ANATOMY AND PHYSIOLOGY OF
NEURONSAP BiologyChapter 48
Objectives
•Describe the different types of neurons
•Describe the structure and function of
dendrites, axons, a synapse, types of
ion channels, and neurotransmitters.
•Describe resting potential and the
sequence of events that occur during
an action potential.
Overview
• The human brain contains an estimated 1011 (100 billion) neurons.
•Each neuron may communicate with thousands of other neurons in complex information-processing circuits.
•Results of brain imaging and other research methods show that groups of neurons function in specialized circuits dedicated to different tasks.
Structural Organization of the Nervous System
•Central nervous system (CNS) – brain and spinal cord
• Responsible for integration of sensory input and associating stimuli with appropriate motor output
•Peripheral nervous system (PNS) –network of nerves extending into different parts of the body that carry sensory input to the CNS and motor output away from the CNA
Nervous systems consist of circuits of neurons and supporting cells
•All animals except sponges (Phylum
Porifera) have a nervous system
•What distinguishes nervous systems of
different animal groups is how
neurons are organized into circuits
Organization of Nervous Systems: Cnidarians
•The simplest
animals with
nervous systems,
the cnidarians,
have neurons
arranged in nerve
nets
Echinoderms
•Sea stars have a
nerve net in each
arm connected by
radial nerves to a
central nerve ring
•What is the
difference?
Platyhelmenthyes
•Relatively simple
cephalized animals, such
as flatworms, have a
central nervous system
(CNS)
•What changed?
Annelids and Arthropods
•Annelids and arthropods have segmentally arranged clusters of neurons called ganglia
• These ganglia connect to the CNS and make up a peripheral nervous system (PNS)
•Change?
Mollusks
•Nervous systems in mollusks
correlate with lifestyles
•Sessile mollusks have simple
systems, whereas more
complex mollusks have
more sophisticated systems
•Why?
Vertebrates
• In vertebrates, the central nervous system consists of a brain and dorsal spinal cord
•The PNS connects to the CNS
•What has been selected for?
Information Processing• Nervous systems process information in three stages:
Information Processing
•Sensory neurons pick up and transmit information from sensors that detect external stimuli (light, heat, touch) and internal conditions (blood pressure, muscle tension).
• Interneurons, in the CNS, integrate the sensory input
•Motor output leaves the CNS via motor neurons, which communicate with effector cells (muscle or endocrine cells).• Effector cells carry out the body’s response to a
stimulus.
Reflexes
•The stages of sensory input,
integration, and motor output are
easy to study in the simple nerve
circuits that produce reflexes, the body’s automatic responses to
stimuli.
Neuron Structure
•Most of a neuron’s organelles are in the cell body
•Most neurons have dendrites, highly branched extensions that receive signals from other neurons
• The axon is typically a much longer extension that transmits signals to other cells at synapses
•Many axons are covered with a myelinsheath• Which speeds up transmission
•Neurons have a wide variety of shapes that reflect input and output interactions
LE 48-6
Dendrites
Cell
body
Axon
InterneuronsSensory neuron Motor neuron
Neurons have a wide variety of shapes that reflect input and output interactions
NEURON STRUCTURE POGIL
Ion pumps and ion channels maintain the resting potential of a neuron
•Across its plasma membrane, every
cell has a voltage difference called
a membrane potential
•The cell’s inside is negative relative
to the outside
The Resting Potential
•Resting potential is the membrane potential of a neuron that is not transmitting signals
•Resting potential depends on ionic gradients across the plasma membrane
•Concentration of Na+ is higher in the extracellular fluid than in the cytosol
• The opposite is true for K+
Animation: Resting Potential
LE 48-10
CYTOSOL EXTRACELLULARFLUID
Plasmamembrane
NEURON FUNCTION POGIL
#s 1-6
Gated Ion Channels
•Gated ion channels open or close in response to one of three stimuli:• Stretch-gated ion channels
open when the membrane is mechanically deformed
• Ligand-gated ion channelsopen or close when a specific chemical binds to the channel
• Voltage-gated ion channels respond to a change in membrane potential
NEURON FUNCTION POGIL
#s 7-9
Action potentials are the signals conducted by axons
• If a cell has gated ion channels, its membrane potential may change in response to stimuli that open or close those channels
•Action potential: rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered selective opening and closing of gated ion channels.
•Once generated, the impulse travels rapidly down the axon away from the cell body and toward the axon terminals.
Production of Action Potentials
•Depolarizations are usually graded
only up to a certain membrane
voltage, called the threshold
•A stimulus strong enough to produce
depolarization that reaches the
threshold triggers a response called
an action potential
Action Potential
•An action potential is a brief all-or-none depolarization of a neuron’s
plasma membrane
• It carries information along axons
Action Potential
•Voltage-gated Na+ and K+ channels are involved in producing an action potential
•When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell
•As the action potential subsides, K+
channels open, and K+ flows out of the cell
Conduction of Action Potentials
•An action potential can travel long
distances by regenerating itself
along the axon
•At the site where the action
potential is generated, an electrical
current depolarizes the neighboring
region of the axon membrane
LE 48-14C
An action potential is generated as Na+ flows inward across the membrane at one location.
