Ch. 44 Nervous System

7/28/2019 Ch. 44 Nervous System

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The Nervous System

Chapter 44


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Nervous System Organization

All animals must be able to respond to

environmental stimuli

-Sensory receptors = Detect stimulus

-Motor effectors = Respond to it

-The nervous system links the two-Consists of neurons and supporting cells


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Vertebrates have three types of neurons

-Sensory neurons (afferent neurons) carry

impulses to central nervous system (CNS)

-Motor neurons (efferent neurons) carry

impulses from CNS to effectors (muscles

and glands)

-Interneurons (association neurons)

provide more complex reflexes and

associative functions (learning and memory)


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The CNS consists of the brain and spinal cord

The peripheral nervous system (PNS)

consists of sensory and motor neurons

-Somatic NS stimulates skeletal muscles

-Autonomic NS stimulates smooth and

cardiac muscles, as well as glands-Sympathetic and parasympathetic NS

-Counterbalance each other


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P

NS

CNS

Brain and Spinal Cord

Sympathetic nervous

system

"fight or flight"

Parasympathetic nervous

system

"rest and repose"

Somatic nervous

system

(voluntary)

Sensory neurons

registering external

stimuli

Autonomic nervous

system

(involuntary)

Sensory Pathways Motor Pathways

central nervous system (CNS)peripheral nervous system (PNS)

Sensory neurons

registering external

stimuli


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Neurons have the same basic structure

-Cell body = Enlarged part containing

nucleus

-Dendrites = Short, cytoplasmic extensions

that receive stimuli

-Axon = Single, long extension thatconducts impulses away from cell body


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Neurons are supported both structurally and

functionally by cells called neuroglia

-Schwann cells and oligodendrocytes

produce myelin sheaths surrounding axons

-In the CNS, myelinated axons form white

matter

-Dendrites/cell bodies form gray matter

-In the PNS, myelinated axons are bundled

to form nerves


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Nerve Impulse Transmission

A potential difference exists across every

cells plasma membrane

-Negative pole = Cytoplasmic side

-Positive pole = Extracellular fluid side

When a neuron is not being stimulated, it

maintains a resting potential-Ranges from -40 to -90 millivolts (mV)

-Average about -70 mV


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The inside of the cell is more negatively

charged than the outside because of:

1. Sodium-potassium pump = Brings two

K+ into cell for every three Na+ it pumps out

2. Ion leakage channels = Allow more K+ to

diffuse out than Na+ to diffuse in


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There is a buildup of positive charge outside

and negative charge inside the membrane

-This electrical potential is an attractive force

to bring K+ ions back into the cell

-Balance between diffusional and electrical

forces leads to the equilibrium potential

The resting membrane potential can be

viewed using a voltmeter and two electrodes


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There are two types of potentials:

-Graded potentials and action potentials

Graded potentials are small transient

changes in membrane potential due to

activation ofgated ion channels-Most are closed in the normal resting cell


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Chemically-gated orligand-gated channels

-Ligands are hormones or neurotransmitters

-Induce opening

and cause changes

in cell membranepermeability


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Depolarization makes the membrane

potential more positive, whereas a

hyperpolarization makes it more negative

-These small changes result in graded

potentials

-Can reinforce or negate each other

Summation is the ability of graded potentials

to combine


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Action potentials result when depolarization

reaches the threshold potential

The action potential is caused by voltage-

gated ion channels

-Two different channels are used:-Voltage-gated Na+ channels

-Voltage-gated K+ channels


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When the threshold voltage is reached,

sodium channels open rapidly

-Transient influx of Na+ causes the

membrane to depolarize

In contrast, potassium channel opens slowly-Efflux of K+ repolarizes the membrane


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The action potential has three phases:

-Rising, falling and undershoot

Action potentials are always separate, all-or-none events with the same amplitude

-Do not add up or interfere with each other

The intensity of a stimulus is coded by thefrequency, not amplitude, of action

potentials


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Each action potential, in its rising phase,

reflects a reversal in membrane polarity

-Positive charges due to influx of Na+ can

depolarize the adjacent region to threshold

-And so the next region produces its own

action potential

-Meanwhile, the previous region repolarizes

back to the resting membrane potential


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Two ways to increase velocity of conduction:

