47 and 48 The Nervous System

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47 and 48 The Nervous System

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35-2 The Nervous System

Slide 2 of 38

Copyright Pearson Prentice Hall

The nervous system controls and

coordinates functions throughout the body and responds to internal and external

stimuli.

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Neurons

Neurons

The messages carried by the nervous system are electrical signals called impulses.

The cells that transmit these impulses are called

neurons.

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Neurons

Neurons are classified according to the direction in which an impulse travels.

• Sensory neurons carry impulses from the sense organs to the spinal cord and brain.

• Motor neurons carry impulses from the brain and spinal cord to muscles and glands.

• Interneurons connect sensory and motor

neurons and carry impulses between them.

(5)

Figure 40.6

(6)

Sensory Neuron

carry impulses from sense organs to spinal cord & brain

Fun Fact:

Where can the largest cells in the

world be found?

The giraffe’s sensory and motor neurons! Some must bring impulses from

the bottom of their legs

to their spinal cord

several meters away!!

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• Nerves work together with muscles for movement. An impulse begins when one

neuron is stimulated by another neuron or by the sense organs.

• The impulse travels down the axons of

Sensory neurons to the brain cells called Interneurons.

• The brain will then send an impulse through

motor neurons to the necessary muscle or

organs, telling it to contract.

(8)

Nerves

 Nerves  Collections of neurons that are joined together by connective tissue.

 Responsible for transferring impulses from

receptors to CNS and back to effectors.

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Neurons

The largest part of a typical neuron is the cell body.

It contains the nucleus and much of the cytoplasm.

Cell body

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Neurons

Dendrites extend from the cell body and carry

impulses from the environment toward the cell body.

Dendrites

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Neurons

The axon is the long fiber that carries impulses away from the cell body.

Axon

terminals

Axon

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Neurons

The axon ends in axon terminals.

Axon

terminals

Axon

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Neurons

The axon is sometimes surrounded by an insulating membrane called the myelin sheath.

There are gaps in the myelin sheath, called nodes, where the membrane is exposed.

Impulses jump from one node to the next.

Myelin sheath

Nodes

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Neurons

Structures of a Neuron

Axon

terminals

Myelin sheath Cell body

Nodes Axon

Dendrites

Nucleus

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

Dendrite  Fine hair-like extensions on the end of a neuron.



Function: receive incoming stimuli.



Cell Body or Soma  The control center of the neuron.



Function: Directs impulses from the dendrites to the axon.



Nucleus  Control center of the Soma.



Function: Tells the soma what to do.



Axon Pathway for the nerve impulse (electrical message) from the soma to the opposite end of the neuron.



Myelin Sheath  An insulating layer around an axon. Made up of Schwann cells.



Node of Ranvier  Gaps in the myelin sheath

where voltage-gated Na

⁺

channels are found

(16)

Resting Potential

The electrical charge across the cell

membrane of a neuron at rest is known as the resting potential.

It is negative

(approximately -70mV) due to the

accumulation of

overall more positive ions outside than

inside.

(17)

Key

Na^ K^

Sodium- potassium pump

Potassium channel

Sodium channel

OUTSIDE OF CELL

INSIDE OF CELL

Figure 48.7

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Slide 18 of 38

The Nerve Impulse

How is a nerve impulse transmitted?

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The Moving Impulse

An impulse begins when a neuron is

stimulated by another neuron or by the environment.

Changes in membrane potential occur because neurons contain gated ion

channels that open or close in response to

stimuli

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Threshold

A stimulus must be of adequate strength to cause a neuron to transmit an impulse.

The minimum level of a stimulus that is required to

activate a neuron is called the threshold.

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A stimulus that is stronger than the threshold produces an impulse or action potential.

A stimulus that is weaker than the threshold produces no impulse.

An action potential (nerve impulse) is ALL OR

NONE event.

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Action Potential involves cell moving through following three potential changes:

1. Depolarization is increase in magnitude of membrane

potential (approximately +40 mV) tending to make the inside

potential higher than resting potential (-70mV)

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Action Potential involves cell moving through following three potential changes:

2. Repolarization is change in membrane potential that returns it

to a negative value just after the depolarization phase of an action

potential has changed the potential to a positive value.

(24)

Action Potential involves cell moving through following three potential changes:

1. Hyperpolarization is a change in membrane potential tending

to make the inside even more negative than resting potential

(25)

At resting potential most voltage-gated sodium (Na

⁺

)

channels are closed; most of the voltage-gated potassium

(K

⁺

) channels are also closed. The Na

⁺

/K

⁺

pump actively

pumps K

⁺

into the cell and Na

⁺

ions out.

(26)

Some Na

⁺

channels open in response to a stimulus,

allowing Na

⁺

ions to enter the cell. The membrane starts to depolarize (the charge across the membrane

lessens).

