Transcript Chapter 12 Nervous Tissue

Nervous
Tissue
Nervous System
Controls and integrates all body activities
Basic functions:
Sense change
Interpret and remember change
React to changes
Nervous vs Endocrine System
Nervous system
Endocrine system
electrical
chemical
fast
slow
local
general
Nervous System
CNS – brain/spinal cord
Integration
Processing
input
output
Sensory
stimulus
Motor
PNS
response
Organization
Central Nervous System – CNS
brain
spinal cord
Peripheral Nervous System - PNS
somatic (SNS)
sensory
motor
autonomic (ANS)
sensory
motor
parasympathetic
sympathetic
Neurons
Functional unit of the Nervous System
Mitochondrion
Dendrite
Cell
body
Nissl substance
Axon
hillock
Axon
Neurofibrils
Nucleus
Collateral
branch
One
Schwann cell
Axon
terminal
(a)
Transmit electrical impulses (action potentials)
Node of
Ranvier
Schwann cells,
forming the myelin
sheath on axon
Structural Classes of Neurons
Functional Classes of Neurons
Afferent
Functional Classes of Neurons
Efferent
Functional Classes of Neurons
Interneurons
Functional Classes of Neurons
Central process (axon)
Cell
body
Sensory
neuron
Ganglion
Dendrites
Peripheral
process (axon)
Afferent
transmission
Interneuron
(association
neuron)
Peripheral
nervous system
Receptors
Efferent transmission
Motor neuron
To effectors
(muscles and glands)
Spinal cord
(central nervous system)
Neuroglia
Schwann
Cells
Schwann cell
cytoplasm
Axon
Schwann cell
plasma membrane
Schwann cell
nucleus
Myelin:
(a)
‘Insulates’ axon.
Increases
transmission of
signal.
(b)
Node of Ranvier:
Neurilemma
Exposed axon
between Schwann
cells
Myelin
sheath
(c)
Gray and White Matter
Right side of brain
Overview
of
Nervous
Function
Left side of brain
5
Cerebral cortex
Brain
Interneuron
Upper motor neuron
4
6
Thalamus
3
Interneuron
Sensory
neuron
7
2
Sensory
receptor
1
Lower motor
neuron
Key:
8
Neuromuscular
junction
Skeletal muscles
Spinal cord
Graded potential
Nerve action potential
Muscle action potential
Ion Channels
Leakage Channel
Ligand-gated channels
Mechanically gated channels
Voltage-gated channels
Ion Channels Animation
Ion Channels
Extracellular fluid
K+ leak channel
closed
Plasma membrane
Cytosol
K+ leak channel
open
K+
Channel randomly
K+
opens and closes
(a) Leakage channel
Extracellular fluid
Ligand-gated
channel closed
Plasma membrane
Ca2+
Na+
Acetylcholine
Chemical stimulus
opens the channel
K+
Cytosol
(b) Ligand-gated channel
Ligand-gated
channel open
Ion Channels
Extracellular fluid
Mechanically gated channel
closed
Plasma membrane
Cytosol
Mechanically gated
channel open
Ca2+
Na+
Mechanical stimulus
opens the channel
(c) Mechanically gated channel
Extracellular fluid
Voltage-gated K+
channel closed
Plasma membrane
Cytosol
K+
K+
Change in
membrane potential
opens the channel
Voltage = –70 mV
Voltage = –50 mV
(d) Voltage-gated channel
Voltage-gated
K+ channel open
Ion Channels
Electrical Signals in Neurons

Like muscle fibers, neurons are electrically excitable. They
communicate with one another using two types of
electrical signals:
 Graded potentials are used
for short-distance
communication only.
 Action potentials allow
communication over long
distances within the body.
Copyright © John Wiley & Sons, Inc. All rights reserved.
Resting Membrane Potential
• Negative ions along inside of cell membrane &
positive ions along outside
– potential energy difference at rest is -70 mV
• Resting potential exists because
– concentration of ions different inside & outside
• extracellular fluid rich in Na+ and Cl• cytosol full of K+, organic phosphate & proteins
– membrane permeability differs for Na+ and K+
• 50-100x’s greater permeability for K+
• inward flow of Na+ can’t keep up with outward flow of K+
• Na+/K+ pump removes Na+ as fast as it leaks in
Resting Membrane Potential
Extracellular
fluid
Extracellular
fluid
Plasma membrane
Cytosol
Equal numbers of +
and – charges in
most of ECF
Resting membrane
potential
(an electrical potential
difference across the
plasma membrane)
Cytosol
Equal numbers of +
and – charges in most
of cytosol
(a) Distribution of charges that produce the resting membrane potential of a neuron
Resting Membrane Potential
Graded Potential
Typically on dendrites or cell body
Graded means that potential varies in amplitude.
Stronger the stimulus, greater the amplitude.
Stronger the stimulus the farther it will travel.
Decreases as it gets farther away from the stimulus
point.