1. Axon has a large diameter

-Less resistance to current flow

-Found primarily in invertebrates

2. Axon is myelinated

-Action potential is only produced at the

nodes of Ranvier

-Impulse jumps from node to node

-Saltatory conduction


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Synapses

Synapses are intercellular junctions

-Presynaptic cell transmits action potential

-Postsynaptic cell receives itTwo basic types: electrical and chemical

Electrical synapses involve direct

cytoplasmic connections between the twocells formed by gap junctions

-Relatively rare in vertebrates


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Synapses

Chemical synapses have a synaptic cleft

between the two cells

-End of presynaptic

cell contains

synaptic vesicles

packed with

neurotransmitters


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Synapses

Action potential triggers influx of Ca2+

-Synaptic vesicles fuse with cell membrane

-Neurotransmitter is released by exocytosis-Diffuses to other side of cleft and binds

to chemical- or ligand-gated receptor

proteins

-Neurotransmitter action is terminated by

enzymatic cleavage or cellular uptake


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Synapses


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Neurotransmitters

Acetylcholine (ACh)

-Crosses the synapse

between a motor

neuron and a muscle

fiber

-Neuromuscular

junction


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Neurotransmitters

Acetylcholine (ACh)

-Binds to ligand-gated receptor in the

postsynaptic membrane

-Produces a depolarization called an

excitatory postsynaptic potential (EPSP)

-Stimulates muscle contraction

-Acetylcholinesterase (AChE) degrades

ACh

-Causes muscle relaxation


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Neurotransmitters

Amino acids

-Glutamate is the major excitatory

neurotransmitter in the vertebrate CNS

-Glycine and GABA (g-aminobutyric acid)

are inhibitory neurotransmitters

-Open ligand-gated channels for Cl

-Produce a hyperpolarization called an

inhibitory postsynaptic potential

(IPSP)


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Neurotransmitters


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Neurotransmitters (Cont.)


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Neurotransmitters

Biogenic amines

-Epinephrine (adrenaline)and

norepinephrine are responsible for the

fight or flight response

-Dopamine is used in some areas of the

brain that control body movements

-Serotonin is involved in the regulation of

sleep


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Neurotransmitters

Neuropeptides

-Substance P is released from sensory

neurons activated by painful stimuli

-Intensity of pain perception depends on

enkephalins and endorphins

Nitric oxide (NO)

-A gas ; produced as needed from arginine

-Causes smooth muscle relaxation


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Synaptic Integration

Integration of EPSPs (depolarization) and

ISPSs (hyperpolarization) occurs on the

neuronal cell body

-Small EPSPs add together to bring the

membrane potential closer to the threshold

-IPSPs subtract from the depolarizing effect

of EPSPs

-And will therefore deter the membrane

potential from reaching threshold


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There are two ways that the membrane can

reach the threshold voltage

-Spatial summation

-Many different dendrites produce EPSPs

-Temporal summation-One dendrite produces repeated EPSPs


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Drug Addiction

Prolonged exposure to a stimulus may cause

cells to lose the ability to respond to it

-This process is called habituation

-The cell decreases the number of

receptors because there is an

abundance of neurotransmitters


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Drug Addiction

Cocaineaffects neurons in the brains

pleasure pathways (limbic system)

-Binds dopamine transporters and prevents

the reuptake of dopamine

-Dopamine survives longer in the synapse

and fires pleasure pathways more and more

-Prolonged exposure triggers the limbic

system neurons to reduce receptor numbers

-The cocaine user is now addicted


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Drug Addiction

Nicotine binds directly to a specific receptor

on postsynaptic neurons of the brain

-Brain adjusts to prolonged exposure by

turning down the volume in two ways:

1. Making fewer nicotine receptors

2. Altering the pattern of activation of

the nicotine receptors


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The Central Nervous System

Sponges are only major phylum without nerves

Cnidarians have the simplest nervous system

-Neurons linked to each other in a nerve net

-No associative activity

Free-living flatworms (phylum Platyhelminthes)

are simplest animals with associative activity-Two nerve cords run down the body

-Permit complex muscle control


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Vertebrate Brains

All vertebrate brains have three basic divisions:

-Hindbrain or rhombencephalon

-Midbrain or mesencephalon

-Forebrain or prosencephalon

In fishes,

-Hindbrain = Largest portion-Midbrain = Processes visual information

-Forebrain = Processes olfactory information


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Vertebrate Brains


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Vertebrate Brains

The relative sizes of different brain regions

have changed as vertebrates evolved

-Forebrain became the dominant feature


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Vertebrate Brains

Forebrain is composed of two elements:

