Transcript Nervous System

Structures and Processes of
the Nervous System – Part 2
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
The Electrical Nature of Nerves
 Neurons use electrical signals to communicate with
other neurons, muscles, and glands.
 The signals, called nerve impulses, involve changes
in the amount of electric charge across a cell’s
plasma membrane.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Resting Membrane Potential
 In a resting neuron, the cytoplasmic side of the
membrane (inside the cell) is negative, relative to
the extracellular side (outside the cell)
 This charge separation across the membrane is a
form of potential energy called membrane
potential.
When microelectrodes are inserted in a
resting neuron, a voltmeter can indicate an
electrical potential difference (voltage) across
a neural membrane.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Resting Membrane Potential
 The resting membrane potential is the potential
difference across a membrane in a resting neuron
(about -70 mV).
 It provides energy for generating a nerve impulse.
 The process of generating a resting membrane
potential of -70 mV is called polarization.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Sodium-Potassium Pump
 The sodium-potassium pump is the most important factor
that contributes to the resting membrane potential.
 This system uses ATP to transport three sodium ions (Na+) out
of the cell and two potassium ions (K+) into the cell.
 The overall result of this process is a constant membrane
potential of -70 mV.
Na/K Pump
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Action Potential
 Recall that a neuron is polarized due to the
charge difference across the membrane.
 Depolarization occurs when the cell becomes
less polarized
 During depolarization, the inside of the cell
becomes less negative relative to the outside
of the cell.
 An action potential causes depolarization to
occur.
Continued…
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Action Potential
 An action potential is the movement of an
electrical impulse along the plasma
membrane of an axon.
 It is an all-or-none phenomenon: if a stimulus
causes the axon to depolarize to a certain
level (the threshold potential), an action
potential occurs.
 Threshold potentials are usually close to -50
mV.
 The strength of an action potential does not
change based on the strength of the stimulus.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Action Potential
The graph shows the changes that occur to membrane potential as an action potential
travels down an axon.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Action Potential: Step 1
An action potential
is triggered when
the threshold
potential is reached.
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Action Potential: Step 2 –
depolarization
Voltage-gated sodium
(Na+) channels open
when the threshold
potential is reached.
Sodium ions move
down their
concentration gradient
and rush into the axon,
causing depolarization
of the membrane. The
membrane potential
difference is now
+40 mV.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Action Potential: Step 3 hyperpolarization
Voltage-gated sodium
channels close due to
change in membrane
potential. Voltage-gated
potassium (K+)
channels open.
Potassium ions move
down their
concentration gradient
and exit the axon,
causing the membrane
to be hyperpolarized to
-90 mV.
Section 8.2
UNIT
4
Action
Potential: Step 4 –
resting membrane restored
Voltage-gated
potassium channels
close. The sodiumpotassium pump and
naturally occurring
diffusion restore the
resting membrane
potential of -70 mV.
The membrane is now
repolarized.
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Action Potential
Section 8.2
 After an action potential occurs, the membrane cannot be
stimulated to undergo another action potential.
 This brief period of time (usually a few milliseconds) is
called the refractory period of the membrane.
 The events that occur in an action potential continue down
the length of the axon until it reaches the end, where it
initiates a response at the next cell.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Myelinated Nerve Impulse
 A nerve impulse consists of a
series of action potentials.
 Conduction of a nerve
impulse along a myelinated
neuron is called saltatory
conduction because action
potentials “jump” from one
node of Ranvier to the next.
 Saltatory conduction is faster
(120 m/s) than the
conduction of nerve impulses
in unmyelinated neurons (0.5
m/s).
Section 8.2
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a
Synapse
 synapse: the junction between two neurons, or between a
neuron and an effector)

neurons are not directly connected; the small gap
between them is called the synaptic cleft

A nerve impulse cannot jump from one neuron to another
Q: How does the nerve impulse proceed from the
presynaptic neuron (which sends out the information) to
the postsynaptic neuron (which receives the information)?
A: Chemical messengers called neurotransmitters carry
the nerve impulse across the synapse from one neuron to
another, or from a neuron to an effector.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a
Synapse: Step 1
The nerve impulse travels to the synaptic terminal.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a
Synapse: Step 2
Synaptic vesicles containing neurotransmitters move
toward and fuse with the presynaptic membrane.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a
Synapse: Step 3
Synaptic vesicles release neurotransmitters into the synaptic
cleft by exocytosis. Neurotransmitters diffuse across the
synapse to reach the postsynaptic neuron or the cell
membrane of an effector.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Signal Transmission Across a
Synapse: Step 4
Neurotransmitters bind to specific receptor proteins on the
postsynaptic membrane. The receptor proteins trigger ion
channels to open. Depolarization of the postsynaptic
membrane occurs, and an action potential is initiated if the
threshold potential is reached.
UNIT 4 Chapter 8: The Nervous System and Homeostasis
Section 8.2
Neurotransmitters
 Neurotransmitters have either excitatory or
inhibitory effects on the postsynaptic membrane.
 Excitatory molecules, like acetylcholine, cause
action potentials by opening sodium channels.
 Inhibitory molecules cause potassium channels to
open, causing hyperpolarization.
Neurotransmitters