(27)

If the threshold of excitation is reached, all the Na

⁺

channels open. Most often the threshold potential is a

membrane potential value between –40 and –55 mV , but

it can vary.

(28)

Rising phase of Action Potential – Na

⁺

influx

makes the inside of the membrane positive with

respect to the outside.

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Falling phase of Action Potential – At the peak action potential, Na

⁺

channels close while K

⁺

channels open. K

⁺

leaves the cell, and inside of cell becomes negative again.

Repolarization

(30)

Hyperpolarization – Last phase of action potential

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• Hyperpolarization overshoots resting potential (-70mV) making it even more negative.

• During the refractory period caused by

hyperpolarization after an action potential, a second action potential cannot be initiated.

• Inactivated Na

⁺

channels behind the zone of

depolarization prevent the action potential from traveling backwards

(32)

& hyperpolarization

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The depolarization-repolarization

process is repeated in the next region of neuronal membrane. In this way, local currents of ions across the plasma

membrane cause the action potential to

be propagated along the length of the

axon.

(35)

(36)

• Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where

voltage-gated Na

⁺

channels are found

(37)

A Chemical Synapse

Synapse is a junction

between two nerve cells,

consisting of a minute gap

across which impulses pass

by diffusion of a chemical

signal.

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Presynaptic

cell Postsynaptic cell

Axon

Presynaptic membrane

Synaptic vesicle containing

neurotransmitter

Postsynaptic membrane Synaptic

cleft

Voltage-gated Ca^2 channel

Ligand-gated ion channels Ca^2

Na^ K^

2 1

3

4

Figure 48.15

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The Synapse

The Synapse

At the end of the neuron, the impulse reaches an axon terminal. Usually the neuron makes contact with another cell at this site.

The neuron may pass the impulse along to the

second cell.

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The Synapse

The location at which a neuron can transfer an impulse to another cell is called a synapse.

The synaptic cleft separates the axon terminal from the dendrites of the adjacent cell.

Synaptic cleft

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The Synapse

Terminals contain vesicles filled with neurotransmitters.

Neurotransmitters

are chemicals used by a neuron to transmit an impulse across a synapse to another cell.

Vesicle

Neurotransmitter

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Slide 42 of 38

The Synapse

As an impulse

reaches a terminal, influx of Ca ions

make vesicles send neurotransmitters into the synaptic cleft.

These diffuse across the cleft and attach to membrane

receptors on the ligand gated ion

channels of the next cell.

Receptor

(43)

Specific ions then rush across the membrane into second neuron causing change in its

potential.

If the stimulation exceeds the cell’s threshold, a

new impulse begins.

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Moments after binding to receptors, neurotransmitters are released from the cell surface.

The neurotransmitters may then be

broken down by enzymes, or taken up

and recycled by the axon terminal.

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Presynaptic

cell Postsynaptic cell

Axon

Presynaptic membrane

Synaptic vesicle containing

neurotransmitter

Postsynaptic membrane Synaptic

cleft

Voltage-gated Ca^2 channel

Ligand-gated ion channels Ca^2

Na^ K^

2 1

3

4

Figure 48.15

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Neurotransmitters fall into two categories

Excitatory Neurotransmitters cause depolarizations that bring the membrane potential toward threshold

Inhibitory Neurotransmitters cause

hyperpolarizations that move the membrane potential farther from threshold

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Slide 47 of 38

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Neurotransmitters

• There are more than 100 neurotransmitters,

belonging to five groups: acetylcholine, biogenic amines (organic bases), amino acids,

neuropeptides, and gases

• A single neurotransmitter may have more than a dozen different receptors

(49)

Table 48.2

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Neurotrnasmitter

Sites where

released Principal Actions

Acetylcholine

Brain Neuromuscular junction Autonomic nervous system

Generally Excitatory Excitatory on skeletal muscles and Excitatory or inhibitory on

internal organs Deficit cause alzheimer

Norepinephrine or noradrenaline

CNS and PNS: Most neuromuscular and neuroglandular junctions of sympathetic division of autonomic nervous

system.

Excitatory or inhibitory, depending on receptors

Epinephrine or

adrenaline ^CNS Generally Excitatory along autonomic nervous system

Dopamine

Areas of brain

Parts of peripheral nervous system

Generally Excitatory Cause feeling of well being Surplus cause schizophrenia and deficit cause parkinson’s Methamphetamine (Speed) is a drug very similar on a molecular level to Dopamine. Speed can enter the Dopamine transporter and it then expels the dopamine out into the synaptic space where it remains longer as a result.

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Neurotrnasmitter

Sites where

released Principal Actions

Serotonin CNS Usually inhibitory Inhibits pain receptors

Gamma-

aminobutyric acid GABA

CNS

Principal inhibitory neurotransmitter in brain Deficit cause epilepsy

Somatostatin Areas of brain Pancreas

Usually inhibitory

Inhibits release of growth hormone

(52)

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There are two types of acetylcholine receptors (AChR) that bind acetylcholine and transmit its signal: muscarinic AChRs and nicotinic AChRs.