-Diencephalon

-Thalamus: Integration and relay center

-Hypothalamus: Participates in basic

drives & emotions; controls pituitary gland

-Telencephalon (end brain)-Devoted largely to associative activity

-Called the cerebrum in mammals


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Cerebrum

The increase in brain size in mammals reflects

the great enlargement of the cerebrum

-Split into right and left cerebral

hemispheres, which are connected by a

tract called the corpus callosum

-Each hemisphere receives sensory input

from the opposite side

-Hemispheres are divided into: frontal,

parietal, temporal and occipital lobes


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Cerebrum


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Cerebrum

Cerebral cortex

-Outer layer of the cerebrum

-Contains about 10% of all neurons in brain

-Highly convoluted surface

-Increases threefold the surface area of

the human brain-Divided into three regions, each with a

specific function


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Cerebrum

Cerebral cortex

-Primary motor cortex: Movement control

-Primary somatosensory cortex: Sensory

control

-Association cortex: Higher mental functions

Basal ganglia

-Aggregates of neuron cell bodies

-Form islands of grey matter within the

cerebrums white matter


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Cerebrum


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Cerebrum


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Other Brain Structures

Thalamus

-Integrates visual, auditory and

somatosensory information

Hypothalamus

-Integrates visceral activities

-Controls pituitary gland-Forms limbic system,withhippocampus

and amygdala

-Responsible for emotional responses


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Complex Functions of the Brain

Sleep and arousal

-One section of reticular formation controls

consciousness and alertness

-Reticular-activating systemcontrols

both sleep and the waking state

-Brain state can be monitored by means of

an electroencephalogram (EEG)

-Records electrical activity


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Language

-Left hemisphere is dominant hemisphere

-Adept at sequential reasoning

Spatial recognition

-Right hemisphere is adept at spatialreasoning

-Primarily involved in musical ability


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Memory

-Appears dispersed across the brain

-Short-term memory is stored in the form of

transient neural excitations

-Long-term memory appears to involve

structural changes in neural connections


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Alzheimer disease is a condition where

memory and thought become dysfunctional

-Two causes have been proposed

1. Nerve cells are killed from the outside in

-External protein: b-amyloid

2. Nerve cells are killed from the inside out-Internal proteins: tau (t)


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Spinal Cord

The spinal cord is a cable of neurons

extending from the brain down through the

backbone

-Enclosed and

protected by

the vertebralcolumn and

the meninges


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Spinal Cord

It serves as the bodys information highway

-Relays messages between the body and

the brain

It also functions in reflexes

-The knee-jerk reflex is monosynaptic-However, most reflexes in vertebrates

involve a single interneuron


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The Peripheral Nervous System

The PNS consists of nerves and ganglia

-Nerves are bundles

of axons bound byconnective tissue

-Ganglia areaggregates of

neuron cell bodies


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The Peripheral Nervous System

Sensory neurons:

-Axons enter the dorsal surface of the spinalcord and form dorsal root of spinal nerve

-Cell bodies are grouped outside the spinalcord in dorsal root ganglia

Motor neurons:

-Axons leave from the ventral surface andform ventral root of spinal nerve

-Cell bodies are located in the spinal cord


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The Somatic Nervous System

Somatic motor neurons stimulate the skeletal

muscles to contract

-In response to conscious command or

reflex actions

The antagonist of the muscle is inhibited by

hyperpolarization (IPSPs) of spinal motor

neurons


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The Autonomic Nervous System

Composed of the sympathetic and

parasympatheticdivisions, plus the

medulla oblongata

In both, efferent motor pathway has 2 neurons

-Preganglionicneuron: exits the CNS and

synapses at an autonomic ganglion

-Postganglionicneuron: exits the ganglion

and regulates visceral effectors

-Smooth or cardiac muscle or glands


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Sympathetic division

-Preganglionic neurons originate in thethoracic and lumbar regions of spinal cord

-Most axons synapse in two parallel chainsof ganglia right outside the spinal cord

Parasympathetic division

-Preganglionic neurons originate in the brainand sacral regions of spinal cord

-Axons terminate in ganglia near or evenwithin internal organs


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Autonomic effects are mediated by the action

ofG protein-coupled receptors

-The receptor is activated by binding to its

ligand (Ach, for example)

-The G protein is activated

-It activates the effector protein


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Ch. 44 Nervous System