Nervous Tissue
Overview of Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• endocrine and nervous system work together to
maintain homeostasis
• overview of the nervous
system
– endocrine system - communicates by chemical messengers
(hormones) secreted into blood
– nervous system - employs electrical and chemical means to send
messages from cell to cell
• properties of neurons
• supportive cells (neuroglia)
• nervous system carries out its task in three basic steps:
• Sensory receptors detect external/internal stimuli and
transmits messages to spinal cord and brain
• brain and spinal cord processes this information
• Brain and spinal cord send commands (or not) to carry out a
response
• electrophysiology of neurons
• synapses
Neurofibrils
• neural integration
Axon
(d)
12-1
12-2
2 Major Subdivisions of Nervous System
Central nervous
system (CNS)
Subdivisions of Nervous System
Peripheral nervous
system (PNS)
Brain
Peripheral nervous system
Central nervous system
Spinal
cord
Nerves
Brain
Ganglia
Spinal
cord
Visceral
sensory
division
Sensory
division
Somatic
sensory
division
Motor
division
Autonomic
Nervous
System
Somatic
motor
division
Skin, bones,
Muscles, Joints,
Senses
Sympathetic
division
12-3
Parasympathetic
division
12-4
Universal Properties of Neurons
Dendrites
•
2 types of cells make up nervous
tissue:
1. Neurons
2. Neuroglial cells (aka Glial cells)
• excitability (irritability)
– respond to stimuli
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
• conductivity
– Cells produce electrical signals that are conducted
to other cells
Initial segment
Structure of a Neuron
Axon collateral
• soma – Cell body, neurosoma, cell body, or
Axon
perikaryon
– nucleus
– cytoplasm contains organelles
• Nissl bodies
– Cytoskeleton: microtubules & neurofibrils
(bundles of actin filaments)
– no centrioles – no further cell division
• secretion
– when electrical signal reaches end of nerve fiber, a
chemical neurotransmitter that can stimulate next
cell
•
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Dendrites
Terminal
arborization
Figure 12.4a
Figure 12.4a
Synaptic knobs
12-5
(a)
12-6
1
Structure of a Neuron
• axon (nerve fiber) –
– cylindrical, relatively
unbranched for most of its
length
– specialized for conduction of
nerve signals
– axoplasm – cytoplasm of axon
– axolemma – plasma membrane
of axon
– one axon per neuron
– Schwann cells and myelin
sheath enclose axon
– terminal ends
– synaptic knob
Functional Classes of Neurons
Peripheral nervous system
Dendrites
Central nervous system
1 Sensory (afferent)
neurons.
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
3 Motor (efferent)
neurons.
Direction of
signal transmission
2 Interneurons
(association
Neurons)
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Terminal
arborization
Figure 12.3
Figure 12.4a
Synaptic knobs
12-7
12-8
(a)
Axonal Transport
Classification according to Structure
• multipolar neuron
• axonal transport – two-way passage of proteins,
organelles, and other material along an axon
– one axon and multiple dendrites
• bipolar neuron
Dendrites
– anterograde transport – movement down the axon away from
soma
– retrograde transport – movement up the axon toward soma
Axon
– one axon and one dendrite
Multipolar neurons
• unipolar neuron
Dendrites
– single process leading away from the
soma
• microtubules guide materials along axon
• anaxonic neuron
– motor proteins (kinesin and dynein) carry materials “on their backs”
while they “crawl” along microtubules
Axon
– many dendrites but no axon
Bipolar neurons
Dendrites
Axon
Unipolar neuron
Dendrites
Anaxonic neuron
Figure 12.5
12-9
12-10
Six Types of Neuroglial Cells
Neuroglial Cells
• 4 types occur only in CNS
– oligodendrocytes
• about a trillion (1012) neurons in nervous system
• form myelin sheaths in CNS
• neuroglia outnumber neurons by as much as 50 to 1!!!!!!!!!!!!!
