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Signal transduction How does one cell send a message to another? How does the cell respond? A simple signal transduction pathway The cell “signal” is a chemical (can be a gas) It binds to a specific receptor on the target cell Response mechanisms are activated in the target cell

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Signal transduction

How does one cell send a message to another?

How does the cell respond?



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The cell “signal” is a chemical (can be a gas)

It binds to a specific receptor on the target cell

Response mechanisms are activated in the

target cell

Response often involves gene transcription


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Paracrine signaling

cytokines

inflammation

“Autocrine”- sometimes released chemical

acts on the cell itself

Endocrine (hormonal) signaling

chemical is released into blood or

other circulatory fluid

often produced by specialized glands


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Synaptic signaling- in nervous system

electrical signal transmitted along

specialized nerve cell membrane

Conducting cell: neuron

Most animals have a nervous system: they

vary in complexity but they all

a.receive information from the

environment

b. process the information

c. trigger a response (usually from

muscles or glands)


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Types of neurons (in vertebrates):

Interneuron- located entirely within CNS,

integrates functions in CNS

Sensory (from sensory receptor to CNS)

Motor (from CNS to effector organ)

somatic- stimulates skeletal muscles

autonomic- affects smooth and cardiac

muscle, also glandular secretion


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Parts of a neuron

Cell body- contains the nucleus and other

organelles

Dendrites- transmit electrical impulses TO the

cell body

Axon- transmits impulse AWAY from the cell body

axons can be several feet long

“Axon hillock” is located near the cell body

nerve impulses originate there


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Structures of neurons

sensory

retina

motor


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Electrical activity in axons

Resting membrane potential in neurons

is –70 mV

Large negatively charged molecules inside

the cell

Positively charged ions outside the cell

(more Na out than K in)

All cells have a membrane potential, but nerve,

muscle and a few other types of cells have

electrical excitability


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Permeability to ion changes

Occurs in a very small area on the membrane

Depolarization- potential difference approaches

zero

Repolarization- back to the resting potential

Hyperpolarization- potential difference increases

positive charges leave cell

negative charges enter cell


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Gated ion channels for K and Na

(lots of these at axon hillock)

Resting cell is more permeable to K than Na

Depolarization- membrane becomes permeable

to Na, and Na can diffuse into cell

After Na gates close, K gates open and K diffuses

out of the cell


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Action potentials

When completed, Na/K pumps restore balance

of the ions

Takes place on a very small part of membrane

occurrence is rapid


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Action potentials are very rapid

Inactivation occurs until membranes are

repolarized

Stronger stimuli stimulate more and more axons

(more action potentials are stimulated, but

their amplitude does not change)



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Conduction of nerve impulses

Unmyelinated axon- wave of action potentials

spreads along length of axon

Amplitude of action potential does not change

Myelinated axon- conduction rate is much

faster


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Synapse- connection between a neuron and

a second cell

From presynaptic to postsynaptic neuron

Release of neurotransmitters (chemicals)

A few electrical synapses in nervous system,

In smooth muscle and heart

gap junctions


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Chemical synapses

One-way

Presynaptic neuron has synaptic vesicles

Fusion of vesicles is mediated by calcium

Calmodulin is activated

Protein kinase activated

Synaptic vesicles fuse with membrane

Neurotransmitters diffuse across cleft and

bind to receptors


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Voltage-regulated channels in presynaptic axon

Chemically regulated channels in

postsynaptic membrane (i.e., binding of

neurotransmitter is triggering event)

Ion channels are opened, depolarization occurs

Can be excitatory or inhibitory

Depends on which receptors are engaged

Integration of impulses in dendrites and cell

body of postsynaptic neuron


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What are some chemicals that act as neuro-

transmitters?

Acetylcholine

Norepinephrine

Dopamine

Serotonin

They are classified structurally (monoamine,

catecholamine, peptide, etc.)

and have different functions


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What do specific neurotransmitters do?

Depends on:

region of brain affected (where neurotrans-

mitters are made)

which cells have receptors

what kind of receptor: excitatory or inhibitory

Example: dopamine (a monoamine, derived from

tyrosine)


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Dopamine is active in different regions of brain

Mesolimbic- limbic system; behavior and reward

(in midbrain)

Many addictive drugs activate these pathways

(i.e., enhance dopamine effects; cocaine

blocks dopamine reuptake. Addiction occurs

because cells become unable to respond to

“normal” levels)

Nicotine seems to promote dopamine release in

this region


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Schizophrenia: too much dopamine in these

pathways

Treatments block a particular type of

dopamine receptor


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Dopamine

Nigrostriatal- neurons in substantia nigra region

of brain involved in initiation of skeletal

movement

Parkinson’s disease- degeneration of neurons

in this region that produce and/or

respond to dopamine

L-DOPA and MAO inhibitors- increase dopamine

transmission


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Control mechanisms:

Neurotransmitter must be cleared quickly or

it will overstimulate target cell

-Uptake by presynaptic neuron (by transporter

proteins)

-Degradation by specific enzyme in presynaptic

neuron

-By post-synaptic neuron (produces an enzyme

that degrades the neurotransmitter)


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  • Summary

  • The nervous system is comprised of the

    • central nervous system (brain, spinal cord)

    • and the periphery (cranial and spinal nerves)

    • Periphery is divided into autonomic and motor

    • neurons.

    • 2. Cells of the nervous system are glial cells and

    • neurons. Neurons conduct nervous impulses,

    • glial cells “support” neurons.

    • 3. Myelination affects the speed at which impulse

    • is delivered.


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4. Neurons conduct electrical and chemical

signaling. Action potential starts at a very

small area of the membrane and is conducted

along the length of the membrane.

Action potential rises with Na influx and falls

with K efflux.

5. Speed of transmission is affected by a.) presence

of myelin, and b.) the diameter of the neuron.

(faster in larger neurons)

6. Neurotransmitters deliver signals across

synapses.


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7. Sometimes signal is excitatory, sometimes

inhibitory.

8. Neurotransmitters are typically small fast-

acting molecules. Effect depends on the

type of target cell and the type of receptor

9. Regulation is very important. Both presynaptic

and postsynaptic cells may produce

substances that either promote reuptake of

or inactivate the neurotransmitter.


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