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The Nervous SystemThe Nervous SystemThe Nervous SystemThe Nervous System

Ch. 7Ch. 7Ch. 7Ch. 7Organization, Structure & FunctionOrganization, Structure & FunctionOrganization, Structure & FunctionOrganization, Structure & Function

Did you know…Did you know…Did you know…Did you know…

•Nerve impulses travel over 250 mph!Nerve impulses travel over 250 mph!Nerve impulses travel over 250 mph!Nerve impulses travel over 250 mph!

•There are 45 miles of nerves just in the skin!There are 45 miles of nerves just in the skin!There are 45 miles of nerves just in the skin!There are 45 miles of nerves just in the skin!

3 Major Functions

1. Sensory Input – detecting stimuli from the

5 senses

2. Integration – processing & interpreting

stimuli in the brain

3. Motor Output – responding to stimuli via

glands or muscles

Nervous System Fucntions

Organization of

the

Nervous System

(CNS)

(PNS)

Structural Organization

Central Nervous System (CNS)

Brain &

Spinal cord

Peripheral Nervous System (PNS)

Nerves that

extend from

brain and spinal

cord

Peripheral NS – Functional Organization

Sensory (afferent) Div.

Carries impulses TOTOTOTO the CNS

from sensory organs:

Motor (efferent) Div.

Carries impulses FROMFROMFROMFROM

the CNS

to other organs:

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Nerves are organized like an interstate….

”traffic” only goes in 1 direction

Afferent

Pathway

(sensory nerves)

Efferent

Pathway

(motor nerves)

The Motor (efferent) Division

Somatic Nervous

System(“voluntary” NS)

Controls voluntary

movement

(skeletal muscles)

Autonomic

Nervous System(“involuntary” NS)

Controls involuntary actions

(smooth & cardiac muscle, glands)

2 parts…

Autonomic NS has 2 parts…

Sympathetic

prepares body for

emergency

(“fight-or-flight” mechanism)

Parasympathetic

Each regulates

the other

calms the body

down

MAIN TYPES OF NERVE CELLS

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NEUROGLIA� Made of connective tissue

� Functions: protection, support, connection; do not transmit impulses like neurons, but can reproduce

� 4 main types:– OLIGODENDROGLIA--form myelin sheath in CNS; hold nerve fibers together

– MICROGLIA--smallest; phagocytosis

– ASTROCYTES--star-shaped; largest & most numerous; forms blood-brain barrier

– SCHWANN CELLS--form myelin sheath in PNS; found around nerve fibers in PNS ONLY

CNS

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NEURONS� Function is to transmit impulses

� 3 main parts:– Cell Body--made of gray matter; clusters are called nuclei in CNS--ganglia in PNS; largest part of neuron; contains organelles found in all cells:

– Dendrites--”branches” off cell body; distal ends are receptors; conduct impulses to cell body

– Axon--usually one/neuron; carries messages away from cell body; myelin sheath covers axon; collaterals--branches of axon; Length of axons?

Dendrites & axons determine the direction of an impulse

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Other Structural Features of Nerve Cells

� Myelin Sheath

– Segmented fat-like membrane (white matter)

– Made of Schwann cells in PNS; oligo. in CNS

– One segment of sheath extends from one Node of Ranvier to another.

– Saltatory Conduction--impulses jump from one Node of Ranvier to another; allows for faster conduction of impulses

� Axon terminals-where vesicles release

neurotransmitters

� Synapses/synaptic cleft- space b/t neurons

Label the diagram:

Anatomy of Anatomy of Anatomy of Anatomy of MultipolarMultipolarMultipolarMultipolar Motor Motor Motor Motor

NeuronNeuronNeuronNeuron

CLASSIFICATION OF NEURONS

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By the number of processes

(Structural Classification)

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�Unipolar—

�Bipolar—

�Multipolar—

Structural Classfication

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Structural Classification of

Neurons

Based on the number of

processes that extend from the cell body of the neuron

By their direction & destination

(Functional Classification)

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Sensory Neurons

� cell body is always outside the CNS

� carry impulses to CNS

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Associative Neurons

�Cell body is in CNS

�provides a link b/t sensory & motor neurons

–Also called Interneurons

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Motor neurons

�Cell body is always in the CNS

�carries impulses away from CNS to the muscles or glands (effectors)

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Other Important Terms:

Definition CNS PNS

Bundles of CELL BODIES “nuclei” “ganglia”

Bundles of AXONS “tracts” “nerves”

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Myelinated parts of neurons “white matter”

Unmyelinated parts of neurons

“gray matter”

Definition CNS PNS

These terms are mainly associated with the brain: Physiology of a Neuron

� There are 3 characteristics of all neurons:

– Excitability/Irritability—

– Conductivity—

– Integration—

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Transmission of a Nerve Impulse

� The plasma membrane of a resting neuron is polarized. This means that there are less positive ions on the inside of the membrane than the outside of the membrane. The INSIDE is more NEGATIVE.

� There are 3 sodium ions (Na+) on the outside of the membrane while there are 2 potassium ions (K+) on the inside.

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Depolarization

� When an impulse is generated by stimuli, the permeability of the membrane changes.

� Since the permeability has changed, Na+ flow into the neuron. This causes the polarity to change and the inside of the membrane is now more positive. This is called depolarization which causes the neuron to transmit an action potential.

� Action potential occurs following the “all-or-none” principle.

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Depolarization (cont.)

� Once action potential occurs, it causes a domino effect down the length of the axon—switching the polarity of the membrane.

� Main point: No sodium ion entry-no impulse transmission.

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Repolarization

� Once the impulse passes through a part of the axon, the permeability changes again and the K+ flow out of the neuron causing the polarity to change as well. This is called repolarization.

� A new impulse cannot be transmitted until repolarization occurs.

� The sodium-potassium pump restores the concentration of sodium/potassium ions on either side of the membrane.

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Saltatory Conduction

� Nerve impulse transmission as just described only happens in unmyelinated nerve fibers.

� Myelinated nerve fibers transmit impulses through saltatory conduction—impulses jumping from one Node of Ranvier to another.

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Synaptic Transmission

� A synapse is the area/space between neurons.� Axonal terminals are found at the end of the axon. These terminals have bulbs called synaptic end knobs. Neurotransmitters accumulate here.

� Impulse travels through axon and reaches the end knobs. Vesicles discharge the neurotransmitter through presynaptic membrane.

� Neurotransmitter diffuses through postsynpatic membrane on receptor of next neuron and changes the polarity of that membrane, creating new action potential moving through neuron.

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Reflex Arcs

� Routes of conduction to and from CNS

� 2-neuron arc--Consists of afferent and efferent neurons

� 3-neuron arc—Consists of afferent neurons, interneurons, and efferent neurons.

� A reflex is a response to impulse conduction.

� There are 2 types of reflexes: muscle contraction and gland secretion.

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