– ependymal cells
• Neuroglia or glial cells:
• line cavities of the brain
• secrete cerebrospinal fluid (CSF)
– 6 types (4 types occur in CNS & 2 types occur in PNS)
– support and protect neurons
– bind neurons together and form framework for nervous
tissue
– microglia
• small, wandering macrophages (derived from WBCs)
• search for cellular debris to phagocytize
- Astrocytes (most abundant glial cell in CNS)
• BBB
• Nerve growth factor
• For scar tissue
12-11
12-12
2
Neuroglial Cells
Neuroglial Cells of CNS
• 2 types occur only in PNS
– Schwann cells
Capillary
Neurons
Astrocyte
Oligodendrocyte
Perivascular feet
Myelinated axon
Ependymal cell
Myelin (cut)
Cerebrospinal fluid
Microglia
• produce a myelin sheaths
• assist in regeneration of damaged fibers
– satellite cells
• surround cell bodies in ganglia of PNS
Figure 12.6
12-13
12-14
Myelin Sheath in PNS
Myelination in PNS
Schwann cell
Axoplasm
Schwann cell
nucleus
Axon
Axolemma
Endoneurium
Basal lamina
Neurilemma
Nucleus
(a)
Neurilemma
Myelin sheath
(c)
Myelin sheath
Figure 12.7a
12-15
Myelination in CNS
12-16
Speed of nerve signal along a
nerve fiber depends on two
factors:
.
….
Oligodendrocyte
Soma
Nucleus
1. diameter of fiber
2. presence or absence of myelin
sheath
Signal conduction occurs along the
surface of a fiber
- larger fibers have more surface
area and conduct signals more
rapidly
- myelin acts as a conductor
Myelin
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Nerve fiber
Terminal
arborization
(b)
Synaptic knobs
12-17
(a)
3
Regeneration of Nerve Fiber
•
soma is intact & some neurilemma
remains
Resting Membrane Potential
Neuromuscular
junction
Myelin sheath
Endoneurium
• RMP exists because of unequal electrolyte (ions) distribution
between extracellular fluid (ECF) and intracellular fluid (ICF)
Muscle fiber
1 Normal nerve fiber
• fiber distal to injury degenerates
– macrophages clean up tissue debris at
the point of injury and beyond
Local trauma
Macrophages
• soma swells, ER breaks up, and
nucleus moves off center
– due to loss of nerve growth factor from
neuron’s target cell
Degenerating
Schwann cells
Degenerating axon
3 Degeneration of severed fiber
Growth processes
• axon stump sprouts growth processes
4 Early regeneration
• regeneration tube – formed by
Schwann cells, basal lamina, and the
neurilemma near injury
• Three factors about a resting neuron
– ions diffuse down their concentration gradient through the
membrane
– plasma membrane is selectively permeable and allows some
ions to pass easier than others
– electrical attraction of cations and anions to each other
Degenerating
terminal
2 Injured fiber
Schwann cells
Regeneration
tube
Atrophy of
muscle fibers
Retraction of
growth processes
Growth processes
5 Late regeneration
– tube guides the growing sprout back to
original target cells and reestablishes
synaptic contact
6 Regenerated fiber
Regrowth of
muscle fibers
• NOT POSSIBLE IN CNS
12-19
12-20
Resting Membrane Potential
Plasma membrane has ion channels
Voltage-activated
Chemically activated
Na+
Passive
K+
CB
K+
K+
Axon
Depolarization occurs when MP becomes more positive
Hyperpolarization: MP becomes more negative than RMP
Repolarization: MP returns to RMP
Resting potential or membrane potential
Creating the Resting Membrane Potential
(RMP)
-
1. Proteins
• At rest, all cells have a negative internal charge and
unequal distribution of ions:
– Results from:
• Large anions are trapped inside cell
• Na+/K+ pump and limited permeability keep Na+ high
outside cell
• K+ is very permeable and is highly concentrated
inside cell (i.e., moves down gradient to outside of
cell)
-
K+ -
2 K+
CB
-
3 Na+
2. Sodium-potassium pump
3. K+ can leak out (diffusion)
7-27
4
Resting Membrane Potential
ECF
Na+ 145 mEq/L
K+
4 mEq/L
K+
channel
Na+
channel
Na+ 12 mEq/L
K+ 150 mEq/L
Large anions
that cannot
escape cell
ICF
• Na+ concentrated outside of cell (ECF)
• K+ concentrated inside cell (ICF)
Resting potential
12-26
Nerve impulse or Action potential
Stimulus
CB
Na
++++++++++++++++++++++++++++++++++++
--
- - - - - - - - - - - - - - - - - - - - -- - - - - - - - - -------------------------- ---- - -
++ +
CB
+++++++++++++++++++++++++++++++++++++
--
++
+ ++
- - Na - -
Polarized
+++++++++++++++
--------------------------------+++++++++++++++
Depolarized
Action potential
Action potential
Na
CB
+++++ --- - - - - - - + ++
- - - - - - - + ++ + +
-+++++ -Na
+++++++++
----------------------+++++++++
Na
CB
++++++++++++ -- - - - - - - - - - - - - - - - - - ++
------------------+ +
++++++++++++ --
Na
-+
+
+ +
--
K+ K+ K+
Repolarization
Polarized
5
Stimulus
Axon
Area of repolarization
Area of depolarization
(a)
Area of depolarization
Potassium
channel
Action potential
Sodium
channel
Action potential
(c)
(b)
Fig. 39.08ab
Fig. 39.08c
Action Potentials
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
– follows an all-or-none
law!
