Transcript The Nervous System 35-2

The Nervous
System
The nervous system controls and
coordinates functions throughout the
body and responds to internal and
external stimuli.
Neurons

Messages carried by the nervous system
are electrical signals called impulses.

Impulses are transmitted by neurons
3 Types of Neurons
Based on the direction in which an
impulse travels
 Sensory neurons – carry impulses from
the sense organ s to the spinal cord
 Motor neurons – carry impulses from the
brain and the spinal cord to muscles and
glands
 Interneurons – connect sensory and motor
neurons and carry impulses between them

Neurons (nerve cells)
Parts of a neuron

Cell body – The largest part of the neuron,
contains the nucleus and cytoplasm

Dendrites – short branches off the cell
body that carry impulses from the
environment or from other neurons
toward the cell body.

Axon – long fiber that carries impulses
away from the cell body

Myelin sheath – insulating membrane
located on the axon
– has gaps in it called nodes, these nodes
speed up the rate the impulse moves
The Nerve Impulse
The resting impulse
The resting neuron has a positive charge
outside the cell and a negative charge
inside the cell.
 This is caused by sodium and potassium
pump which pumps sodium out and
potassium into the cell.
 Potassium can leak out of the cell faster
than sodium which leaves a negative
charge on the inside of the cell.


This leaves a positive charge outside the
cell and a negative charge inside the cell
membrane.

Resting potential – the electrical charge
across the cell membrane of a neuron in
its resting state
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hapter2/animation__how_the_sodium_potassium_pump_works.html
The Moving Impulse
An impulse begins when a neuron is
stimulated by another neuron or by the
environment.
 The impulse travels down the axon away
from the cell body and toward the axon
terminals
 Na+ flows into the cell reversing the
resting potential

Action potential
Action potential – the change from a
negative to a positive charge along the
axon
 As the impulse passes the K+ gates open
up an allow K+ to flow out
 This restores the resting potential

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ew0/chapter14/animation__the_nerve_impulse.html
Threshold

The strength of an impulse is always the
same
– It either happens or it doesnt

Threshold – the minimum level of a
stimulus that is required to activate a
neuron.
The Synapse
At the end of the neuron, the impulse
reaches an axon terminal.
 This is where the neuron touches another
cell.
 The neuron can pass the impulse along to
another cell.
 This location is called a synapse.

Neurotransmitters

The chemicals used by neurons to
transmit an impulse across a synapse to
another cell