Mammary glands, uterine muscles

Figure 45.15

Anterior Pituitary Hormones

• Hormone production in the anterior pituitary is

controlled by releasing and inhibiting hormones

from the hypothalamus

• For example, prolactin-releasing hormone from

the hypothalamus stimulates the anterior pituitary

to secrete prolactin (PRL), which has a role in

milk production


Tropic effects only: FSH LH TSH ACTH

Nontropic effects only: Prolactin MSH

Nontropic and tropic effects: GH Hypothalamic

releasing and inhibiting hormones

Posterior pituitary

Neurosecretory cells of the hypothalamus

Portal vessels

Endocrine cells of the anterior pituitary

Pituitary hormones

HORMONE FSH and LH TSH ACTH Prolactin MSH GH

TARGET Thyroid Melanocytes Testes or ovaries

Adrenal cortex

Mammary glands

Liver, bones, other tissues

Figure 45.16

Table 45.1a

Table 45.1b

Thyroid Regulation: A Hormone Cascade

Pathway

• A hormone can stimulate the release of a series

of other hormones, the last of which activates a

nonendocrine target cell; this is called a hormone

cascade pathway

• The release of thyroid hormone results from a

hormone cascade pathway involving the

hypothalamus, anterior pituitary, and thyroid

gland

• Hormone cascade pathways typically involve

negative feedback


Figure 45.17 Pathway Example

Stimulus Cold

Sensory neuron

Hypothalamus

Neurosecretory cell

Releasing hormone

Blood vessel

Anterior pituitary

Tropic hormone

Endocrine cell

Hormone

Target

cells

Response

Neg

ati

ve

fee

db

ac

k

Hypothalamus secretes

thyrotropin-releasing

hormone (TRH ).

Anterior pituitary secretes

thyroid-stimulating

hormone (TSH, also known

as thyrotropin ).

Thyroid gland secretes

thyroid hormone

(T3 and T4 ).

Body tissues

Increased cellular

metabolism

Disorders of Thyroid Function and

Regulation

• Hypothyroidism, too little thyroid function, can

produce symptoms such as

– Weight gain, lethargy, cold intolerance

• Hyperthyroidism, excessive production of

thyroid hormone, can lead to

– High temperature, sweating, weight loss,

irritability, and high blood pressure

• Malnutrition can alter thyroid function


• Graves disease, a form of hyperthyroidism

caused by autoimmunity, is typified by

protruding eyes

• Thyroid hormone refers to a pair of hormones

– Triiodothyronin (T3), with three iodine atoms

– Thyroxine (T4), with four iodine atoms

• Insufficient dietary iodine leads to an enlarged

thyroid gland, called a goiter


Figure 45.18

Low level of iodine uptake

High level of iodine uptake

Evolution of Hormone Function

• Over the course of evolution the function of a

given hormone may diverge between species

• For example, thyroid hormone plays a role in

metabolism across many lineages, but in frogs

has taken on a unique function: stimulating the

resorption of the tadpole tail during

metamorphosis

• Prolactin also has a broad range of activities in

vertebrates


• Melanocyte-stimulating hormone (MSH)

regulates skin color in amphibians, fish, and

reptiles by controlling pigment distribution in

melanocytes

• In mammals, MSH plays additional roles in

hunger and metabolism in addition to

coloration


Tropic and Nontropic Hormones

• A tropic hormone regulates the function of

endocrine cells or glands

• Three primarily tropic hormones are

– Follicle-stimulating hormone (FSH)

– Luteinizing hormone (LH)

– Adrenocorticotropic hormone (ACTH)


• Growth hormone (GH) is secreted by the

anterior pituitary gland and has tropic and

nontropic actions

• It promotes growth directly and has diverse

metabolic effects

• It stimulates production of growth factors

• An excess of GH can cause gigantism, while a

lack of GH can cause dwarfism


• Endocrine signaling regulates homeostasis,

development, and behavior

Concept 45.4: Endocrine glands respond to

diverse stimuli in regulating homeostasis,

development, and behavior


Parathyroid Hormone and Vitamin D:

Control of Blood Calcium

• Two antagonistic hormones regulate the

homeostasis of calcium (Ca2+) in the blood of

mammals

– Parathyroid hormone (PTH) is released by the

parathyroid glands

– Calcitonin is released by the thyroid gland


Active vitamin D

Increases Ca2 uptake in intestines

Stimulates Ca2 uptake in kidneys

Stimulates Ca2 release from bones

Parathyroid gland (behind thyroid)

PTH

Blood Ca2 level rises.