Graded Potentials Animation
Graded Potential
Graded Potential
Extracellular fluid
Plasma membrane
Ca2+
Acetylcholine
Ligand-gated channel Na+
closed
Cytosol
Ligand-gated channel
open
Binding of acetylcholine
Resting
membrane
potential
Depolarizing
graded
potential
K+
(b) Depolarizing graded potential caused by the neurotransmitter acetylcholine, a ligand stimulus
Extracellular fluid
Ligand-gated channel
closed
Resting
membrane
potential
Plasma membrane
Cytosol
Glycine
Cl–
Ligand-gated channel
open
Binding of glycine
(c) Hyperpolarizing graded potential caused by the neurotransmitter glycine, a ligand stimulus
Hyperpolarizing
graded
potential
Graded Potential
Graded Potential
Action Potential
Action Potential
Extracellular fluid
Na+
Na+ channel
Plasma membrane
K+ channel
Activation
gate closed
Inactivation
gate open
K+
mV
Time
Cytosol
1. Resting state:
All voltage-gated Na+ and K+
channels are closed. Axon plasma
membrane is at resting membrane
potential: small buildup of negative
charges along inside surface of
membrane and equal buildup of
positive charges along outside surface
of membrane.
Action Potential
2. Depolarizing phase:
When membrane potential of axon reaches threshold, Na+
channel activation gates open. As Na+ ions move through
these channels into neuron, buildup of positive charges forms
along inside surface of membrane and membrane becomes
depolarized.
Na+
mV
Time
K+
Action Potential
mV
Time
Na+
3. Repolarizing phase begins:
Na+ channel inactivation gates close and
K+ channels open. Membrane starts to
become repolarized as some K+ ions
leave neuron and few negative charges
begin to build up along inside surface of
membrane.
K+
Action Potential
Na+
K+
4. Repolarization phase continues:
K+ outflow continues. As more K+ ions leave neuron, more
negative charges build up along inside surface of membrane.
K+ outflow eventually restores resting membrane potential.
Na+ channel inactivation gates open. Return to resting state
occurs when K+ gates close.
mV
Time
Action Potential
Comparison of Graded & Action
Potentials
Continuous Conduction
Cell body
Time
Na+
1
msec
Na+
Current flow due to
opening of Na+ channels
Trigger zone
Na+
5
msec
Na+
Na+
10
msec
Na+
Leading edge of
action potential
(a) Continuous conduction
Saltatory Conduction
Cell body
Time
Na+
1
msec
Nodes of Ranvier
Na+
Current flow due to
opening of Na+ channels
Trigger zone
Na+
5
msec
Na+
Na+
10
msec
Na+
Leading edge of
action potential
(b) Saltatory conduction
Stimulus Intensity
How do we differentiate a light touch from a firmer
touch?
– frequency of impulses
• firm pressure generates impulses at a higher frequency
– number of sensory neurons activated
• firm pressure stimulates more neurons than does a light touch
Signal Transmission at Synapses
2 Types of synapses
– electrical
• ionic current spreads to next cell through gap junctions
• faster, two-way transmission & capable of synchronizing
groups of neurons
– chemical
• one-way information transfer from a presynaptic neuron to
a postsynaptic neuron
– axodendritic -- from axon to dendrite
– axosomatic -- from axon to cell body
– axoaxonic -- from axon to axon
Chemical Synapse
Presynaptic
neuron
1
Nerve impulse
2
2
Ca2+
Ca2+
Voltage-gated Ca2+ channel
Synaptic end bulb
Cytoplasm
Synaptic
vesicles
Synaptic cleft
Ca2+
3
Neurotransmitter
4
Neurotransmitter
receptor
Ligand-gated
channel closed
Na+
Ligand-gated
channel open
5
Postsynaptic neuron
6 Postsynaptic
potential
7
Nerve impulse
Neurotransmitters
Acetylcholine
ATP and Other Purines
Amino Acids
Nitric oxide
glutamate and aspartate
GABA and glycine
Biogenic amines
norepinephrine
epinephrine
dopamine
serotonin
Neuropeptides
endorphins
enkephalin
dynorphins
substance P
Neurotransmitters
Postsynaptic potentials
Excitatory postsynaptic
potential (EPSP)
Na+ and K+ gates open at
the same time, Na+ diffuses
faster results in a
depolarizing potential
Postsynaptic Potential
Inhibitory postsynaptic
potential (IPSP)
Membrane made more
permeable to K+ and Cl-,
Na+ not affected results in
a hyperpolarization
Removal of Neurotransmitter
Neurotransmitter must be removed from the synapse for
normal synaptic function.
- Diffusion
- Enzymatic degradation
- Uptake by cell
Events at the Synapse
Summation
Summation
Presynaptic neuron 3
Cell body
Dendrites
Presynaptic neuron 2
Presynaptic neuron 4
Axon
Axon
terminal
Presynaptic neuron 1
EPSP
Excitatory
neurotransmitter
Postsynaptic neuron
Presynaptic neuron 5
Inhibitory
neurotransmitter
Trigger zone (net summation of
EPSPs and IPSPs determines
whether an action potential is
generated here)