These receptors are functionally different:

1. Muscarinic type being G-protein coupled receptors (GPCRs) that mediate a slow

metabolic response via second messenger cascades.

2. Nicotinic type are ligand-gated ion channels

that mediate a fast synaptic transmission of the

neurotransmitter.

(54)

Muscarinic type acetylcholine receptors

(55)

Nicotinic type acetylcholine receptors

(56)

Figure 49.23

Nicotine stimulates dopamine- releasing VTA neuron.

Inhibitory neuron

Dopamine- releasing VTA neuron

Cerebral neuron of reward pathway

Opium and heroin decrease activity of inhibitory

neuron.

Cocaine and amphetamines block removal of dopamine from synaptic cleft.

Reward system response

(57)

Methamphetamines damage nerve endings, which cause naturally occurring

neurotransmitters—Dopamine and Serotonin—

to be ineffective. Because Dopamine and

Serotonin are responsible for easing pain (like the pain which comes from drug withdrawal), withdrawal periods are extremely painful

because the body does not have its natural pain reliever to administer.

Methamphetamine withdrawal is said to be one

of the most painful experiences a human being

can endure

(58)

Awakenings is a film based on a true story of

British neurologist Oliver Sacks who administered L-Dopa to awake patients who were in decades of catatonia.

L-DOPA is the precursor to

the neurotransmitters dopamine, norepinephrine

(noradrenaline), and epinephrine (adrenaline). L-

DOPA crosses the protective blood–brain barrier,

whereas dopamine itself cannot. Thus, L-DOPA

is used to increase dopamine concentrations in

the treatment of Parkinson's disease

(59)

(60)

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How is sympathetic nervous system caused?

Sympathetic nervous system response is caused by epinephrine (adrenalin) and norepinephrine (noradrenalin).

Epinephrine is mainly produced by the adrenal medulla as a hormone, although small amounts are produced in the nerves and act as a

neurotransmitter.

Noradrenaline is mainly produced in the nerves, although small amounts are also produced in the adrenal medulla.

Both norepinephrine and epinephrine are

released during a fight-or-flight response.

(62)

How is parasympathetic nervous system caused?

Neurons in the parasympathetic nervous system utilize acetylcholine, a

neurotransmitter for cell to cell

communication. Any tissues that are

controlled by parasympathetic system will

have receptors for acetylcholine so that this

system can communicate with them.

(63)

brain

Spinal Cord

Cerebellum Cerebrum

Medulla Oblongata

Consists of: Brain and Spinal Cord

(64)

Figure 49.UN04

Spinal cord Cerebral cortex

Cerebellum Medulla oblongata

Pons Hindbrain

Midbrain Forebrain

Cerebrum Thalamus

Hypothalamus Pituitary gland

(65)

Cerebrum Voluntary or conscious activities of the body-learning, judgment

Cerebellum Coordinates and balances the actions of the muscles

Medulla Oblongata (Brain Stem)

Controls involuntary actions like blood pressure, heart rate,

breathing, and swallowing

Spinal Cord

The main communications link between the brain and the rest of

the body

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35-3 Divisions of the Nervous System

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The Brain

The Thalamus and Hypothalamus

The thalamus receives messages from all sensory receptors throughout the body and relays the

information to the proper region of the cerebrum

for further processing.

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The Brain

The hypothalamus controls recognition and

analysis of hunger, thirst, fatigue, anger, and body temperature.

It controls coordination of the nervous and

endocrine systems.

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The Spinal Cord

The Spinal Cord

The spinal cord is the main communications link between the brain and the rest of the body.

Certain information, including some kinds of

reflexes, are processed directly in the spinal cord.

A reflex is a quick, automatic response to a

stimulus.

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The Peripheral Nervous System

The Peripheral Nervous System

The peripheral nervous system is all of the nerves

and associated cells that are not part of the brain

and the spinal cord.

(70)

Consists of:

Sensory division and Motor

division

-includes all

sensory neurons,

motor neurons,

and sense organs

(71)

Figure 49.15

Motor cortex (control of

skeletal muscles) Frontal lobe

Prefrontal cortex (decision making, planning)

Broca’s area

(forming speech) Temporal lobe

Auditory cortex (hearing) Wernicke’s area

(comprehending language)

Somatosensory cortex (sense of touch)

Parietal lobe

Sensory association cortex (integration of sensory information)

Visual association cortex (combining images and object recognition)

Occipital lobe

Cerebellum

Visual cortex

(processing visual stimuli and pattern recognition)

(72)

A reflex is an involuntary

response that is processed in the

spinal cord not the brain.

Reflexes protect the body before the brain knows

what is going on.

Reflex Arc

(73)

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The Peripheral Nervous System