Homeostasis: Blood Ca2 level

(about 10 mg/100 mL)

STIMULUS: Falling blood

Ca2 level

Figure 45.20-2

• PTH increases the level of blood Ca2+

– It releases Ca2+ from bone and stimulates

reabsorption of Ca2+ in the kidneys

– It also has an indirect effect, stimulating the

kidneys to activate vitamin D, which promotes

intestinal uptake of Ca2+ from food

• Calcitonin decreases the level of blood Ca2+

– It stimulates Ca2+ deposition in bones and

secretion by kidneys


Adrenal Hormones: Response to Stress

• The adrenal glands are adjacent to the kidneys

• Each adrenal gland actually consists of two

glands: the adrenal medulla (inner portion) and

adrenal cortex (outer portion)


Catecholamines from the Adrenal Medulla

• The adrenal medulla secretes epinephrine

(adrenaline) and norepinephrine (noradrenaline)

• These hormones are members of a class of

compounds called catecholamines

• They are secreted in response to stress-activated

impulses from the nervous system

• They mediate various fight-or-flight responses


• Epinephrine and norepinephrine

– Trigger the release of glucose and fatty acids into the blood

– Increase oxygen delivery to body cells

– Direct blood toward heart, brain, and skeletal muscles and away from skin, digestive system, and kidneys

• The release of epinephrine and norepinephrine occurs in response to involuntary nerve signals


Spinal cord (cross section)

(a) (b) Long-term stress response and the adrenal cortex

Short-term stress response and the adrenal medulla

Stress

Nerve signals

Nerve cell

Releasing hormone

Hypothalamus

Anterior pituitary

Blood vessel

Nerve cell ACTH Adrenal medulla secretes epinephrine and norepinephrine.

Adrenal gland

Kidney

Adrenal cortex secretes mineralocorticoids and glucocorticoids.

Effects of epinephrine and norepinephrine: Effects of mineralocorticoids:

Effects of glucocorticoids:

• Glycogen broken down to glucose; increased blood glucose

• Increased blood pressure

• Increased breathing rate

• Increased metabolic rate

• Change in blood flow patterns, leading to increased alertness and decreased digestive, excretory, and reproductive system activity

• Retention of sodium ions and water by kidneys

• Increased blood volume and blood pressure

• Proteins and fats broken down and converted to glucose, leading to increased blood glucose

• Partial suppression of immune system

Figure 45.21

Spinal cord (cross section)

(a) Short-term stress response and the adrenal medulla

Stress

Nerve signals

Nerve cell

Nerve cell Adrenal medulla secretes epinephrine and norepinephrine.

Adrenal gland

Kidney

Effects of epinephrine and norepinephrine:

• Glycogen broken down to glucose; increased blood glucose

• Increased blood pressure

• Increased breathing rate

• Increased metabolic rate

• Change in blood flow patterns, leading to increased alertness and decreased digestive, excretory, and reproductive system activity

Hypo- thalamus

Figure 45.21a

Steroid Hormones from the Adrenal Cortex

• The adrenal cortex releases a family of steroids

called corticosteroids in response to stress

• These hormones are triggered by a hormone

cascade pathway via the hypothalamus and

anterior pituitary (ACTH)

• Humans produce two types of corticosteroids:

glucocorticoids and mineralocorticoids


(b) Long-term stress response and the adrenal cortex

Stress

Releasing hormone

Hypothalamus

Anterior pituitary

Blood vessel

ACTH

Adrenal cortex secretes mineralocorticoids and glucocorticoids.

Effects of mineralocorticoids:

Effects of glucocorticoids:

• Retention of sodium ions and water by kidneys

• Increased blood volume and blood pressure

• Proteins and fats broken down and converted to glucose, leading to increased blood glucose

• Partial suppression of immune system

Adrenal gland

Kidney

Figure 45.21b

• Glucocorticoids, such as cortisol, influence

glucose metabolism and the immune system

• Mineralocorticoids, such as aldosterone, affect

salt and water balance

• The adrenal cortex also produces small amounts

of steroid hormones that function as sex

hormones


Gonadal Sex Hormones

• The gonads, testes and ovaries, produce most of

the sex hormones: androgens, estrogens, and

progestins

• All three sex hormones are found in both males

and females, but in significantly different

proportions


• The testes primarily synthesize androgens,

mainly testosterone, which stimulate

development and maintenance of the male

reproductive system

• Testosterone causes an increase in muscle and

bone mass and is often taken as a supplement to

cause muscle growth, which carries health risks


Chromosome Set

Appearance of Genitalia

No surgery Embryonic gonad

removed

XY (male)

XX (female)

Male

Female

Female

Female

RESULTS

Figure 45.22

• Estrogens, most importantly estradiol, are

responsible for maintenance of the female

reproductive system and the development of

female secondary sex characteristics

• In mammals, progestins, which include

progesterone, are primarily involved in preparing

and maintaining the uterus

• Synthesis of the sex hormones is controlled by

FSH and LH from the anterior pituitary


• Between 1938 and 1971 some pregnant women

at risk for complications were prescribed a

synthetic estrogen called diethylstilbestrol (DES)

• Daughters of women treated with DES are at

higher risk for reproductive abnormalities,

including miscarriage, structural changes, and

cervical and vaginal cancers


Endocrine Disruptors

• DES is an endocrine disruptor, a molecule that

interrupts the normal function of a hormone

pathway, in this case, that of estrogen


Melatonin and Biorhythms

• The pineal gland, located in the brain, secretes

melatonin

• Light/dark cycles control release of melatonin

• Primary functions of melatonin appear to relate to

biological rhythms associated with reproduction


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