– nondecremental - do not get
weaker with distance
– irreversible - once started goes
to completion and can not be
stopped
4
+35
3
5
0
Depolarization
mV
• only a thin layer of the cytoplasm
next to the cell membrane is
affected
• action potential is often called a
spike
• Characteristics of action potential
Repolarization
Action
potential
Threshold
2
–55
Local
potential
1
7
–70
Resting membrane
potential
6
Hyperpolarization
Time
(a)
12-34
Sodium and Potassium Gates
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
Na+
K+
gate
1 Na+ and K+ gates closed
35
0
–70
Resting membrane
potential
mV
mV
Na+
gate
2 Na+ gates open, Na+
enters cell, K+ gates
beginning to open
35
0
–70
Depolarization begins
Figure 12.14
mV
–70
Depolarization ends,
repolarization begins
4 Na+ gates closed,
K+ gates closing
mV
35
0
35
0
3 Na+ gates closed, K+ gates
fully open, K+ leaves cell
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
–70
Repolarization complete
12-35
6
Refractory Period
Absolute
refractory
period
Relative
refractory
period
+35
0
mV
• Refractory period – period of
resistance to stimulation
• two phases of the refractory
period
– absolute refractory period
• no stimulus of any
strength will trigger AP
• as long as Na+ gates are
open
– relative refractory period
• only especially strong
stimulus will trigger new
AP
Threshold
–55
Resting membrane
potential
–70
Time
12-38
Saltatory Conduction
Myelinated Fibers
Nerve Signal Conduction Unmyelinated Fibers
• Voltage-gated channels needed for APs
Dendrites
Cell body
– fewer than 25 per m2 in myelin-covered regions (internodes)
– up to 12,000 per m2 in nodes of Ranvier
Axon
• fast Na+ diffusion occurs between nodes
– signal weakens under myelin sheath, but still strong enough to stimulate an
action potential at next node
Signal
Action potential
in progress
++++–––+++++++++++
––––+++–––––––––––
• saltatory conduction – the nerve signal seems to jump from node to
node
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Refractory
membrane
Excitable
membrane
–––– +++–––––––––––
++++–––+++++++++++
+++++++++–––++++++
–––––––––+++––––––
–––––––––+++––––––
+++++++++–––++++++
+++++++++++++–––++
–––––––––––––+++––
Figure 12.17a
12-39
–––––––––––––+++––
+++++++++++++–––++
(a)
Saltatory Conduction
Na+ inflow at node
generates action potential
(slow but nondecremental)
Na+ diffuses along inside
of axolemma to next node
(fast but decremental)
Excitation of voltageregulated gates will
generate next action
potential here
12-40
Local Potentials
+ +
– –
– –
+ +
– –
+ +
+ +
– –
+ +
– –
– –
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
– –
+ +
+ +
– –
+ +
– –
– –
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
– –
+ +
+ +
– –
+ +
– –
– –
+ +
+ +
– –
– –
+ +
• Neuron response begins at the dendrite
• when neuron is stimulated at dendrite (or soma)
– opens Na+ gates and allows Na+ to rush in to the cell
– Na+ inflow neutralizes some of the internal negative charge
• Depolarization!
(b)
Action potential
in progress
Refractory
membrane
Excitable
membrane
12-41
• for short distance on the inside of the plasma membrane
producing a current that travels towards the cell’s trigger zone –
this short-range change in voltage is called a local potential
12-42
7
EPSPs
Characteristic of Local Potentials
• They are different than APs:
– graded - vary in magnitude with stimulus strength
• stronger stimuli open more Na+ gates!
– decremental - get weaker the farther they travel from
the point of stimulation
– reversible - when stimulation ceases, K+ diffusion out
of cell returns cell to normal resting potential
– can be either excitatory (EPSP) or inhibitory (IPSP)
• Graded in
magnitude
• Have no threshold
• Cause
depolarization
• Have no refractory
period
• Summate
- Stimuli (neurotransmitters) make the membrane
potential more negative – hyperpolarize it – becomes
less sensitive and less likely to produce an action
potential
12-43
7-76
• http://www.sinauer.com/neuroscience4
e/animations5.2.html
8