Na+
Action potential
Axon
Na+
Action potentialK+
The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.
K+
Na+
Action potentialK+
The depolarization-repolarization process is repeated in the next region of the 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.
K+
Conduction Speed
•The speed of an action potential
increases with the axon’s diameter
• In vertebrates, axons are myelinated,
also causing an action potential’s speed
to increase
LE 48-15
Cell body
Schwann cell
Depolarized region(node of Ranvier)
Myelinsheath
Axon
NEURON FUNCTION POGIL
#s 10-15
Four Phases of an Action Potential
•Resting state: no channels are open
•Depolarizing phase: membrane briefly reverses polarity
• Cell interior becomes positive to the exterior
•Repolarizing phase: returns membrane to its resting level
•Hyperpolarized phase: refractory period
Depolarization phase
•Na+ activation gates open allowing
an influx of Na+
•Potassium gates remain closed
• Interior of the cell becomes more
positive charged than the exterior
Repolarization phase
•Returns membrane to its resting level
•Gates close sodium channels and
opens potassium channels
•The inside of the cell becomes more
negative compared to the outside
of the cell
Hyperpolarization phase
•Membrane potential is temporarily more negative than the resting state
•Sodium channels remain closed by potassium channels remain open
•Refractory period occurs during this phase
• Neuron is insensitive to depolarizing stimuli
• This limits the maximum rate at which action potentials can be stimulated in a neuron.
LE 48-13_5
Resting potential
Threshold
Mem
bra
ne p
ote
nti
al
(mV
)
Actionpotential
Time–100
–50
+50
0
Potassiumchannel
Extracellular fluid
Plasma membrane
Na+
Resting state
Inactivationgate
Activationgates
Sodiumchannel K+
Cytosol
Na+
Depolarization
K+
Na+
Na+
Rising phase of the action potential
K+
Na+
Na+
Falling phase of the action potential
K+
Na+
Na+
Undershoot
K+
Na+
NEURON FUNCTION POGIL
#s 16-20
Neurons communicate with other cells at synapses
• In an electrical synapse, current flows directly from one cell to another via a gap junction
• The vast majority of synapses are chemical synapses
• In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters stored in the synaptic terminal
LE 48-17
Postsynaptic cellPresynaptic cell
Synaptic vesicles
containing
neurotransmitter
Presynaptic
membrane
Voltage-gated
Ca2+ channel
Ca2+Postsynaptic
membrane
Postsynaptic
membrane
Neuro-
transmitter
Ligand-
gated
ion channel
Na+
K+
Ligand-gated
ion channels
Synaptic cleft
When an action potential reaches a terminal, the final result is
release of neurotransmitters into the synaptic cleft
Synaptic Transmission
•Synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels
•Neurotransmitter binding causes ion channels to open, generating a new postsynaptic potential
•After release, the neurotransmitter diffuses out of the synaptic cleft
• It may be taken up by surrounding cells (reuptake) and/or degraded by enzymes
Neurotransmitters
•The same neurotransmitter can
produce different effects in different
types of cells
Generation of Postsynaptic Potentials
•Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell
•Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential
Postsynaptic Potentials
•Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the
membrane potential toward threshold
• This is what you modeled
• Inhibitory postsynaptic potentials (IPSPs)
are hyperpolarizations that move the
membrane potential farther from
threshold
Model 1: Action Potential
•The model will incorporate the
sodium-potassium pump previously
created.
• The model must include:
•Ligand gated channel (and a ligand)
•Voltage gated Na+ channel
•Voltage gated K+ channels
Model 2: Synaptic Transmission
•This model will use pipe cleaners
for the vesicles. Only the
neurotransmitters will interact with
the ligand gates of Model 1.
•Model 2 must include:
•Ca2+ Channels and Ca2+
•Neurotransmitter Vesicles and NTs
Neuron Modeling
•Model 1 must include:•Ligand gated channel (and a ligand/NT)
•Voltage gated Na+ channel
•Voltage gated K+ channels
•Model 2 must include: •Ca2+ Channels and Ca2+
•Neurotransmitter Vesicles and NTs
•NTs will go to the ligand gates of Model 1
Neuron Quiz Tomorrow
•Sodium-Potassium pump and establishment of Resting Potential
•Anatomy of a neuron and types of neurons
•Action Potential! (the whole thing)
•Membrane Potential Graph
•Synaptic Transmission, neurotransmitters and reuptake
•Myelin (just know what it does)
• IPSP and EPSP
NEURON REVIEW KAHOOT
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